Abstract PL06-02: Inflammation, DNA mismatch repair, and colon cancer.

Abstract

Abstract Colorectal neoplasia is driven by at least two pathogenic pathways for cancer development. Chromosomal instability, observed in ~80% of sporadic colorectal cancers, is named due to the rearrangement of chromosomes, resulting in aneuploid nuclei. The etiology for CIN is not known. A common finding is allelic mutation of a tumor suppressor gene, followed by loss of the remaining normal allele (termed loss of heterozygosity, LOH). Microsatellite instability (MSI), observed in up to 20% of sporadic colorectal cancers, is named for frameshifts (or instability) that occur at DNA microsatellite sequences. Cells remain diploid. The etiology for MSI is inactivation of DNA mismatch repair (MMR), an enzyme system functioning in the nucleus to repair interstrand nucleotide mispairs and loops of DNA that are formed on one strand at microsatellites. The specificity of mismatch recognition is directed by hMutSa and hMutSb; hMutSa (hMSH2-hMSH6 heterodimer) binds to single mispairs and single insertion-deletion loops, whereas hMutSb (hMSH2-hMSH3 heterodimer) binds larger insertion-deletion loops. MSI sporadic cancers are caused by hypermethylation of the MMR gene hMLH1's promoter, preventing its transcription and translation. With loss of hMLH1, MMR completely fails and mutations accumulate in the genome due to non-repair of DNA polymerase errors. Clinically, MSI tumors contain surrounding and intratumoral mononuclear inflammatory cells which are thought responsible in containing the tumor in place, leading to improved survival over same staged patients with non-MSI tumors. Although the types of inflammatory cells within MSI tumors have not been characterized, it is believed that neo-peptides from frameshifted proteins as a result of MMR failure are generated to attract mononuclear cells into the tumor, producing a protective immune response. Elevated microsatellite instability at selected tetranucleotide repeats (EMAST) is a genetic signature observed in 60% of sporadic colon cancers. Unlike MSI colorectal cancers in which hypermethylation of hMLH1 drives multiple target gene mutations, the cause of EMAST has been recently associated with reduced expression of hMSH3, which we have also observed in hamartomatous polyps and during the adenoma-to-carcinoma progression, and is hypothesized to be triggered as a consequence of inflammation with subsequent relaxation of MMR function. EMAST-positive human colorectal cancers are strongly associated with inflammation, particularly within tumor nests, but unlike MSI tumors, this inflammation is associated with advance staged colorectal cancer, implying reduced survival. Citation Format: John M. Carethers. Inflammation, DNA mismatch repair, and colon cancer. [abstract]. In: Proceedings of the Fifth AACR Conference on the Science of Cancer Health Disparities in Racial/Ethnic Minorities and the Medically Underserved; 2012 Oct 27-30; San Diego, CA. Philadelphia (PA): AACR; Cancer Epidemiol Biomarkers Prev 2012;21(10 Suppl):Abstract nr PL06-02.