The Landscape of Somatic Genetic Alterations in Breast Cancers From ATM Germline Mutation Carriers

ERBB2 mutations define a subgroup of endometrial carcinomas associated with high tumor mutational burden and the microsatellite instability-high (MSI-H) molecular subtype

Anti-HER2 therapy is indicated for erb-b2 receptor tyrosine kinase 2 (ERBB2)-amplified/overexpressing endometrial carcinoma (EC). Mutations constitute another mode of ERBB2 activation, but only rare ERBB2-mutated ECs have been reported. We sought to characterize the clinicopathologic and genetic features of ERBB2-mutated EC. From an institutional cohort of 2638 ECs subjected to clinical tumor-normal panel sequencing, 69 (2.6%) with pathogenic ERBB2 mutation(s) were identified, of which 11 were also ERBB2-amplified. The most frequent ERBB2 hotspot mutations were V842I (38%) and R678Q (25%). ERBB2 mutations were clonal in 87% of evaluable cases. Immunohistochemistry revealed low HER2 protein expression in most ERBB2-mutated ECs (0/1+ in 66%, 2+ in 27%); all 3+ tumors (7.3%) were also ERBB2-amplified. Compared to ERBB2-wildtype ECs (with or without ERBB2 amplification), ERBB2-mutated/non-amplified ECs were enriched for the microsatellite instability-high (MSI-H) and, to a lesser extent, DNA polymerase epsilon, catalytic subunit (POLE) molecular subtypes, and associated with high tumor mutational burden and low chromosomal instability. Survival outcomes were similar between patients with ERBB2-mutated/non-amplified versus wildtype EC, whereas ERBB2 amplification was associated with worse prognosis on univariate, but not multivariate, analyses. In conclusion, ERBB2 mutation defines a rare subgroup of ECs that is pathogenically distinct from ERBB2-wildtype and ERBB2-amplified ECs.

Co-observation of germline pathogenic variants in breast cancer predisposition genes: Results from analysis of the BRIDGES sequencing dataset

Co-observation of a gene variant with a pathogenic variant in another gene that explains the disease presentation has been designated as evidence against pathogenicity for commonly used variant classification guidelines. Multiple variant curation expert panels have specified, from consensus opinion, that this evidence type is not applicable for the classification of breast cancer predisposition gene variants. Statistical analysis of sequence data for 55,815 individuals diagnosed with breast cancer from the BRIDGES sequencing project was undertaken to formally assess the utility of co-observation data for germline variant classification. Our analysis included expected loss-of-function variants in 11 breast cancer predisposition genes and pathogenic missense variants in BRCA1, BRCA2, and TP53. We assessed whether co-observation of pathogenic variants in two different genes occurred more or less often than expected under the assumption of independence. Co-observation of pathogenic variants in each of BRCA1, BRCA2, and PALB2 with the remaining genes was less frequent than expected. This evidence for depletion remained after adjustment for age at diagnosis, study design (familial versus population-based), and country. Co-observation of a variant of uncertain significance in BRCA1, BRCA2, or PALB2 with a pathogenic variant in another breast cancer gene equated to supporting evidence against pathogenicity following criterion strength assignment based on the likelihood ratio and showed utility in reclassification of missense BRCA1 and BRCA2 variants identified in BRIDGES. Our approach has applicability for assessing the value of co-observation as a predictor of variant pathogenicity in other clinical contexts, including for gene-specific guidelines developed by ClinGen Variant Curation Expert Panels.

Ataxia-Telangiectasia Mutated Loss-of-Function Displays Variant and Tissue-Specific Differences across Tumor Types

PURPOSE

Mutations in the ATM gene are common in multiple cancers, but clinical studies of therapies targeting ATM-aberrant cancers have yielded mixed results. Refinement of ATM loss of function (LOF) as a predictive biomarker of response is urgently needed.

EXPERIMENTAL DESIGN

We present the first disclosure and preclinical development of a novel, selective ATR inhibitor, ART0380, and test its antitumor activity in multiple preclinical cancer models. To refine ATM LOF as a predictive biomarker, we performed a comprehensive pan-cancer analysis of ATM variants in patient tumors and then assessed the ATM variant-to-protein relationship. Finally, we assessed a novel ATM LOF biomarker approach in retrospective clinical data sets of patients treated with platinum-based chemotherapy or ATR inhibition.

RESULTS

ART0380 had potent, selective antitumor activity in a range of preclinical cancer models with differing degrees of ATM LOF. Pan-cancer analysis identified 10,609 ATM variants in 8,587 patient tumors. Cancer lineage-specific differences were seen in the prevalence of deleterious (Tier 1) versus unknown/benign (Tier 2) variants, selective pressure for loss of heterozygosity, and concordance between a deleterious variant and ATM loss of protein (LOP). A novel ATM LOF biomarker approach that accounts for variant classification, relationship to ATM LOP, and tissue-specific penetrance significantly enriched for patients who benefited from platinum-based chemotherapy or ATR inhibition.

CONCLUSIONS

These data help to better define ATM LOF across tumor types in order to optimize patient selection and improve molecularly targeted therapeutic approaches for patients with ATM LOF cancers.

Targeting ATR in patients with cancer

Pharmacological inhibition of the ataxia telangiectasia and Rad3-related protein serine/threonine kinase (ATR; also known as FRAP-related protein (FRP1)) has emerged as a promising strategy for cancer treatment that exploits synthetic lethal interactions with proteins involved in DNA damage repair, overcomes resistance to other therapies and enhances antitumour immunity. Multiple novel, potent ATR inhibitors are being tested in clinical trials using biomarker-directed approaches and involving patients across a broad range of solid cancer types; some of these inhibitors have now entered phase III trials. Further insight into the complex interactions of ATR with other DNA replication stress response pathway components and with the immune system is necessary in order to optimally harness the potential of ATR inhibitors in the clinic and achieve hypomorphic targeting of the various ATR functions. Furthermore, a deeper understanding of the diverse range of predictive biomarkers of response to ATR inhibitors and of the intraclass differences between these agents could help to refine trial design and patient selection strategies. Key challenges that remain in the clinical development of ATR inhibitors include the optimization of their therapeutic index and the development of rational combinations with these agents. In this Review, we detail the molecular mechanisms regulated by ATR and their clinical relevance, and discuss the challenges that must be addressed to extend the benefit of ATR inhibitors to a broad population of patients with cancer.

Converging and evolving immuno-genomic routes toward immune escape in breast cancer

The interactions between tumor and immune cells along the course of breast cancer progression remain largely unknown. Here, we extensively characterize multiple sequential and parallel multiregion tumor and blood specimens of an index patient and a cohort of metastatic triple-negative breast cancers. We demonstrate that a continuous increase in tumor genomic heterogeneity and distinct molecular clocks correlated with resistance to treatment, eventually allowing tumors to escape from immune control. TCR repertoire loses diversity over time, leading to convergent evolution as breast cancer progresses. Although mixed populations of effector memory and cytotoxic single T cells coexist in the peripheral blood, defects in the antigen presentation machinery coupled with subdued T cell recruitment into metastases are observed, indicating a potent immune avoidance microenvironment not compatible with an effective antitumor response in lethal metastatic disease. Our results demonstrate that the immune responses against cancer are not static, but rather follow dynamic processes that match cancer genomic progression, illustrating the complex nature of tumor and immune cell interactions.

Functional Homologous Recombination (HR) Screening Shows the Majority of BRCA1/2-Mutant Breast and Ovarian Cancer Cell Lines Are HR-Proficient

Tumors with a pathogenic BRCA1/2 mutation are homologous recombination (HR)-deficient (HRD) and consequently sensitive to platinum-based chemotherapy and Poly-[ADP-Ribose]-Polymerase inhibitors (PARPi). We hypothesized that functional HR status better reflects real-time HR status than BRCA1/2 mutation status. Therefore, we determined the functional HR status of 53 breast cancer (BC) and 38 ovarian cancer (OC) cell lines by measuring the formation of RAD51 foci after irradiation. Discrepancies between functional HR and BRCA1/2 mutation status were investigated using exome sequencing, methylation and gene expression data from 50 HR-related genes. A pathogenic BRCA1/2 mutation was found in 10/53 (18.9%) of BC and 7/38 (18.4%) of OC cell lines. Among BRCA1/2-mutant cell lines, 14/17 (82.4%) were HR-proficient (HRP), while 1/74 (1.4%) wild-type cell lines was HRD. For most (80%) cell lines, we explained the discrepancy between functional HR and BRCA1/2 mutation status. Importantly, 12/14 (85.7%) BRCA1/2-mutant HRP cell lines were explained by mechanisms directly acting on BRCA1/2. Finally, functional HR status was strongly associated with COSMIC single base substitution signature 3, but not BRCA1/2 mutation status. Thus, the majority of BRCA1/2-mutant cell lines do not represent a suitable model for HRD. Moreover, exclusively determining BRCA1/2 mutation status may not suffice for platinum-based chemotherapy or PARPi patient selection.

Pathogenic germline variants in non-BRCA1/2 homologous recombination genes in ovarian cancer: Analysis of tumor phenotype and survival

OBJECTIVE

To define molecular features of ovarian cancer (OC) with germline pathogenic variants (PVs) in non-BRCA homologous recombination (HR) genes and analyze survival compared to BRCA1/2 and wildtype (WT) OC.

METHODS

We included patients with OC undergoing tumor-normal sequencing (MSK-IMPACT) from 07/01/2015-12/31/2020, including germline assessment of BRCA1/2 and other HR genes ATM, BARD1, BRIP1, FANCA, FANCC, NBN, PALB2, RAD50, RAD51B, RAD51C, and RAD51D. Biallelic inactivation was assessed within tumors. Progression-free (PFS) and overall survival (OS) were calculated from pathologic diagnosis using the Kaplan-Meier method with left truncation. Whole-exome sequencing (WES) was performed in a subset.

RESULTS

Of 882 patients with OC, 56 (6.3%) had germline PVs in non-BRCA HR genes; 95 (11%) had BRCA1-associated OC (58 germline, 37 somatic); and 59 (6.7%) had BRCA2-associated OC (40 germline, 19 somatic). High rates of biallelic alterations were observed among germline PVs in BRIP1 (11/13), PALB2 (3/4), RAD51B (3/4), RAD51C (3/4), and RAD51D (8/10). In cases with WES (27/35), there was higher tumor mutational burden (TMB; median 2.5 [1.1-6.0] vs. 1.2 mut/Mb [0.6-2.6]) and enrichment of HR-deficient (HRD) mutational signatures in tumors associated with germline PALB2 and RAD51B/C/D compared with BRIP1 PVs (p < 0.01). Other features of HRD, including telomeric-allelic imbalance (TAI) and large-scale state transitions (LSTs), were similar. Although there was heterogeneity in PFS/OS by gene group, only BRCA1/2-associated OC had improved survival compared to WT OC (p < 0.01).

CONCLUSIONS

OCs associated with germline PVs in non-BRCA HR genes represent a heterogenous group, with PALB2 and RAD51B/C/D associated with an HRD phenotype.

Performance of a RAD51-based functional HRD test on paraffin-embedded breast cancer tissue

PURPOSE

BRCA-deficient breast cancers (BC) are highly sensitive to platinum-based chemotherapy and PARP inhibitors due to their deficiency in the homologous recombination (HR) pathway. However, HR deficiency (HRD) extends beyond BRCA-associated BC, highlighting the need for a sensitive method to enrich for HRD tumors in an alternative way. A promising approach is the use of functional HRD tests which evaluate the HR capability of tumor cells by measuring RAD51 protein accumulation at DNA damage sites. This study aims to evaluate the performance of a functional RAD51-based HRD test for the identification of HRD BC.

METHODS

The functional HR status of 63 diagnostic formalin-fixed paraffin-embedded (FFPE) BC samples was determined by applying the RAD51-FFPE test. Samples were screened for the presence of (epi)genetic defects in HR and matching tumor samples were analyzed with the RECAP test, which requires ex vivo irradiated fresh tumor tissue on the premise that the HRD status as determined by the RECAP test faithfully represented the functional HR status.

RESULTS

The RAD51-FFPE test identified 23 (37%) of the tumors as HRD, including three tumors with pathogenic variants in BRCA1/2. The RAD51-FFPE test showed a sensitivity of 88% and a specificity of 76% in determining the HR-class as defined by the RECAP test.

CONCLUSION

Given its high sensitivity and compatibility with FFPE samples, the RAD51-FFPE test holds great potential to enrich for HRD tumors, including those associated with BRCA-deficiency. This potential extends to situations where DNA-based testing may be challenging or not easily accessible in routine clinical practice. This is particularly important considering the potential implications for treatment decisions and patient stratification.

Comprehensive profiling of pathogenic germline large genomic rearrangements in a pan-cancer analysis

The presence of large genomic rearrangements (LGRs) has been heavily investigated in breast and ovarian cancer. However, correlations between LGRs and cancer types beyond these two have not been extensively profiled, likely due to the highly inefficient methods of detecting these types of alterations. This study utilized next-generation sequencing (NGS) to analyze and classify the germline LGR profile in 17 025 cancer patients across 22 cancer types. We characterized newly identified LGRs based on predicted pathogenicity and took a closer look at genes that acquire both germline and somatic mutations within our samples. The detection method for LGRs was validated using droplet digital polymerase chain reaction (ddPCR) assay of commonly investigated LGR genes. In total, 15 659 samples from across 22 cancer types were retained for analysis after filtering. We observed that, in our cohort, the cancer types with the highest proportion of germline LGRs were ovarian cancer (4.7%), renal cell carcinoma (2.5%), breast cancer (2%), glioma (1.8%) and thyroid carcinoma (1.8%). Annotation of detected germline variants revealed several genes-MSH2, FANCA and PMS2-that contain novel LGRs. We observed co-occurrences between germline LGRs in MSH2 and somatic single nucleotide variants/insertion and deletions (SNVs/InDels) in BRCA2, KTM2B, KDM5A, CHD8, and HNF1A. Furthermore, our analysis showed that samples with pathogenic and likely pathogenic germline LGRs tended to also have higher mutational burden, chromosomal instability, and microsatellite instability ratio compared to samples with pathogenic germline SNVs/InDels. In this study, we demonstrated the prevalence of pathogenic germline LGRs beyond breast and ovarian cancer. The profiles of these pathogenic or likely pathogenic alterations will fuel further investigations and highlight new understanding of LGRs across multiple cancer types.

Genetic characteristics of platinum-sensitive ovarian clear cell carcinoma

OBJECTIVE

Most ovarian clear cell carcinomas are resistant to platinum-based chemotherapy, while a small subset shows a positive response. The aim of this study was to clarify the clinical, pathological and genetic characteristics of platinum-sensitive ovarian clear cell carcinomas.

METHODS

The study included 53 patients with stage III-IV ovarian clear cell carcinoma who had residual tumours after primary surgery and received platinum-based therapy between 2009 and 2018. A retrospective examination of platinum sensitivity was performed using the criterion of ≥6 months from the last day of first-line platinum therapy until recurrence/progression. Cases determined to be platinum-sensitive were subjected to immunohistochemical staining, genomic analyses using target sequencing (i.e. NCC Oncopanel) and homologous recombination deficiency (myChoice® HRD Plus) assays.

RESULTS

Of the 53 stage III-IV ovarian clear cell carcinoma cases, 11 (21%) were platinum-sensitive. These cases showed better progression-free and overall survival than platinum-resistant cases (hazard ratio = 0.16, P < 0.001). Among the seven sensitive cases whose tumour tissues were available for molecular profiling, five were pure ovarian clear cell carcinoma based on pathological and genetic features, whereas the remaining two cases were re-diagnosed as high-grade serous ovarian carcinoma. The pure ovarian clear cell carcinomas lacked BRCA1 and BRCA2 mutations, consistent with the absence of the homologous recombination deficiency phenotype, whereas two cases (40%) had ATM mutations. By contrast, the two high-grade serous ovarian carcinoma cases had BRCA1 or BRCA2 mutations associated with the homologous recombination deficiency phenotype.

CONCLUSION

The subset of platinum-sensitive ovarian clear cell carcinomas includes a majority with pure ovarian clear cell carcinoma features that lack the homologous recombination deficiency phenotype.

A Multiparameter Molecular Classifier to Predict Response to Neoadjuvant Lapatinib plus Trastuzumab without Chemotherapy in HER2+ Breast Cancer

PURPOSE

Clinical trials reported 25% to 30% pathologic complete response (pCR) rates in HER2+ patients with breast cancer treated with anti-HER2 therapies without chemotherapy. We hypothesize that a multiparameter classifier can identify patients with HER2-"addicted" tumors who may benefit from a chemotherapy-sparing strategy.

EXPERIMENTAL DESIGN

Baseline HER2+ breast cancer specimens from the TBCRC023 and PAMELA trials, which included neoadjuvant treatment with lapatinib and trastuzumab, were used. In the case of estrogen receptor-positive (ER+) tumors, endocrine therapy was also administered. HER2 protein and gene amplification (ratio), HER2-enriched (HER2-E), and PIK3CA mutation status were assessed by dual gene protein assay (GPA), research-based PAM50, and targeted DNA-sequencing. GPA cutoffs and classifier of response were constructed in TBCRC023 using a decision tree algorithm, then validated in PAMELA.

RESULTS

In TBCRC023, 72 breast cancer specimens had GPA, PAM50, and sequencing data, of which 15 had pCR. Recursive partitioning identified cutoffs of HER2 ratio ≥ 4.6 and %3+ IHC staining ≥ 97.5%. With PAM50 and sequencing data, the model added HER2-E and PIK3CA wild-type (WT). For clinical implementation, the classifier was locked as HER2 ratio ≥ 4.5, %3+ IHC staining ≥ 90%, and PIK3CA-WT and HER2-E, yielding 55% and 94% positive (PPV) and negative (NPV) predictive values, respectively. Independent validation using 44 PAMELA cases with all three biomarkers yielded 47% PPV and 82% NPV. Importantly, our classifier's high NPV signifies its strength in accurately identifying patients who may not be good candidates for treatment deescalation.

CONCLUSIONS

Our multiparameter classifier differentially identifies patients who may benefit from HER2-targeted therapy alone from those who need chemotherapy and predicts pCR to anti-HER2 therapy alone comparable with chemotherapy plus dual anti-HER2 therapy in unselected patients.

Germline ATM Mutations Detected by Somatic DNA Sequencing in Lethal Prostate Cancer

Background

Germline mutations in the ataxia telangiectasia mutated (ATM) gene occur in 0.5-1% of the overall population and are associated with tumour predisposition. The clinical and pathological features of ATM-mutated prostate cancer (PC) are poorly defined but have been associated with lethal PC.

Objective

To report on the clinical characteristics including family history and clinical outcomes of a cohort of patients with advanced metastatic castration-resistant PC (CRPC) who were found to have germline ATM mutations after mutation detection by initial tumour DNA sequencing.

Design setting and participants

We acquired germline ATM mutation data by saliva next-generation sequencing from patients with ATM mutations in PC biopsies sequenced between January 2014 and January 2022. Demographics, family history, and clinical data were collected retrospectively.

Outcome measurements and statistical analysis

Outcome endpoints were based on overall survival (OS) and time from diagnosis to CRPC. Data were analysed using R version 3.6.2 (R Foundation for Statistical Computing, Vienna, Austria).

Results and limitations

Overall, seven patients (n = 7/1217; 0.6%) had germline ATM mutations detected, with five of them having a family history of malignancies, including breast, prostate, pancreas, and gastric cancer; leukaemia; and lymphoma. Two patients had concomitant somatic mutations in tumour biopsies in genes other than ATM, while two patients were found to carry more than one ATM pathogenic mutation. Five tumours in germline ATM variant carriers had loss of ATM by immunohistochemistry. The median OS from diagnosis was 7.1 yr (range 2.9-14 yr) and the median OS from CRPC was 5.3 yr (range 2.2-7.3 yr). When comparing these data with PC patients sequenced by The Cancer Genome Atlas, we found that the spatial localisation of mutations was similar, with distribution of alterations occurring on similar positions in the ATM gene. Interestingly, these include a mutation within the FRAP-ATM-TRRAP (FAT) domain, suggesting that this represents a mutational hotspot for ATM.

Conclusions

Germline ATM mutations are rare in patients with lethal PC but occur at mutational hotspots; further research is warranted to better characterise the family histories of these men and PC clinical course.

Patient summary

In this report, we studied the clinical and pathological features of advanced prostate cancers associated with germline mutations in the ATM gene. We found that most patients had a strong family history of cancer and that this mutation might predict the course of these prostate cancers, as well as response to specific treatments.

Camonsertib in DNA damage response-deficient advanced solid tumors: phase 1 trial results

Predictive biomarkers of response are essential to effectively guide targeted cancer treatment. Ataxia telangiectasia and Rad3-related kinase inhibitors (ATRi) have been shown to be synthetic lethal with loss of function (LOF) of ataxia telangiectasia-mutated (ATM) kinase, and preclinical studies have identified ATRi-sensitizing alterations in other DNA damage response (DDR) genes. Here we report the results from module 1 of an ongoing phase 1 trial of the ATRi camonsertib (RP-3500) in 120 patients with advanced solid tumors harboring LOF alterations in DDR genes, predicted by chemogenomic CRISPR screens to sensitize tumors to ATRi. Primary objectives were to determine safety and propose a recommended phase 2 dose (RP2D). Secondary objectives were to assess preliminary anti-tumor activity, to characterize camonsertib pharmacokinetics and relationship with pharmacodynamic biomarkers and to evaluate methods for detecting ATRi-sensitizing biomarkers. Camonsertib was well tolerated; anemia was the most common drug-related toxicity (32% grade 3). Preliminary RP2D was 160 mg weekly on days 1-3. Overall clinical response, clinical benefit and molecular response rates across tumor and molecular subtypes in patients who received biologically effective doses of camonsertib (>100 mg d-1) were 13% (13/99), 43% (43/99) and 43% (27/63), respectively. Clinical benefit was highest in ovarian cancer, in tumors with biallelic LOF alterations and in patients with molecular responses. ClinicalTrials.gov registration: NCT04497116 .

NR4A3 Expression Is Consistently Absent in Acinic Cell Carcinomas of the Breast: A Potential Nosologic Shift

Acinic cell carcinoma (AciCC) is a tumor that is recognized in both the breast and salivary glands. Recently, the recurrent genomic rearrangement, t(4;9)(q13;q31) was identified in salivary AciCC that results in constitutive upregulation of the nuclear transcription factor NR4A3, which can be detected by immunohistochemistry. In this study, we sought to evaluate NR4A3 expression in breast AciCC using immunohistochemistry. Strong and diffuse nuclear staining was considered a positive result. Sixteen AciCCs were studied, including 8 pure AciCCs and 8 AciCCs admixed with other types (invasive carcinoma of no special type in 5 cases and metaplastic carcinoma in 3 cases). All 16 AciCCs showed negative results for NR4A3 expression. Four cases with available material were evaluated for rearrangements of the NR4A3 gene by fluorescence in situ hybridization and no rearrangements were observed. Whole-genome sequencing of 1 AciCC revealed a TP53 splice-site mutation, high levels of genomic instability, and genomic features of homologous recombination DNA repair defects; a structural variant analysis of this case did not reveal the presence of a t(4;9) rearrangement. We conclude that breast AciCCs consistently lack NR4A3 rearrangement or overexpression, unlike most of the salivary AciCCs, and that consistent with previous results, breast AciCCs are associated with genomic alterations more similar to those seen in triple-negative breast carcinomas than salivary gland AciCCs. These results suggest that unlike other salivary gland-like tumors that occur in the breast, the molecular underpinnings of the salivary gland and breast AciCCs are different and that the salivary gland and breast AciCCs likely represent distinct entities.

Breast cancer genomes from CHEK2 c.1100delC mutation carriers lack somatic TP53 mutations and display a unique structural variant size distribution profile

BACKGROUND

CHEK2 c.1100delC was the first moderate-risk breast cancer (BC) susceptibility allele discovered. Despite several genomic, transcriptomic and functional studies, however, it is still unclear how exactly CHEK2 c.1100delC promotes tumorigenesis. Since the mutational landscape of a tumor reflects the processes that have operated on its development, the aim of this study was to uncover the somatic genomic landscape of CHEK2-associated BC.

METHODS

We sequenced primary BC (pBC) and normal genomes of 20 CHEK2 c.1100delC mutation carriers as well as their pBC transcriptomes. Including pre-existing cohorts, we exhaustively compared CHEK2 pBC genomes to those from BRCA1/2 mutation carriers, those that displayed homologous recombination deficiency (HRD) and ER- and ER+ pBCs, totaling to 574 pBC genomes. Findings were validated in 517 metastatic BC genomes subdivided into the same subgroups. Transcriptome data from 168 ER+ pBCs were used to derive a TP53-mutant gene expression signature and perform cluster analysis with CHEK2 BC transcriptomes. Finally, clinical outcome of CHEK2 c.1100delC carriers was compared with BC patients displaying somatic TP53 mutations in two well-described retrospective cohorts totaling to 942 independent pBC cases.

RESULTS

BC genomes from CHEK2 mutation carriers were most similar to ER+ BC genomes and least similar to those of BRCA1/2 mutation carriers in terms of tumor mutational burden as well as mutational signatures. Moreover, CHEK2 BC genomes did not show any evidence of HRD. Somatic TP53 mutation frequency and the size distribution of structural variants (SVs), however, were different compared to ER+ BC. Interestingly, BC genomes with bi-allelic CHEK2 inactivation lacked somatic TP53 mutations and transcriptomic analysis indicated a shared biology with TP53 mutant BC. Moreover, CHEK2 BC genomes had an increased frequency of > 1 Mb deletions, inversions and tandem duplications with peaks at specific sizes. The high chromothripsis frequency among CHEK2 BC genomes appeared, however, not associated with this unique SV size distribution profile.

CONCLUSIONS

CHEK2 BC genomes are most similar to ER+ BC genomes, but display unique features that may further unravel CHEK2-driven tumorigenesis. Increased insight into this mechanism could explain the shorter survival of CHEK2 mutation carriers that is likely driven by intrinsic tumor aggressiveness rather than endocrine resistance.

Pathogenic Variants in Adult-Onset Cancer Predisposition Genes in Pediatric Cancer: Prevalence and Impact on Tumor Molecular Features and Clinical Management

PURPOSE

Clinical genomic sequencing of pediatric tumors is increasingly uncovering pathogenic variants in adult-onset cancer predisposition genes (aoCPG). Nevertheless, it remains poorly understood how often aoCPG variants are of germline origin and whether they influence tumor molecular profiles and/or clinical care. In this study, we examined the prevalence, spectrum, and impacts of aoCPG variants on tumor genomic features and patient management at our institution.

EXPERIMENTAL DESIGN

This is a retrospective study of 1,018 children with cancer who underwent clinical genomic sequencing of their tumors. Tumor genomic data were queried for pathogenic variants affecting 24 preselected aoCPGs. Available tumor whole-genome sequencing (WGS) data were evaluated for second hit mutations, loss of heterozygosity (LOH), DNA mutational signatures, and homologous recombination deficiency (HRD). Patients whose tumors harbored one or more pathogenic aoCPG variants underwent subsequent germline testing based on hereditary cancer evaluation and family or provider preference.

RESULTS

Thirty-three patients (3%) had tumors harboring pathogenic variants affecting one or more aoCPGs. Among 21 tumors with sufficient WGS sequencing data, six (29%) harbored a second hit or LOH affecting the remaining aoCPG allele with four of these six tumors (67%) also exhibiting a DNA mutational signature consistent with the altered aoCPG. Two additional tumors demonstrated HRD, of uncertain relation to the identified aoCPG variant. Twenty-one of 26 patients (81%) completing germline testing were positive for the aoCPG variant in the germline. All germline-positive patients were counseled regarding future cancer risks, surveillance, and risk-reducing measures. No patients had immediate cancer therapy changed due to aoCPG data.

CONCLUSIONS

AoCPG variants are rare in pediatric tumors; however, many originate in the germline. Almost one third of tumor aoCPG variants examined exhibited a second hit and/or conferred an abnormal DNA mutational profile suggesting a role in tumor formation. aoCPG information aids in cancer risk prediction but is not commonly used to alter the treatment of pediatric cancers.

Hereditary breast cancer: syndromes, tumour pathology and molecular testing

Hereditary factors account for a significant proportion of breast cancer risk. Approximately 20% of hereditary breast cancers are attributable to pathogenic variants in the highly penetrant BRCA1 and BRCA2 genes. A proportion of the genetic risk is also explained by pathogenic variants in other breast cancer susceptibility genes, including ATM, CHEK2, PALB2, RAD51C, RAD51D and BARD1, as well as genes associated with breast cancer predisposition syndromes - TP53 (Li-Fraumeni syndrome), PTEN (Cowden syndrome), CDH1 (hereditary diffuse gastric cancer), STK11 (Peutz-Jeghers syndrome) and NF1 (neurofibromatosis type 1). Polygenic risk, the cumulative risk from carrying multiple low-penetrance breast cancer susceptibility alleles, is also a well-recognised contributor to risk. This review provides an overview of the established breast cancer susceptibility genes as well as breast cancer predisposition syndromes, highlights distinct genotype-phenotype correlations associated with germline mutation status and discusses molecular testing and therapeutic implications in the context of hereditary breast cancer.

Comprehensive Genomic Profiling of Cell-Free Circulating Tumor DNA Detects Response to Ribociclib Plus Letrozole in a Patient with Metastatic Breast Cancer

Analysis of cell-free circulating tumor DNA obtained by liquid biopsy is a non-invasive approach that may provide clinically actionable information when conventional tissue biopsy is inaccessible or infeasible. Here, we followed a patient with hormone receptor-positive and human epidermal growth factor receptor (HER) 2-negative breast cancer who developed bone metastases seven years after mastectomy. We analyzed circulating cell-free DNA (cfDNA) extracted from plasma using high-depth massively parallel sequencing targeting 468 cancer-associated genes, and we identified a clonal hotspot missense mutation in the PIK3CA gene (3:178952085, A > G, H1047R) and amplification of the CCND1 gene. Whole-exome sequencing revealed that both alterations were present in the primary tumor. After treatment with ribociclib plus letrozole, the genetic abnormalities were no longer detected in cfDNA. These results underscore the clinical utility of combining liquid biopsy and comprehensive genomic profiling to monitor treatment response in patients with metastasized breast cancer.

APOBEC mutagenesis, kataegis, chromothripsis in EGFR-mutant osimertinib-resistant lung adenocarcinomas

BACKGROUND

Studies of targeted therapy resistance in lung cancer have primarily focused on single-gene alterations. Based on prior work implicating apolipoprotein b mRNA-editing enzyme, catalytic polypeptide-like (APOBEC) mutagenesis in histological transformation of epidermal growth factor receptor (EGFR)-mutant lung cancers, we hypothesized that mutational signature analysis may help elucidate acquired resistance to targeted therapies.

PATIENTS AND METHODS

APOBEC mutational signatures derived from an Food and Drug Administration-cleared multigene panel [Memorial Sloan Kettering Cancer Center Integrated Mutation Profiling of Actionable Cancer Targets (MSK-IMPACT)] using the Signature Multivariate Analysis (SigMA) algorithm were validated against the gold standard of mutational signatures derived from whole-exome sequencing. Mutational signatures were decomposed in 3276 unique lung adenocarcinomas (LUADs), including 93 paired osimertinib-naïve and -resistant EGFR-mutant tumors. Associations between APOBEC and mechanisms of resistance to osimertinib were investigated. Whole-genome sequencing was carried out on available EGFR-mutant lung cancer samples (10 paired, 17 unpaired) to investigate large-scale genomic alterations potentially contributing to osimertinib resistance.

RESULTS

APOBEC mutational signatures were more frequent in receptor tyrosine kinase (RTK)-driven lung cancers (EGFR, ALK, RET, and ROS1; 25%) compared to LUADs at large (20%, P < 0.001); across all subtypes, APOBEC mutational signatures were enriched in subclonal mutations (P < 0.001). In EGFR-mutant lung cancers, osimertinib-resistant samples more frequently displayed an APOBEC-dominant mutational signature compared to osimertinib-naïve samples (28% versus 14%, P = 0.03). Specifically, mutations detected in osimertinib-resistant tumors but not in pre-treatment samples significantly more frequently displayed an APOBEC-dominant mutational signature (44% versus 23%, P < 0.001). EGFR-mutant samples with APOBEC-dominant signatures had enrichment of large-scale genomic rearrangements (P = 0.01) and kataegis (P = 0.03) in areas of APOBEC mutagenesis.

CONCLUSIONS

APOBEC mutational signatures are frequent in RTK-driven LUADs and increase under the selective pressure of osimertinib in EGFR-mutant lung cancer. APOBEC mutational signature enrichment in subclonal mutations, private mutations acquired after osimertinib treatment, and areas of large-scale genomic rearrangements highlights a potentially fundamental role for APOBEC mutagenesis in the development of resistance to targeted therapies, which may be potentially exploited to overcome such resistance.

Clinico-genomic Characterization of ATM and HRD in Pancreas Cancer: Application for Practice

PURPOSE

Characterizing germline and somatic ATM variants (gATMm, sATMm) zygosity and their contribution to homologous recombination deficiency (HRD) is important for therapeutic strategy in pancreatic ductal adenocarcinoma (PDAC).

EXPERIMENTAL DESIGN

Clinico-genomic data for patients with PDAC and other cancers with ATM variants were abstracted. Genomic instability scores (GIS) were derived from ATM-mutant cancers and overall survival (OS) was evaluated.

RESULTS

Forty-six patients had PDAC and pathogenic ATM variants including 24 (52%) stage III/IV: gATMm (N = 24), and sATMm (N = 22). Twenty-seven (59%) had biallelic, 15 (33%) monoallelic, and 4 indeterminate (8%) variants. Median OS for advanced-stage cohort at diagnosis (N = 24) was 19.7 months [95% confidence interval (CI): 12.3-not reached (NR)], 27.1 months (95% CI: 22.7-NR) for gATMm (n = 11), and 12.3 months for sATMm (n = 13; 95% CI: 11.9-NR; P = 0.025). GIS was computed for 33 patients with PDAC and compared with other ATM-mutant cancers enriched for HRD. The median was lower (median, 11; range, 2-29) relative to breast (18, 3-55) or ovarian (25, 3-56) ATM-mutant cancers (P < 0.001 and P = 0.003, respectively). Interestingly, biallelic pathogenic ATM variants were mutually exclusive with TP53. Other canonical driver gene (KRAS, CDKN2A, SMAD4) variants were less frequent in ATM-mutant PDAC.

CONCLUSIONS

ATM variants in PDAC represent a distinct biologic group and appear to have favorable OS. Nonetheless, pathogenic ATM variants do not confer an HRD signature in PDAC and ATM should be considered as a non-core HR gene in this disease.

Somatic inactivation of breast cancer predisposition genes in tumors associated with pathogenic germline variants

BACKGROUND

Breast cancers (BCs) that arise in individuals heterozygous for a germline pathogenic variant in a susceptibility gene, such as BRCA1 and BRCA2, PALB2, and RAD51C, have been shown to exhibit biallelic loss in the respective genes and be associated with triple-negative breast cancer (TNBC) and distinctive somatic mutational signatures. Tumor sequencing thus presents an orthogonal approach to assess the role of candidate genes in BC development.

METHODS

Exome sequencing was performed on paired normal-breast tumor DNA from 124 carriers of germline loss-of-function (LoF) or missense variant carriers in 15 known and candidate BC predisposition genes identified in the BEACCON case-control study. Biallelic inactivation and association with tumor genome features including mutational signatures and homologous recombination deficiency (HRD) score were investigated.

RESULTS

BARD1-carrying TNBC (4 of 5) displayed biallelic loss and associated high HRD scores and mutational signature 3, as did a RAD51D-carrying TNBC and ovarian cancer. Biallelic loss was less frequent in BRIP1 BCs (4 of 13) and had low HRD scores. In contrast to other established BC genes, BCs from carriers of CHEK2 LoF (6 of 17) or missense (2 of 20) variant had low rates of biallelic loss. Exploratory analysis of BC from carriers of LoF variants in candidate genes such as BLM, FANCM, PARP2, and RAD50 found little evidence of biallelic inactivation.

CONCLUSIONS

BARD1 and RAD51D behave as classic BRCA-like predisposition genes with biallelic inactivation, but this was not observed for any of the candidate genes. However, as demonstrated for CHEK2, the absence of biallelic inactivation does not provide definitive evidence against the gene's involvement in BC predisposition.

To indel or not to indel: Factors influencing mutagenesis during chromosomal break end joining

Chromosomal DNA double-strand breaks (DSBs) are the effective lesion of radiotherapy and other clastogenic cancer therapeutics, and are also the initiating event of many approaches to gene editing. Ligation of the DSBs by end joining (EJ) pathways can restore the broken chromosome, but the repair junctions can have insertion/deletion (indel) mutations. The indel patterns resulting from DSB EJ are likely defined by the initial structure of the DNA ends, how the ends are processed and synapsed prior to ligation, and the factors that mediate the ligation step. In this review, we describe key factors that influence these steps of DSB EJ in mammalian cells, which is significant both for understanding mutagenesis resulting from clastogenic cancer therapeutics, and for developing approaches to manipulating gene editing outcomes.

Risk Stratification of Stage I Grade 3 Endometrioid Endometrial Carcinoma in the Era of Molecular Classification

PURPOSE

The role of adjuvant therapy in stage I grade 3 endometrioid endometrial carcinoma (EEC) is debatable. We sought to define the agreement between Post Operative Radiation Therapy in Endometrial Carcinoma 1 (PORTEC-1) high-intermediate risk (HIR) and Gynecologic Oncology Group (GOG)-99 HIR criteria, assess their concordance with The Cancer Genome Atlas molecular subtypes, and evaluate oncologic outcomes in this population.

METHODS

We identified patients with stage I grade 3 EECs who underwent surgical staging at our institution from January 2014 to January 2020. Patients were stratified into PORTEC-1 HIR, GOG-99 HIR, and The Cancer Genome Atlas molecular subtypes. Adjuvant treatment, and progression-free survival (PFS), and overall survival (OS) were analyzed.

RESULTS

Seventy-five patients were included. The agreement between PORTEC-1 and GOG-99 HIR classification was 68% (95% CI, 56.2 to 78.3), with a kappa of 0.36 (P = .001). There was no agreement between PORTEC-1 or GOG-99 HIR classification and a dichotomized molecular classification (copy number-high [CN-H] v other subtypes), with a kappa of 0.03 (P = .39) and -0.03 (P = .601), respectively. There was no difference in PFS between PORTEC-1 HIR and non-HIR (HR, 10.9; 95% CI, 0.28 to 4.21) or between GOG-99 HIR and non-HIR (HR, 1.22; 95% CI, 0.32 to 4.6) stage I grade 3 EECs. Patients with CN-H compared with non-CN-H EEC had worse PFS (HR, 5.67; 95% CI, 1.73 to 18.63) and OS (HR, 5.05; 95% CI, 1.13 to 22.5).

CONCLUSION

In surgically staged patients with stage I grade 3 EEC, PORTEC-1 and GOG-99 HIR criteria were not prognostic and did not identify CN-H patients. Patients with CN-H EEC had worse PFS and OS compared with those with other molecular subtypes. The integration of the molecular classification with recognized clinicopathologic factors may identify patients with higher-risk stage I grade 3 EEC who benefit from additional therapy.

Techniques for Profiling the Cellular Immune Response and Their Implications for Interventional Oncology

In recent years there has been increased interest in using the immune contexture of the primary tumors to predict the patient's prognosis. The tumor microenvironment of patients with cancers consists of different types of lymphocytes, tumor-infiltrating leukocytes, dendritic cells, and others. Different technologies can be used for the evaluation of the tumor microenvironment, all of which require a tissue or cell sample. Image-guided tissue sampling is a cornerstone in the diagnosis, stratification, and longitudinal evaluation of therapeutic efficacy for cancer patients receiving immunotherapies. Therefore, interventional radiologists (IRs) play an essential role in the evaluation of patients treated with systemically administered immunotherapies. This review provides a detailed description of different technologies used for immune assessment and analysis of the data collected from the use of these technologies. The detailed approach provided herein is intended to provide the reader with the knowledge necessary to not only interpret studies containing such data but also design and apply these tools for clinical practice and future research studies.

Uncovering variable neoplasms between ATM protein-truncating and common missense variants using 394,694 UK Biobank exomes

As an essential regulator of DNA damage, Ataxia-telangiectasia mutated (ATM) gene has been widely studied in oncology. However, the independent effects of ATM missense variants and protein-truncating variants (PTVs) on neoplasms have not been heavily studied. Whole-exome sequencing data and the clinical health records of ~400K UK Biobank European participants were used in this analysis. We mined genetic associations from gene-level and variant-level phenome-wide association studies, and conducted a variant-level conditional association study to test whether the effects of ATM missense variants on neoplasms were independent of ATM PTV carrier status. The gene-level PTV collapsing analysis was consistent with established ATM PTV literature showing that the aggregated impact of 286 ATM PTVs significantly (P<2x10^-9 ) associated with 31 malignant neoplasm phenotypes. Of 773 distinct protein-coding variants in ATM, three individual missense variants significantly (P<2x10^-9 ) associated with nine phenotypes. Remarkably, although the nine phenotypes were tumour-related, none overlapped the established ATM PTV-linked malignancies. A subsequent conditional analysis identified that the missense signals were acting independently of the known clinically relevant ATM PTVs.

First international workshop of the ATM and cancer risk group (4-5 December 2019)

The first International Workshop of the ATM and Cancer Risk group focusing on the role of Ataxia-Telangiectasia Mutated (ATM) gene in cancer was held on December 4 and 5, 2019 at Institut Curie in Paris, France. It was motivated by the fact that germline ATM pathogenic variants have been found to be associated with different cancer types. However, due to the lack of precise age-, sex-, and site-specific risk estimates, no consensus on management guidelines for variant carriers exists, and the clinical utility of ATM variant testing is uncertain. The meeting brought together epidemiologists, geneticists, biologists and clinicians to review current knowledge and on-going challenges related to ATM and cancer risk. This report summarizes the meeting sessions content that covered the latest results in family-based and population-based studies, the importance of accurate variant classification, the effect of radiation exposures for ATM variant carriers, and the characteristics of ATM-deficient tumors. The report concludes that ATM variant carriers outside of the context of Ataxia-Telangiectasia may benefit from effective cancer risk management and therapeutic strategies and that efforts to set up large-scale studies in the international framework to achieve this goal are necessary.

Intratumor genetic heterogeneity and clonal evolution to decode endometrial cancer progression

Analyzing different tumor regions by next generation sequencing allows the assessment of intratumor genetic heterogeneity (ITGH), a phenomenon that has been studied widely in some tumor types but has been less well explored in endometrial carcinoma (EC). In this study, we sought to characterize the spatial and temporal heterogeneity of 9 different ECs using whole-exome sequencing, and by performing targeted sequencing validation of the 42 primary tumor regions and 30 metastatic samples analyzed. In addition, copy number alterations of serous carcinomas were assessed by comparative genomic hybridization arrays. From the somatic mutations, identified by whole-exome sequencing, 532 were validated by targeted sequencing. Based on these data, the phylogenetic tree reconstructed for each case allowed us to establish the tumors’ evolution and correlate this to tumor progression, prognosis, and the presence of recurrent disease. Moreover, we studied the genetic landscape of an ambiguous EC and the molecular profile obtained was used to guide the selection of a potential personalized therapy for this patient, which was subsequently validated by preclinical testing in patient-derived xenograft models. Overall, our study reveals the impact of analyzing different tumor regions to decipher the ITGH in ECs, which could help make the best treatment decision.

ATM Germline Mutated Gastroesophageal Junction Adenocarcinomas: Clinical Descriptors, Molecular Characteristics and Potential Therapeutic Implications

Background

Gastroesophageal junction (GEJ) adenocarcinoma is a rare cancer associated with poor prognosis. The genetic factors conferring predisposition to GEJ adenocarcinoma have yet to be identified.

Methods

We analyzed germline testing results from 23 381 cancer patients undergoing tumor-normal sequencing, of which 312 individuals had GEJ adenocarcinoma. Genomic profiles and clinico-pathologic features were analyzed for the GEJ adenocarcinomas. Silencing of ATM and ATR was performed using validated short-interfering RNA species in GEJ, esophageal, and gastric adenocarcinoma cell lines. All statistical tests were 2-sided.

Results

Pathogenic or likely pathogenic ATM variants were identified in 18 of 312 patients (5.8%), and bi-allelic inactivation of ATM through loss of heterozygosity of the wild-type allele was detected in all (16 of 16) samples with sufficient tumor content. Germline ATM-mutated GEJ adenocarcinomas largely lacked somatic mutations in TP53, were more likely to harbor MDM2 amplification, and harbored statistically significantly fewer somatic single nucleotide variants (2.0 mutations/Mb vs 7.9 mutations/Mb; P < .001). A statistically significantly higher proportion of germline ATM-mutated than ATM–wild-type GEJ adenocarcinoma patients underwent a curative resection (10 [100%] vs 92 [86.8%], P = .04; Fisher’s exact test.), A synthetic lethal interaction between short-interfering RNA silencing of ATM and ATR was observed in the models analyzed.

Conclusions

Our results indicate that germline pathogenic variants in ATM drive oncogenesis in GEJ adenocarcinoma and might result in a distinct clinical phenotype. Given the high prevalence of germline ATM-mutated GEJ adenocarcinomas, genetic testing for individuals with GEJ adenocarcinomas may be considered to better inform prognostication, treatment decisions, and future cancer risk.

Pathogenesis of Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) encompasses a heterogeneous group of fundamentally different diseases with different histologic, genomic, and immunologic profiles, which are aggregated under this term because of their lack of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2 expression. Massively parallel sequencing and other omics technologies have demonstrated the level of heterogeneity in TNBCs and shed light into the pathogenesis of this therapeutically challenging entity in breast cancer. In this review, we discuss the histologic and molecular classifications of TNBC, the genomic alterations these different tumor types harbor, and the potential impact of these alterations on the pathogenesis of these tumors. We also explore the role of the tumor microenvironment in the biology of TNBCs and its potential impact on therapeutic response. Dissecting the biology and understanding the therapeutic dependencies of each TNBC subtype will be essential to delivering on the promise of precision medicine for patients with triple-negative disease.

Neuroendocrine tumours of the breast: a genomic comparison with mucinous breast cancers and neuroendocrine tumours of other anatomic sites

AIMS

Breast neuroendocrine tumours (NETs) constitute a rare histologic subtype of oestrogen receptor (ER)-positive breast cancer, and their definition according to the WHO classification was revised in 2019. Breast NETs display histologic and transcriptomic similarities with mucinous breast carcinomas (MuBCs). Here, we sought to compare the repertoire of genetic alterations in breast NETs with MuBCs and NETs from other anatomic origins.

METHODS

On histologic review applying the new WHO criteria, 18 breast tumours with neuroendocrine differentiation were reclassified as breast NETs (n=10) or other breast cancers with neuroendocrine differentiation (n=8). We reanalysed targeted sequencing or whole-exome sequencing data of breast NETs (n=10), MuBCs type A (n=12) and type B (n=11).

RESULTS

Breast NETs and MuBCs were found to be genetically similar, harbouring a lower frequency of PIK3CA mutations, 1q gains and 16q losses than ER-positive/HER2-negative breast cancers. 3/10 breast NETs harboured the hallmark features of ER-positive disease (ie, PIK3CA mutations and concurrent 1q gains/16q losses). Breast NETs showed an enrichment of oncogenic/likely oncogenic mutations affecting transcription factors compared with common forms of ER-positive breast cancer and with pancreatic and pulmonary NETs.

CONCLUSIONS

Breast NETs are heterogeneous and are characterised by an enrichment of mutations in transcription factors and likely constitute a spectrum of entities histologically and genomically related to MuBCs. While most breast NETs are distinct from ER-positive/HER2-negative IDC-NSTs, a subset of breast NETs appears to be genetically similar to common forms of ER-positive breast cancer, suggesting that some breast cancers may acquire neuroendocrine differentiation later in tumour evolution.

Multiple Primary Cancers in Patients Undergoing Tumor-Normal Sequencing Define Novel Associations

BACKGROUND

Cancer survivors are developing more subsequent tumors. We sought to characterize patients with multiple (≥2) primary cancers (MPC) to assess associations and genetic mechanisms.

METHODS

Patients were prospectively consented (01/2013-02/2019) to tumor-normal sequencing via a custom targeted panel (MSK-IMPACT). A subset consented to return of results of ≥76 cancer predisposition genes. International Agency for Research on Cancer (IARC) 2004 rules for defining MPC were applied. Tumor pairs were created to assess relationships between cancers. Age-adjusted, sex-specific, standardized incidence ratios (SIR) for first to second cancer event combinations were calculated using SEER rates, adjusting for confounders and time of ascertainment. Associations were made with germline and somatic variants.

RESULTS

Of 24,241 patients, 4,340 had MPC (18%); 20% were synchronous. Most (80%) had two primaries; however, 4% had ≥4 cancers. SIR analysis found lymphoma-lung, lymphoma-uterine, breast-brain, and melanoma-lung pairs in women and prostate-mesothelioma, prostate-sarcoma, melanoma-stomach, and prostate-brain pairs in men in excess of expected after accounting for synchronous tumors, known inherited cancer syndromes, and environmental exposures. Of 1,580 (36%) patients who received germline results, 324 (21%) had 361 pathogenic/likely pathogenic variants (PV), 159 (44%) in high penetrance genes. Of tumor samples analyzed, 55% exhibited loss of heterozygosity at the germline variant. In those with negative germline findings, melanoma, prostate, and breast cancers were common.

CONCLUSIONS

We identified tumor pairs without known predisposing mutations that merit confirmation and will require novel strategies to elucidate genetic mechanisms of shared susceptibilities.

IMPACT

If verified, patients with MPC with novel phenotypes may benefit from targeted cancer surveillance.

Recurrence biomarkers of triple negative breast cancer treated with neoadjuvant chemotherapy and anti-EGFR antibodies

To find metastatic recurrence biomarkers of triple-negative breast cancer (TNBC) treated by neoadjuvant chemotherapy and anti-EGFR antibodies (NAT), we evaluated tumor genomic, transcriptomic, and immune features, using MSK-IMPACT assay, gene arrays, Nanostring technology, and TIL assessment on H&E. Six patients experienced a rapid fatal recurrence (RR) and other 6 had later non-fatal recurrences (LR). Before NAT, RR had low expression of 6 MHC class I and 13 MHC class II genes but were enriched in upregulated genes involved in the cell cycle-related pathways. Their TIL number before NAT in RR was very low (<5%) and did not increase after treatment. In post-NAT residual tumors, RR cases showed high expression of SOX2 and CXCR4. Our results indicate that high expression of cell cycle genes, combined with cold immunological phenotype, may predict strong TNBC resistance to NAT and rapid progression after it. This biomarker combination is worth validation in larger studies.

Mesonephric and mesonephric-like carcinomas of the female genital tract: molecular characterization including cases with mixed histology and matched metastases

Mesonephric carcinoma of the cervix is a rare tumor derived from Wolffian remnants. Mesonephric-like carcinomas of the ovary and endometrium, while morphologically similar, do not have obvious Wolffian derivation. Here, we sought to characterize the repertoire of genetic alterations in primary mesonephric and mesonephric-like carcinomas, in the distinct histologic components of mixed cases, as well as in matched primary tumors and metastases. DNA from microdissected tumor and normal tissue from mesonephric carcinomas (cervix, n = 8) and mesonephric-like carcinomas (ovarian n = 15, endometrial n = 13) were subjected to sequencing targeting 468 cancer-related genes. The histologically distinct components of four cases with mixed histology and four primary tumors and their matched metastases were microdissected and analyzed separately. Mesonephric-like carcinomas were underpinned by somatic KRAS mutations (25/28, 89%) akin to mesonephric carcinomas (8/8, 100%), but also harbored genetic alterations more frequently reported in Müllerian tumors. Mesonephric-like carcinomas that lacked KRAS mutations harbored NRAS (n = 2, ovary) or BRAF (n = 1, endometrium) hotspot mutations. PIK3CA mutations were identified in both mesonephric-like (8/28, 28%) and mesonephric carcinomas (2/8, 25%). Only mesonephric-like tumors harbored CTNNB1 hotspot (4/28, 14%) and PTEN (3/13, 23%) mutations. Copy number analysis revealed frequent gains of chromosomes 1q and 10 in both mesonephric (87% 1q; 50% chromosome 10) and mesonephric-like tumors (89% 1q; 43% chromosome 10). Chromosome 12 gains were more frequent in ovarian mesonephric-like carcinomas, and losses of chromosome 9 were more frequent in mesonephric than in mesonephric-like carcinomas (both p = 0.01, Fisher's exact test). The histologically distinct components of four mixed cases were molecularly related and shared similar patterns of genetic alterations. The progression from primary to metastatic lesions involved the acquisition of additional mutations, and/or shifts from subclonal to clonal mutations. Our findings suggest that mesonephric-like carcinomas are derived from a Müllerian substrate with differentiation along Wolffian/mesonephric lines.

Synthetic Lethality in Cancer Therapeutics: The Next Generation

Synthetic lethality (SL) provides a conceptual framework for tackling targets that are not classically "druggable," including loss-of-function mutations in tumor suppressor genes required for carcinogenesis. Recent technological advances have led to an inflection point in our understanding of genetic interaction networks and ability to identify a wide array of novel SL drug targets. Here, we review concepts and lessons emerging from first-generation trials aimed at testing SL drugs, discuss how the nature of the targeted lesion can influence therapeutic outcomes, and highlight the need to develop clinical biomarkers distinct from those based on the paradigms developed to target activated oncogenes. SIGNIFICANCE: SL offers an approach for the targeting of loss of function of tumor suppressor and DNA repair genes, as well as of amplification and/or overexpression of genes that cannot be targeted directly. A next generation of tumor-specific alterations targetable through SL has emerged from high-throughput CRISPR technology, heralding not only new opportunities for drug development, but also important challenges in the development of optimal predictive biomarkers.

Investigation of monogenic causes of familial breast cancer: data from the BEACCON case-control study

Breast cancer (BC) has a significant heritable component but the genetic contribution remains unresolved in the majority of high-risk BC families. This study aims to investigate the monogenic causes underlying the familial aggregation of BC beyond BRCA1 and BRCA2, including the identification of new predisposing genes. A total of 11,511 non-BRCA familial BC cases and population-matched cancer-free female controls in the BEACCON study were investigated in two sequencing phases: 1303 candidate genes in up to 3892 cases and controls, followed by validation of 145 shortlisted genes in an additional 7619 subjects. The coding regions and exon–intron boundaries of all candidate genes and 14 previously proposed BC genes were sequenced using custom designed sequencing panels. Pedigree and pathology data were analysed to identify genotype-specific associations. The contribution of ATM, PALB2 and CHEK2 to BC predisposition was confirmed, but not RAD50 and NBN. An overall excess of loss-of-function (LoF) (OR 1.27, p = 9.05 × 10⁻⁹) and missense (OR 1.27, p = 3.96 × 10⁻⁷³) variants was observed in the cases for the 145 candidate genes. Leading candidates harbored LoF variants with observed ORs of 2–4 and individually accounted for no more than 0.79% of the cases. New genes proposed by this study include NTHL1, WRN, PARP2, CTH and CDK9. The new candidate BC predisposition genes identified in BEACCON indicate that much of the remaining genetic causes of high-risk BC families are due to genes in which pathogenic variants are both very rare and convey only low to moderate risk.

Homologous recombination deficiency (HRD) score in germline BRCA2- versus ATM-altered prostate cancer

The homologous recombination deficiency (HRD) score integrates three DNA-based measures of genomic instability, and has been understudied in prostate cancer. Given the recent FDA approval of two PARP inhibitors for prostate cancer, HRD score analysis could help to refine treatment selection. We assessed HRD score (defined as the sum of loss-of-heterozygosity, telomeric allelic imbalance, and large-scale state transitions) in three cohorts of primary prostate cancer, including a Johns Hopkins University (JHU) cohort with germline mutations in BRCA2, ATM, or CHEK2 (n = 64), the TCGA cohort (n = 391), and the PROGENE cohort (n = 102). In the JHU cohort, tumors with germline BRCA2 mutations had higher HRD scores (median = 27) than those with germline ATM or CHEK2 mutations (median = 16.5 [p = 0.029] and 9 [p < 0.001], respectively). For TCGA tumors without underlying HR pathway mutations, the median HRD score was 11, significantly lower than ovarian carcinoma lacking BRCA1/2 mutations (median = 28). In the absence of HR gene mutations, the median HRD score was unexpectedly higher among prostate cancers with TP53 mutations versus those without (17 vs. 11; p = 0.015); this finding was confirmed in the PROGENE cohort (24 vs. 16; p = 0.001). Finally, among eight BRCA2-altered patients who received olaparib, progression-free survival trended longer in those with HRD scores above versus below the median (14.9 vs. 9.9 months). We conclude that HRD scores are low in primary prostate cancer and higher in cases with germline BRCA2 or somatic TP53 mutations. Germline BRCA2-altered cases have significantly higher HRD scores than germline ATM-altered or CHEK2-altered cases, consistent with the lower efficacy of PARP inhibitors among the latter.

Mutations in BRCA1/2 and Other Panel Genes in Patients With Metastatic Breast Cancer -Association With Patient and Disease Characteristics and Effect on Prognosis

PURPOSE

Among patients with metastatic breast cancer (mBC), the frequency of germline mutations in cancer susceptibility genes and the clinical relevance of these mutations are unclear. In this study, a prospective cohort of patients with mBC was used to determine mutation rates for breast cancer (BC) predisposition genes, to evaluate the clinical characteristics of patients with mutations, and to assess the influence of mutations on patient outcome.

PATIENTS AND METHODS

Germline DNA from 2,595 patients with mBC enrolled in the prospective PRAEGNANT registry was evaluated for mutations in cancer predisposition genes. The frequencies of mutations in known BC predisposition genes were compared with results from a prospective registry of patients with nonmetastatic BC sequenced using the same QIAseq method and with public reference controls. Associations between mutation status and tumor characteristics, progression-free survival, and overall survival were assessed.

RESULTS

Germline mutations in 12 established BC predisposition genes (including BRCA1 and BRCA2) were detected in 271 (10.4%) patients. A mutation in BRCA1 or BRCA2 was seen in 129 patients (5.0%). BRCA1 mutation carriers had a higher proportion of brain metastasis (27.1%) compared with nonmutation carriers (12.8%). Mutations were significantly enriched in PRAEGNANT patients with mBC compared with patients with nonmetastatic BC (10.4% v 6.6%, P < .01). Mutations did not significantly modify progression-free survival or overall survival for patients with mBC.

CONCLUSION

Multigene panel testing may be considered in all patients with mBC because of the high frequency of germline mutations in BRCA1/2 and other BC predisposition genes. Although the prognosis of mutation carriers and nonmutation carriers with mBC was similar, differences observed in tumor characteristics have implications for treatment and for future studies of targeted therapies.

Clonal relationship and directionality of progression of synchronous endometrial and ovarian carcinomas in patients with DNA mismatch repair-deficiency associated syndromes

Sporadic synchronous endometrial (ECs) and ovarian cancers (OCs), although clinically considered to be independent primaries, have been shown to be clonally related and likely constitute metastases from each other. We sought to define whether synchronous ECs/OCs in patients with DNA mismatch repair (MMR)-deficiency syndromes would be clonally related. We subjected synchronous ECs/OCs from four patients (LS3-LS6) with clinically confirmed Lynch syndrome (LS) and one patient with constitutional mismatch repair-deficiency syndrome (CMMRD) to massively parallel sequencing targeting 468 cancer-related genes. Somatic mutations, copy number alterations (CNAs), clonal relatedness and clonal decomposition analyses were performed using previously described bioinformatics methods. All synchronous ECs/OCs analyzed were considered independent primaries based on clinicopathologic criteria. Sequencing analysis revealed that the ECs/OCs of three cases (LS2-CMMRD, L3, L4) harbored similar repertoires of somatic mutations and CNAs and were clonally related. In these three cases, a subset of subclonal mutations in the EC became clonal in the OC, suggesting that the EC was likely the substrate from which the OC developed. LS5's EC/OC had distinct mutational profiles but shared specific CNAs. In contrast, LS6's EC/OC harbored distinct somatic mutations and lacked CNAs, consistent with each tumor constituting an independent primary lesion. In LS5 and LS6, PTEN mutations and PTEN loss of protein expression were found to be restricted to the EC. Finally, re-analysis of sequencing data of sporadic synchronous ECs/OCs supported the observations made in the current study that the directionality of progression is likely from the endometrium to the ovary. In conclusion, contrary to sporadic synchronous ECs/OCs, which are almost invariably clonally related, ECs/OCs simultaneously involving the uterus and ovary in LS patients may represent distinct primary tumors. A subset of MMR-deficiency syndrome-related synchronous ECs/OCs, however, may originate from a single primary tumor at variance with their clinical diagnosis, with the endometrium being the likeliest site of origin.

Genomic Features and Classification of Homologous Recombination Deficient Pancreatic Ductal Adenocarcinoma

BACKGROUND AND AIMS

Homologous recombination deficiency (HRD) in pancreatic ductal adenocarcinoma (PDAC), remains poorly defined beyond germline (g) alterations in BRCA1, BRCA2, and PALB2.

METHODS

We interrogated whole genome sequencing (WGS) data on 391 patients, including 49 carriers of pathogenic variants (PVs) in gBRCA and PALB2. HRD classifiers were applied to the dataset and included (1) the genomic instability score (GIS) used by Myriad's MyChoice HRD assay; (2) substitution base signature 3 (SBS3); (3) HRDetect; and (4) structural variant (SV) burden. Clinical outcomes and responses to chemotherapy were correlated with HRD status.

RESULTS

Biallelic tumor inactivation of gBRCA or PALB2 was evident in 43 of 49 germline carriers identifying HRD-PDAC. HRDetect (score ≥0.7) predicted gBRCA1/PALB2 deficiency with highest sensitivity (98%) and specificity (100%). HRD genomic tumor classifiers suggested that 7% to 10% of PDACs that do not harbor gBRCA/PALB2 have features of HRD. Of the somatic HRDetect^hi cases, 69% were attributed to alterations in BRCA1/2, PALB2, RAD51C/D, and XRCC2, and a tandem duplicator phenotype. TP53 loss was more common in BRCA1- compared with BRCA2-associated HRD-PDAC. HRD status was not prognostic in resected PDAC; however in advanced disease the GIS (P = .02), SBS3 (P = .03), and HRDetect score (P = .005) were predictive of platinum response and superior survival. PVs in gATM (n = 6) or gCHEK2 (n = 2) did not result in HRD-PDAC by any of the classifiers. In 4 patients, BRCA2 reversion mutations associated with platinum resistance.

CONCLUSIONS

Germline and parallel somatic profiling of PDAC outperforms germline testing alone in identifying HRD-PDAC. An additional 7% to 10% of patients without gBRCA/PALB2 mutations may benefit from DNA damage response agents.

Genetic characterisation of adult primary pleomorphic uterine rhabdomyosarcoma and comparison with uterine carcinosarcoma

AIMS

To characterise the genetic alterations in adult primary uterine rhabdomyosarcomas (uRMSs) and to investigate whether these tumours are genetically distinct from uterine carcinosarcomas (UCSs).

METHODS AND RESULTS

Three tumours originally diagnosed as primary adult pleomorphic uRMS were subjected to massively parallel sequencing targeting 468 cancer-related genes and RNA-sequencing. Mutational profiles were compared with those of UCSs (n = 57) obtained from The Cancer Genome Atlas. Sequencing data analyses were performed using validated bioinformatic approaches. Pathogenic TP53 mutations and high levels of genomic instability were detected in the three cases. uRMS1 harboured a likely pathogenic YTHDF2-FOXR1 fusion. uRMS2 harboured a PPP2R1A hotspot mutation and amplification of multiple genes, including WHSC1L1, FGFR1, MDM2, and CCNE1, whereas uRMS3 harboured an FBXW7 hotspot mutation and an ANKRD11 homozygous deletion. Hierarchical clustering of somatic mutations and copy number alterations revealed that these tumours initially diagnosed as pleomorphic uRMSs and UCSs were similar. Subsequent comprehensive pathological re-review of the three uRMSs revealed previously unidentified minute pan-cytokeratin-positive atypical glands in one case (uRMS3), favouring its reclassification as UCS with extensive rhabdomyosarcomatous overgrowth.

CONCLUSIONS

Adult pleomorphic uRMSs harbour TP53 mutations and high levels of copy number alterations. Our findings underscore the challenge in discriminating between uRMS and UCS with rhabdomyosarcomatous differentiation.

TERT promoter hotspot mutations and gene amplification in metaplastic breast cancer

Metaplastic breast cancers (MBCs) are characterized by complex genomes, which seem to vary according to their histologic subtype. TERT promoter hotspot mutations and gene amplification are rare in common forms of breast cancer, but present in a subset of phyllodes tumors. Here, we sought to determine the frequency of genetic alterations affecting TERT in a cohort of 60 MBCs with distinct predominant metaplastic components (squamous, 23%; spindle, 27%; osseous, 8%; chondroid, 42%), and to compare the repertoire of genetic alterations of MBCs according to the presence of TERT promoter hotspot mutations or gene amplification. Forty-four MBCs were subjected to: whole-exome sequencing (WES; n = 27) or targeted sequencing of 341-468 cancer-related genes (n = 17); 16 MBCs were subjected to Sanger sequencing of the TERT promoter, TP53 and selected exons of PIK3CA, HRAS, and BRAF. TERT promoter hotspot mutations (n = 9) and TERT gene amplification (n = 1) were found in 10 of the 60 MBCs analyzed, respectively. These TERT alterations were less frequently found in MBCs with predominant chondroid differentiation than in other MBC subtypes (p = 0.01, Fisher's exact test) and were mutually exclusive with TP53 mutations (p < 0.001, CoMEt). In addition, a comparative analysis of the MBCs subjected to WES or targeted cancer gene sequencing (n = 44) revealed that MBCs harboring TERT promoter hotspot mutations or gene amplification (n = 6) more frequently harbored PIK3CA than TERT wild-type MBCs (n = 38; p = 0.001; Fisher's exact test). In conclusion, TERT somatic genetic alterations are found in a subset of TP53 wild-type MBCs with squamous/spindle differentiation, highlighting the genetic diversity of these cancers.

Landscape of somatic mutations in breast cancer: new opportunities for targeted therapies in Saudi Arabian patients

Breast cancer (BCa) ranks first in incidence rate among cancers in Arab females. The association between genetic polymorphisms in tumor suppressor genes and the risk of BCa has been studied in many ethnic populations with conflicting conclusions while Arab females and Saudi Arabian studies are still lacking. We screened a cohort of Saudi BCa patients by NGS using a bespoke gene panel to clarify the genetic landscape of this population, correlating and assessing genetic findings with clinical outcomes. We identified a total of 263 mutations spanning 51 genes, including several frequently mutated. Among the genes analyzed, the highest mutation rates were found in PIK3CA (12.9%), BRCA2 (11.7%), BRCA1 (10.2%), TP53 (6.0%), MSH2 (3.8%), PMS2 (3.8%), BARD1 (3.8%), MLH1 (3.4%), CDH1 (3.0%), RAD50 (3.0%), MSH6 (3.0%), NF1 (2.6%), in addition to others. We identified multiple common recurrent variants and previously reported mutations. We also identified 46 novel variants in 22 genes that were predicted to have a pathogenic effect. Survival analysis according to the four most common mutations (BRCA1, BRCA2, TP53, and PIK3CA) showed reduced survival in BRCA1 and BRCA2-mutant patients compared to total patients. Moreover, BRCA2 was demonstrated as an independent predictor of reduced survival using independent Cox proportional hazard models.

We reveal the landscape of the mutations associated with BCa in Saudi women, highlighting the importance of routine genetic sequencing in implementation of precision therapies in KSA.

Extended gene panel testing in lobular breast cancer

Purpose: Lobular breast cancer (LBC) accounts for ~ 15% of breast cancer. Here, we studied the frequency of pathogenic germline variants (PGVs) in an extended panel of genes in women affected with LBC. Methods: 302 women with LBC and 1567 without breast cancer were tested for BRCA1/2 PGVs. A subset of 134 LBC affected women who tested negative for BRCA1/2 PGVs underwent extended screening, including: ATM, CDH1, CHEK2, NBN, PALB2, PTEN, RAD50, RAD51D, and TP53.Results: 35 PGVs were identified in the group with LBC, of which 22 were in BRCA1/2. Ten actionable PGVs were identified in additional genes (ATM(4), CDH1(1), CHEK2(1), PALB2(2) and TP53(2)). Overall, PGVs in three genes conferred a significant increased risk for LBC. Odds ratios (ORs) were: BRCA1: OR = 13.17 (95%CI 2.83–66.38; P = 0.0017), BRCA2: OR = 10.33 (95%CI 4.58–23.95; P < 0.0001); and ATM: OR = 8.01 (95%CI 2.52–29.92; P = 0.0053). We did not detect an increased risk of LBC for PALB2, CDH1 or CHEK2. Conclusion: The overall PGV detection rate was 11.59%, with similar rates of BRCA1/2 (7.28%) PGVs as for other actionable PGVs (7.46%), indicating a benefit for extended panel genetic testing in LBC. We also report a previously unrecognised association of pathogenic variants in ATM with LBC.

A P53-Independent DNA Damage Response Suppresses Oncogenic Proliferation and Genome Instability

The Mre11-Rad50-Nbs1 complex is a DNA double-strand break sensor that mediates a tumor-suppressive DNA damage response (DDR) in cells undergoing oncogenic stress, yet the mechanisms underlying this effect are poorly understood. Using a genetically inducible primary mammary epithelial cell model, we demonstrate that Mre11 suppresses proliferation and DNA damage induced by diverse oncogenic drivers through a p53-independent mechanism. Breast tumorigenesis models engineered to express a hypomorphic Mre11 allele exhibit increased levels of oncogene-induced DNA damage, R-loop accumulation, and chromosomal instability with a characteristic copy number loss phenotype. Mre11 complex dysfunction is identified in a subset of human triple-negative breast cancers and is associated with increased sensitivity to DNA-damaging therapy and inhibitors of ataxia telangiectasia and Rad3 related (ATR) and poly (ADP-ribose) polymerase (PARP). Thus, deficiencies in the Mre11-dependent DDR drive proliferation and genome instability patterns in p53-deficient breast cancers and represent an opportunity for therapeutic exploitation.

Molecular characterization of high-grade serous ovarian cancers occurring in younger and older women

OBJECTIVES

To determine if the mutational landscapes and genomic features of homologous recombination DNA repair defects (HRD) vary between younger and older patients with high-grade serous ovarian cancer (HGSOC).

METHODS

Younger and older women were defined as bottom and top age quartiles, respectively. HGSOCs from 15 younger (median 49 years, range 35-53) and 15 older women (median 72 years, range 70-87) were subjected to whole-exome sequencing (WES). For validation, HGSOC WES data were obtained from The Cancer Genome Atlas (TCGA), including 38 younger (median 45 years, range 34-50) and 30 older women (median 74 years, range 68-84). Mutational profiles, BRCA1/2 status, genomic HRD features, and for TCGA cases RNA-sequencing-based HRD transcriptomic signatures were assessed.

RESULTS

In the institutional cohort, pathogenic germline BRCA1/2 mutations were more frequent in younger (5/15) than older women (0/15, p = 0.042). No somatic BRCA1/2 mutations were identified. HGSOCs from older patients preferentially displayed aging-related mutational signatures and, in contrast to younger patients, harbored CCNE1 amplifications (3/15, 20%). In the TCGA cohort, pathogenic germline BRCA1 (younger 8/38, older 0/30, p = 0.007) but not BRCA2 mutations (young 3/38, older 4/30, p = 0.691) were more frequent in younger patients. Again, no somatic BRCA1/2 mutations were identified. HGSOCs from younger women more frequently displayed genomic features of HRD (all, p < 0.05), a significant HRD gene-signature enrichment, but less frequently CCNE1 amplification (p = 0.05). Immunoreactive CLOVAR subtypes were more common in HGSOCs from younger women, and proliferative subtypes in HGSOCs from older women (p = 0.041).

CONCLUSIONS

HGSOC patients diagnosed at an older age less frequently harbor pathogenic BRCA1 germline mutations and genomic features of HRD than younger women. Individualized treatment options, particularly pertaining to use of PARP inhibitors, in older women may be warranted.

Pathogenic ATM Mutations in Cancer and a Genetic Basis for Radiotherapeutic Efficacy

BACKGROUND

Radiation therapy is one of the most commonly used cancer therapeutics but genetic determinants of clinical benefit are poorly characterized. Pathogenic germline variants in ATM are known to cause ataxia-telangiectasia, a rare hereditary syndrome notable for marked radiosensitivity. In contrast, somatic inactivation of ATM is a common event in a wide variety of cancers, but its clinical actionability remains obscure.

METHODS

We analyzed 20 107 consecutively treated advanced cancer patients who underwent targeted genomic sequencing as part of an institutional genomic profiling initiative and identified 1085 harboring a somatic or germline ATM mutation, including 357 who received radiotherapy (RT). Outcomes of irradiated tumors harboring ATM loss-of-function (LoF) mutations were compared with those harboring variants of unknown significance. All statistical tests were 2-sided.

RESULTS

Among 357 pan-cancer patients who received 727 courses of RT, genetic inactivation of ATM was associated with improved radiotherapeutic efficacy. The 2-year cumulative incidence of irradiated tumor progression was 13.2% vs 27.5% for tumors harboring an ATM LoF vs variant of unknown significance allele, respectively (hazard ratio [HR] = 0.51, 95% confidence interval [CI] = 0.34 to 0.77, P = .001). The greatest clinical benefit was seen in tumors harboring biallelic ATM inactivation (HR = 0.19, 95% CI = 0.06 to 0.60, P = .005), with statistically significant benefit also observed in tumors with monoallelic ATM inactivation (HR = 0.57, 95% CI = 0.35 to 0.92, P = .02). Notably, ATM LoF was highly predictive of outcome in TP53 wild-type tumors but not among TP53-mutant tumors.

CONCLUSIONS

We demonstrate that somatic ATM inactivation is associated with markedly improved tumor control following RT. The identification of a radio-sensitive tumor phenotype across multiple cancer types offers potential clinical opportunities for genomically guided RT.

Homologous recombination deficiency: how genomic signatures are generated

Cancer genomes harbor mutational and structural rearrangements that are jointly shaped by DNA damage and repair mechanisms. Accumulating evidence suggests that genetic alterations in DNA repair-defective tumors reflect the scars caused by the use of backup DNA repair mechanisms needed to maintain cellular viability. Detailed analysis of the patterns of mutations and structural rearrangements present in BRCA1/2-deficient tumors has allowed for the delineation of genomic signatures that reflect alternative repair with inactive homologous recombination (HR). Here we aim to summarize recent advances in the analysis of genomic signatures associated with HR-deficiency and examine recent studies that have shed light on the backup repair mechanisms responsible for genomic scarring in HR-deficient tumors.

BRCA2, ATM, and CDK12 Defects Differentially Shape Prostate Tumor Driver Genomics and Clinical Aggression

PURPOSE

DNA damage repair (DDR) defects are common across cancer types and can indicate therapeutic vulnerability. Optimal exploitation of DDR defects in prostate cancer requires new diagnostic strategies and a better understanding of associated clinical genomic features.

EXPERIMENTAL DESIGN

We performed targeted sequencing of 1,615 plasma cell-free DNA samples from 879 patients with metastatic prostate cancer. Depth-based copy-number calls and heterozygous SNP imbalance were leveraged to expose DDR-mutant allelic configuration and categorize mechanisms of biallelic loss. We used split-read structural variation analysis to characterize tumor suppressor rearrangements. Patient-matched archival primary tissue was analyzed identically.

RESULTS

BRCA2, ATM, and CDK12 were the most frequently disrupted DDR genes in circulating tumor DNA (ctDNA), collectively mutated in 15% of evaluable cases. Biallelic gene disruption via second somatic alteration or mutant allele-specific imbalance was identified in 79% of patients. A further 2% exhibited homozygous BRCA2 deletions. Tumor suppressors TP53, RB1, and PTEN were controlled via disruptive chromosomal rearrangements in BRCA2-defective samples, but via oncogene amplification in context of CDK12 defects. TP53 mutations were rare in cases with ATM defects. DDR mutations were re-detected across 94% of serial ctDNA samples and in all available archival primary tissues, indicating they arose prior to metastatic progression. Loss of BRCA2 and CDK12, but not ATM, was associated with poor clinical outcomes.

CONCLUSIONS

BRCA2, ATM, and CDK12 defects are each linked to distinct prostate cancer driver genomics and aggression. The consistency of DDR status in longitudinal samples and resolution of allelic status underscores the potential for ctDNA as a diagnostic tool.