Mutational analysis of the nucleotide binding domain of the mismatch repair enzyme hMSH-2

Mutator phenotypes of yeast strains heterozygous for mutations in the MSH2 gene

Heterozygosity for germ-line mutations in the DNA mismatch repair gene MSH2 predisposes humans to cancer. Here we use a highly sensitive reporter to describe a spontaneous mutator phenotype in diploid yeast cells containing a deletion of only one MSH2 allele. We also identify five MSH2 missense mutations that have dominant mutator effects in heterozygous cells when expressed at normal levels from the natural MSH2 promoter. For example, a 230-fold mutator effect is observed in an MSH2/msh2 diploid strain in which Gly693, which is invariant in MutS homologs and involved in ATP hydrolysis, is changed to alanine. DNA binding data suggest that mismatch repair is suppressed by binding of a mutant Msh2-Msh6 heterodimer to a mismatch with subsequent inability to dissociate from the mismatch in the presence of ATP. A dominant mutator effect also is observed in yeast when Gly693 is changed to serine. An early onset colorectal tumor is heterozygous for the analogous Gly --> Ser mutation in hMSH2, and a second hMSH2 mutation was not found, suggesting that this missense mutation may predispose to cancer via a dominant mutator effect. The mutator effects of the deletion mutant and the Gly --> Ala missense mutant in yeast MSH2 are enhanced by heterozygosity for a missense mutation in DNA polymerase delta that reduces its proofreading activity but is not a mutator in the heterozygous state. The synergistic effects of heterozygosity for mutations in two different genes that act in series to correct replication errors may be relevant to cancer predisposition.

Prediction-based threading of the hMSH2 DNA mismatch repair protein

Mutations in the genes whose products participate in DNA mismatch repair underlie the increased risk of cancer in families with hereditary nonpolyposis colon carcinoma. Mutations in hMSH2 account for approximately 50% of the mutations found in these families. We sought to predict the 3-dimensional structure of hMSH2 by identifying structural homologues using prediction-based threading and by computer modeling using information from these putative structurally related proteins. Prediction-based threading identified three candidate structural homologues: glycogen phosphorylase (gpb), a 70 kDa soluble lytic transglycosylase, and ribonucleotide reductase protein R1. An independent approach utilizing a potential-based threading program also identified gpb as a structural homologue. The models based on the structures of these proteins suggest that the ATP binding domain and helix-turn-helix domain are exposed on the outside of the protein. All known bacterial MutS and hMSH2 mutations appear to be clustered in similar vicinities in the theoretical models of hMSH2; the major site is within the ATP binding domain and near the carboxyl-terminal end, whereas a smaller number map to the region coding for exon 5 and the amino-terminal domain. All point mutations also appear to affect amino acids that are exposed on the outside surface of the protein.

Mapping the minimal domain of hMSH-2 sufficient for binding mismatched oligonucleotides

The human MSH-2 gene product is a member of a highly conserved family of proteins involved in post-replication mismatch repair. Germline mutations in this gene have been implicated in hereditary non-polyposis colorectal cancer (HNPCC). Alterations in the coding region of the hMSH-2 gene result in a mutator phenotype with marked instability of microsatellite sequences, indicative of a deficiency in DNA repair. We have previously shown that a region of high homology between MutS proteins of different species containing a nucleotide binding domain, is sufficient to bind DNA containing specific mismatched residues. In order to determine the minimal domain of hMSH-2 necessary for binding mismatch-containing oligonucleotides, deletion analysis of the C-terminal region was performed. We have constructed a 5' and 3' deletion series, expressed each deletion as a bacterial fusion protein and assessed it for ATPase activity and its ability to identify mismatch containing DNA. Here we demonstrate that a 585 bp fragment encoding 195 amino acids within the C-terminal domain of hMSH-2 is sufficient to bind to DNA containing mismatches.