ABSTRACT
To understand the evolutionary process of the DNA mismatch repair system, we conducted systematic phylogenetic analysis of its key components, the bacterial MutS and MutL genes and their eukaryotic homologs. Based on genome-wide homolog searches, we identified three new MutS subfamilies (MutS3-5) in addition to the previously studied MutS1 and MutS2 subfamilies. Detailed evolutionary analysis strongly suggests that frequent ancient horizontal gene transfer (HGT) occurred with both MutS and MutL genes from bacteria to eukaryotes and/or archaea. Our results further imply that the origins of mismatch repair system in eukaryotes and archaea are largely attributed to ancient HGT from bacteria instead of vertical evolution. Specifically, the eukaryotic MutS and MutL homologs likely originated from endosymbiotic ancestors of mitochondria or chloroplasts, indicating that not only archaea, but also bacteria are important sources of eukaryotic DNA metabolic genes. The archaeal MutS1 and MutL homologs were also acquired from bacteria simultaneously through HGT. Moreover, the distribution and evolution profiles of the MutS1 and MutL genes suggest that they have undergone long-term coevolution. Our work presents an overall portrait of the evolution of these important genes in DNA metabolism and also provides further understanding about the early evolution of cellular organisms.
Subject(s)
DNA Mismatch Repair , Evolution, Molecular , Gene Transfer, Horizontal , Multigene Family , MutS DNA Mismatch-Binding Protein/genetics , Animals , Genes, Archaeal , Genes, Bacterial , Genes, Plant , Genomics , Models, Genetic , MutS DNA Mismatch-Binding Protein/classification , MutS Homolog 2 Protein/classification , MutS Homolog 2 Protein/genetics , PhylogenyABSTRACT
Missense changes constitute approximately 1/3 and 1/5 of all rare sequence constitutional variations identified in the MLH1 and MSH2 genes by mutation screening. They represent a challenge for the clinician and for the genetic counselor, who often cannot use them for the management of Lynch syndrome families. Several parameters can be evaluated to gain insight into the significance of such unclassified variants (UVs). These include analysis of microsatellite instability (MSI), immunohistochemistry of mismatch repair (MMR) proteins, segregation data, frequency of the variants in control samples, presence of other pathogenic mutations, and functional and mRNA analyses. While none of these variables can be used alone to predict the significance of UVs in a single case, combined evaluation can lead to clinically useful conclusions. This review reports available information on a sample of MLH1 and MSH2 missense UVs, for which MSI and immunohistochemical data could be retrieved from the literature. Currently, since MSI analysis is routinely performed as a diagnostic test for Lynch syndrome, tumor MSI status represents the most important factor for determining the pathogenicity of UVs in MMR genes.
Subject(s)
Carrier Proteins/genetics , Colorectal Neoplasms, Hereditary Nonpolyposis/chemistry , Colorectal Neoplasms, Hereditary Nonpolyposis/genetics , Genetic Variation , Microsatellite Instability , Microsatellite Repeats/genetics , MutS Homolog 2 Protein/genetics , Nuclear Proteins/genetics , Adaptor Proteins, Signal Transducing , Carrier Proteins/biosynthesis , Carrier Proteins/classification , Colorectal Neoplasms, Hereditary Nonpolyposis/diagnosis , Colorectal Neoplasms, Hereditary Nonpolyposis/epidemiology , Humans , Immunohistochemistry , MutL Protein Homolog 1 , MutS Homolog 2 Protein/biosynthesis , MutS Homolog 2 Protein/classification , Nuclear Proteins/biosynthesis , Nuclear Proteins/classificationABSTRACT
BACKGROUND: The treatment of ovarian cancer is hindered by intrinsic or acquired resistance to platinum-based chemotherapy. The aim of this study is to determine the frequency of mismatch repair (MMR) inactivation in ovarian cancer and its association with resistance to platinum-based chemotherapy. METHODS: We determined, microsatellite instability (MSI) as a marker for MMR inactivation (analysis of BAT25 and BAT26), MLH1 promoter methylation status (methylation specific PCR on bisulfite treated DNA) and mRNA expression of MLH1, MSH2, MSH3, MSH6 and PMS2 (quantitative RT-PCR) in 75 ovarian carcinomas and eight ovarian cancer cell lines RESULTS: MSI was detected in three of the eight cell lines i.e. A2780 (no MLH1 mRNA expression due to promoter methylation), SKOV3 (no MLH1 mRNA expression) and 2774 (no altered expression of MMR genes). Overall, there was no association between cisplatin response and MMR status in these eight cell lines. Seven of the 75 ovarian carcinomas showed MLH1 promoter methylation, however, none of these showed MSI. Forty-six of these patients received platinum-based chemotherapy (11 non-responders, 34 responders, one unknown response). The resistance seen in the eleven non-responders was not related to MSI and therefore also not to MMR inactivation. CONCLUSION: No MMR inactivation was detected in 75 ovarian carcinoma specimens and no association was seen between MMR inactivation and resistance in the ovarian cancer cell lines as well as the ovarian carcinomas. In the discussion, the results were compared to that of twenty similar studies in the literature including in total 1315 ovarian cancer patients. Although no association between response and MMR status was seen in the primary tumor the possible role of MMR inactivation in acquired resistance deserves further investigation.
