Molecular Genetics

Procaryotes and eucaryotes have evolved similar systems to repair DNA by mismatch repair.

Facts

• Some mutant bacteria, yeast and mammalian cell lines exhibit high frequencies of spontaneous mutation. In E. coli, the mutability-causing mutations are mutS, mut L and mutH.
• The mutS protein recognizes and binds to mismatches in DNA duplexes. The mutL protein binds to mutS. The mutH protein binds to hemimethylated dam methylation sites. The mutS-mutL-DNA complex stimulates mutH to cleave the unmethylated DNA strand at the GATC sequence. The GATC site can be either 5' or 3' of the recognized mismatch. One of two exonucleases (depending on whether cleavage was 5' or 3') chews away at the DNA to beyond the mismatch site. Long patch repair synthesis follows.
• Eucaryotes have proteins with sequence similarity to mutS and mutL that are involved in a similar repair pathway. The eucaryotic mutS is a dimer of MSH2 and GTBP (now known as MSH3 or MSH6) proteins. Eucaryotic mutL also consists of two polypeptides, MLH1 and PMS2. The PMS2 gene was originally identified in yeast where its mutation causes abnormalities in post-meiotic segregation.
• Extracts of wild type eucaryotic cells will repair mismatched base pairs and small indels in a nicked heteroduplex. The nucleotide on the strand with the nick is the one that is replaced. Extracts of cells mutant in genes for MSH2, GTBP, MLH1 and PMS2 are unable to perform this repair.
• Mutations in the human MSH2 gene are associated with a predisposition to colon cancer (HNPCC). The mutant cells have unstable microsatellite repeats (An, (GGC)n, (CA)n).
• Cell lines have been selected for resistance to the lethal effects of methylating agents MNNG and MNU from lines that are Mer- (lack the O-methylguaninetransferase). These cell lines have mutations in GTBP genes or in MSH2.

Interpretations

• Mismatch repair systems in procaryotes and eucaryotes are similar in the recognition of damage and in the mode of damage removal.
• The systems primarily repair undamaged, but mismatched, base pairs and small insertions or deletions. Some modified bases, such as O-methylguanine are also recognized.
• In E. coli, mismatch repair is also methyl-directed repair, the methylation pattern directing the repair system to correct the newly synthesized strand, sparing the template strand. In eucaryotes, the role of methylation in directing repair is uncertain. Repair is directed by nicks. Nicks are normally present in the daughter strands during DNA replication and could thus provide the needed specificity.
• Microsatellite instability associated with mutations in mismatch repair proteins is consistent with DNA replication stuttering frequently at simple repeat sequences and these stutters (small indels) being efficiently repaired by the mismatch repair system.
• Mismatches resulting from base damage may lead to mutations if recognized by the mismatch repair system instead of BER or NER. This may account for the effectiveness of MNU and MNNG as mutagens.

Further information

• Heteroduplexes resulting from strand exchange in recombination are also substrates for mismatch repair.
• The GTBP partner of MSH2 is one of two proteins, now designated MSH3 and MSH6.
• No eukaryotic homologue of mutH is known.
• Other repair systems include: damage reversal; nucleotide excision repair; and base excision repair.
• Mismatch repair may be responsible for the biotechnological useful technique of chimeroplasty.

This is page 3134 of Molecular Genetics by Ulrich Melcher, © 1997-2002