Prostate cancer is the most common non-skin cancer in America, affecting 1 in 6 men (for more information visit the Prostate Cancer Foundation website). As with any illness, early detection of the disease is important for disease control by therapeutic intervention. A common chromosomal rearrangement in prostate cancers involves the fusion of the TMPRSS2 gene to the ERG oncogene, both genes located in close proximity on chromosome 21. This genomic alteration results in the deletion of intervening genomic sequences and leads to increased ERG expression, disrupting androgen receptor signaling to promote disease progression.
Several fluorescence in situ hybridization (FISH) techniques have been previously developed to investigate TRMPRSS2/ERG rearrangements with relatively low resolution. For instance, a dual-color FISH assay can detect chromosome changes at the ERG gene but cannot ascertain whether a fusion event of ERG to TRMPRSS2 or any other gene has occurred. To overcome this weakness in identifying ERG or TRMPRSS2 rearrangements, the Fang Lab at Seattle Cancer Care Alliance and Nelson Lab in the Division of Human Biology at the Hutchinson Center developed a FISH assay that can detect a variety of TRMPRSS2/ERG rearrangements in prostate cancer cells.
The researchers developed four color-distinct probes, two of which label the 5’ and 3’ ends of the TMPRSS2 gene and the other two label the 5’ and 3’ region of the ERG gene. They used these probes to stain normal and prostate cancer cells to analyze the different genomic rearrangements that occurred with the TMPRSS2 and ERG genes. In normal cells, all four probes recognize the intact chromosome and produce a cluster of four-colored signals. In prostate tumors, a common gene fusion event involves the fusion of the 5’ region of TMPRSS2 with the 3’ region of ERG. The intervening chromosomal sequences can become deleted or translocated to a different chromosome. Indeed, 50% of the scored prostate tumors showed this common TRMPRSS2/ERG fusion, as indicated by the loss of hybridization of two of the probes due to chromosomal deletion. The four-colored FISH assay also efficiently detected other less common TRMPRSS2/ERG fusions including dual and multiple-deletion fusions (see figure).
To prove the applicability of their novel FISH assay to detect gene fusions in a variety of cells, the researchers stained prostate cell lines, and also primary and metastatic prostate cancer samples. Indeed, the four-color FISH assay revealed the presence of the TRMPRSS2/ERG fusion in ~50% of primary and metastatic prostate cancer samples. More importantly, additional and alternative genetic rearrangements at the TRMPRSS2 and ERG loci were apparent. These alternative rearrangements would have been missed by other FISH-based approaches, highlighting the ability of the FISH approach devised by Qu et. al. to detect novel genomic alterations within genes that promote prostate cancer progression. The researchers also showed that the TRMPRSS2/ERG fusion status identified by their novel FISH assay was consistent with the fusion status defined by two complementary approaches: RT-PCR and Gen-Probe.
In summary, Qu et. al. have generated a robust, analytically sensitive and highly specific assay to identify typical and atypical TRMPRSS2/ERG fusions in prostate cancer cells. This assay will aid in the elucidation of the complex genomic alterations that are involved in prostate cancer pathogenesis and prognosis.
Qu X, Randhawa G, Friedman C, O'Hara-Larrivee S, Kroeger K, Dumpit R, True L, Vakar-Lopez F, Porter C, Vessella R, Nelson P, Fang M. 2013. A novel four-color fluorescence in situ hybridization assay for the detection of TMPRSS2 and ERG rearrangements in prostate cancer. Cancer Genet. Epub ahead of print, doi: 10.1016.