The identification of chromosomal translocation, t(4;6)(q22;q15), in prostate cancer

The complexity of prostate cancer: genomic alterations and heterogeneity

Although prostate cancer is the most common malignancy to affect men in the Western world, the molecular mechanisms underlying its development and progression remain poorly understood. Like all cancers, prostate cancer is a genetic disease that is characterized by multiple genomic alterations, including point mutations, microsatellite variations, and chromosomal alterations such as translocations, insertions, duplications, and deletions. In prostate cancer, but not other carcinomas, these chromosome alterations result in a high frequency of gene fusion events. The development and application of novel high-resolution technologies has significantly accelerated the detection of genomic alterations, revealing the complex nature and heterogeneity of the disease. The clinical heterogeneity of prostate cancer can be partly explained by this underlying genetic heterogeneity, which has been observed between patients from different geographical and ethnic populations, different individuals within these populations, different tumour foci within the same patient, and different cells within the same tumour focus. The highly heterogeneous nature of prostate cancer provides a real challenge for clinical disease management and a detailed understanding of the genetic alterations in all cells, including small subpopulations, would be highly advantageous.

Chromosome rearrangement associated inactivation of tumour suppressor genes in prostate cancer

Prostate cancer, the most common male cancer in Western countries, is commonly detected with complex chromosomal rearrangements. Following the discovery of the recurrent TMPRSS2:ETS fusions in prostate cancer and EML4:ALK in non-small-cell lung cancer, it is now accepted that fusion genes not only are the hallmark of haematological malignancies and sarcomas, but also play an important role in epithelial cell carcinogenesis. However, previous studies aiming to identify fusion genes in prostate cancer were mainly focused on expression changes and fusion transcripts. To investigate the genes recurrently affected by the chromosome breakpoints in prostate cancer, we analysed Affymetrix array 6.0 and 500K SNP microarray data from 77 prostate cancer samples. While the two genes most frequently affected by genomic breakpoints were, as expected, ERG and TMPRSS2, surprisingly more known tumour suppressor genes (TSGs) than known oncogenes were identified at recurrent chromosome breakpoints. Certain well-characterised TSGs, including p53, PTEN, BRCA1 and BRCA2 are recurrently truncated as a result of chromosome rearrangements in prostate cancer. Interestingly, many of the genes residing at recurrent breakpoint sites have not yet been implicated in prostate carcinogenesis such as HOOK3, PPP2R2A and TCBA1. We have confirmed the generally reduced expression of selected genes in clinical samples using quantitative RT-PCR analysis. Subsequently, we further investigated the genes associated with the t(4:6) translocation in LNCaP cells and reveal the genomic fusion of SNX9 and putative TSG UNC5C, which led to the reduced expression of both genes. This study reveals another common mechanism that leads to the inactivation of TSGs in prostate cancer and the identification of multiple TSGs inactivated by chromosome rearrangements will lead to new direction of research for the molecular basis of prostate carcinogenesis.