Clonality and genetic divergence in multifocal low-grade superficial urothelial carcinoma as determined by chromosome 9 and p53 deletion analysis

Multifocality and recurrence are clinically important features of urothelial carcinomas of the urinary bladder. Recent molecular genetic studies have suggested that multifocal urothelial carcinomas are monoclonally derived from an identical transformed progenitor cell. However, most of these studies investigated advanced and poorly differentiated tumors. The study presented focuses on early papillary tumors, including 52 superficial well-differentiated multifocal and recurrent bladder carcinomas from 10 patients. Microdissection separating urothelium from stromal cells was considered essential to obtain pure tumor cell populations. Genetic analysis was carried out by applying two different methods. Dual color fluorescence in situ hybridization (FISH) with centromeric probes for chromosomes 9 and 17 and gene-specific probes for chromosome loci 9q22, 9p21, and 17p13 was carried out in parallel to loss of heterozygosity (LOH) analyses applying 5 microsatellite markers on these chromosomes. Overall, deletions on chromosome 9p were found in 47 tumors (90%), at chromosome 9q in 36 tumors (69%) and at chromosome 17p in 3 tumors (6%). There was a very high correlation of the results between FISH and LOH analysis. Ten early superficial papillary tumors showed deletion of chromosome 9p without deletion of 9q, suggesting 9p deletions as a very early event in the development of papillary urothelial carcinoma. Although in four patients, all investigated tumors showed identical genetic alterations and one patient showed no genetic alterations at the loci investigated, in five patients, two or more clones with different deletions were found. In four of these patients, the results are compatible with clonal divergence and selection of different cell subpopulations derived from a common progenitor cell. However, in one patient different alleles in two markers at chromosome 9 were deleted, favoring an independent evolution of two recurring tumor cell clones. In summary, we could show that there is considerable genetic heterogeneity in early multifocal and recurring urothelial carcinoma and demonstrated the occurrence of two independent clones in at least one patient as an indicator of possible initial oligoclonality of bladder cancer.