Mutational analysis of field cancerization in bladder cancer

The multifocal and recurrent nature of bladder cancer has been explained by field cancerization of the bladder urothelium. To shed light on field cancerization in the bladder, we investigated the mutational landscape of normal appearing urothelium and paired bladder tumors from four patients. Sequencing of 509 cancer driver genes revealed the presence of 2-16 mutations exclusively localized in normal tissue (average target read depth 634x). Furthermore, 6-13 mutations were shared between tumor and normal samples and 8-75 mutations were exclusively detected in tumor samples. More mutations were observed in normal samples from patients with multifocal disease compared to patients with unifocal disease. Mutations in normal samples had low allele frequencies compared to tumor mutations (p<2.2*10-16). Furthermore, significant differences in the type of nucleotide changes between tumor, normal and shared mutations (p=2.7*10-8) were observed, and mutations in APOBEC context were observed primarily among tumor mutations (p=0.026). No differences in functional impact between normal, shared and tumor mutations were observed (p=0.23). Overall, these findings support the theory of multiple fields in the bladder, and document non-tumor specific driver mutations to be present in normal appearing bladder tissue.