Longitudinal measurement of subcutaneous and intratibial human prostate cancer xenograft growth and response to ionizing radiation by plasma Alu and LINE-1 ctDNA: A comparison to standard methods

BACKGROUND

Current preclinical models of metastatic prostate cancer (PCa) require sophisticated technologies and/or genetically engineered cells for the noninvasive monitoring of tumors in remote sites, such as bone. Recent developments in circulating tumor DNA (ctDNA) analysis provide an alternative method for noninvasive tumor monitoring at a low cost. Here, we sought to evaluate human Alu and LINE-1 ctDNA for the longitudinal measurement of subcutaneous and intratibial human PCa xenograft growth and response to ionizing radiation (IR) through comparison with standard slide caliper and bioluminescence measurements.

MATERIAL AND METHODS

Intratibial and subcutaneous xenografts were established in male athymic nude mice using LNCaP cells that stably express firefly luciferase. A subset of tumors was treated with a single dose of IR (CT-guided focal IR, 6 Gy). Tumor measurements were simultaneously taken by slide caliper (subcutaneous only), in vivo bioluminescence imaging, and quantitative real-time PCR (qPCR) of human-specific Alu and LINE-1 ctDNA for several weeks.

RESULTS

Levels of ctDNA and bioluminescence increased concordantly with subcutaneous and intratibial tumor growth. A statistically significant correlation (Spearman) was observed between ctDNA and subcutaneous tumor volume (LINE-1, r = .94 and Alu, r = .95, p < .0001), ctDNA and bioluminescence (LINE-1, r = .66 and Alu, r = .60, p < .002), and bioluminescence and tumor volume (r = .66, p = .0003). Bioluminescence and ctDNA were also significantly correlated in intratibial tumors (LINE-1, r = .82 and Alu, r = .81, p < .0001). Following external beam IR, the tumor responses varied briefly by method of measurement, but followed a similar trend. Statistically significant correlations were maintained between ctDNA and slide caliper measurement in irradiated subcutaneous tumors (LINE-1, r = .64 and Alu, r = .44, p < .02), and ctDNA and bioluminescence in intratibial tumors (LINE-1, r = .55, p = .018).

CONCLUSIONS

Real-time qPCR of circulating human Alu and LINE-1 DNA provides an accurate measurement of subcutaneous and intratibial xenograft burden that is comparable with conventional bioluminescence imaging and slide caliper measurement. Transient differences in measurements were observed following tumor-targeted IR, but overall all measurements mirrored tumor growth and response.

Abstract 2929: Comparative analysis of circulating human tumor DNA and bioluminescent imaging in monitoring tumor burden and therapeutic response of ionizing radiation in intratibial human prostate tumor xenografts

Purpose/Objective: Pre-clinical models of prostate cancer (PCa) metastases provide critical insight into tumor biology and therapeutic response. Current models require sophisticated reporter transgene models and/or expensive tools to noninvasively monitor tumors in metastatic sites, such as bone. Recent developments in the measurement of circulating tumor DNA (ctDNA) provide an alternative method to noninvasively assess tumor burden at a low cost. Here we present a comparative analysis of human PCa xenograft tumor growth and therapeutic response by simultaneous measurement of ctDNA and bioluminescence.

Materials and methods: Male athymic nude mice were used for intra-tibial (n=10) modeling. LNCaP-CMV-Luc cells were implanted in the tibial medullary canal. Tumor burden was evaluated at pre and post-IR (focal exposure 6 Gy on day 28 post-implantation) time-points by in vivo bioluminescent imaging and quantitative real-time polymerase chain reaction (qPCR) for human LINE1/Alu DNA. qPCR was performed on plasma-derived DNA consecutively on the same day as bioluminescence.

Results: Human genomic DNA quantification was sensitive to 0.1 pg and linear by hAlu (R2=0.98, for 0.1pg-150ng) and LINE DNA (R2=0.99, for 0.1pg-150ng) qPCR. hAlu DNA qPCR produces a much lower Ct value when compared to LINE1; possibly due to its higher copies in the human genome (Ct value 28 versus 22/0.1 pg). Following tumor inoculation, ctDNA level and tumor bioluminescence were strongly and significantly correlated over time (LINE1, r=0.65 and Alu, r=0.59, p&lt;0.002). Following radiation, we observed an immediate (day 32) but transient decrease in the levels of ctDNA and bioluminescence, followed by a gradual increase due to tumor relapse. The correlation between ctDNA level and bioluminescence was not maintained, but responses followed a similar trend.

Conclusions: In the present study, we report that the volume of human PCa xenografts can be accurately measured in the intra-tibial compartments of athymic nude mice by qPCR of circulating human Alu and LINE1 elements. Plasma-derived ctDNA levels highly correlated with tumor bioluminescence over time. Tumor directed radiation therapy exhibited slightly differential responses in ctDNA level and tumor bioluminescence, but overall both measurements followed a similar trend.

Citation Format: Alok Mishra, Kenji Zennami, Esteban Velarde, Jonathan B. Coulter, Srinivasan Yegnasubramanian, Shawn E. Lupold, Theodore L. DeWeese. Comparative analysis of circulating human tumor DNA and bioluminescent imaging in monitoring tumor burden and therapeutic response of ionizing radiation in intratibial human prostate tumor xenografts [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2929.

Abstract 2404: Increased mitochondrial DNA copy number occurs during prostate cancer progression and in cancer precursor lesions across multiple organs

Accurately measuring mitochondrial DNA copy number (mtDNAcn) is important for improving our understanding of cancer biology. Prior studies have revealed differences in mtDNAcn between matched bulk tumor-normal pairs from a range of different organs. However, there is little consistency across tumor types; compared to their normal tissue counterparts, some cancers appear to have increased mtDNAcn, others appear to have decreased mtDNAcn, and in others, such as prostate cancer, studies have shown contradictory results. The lack of consistency of mtDNAcn changes in tumor-normal pairs may reflect a failure to address the marked heterogeneity in mtDNAcn across different cell types within normal tissues. In addition, tumor cell heterogeneity in mtDNAcn has also not been taken into account in most prior studies, which can further confound tumor-normal comparisons. Finally, little is known about mtDNAcn alterations in cancer precursor lesions. To address these technical limitations, we combined a recently introduced quantitative in situ hybridization method with immunohistochemistry in a multiplex assay. This enabled us to measure mtDNAcn using digital image analysis in specific cell populations of interest on formalin fixed paraffin embedded (FFPE) and frozen tissue samples. We determined that normal prostate basal cells have approximately 3-fold higher mtDNA levels than normal prostate luminal cells. Compared to the adjacent normal prostate glands, we found markedly increased levels of mtDNA in high-grade prostatic intraepithelial neoplasia (HGPIN), the presumptive precursor lesions to most prostate cancers. We also found increased levels as well as heterogeneity of mtDNAcn in invasive prostate cancer lesions, and more consistently elevated levels in castration-resistant metastatic prostate cancer (CRPC). Increased mtDNA copy number was also identified by whole genome sequencing and quantitative PCR from laser capture micro-dissected human prostate tumor-normal pairs. We also observed higher mtDNAcn in two other precancer lesion types, pancreatic intraepithelial neoplasia and colorectal adenomas. Our approach represents a technological advance as it facilitates mtDNAcn measurements in a manner that preserves morphology, allowing for the evaluation of specific cell populations of interest. These findings raise the hypothesis that increased mitochondrial mass and function may drive the development of precancerous lesions in the prostate, pancreas and colon, as well as invasive prostate cancer development and progression.

Citation Format: Jiayu Chen, Qizhi Zheng, Jessica L. Hicks, Levent Trabzonlu, Emmanuel S. Antonarakis, Mark C. Markowski, Samuel R. Denmeade, Busra Ozbek, Anuj Gupta, Tatianna C. Larman, Ralph H. Hruban, Sarah Wheelan, Srinivasan Yegnasubramanian, Angelo M. De Marzo. Increased mitochondrial DNA copy number occurs during prostate cancer progression and in cancer precursor lesions across multiple organs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2404.

Abstract 2198: Allelic phasing of genomic alterations through linked read whole genome sequencing in human prostate cancer cell lines

Prostate cancer research has relied on a limited number of prostate cancer cell lines despite decades of effort in establishing them from human prostate cancers. These cell lines thus represent precious resources for in vitro and in vivo preclinical studies of prostate cancer and have fueled many advances in the field. While there has been tremendous effort in characterizing their molecular profiles and genomic features, comprehensive whole genome sequencing with allelic phasing of genomic alterations has not been undertaken to date. Here, we utilized powerful whole genome linked-read sequencing of high molecular weight DNA (&gt;60Kbp) from eight common prostate cancer cell lines as well as four castrate resistant (CR) subclones derived from three of those cell lines. Long range information enabled assembly of mutations (single nucleotide variants-SNVs and small indels) and structural variants (SVs) across the genome into large phased blocks. This allowed accounting of multiple mutations within a given gene to assess whether they were present on a single allele, or on opposite alleles, to assess the “allelic status” of phased mutations. Using this phased mutational analysis, we found that in cell lines with microsatellite instability (MSI; LNCaP, LAPC4, DU145, CWR22Rv1), 10-20% of a priori defined driver genes are inactivated by at least two heterozygous mutations on opposite alleles. Overall, MSI cell lines had higher levels of mutations and fewer SVs than those that were microsatellite stable (MSS; PC3, VCaP). The phased SV analysis allowed identification of complex rearrangement chains consistent with chromothripsis (PC3, DU145) and chromoplexy (VCAP, LnCAP-C42b), including reconstruction of the complex chained TMPRSS2-ERG rearrangement in VCaP. Additionally, comparison of parental and CR subclones revealed previously known as well as several novel genomic alterations that were associated with the CR subclones, including chromosome 8 amplification (in LNCaP-Abl and LAPC4-CR), further androgen receptor amplification amongst a host of increased complex SVs (in VCAP-CR), SWI/SNF mutations and increased complex SVs (in LNCAP-C42b), CDK12 biallelic loss and numerous tandem duplications (in LAPC4-CR). This study therefore comprehensively characterizes allelically phased genomic alterations in the commonly used prostate cancer cell lines and provides a useful resource for future cancer research.

Citation Format: Minh-Tam Pham, Harshath Gupta, Anuj Gupta, Ajay Vaghasia, Alyza M. Skaist, McKinzie A. Garrison, Roshan Chikamarne, Sarah J. Wheelan, William G. Nelson, Srinivasan Yegnasubramanian. Allelic phasing of genomic alterations through linked read whole genome sequencing in human prostate cancer cell lines [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 2198.

GSTP1 positive prostatic adenocarcinomas are more common in Black than White men in the United States

GSTP1 is a member of the Glutathione-S-transferase (GST) family silenced by CpG island DNA hypermethylation in 90-95% of prostate cancers. However, prostate cancers expressing GSTP1 have not been well characterized. We used immunohistochemistry against GSTP1 to examine 1673 primary prostatic adenocarcinomas on tissue microarrays (TMAs) with redundant sampling from the index tumor from prostatectomies. GSTP1 protein was positive in at least one TMA core in 7.7% of cases and in all TMA cores in 4.4% of cases. The percentage of adenocarcinomas from Black patients who had any GSTP1 positive TMA cores was 14.9%, which was 2.5 times higher than the percentage from White patients (5.9%; P < 0.001). Further, the percentages of tumors from Black patients who had all TMA spots positive for GSTP1 (9.5%) was 3-fold higher than the percentage from White patients (3.2%; P<0.001). In terms of association with other molecular alterations, GSTP1 positivity was enriched in ERG positive cancers among Black men. By in situ hybridization, GSTP1 mRNA expression was concordant with protein staining, supporting the lack of silencing of at least some GSTP1 alleles in GSTP1-positive tumor cells. This is the first report revealing that GSTP1-positive prostate cancers are substantially over-represented among prostate cancers from Black compared to White men. This observation should prompt additional studies to determine whether GSTP1 positive cases represent a distinct molecular subtype of prostate cancer and whether GSTP1 expression could provide a biological underpinning for the observed disparate outcomes for Black men.

Genomic profiles and clinical outcomes in primary versus secondary metastatic hormone-sensitive prostate cancer

BACKGROUND

Clinical outcomes may differ among patients presenting with primary (de novo) metastatic hormone-sensitive prostate cancer (mHSPC) versus secondary (metachronous) mHSPC occurring after local therapy. It is unknown what molecular features distinguish these potentially distinct presentations.

METHODS

A single-center retrospective study of mHSPC patients classified as primary mHSPC (n = 121) or secondary mHSPC (n = 106). A targeted set of genes was analyzed: BRCA2, PTEN, RB1, TP53, SPOP, CDK12, any two out of PTEN/RB1/TP53 alterations, and homologous recombination deficiency mutations. TP53 mutations were categorized as loss-of-function (LOF) versus dominant-negative (DN). The impacts of genetic features on progression-free survival (PFS) and overall survival (OS) were assessed using univariate and multivariate Cox proportional hazards regression.

RESULTS

Median PFS was 15 and 30 months for men with primary and secondary mHSPC, respectively (hazard ratio: 0.57, 95% confidence interval: 0.41-0.78; p < .01). OS did not show a significant difference between groups. There were more men with Gleason 8-10 disease in the primary versus secondary mHSPC groups (83% vs. 68%; p < .01). In univariate and multivariate analyses, TP53 DN mutations showed a statistically significant association with OS for the entire mHSPC population. Conversely, SPOP mutations were associated with improved OS. Additionally, TP53 mutations (DN and LOF) were associated with worse OS for secondary mHSPC. A combination of PTEN/RB1/TP53 mutations was associated with worse OS and PFS for secondary mHSPC, while no genomic alteration affected outcomes for primary mHSPC.

CONCLUSIONS

TP53 DN mutations, but not all TP53 alterations, were the strongest predictor of negative outcomes in men with mHSPC, while SPOP mutations were associated with improved outcomes. In subgroup analyses, specific alterations were prognostic of outcomes in secondary, but not primary, mHSPC.

Functional characterization of CD4+ T cell receptors crossreactive for SARS-CoV-2 and endemic coronaviruses

BACKGROUNDRecent studies have reported T cell immunity to the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) in unexposed donors, possibly due to crossrecognition by T cells specific for common cold coronaviruses (CCCs). True T cell crossreactivity, defined as the recognition by a single TCR of more than one distinct peptide-MHC ligand, has never been shown in the context of SARS-CoV-2.METHODSWe used the viral functional expansion of specific T cells (ViraFEST) platform to identify T cell responses crossreactive for the spike (S) glycoproteins of SARS-CoV-2 and CCCs at the T cell receptor (TCR) clonotype level in convalescent COVID-19 patients (CCPs) and SARS-CoV-2-unexposed donors. Confirmation of SARS-CoV-2/CCC crossreactivity and assessments of functional avidity were performed using a TCR cloning and transfection system.RESULTSMemory CD4+ T cell clonotypes that crossrecognized the S proteins of SARS-CoV-2 and at least one other CCC were detected in 65% of CCPs and unexposed donors. Several of these TCRs were shared among multiple donors. Crossreactive T cells demonstrated significantly impaired SARS-CoV-2-specific proliferation in vitro relative to monospecific CD4+ T cells, which was consistent with lower functional avidity of their TCRs for SARS-CoV-2 relative to CCC.CONCLUSIONSOur data confirm, for what we believe is the first time, the existence of unique memory CD4+ T cell clonotypes crossrecognizing SARS-CoV-2 and CCCs. The lower avidity of crossreactive TCRs for SARS-CoV-2 may be the result of antigenic imprinting, such that preexisting CCC-specific memory T cells have reduced expansive capacity upon SARS-CoV-2 infection. Further studies are needed to determine how these crossreactive T cell responses affect clinical outcomes in COVID-19 patients.FUNDINGNIH funding (U54CA260492, P30CA006973, P41EB028239, R01AI153349, R01AI145435-A1, R21AI149760, and U19A1088791) was provided by the National Institute of Allergy and Infectious Diseases, the National Cancer Institute, and the National Institute of Biomedical Imaging and Bioengineering. The Bloomberg~Kimmel Institute for Cancer Immunotherapy, The Johns Hopkins University Provost, and The Bill and Melinda Gates Foundation provided funding for this study.

The somatic mutation landscape of germline CHEK2-altered prostate cancer

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Background: The intersection between germline and somatic genomics is an evolving field in which germline mutations may predispose to unique patterns of subsequent somatic mutations in cancer. Germline mutations in CHEK2, involved in cell cycle regulation and DNA damage response, are associated with an increased risk of prostate cancer (PCa), while somatic-only CHEK2 alterations in PCa are rare. The association of germline CHEK2 (g CHEK2)-altered PCa with somatic mutations is unknown, and may inform hypotheses about the etiology of these cancers. Methods: Germline DNA sequencing of 1,042 consecutive PCa patients (pts) from the public SignalDB database (www.signaldb.org) was analyzed for prevalence of pathogenic g CHEK2 mutations and was compared to individuals from the general population estimated by the ExAC database (containing 53,105 germline exomes). A separate cohort of 33 PCa pts from Johns Hopkins (JH) with known g CHEK2 mutations and available somatic tumor DNA sequencing (from primary prostatic tissue) was used to assess the association of g CHEK2 mutations with somatic mutations in genes that are recurrently altered in PCa ( TP53, RB1, PTEN, ATM, BRCA1/2, and CDK12); the prevalence of these somatic alterations was compared to those in 333 unselected PCa pts from the TCGA cohort. Somatic biallelic inactivation of CHEK2 was analyzed in a subset of pts. After uncovering a potential link between g CHEK2 and somatic CDK12 mutations, we studied a cohort of 69 pts with somatic CDK12 mutations where germline data were also available. Results: 28 of 1,042 (2.7%) PCa pts from SignalDB had a pathogenic g CHEK2 mutation, compared to a population prevalence (in ExAC) of 1.4% (750 of 53,105) (RR 1.9, 95%CI 1.3–2.8, P< 0.001). Strikingly, only 23.8% of pts from SignalDB with g CHEK2 mutations had biallelic inactivation in the tumor . Furthermore, none of the 33 g CHEK2 pts from the JH cohort had evidence of somatic LOH. There were no differences in mutation prevalences involving TP53, RB1, PTEN, ATM, and BRCA1/2 between g CHEK2-altered and non-altered PCa pts. Unexpectedly, 5 of 33 (15%) g CHEK2-altered pts from the JH cohort had a somatic CDK12 mutation, compared to only 3 of 333 CDK12 mutations (1%) in unselected PCa pts from the TCGA cohort (RR 16.8, 95%CI 4.2–67, P< 0.001). Conversely, 11 of 69 (16%) pts with a somatic CDK12 mutation harbored a pathogenic g CHEK2 mutation, compared to 28 of 1,042 (2.7%) unselected PCa pts from SignalDB (RR 5.9, 95%CI 3.1–11.4, P< 0.001). Conclusions: Prostate cancers from g CHEK2-altered pts are infrequently characterized by biallelic CHEK2 inactivation and may be enriched for somatic CDK12 mutations, suggesting a unique mechanism of carcinogenesis that is different from g BRCA2-altered pts. Conversely, somatic CDK12-mutated cancers may be enriched for g CHEK2 mutations. The co-occurrence of CHEK2 and CDK12 mutations suggests a synergistic role in promoting cancer growth.

Resistance to androgen receptor signaling inhibition does not necessitate development of neuroendocrine prostate cancer

Resistance to AR signaling inhibitors (ARSis) in a subset of metastatic castration-resistant prostate cancers (mCRPCs) occurs with the emergence of AR^– neuroendocrine prostate cancer (NEPC) coupled with mutations/deletions in PTEN, TP53, and RB1 and the overexpression of DNMTs, EZH2, and/or SOX2. To resolve whether the lack of AR is the driving factor for the emergence of the NE phenotype, molecular, cell, and tumor biology analyses were performed on 23 xenografts derived from patients with PC, recapitulating the full spectrum of genetic alterations proposed to drive NE differentiation. Additionally, phenotypic response to CRISPR/Cas9-mediated AR KO in AR⁺ CRPC cells was evaluated. These analyses document that (a) ARSi-resistant NEPC developed without androgen deprivation treatment; (b) ARS in ARSi-resistant AR⁺/NE⁺ double-positive “amphicrine” mCRPCs did not suppress NE differentiation; (c) the lack of AR expression did not necessitate acquiring a NE phenotype, despite concomitant mutations/deletions in PTEN and TP53, and the loss of RB1 but occurred via emergence of an AR^–/NE^– double-negative PC (DNPC); (d) despite DNPC cells having homogeneous genetic driver mutations, they were phenotypically heterogeneous, expressing basal lineage markers alone or in combination with luminal lineage markers; and (e) AR loss was associated with AR promoter hypermethylation in NEPCs but not in DNPCs.

Epigenetic and transcriptional analysis reveals a core transcriptional program conserved in clonal prostate cancer metastases

The epigenomic regulation of transcriptional programs in metastatic prostate cancer is poorly understood. We studied the epigenomic landscape of prostate cancer drivers using transcriptional profiling and ChIP‐seq in four clonal metastatic tumors derived from a single prostate cancer patient. Our epigenomic analyses focused on androgen receptor (AR), which is a key oncogenic driver in prostate cancer, the AR pioneer factor FOXA1, chromatin insulator CCCTC‐Binding Factor, as well as for modified histones H3K27ac and H3K27me3. The vast majority of AR binding sites were shared among healthy prostate, primary prostate cancer, and metastatic tumor samples, signifying core AR‐driven transcriptional regulation within the prostate cell lineage. Genes associated with core AR‐binding events were significantly enriched for essential genes in prostate cancer cell proliferation. Remarkably, the metastasis‐specific active AR binding sites showed no differential transcriptional output, indicating a robust transcriptional program across metastatic samples. Combined, our data reveal a core transcriptional program in clonal metastatic prostate cancer, despite epigenomic differences in the AR cistrome.

Metabolic programs define dysfunctional immune responses in severe COVID-19 patients

It is unclear why some SARS-CoV-2 patients readily resolve infection while others develop severe disease. By interrogating metabolic programs of immune cells in severe and recovered coronavirus disease 2019 (COVID-19) patients compared with other viral infections, we identify a unique population of T cells. These T cells express increased Voltage-Dependent Anion Channel 1 (VDAC1), accompanied by gene programs and functional characteristics linked to mitochondrial dysfunction and apoptosis. The percentage of these cells increases in elderly patients and correlates with lymphopenia. Importantly, T cell apoptosis is inhibited in vitro by targeting the oligomerization of VDAC1 or blocking caspase activity. We also observe an expansion of myeloid-derived suppressor cells with unique metabolic phenotypes specific to COVID-19, and their presence distinguishes severe from mild disease. Overall, the identification of these metabolic phenotypes provides insight into the dysfunctional immune response in acutely ill COVID-19 patients and provides a means to predict and track disease severity and/or design metabolic therapeutic regimens.

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T cells with a unique metabolic profile are expanded in acute COVID-19

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These T cells are prone to mitochondrial apoptosis, correlating with lymphopenia

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Metabolically distinct myeloid-derived suppressor cells increase in acute COVID-19

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The presence of these M-MDSCs in acute COVID-19 correlates with disease severity

Genomic and phenotypic heterogeneity in prostate cancer

From a clinical, morphological and molecular perspective, prostate cancer is a heterogeneous disease. Primary prostate cancers are often multifocal, having topographically and morphologically distinct tumour foci. Sequencing studies have revealed that individual tumour foci can arise as clonally distinct lesions with no shared driver gene alterations. This finding demonstrates that multiple genomically and phenotypically distinct primary prostate cancers can be present in an individual patient. Lethal metastatic prostate cancer seems to arise from a single clone in the primary tumour but can exhibit subclonal heterogeneity at the genomic, epigenetic and phenotypic levels. Collectively, this complex heterogeneous constellation of molecular alterations poses obstacles for the diagnosis and treatment of prostate cancer. However, advances in our understanding of intra-tumoural heterogeneity and the development of novel technologies will allow us to navigate these challenges, refine approaches for translational research and ultimately improve patient care.

A conserved intratumoral regulatory T cell signature identifies 4-1BB as a pan-cancer target

Despite advancements in targeting the immune checkpoints program cell death protein 1 (PD-1), programmed death ligand 1 (PD-L1), and cytotoxic T lymphocyte-associated protein 4 (CTLA-4) for cancer immunotherapy, a large number of patients and cancer types remain unresponsive. Current immunotherapies focus on modulating an antitumor immune response by directly or indirectly expanding antitumor CD8 T cells. A complementary strategy might involve inhibition of Tregs that otherwise suppress antitumor immune responses. Here, we sought to identify functional immune molecules preferentially expressed on tumor-infiltrating Tregs. Using genome-wide RNA-Seq analysis of purified Tregs sorted from multiple human cancer types, we identified a conserved Treg immune checkpoint signature. Using immunocompetent murine tumor models, we found that antibody-mediated depletion of 4-1BB-expressing cells (4-1BB is also known as TNFRSF9 or CD137) decreased tumor growth without negatively affecting CD8 T cell function. Furthermore, we found that the immune checkpoint 4-1BB had a high selectivity for human tumor Tregs and was associated with worse survival outcomes in patients with multiple tumor types. Thus, antibody-mediated depletion of 4-1BB-expressing Tregs represents a strategy with potential activity across cancer types.

Metabolic programs define dysfunctional immune responses in severe COVID-19 patients

It remains unclear why some patients infected with SARS-CoV-2 readily resolve infection while others develop severe disease. To address this question, we employed a novel assay to interrogate immune-metabolic programs of T cells and myeloid cells in severe and recovered COVID-19 patients. Using this approach, we identified a unique population of T cells expressing high H3K27me3 and the mitochondrial membrane protein voltage-dependent anion channel (VDAC), which were expanded in acutely ill COVID-19 patients and distinct from T cells found in patients infected with hepatitis c or influenza and in recovered COVID-19. Increased VDAC was associated with gene programs linked to mitochondrial dysfunction and apoptosis. High-resolution fluorescence and electron microscopy imaging of the cells revealed dysmorphic mitochondria and release of cytochrome c into the cytoplasm, indicative of apoptosis activation. The percentage of these cells was markedly increased in elderly patients and correlated with lymphopenia. Importantly, T cell apoptosis could be inhibited in vitro by targeting the oligomerization of VDAC or blocking caspase activity. In addition to these T cell findings, we also observed a robust population of Hexokinase II⁺ polymorphonuclear-myeloid derived suppressor cells (PMN-MDSC), exclusively found in the acutely ill COVID-19 patients and not the other viral diseases. Finally, we revealed a unique population of monocytic MDSC (M-MDSC) expressing high levels of carnitine palmitoyltransferase 1a (CPT1a) and VDAC. The metabolic phenotype of these cells was not only highly specific to COVID-19 patients but the presence of these cells was able to distinguish severe from mild disease. Overall, the identification of these novel metabolic phenotypes not only provides insight into the dysfunctional immune response in acutely ill COVID-19 patients but also provide a means to predict and track disease severity as well as an opportunity to design and evaluate novel metabolic therapeutic regimens.

Pervasive promoter hypermethylation of silenced TERT alleles in human cancers

BACKGROUND

In cancers, maintenance of telomeres often occurs through activation of the catalytic subunit of telomerase, encoded by TERT. Yet, most cancers show only modest levels of TERT gene expression, even in the context of activating hotspot promoter mutations (C228T and C250T). The role of epigenetic mechanisms, including DNA methylation, in regulating TERT gene expression in cancer cells is as yet not fully understood.

METHODS

Here, we have carried out the most comprehensive characterization to date of TERT promoter methylation using ultra-deep bisulfite sequencing spanning the CpG island surrounding the core TERT promoter in 96 different human cell lines, including primary, immortalized and cancer cell types, as well as in control and reference samples.

RESULTS

In general, we observed that immortalized and cancer cell lines were hypermethylated in a region upstream of the recurrent C228T and C250T TERT promoter mutations, while non-malignant primary cells were comparatively hypomethylated in this region. However, at the allele-level, we generally found that hypermethylation of promoter sequences in cancer cells is associated with repressed expression, and the remaining unmethylated alleles marked with open chromatin are largely responsible for the observed TERT expression in cancer cells.

CONCLUSIONS

Our findings suggest that hypermethylation of the TERT promoter alleles signals transcriptional repression of those alleles, leading to attenuation of TERT activation in cancer cells. This type of fine tuning of TERT expression may account for the modest activation of TERT expression in most cancers.

Dickkopf-1 Can Lead to Immune Evasion in Metastatic Castration-Resistant Prostate Cancer

PURPOSE

Metastatic castration-resistant prostate cancer (mCRPC) with low androgen receptor (AR) and without neuroendocrine signaling, termed double-negative prostate cancer (DNPC), is increasingly prevalent in patients treated with AR signaling inhibitors and is in need of new biomarkers and therapeutic targets.

METHODS

Candidate genes enriched in DNPC were determined using differential gene expression analysis of discovery and validation cohorts of mCRPC biopsies. Laboratory studies were carried out in human mCRPC organoid cultures, prostate cancer (PCa) cell lines, and mouse xenograft models. Epigenetic studies were carried out in a rapid autopsy cohort.

RESULTS

Dickkopf-1 (DKK1) expression is increased in DNPC relative to prostate-specific antigen (PSA)-expressing mCRPC in the Stand Up to Cancer/Prostate Cancer Foundation discovery cohort (11.2 v 0.28 reads per kilobase per million mapped reads; q < 0.05; n = 117) and in the University of Washington/Fred Hutchinson Cancer Research Center cohort (9.2 v 0.99 fragments per kilobase of transcript per million mapped reads; P < .0001). DKK1 expression can be regulated by activated Wnt signaling in vitro and correlates with activating canonical Wnt signaling mutations and low PSA mRNA in mCRPC biopsies (P < .05). DKK1 hypomethylation was associated with increased DKK1 mRNA expression (Pearson r = -0.66; P < .0001) in a rapid autopsy cohort (n = 7). DKK1-high mCRPC biopsies are infiltrated with significantly higher numbers of quiescent natural killer (NK) cells (P < .005) and lower numbers of activated NK cells (P < .0005). Growth inhibition of the human PCa model PC3 by the anti-DKK1 monoclonal antibody DKN-01 depends on the presence of NK cells in a severe combined immunodeficient xenograft mouse model.

CONCLUSION

These results support DKK1 as a contributor to the immunosuppressive tumor microenvironment of DNPC. These data have provided the rationale for a clinical trial targeting DKK1 in mCRPC (ClinicalTrials.gov identifier: NCT03837353).

Abstract IA09: A phase II trial to promote recovery from COVID-19 with endocrine therapy

Background: Death from COVID-19 disproportionately affects men, with up to 80% of deaths in severe COVID-19 cases being in men. There are a number of potential differences that might contribute to these sex differences. TMPRSS2 is a serine protease that primes the spike protein of SARS-CoV-2, a critical step in viral entry. TMPRSS2 is most highly expressed in the prostate where it is under androgen control, upregulated by testosterone and downregulated by antiandrogens. ACE2, the receptor used for entry into the host cell, is located on the X chromosome and may also have levels that are altered by hormones, with estradiol downregulating its expression. Previous research on acute lung injury demonstrated that estradiol seems to have beneficial effects on repair of lung injury. Therefore, our central hypothesis is that hormones may partially contribute to the gender disparity seen in COVID-19 patients, with high levels of testosterone being harmful and high levels of estrogen being helpful. Bicalutamide is a nonsteroidal antiandrogen that inhibits the action of androgens and, via feedback on the hypothalamic-pituitary axis, upregulates estradiol. We are conducting a phase II clinical trial to determine if bicalutamide improves the percentage of COVID+ patients with clinical improvement by 7 days.

Methods: We will enroll 40 patients who are hospitalized for COVID-19 with minimal respiratory symptoms (respiratory rate &lt;30 and &lt; 6L oxygen by nasal canula). Patients with more severe symptoms or oxygen requirements, who have taken hormones within the past month, or have pre-existing liver or cardiac disease will be excluded. Patients will be randomized 1:1 (20 in each arm) to bicalutamide or standard of care and will be stratified by gender. The primary outcome is comparing the percentage of patients with clinical improvement at day 7, compared to historical controls based on the World Health Organization categorical scale of clinical improvement. Key secondary clinical endpoints include all-cause mortality at 28 and 60 days, need for mechanical ventilation or ICU care, and safety of bicalutamide in this population. We will also determine the impact of bicalutamide therapy on viral infectivity by studying the reduction in viral load, hormone modulation and engagement of the endocrine axis, and immune response modulation promoting pro-repair immune function in patients with COVID-19. Clinical trial registration number: NCT04374279.

Citation Format: Catherine H. Marshall, Srinivasan Yegnasubramanian, Hao Wang, Jennifer Durham, Ting Wang, Rachel Damico, Franco R. D'Alessio, Venkataramana K. Sidhaye, Andrew Pekosz, Joseph L. Mankowski, Sabra L. Klein, Sumati Murli, Elizabeth M. Jaffee, Samuel R. Denmeade. A phase II trial to promote recovery from COVID-19 with endocrine therapy [abstract]. In: Proceedings of the AACR Virtual Meeting: COVID-19 and Cancer; 2020 Jul 20-22. Philadelphia (PA): AACR; Clin Cancer Res 2020;26(18_Suppl):Abstract nr IA09.

Abstract 4790: An in situ hybridization assay reveals high mitochondrial DNA copy number in stem cell/proliferative cell compartments

Mitochondria are critical regulators of energy production, metabolism and cell death. Warburg observed that in the presence of oxygen, many cancer cells take up abundant amounts of glucose, and excrete the carbon mainly as lactate. Warburg postulated that this aerobic glycolysis was essential and universal in cancer, and was caused by mitochondrial dysfunction. Yet, more recent findings suggest that aerobic glycolysis also occurs in rapidly dividing normal cells to support anabolic reactions needed for cell growth and replication. Furthermore, a number of metabolites generated by the tricarboxylic acid cycle and the electron transport chain are also used for macromolecular synthesis in proliferating cancer and normal cells. This suggests that an increase in mitochondrial activity, especially those functions related to anabolic metabolism, may be required for cancer development and progression. One mechanism to increase mitochondrial function is through mitochondrial biogenesis, which is partly regulated by regulating mitochondrial DNA (mtDNA) copy number.

Alterations in mtDNA sequence and copy number have been implicated in various cancers. Since most prior measurements of mtDNA copy number have been performed using homogenates of complex tissues composed of multiple cell types, very little is known about cell type-specific mtDNA copy number heterogeneity. Here, we validated a quantitative in situ hybridization approach to study mtDNA levels at the single cell level in tissue sections. In hierarchically organized self-renewing tissues, higher levels of mtDNA were observed in stem/proliferative compartments than in differentiated compartments. Also, we found zonal heterogeneity of mtDNA levels in the liver that mirrors the known oxygen tension gradient. In the kidney, we observed markedly lower mtDNA levels in glomeruli and the collecting ducts compared to cortical proximal and distal tubules. This work provides visualization of mtDNA copy number in distinct cell populations in situ for the first time across normal mammalian tissues, and can serve as a foundation for quantifying cell type-specific mtDNA copy number dynamics in any relevant diseases, especially in cancers and their precursor lesions.

Citation Format: JIAYU CHEN, Qizhi Zheng, Lauren B. Peiffer, Jessica L. Hicks, Michael C. Haffner, Avi Z. Rosenberg, Moshe Levi, Xiaoxin Wang, Busra Ozbek, Srinivasan Yegnasubramanian, Angelo M. De Marzo. An in situ hybridization assay reveals high mitochondrial DNA copy number in stem cell/proliferative cell compartments [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 4790.

Abstract 5303: GSTP1 positive prostate cancer

Introduction: GSTP1 is a member of the Glutathione-S-transferase family, known to be silenced by CpG island DNA methylation in approximately 90% of prostatic adenocarcinomas. However, the cases that are not densely methylated have not been extensively characterized in terms GSTP1 expression or clinico-pathological features.

Description of experimental procedures: We analytically validated an automated IHC assay against human GSTP1 using well-known cell line controls and human prostate cancer tissues. We examined 33 TMAs from 1296 prostatectomy patients using the index tumor with 4-fold redundancy from patients from 1993 to 2015. TMA slides stained for GSTP1 were scanned and uploaded to a web based image scoring platform (TMAJ). The images were reviewed by two different pathologists with expertise in prostate cancer. The scoring system followed a two-tiered approach, noted as positive (carcinomas with any nuclear and/or cytoplasmic staining), and as negative (carcinoma cells with no nuclear or cytoplasmic stain) in the presence of benign stromal or inflammatory cells that served as internal controls. The GSTP1 positive staining was correlated with Gleason grade groups, pathological stage, and age.

Results: After exclusion for inadequate tissue staining, GSTP1 was evaluated in 4326 TMA cores with carcinoma. GSTP1 IHC staining was positive in 190 cores (4.3%), corresponding to 75 patients (5.9% of patients had GSTP1 positive cancer). Of these 190 positive cores, 109 showed homogeneous (100% positive) tumor cell staining while 81 showed heterogeneous staining, with mixed positive and negative regions. In 43 patients (3.3%), all TMA cores containing carcinoma stained positively. In a number of standard slides from prostatectomies, GSTP1 positive IHC staining cases were also found to be positive for GSTP1 mRNA by in situ hybridization. There was no correlation between GSTP1 positive staining, Gleason grade group, pathological stage, margin status or patient age. Preliminary analysis in a subset of cases showed no correlation with TMPRSS2-ERG status or PTEN loss.

Conclusion: GSTP1 protein is present in approximately 5.9% of clinically localized prostate cancers and in approximately 50% of these positive cases, all TMA cores with carcinoma stained positively.

Citation Format: Igor D. Vidal, Stephanie Glavaris, Tracy Jones, Jessica Hicks, Bruce J. Trock, Srinivasan Yegnasubramanian, William Nelson, Angelo M. De Marzo. GSTP1 positive prostate cancer [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 5303.

Abstract 157: Pervasive promoter hypermethylation of silenced TERT alleles in human cancers

In cancers, the epigenetic regulation of telomerase expression is not fully characterized, with multiple conflicting reports. In particular, the role of DNA methylation of cytosine residues in the control of TERT expression remains unclear. Here, we have carried out the most comprehensive characterization of TERT promoter methylation using ultra-deep bisulfite sequencing spanning more than 310 CpGs within and surrounding the core TERT promoter in 96 different cell lines, including primary, immortalized, and cancer cell types, as well as in control and reference samples. In general, we observed that immortalized and cancer cell lines were hypermethylated in a region upstream of the recurrent G228A and G250A TERT promoter mutations, while non-malignant primary cells were comparatively hypomethylated in this region. Notably, in cancers, the hypermethylated alleles were associated with repressed expression while remaining unmethylated alleles were enriched with the open chromatin marks, H3K4me3 and H3K27ac. Furthermore, the hypomethylated alleles of the TERT promoter were responsible for the observed TERT expression in cancer. This association of hypomethylated alleles with active expression and hypermethylated alleles with repressed expression is particularly evident in cancers harboring TERT promoter mutations, and cancers displaying allele-specific expression. Reporter assays confirmed that DNA methylation of the TERT promoter greatly suppressed reporter expression in heterologous TERT promoter-reporter constructs. Overall, our findings suggest that hypermethylation of the TERT promoter alleles signals transcriptional repression of those alleles, leading to the attenuation of TERT activation in cancer cells.

Citation Format: Mindy K. Graham, David Esopi, Jacqueline A. Brosnan-Cashman, Anuj Gupta, Angelo De Marzo, William G. Nelson, Christopher M. Heaphy, Alan K. Meeker, Sarah J. Wheelan, Srinivasan Yegnasubramanian. Pervasive promoter hypermethylation of silenced TERT alleles in human cancers [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 157.

Abstract 1555: Single cell level treatment-specific characterization of HER2+ breast cancers treated with immune checkpoint inhibitors and entinostat

HER2+ breast cancers are known to be less-immunogenic and associated with low response rates to ICIs such as anti-PD-1, anti-PDL-1 or anti-CTLA-4. A combination of immunosuppressive signals that prevent cytotoxic T cells from infiltrating the tumor microenvironment (TME) and low tumor antigen expression contribute to immunotherapy resistance in this population. Epigenetic modulators can both reexpress tumor antigens and rewire the immunosuppressive environment. We previously used a benzamide histone deacetylase inhibitor, entinostat, in combination with ICIs to reverse the immunosuppressive TME and increase tumor antigen expression in a NeuN HER2+ mouse model of breast cancer. Our results showed that entinostat in combination with anti-PD-1, anti-CTLA-4, or both provided a significant survival benefit compared to either treatment alone. This current study employs single cell RNA-seq on whole tumor samples from mice treated with ICIs and entinostat to investigate the role of epigenetic inhibitors in rewiring the expression of tumor antigens and the cellular landscape of the TME. We generate single cell data of over 54,000 cells from 20 tumors treated with entinostat alone or in combination with anti-PD-1 and anti-CTLA-4 and their combination. Analysis of cells in the TME identifies consistent proportion of monocytes, macrophages, T-cells, Myeloid Derived Suppressor Cells (MDSCs) and Cancer Associated Fibroblasts (CAFs) before and after treatment. Differential expression analysis within the cell types identifies distinct subpopulations and we explore those that are either proportionally higher or lower in each treatment group. Notably, pathway analysis on differentially expressed genes of each cell type identified that combination entinostat and checkpoint treatment increased T cell activation, leukocyte proliferation, myeloid leukocyte and neutrophil migration, and decreased Wnt signaling and histone modifications in tumor cells. Further analysis of the tumor cells from these data and additional ATAC-seq data will enable us to further test the role of antigen reexpression in this TME of activated tumors. We also used the CoGAPS matrix factorization algorithm and RNA velocity analysis to identify transcriptional patterns that are enriched in response to combination treatment. Our current work provides insights into the transcriptional network within a breast tumor after treatment with entinostat. Our follow-up studies in patient samples from a corresponding clinical trial will allow us to map the role of epigenetic modulation in breast tumors. We predict our findings will bring us closer to identifying additional therapeutic targets and ultimately improve survival rates of patients with less-immunogenic tumors. NCT02453620.

Citation Format: Dimitrios N. Sidiropoulos, Emily Davis-Marcisak, Christine Rafie, Luciane T. Kagohara, Gaurav Sharma, Roisin M. Connolly, Vered Stearns, Srinivasan Yegnasubramanian, Elizabeth M. Jaffee, Elana J. Fertig, Evanthia T. Roussos Torres. Single cell level treatment-specific characterization of HER2+ breast cancers treated with immune checkpoint inhibitors and entinostat [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 1555.

Abstract 2383: RepairSwitch: a novel cell-based functional assay for simultaneous measurement of homologous recombination and non-homologous end joining mediated DNA repair

DNA repair pathways are frequently defective in human cancers. DNA double stranded breaks (DSBs) are most often repaired by either homologous recombination (HR) or non-homologous end joining (NHEJ). Alterations in repair pathways can indicate sensitivity to therapeutic agents such as PARP inhibitors, cisplatin, and immunotherapy. Thus, functional assays to measure rates of HR and NHEJ is of significant interest. Several methods have been developed to measure rates of HR or NHEJ; however, there is a need for functional cell-based assays that can measure rates by both DNA DSB pathways simultaneously. Here, we have developed the RepairSwitch assay, a flow cytometry assay to assess rates of HR and NHEJ mediated repair of CAS9 programmed DSB simultaneously using a novel fluorescence switching reporter system. The assay exhibited low background signal and was capable of detecting rare repair events in the 1 in 10,000 range. We demonstrate the utility of RepairSwitch by measuring the potency of inhibitors of ATM (KU-60019, KU-55933), DNA-PK (NU7441), and PARP (olaparib) on modulating DSB repair rates in HEK293FT cells. The selective ATM inhibitor KU-60019 inhibited HR rates with IC50 of 915 nM. Interestingly, KU-60019 exposure led to a dose responsive increase in rates of NHEJ. In contrast, the less selective ATM inhibitor KU-55933, which also has activity on DNA-PK, showed inhibition of both HR and NHEJ. The selective DNA-PK inhibitor NU7441 inhibited NHEJ efficiency with an IC50 of 299 nM, and showed a dose responsive increase in HR. The PARP inhibitor olaparib showed lower potency in modulating HR and NHEJ. We next used the RepairSwitch assay to assess how pharmacological and genetic inhibition of DNA methyltransferases (DNMT) impacted rates of HR and NHEJ. The DNMT inhibitor decitabine reduced HR, but increased rates of NHEJ, both in a dose responsive manner, in both HEK293FT and HCT-116 cells (IC50 for HR of 187 nM and 1.4 uM respectively). Knockout of DNMT1 and DNMT3B increased NHEJ, while knockout of DNMT3B, but not DNMT1, reduced HR. These results illustrate the utility of RepairSwitch as a functional assay for measuring changes in rates of DSB repair induced by pharmacological or genetic perturbation. Furthermore, the findings illustrate the potential for one DNA repair mechanism to compensate in part for loss of another. Finally, we showed that inhibition of DNMT can lead to reduction of HR and increase in NHEJ, providing some additional insight into recently observed synergy of DNMT inhibitors with PARP inhibitors for cancer treatment.

Citation Format: Ruchama C. Steinberg, Jianyong Liu, Ajay Vaghasia, Hugh Giovinazzo, Dimitri Tselenchuk, Minh-Tam Pham, Roshan V. Chikarmane, William B. Nelson, Srinivasan Yegnasubramanian. RepairSwitch: a novel cell-based functional assay for simultaneous measurement of homologous recombination and non-homologous end joining mediated DNA repair [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 2383.

Lactoferrin CpG Island Hypermethylation and Decoupling of mRNA and Protein Expression in the Early Stages of Prostate Carcinogenesis

Lactoferrin (LTF) is an iron-binding protein canonically known for its innate and adaptive immune functions. LTF may also act as a tumor suppressor with antiproliferative action. LTF is inactivated genetically or epigenetically in various cancers, and a CpG island spanning the transcriptional start site of LTF is hypermethylated in prostate cancer cell lines. We, therefore, hypothesized that LTF expression is silenced via CpG island hypermethylation in the early stages of prostate tumorigenesis carcinogenesis. Targeted methylation analysis was performed using a combination of methylated-DNA precipitation and methylation-sensitive restriction enzymes, and laser-capture microdissection followed by bisulfite sequencing on DNA isolated from prostate tissue samples, including both primary and metastatic disease. LTF mRNA in situ hybridization and LTF protein immunohistochemistry were also performed. We report that the LTF CpG island is frequently and densely methylated in high-grade prostatic intraepithelial neoplasia, primary prostate carcinoma, and metastases. We further report a decoupling of lactoferrin mRNA and protein expression, including in lesions where LTF mRNA has presumably been silenced via CpG island methylation. We conclude that LTF mRNA expression is silenced in prostate tumorigenesis via hypermethylation, supporting a role for LTF as a prostate cancer tumor suppressor gene. Likewise, the frequency at which the LTF CpG island is methylated across samples suggests it is an important and conserved step in prostate cancer initiation.

Immunogenicity of prostate cancer is augmented by BET bromodomain inhibition

Background

Prostate cancer responds poorly to current immunotherapies. Epigenetic therapies such as BET Bromodomain inhibition can change the transcriptome of tumor cells, possibly making them more immunogenic and thus susceptible to immune targeting.

Methods

We characterized the effects of BET bromodomain inhibition using JQ1 on PD-L1 and HLA-ABC expression in two human prostate cell lines, DU145 and PC3. RNA-Seq was performed to assess changes on a genome-wide level. A cytotoxic T cell killing assay was performed in MC38-OVA cells treated with JQ1 to demonstrate increased immunogenicity. In vivo experiments in the Myc-Cap model were conducted to show the effects of JQ1 administration in concert with anti-CTLA-4 checkpoint blockade.

Results

Here, we show that targeting BET bromodomains using the small molecule inhibitor JQ1 decreased PD-L1 expression and mitigated tumor progression in prostate cancer models. Mechanistically, BET bromodomain inhibition increased MHC I expression and increased the immunogenicity of tumor cells. Transcriptional profiling showed that BET bromodomain inhibition regulates distinct networks of antigen processing and immune checkpoint molecules. In murine models, treatment with JQ1 was additive with anti-CTLA-4 immunotherapy, resulting in an increased CD8/Treg ratio.

Conclusions

BET Bromodomain inhibition can mediate changes in expression at a genome wide level in prostate cancer cells, resulting in an increased susceptibility to CD8 T cell targeting. These data suggest that combining BET bromodomain inhibition with immune checkpoint blockade may have clinical activity in prostate cancer patients.