In the 214 prostate cancer patients scored for fusion status, we found that the presence of the TMPRSS2-ERG fusion was not associated with prostate cancer-specific mortality. Similarly, no statistically significant association was found between prostate cancer-specific mortality and fusion type (translocation vs. deletion) or number (single vs. multiple). However, there was a suggestion of higher prostate cancer-specific mortality in those patients with multiple fusion products. In addition, we found the rs12329760 SNP in TMPRSS2 to be significantly associated with fusion by translocation and with multiple copies of the fusion protein.
Currently, only two previous studies have investigated the relationship between fusion status and prostate cancer-specific mortality [15, 19]. Consistent with the results presented here, Demichelis and colleagues (2007) observed no significant association between TMPRSS2-ERG gene fusion and disease-specific mortality when results were adjusted for Gleason score and age (p = 0.2) . By comparison, Attard and colleagues (2007) did not present results for overall fusion status however, they did show that fusions caused by deletion had significantly worse disease-specific mortality and this association was largely driven by tumors with two or more copies of the fusion product . While we also observed suggestive evidence for higher prostate cancer-specific mortality in patients with multiple fusion products, we did not observe an association with fusion type (translocation or deletion). This could be due to a number of differences between our study and Attard and colleagues' . Although the follow-up time of this study was greater than that of Attard et al. (median 12.3 years vs. 7.5 years), there were fewer prostate cancer-specific deaths during this time that could be attributed to the younger median age of diagnosis (60 years) and fewer cases with a Gleason score of > 7. As Gleason score is a strong, independent predictor of adverse outcomes and we have a relatively young group of patients, it may require a longer follow-up time to observe a potential association between prostate cancer-specific death and TMPRSS2-ERG fusion type (translocation/deletion) or number (single/multiple).
While not statistically significant, when unadjusted for Gleason score, we found that there was some evidence for higher prostate cancer-specific mortality in patients with fusions caused by translocation but not deletion. Attard and colleagues (2007) suggest that the sequence intervening the TMPRSS2 and ERG genes may contain tumor-suppressor genes which when lost, increase disease aggressiveness . In addition, Birger and colleagues (2006) identified significantly down-regulated genes located in the area of the common deletion site, of which at least one, HMGN1, has been associated with tumor growth in mice and primary mouse embryonic fibroblast cell lines . While deletion of this intervening region could explain the poorer outcome, it is not implausible that translocation could also disrupt the expression of these intervening genes in such a way to cause adverse outcomes. Clearly more work needs to be done, first to determine whether one particular fusion type is associated with poor outcomes and second, to determine whether intervening genes do play a role in the biological effect of the TMPRSS2-ERG fusion.
Few studies have addressed the issue of how the TMPRSS2-ERG fusion is initiated in prostate carcinogenesis. Using a bioinformatics approach, Liu and colleagues (2007) found Alu repeats in the TMPRSS2 and ERG breakpoint regions and that the distribution of these repeats correlates with the structure of the multiple TMPRSS2-ERG fusion transcripts identified to date . This finding as well as the fact that genomic alterations associated with Alu repeats have been observed to be associated with various other cancers, led Liu and colleagues (2007) to suggest that these Alu elements may facilitate recombination that leads to the fusion of the TMPRSS2 and ERG genes in prostate cancer. To our knowledge, this is the first study to investigate whether common genetic polymorphisms present in the TMPRSS2 and ERG genes in germline DNA are associated with the presence and/or type of TMPRSS2-ERG fusion in prostate tumor tissue. No associations were detected between the ERG SNPs and gene fusion, however we did find an association between the TMPRSS2 SNP rs12329760 and particular forms of the TMPRSS2-ERG fusion. While this finding needs to be replicated, it is interesting to speculate on how this SNP could influence the formation of the fusion protein. The Met160Val amino acid is highly conserved across mammals (ancestral form is the C allele or Val amino acid)  suggesting that it may be less tolerant to substitutions. The SNP is present in an exonic splicing enhancer (ESE) srp40 site and the presence of the A allele is predicted to disrupt the ESE, potentially resulting in an increased chance of exon skipping or protein malformation . It is also interesting to speculate on whether other gene fusions are contributing to prostate cancer development and progression. There are a great number of proteins that have been found to be over or under-expressed in prostate cancer and to be associated with various stages of tumor development. It is possible that like the fusions between TMPRSS2 and ERG, ETV1, ETV4 and ETV5, other functionally identical fusions are involved in changes in gene expression and prostate cancer development but are yet to be discovered [2, 35, 36].
If the association between the TMPRSS2 SNP and fusion type is replicated and in particular, if other SNPs associated with the acquisition of this gene fusion are identified, these data may present opportunities to augment or further current prostate cancer diagnostic abilities. As the gene fusion has been associated with early forms of prostate cancer, a germline genetic test could be developed to augment current screening procedures. In addition, clinicians are currently unable to distinguish men who will go on to develop aggressive metastatic prostate cancer from those whose disease will remain indolent. This may change with recent suggestions that particular TMPRSS2-ERG fusion types are a predictor of aggressive disease and prostate cancer-specific mortality [4, 5, 7, 15, 19]. Again a genetic test may be able to alert clinicians to those men who are more at risk for aggressive disease and therefore treatment strategies could be tailored accordingly.
There are both advantages and limitations to this study that must be taken into consideration when interpreting the results. Cases in this study were population-based unlike several previous TMPRSS2-ERG studies [4, 5, 18, 19], there was a mean surveillance period of 11.6 years after diagnosis, and prostate cancer-specific death was confirmed by death certificate. However, due to an average 5-year relative survival rate for prostate cancer of 98%, there were few prostate cancer-specific deaths in this cohort and therefore limited power. In addition, due to the technical problems inherent to assaying TMAs using FISH , only 57% of the cases could be scored. As a consequence, while there was some evidence of an association between multiple fusions and cancer-specific survival, there were insufficient events to observe a statistically significant association. Lack of power is also a concern in the SNP analyses and overall replication of the study is a priority before any translational studies are initiated. One final concern is that only one tumor focus was investigated per case in this study. As noted in the Introduction, focal heterogeneity is typically observed so it is possible cases were scored as normal when they did in fact have fusion transcripts present at other foci. Key future studies need to address the issue of whether results from one focus are predictive of tumor behavior overall.