Chromosome mechanisms giving rise to the TMPRSS2-ERG fusion oncogene in prostate cancer and HGPIN lesions.

To the Editor: I read with great interest the article by Perner et al recently published in this Journal, in which they report the analysis of the TMPRSS2-ERG rearrangement in a large sample set that included clinically localized prostate carcinomas, hormone naive and refractory metastases, high-grade prostatic intraepithelial neoplasia (HGPIN), benign prostatic hyperplasia, and proliferative inflammatory atrophy lesions, and also normal prostatic tissue. I wish to address 2 points raised by this comprehensive fluorescence in situ hybridization (FISH) study. The first issue has to do with the detection of the TMPRSS2-ERG rearrangement in 19% of the HGPIN lesions. These data corroborate our findings published last year, in which we have for the first time identified the TMPRSS2-ERG fusion oncogene in 21% of the HGPIN lesions and confirmed its presence in 50% of prostate carcinomas. Our findings were further validated by sequencing analysis and by real-time polymerase chain reaction quantification of the TMPRSS2-ERG fusion transcript. Furthermore, we demonstrated chromosome copy number changes by comparative genomic hybridization in 42% of the clinically confined prostate carcinomas but in none of the 16 HGPIN lesions analyzed, showing that the TMPRSS2-ERG rearrangement is an early event that may precede large chromosome alterations in prostate carcinogenesis. However, whereas Perner et al reported the HGPIN lesions positive for TMPRSS2-ERG to be in tight proximity to prostate adenocarcinomas presenting the same rearrangement, 2 of our 4 patients with the TMPRSS2-ERG fusion transcript in the HGPIN lesion did not show that genetic change in the adenocarcinoma identified in the same gland. This observation not only supports the hypothesis that HGPIN is a precursor lesion of at least some prostate adenocarcinomas, but also that prostate carcinogenesis may be a multicentric process, in which at least 2 independent pathogenetic pathways may coexist in the same prostate, leading to clonally unrelated neoplasias with or without involvement of the avian erythroblastosis virus E26 homologue pathway. The second issue I wish to discuss has to do with the misuse of cytogenetic nomenclature when the authors attempt to describe the chromosomal mechanisms behind the TMPRSS2-ERG fusion. To detect the rearrangement indirectly, Perner et al have used a dual-color, breakapart FISH strategy that employs 2 probes flanking the gene of interest (ERG). Their Figure 1 illustrates well this FISH strategy to detect translocations between nonhomologous chromosomes, but it is misleading regarding the detection of the TMPRSS2-ERG fusion because the 2 genes are located in the same chromosome just 2.9Mb apart (21q22). As the authors state, the main mechanism causing the TMPRSS2-ERG fusion is an intronic deletion between the 2 genes, which can indirectly be detected by the loss of the probe targeting the region 50 of ERG (as seen in about 62% of the 115 positive clinically localized prostate adenocarcinomas and illustrated in their Figs. 2B, F). The problem arises when the authors affirm that the mechanism behind the remaining 38% fusionpositive cases is a translocation, on the basis of the split signal pattern detected by FISH (as seen in their Fig. 3D). Purely on theoretical grounds, 2 additional chromosome mechanisms could give rise to the TMPRSS2-ERG fusion without loss of the DNA segment between the 2 genes, that is, a translocation involving the 2 chromosomes 21 and an insertion of the intervening genetic material into another chromosome. Although both mechanisms are balanced (no net loss or gain of chromosome material), by definition a translocation requires only 2 chromosome break points, whereas an insertion requires 3. As I illustrate in Figure 1, the signal pattern observed in Figure 3D of Perner et al is not compatible with a translocation but with an insertion, as the genetic material between TMPRSS2 and ERG must be inserted into a third chromosome (besides the 2 chromosome 21 homologs) to give rise to the observed split red and green signals (Fig. 1B). Although it is theoretically possible to obtain a TMPRSS2-ERG fusion by a reciprocal translocation (with a break point on ERG in 1 chromosome homolog and a break point in TMPRSS2 on the other; Fig. 1C), this has to my knowledge not yet been demonstrated. In fact, the FISH strategy used by Perner et al is not appropriate to distinguish a translocation from a deletion as mechanisms causing a TMPRSS2ERG fusion, because the former mechanism would originate the juxtaposition of 2 green signals that most likely are not spatially resolvable on tissue sections. The misinterpretation of cytogenetic nomenclature I here refer to has now been made in several publications, both from this group (who made the seminal discovery of prostate cancer fusion genes) and from others. It would be useful that everyone working in this exciting and expanding field of knowledge becomes familiar with the International System for Cytogenetic Nomenclature (2005), so that we more easily may understand each others’ scientific findings by speaking the same language.