Genomic instability and DNA damage repair in clear cell renal cell carcinoma.

Abstract

4581 Background: Kidney cancer accounts for 2-3% of all new cancers with clear cell renal cell carcinoma (ccRCC) the most common subtype. ccRCC is characterized by a high level of genomic instability, suggesting defective DNA damage repair (DDR). The most frequent genomic alteration in ccRCC involves loss of the 3p chromosomal arm which harbors the von Hippel Lindau gene ( VHL), in addition to nearby genes SETD2, BAP1, and PBRM. We hypothesized that VHL loss leads to defective DDR as an early event in ccRCC carcinogenesis, giving way to a mutator phenotype. We posited that assessment of very early ccRCC tumors would inform us regarding the core mutations required to drive tumorigenesis in ccRCC, and that we could confirm these findings in appropriate model systems. Methods: We performed whole-exome (WES) DNA sequencing on 11 early-stage ccRCC tumors from 5 individuals, along with their matched normal DNA. We then analyzed ccRCC samples with and without somatic VHL mutations from the Cancer Genome Atlas (TCGA) for mutational load. Finally we assessed DDR signaling activity in renal proximal tubular cell lines (RPTEC) with VHL/SETD2 knockdown and in murine embryo fibroblasts (MEFs) from Vhl and Setd2knockout mice treated with etoposide via γH2AX expression and direct repeat-green fluorescent protein reporter assay. Results: All 11 samples revealed loss of 3p with pathogenic germline or somatic mutation in remaining VHL allele. No mutations were found in genes frequently mutated in larger ccRCC, including PBRM1, BAP1 or SETD2. WES revealed ~100 mutations/tumor, with no shared mutations across samples, even within the same individual. TCGA analysis showed similar mutational loads across ccRCC samples. MEFs with biallelic loss of Vhl and monoallelic loss of Setd2 and RPTEC with VHL and SETD2knockdown displayed increased DNA damage with impaired homologous repair and increased non-homologous end joining (NHEJ). Conclusions: Early stage ccRCC tumors with loss of VHL and chr 3p demonstrate genomic instability and a mutator phenotype similar to more advanced ccRCC. Cell line models of early ccRCC show increased DNA damage with a greater reliance on error-prone NHEJ machinery. These defects could be targeted for synthetic lethal treatment strategies.