BRCA1 loss preexisting in small subpopulations of prostate cancer is associated with advanced disease and metastatic spread to lymph nodes and peripheral blood

Abstract

Purpose: A preliminary study performed on a small cohort of multifocal prostate cancer (PCa) detected BRCA1 allelic imbalances among circulating tumor cells (CTC). The present analysis was aimed to elucidate the biological and clinical roles of BRCA1 losses in metastatic spread and tumor progression in PCa patients.

Experimental design: To map molecular progression in PCa outgrowth, we used fluorescence in situ hybridization analysis of primary tumors and lymph node sections, and CTCs from peripheral blood.

Results: We found that 14% of 133 tested patients carried monoallelic BRCA1 loss in at least one tumor focus. Extended molecular analysis of chr17q revealed that this aberration was often a part of larger cytogenetic rearrangement involving chr17q21 accompanied by allelic imbalance of the tumor suppressor gene PTEN and lack of BRCA1 promoter methylation. The BRCA1 losses correlated with advanced T stage (P < 0.05), invasion to pelvic lymph nodes (P < 0.05), as well as biochemical recurrence (P < 0.01). Their prevalence was twice as high within 62 lymph node metastases (LNM) as in primary tumors (27%, P < 0.01). The analysis of 11 matched primary PCa-LNM pairs confirmed the suspected transmission of genetic abnormalities between these two sites. In four of seven patients with metastatic disease, BRCA1 losses appeared in a minute fraction of cytokeratin- and vimentin-positive CTCs.

Conclusions: Small subpopulations of PCa cells bearing BRCA1 losses might be one confounding factor initiating tumor dissemination and might provide an early indicator of shortened disease-free survival.

Figure 1

A – BRCA1 loss in primary tumor: BRCA1 – Spectrum Orange, CEP17 – Spectrum Aqua displayed in green; magnification 1000x. B – The frequencies of ERB2/HER2, 17q GRR and BRCA1 loci losses, normal copy numbers or gains in 144, 149 and 133 patients informative for each locus. C – A simplified scheme of the DNA probes, genes and genetic distances within 17q12–21.1 investigated in this study.

Figure 2

A – Disease-free survival of prostate cancer patients with the BRCA1 gene dosage 0.75 vs. >0.75 and ≤1.25. B – Multivariate analysis for the prognostic impact of T, N, Gleason score and BRCA1 gene dosage classifiers defined as shown in the table (Cox Regression Model).

Figure 3

Involvement of BRCA1 loss in PCa dissemination: A – CTCs with BRCA1 loss in metastatic PCa patients. Bar chart: Frequency (%) of cells carrying BRCA1 loss in total 500 cells scored for BRCA1 gene dosage and compared to control (mean score for 3 healthy volunteers). Table: The presence of CK(+) and vim(+++) cells and the most frequently observed type of aberration presented as a ratio of BRCA1 and CEP17 probes. B – The examples of CTCs carrying BRCA1 loss: BRCA1 – Spectrum Orange, CEP17 – Spectrum Aqua displayed in green, vimentin or CK staining displayed in green; magnification 630x; arrows indicate the BRCA1 loss found in vim(+++) or CK(+) CTCs, asterisks – in vim(−) or CK(−) cells.

Figure 4

A hypothetical model of the involvement of BRCA1 loss in prostate cancer dissemination and metastasis.