An Experimenter's Guide to Glioblastoma Invasion Pathways

Glioblastoma is a highly aggressive brain tumor that is characterized by its unparalleled invasiveness. Invasive glioblastoma cells not only escape surgery and focal therapies but also are more resistant to current radio- and chemo-therapeutic approaches. Thus, any curative therapy for this deadly disease likely should include treatment strategies that interfere with glioblastoma invasiveness. Understanding glioblastoma invasion mechanisms is therefore critical. We discuss the strengths and weaknesses of various glioblastoma invasion models and conclude that robust experimental evidence has been obtained for a pro-invasive role of Ephrin receptors, Rho GTPases, and casein kinase 2 (CK2). Extensive interplay occurs between these proteins, suggesting the existence of a glioblastoma invasion signaling network that comprises several targets for therapy.

Abstract 5791: Bromodomain inhibitors for the treatment of invasive lobular carcinoma

Invasive lobular carcinoma (ILC) is the second most common type of breast cancer after invasive ductal carcinoma (IDC), accounting for approximately 10-15% of all breast tumors. ILC is characterized by inactivation of E-Cadherin and neoplastic cells that invade the stroma in a "single-file" pattern. Women with ILC are usually older, have used hormone replacement therapy and are more likely to have hormone receptor-positive disease. ILCs have similar survival to IDCs at both five and 10 years, but despite this, the clinical course is distinct: ILCs are three times more likely to metastasize to the peritoneum, gastrointestinal tract, and ovaries and are more frequently bilateral. Therefore, tailored therapeutic options for this distinct, hard-to-treat subtype of breast cancer are required.

As part of the RATHER FP7 HEALTH consortium (www.ratherproject.com), we carried out RNA-Seq analysis of 61 primary ILC samples and identified that high expression of the BET family protein Brd3 (uniquely among BRD family members) was associated with poor recurrence free survival (p=0.03, HR 8.63, CI 1.22-60.85). This observation was further validated in the independent METABRIC cohort (n=99), where again, high Brd3 expression (and not other BRD members) was associated with poor recurrence-free survival (p<0.01, HR=3.16, CI 1.24-8.03). Using a two ILC cell lines (SUM44PE and MDA-MB134VI) we found that ILC cells were relatively resistant to the anti-estrogen therapies tamoxifen and fulvestrant compared to those derived from IDC. Next, we tested whether the ILC cell lines were sensitive to BET protein inhibition using the pan-BET family inhibitor JQ1. Interestingly, while JQ1 inhibited cell growth in both ILC cell lines tested, apoptosis was only induced in SUM44PE cells, while MDA-MB134VI cells exhibited G1 arrest. Dynamic BH3 profiling was used to dissect the underlying anti-apoptotic dependencies in each ILC cell type and showed that in the JQ1-resistant MDA-MB134VI cells, survival was predominantly Bcl2-dependent. Combination of JQ1 and the Bcl2-inhibitor venetoclax (ABT-199) synergistically killed MDA-MB134V1 cells compared to treatment with JQ1 alone, while combination with the Bcl2/Bcl-Xl/Bcl-W inhibitor navitoclax (ABT-263) added further synergy.

With a number of BET inhibitors now entering clinical trials, the data described here suggest that BET inhibition is a rational therapeutic option for some ILC cases, and for those that do not respond, combination with venetoclax may be a suitable therapeutic strategy. In our cell line models, baseline Bcl-2 expression was sufficient to predict induction of apoptosis in response to JQ1 and could be used to guide therapeutic choice.

Citation Format: Darran P. O'connor, Louise Walsh, Yue Fan, Finbarr Tarrant, Suet-Feung Chin, Philip Schouten, Carlos Caldas, Rene Bernards, Triona ni Chonghaile, William Gallagher. Bromodomain inhibitors for the treatment of invasive lobular carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 5791.

Abstract P2-05-02: Functional genomic screening identifies ubiquitin-specific protease 11 (USP11) as a novel regulator of ER-alpha transcription in breast cancer

Approximately 70% of breast cancers overexpress the estrogen receptor α (ERα) and depend on this key transcriptional regulator for growth and differentiation. The discovery of novel mechanisms controlling ERα function represent major advances in our understanding of breast cancer progression and potentially offer attractive new therapeutic opportunities. Here, we investigated the role of deubiquitinating enzymes (DUBs), which act to remove ubiquitin moieties from proteins, in regulating transcriptional activity of ERα in breast cancer.

To identify DUBs involved in the regulation of ERα transcriptional activity, we performed an RNAi loss-of-function screen using a library of shRNA vectors targeting all human DUB genes. The DUB library consisted of pools of four non-overlapping shRNAs targeting all 108 known or putative DUBs (432 shRNAs in total). We found that suppression of a number of DUBs markedly repressed or enhanced the activity of an estrogen-response-element (ERE) luciferase reporter following estradiol (E2) stimulation. Of particular interest, suppression of the BRCA2-associated DUB, USP11, was found to down-regulate ERα transcriptional activity.

Subsequent validation using two individual siRNAs targeted to USP11 revealed a notable reduction in expression of endogenous ERα target genes in the ZR-75-1 cell line, as quantified using qRT-PCR. Further validation was carried out in a HEK293T USP11 knockout cell line, where reduced activity of an ERE-luciferase reporter was detected when compared to wild-type cells. This phenotype was rescued with a USP11 overexpression vector, both in the presence and absence of E2. Furthermore, USP11 expression was found to be upregulated in the estrogen-independent cell line LCC1 when compared to their parental MCF7 cells. Knockdown of USP11 in LCC1 cells resulted in decreased mRNA expression of a panel of ERα target genes, while RNA-seq revealed a downregulation of several putative ERα target genes and a downregulation of many cell cycle-associated proteins.

To support the prognostic relevance of USP11, immunohistochemical staining of a breast cancer tissue microarray (103 ER+ patients available for final analysis) was performed. Kaplan-Meier analysis of this cohort revealed a highly significant association between high USP11 expression and poor overall (p=0.030) and breast cancer-specific survival (p=0.041). In silico analysis of publically available breast cancer gene expression datasets further supported an association between high USP11 mRNA levels and poor prognosis. We observed a significant correlation between high expression of USP11 mRNA in ER-positive patients and poor distant metastasis-free survival (HR 2, CI 1.37-2.91, p=0.00023). This correlation was also significant in ER-positive patients who had received tamoxifen only (HR 2.9, CI 1.63-5.15, p=0.00015).

These results suggest a role for USP11 in driving cellular growth and identify USP11 as novel therapeutic target in breast cancer.

Citation Format: Dwane L, Das S, Moran B, O'Connor AE, Mulrane L, Dirac AM, Jirstrom K, Crown JP, Bernards R, Gallagher WM, Ní Chonghaile T, O'Connor DP. Functional genomic screening identifies ubiquitin-specific protease 11 (USP11) as a novel regulator of ER-alpha transcription in breast cancer [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P2-05-02.

Abstract P1-09-02: Bromodomain inhibitors for the treatment of invasive lobular carcinoma

Invasive lobular carcinoma (ILC) is the second most common type of breast cancer after invasive ductal carcinoma (IDC), accounting for approximately 10-15% of all breast tumors. ILC is characterized by inactivation of E-Cadherin and neoplastic cells that invade the stroma in a "single-file" pattern. Women with ILC are usually older, have used hormone replacement therapy and are more likely to have hormone receptor–positive disease. ILCs have similar survival to IDCs at both five and 10 years, but despite this, the clinical course is distinct: ILCs are three times more likely to metastasize to the peritoneum, gastrointestinal tract, and ovaries and are more frequently bilateral. Therefore, tailored therapeutic options for this distinct, hard-to-treat subtype of breast cancer are required.

As part of the RATHER FP7 HEALTH consortium (www.ratherproject.com), we carried out RNA-Seq analysis of 61 primary ILC samples and identified that high expression of the BET family protein Brd3 (uniquely among BRD family members) was associated with poor recurrence free survival (p=0.03, HR 8.63, CI 1.22-60.85). This observation was further validated in the independent METABRIC cohort (n=99), where again, high Brd3 expression (and not other BRD members) was associated with poor recurrence-free survival (p<0.01, HR=3.16, CI 1.24-8.03). Using a two ILC cell lines (SUM44PE and MDA-MB134VI) we found that ILC cells were relatively resistant to the anti-estrogen therapies tamoxifen and fulvestrant compared to those derived from IDC. Next, we tested whether the ILC cell lines were sensitive to BET protein inhibition using the pan-BET family inhibitor JQ1. Interestingly, while JQ1 inhibited cell growth in both ILC cell lines tested, apoptosis was only induced in SUM44PE cells, while MDA-MB134VI cells exhibited G1 arrest. Dynamic BH3 profiling was used to dissect the underlying anti-apoptotic dependencies in each ILC cell type and showed that in the JQ1-resistant MDA-MB134VI cells, survival was predominantly Bcl2-dependent. Combination of JQ1 and the Bcl2-inhibitor venetoclax (ABT-199) synergistically killed MDA-MB134V1 cells compared to treatment with JQ1 alone, while combination with the Bcl2/Bcl-Xl/Bcl-W inhibitor navitoclax (ABT-263) added further synergy.

With a number of BET inhibitors now entering clinical trials, the data described here suggest that BET inhibition is a rational therapeutic option for some ILC cases, and for those that do not respond, combination with venetoclax may be a suitable therapeutic strategy. In our cell line models, baseline Bcl-2 expression was sufficient to predict induction of apoptosis in response to JQ1 and could be used to guide therapeutic choice. These results should now be investigated in vivo before a prospective clinical trial.

This material is based upon works supported by the Irish Cancer Society Collaborative Cancer Research Centre BREAST-PREDICT Grant CCRC13GAL" and the SFi CDA Award 15/CDA/3438

Citation Format: O'Connor DP, Walsh L, Fan Y, Tarrant F, Chin S-F, Schouten P, Caldas C, Bernards R, Ni Chonghaile T, Gallagher WM. Bromodomain inhibitors for the treatment of invasive lobular carcinoma [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P1-09-02.

With great power comes great vulnerability

The clinical responses to targeted drugs are often transient and do not always translate into meaningful overall survival due to the development of resistance. We discuss here that the greater power of drug resistant cells can be associated with significant newly-acquired vulnerabilities that can be exploited therapeutically.

Proteomic Features of Colorectal Cancer Identify Tumor Subtypes Independent of Oncogenic Mutations and Independently Predict Relapse-Free Survival

BACKGROUND

The directed study of the functional proteome in colorectal cancer (CRC) has identified critical protein markers and signaling pathways; however, the prognostic relevance of many of these proteins remains unclear.

METHODS

We determined the prognostic implications of the functional proteome in 263 CRC tumor samples from patients treated at MD Anderson Cancer Center (MDACC) and 462 patients from The Cancer Genome Atlas (TCGA) to identify patterns of protein expression that drive tumorigenesis. A total of 163 validated proteins were analyzed by reverse phase protein array (RPPA). Unsupervised hierarchical clustering of the tumor proteins from the MDACC cohort was performed, and clustering was validated using RPPA data from TCGA CRC. Cox regression was used to identify predictors of tumor recurrence.

RESULTS

Clustering revealed dichotomization, with subtype A notable for a high epithelial-mesenchymal transition (EMT) protein signature, while subtype B was notable for high Akt/TSC/mTOR pathway components. Survival data were only available for the MDACC cohort and were used to evaluate prognostic relevance of these protein signatures. Group B demonstrated worse relapse-free survival (hazard ratio 2.11, 95% confidence interval 1.04-4.27, p = 0.039), although there was no difference in known genomic drivers between the two proteomic groups. Proteomic grouping and stage were significant predictors of recurrence on multivariate analysis. Eight proteins were found to be significant predictors of tumor recurrence on multivariate analysis: Collagen VI, FOXO3a, INPP4B, LcK, phospho-PEA15, phospho-PRAS40, Rad51, phospho-S6.

CONCLUSION

CRC can be classified into distinct subtypes by proteomic features independent of common oncogenic driver mutations. Proteomic analysis has identified key biomarkers with prognostic importance, however these findings require further validation in an independent cohort.

Abstract 1219: Integrative kinome exome sequencing and copy number profiling of ovarian clear cell carcinoma

Introduction: High stage ovarian clear cell carcinoma (OCCC) is less responsive to chemotherapy and has a worse prognosis than other ovarian cancer subtypes. The most frequently mutated genes in OCCC have been characterized in recent years. These include deleterious mutations in the tumor suppressor gene ARID1A, epigenetic silencing and deleterious mutations in the PI3K antagonist PTEN as well as activating mutations in the PI3K catalytic subunit PIK3CA. Yet, therapeutic strategies that utilize these genetic aberrations are lacking. In the present study, we aimed to identify and validate new kinase targets in OCCC.

Methods: To determine new mutations and copy number gains and losses, kinome sequencing was performed on DNA isolated from tumor samples (n=124) and matched controls (n=47) and high-coverage SNP analysis was implemented for 109 of the collected OCCC tumor samples. Inhibitor sensitivity screening was performed on 17 OCCC cell lines to validate potential kinase targets, following in vivo validation in OCCC patient-derived xenografts (PDX).

Results: Mutations in ARID1A, PIK3CA, PTEN, KRAS and TP53 corresponded to frequencies found in literature. Most identified mutations could be designated to the PI3K/Akt/mTOR and MAPK signal transduction pathway, and HER family of receptor tyrosine kinases. The PI3K pathway related genes PIK3R1, ERBB3 and AKT1 were significantly mutated in our dataset and not described in OCCC before. EGFR, ERBB2 and PIK3CA emerged as most frequently amplified kinases. Combining mutations and copy number alterations in these pathways, 91% of all tumors were affected. The highly positive staining of p-S6 (90% of OCCC tumors) indeed suggests high mTORC1/2 activity in these tumors. Inhibitor screening demonstrated subsets of cell lines to be sensitive to EGFR or ERBB2 inhibition. Furthermore, the vast majority of OCCC cell lines was susceptible to inhibition of PI3K and MAPK downstream target mTORC1/2 using AZD8055, which efficacy was further demonstrated in several OCCC PDX models. In contrast, mTORC1 inhibition alone was less effective.

Conclusion: Most kinase mutations and copy number alterations in OCCC can be related to PI3K and MAPK pathway activation. In vitro and in vivo data suggest mTORC1/2 inhibition as a novel treatment strategy in OCCC. Combinations of EGFR or ERBB2 inhibition with mTORC1/2 targeted therapy can be envisioned for OCCC tumors with EGFR or ERBB2 alterations. Supported by a grant from the Dutch Cancer Foundation: RUG 2012-5477

Citation Format: J.j. Caumanns, G.b.a Wisman, K Berns, T. Tomar, R.s.n. Fehrmann, R Bernards, A.g.j. Van der Zee, S De Jong. Integrative kinome exome sequencing and copy number profiling of ovarian clear cell carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1219. doi:10.1158/1538-7445.AM2017-1219

Abstract 4674: Epigenetic modulators for the treatment of invasive lobular carcinoma breast cancer

Invasive lobular carcinoma (ILC) is a breast cancer subtype comprising 10% of breast tumors. The majority of ILC (90%) are estrogen receptor (ER)-positive and therefore candidates for endocrine therapy. Unfortunately, de novo resistant to endocrine therapies occurs in 33% of women and a further 40% will relapse on treatment. Therefore, novel therapeutic targets are required for ILC. Deregulated transcription is a recurring theme in cancer, which can be due to epigenetic events. The bromdomain & extra-terminal domain (BET) family of proteins (BRD2, BRD3, BRD4, BRDT) function as chromatin readers that bind acetylated lysine residues on histones and regulate transcription. We performed RNA-Sequencing analysis on 61 primary ILC samples and found that high expression of BRD3 is associated with poor survival in ILC (log rank test, p=0.037). We validated this finding, that high expression of BRD3 is associated with poor survival, in a second cohort of 99 ILC primary samples from the METABRIC dataset (log rank test, p=0.0157). Next, we tested if ILC cell lines were sensitive to BET inhibition using the small molecule inhibitor JQ1, which inhibits all BET family proteins. JQ1 downregulates growth and survival genes in ILC cell lines including MYC, ER and BCL-XL. Pathway analysis following RNA sequencing revealed that JQ1 targets the apoptotic and Wnt signalling pathways in ILC cell lines. Interestingly, JQ1 inhibited the cell growth in all ILC cell lines tested, however apoptosis was only induced in two ILC cell lines. Furthermore, ILC cell lines which were relatively resistant to JQ1-induced apoptosis expressed both the BCL-2 and BCL-XL anti-apoptotic proteins. This led us to assess the combination of JQ1 and the BH3 mimetics, ABT-199 and ABT-263. ABT-199 is a selective small molecule inhibitor of BCL-2, whereas ABT-263 is an inhibitor of BCL-2, BCL-XL and BCL-W proteins. We found the combination of JQ1 and ABT-263, but not the combination of JQ1 & ABT-199, to be synergistic and enhance apoptosis in ILC cell lines. This is in accordance with BH3 profiling of ILC cell lines which indicated that ILC cell lines are dependent on BCL-2/ BCL-XL proteins for cell survival. As JQ1 is a pan-BET family inhibitor, we also wish to determine which BET protein is responsible for sensitivity to JQ1. Following knockdown of each BET protein in the CAMA-1 cell line we found that BRD3 and BRD4 were responsible for loss of cell viability. Future work will include determining the specific role of BRD3 in ILC and the effectiveness of the JQ1 and ABT-263 combination in vivo. Our work suggests that inhibition of BET proteins in combination with BH3 mimetics may be a rational therapeutic combination for ILC.

Citation Format: Louise Walsh, Bruce Moran, Sudipto Das, Finbarr Tarrant, Philip Schouten, Suet-Feung Chin, Rene Bernards, Carlos Caldas, William Gallagher, Tríona Ní Chonghaile*, Darran O'Connor*. Epigenetic modulators for the treatment of invasive lobular carcinoma breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4674. doi:10.1158/1538-7445.AM2017-4674

Abstract 3182: Cytoskeletal modulation results in increased tumor survival and drug resistance through attenuation of p53 dependent apoptosis

One of the major challenges to eradicating cancer involves the evolution of drug resistant clones and persistence of residual disease that escapes our current limit of detection but may proliferate upon cessation of therapy. These microscopic foci of residual diseases often exhibit stem-cell like properties and are intrinsically more resistant to drug therapy. A better understanding of the mechanisms underlying the innate drug resistance of these cellular populations may lead to improved treatment strategies, resulting in more durable remissions and ultimately improved patient survival.

We utilize functional genomic platforms to identify mechanisms responsible for the persistence of drug tolerant cells. A genome wide shRNA enrichment screen was performed using the c-Met addicted cell line GTL-16 to identify genes whose knockdown conferred DTP survival. Inhibition of the RhoA-ROCK-myosin pathway promotes drug resistance in a variety of tumor models, including those treated with either targeted therapy or conventional chemotherapy. Furthermore, pharmacological validation using multiple small molecule inhibitors of ROCK1 phenocopied both myosin heavy chain (MYH9) and light chain (MYL12) shRNA knockdown. More recently, utilizing a combination of gene expression and biochemical approaches, we have identified attenuation of p53 induced apoptosis to be a key event in mediating survival of these drug tolerant persisters. Reactivation of p53 using nutlin results in increased cell death.

These findings demonstrate that modulation of cytoskeleton is an important, but underappreciated, mechanism of drug resistance across many tumor types. These downstream effectors may serve as novel therapeutic targets for intervention and also biomarkers to stratify patients and their response to treatment.

Citation Format: Victoria E. Wang, John Doench, David Root, Rene Bernards, Jeffrey Settleman, Frank McCormick. Cytoskeletal modulation results in increased tumor survival and drug resistance through attenuation of p53 dependent apoptosis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3182. doi:10.1158/1538-7445.AM2017-3182

Abstract 5409: Assessment of the MammaPrint 70-gene profile using RNA sequencing technology

Introduction: Improvements in RNA processing have enabled microarray diagnostics for formalin-fixed, paraffin-embedded (FFPE) tissue. Recently, MammaPrint, a prognostic 70-gene profile for early-stage breast cancer, was successfully translated to FFPE tissue showing to be substantially equivalent to fresh tissue. In recent years, RNA-sequencing (RNA-Seq) became the standard method for transcriptome analysis, because of its low background signal and its ability of quantifying a large dynamic range of expression levels. Here we report a preliminary analysis of the FFPE MammaPrint 70-gene profile using RNA-Seq technology and the comparison with the MammaPrint® microarray diagnostic test in a series of FFPE samples.

Methods: RNA-Seq was carried out using a strand-specific RNA library preparation followed by target enrichment of the coding region of the human transcriptome without relying on the presence of poly-A tail. RNA sequencing libraries were prepared starting from a minimal amount of 20 ng of total RNA based on the DV200 metric assessment. The library pools were single-end sequenced on the Illumina HiSeq 2500 instrument at the length of 65bp. The resulting sequences were mapped to the human reference genome (build 38) using TopHat v2.1. Tophat was guided by using a transcriptome index from Ensembl (version 77). The HTSeq-count tool was used to generate the total number of uniquely mapped reads for each gene. Gene expressions were normalized with Count Per Million (CPM) normalization and log2 transformed afterwards. Microarray data of the sample were available for analysis comparison.

Results: On average, we obtained 22 million reads assigned to gene per sample (min=15M, max=28M). The number of reads assigned to genes vary from 61% to 70% of the total number of reads. Between 80% and 90% of the reads assigned to genes mapped to protein coding genes which is comparable to fresh frozen material. The 70-gene signature was successfully mapped to the RNA-Seq genes. A median raw read-count of 384 was observed for the 70-gene profile among the samples. Importantly, we observed a high concordance (R2 Pearson correlation=0.97) between the MammaPrint index calculated using the RNA-Seq data and the correspondent Microarray MammaPrint index. Additionally, the BluePrint profile, a microarray diagnostic test for breast cancer molecular subtyping, was successfully translated to the RNA-Seq platform. As with the MammaPrint profile, BluePrint showed high concordance between the two technologies with high correlation values for each of the subtypes (Luminal R2 Pearson correlation=0.98, Basal R2 Pearson correlation=0.97, HER2 R2 Pearson correlation=0.77). Conclusions: Next Generation RNA-sequencing is a feasible technology to assess diagnostic signatures, such as the 70 gene MammaPrint and BluePrint profiles.

Citation Format: Lorenza Mittempergher, Jacob B. Spangler, Mireille H. Snel, Leonie J. Delahaye, Iris de Rink, Sun Tian, Annuska M. Glas, Rene Bernards. Assessment of the MammaPrint 70-gene profile using RNA sequencing technology [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5409. doi:10.1158/1538-7445.AM2017-5409

Abstract 3040: Functional genomic screening identifies USP11 as a novel regulator of ERα transcription in breast cancer

Approximately 70% of breast cancers overexpress the estrogen receptor α (ERα) and depend on this key transcriptional regulator for growth and differentiation. The discovery of novel mechanisms controlling ERα function represent major advances in our understanding of breast cancer progression and potentially offer attractive new therapeutic opportunities. Here, we investigated the role of deubiquitinating enzymes (DUBs), which act to remove ubiquitin moieties from proteins, in regulating transcriptional activity of ERα in breast cancer.

To identify DUBs involved in the regulation of ERα transcriptional activity, we performed an RNAi loss-of-function screen using a library of shRNA vectors targeting all human DUB genes. The DUB library consisted of pools of four non-overlapping shRNAs targeting all 108 known or putative DUBs (432 shRNAs in total). We found that suppression of a number of DUBs markedly repressed or enhanced the activity of an estrogen-response-element (ERE) luciferase reporter following estradiol (E2) stimulation. Of particular interest, suppression of the BRCA2-associated DUB, USP11, was found to down-regulate ERα transcriptional activity.

Subsequent validation using two individual siRNAs targeted to USP11 revealed a notable reduction in expression of endogenous ERα target genes in the ZR-75-1 cell line, as quantified using qRT-PCR. Immunoprecipitation of ERα revealed no physical interaction with USP11, however E2 stimulation resulted in translocation of USP11 to the nucleus, suggesting a potential role in E2-induced transcription. Furthermore, USP11 expression was found to be upregulated in the estrogen-independent cell line LCC1 when compared to their parental MCF7 cells. Knockdown of USP11 in LCC1 cells resulted in decreased mRNA expression of a panel of ERα target genes, suggesting a role for USP11 in an estrogen independent setting.

To support the prognostic relevance of USP11, immunohistochemical staining of a breast cancer tissue microarray (n=144) was performed. Kaplan-Meier analysis of this cohort revealed a highly significant association between poor overall survival (OS) (p=0.030) and breast cancer-specific survival (BCSS) (p=0.041). In silico analysis of publically available breast cancer gene expression datasets further supported an association between high USP11 mRNA levels and poor prognosis. We observed a significant correlation between high expression of USP11 mRNA in ER-positive patients and poor distant metastasis-free survival (DMFS) (HR 2, CI 1.37-2.91, p=0.00023). This correlation was also significant in ER-positive patients who had received endocrine therapy only (HR 2.9, CI 1.63-5.15, p=0.00015).

These results suggest a role for USP11 in driving cellular growth and identify USP11 as novel therapeutic target in breast cancer.

Citation Format: Lisa Dwane, Aisling E. O'Connor, Laoighse Mulrane, Annette M. Dirac, Karin Jirstrom, John P. Crown, Rene Bernards, William M. Gallagher, Tríona Ní Chonghaile, Darran P. O'Connor. Functional genomic screening identifies USP11 as a novel regulator of ERα transcription in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3040. doi:10.1158/1538-7445.AM2017-3040

Abstract 575: PD-L1 positive tumor-infiltrating lymphocytes and mutational load in breast cancer

Background: PD-1 blockade has emerged as an effective treatment for a subset of cancer patients. Studies have shown that PD-L1 expression is associated with likelihood of response to PD-1 blockade. In order to select the right breast cancer patient for immunotherapy, characterization of the immune landscape of breast tumors is required. Therefore, we assessed PD-L1 expression and tumor-infiltrating lymphocytes (TILs) in different breast tumor subtypes and the link with prognosis. We also sequenced a panel of genes to assess the mutational load in triple negative tumors (TNBC) and investigate the association with PD-L1 positive TILs.

Material and methods: We analyzed 438 tumor samples from breast cancer patients of all ages treated between 1986 and 2007 with surgery, with or without adjuvant therapy. PD-L1 was stained using whole slide specimens (E1L3N® antibody) after methodological validation. Pathologists quantified TILs based on International TILs Working Group recommendations and scored PD-L1 based on the percentage of positive (tumor and/or immune) cells; as negative if 0%, positive if ≥1%, and high if >50%. Mutational load was assessed based on DNA kinome sequencing. Associations were measured by Cox/logistic regression model, including pathological variables. Multiplex imaging of 20 immune-infiltrated areas from four ER negative tumors were performed using the Vectra® system based on immunofluorescence staining panel of: CD4, CD68, CD8, FOXP3 and PD-L1.

Results: PD-L1 expression and TILs were higher in ductal (compared with lobular), high grade and estrogen receptor (ER)-negative tumors (p<0.001). TILs (density ≥5%) were significantly associated with worse distant metastasis-free survival (DMFS) only in ER-positive tumors (n=204): HR=2.72; 95%CI: 1.07-6.94. PD-L1 positivity (≥1%) followed the same trend: HR=1.66; 95%CI: 0.87-3.15. However, in ER-negative tumors (n=171), high PD-L1 expression (>50%) was significantly associated with better DMFS: HR=0.51; 95%CI: 0.27-0.98. TNBC with high PD-L1 expression of TILs (>50%) showed an association with increased mutation load (p=0.019) and a trend for better DMFS (HR=0.41; 95%CI: 0.16-1.04) compared with tumors lacking TILs. Further characterization of PD-L1 positivity in the immune-infiltrated cells was conducted by a multiplex imaging analysis. Preliminary results indicated that PD-L1 is expressed in CD68+, CD4+, FOXP3+ and CD8+ immune-cells.

Conclusion: Our findings suggest that PD-L1 positive TILs are associated with worse prognosis in ER-positive breast cancer and with better outcome in ER-negative group. In TNBC, high mutational load correlates with high PD-L1 positive TILs.

Citation Format: Marcelo Sobral-Leite, Koen Van de Vijver, Magali Michaut, Hugo M. Horlings, Tesa M. Severson, Philip C. Schouten, Rianne van der Linden, Kelly Kersten, Anna Marie Mulligan, Nayana Weerasooriya, Joyce Sanders, Ashley Cimino-Mathews, Dennis Peters, Gerrit K. Hooijer, Erik Hooijberg, Annegien Broeks, Rene Bernards, Sabine Linn, Irene L. Andrulis, Marc J. van de Vijver, Lodewyk F. Wessels, Marleen Kok, Karin E. de Visser, Marjanka K. Schmidt. PD-L1 positive tumor-infiltrating lymphocytes and mutational load in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 575. doi:10.1158/1538-7445.AM2017-575

A breast cancer gene signature for indolent disease

PURPOSE

Early-stage hormone-receptor positive breast cancer is treated with endocrine therapy and the recommended duration of these treatments has increased over time. While endocrine therapy is considered less of a burden to patients compared to chemotherapy, long-term adherence may be low due to potential adverse side effects as well as compliance fatigue. It is of high clinical utility to identify subgroups of breast cancer patients who may have excellent long-term survival without or with limited duration of endocrine therapy to aid in personalizing endocrine treatment.

METHODS

We describe a new ultralow risk threshold for the 70-gene signature (MammaPrint) that identifies a group of breast cancer patients with excellent 20 year, long-term survival prognosis. Tumors of these patients are referred to as "indolent breast cancer." We used patient series on which we previously established and assessed the 70-gene signature high-low risk threshold.

RESULTS

In an independent validation cohort, we show that patients with indolent breast cancer had 100% breast cancer-specific survival at 15 years of follow-up.

CONCLUSIONS

Our data indicate that patients with indolent disease may be candidates for limited treatment with adjuvant endocrine therapy based on their very low risk of distant recurrences or death of breast cancer.

Abstract OT2-01-11: Phase II of POSEIDON: A phase Ib / randomized phase II trial of tamoxifen plus taselisib or placebo in hormone receptor positive, HER2 negative, metastatic breast cancer patients with prior exposure to endocrine treatment

Background: The combination of PI3K-AKT-mTOR pathway inhibitors with endocrine therapy can improve clinical outcomes of hormone receptor positive (HR+) metastatic breast cancer (MBC) patients. Taselisib is a potent and selective PI3K inhibitor, with greater selectivity against mutant (MUT) PI3Kα isoforms than wild-type (WT) via a unique mechanism. Phase Ib data of POSEIDON with Taselisib + tamoxifen (TAM) demonstrated encouraging activity in patients with heavily pre-treated MBC, with an acceptable toxicity profile (Baird et al, ASCO 2016). The recommended phase II dose (RP2D) was Taselisib 4mg plus TAM 20mg, both administered on a daily continuous schedule. ctDNA monitoring may have value in drug development by (1) assessing predictive biomarkers to therapy, (2) providing an early indication of treatment response, and (3) shedding light on potential mechanisms of acquired drug resistance. In some patients included in phase Ib of POSEIDON, tumor response was preceded by a corresponding early change in plasma PIK3CA ctDNA levels. Methods: The phase II portion of the POSEIDON trial is a two-arm, randomized, double blind study of Taselisib plus TAM versus placebo (PLA) plus TAM in pre- and postmenopausal women with HR+/HER2- MBC. In the first part of the Phase II, 180 patients will be randomized (1:1) to receive continuous TAM with either Taselisib at the RP2D or PLA until disease progression, unacceptable toxicity or patient / physician decision. Crossover is allowed upon progressive disease in those patients receiving PLA plus TAM, after collection of tumor and blood samples for exploratory biomarker analysis. Stratification is based on menopausal status, histology [lobular breast cancer (LBC) vs. ductal/others], PIK3CA mutation (WT vs. exon 9 vs. exon 20), prior everolimus, timing of recurrence/progression after prior endocrine therapy, number of prior chemotherapy (CT) lines, and treatment center. After recruiting the initial 180 patients, trial will focus in LBC, until a total number of 110 patients with LBC are enrolled. Other key eligibility criteria include presence of measurable or evaluable disease (RECIST 1.1), prior progression to endocrine treatment, maximum of 5 prior CT lines in the metastatic setting, absence of diabetes under medical treatment, and absence of chronic inflammatory bowel disease. Primary endpoint is investigator-assessed PFS. Key secondary endpoints are PFS in LBC, objective response rate, clinical benefit rate, safety, and exploratory biomarker analysis (including ctDNA). The study has a 90% power at a two-sided log-rank test significance level of 0.2 to detect an HR of 0.64, which corresponds to an increase in median PFS from 4.5 months in the PLA plus TAM arm to 7 months in the Taselisib plus TAM arm. Enrollment to POSEIDON Phase II started in June 2016 (Clinicaltrials.gov NCT02285179).

Citation Format: Oliveira M, Baird RD, van Rossum AGJ, Beelen K, Garcia-Corbacho J, Mandjes IAM, Vallier AL, van Werkhoven E, Garrigós L, Kumar S, van Tinteren H, Muñoz S, Linossi C, Rosing H, Miquel JM, Schrier M, de Vries Schultink A, Saura C, Gallagher WM, Bernards R, Tabernero J, Cortés J, Caldas C, Linn SC. Phase II of POSEIDON: A phase Ib / randomized phase II trial of tamoxifen plus taselisib or placebo in hormone receptor positive, HER2 negative, metastatic breast cancer patients with prior exposure to endocrine treatment [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr OT2-01-11.

Abstract P6-11-06: Bromodomain inhibitors represent a rational therapeutic option for the treatment of invasive lobular carcinoma

Invasive lobular carcinoma (ILC) is the second most common type of breast cancer after invasive ductal carcinoma (IDC), accounting for approximately 10-15% of all breast tumors. ILC is characterized by inactivation of E-Cadherin and neoplastic cells that invade the stroma in a "single-file" pattern. Women with ILC are usually older, have used hormone replacement therapy and are more likely to have hormone receptor–positive disease. ILCs have similar survival to IDCs at both five and 10 years, but despite this, the clinical course is distinct: ILCs are three times more likely to metastasize to the peritoneum, gastrointestinal tract, and ovaries and are more frequently bilateral. Therefore, tailored therapeutic options for this distinct, hard-to-treat subtype of breast cancer are required.

As part of the RATHER FP7 HEALTH consortium (www.ratherproject.com), we carried out RNA-Seq analysis of 61 primary ILC samples and identified that high expression of the BET family protein Brd3 was associated with poor recurrence free survival (p=0.03, HR 8.63, CI 1.22-60.85). This observation was further validated in the independent METABRIC cohort (n=99), where again, high Brd3 expression was associated with poor recurrence-free survival (p<0.01, HR=3.16, CI 1.24-8.03). Using a two ILC cell lines (SUM44PE and MDA-MB134VI) we found that ILC cells were relatively resistant to the anti-estrogen therapies tamoxifen and fulvestrant compared to those derived from IDC. Next, we tested whether the ILC cell lines were sensitive to BET protein inhibition using the pan-BET family inhibitor JQ1. Interestingly, while JQ1 inhibited cell growth in both ILC cell lines tested, apoptosis was only induced in SUM44PE cells, while MDA-MB134VI cells exhibited G1 arrest. Dynamic BH3 profiling was used to dissect the underlying anti-apoptotic dependencies in each ILC cell type and showed that in the JQ1-resistant MDA-MB134VI cells, survival was predominantly Bcl2-dependent. Combination of JQ1 and the Bcl2-inhibitor venetoclax (ABT-199) synergistically killed MDA-MB134V1 cells compared to treatment with JQ1 alone, while combination with the Bcl2/Bcl-Xl/Bcl-W inhibitor navitoclax (ABT-263) added further synergy .

With a number of BET inhibitors now entering clinical trials, the data described here suggest that BET inhibition is a rational therapeutic option for some ILC cases, and for those that do not respond, combination with venetoclax may be a suitable therapeutic strategy. In our cell line models, baseline Bcl-2 expression was sufficient to predict induction of apoptosis in response to JQ1 and could be used to guide therapeutic choice. These results should now be investigated in vivo before a prospective clinical trial.

"This material is based upon works supported by the Irish Cancer Society Collaborative Cancer Research Centre BREAST-PREDICT Grant CCRC13GAL".

Citation Format: O'Connor DP, Walsh L, Tarrant F, Chin S-F, Schouten P, Linn S, Bernards R, Caldas C, Gallagher WM, ni Chonghaile T. Bromodomain inhibitors represent a rational therapeutic option for the treatment of invasive lobular carcinoma [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-11-06.

Abstract A27: Systematic discovery of mutation-specific synthetic lethals by mining pan-cancer primary tumor data

Synthetic lethality, in which a single gene defect leads to dependency on a second gene that is otherwise not essential, is an attractive paradigm to identify targeted therapies for somatic mutations. Current methods to detect synthetic lethal (SL) partners for somatic mutations use large-scale shRNA screens in cell lines, combine shRNA data with tumor genomic data or use human orthologs of yeast SL interactions. These approaches are limited as they rely on cell line or yeast data, which are not representative of primary tumors.

We have developed MiSL, a novel computational algorithm that utilizes large pan-cancer patient datasets (mutation, copy number and gene expression) to identify SL partners for specific mutations in specific cancer types. The underlying assumption of our approach is that, across multiple cancers, SL partners of a mutation will be amplified more frequently or deleted less frequently, with concordant changes in expression, in primary tumor samples harboring the mutation. Application of MiSL produced candidate SL partners for 30-80% of recurrent mutations in 12 cancers. Importantly, MiSL identified candidate SL partners for mutations (mut) in genes such as IDH1 that are not well-represented in existing cell lines. This is a distinct advantage over recent computational methods that combine shRNA data along with genomic data to make their predictions. Since MiSL uses only genomic and gene expression data, it allows assessment of a wide range of primary human tumors and mutations found in large primary tumor data sets such as TCGA.

We validated MiSL using existing data and large-scale shRNA experiments we performed in doxycycline-inducible expression systems. We found that IDH1mut MiSL candidates in acute myeloid leukemia (AML) were enriched (p=0.004) for essential genes specific to IDH1mut but not IDH1 wildtype cells determined by a DECIPHER shRNA screen covering 9,965 human genes performed in doxycycline-inducible IDH1 (R132) THP-1 cells. Importantly, 1 out of 5 MiSL candidates was a SL partner of IDH1mut in AML cells as per the shRNA screen, indicating MiSL's strong predictive power. Also, for multiple mutations in colorectal cancer, MiSL candidates were enriched (p<0.05) with genes that were selectively essential in the mutated colorectal cell-lines in Achilles data.

Next, we used MiSL to identify novel and druggable SL partners in (i) AML and (ii) breast cancer. MiSL predicted a novel SL interaction in AML between IDH1mut and ACACA, the rate-limiting enzyme of fatty acid synthesis. Consistent with our prediction, pharmacologic or genetic blockade of ACACA prevented cell proliferation in the presence of IDH1mut, but not with IDH1 wildtype, in AML cell lines.

Furthermore, when transduced with lentivirus encoding RFP-marked shRNA to ACACA, primary IDH1mut AML cells exhibited markedly reduced engraftment of RFP-positive human CD45+CD33+ leukemic cells compared to scrambled non-targeting shRNA (p<0.05) at 12 weeks post-engraftment, validating the SL interaction between mutant IDH1 and ACACA. This vulnerability indicates a novel role for IDH1mut in reprogramming lipid metabolism. MiSL also predicted that AKT1 is a SL partner of PIK3CAmut in breast cancer which we experimentally confirmed using 8 breast cancer lines. All four PIK3CAmut (but not wildtype) breast cancers were sensitive to AKT1 inhibition in viability and colony assays.

In summary, MiSL is a general computational solution that finds novel SL interactions. Specifically, IDH1mut-ACACA is the first in vivo validated synthetic lethal in human tumor cells discovered purely by computational analysis of tumor genomic data. MiSL can greatly accelerate identification of pharmacologic targets associated with specific somatic mutations in specific tumor types for all kinds of mutations, thereby making it directly translatable to clinical applications. MiSL can also pinpoint predictive genetic biomarkers that can identify/extend indications for targeted therapies.

Citation Format: Subarna Sinha, Daniel Thomas, Steven Chan, Yang Gao, Diede Brunen, Damoun Torabi, Andreas Reinisch, Rene Bernards, Ravindra Majeti, David L. Dill. Systematic discovery of mutation-specific synthetic lethals by mining pan-cancer primary tumor data. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Targeting the Vulnerabilities of Cancer; May 16-19, 2016; Miami, FL. Philadelphia (PA): AACR; Clin Cancer Res 2017;23(1_Suppl):Abstract nr A27.

Prognostic Value of MammaPrint® in Invasive Lobular Breast Cancer

BACKGROUND

MammaPrint® is a microarray-based gene expression test cleared by the US Food and Drug Administration to assess recurrence risk in early-stage breast cancer, aimed to guide physicians in making neoadjuvant and adjuvant treatment decisions. The increase in the incidence of invasive lobular carcinomas (ILCs) over the past decades and the modest representation of ILC in the MammaPrint development data set calls for a stratified survival analysis dedicated to this specific subgroup.

STUDY AIM

The current study aimed to validate the prognostic value of the MammaPrint test for breast cancer patients with early-stage ILCs.

MATERIALS AND METHODS

Univariate and multivariate survival associations for overall survival (OS), distant metastasis-free interval (DMFI), and distant metastasis-free survival (DMFS) were studied in a study population of 217 early-stage ILC breast cancer patients from five different clinical studies.

RESULTS AND DISCUSSION

A significant association between MammaPrint High Risk and poor clinical outcome was shown for OS, DMFI, and DMFS. A subanalysis was performed on the lymph node-negative study population. In the lymph node-negative study population, we report an up to 11 times higher change in the diagnosis of an event in the MammaPrint High Risk group. For DMFI, the reported hazard ratio is 11.1 (95% confidence interval = 2.3-53.0).

CONCLUSION

Study results validate MammaPrint as an independent factor for breast cancer patients with early-stage invasive lobular breast cancer. Hazard ratios up to 11 in multivariate analyses emphasize the independent value of MammaPrint, specifically in lymph node-negative ILC breast cancers.