Nallasivam Palanisamy

The mutational landscape of lethal castration-resistant prostate cancer

Characterization of the prostate cancer transcriptome and genome has identified chromosomal rearrangements and copy number gains and losses, including ETS gene family fusions, PTEN loss and androgen receptor (AR) amplification, which drive prostate cancer development and progression to lethal, metastatic castration-resistant prostate cancer (CRPC). However, less is known about the role of mutations. Here we sequenced the exomes of 50 lethal, heavily pre-treated metastatic CRPCs obtained at rapid autopsy (including three different foci from the same patient) and 11 treatment-naive, high-grade localized prostate cancers. We identified low overall mutation rates even in heavily treated CRPCs (2.00 per megabase) and confirmed the monoclonal origin of lethal CRPC. Integrating exome copy number analysis identified disruptions of CHD1 that define a subtype of ETS gene family fusion-negative prostate cancer. Similarly, we demonstrate that ETS2, which is deleted in approximately one-third of CRPCs (commonly through TMPRSS2:ERG fusions), is also deregulated through mutation. Furthermore, we identified recurrent mutations in multiple chromatin- and histone-modifying genes, including MLL2 (mutated in 8.6% of prostate cancers), and demonstrate interaction of the MLL complex with the AR, which is required for AR-mediated signalling. We also identified novel recurrent mutations in the AR collaborating factor FOXA1, which is mutated in 5 of 147 (3.4%) prostate cancers (both untreated localized prostate cancer and CRPC), and showed that mutated FOXA1 represses androgen signalling and increases tumour growth. Proteins that physically interact with the AR, such as the ERG gene fusion product, FOXA1, MLL2, UTX (also known as KDM6A) and ASXL1 were found to be mutated in CRPC. In summary, we describe the mutational landscape of a heavily treated metastatic cancer, identify novel mechanisms of AR signalling deregulated in prostate cancer, and prioritize candidates for future study.

Genomic loss of microRNA-101 leads to overexpression of histone methyltransferase EZH2 in cancer.

Enhancer of zeste homolog 2 (EZH2) is a mammalian histone methyltransferase that contributes to the epigenetic silencing of target genes and regulates the survival and metastasis of cancer cells. EZH2 is overexpressed in aggressive solid tumors by mechanisms that remain unclear. Here we show that the expression and function of EZH2 in cancer cell lines are inhibited by microRNA-101 (miR-101). Analysis of human prostate tumors revealed that miR-101 expression decreases during cancer progression, paralleling an increase in EZH2 expression. One or both of the two genomic loci encoding miR-101 were somatically lost in 37.5% of clinically localized prostate cancer cells (6 of 16) and 66.7% of metastatic disease cells (22 of 33). We propose that the genomic loss of miR-101 in cancer leads to overexpression of EZH2 and concomitant dysregulation of epigenetic pathways, resulting in cancer progression.

Transcriptome Sequencing to Detect Gene Fusions in Cancer

Recurrent gene fusions, typically associated with haematological malignancies and rare bone and soft-tissue tumours, have recently been described in common solid tumours. Here we use an integrative analysis of high-throughput long- and short-read transcriptome sequencing of cancer cells to discover novel gene fusions. As a proof of concept, we successfully used integrative transcriptome sequencing to ‘re-discover’ the BCR–ABL1 (ref. 10) gene fusion in a chronic myelogenous leukaemia cell line and the TMPRSS2–ERG gene fusion in a prostate cancer cell line and tissues. Additionally, we nominated, and experimentally validated, novel gene fusions resulting in chimaeric transcripts in cancer cell lines and tumours. Taken together, this study establishes a robust pipeline for the discovery of novel gene chimaeras using high-throughput sequencing, opening up an important class of cancer-related mutations for comprehensive characterization.

Transcriptome sequencing across a prostate cancer cohort identifies PCAT-1 , an unannotated lincRNA implicated in disease progression

Noncoding RNAs (ncRNAs) are emerging as key molecules in human cancer, with the potential to serve as novel markers of disease and to reveal uncharacterized aspects of tumor biology. Here we discover 121 unannotated prostate cancer–associated ncRNA transcripts (PCATs) by ab initio assembly of high-throughput sequencing of polyA+ RNA (RNA-Seq) from a cohort of 102 prostate tissues and cells lines. We characterized one ncRNA, PCAT-1, as a prostate-specific regulator of cell proliferation and show that it is a target of the Polycomb Repressive Complex 2 (PRC2). We further found that patterns of PCAT-1 and PRC2 expression stratified patient tissues into molecular subtypes distinguished by expression signatures of PCAT-1–repressed target genes. Taken together, our findings suggest that PCAT-1 is a transcriptional repressor implicated in a subset of prostate cancer patients. These findings establish the utility of RNA-Seq to identify disease-associated ncRNAs that may improve the stratification of cancer subtypes.High-throughput sequencing of polyA+ RNA (RNA-Seq) in human cancer shows remarkable potential to identify both novel markers of disease and uncharacterized aspects of tumor biology, particularly non-coding RNA (ncRNA) species. We employed RNA-Seq on a cohort of 102 prostate tissues and cells lines and performed ab initio transcriptome assembly to discover unannotated ncRNAs. We nominated 121 such Prostate Cancer Associated Transcripts (PCATs) with cancer-specific expression patterns. Among these, we characterized PCAT-1 as a novel prostate-specific regulator of cell proliferation and target of the Polycomb Repressive Complex 2 (PRC2). We further found that high PCAT-1 and PRC2 expression stratified patient tissues into molecular subtypes distinguished by expression signatures of PCAT-1-repressed target genes. Taken together, the findings presented herein identify PCAT-1 as a novel transcriptional repressor implicated in subset of prostate cancer patients. These findings establish the utility of RNA-Seq to identify disease-associated ncRNAs that may improve the stratification of cancer subtypes.

Induced chromosomal proximity and gene fusions in prostate cancer.

Androgen signaling facilitates the formation of an oncogenic fusion gene in prostate cancer cells. Gene fusions play a critical role in cancer progression. The mechanisms underlying their genesis and cell type specificity are not well understood. About 50% of human prostate cancers display a gene fusion involving the 5′ untranslated region of TMPRSS2, an androgen-regulated gene, and the protein-coding sequences of ERG, which encodes an erythroblast transformation–specific (ETS) transcription factor. By studying human prostate cancer cells with fluorescence in situ hybridization, we show that androgen signaling induces proximity of the TMPRSS2 and ERG genomic loci, both located on chromosome 21q22.2. Subsequent exposure of the cells to gamma irradiation, which causes DNA double-strand breaks, facilitates the formation of the TMPRSS2-ERG gene fusion. These results may help explain why TMPRSS2-ERG fusions are restricted to the prostate, which is dependent on androgen signaling.

Mechanistic Rationale for Inhibition of Poly(ADP-Ribose) Polymerase in ETS Gene Fusion-Positive Prostate Cancer

Recurrent fusions of ETS genes are considered driving mutations in a diverse array of cancers, including Ewings sarcoma, acute myeloid leukemia, and prostate cancer. We investigate the mechanisms by which ETS fusions mediate their effects, and find that the product of the predominant ETS gene fusion, TMPRSS2:ERG, interacts in a DNA-independent manner with the enzyme poly (ADP-ribose) polymerase 1 (PARP1) and the catalytic subunit of DNA protein kinase (DNA-PKcs). ETS gene-mediated transcription and cell invasion require PARP1 and DNA-PKcs expression and activity. Importantly, pharmacological inhibition of PARP1 inhibits ETS-positive, but not ETS-negative, prostate cancer xenograft growth. Finally, overexpression of the TMPRSS2:ERG fusion induces DNA damage, which is potentiated by PARP1 inhibition in a manner similar to that of BRCA1/2 deficiency.

Chimeric transcript discovery by paired-end transcriptome sequencing

Recurrent gene fusions are a prevalent class of mutations arising from the juxtaposition of 2 distinct regions, which can generate novel functional transcripts that could serve as valuable therapeutic targets in cancer. Therefore, we aim to establish a sensitive, high-throughput methodology to comprehensively catalog functional gene fusions in cancer by evaluating a paired-end transcriptome sequencing strategy. Not only did a paired-end approach provide a greater dynamic range in comparison with single read based approaches, but it clearly distinguished the high-level “driving” gene fusions, such as BCR-ABL1 and TMPRSS2-ERG, from potential lower level “passenger” gene fusions. Also, the comprehensiveness of a paired-end approach enabled the discovery of 12 previously undescribed gene fusions in 4 commonly used cell lines that eluded previous approaches. Using the paired-end transcriptome sequencing approach, we observed read-through mRNA chimeras, tissue-type restricted chimeras, converging transcripts, diverging transcripts, and overlapping mRNA transcripts. Last, we successfully used paired-end transcriptome sequencing to detect previously undescribed ETS gene fusions in prostate tumors. Together, this study establishes a highly specific and sensitive approach for accurately and comprehensively cataloguing chimeras within a sample using paired-end transcriptome sequencing.

Derivation of clinically compliant MSCs from CD105+, CD24- differentiated human ESCs.

Adult tissue‐derived mesenchymal stem cells (MSCs) have demonstrated therapeutic efficacy in treating diseases or repairing damaged tissues through mechanisms thought to be mediated by either cell replacement or secretion of paracrine factors. Characterized, self‐renewing human ESCs could potentially be an invariable source of consistently uniform MSCs for therapeutic applications. Here we describe a clinically relevant and reproducible manner of generating identical batches of hESC‐derived MSC (hESC‐MSC) cultures that circumvents exposure to virus, mouse cells, or serum. Trypsinization and propagation of HuES9 or H1 hESCs in feeder‐ and serum‐free selection media generated three polyclonal, karyotypically stable, and phenotypically MSC‐like cultures that do not express pluripotency‐associated markers but displayed MSC‐like surface antigens and gene expression profile. They differentiate into adipocytes, osteocytes, and chondrocytes in vitro. Gene expression and fluorescence‐activated cell sorter analysis identified CD105 and CD24 as highly expressed antigens on hESC‐MSCs and hESCs, respectively. CD105+, CD24− monoclonal isolates have a typical MSC gene expression profiles and were identical to each other with a highly correlated gene expression profile (r2 > .90). We have developed a protocol to reproducibly generate clinically compliant and identical hESC‐MSC cultures.

Urine TMPRSS2:ERG Fusion Transcript Stratifies Prostate Cancer Risk in Men with Elevated Serum PSA

Urine TMPRSS2:ERG gene fusion could be used for stratification of patients at higher risk for prostate cancer. Old Gene Fusion, New Diagnostic Tricks The “PSA test” is a routine test for men over the age of 50 or for those at risk for prostate cancer. It measures the level of prostate-specific antigen (PSA) in the blood, and if that level is above a predefined cutoff, a biopsy is recommended for definitive diagnosis. This test is not perfect; benign conditions, such as an enlarged prostate, can contribute to high levels of PSA, resulting in a “false-positive” and subsequent overdiagnosis and overtreatment. Because of the high prevalence of prostate cancer (it is estimated that nearly 250,000 men will be diagnosed with the disease in 2011), it is clear that a more accurate test for prostate cancer is needed. Here, Tomlins et al. improve on the PSA test by taking a new twist on a known gene fusion, using it to stratify more than 1000 men in two multicenter cohorts based on risk for developing the disease. Recently, it was discovered that the fusion of two genes, the transmembrane protease, serine 2 (TMPRSS2) gene and the v-ets erythroblastosis virus E26 oncogene homolog (avian) (ERG) gene, known as TMPRSS2:ERG, is overexpressed in more than 50% of PSA-screened prostate cancers. The protein product of this fusion cannot be detected in serum, so the authors decided to test for the presence of TMPRSS2:ERG mRNA in urine. First, they developed a clinical-grade, transcription-mediated amplification assay for quantifying fusion mRNA—this generated a TMPRSS2:ERG “score.” Urine TMPRSS2:ERG score was linked to the presence of cancer, tumor volume, and clinically significant cancer in patients. Then, the authors combined the TMPRSS2:ERG score with the level of prostate cancer antigen 3 (PCA3) in urine. TMPRSS2:ERG+PCA3 improved the performance of the multivariate Prostate Cancer Prevention Trial risk calculator, thus demonstrating clinical utility. Who said you can’t teach an old gene fusion new tricks? By combining the cancer-specific fusion TMPRSS2:ERG score with levels of PSA (in serum) and PCA3 (in urine), Tomlins and colleagues demonstrated more accurate, individualized stratification of men at high risk for developing clinically significant prostate cancer—an important step in streamlining diagnosis and treatment. Moreover, men with extremes of TMPRSS2:ERG+PCA3 had different risks of cancer on biopsy; in combination with other clinicopathological features, urine TMPRSS2:ERG+PCA3 might also inform the urgency of biopsy after PSA screening. More than 1,000,000 men undergo prostate biopsy each year in the United States, most for “elevated” serum prostate-specific antigen (PSA). Given the lack of specificity and unclear mortality benefit of PSA testing, methods to individualize management of elevated PSA are needed. Greater than 50% of PSA-screened prostate cancers harbor fusions between the transmembrane protease, serine 2 (TMPRSS2) and v-ets erythroblastosis virus E26 oncogene homolog (avian) (ERG) genes. Here, we report a clinical-grade, transcription-mediated amplification assay to risk stratify and detect prostate cancer noninvasively in urine. The TMPRSS2:ERG fusion transcript was quantitatively measured in prospectively collected whole urine from 1312 men at multiple centers. Urine TMPRSS2:ERG was associated with indicators of clinically significant cancer at biopsy and prostatectomy, including tumor size, high Gleason score at prostatectomy, and upgrading of Gleason grade at prostatectomy. TMPRSS2:ERG, in combination with urine prostate cancer antigen 3 (PCA3), improved the performance of the multivariate Prostate Cancer Prevention Trial risk calculator in predicting cancer on biopsy. In the biopsy cohorts, men in the highest and lowest of three TMPRSS2:ERG+PCA3 score groups had markedly different rates of cancer, clinically significant cancer by Epstein criteria, and high-grade cancer on biopsy. Our results demonstrate that urine TMPRSS2:ERG, in combination with urine PCA3, enhances the utility of serum PSA for predicting prostate cancer risk and clinically relevant cancer on biopsy.

Functionally recurrent rearrangements of the MAST kinase and Notch gene families in breast cancer.

Breast cancer is a heterogeneous disease that has a wide range of molecular aberrations and clinical outcomes. Here we used paired-end transcriptome sequencing to explore the landscape of gene fusions in a panel of breast cancer cell lines and tissues. We observed that individual breast cancers have a variety of expressed gene fusions. We identified two classes of recurrent gene rearrangements involving genes encoding microtubule-associated serine-threonine kinase (MAST) and members of the Notch family. Both MAST and Notch-family gene fusions have substantial phenotypic effects in breast epithelial cells. Breast cancer cell lines harboring Notch gene rearrangements are uniquely sensitive to inhibition of Notch signaling, and overexpression of MAST1 or MAST2 gene fusions has a proliferative effect both in vitro and in vivo. These findings show that recurrent gene rearrangements have key roles in subsets of carcinomas and suggest that transcriptome sequencing could identify individuals with rare, targetable gene fusions.