Abstract 5743: Characterization of mitochondrial DNA mutations and tumor evolution in ulcerative colitis-associated colorectal cancer

Abstract

Abstract The role of mitochondrial DNA (mtDNA) mutations in cancer is highly controversial. While specific mtDNA mutations have been associated with certain cancers, it has been recently reported that mtDNA mutations are negatively selected in most tumors. We aimed to clarify the role of mtDNA mutations in cancer progression by: 1) capitalizing on the multi-step sequence of tumorigenesis in ulcerative colitis (UC), a premalignant disease ideal for the study of tumor progression, and 2) employing a highly accurate, ultra-deep sequencing method to fully characterize the mtDNA mutational profile at different stages of tumor evolution. UC is an inflammatory bowel disease characterized by chronic colonic inflammation that increases the risk of colorectal cancer (CRC). UC CRC evolves through a stepwise process that involves the accumulation of molecular alterations in otherwise histologically normal epithelium, low-grade dysplasia (LGD), high-grade dysplasia (HGD), and cancer. Our lab has previously identified mitochondrial alterations as a facet of dysplastic progression in UC. Here we have worked to fully characterize the spectrum and frequency of mtDNA mutations in UC tumorigenesis by applying a novel, highly sensitive NGS technology, Duplex Sequencing (DS), that can accurately detect mutant allele frequencies (MAF) as low as 0.01%. For two patients that developed cancer, we analyzed 15-20 colon biopsies obtained at colectomy, representing all stages of tumorigenesis. The median duplex sequencing depth was 4,000x, which allowed unprecedented resolution to detect subclonal mutations (MAF 1-99%) and rare mutations (MAF<1%). Several subclonal mutations were present in all biopsies, but with variable MAF, thus enabling construction of phylogenetic trees via PHYLIP analysis. Rare mutations were abundant in all biopsies; on average one in every 10,000 nucleotides sequenced was mutated. Most rare mutations were G to A transitions, indicative of replicative errors, but G to T and C to A transversions, the oxidative damage mutational signature, were prevalent in some biopsies. We conclude, therefore, that most mtDNA mutations are caused by proliferation with a contribution by inflammatory processes, and that this may have greater implications for their role in other cancer types. Citation Format: Kathryn Terese Baker, Daniela Nachmanson, Scott R. Kennedy, Teresa A. Brentnall, Rosa Ana Risques. Characterization of mitochondrial DNA mutations and tumor evolution in ulcerative colitis-associated colorectal cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 5743. doi:10.1158/1538-7445.AM2017-5743