Human MutL homolog (MLH1) function in DNA mismatch repair: a prospective screen for missense mutations in the ATPase domain.
Germline mutations in the DNA mismatch repair (MMR) genes MSH2 and MLH1 are responsible for the majority of hereditary non-polyposis colorectal cancer (HNPCC), an autosomal-dominant early-onset cancer syndrome. Genetic testing of both MSH2 and MLH1 from individuals suspected of HNPCC has revealed a considerable number of missense codons, which are difficult to classify as either pathogenic mutations or silent polymorphisms. To identify novel MLH1 missense codons that impair MMR activity, a prospective genetic screen in the yeast Saccharomyces cerevisiae was developed. The screen utilized hybrid human-yeast MLH1 genes that encode proteins having regions of the yeast ATPase domain replaced by homologous regions from the human protein. These hybrid MLH1 proteins are functional in MMR in vivo in yeast. Mutagenized MLH1 fragments of the human coding region were synthesized by error-prone PCR and cloned directly in yeast by in vivo gap repair. The resulting yeast colonies, which constitute a library of hybrid MLH1 gene variants, were initially screened by semi-quantitative in vivo MMR assays. The hybrid MLH1 genes were recovered from yeast clones that exhibited a MMR defect and sequenced to identify alterations in the mutagenized region. This investigation identified 117 missense codons that conferred a 2-fold or greater decreased efficiency of MMR in subsequent quantitative MMR assays. Notably, 10 of the identified missense codons were equivalent to codon changes previously observed in the human population and implicated in HNPCC. To investigate the effect of all possible codon alterations at single residues, a comprehensive mutational analysis of human MLH1 codons 43 (lysine-43) and 44 (serine-44) was performed. Several amino acid replacements at each residue were silent, but the majority of substitutions at lysine-43 (14/19) and serine-44 (18/19) reduced the efficiency of MMR. The assembled data identifies amino acid substitutions that disrupt MLH1 structure and/or function, and should assist the interpretation of MLH1 genetic tests.
Ellison
Aaron R
AR
BitTech, Inc., Westlake Village, CA 91361, USA.
Lofing
Joan
J
Bitter
Grant A
GA
D048868:Adaptor Proteins, Signal Transducing; D000251:Adenosine Triphosphatases / Q000737:chemistry; D000595:Amino Acid Sequence; D019677:Artificial Gene Fusion; D020137:Base Pair Mismatch; D002352:Carrier Proteins; D003062:Codon / Q000235:genetics; D003123:Colorectal Neoplasms, Hereditary Nonpolyposis / Q000175:diagnosis; D004252:DNA Mutational Analysis; D004260:DNA Repair; D005656:Fungal Proteins / Q000235:genetics; D005820:Genetic Testing; D006801:Humans; D008969:Molecular Sequence Data; D000070957:MutL Protein Homolog 1; D016296:Mutagenesis; D020125:Mutation, Missense; D009363:Neoplasm Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism; D009687:Nuclear Proteins; D017434:Protein Structure, Tertiary; D012441:Saccharomyces cerevisiae / Q000166:cytology / Q000235:genetics; D029701:Saccharomyces cerevisiae Proteins; D016415:Sequence Alignment
D048868:Adaptor Proteins, Signal Transducing; D002352:Carrier Proteins; D003062:Codon; D005656:Fungal Proteins; C088859:MLH1 protein, S cerevisiae; C501486:MLH1 protein, human; D009363:Neoplasm Proteins; D009687:Nuclear Proteins; D029701:Saccharomyces cerevisiae Proteins; D000251:Adenosine Triphosphatases; D000070957:MutL Protein Homolog 1