Inflammatory/juvenile-like polyps in neurofibromatosis type 1 associated with epithelial dysplasia

The term "juvenile-like (inflammatory/hyperplastic) mucosal polyps" (JLIHMP) has been recently introduced to describe a spectrum of polypoid lesions in patients with neurofibromatosis type 1 (NF-1). Due to the scarce number of reported cases and histopathological similarities with entities such as sporadic/syndromic juvenile polyps or inflammatory fibroid polyps, this entity remains a subject of debate. We describe herein a case of multiple JLIHMPs in a patient with NF-1, and we document the presence of low-grade dysplasia within one of these polyps.

Neurofibromatosis type 1 mosaicism in patients with constitutional mismatch repair deficiency

Differential diagnosis between constitutional mismatch repair deficiency (CMMRD) and neurofibromatosis type 1 (NF1) is crucial as treatment and surveillance differ. We report the case of a girl with a clinical diagnosis of sporadic NF1 who developed a glioblastoma. Immunohistochemistry for MMR proteins identified PMS2 loss in tumour and normal cells and WES showed the tumour had an ultra-hypermutated phenotype, supporting the diagnosis of CMMRD. Germline analyses identified two variants (one pathogenic variant and one classified as variant(s) of unknown significance) in the PMS2 gene and subsequent functional assays on blood lymphocytes confirmed the diagnosis of CMMRD. The large plexiform neurofibroma of the thigh and the freckling were however more compatible with NF1. Indeed, a NF1 PV (variant allele frequencies of 20%, 3% and 9% and in blood, skin and saliva samples, respectively) was identified confirming a mosaicism for NF1. Retrospective analysis of a French cohort identified NF1 mosaicism in blood DNA in 2 out of 22 patients with CMMRD, underlining the existence of early postzygotic PV of NF1 gene in patients with CMMRD whose tumours have been frequently reported to exhibit somatic NF1 mutations. It highlights the potential role of this pathway in the pathogenesis of CMMRD-associated gliomas and argues in favour of testing MEK inhibitors in this context.

Next-Generation-Sequencing on fine needle aspirates in neck recurrence of thyroid cancers

OBJECTIVES

Tumor molecular genotyping plays a key role in improving the management of advanced thyroid cancers. Molecular tests are classically performed on Formalin-Fixed Paraffin-Embedded (FFPE) carcinoma tissue. However alternative molecular testing strategies are needed when FFPE tumoral tissue is unavailable. The objective of our study was to retrospectively assess the performance of targeted DNA and RNA-based Next Generation Sequencing (NGS) on the fine needle aspirate from thyroid cancer cervical recurrences to determine if this strategy is efficient in clinical practice.

DESIGN/METHODS

A retrospective study of 33 patients who had had DNA and/or RNA-based NGS on ultrasound (US)-guided fine needle aspirates of cervical thyroid cancer recurrences in our Department from July 2019 to September 2022.

RESULTS

In total, 34 DNA and 32 RNA-based NGS analyses were performed. Out of the 34 DNA-based NGS performed, 27 (79%) were conclusive allowing the identification of an oncogenic driver for 18 patients (53%). The most common mutation (n = 13) was BRAF c.1799T>A. Out of the 32 RNA-based NGS performed, 26 were interpretable (81%) and no gene fusion was found. The identification of a BRAFV600E mutation was decisive for one patient in our series, who was prescribed dabrafenib and trametinib.

CONCLUSIONS

NGS performed on fine needle aspirates of neck lymph node metastases enabled the identification of an oncogenic driver alteration in 53% of the cases in our series of advanced thyroid cancer patients and could significantly alter patient management.

Prediction of Response to Immune Checkpoint Blockade in Patients with Metastatic Colorectal Cancer with Microsatellite Instability

BACKGROUND

Mismatch repair deficient (dMMR) tumors displaying microsatellite instability (MSI) represent a paradigm for the success of immune checkpoint inhibitor (ICI)-based immunotherapy, particularly in patients with metastatic colorectal cancer (mCRC). However, a proportion of patients with dMMR/MSI mCRC exhibit resistance to ICI. Identification of tools predicting MSI mCRC patient response to ICI are required for the design of future strategies further improving this therapy.

PATIENTS AND METHODS

We combined high-throughput DNA and RNA sequencing of tumors from 116 patients with MSI mCRC treated with anti-PD-1 +/- anti-CTLA-4 of the NIPICOL phase II trial (C1, NCT03350126, discovery set) and the IMMUNOMSI prospective cohort (C2, validation set). The DNA/RNA predictors whose status was significantly associated with ICI status of response in C1 were subsequently validated in C2. Primary endpoint was iPFS (progression-free survival by iRECIST).

RESULTS

Analyses showed no impact of previously suggested DNA/RNA indicators of resistance to ICI, e.g., MSISensor score, tumor mutational burden, or specific cellular and molecular tumoral contingents. By contrast, iPFS under ICI was shown in C1 and C2 to depend both on a multiplex MSI signature involving the mutations of 19 microsatellites (HR_C2 = 3.63; 95% CI [1.65-7.99] ; p = 1.4x10^-3) and the expression of a set of 182 RNA markers with a non-epithelial TGFB-related desmoplastic orientation (HR_C2 = 1.75 ; 95% CI [1.03-2.98] ; p = 0.035). Both DNA and RNA signatures were independently predictive of iPFS.

CONCLUSIONS

iPFS in patients with MSI mCRC can be predicted by simply analyzing the mutational status of DNA microsatellite-containing genes in epithelial tumor cells together with nonepithelial TGFB-related desmoplastic RNA markers.

AXIN2 germline testing in a French cohort validates pathogenic variants as a rare cause of predisposition to colorectal polyposis and cancer

Only a few patients with germline AXIN2 variants and colorectal adenomatous polyposis or cancer have been described, raising questions about the actual contribution of this gene to colorectal cancer (CRC) susceptibility. To assess the clinical relevance for AXIN2 testing in patients suspected of genetic predisposition to CRC, we collected clinical and molecular data from the French Oncogenetics laboratories analyzing AXIN2 in this context. Between 2004 and June 2020, 10 different pathogenic/likely pathogenic AXIN2 variants were identified in 11 unrelated individuals. Eight variants were from a consecutive series of 3322 patients, which represents a frequency of 0.24%. However, loss-of-function AXIN2 variants were strongly associated with genetic predisposition to CRC as compared with controls (odds ratio: 11.89, 95% confidence interval: 5.103-28.93). Most of the variants were predicted to produce an AXIN2 protein devoid of the SMAD3-binding and DIX domains, but preserving the β-catenin-binding domain. Ninety-one percent of the AXIN2 variant carriers who underwent colonoscopy had adenomatous polyposis. Forty percent of the variant carriers developed colorectal or/and other digestive cancer. Multiple tooth agenesis was present in at least 60% of them. Our report provides further evidence for a role of AXIN2 in CRC susceptibility, arguing for AXIN2 testing in patients with colorectal adenomatous polyposis or cancer.

Evaluation of two new highly multiplexed PCR assays as an alternative to next-generation sequencing for IDH1/2 mutation detection

IDH1 and IDH2 somatic mutations have been identified in solid tumors and blood malignancies. The development of inhibitors of mutant IDH1 and IDH2 in the past few years has prompted the development of a fast and sensitive assay to detect IDH1^R132 , IDH2^R140 and IDH2^R172 mutations to identify patients eligible for these targeted therapies. This study aimed to compare two new multiplexed PCR assays - an automated quantitative PCR (qPCR) on the PGX platform and a droplet digital PCR (ddPCR) with next-generation sequencing (NGS) for IDH1/2 mutation detection. These assays were evaluated on 102 DNA extracted from patient peripheral blood, bone marrow and formalin-fixed paraffin-embedded tissue samples with mutation allelic frequency ranging from 0.6% to 45.6%. The ddPCR assay had better analytical performances than the PGX assay with 100% specificity, 100% sensitivity and a detection limit down to 0.5% on IDH1^R132 , IDH2^R140 and IDH2^R172 codons, and a high correlation with NGS results. Therefore, the new highly multiplexed ddPCR is a fast and cost-effective assay that meets most clinical needs to identify and follow cancer patients in the era of anti-IDH1/2-targeted therapies.

Detection of a pathogenic Alu element insertion in PALB2 gene from targeted NGS diagnostic data

Despite routine analysis of a large panel of genes, pathogenic variants are only detected in approximately 20% of families with hereditary breast and/or ovarian cancer. Mobile element insertions (MEI) are known to cause genetic diseases in humans, but remain challenging to detect. Retrospective analysis of targeted next-generation sequencing (NGS) data from 359 patients was performed using a dedicated MEI detection pipeline. We detected one MEI in exon 9 of the PALB2 gene in a woman with a family history of breast cancer. The pathogenic variant, c.2872_2888delins114AluL2, disrupts the PALB2 coding sequence and leads to the production of a truncated protein, p.(Gln958Valfs*38). This is the first report of a pathogenic MEI in PALB2. This study illustrates that MEI analysis may help to improve molecular diagnostic yield and can be performed from targeted NGS data used for routine diagnosis.

Possible Primary Thyroid Nuclear Protein in Testis Carcinomas with NSD3::NUTM1 Translocation Revealed by RNA Sequencing: A Report of Two Cases

Background: Nuclear protein in testis (NUT) carcinomas (NC) are a rare, highly aggressive, subset of squamous cell carcinomas, characterized by a translocation involving the NUTM1 gene. Thyroid location of NUT carcinomas has rarely been described. Methods: We report here two cases of thyroid NC with NSD3::NUTM1 translocation. Results: The first case presented as a very aggressive undifferentiated thyroid carcinoma in a 38-year-old man who died 21 months after the diagnosis. The second case was diagnosed after multiple lymphadenopathy recurrences mainly in the neck in a 37-year-old woman 7 years after total thyroidectomy for papillary thyroid carcinoma with a classic and a solid/trabecular component. Conclusions: Our case reports highlight the challenges in diagnosing these exceptional carcinomas. The therapeutic impact of the administration of pharmacological compounds with epigenetic action, in line with the physiopathology of these carcinomas, is also discussed.

Detection of BRAFV600E by digital PCR on fine-needle aspirate enables rapid initiation of dabrafenib and trametinib in unresectable anaplastic thyroid carcinoma

INTRODUCTION

Recently, targeted therapies using BRAFV600E and MEK inhibitors (dabrafenib and trametinib, respectively) have been recommended in BRAF-mutated anaplastic thyroid carcinoma (ATC). Considering the fast development of ATC, droplet digital PCR (ddPCR) performed on fine-needle aspirate (FNA), which is a rapid, reliable, and low-cost method, appears interesting for the detection of BRAFV600E mutation in these patients and allows early initiation of targeted therapies.

RESULTS

In our two patients, both presenting extensive cervical masses inaccessible to surgery, ddPCR results were available in less than 24 h. Therefore, dabrafenib and trametinib were started only a few days after first contact.

CONCLUSIONS

We suggest that ddPCR on FNA be used in non-resectable cervical masses for rapid BRAFV600E mutation detection in the hope that starting targeted therapies early might improve outcomes.

Complete resection of ulcerating, infiltrative, voluminous differentiated thyroid carcinoma

An 87-year-old woman was referred to our department for a 15 cm right-sided cervical tumor with bleeding and skin ulceration, developed on a 6 cm papillary thyroid carcinoma diagnosed two years earlier. Surprisingly, there were no other compressive symptoms. Unexpectedly, but successfully, total thyroidectomy and neck dissection were performed. There were no poorly differentiated or anaplastic components in the final histological analysis. Impressive dehiscence occurred shortly after surgery and was also successfully managed. Our case highlights the benefit of considering surgery in the context of a tertiary care center even for an apparent massive aggressive cervical mass and despite old age.

Low-Coverage Whole Genome Sequencing of Cell-Free DNA From Immunosuppressed Cancer Patients Enables Tumor Fraction Determination and Reveals Relevant Copy Number Alterations

Cell-free DNA (cfDNA) analysis is a minimally invasive method that can be used to detect genomic abnormalities by directly testing a blood sample. This method is particularly useful for immunosuppressed patients, who are at high risk of complications from tissue biopsy. The cfDNA tumor fraction (TF) varies greatly across cancer type and between patients. Thus, the detection of molecular alterations is highly dependent on the circulating TF. In our study, we aimed to calculate the TF and characterize the copy number aberration (CNA) profile of cfDNA from patients with rare malignancies occurring in immunosuppressed environments or immune-privileged sites. To accomplish this, we recruited 36 patients: 19 patients with non-Hodgkin lymphoma (NHL) who were either human immunodeficiency virus (HIV)-positive or organ transplant recipients, 5 HIV-positive lung cancer patients, and 12 patients with glioma. cfDNA was extracted from the patients' plasma and sequenced using low-coverage whole genome sequencing (LC-WGS). The cfDNA TF was then calculated using the ichorCNA bioinformatic algorithm, based on the CNA profile. In parallel, we performed whole exome sequencing of patient tumor tissue and cfDNA samples with detectable TFs. We detected a cfDNA TF in 29% of immune-suppressed patients (one patient with lung cancer and six with systemic NHL), with a TF range from 8 to 70%. In these patients, the events detected in the CNA profile of cfDNA are well-known events associated with NHL and lung cancer. Moreover, cfDNA CNA profile correlated with the CNA profile of matched tumor tissue. No tumor-derived cfDNA was detected in the glioma patients. Our study shows that tumor genetic content is detectable in cfDNA from immunosuppressed patients with advanced NHL or lung cancer. LC-WGS is a time- and cost-effective method that can help select an appropriate strategy for performing extensive molecular analysis of cfDNA. This technique also enables characterization of CNAs in cfDNA when sufficient tumor content is available. Hence, this approach can be used to collect useful molecular information that is relevant to patient care.

Classification of 101 BRCA1 and BRCA2 variants of uncertain significance by cosegregation study: A powerful approach

Up to 80% of BRCA1 and BRCA2 genetic variants remain of uncertain clinical significance (VUSs). Only variants classified as pathogenic or likely pathogenic can guide breast and ovarian cancer prevention measures and treatment by PARP inhibitors. We report the first results of the ongoing French national COVAR (cosegregation variant) study, the aim of which is to classify BRCA1/2 VUSs. The classification method was a multifactorial model combining different associations between VUSs and cancer, including cosegregation data. At this time, among the 653 variants selected, 101 (15%) distinct variants shared by 1,624 families were classified as pathogenic/likely pathogenic or benign/likely benign by the COVAR study. Sixty-six of the 101 (65%) variants classified by COVAR would have remained VUSs without cosegregation data. Of note, among the 34 variants classified as pathogenic by COVAR, 16 remained VUSs or likely pathogenic when following the ACMG/AMP variant classification guidelines. Although the initiation and organization of cosegregation analyses require a considerable effort, the growing number of available genetic tests results in an increasing number of families sharing a particular variant, and thereby increases the power of such analyses. Here we demonstrate that variant cosegregation analyses are a powerful tool for the classification of variants in the BRCA1/2 breast-ovarian cancer predisposition genes.

Performance of Next-Generation Sequencing for the Detection of Microsatellite Instability in Colorectal Cancer With Deficient DNA Mismatch Repair

BACKGROUND & AIMS

Next-generation sequencing (NGS) was recently approved by the United States Food and Drug Administration to detect microsatellite instability (MSI) arising from defective mismatch repair (dMMR) in patients with metastatic colorectal cancer (mCRC) before treatment with immune checkpoint inhibitors (ICI). In this study, we aimed to evaluate and improve the performance of NGS to identify MSI in CRC, especially dMMR mCRC treated with ICI.

METHODS

CRC samples used in this post hoc study were reassessed centrally for MSI and dMMR status using the reference methods of pentaplex polymerase chain reaction and immunohistochemistry. Whole-exome sequencing (WES) was used to evaluate MSISensor, the Food and Drug Administration-approved and NGS-based method for assessment of MSI. This was performed in (1) a prospective, multicenter cohort of 102 patients with mCRC (C1; 25 dMMR/MSI, 24 treated with ICI) from clinical trials NCT02840604 and NCT033501260, (2) an independent retrospective, multicenter cohort of 113 patients (C2; 25 mCRC, 88 non-mCRC, all dMMR/MSI untreated with ICI), and (3) a publicly available series of 118 patients with CRC from The Cancer Genome Atlas (C3; 51 dMMR/MSI). A new NGS-based algorithm, namely MSICare, was developed. Its performance for assessment of MSI was compared with MSISensor in C1, C2, and C3 at the exome level or after downsampling sequencing data to the MSK-IMPACT gene panel. MSICare was validated in an additional retrospective, multicenter cohort (C4) of 152 patients with new CRC (137 dMMR/MSI) enriched in tumors deficient in MSH6 (n = 35) and PMS2 (n = 9) after targeted sequencing of samples with an optimized set of microsatellite markers (MSIDIAG).

RESULTS

At the exome level, MSISensor was highly specific but failed to diagnose MSI in 16% of MSI/dMMR mCRC from C1 (4 of 25; sensitivity, 84%; 95% confidence interval [CI], 63.9%-95.5%), 32% of mCRC (8 of 25; sensitivity, 68%; 95% CI, 46.5%-85.1%), and 9.1% of non-mCRC from C2 (8 of 88; sensitivity, 90.9%; 95% CI, 82.9%-96%), and 9.8% of CRC from C3 (5 of 51; sensitivity, 90.2%; 95% CI, 78.6%-96.7%). Misdiagnosis included 4 mCRCs treated with ICI, of which 3 showed an overall response rate without progression at this date. At the exome level, reevaluation of the MSI genomic signal using MSICare detected 100% of cases with true MSI status among C1 and C2. Further validation of MSICare was obtained in CRC tumors from C3, with 96.1% concordance for MSI status. Whereas misdiagnosis with MSISensor even increased when analyzing downsampled WES data from C1 and C2 with microsatellite markers restricted to the MSK-IMPACT gene panel (sensitivity, 72.5%; 95% CI, 64.2%-79.7%), particularly in the MSH6-deficient setting, MSICare sensitivity and specificity remained optimal (100%). Similar results were obtained with MSICare after targeted NGS of tumors from C4 with the optimized microsatellite panel MSIDIAG (sensitivity, 99.3%; 95% CI, 96%-100%; specificity, 100%).

CONCLUSIONS

In contrast to MSISensor, the new MSICare test we propose performs at least as efficiently as the reference method, MSI polymerase chain reaction, to detect MSI in CRC regardless of the defective MMR protein under both WES and targeted NGS conditions. We suggest MSICare may rapidly become a reference method for NGS-based testing of MSI in CRC, especially in mCRC, where accurate MSI status is required before the prescription of ICI.

IDH-wildtype lower-grade diffuse gliomas: the importance of histological grade and molecular assessment for prognostic stratification

BACKGROUND

Isocitrate dehydrogenase (IDH) wildtype (wt) grade II gliomas are a rare and heterogeneous entity. Survival and prognostic factors are poorly defined.

METHODS

We searched retrospectively all patients diagnosed with diffuse World Health Organization (WHO) grades II and III gliomas at our center (1989-2020).

RESULTS

Out of 517 grade II gliomas, 47 were "diffuse astrocytomas, IDHwt." Tumors frequently had fronto-temporo-insular location (28/47, 60%) and infiltrative behavior. We found telomerase reverse transcriptase (TERT) promoter mutations (23/45, 51%), whole chromosome 7 gains (10/37, 27%), whole chromosome 10 losses (10/41, 24%), and EGFR amplifications (4/43, 9%), but no TP53 mutations (0/22, 0%). Median overall survival (OS) was 59 months (vs 19 mo for IDHwt grade III gliomas) (P < 0.0001). Twenty-nine patients (29/43, 67%) met the definition of molecular glioblastoma according to cIMPACT-NOW update 3. Median OS in this subset was 42 months, which was shorter compared with patients with IDHwt grade II gliomas not meeting this definition (median OS: 57 mo), but substantially longer compared with IDHwt grade III gliomas meeting the definition for molecular glioblastoma (median OS: 17 mo, P < 0.0001). Most patients with IDHwt grade II gliomas met cIMPACT criteria because of isolated TERT promoter mutations (16/26, 62%), which were not predictive of poor outcome (median OS: 88 mo). Actionable targets, including 5 gene fusions involving FGFR3, were found in 7 patients (24%).

CONCLUSIONS

Our findings highlight the importance of histological grading and molecular profiling for the prognostic stratification of IDHwt gliomas and suggest some caution when assimilating IDHwt grade II gliomas to molecular glioblastomas, especially those with isolated TERT promoter mutation.

Concordance Between Tumor and Germline BRCA Status in High-Grade Ovarian Carcinoma Patients in the Phase III PAOLA-1/ENGOT-ov25 Trial

BACKGROUND

PAOLA1 is a phase III study assessing olaparib maintenance therapy in advanced high-grade ovarian carcinoma patients responding to first-line platinum-taxane-based chemotherapy plus bevacizumab as standard of care. Randomization was stratified by treatment outcome and tumor BRCA1/2 status (tBRCA) at screening.

METHODS

tBRCA was tested on formalin-fixed, paraffin-embedded tumor blocks on 5 French platforms using 2 next-generation sequencing methods based either on hybrid capture or amplicon technology. One of the exploratory objectives was to assess the concordance between germline (gBRCA) and tBRCA testing in French patients. gBRCA testing was performed on blood samples on the same platforms.

RESULTS

From May 2015 to July 2017, tBRCA tests were performed for 1176 screened patients. Only 52 (4.4%) tumor samples were noncontributive. The median interval between reception of the tumor sample and availability of the tBRCA status result was 37 days (range = 8-260). A pathogenic variant was reported in 27.1% tumor samples (319 of 1176 screened patients). tBRCA and gBRCA testing were performed for 451 French patients with negative results for both tests in 306 patients (67.8%) and positive results for both tests in 85 patients (18.8%). Only 1 large genomic rearrangement of BRCA1 was detected, exclusively in the blood sample. Interestingly, tBRCA testing revealed 6.4% of pathogenic variant (29 of 451) not detected by gBRCA testing.

CONCLUSIONS

tBRCA testing is an appropriate tool with an acceptable turnaround time for clinical practice and a low failure rate, ensuring reliable identification of patients likely to benefit from poly(ADP-ribose) polymerase inhibitor therapy.

Utility of a mainstreamed genetic testing pathway in breast and ovarian cancer patients during the COVID-19 pandemic

INTRODUCTION

Mainstreamed genetic testing (MGT) obviates the need for a cancer genetics consultation, since trained oncologists (O) and gynaecologists (G) provide counseling, prescribe testing and deliver results. We report results from our MGT program and emphasize its utility during the COVID-19 lockdown, when cancer genetics clinics had suspended their activity.

METHODS

An MGT pathway for breast and ovarian cancer (BC/OC) patients was established in Jan-2018 between the Assistance Publique - Hôpitaux de Paris.Sorbonne Université Cancer Genetics team and the Oncology/Gynecology departments at one teaching and two regional hospitals. Trained O + G evaluated patients with the Manchester Scoring System. A 12-point threshold was recommended for testing. Next-generation sequencing of BRCA1, BRCA2, PALB2, RAD51C and RAD51D was performed. Results were delivered to the patient by O/G. Pathogenic variants (PV) carriers were referred to the genetics clinic. Results are reported for the 2nd-Jan-2018 to 1st-June-2020 period. That includes the eight-week COVID-19 lockdown and three-week de-confinement phase 1.

RESULTS

Results were available for 231/234 patients. Twenty-eight (12.1%) carried a PV. Of the 27 patients tested during the COVID-19 period, three carried a PV, two in BRCA1 and one in RAD51C. The clinical impact was immediate for the two BRCA1 BC cases undergoing neo-adjuvant chemotherapy, since double mastectomy and salpingo-oophorectomy will now be performed using two-step strategies.

CONCLUSIONS

MGT guaranteed care continuity in BC/OC patients during the critical phases of the COVID-19 pandemic, with immediate implications for PV carriers. More broadly, we report for the first time the successful implementation of MGT in France.

Abstract 5705: Mechanisms and therapeutic implications of hypermutation in gliomas

High tumor mutational burden (hypermutation) is observed in some gliomas; however, its mechanisms of development and whether it predicts immunotherapy response are poorly understood. Here, we comprehensively analyze the molecular determinants of mutational burden and signatures in 10,294 gliomas including AACR Project GENIE and institutional datasets. We delineate two main pathways to hypermutation: a de novo pathway associated with constitutional defects in DNA polymerase and mismatch repair (MMR) genes, and a more common post-treatment pathway associated with acquired resistance driven by MMR defects in chemotherapy-sensitive gliomas that recur after temozolomide treatment. Experimentally, the mutational signature of post-treatment hypermutated gliomas was only recapitulated by temozolomide-induced damage in cells harboring MMR deficiency. MMR-deficient gliomas exhibited unique features including the lack of prominent T-cell infiltrates, extensive intratumoral heterogeneity, poor survival and low response rate to PD-1 blockade. Moreover, while microsatellite instability in MMR-deficient gliomas was not detected by bulk analyses, single-cell whole-genome sequencing of post-treatment hypermutated glioma cells demonstrated microsatellite mutations. This study shows that chemotherapy can drive acquisition of hypermutated populations without promoting response to PD-1 blockade and supports diagnostic use of mutational burden and signatures in cancer.

Citation Format: Mehdi Touat, Yvonne Y. Li, Adam N. Boynton, Liam F. Spurr, Bryan Iorgulescu, Craig L. Bohrson, Isidro Cortes-Ciriano, Jack E. Geduldig, Kristine Pelton, Mary J. Lim-Fat, Sangita Pal, Shakti H. Ramkissoon, Frank Dubois, Charlotte Bellamy, Naomi Currimjee, Kenin Qian, Seth Malinowski, Aniket Shetty, Kin-Hoe Chow, Maïté Verreault, Erell Guillerm, Samy Ammari, Frédéric Beuvon, Karima Mokhtari, Agusti Alentorn, Caroline Dehais, Caroline Houillier, Florence Laigle-Donadey, Dimitri Psimaras, Alexandre Carpentier, Philippe Cornu, Laurent Capelle, Bertrand Mathon, Jill S. Barnholtz-Sloan, Arnab Chakravarti, Wenya L. Bi, Garrett M. Frampton, Marc Sanson, Brian M. Alexander, Andrew Cherniack, Patrick Y. Wen, David A. Reardon, Aurelien Marabelle, Peter J. Park, Ahmed Idbaih, Rameen Beroukhim, Pratiti Bandopadhayay, Franck Bielle, Keith L. Ligon. Mechanisms and therapeutic implications of hypermutation in gliomas [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5705.

Molecular tumor testing in patients with Lynch-like syndrome reveals a de novo mosaic variant of a mismatch repair gene transmitted to offspring

In Lynch-like syndrome, patients have tumors with microsatellite instability but no germline pathogenic variant in mismatch repair genes or somatic methylation of the MLH1 promoter. Identification of the mechanism that causes these tumors is crucial for guiding screening of the patients and their relatives. Double somatic hits are the usual explanation for these cases; however, we have previously reported a de novo mosaic pathogenic variant in a patient with Lynch-like syndrome. Using tumoral NGS analysis of a series of 16 patients with Lynch-like syndrome, we found six patients with double somatic hits, including one patient with mosaicism of a de novo pathogenic variant in MSH2. This variant was transmitted to the patient's offspring, which has significant implications for genetic counseling.

Mechanisms and therapeutic implications of hypermutation in gliomas

A high tumour mutational burden (hypermutation) is observed in some gliomas1-5; however, the mechanisms by which hypermutation develops and whether it predicts the response to immunotherapy are poorly understood. Here we comprehensively analyse the molecular determinants of mutational burden and signatures in 10,294 gliomas. We delineate two main pathways to hypermutation: a de novo pathway associated with constitutional defects in DNA polymerase and mismatch repair (MMR) genes, and a more common post-treatment pathway, associated with acquired resistance driven by MMR defects in chemotherapy-sensitive gliomas that recur after treatment with the chemotherapy drug temozolomide. Experimentally, the mutational signature of post-treatment hypermutated gliomas was recapitulated by temozolomide-induced damage in cells with MMR deficiency. MMR-deficient gliomas were characterized by a lack of prominent T cell infiltrates, extensive intratumoral heterogeneity, poor patient survival and a low rate of response to PD-1 blockade. Moreover, although bulk analyses did not detect microsatellite instability in MMR-deficient gliomas, single-cell whole-genome sequencing analysis of post-treatment hypermutated glioma cells identified microsatellite mutations. These results show that chemotherapy can drive the acquisition of hypermutated populations without promoting a response to PD-1 blockade and supports the diagnostic use of mutational burden and signatures in cancer.

DRES-08. CLINICAL SIGNIFICANCE OF HYPERMUTATION IN GLIOMAS

BACKGROUND

Hypermutation is an emerging biomarker for predicting response to immunotherapy in cancer patients, however its clinical value in gliomas is not established. We sought to assess the determinants of hypermutation in gliomas, and its value for predicting response to standard of care and immune checkpoint blockade (ICB).

METHODS

We performed comprehensive genomic characterization of 2,420 clinically annotated gliomas. We assessed the clinical and molecular characteristics associated with hypermutation and relationships between hypermutation and response to cancer treatments.

RESULTS

Hypermutation occurred predominantly as an adaptive resistance mechanism to temozolomide in gliomas and was most prevalent in recurrent gliomas with MGMTpromoter methylation (33.8%), IDH1/2mutation (41.0%) or 1p/19q co-deletion (59.5%). Hypermutation was almost always associated with molecular defects in DNA mismatch repair (MMR), and was associated with shorter survival after its appearance based on multivariate analysis (hazard ratio 1.91; 95% CI 1.24–2.94; P=0.004). The molecular mechanisms whereby gliomas undergo hypermutation during therapy with alkylating agents were dissected using patient-derived glioma models in vitro and in vivo. Outcomes of hypermutated gliomas treated with immune checkpoint blockade or with standard of care agents will be presented at the conference.

CONCLUSIONS

Using the largest set of hypermutated gliomas described to date, this study establishes that mutational burden and mutation signatures are clinically and biologically significant biomarkers that can be used to predict therapy response and guide treatment decisions in gliomas

Methylation Tolerance-Based Functional Assay to Assess Variants of Unknown Significance in the MLH1 and MSH2 Genes and Identify Patients With Lynch Syndrome

BACKGROUND & AIMS

Approximately 75% of patients with suspected Lynch syndrome carry variants in MLH1 or MSH2, proteins encoded by these genes are required for DNA mismatch repair (MMR). However, 30% of these are variants of unknown significance (VUS). A assay that measures cell response to the cytotoxic effects of a methylating agent can determine the effects of VUS in MMR genes and identify patients with constitutional MMR-deficiency syndrome. We adapted this method to test the effects of VUS in MLH1 and MSH2 genes found in patients with suspected Lynch syndrome.

METHODS

We transiently expressed MLH1 or MSH2 variants in MLH1- or MSH2-null human colorectal cancer cell lines (HCT116 or LoVo), respectively. The MMR process causes death of cells with methylation-damaged DNA bases, so we measured proportions of cells that undergo death following exposure to the methylating agent; cells that escaped its toxicity were considered to have variants that affect function of the gene product. Using this assay, we analyzed 88 variants (mainly missense variants), comprising a validation set of 40 previously classified variants (19 in MLH1 and 21 in MSH2) and a prospective set of 48 VUS (25 in MLH1 and 23 in MSH2). Prediction scores were calculated for all VUS according to the recommendations of the American College of Medical Genetics and Genomics, based on clinical, somatic, in silico, population, and functional data.

RESULTS

The assay correctly classified 39 of 40 variants in the validation set. The assay identified 12 VUS that did alter function of the gene product and 28 VUS that did not; the remaining 8 VUS had intermediate effects on MMR capacity and could not be classified. Comparison of assay results with prediction scores confirmed the ability of the assay to discriminate VUS that affected the function of the gene products from those that did not.

CONCLUSIONS

Using an assay that measures the ability of the cells to undergo death following DNA damage induction by a methylating agent, we were able to assess whether variants in MLH1 and MSH2 cause defects in DNA MMR. This assay might be used to help assessing the pathogenicity of VUS in MLH1 and MSH2 found in patients with suspected Lynch syndrome.

The Role of BEAMing and Digital PCR for Multiplexed Analysis in Molecular Oncology in the Era of Next-Generation Sequencing

BEAMing polymerase chain reaction (PCR) and digital PCR (dPCR) are used for robust and accurate quantification of nucleic acids. These methods are particularly well suited for the identification of very small fractions (<1%) of variant copies such as the presence of mutant genes in a predominantly wild-type background. BEAMing and dPCR are increasingly used in diverse fields including bacteriology, virology, non-invasive prenatal testing, and oncology, in particular for the molecular analysis of liquid biopsies. In this review, we present the principles of BEAMing and dPCR as well as the trends of future technical development, focusing on the possibility of developing multiplexed mutation analysis. Finally, we will discuss why such techniques will remain useful despite the ever-decreasing costs and increased automatization of next-generation sequencing (NGS), using molecular characterization of cancer cells as an example.

Identification of Positively and Negatively Selected Driver Gene Mutations Associated With Colorectal Cancer With Microsatellite Instability

BACKGROUND & AIMS

Recent studies have shown that cancers arise as a result of the positive selection of driver somatic events in tumor DNA, with negative selection playing only a minor role, if any. However, these investigations were concerned with alterations at nonrepetitive sequences and did not take into account mutations in repetitive sequences that have very high pathophysiological relevance in the tumors showing microsatellite instability (MSI) resulting from mismatch repair deficiency investigated in the present study.

METHODS

We performed whole-exome sequencing of 47 MSI colorectal cancers (CRCs) and confirmed results in an independent cohort of 53 MSI CRCs. We used a probabilistic model of mutational events within microsatellites, while adapting pre-existing models to analyze nonrepetitive DNA sequences. Negatively selected coding alterations in MSI CRCs were investigated for their functional and clinical impact in CRC cell lines and in a third cohort of 164 MSI CRC patients.

RESULTS

Both positive and negative selection of somatic mutations in DNA repeats was observed, leading us to identify the expected true driver genes associated with the MSI-driven tumorigenic process. Several coding negatively selected MSI-related mutational events (n = 5) were shown to have deleterious effects on tumor cells. In the tumors in which deleterious MSI mutations were observed despite the negative selection, they were associated with worse survival in MSI CRC patients (hazard ratio, 3; 95% CI, 1.1-7.9; P = .03), suggesting their anticancer impact should be offset by other as yet unknown oncogenic processes that contribute to a poor prognosis.

CONCLUSIONS

The present results identify the positive and negative driver somatic mutations acting in MSI-driven tumorigenesis, suggesting that genomic instability in MSI CRC plays a dual role in achieving tumor cell transformation. Exome sequencing data have been deposited in the European genome-phenome archive (accession: EGAS00001002477).

Droplet digital PCR of circulating tumor cells from colorectal cancer patients can predict KRAS mutations before surgery

In colorectal cancer (CRC), KRAS mutations are a strong negative predictor for treatment with the EGFR-targeted antibodies cetuximab and panitumumab. Since it can be difficult to obtain appropriate tumor tissues for KRAS genotyping, alternative methods are required. Circulating tumor cells (CTCs) are believed to be representative of the tumor in real time. In this study we explored the capacity of a size-based device for capturing CTCs coupled with a multiplex KRAS screening assay using droplet digital PCR (ddPCR). We showed that it is possible to detect a mutant ratio of 0.05% and less than one KRAS mutant cell per mL total blood with ddPCR compared to about 0.5% and 50-75 cells for TaqMeltPCR and HRM. Next, CTCs were isolated from the blood of 35 patients with CRC at various stage of the disease. KRAS genotyping was successful for 86% (30/35) of samples with a KRAS codon 12/13 mutant ratio of 57% (17/30). In contrast, only one patient was identified as KRAS mutant when size-based isolation was combined with HRM or TaqMeltPCR. KRAS status was then determined for the 26 available formalin-fixed paraffin-embedded tumors using standard procedures. The concordance between the CTCs and the corresponding tumor tissues was 77% with a sensitivity of 83%. Taken together, the data presented here suggest that is feasible to detect KRAS mutations in CTCs from blood samples of CRC patients which are predictive for those found in the tumor. The minimal invasive nature of this procedure in combination with the high sensitivity of ddPCR might provide in the future an opportunity to monitor patients throughout the course of disease on multiple levels including early detection, prognosis, treatment and relapse as well as to obtain mechanistic insight with respect to tumor invasion and metastasis.