MSH6

Discovery

MSH6 was first identified in the budding yeast S. cerevisiae because of its homology to MSH2. The identification of the human GTBP gene and subsequent amino acid sequence availability showed that yeast MSH6 and human GTBP were more related to each other than any other MutS homolog, with a 26.6% amino acid identity. Thus, GTBP took on the name human MSH6, or hMSH6.

Structure

In the human genome, hMSH6 is located on chromosome 2. It contains the Walker-A/B adenine nucleotide binding motif, which is the most highly conserved sequence found in all MutS homologs.

Function

Importance of mismatch repair

Mismatches commonly occur as a result of DNA replication errors, genetic recombination, or other chemical and physical factors. Recognizing those mismatches and repairing them is extremely important for cells, because failure to do so results in microsatellite instability, an elevated spontaneous mutation rate (mutator phenotype), and susceptibility to HNPCC. hMSH6 combines with hMSH2 to form the active protein complex, hMutS alpha, also called hMSH2-hMSH6.

Mismatch recognition

Mismatch recognition by this complex is regulated by the ADP to ATP transformation, which provides evidence that hMutS alpha complex functions as a molecular switch. In normal DNA, adenine (A) bonds with thymine (T) and cytosine (C) bonds with guanine (G). Sometimes there will be a mismatch where T will bind with G, which is called a G/T mismatch. When a G/T mismatch is recognized, hMutS alpha complex binds and exchanges ADP for ATP. The ADP--ATP exchange causes a conformational change to convert hMutS alpha into a sliding clamp that can diffuse along the DNA backbone. The ATP induces a release of the complex from the DNA and allows the hMutS alpha to dissociate along the DNA like a sliding clamp. This transformation helps trigger downstream events to repair the damaged DNA.

Cancer

Although mutations in hMSH2 cause a strong general mutator phenotype, mutations in hMSH6 cause only a modest mutator phenotype. At the gene level, the mutations were found to cause primarily single-base substitution mutations, which suggests that the role of hMSH6 is primarily for correcting single-base substitution mutations and to a lesser extent single base insertion/deletion mutations.

Mutations in the hMSH6 gene cause the protein to be nonfunctional or only partially active, thus reducing its ability to repair mistakes in DNA. The loss of MSH6 function results in instability at mononucleotide repeats.

Epigenetic control of MSH6 in cancer

Two microRNAs, miR21 and miR-155, target the DNA mismatch repair (MMR) genes hMSH6 and hMSH2, to cause reduced expression of their proteins. If one or the other of these two microRNAs is over-expressed, hMSH2 and hMSH6 proteins are under-expressed, resulting in reduced DNA mismatch repair and increased microsatellite instability.

One of these microRNAs, miR21, is regulated by the epigenetic methylation state of the CpG islands in one or the other of its two promoter regions. Hypomethylation of its promoter region is associated with increased expression of an miRNA. High expression of a microRNA causes repression of its target genes (see microRNA silencing of genes). In 66% to 90% of colon cancers, miR-21 was over-expressed, and generally the measured level of hMSH2 was decreased (and hMSH6 is unstable without hMSH2).

The other microRNA, miR-155, is regulated both by epigenetic methylation of the CpG islands in its promoter region and by epigenetic acetylation of histones H2A and H3 at the miR-155 promoter (where acetylation increases transcription). Measured by two different methods, miR-155 was over-expressed in sporadic colorectal cancers by either 22% or 50%. When miR-155 was elevated, hMSH2 was under-expressed in 44% to 67% of the same tissues (and hMSH6 is likely under-expressed as well, and also unstable in the absence of hMSH2).

Interactions

MSH6 has been shown to interact with MSH2, PCNA and BRCA1.

References

References

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2. (1996). "hMSH2 forms specific mispair-binding complexes with hMSH3 and hMSH6". PNAS.
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4. (1995). "Identification of mismatch repair genes and their role in the development of cancer". Current Opinion in Genetics & Development.
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8. (2010). "MicroRNA-21 induces resistance to 5-fluorouracil by down-regulating human DNA MutS homolog 2 (hMSH2)". Proc. Natl. Acad. Sci. U.S.A..
9. (2010). "Modulation of mismatch repair and genomic stability by miR-155". Proc. Natl. Acad. Sci. U.S.A..
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18. (Nov 1996). "hMSH2 forms specific mispair-binding complexes with hMSH3 and hMSH6". Proceedings of the National Academy of Sciences of the United States of America.
19. (Mar 2001). "hMSH3 and hMSH6 interact with PCNA and colocalize with it to replication foci". Genes & Development.
20. (Nov 2000). "Functional interaction of proliferating cell nuclear antigen with MSH2-MSH6 and MSH2-MSH3 complexes". The Journal of Biological Chemistry.
21. (Oct 2002). "A proteomics approach to identify proliferating cell nuclear antigen (PCNA)-binding proteins in human cell lysates. Identification of the human CHL12/RFCs2-5 complex as a novel PCNA-binding protein". The Journal of Biological Chemistry.
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