title: Conditional nuclear localization of hMLH3 suggests a minor activity in mismatch repair and supports its role as a low-risk gene in HNPCC.
abstract: DNA mismatch repair (MMR) mechanism contributes to the maintenance of genomic stability. Loss of MMR function predisposes to a mutator cell phenotype, microsatellite instability (MSI) and cancer, especially hereditary non-polyposis colorectal cancer (HNPCC). To date, five MMR genes, hMSH2, hMSH6, hMLH1, hPMS2, and hMLH3 are associated with HNPCC. Although, hMLH3 is suggested to be causative in HNPCC, its relevance to MMR needs to be confirmed to reliably assess significance of the inherited sequence variations in it. Recently, a human heterodimer hMLH1/hMLH3 (hMutLgamma) was shown to be able to assist hMLH1/hPMS2 (hMutLalpha) in the repair of mismatches in vitro. To repair mismatches in vivo, hMLH3 ought to localize in the nucleus. Our immunofluorescence analyses indicated that when all the three MutL homologues are natively expressed in human cells, endogenous hMLH1 and hPMS2 localize in the nucleus, whereas hMLH3 stays in the cytoplasm. Absence of hPMS2 and co-expression of hMLH3 with hMLH1 results in its partial nuclear localization. Our results are clinically relevant since they show that in the nuclear localization hMLH3 is dependent on hMLH1 and competitive with hPMS2. The continuous nuclear localization of hMLH1 and hPMS2 suggests that in vivo, hPMS2 (hMutLalpha) has a major activity in MMR. In absence of hPMS2, hMLH3 (hMutLgamma) is located in the nucleus, suggesting a conditional activity in MMR and supporting its role as a low-risk gene in HNPCC.
authors.affiliation: Department of Biological and Environmental Sciences, Genetics, University of Helsinki, FI-00014 Helsinki, Finland.
authors.forename: Mari K
authors.initials: MK
authors.lastname: Korhonen
authors.forename: Tiina E
authors.initials: TE
authors.lastname: Raevaara
authors.forename: Hannes
authors.initials: H
authors.lastname: Lohi
authors.forename: Minna
authors.initials: M
authors.lastname: Nyström
mesh_terms: D000251:Adenosine Triphosphatases / Q000096:biosynthesis; D020137:Base Pair Mismatch; D002352:Carrier Proteins / Q000096:biosynthesis*; D045744:Cell Line, Tumor; D002467:Cell Nucleus / Q000378:metabolism*; D015179:Colorectal Neoplasms / Q000378:metabolism* / Q000473:pathology; D003123:Colorectal Neoplasms, Hereditary Nonpolyposis / Q000378:metabolism* / Q000473:pathology; D004260:DNA Repair; D045643:DNA Repair Enzymes / Q000096:biosynthesis; D004268:DNA-Binding Proteins / Q000096:biosynthesis; D018095:Germ-Line Mutation; D006367:HeLa Cells; D006801:Humans; D053842:Microsatellite Instability; D008856:Microscopy, Fluorescence; D000070976:Mismatch Repair Endonuclease PMS2; D000070956:MutL Proteins; D010641:Phenotype; D012306:Risk
chemical_list: D002352:Carrier Proteins; D004268:DNA-Binding Proteins; C403623:MLH3 protein, human; D000251:Adenosine Triphosphatases; C094934:PMS2 protein, human; D000070976:Mismatch Repair Endonuclease PMS2; D000070956:MutL Proteins; D045643:DNA Repair Enzymes