title: Functional significance and clinical phenotype of nontruncating mismatch repair variants of MLH1.
abstract: OBJECTIVE
  Germline mutations in mismatch repair genes are associated with hereditary nonpolyposis colorectal cancer. A significant proportion of mutations are nontruncating and associated with a variability of clinical phenotype and microsatellite instability and with occasional presence of residual protein in tumor tissue that suggests impaired functional activity but not total lack of mismatch repair. To address pathogenic significance and mechanism of pathogenicity, we studied the functionality of 31 nontruncating MLH1 mutations found in clinically characterized colorectal cancer families and 3 other variations listed in a mutation database.
  
  
  METHODS
  Mutations constructed by site-directed mutagenesis were studied for protein expression/stability, subcellular localization, protein-protein interaction, and repair efficiency. The genetic and biochemical data were correlated with clinical data. Finally, comparative sequence analysis was performed to assess the value of sequence homology as a tool for predicting functional results.
  
  
  RESULTS
  Altogether, 22 mutations were pathogenic in more than one assay, 2 variants were impaired in one assay, and 10 variants acted like wild-type protein. Twenty of 34 mutations affected the quantity of MLH1 protein, whereas only 15 mainly amino-terminal mutations were defective in an in vitro repair assay. Comparative sequence analysis correctly predicted functional studies for 82% of variants.
  
  
  CONCLUSIONS
  Pathogenic nontruncating alterations in MLH1 may interfere with different biochemical mechanisms but generally more than one. The severe biochemical defects are mirrored by phenotypic characteristics such as early age at onset and high microsatellite instability, whereas variants with no or mild defects in functionality are associated with variable clinical phenotypes.
authors.affiliation: Department of Biological and Environmental Sciences, University of Helsinki, Finland.
authors.forename: Tiina E
authors.initials: TE
authors.lastname: Raevaara
authors.forename: Mari K
authors.initials: MK
authors.lastname: Korhonen
authors.forename: Hannes
authors.initials: H
authors.lastname: Lohi
authors.forename: Heather
authors.initials: H
authors.lastname: Hampel
authors.forename: Elly
authors.initials: E
authors.lastname: Lynch
authors.forename: Karin E
authors.initials: KE
authors.lastname: Lönnqvist
authors.forename: Elke
authors.initials: E
authors.lastname: Holinski-Feder
authors.forename: Christian
authors.initials: C
authors.lastname: Sutter
authors.forename: Wendy
authors.initials: W
authors.lastname: McKinnon
authors.forename: Sekhar
authors.initials: S
authors.lastname: Duraisamy
authors.forename: Anne-Marie
authors.initials: AM
authors.lastname: Gerdes
authors.forename: Päivi
authors.initials: P
authors.lastname: Peltomäki
authors.forename: Maija
authors.initials: M
authors.lastname: Kohonen-Ccorish
authors.forename: Elisabeth
authors.initials: E
authors.lastname: Mangold
authors.forename: Finlay
authors.initials: F
authors.lastname: Macrae
authors.forename: Marc
authors.initials: M
authors.lastname: Greenblatt
authors.forename: Albert
authors.initials: A
authors.lastname: de la Chapelle
authors.forename: Minna
authors.initials: M
authors.lastname: Nyström
mesh_terms: D048868:Adaptor Proteins, Signal Transducing; D000328:Adult; D017677:Age Distribution; D000368:Aged; D020137:Base Pair Mismatch / Q000235:genetics*; D014408:Biomarkers, Tumor / Q000032:analysis; D015153:Blotting, Western; D002352:Carrier Proteins; D015331:Cohort Studies; D003123:Colorectal Neoplasms, Hereditary Nonpolyposis / Q000453:epidemiology / Q000235:genetics*; D005260:Female; D015972:Gene Expression Regulation, Neoplastic; D020022:Genetic Predisposition to Disease / Q000453:epidemiology*; D018095:Germ-Line Mutation; D006801:Humans; D015994:Incidence; D008297:Male; D008875:Middle Aged; D000070957:MutL Protein Homolog 1; D009363:Neoplasm Proteins / Q000235:genetics*; D009687:Nuclear Proteins / Q000235:genetics*; D010375:Pedigree; D010641:Phenotype; D011336:Probability; D011379:Prognosis; D018570:Risk Assessment; D012494:Sampling Studies; D012680:Sensitivity and Specificity; D012720:Severity of Illness Index; D017678:Sex Distribution; D016019:Survival Analysis
chemical_list: D048868:Adaptor Proteins, Signal Transducing; D014408:Biomarkers, Tumor; D002352:Carrier Proteins; C501486:MLH1 protein, human; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D000070957:MutL Protein Homolog 1