期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1998
卷号：95
期号：15
页码：8568-8573
DOI：10.1073/pnas.95.15.8568
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：We tested the ability of recombinant hMutS (hMSH2/hMSH6) and hMutS{beta} (hMSH2/hMSH3) heterodimers to complement the mismatch repair defect of HEC59, a human cancer cell line whose extracts lack all three MutS homologues. Although repair of both base/base mispairs and insertion-deletion loops was restored by hMutS, only the latter substrates were addressed in extracts supplemented with hMutS{beta}. hMutS was also able to complement a defect in the repair of base/base mispairs in CHO R and HL60R cell extracts. In these cells, methotrexate-induced amplification of the dihydrofolate reductase (DHFR) locus, which also contains the MSH3 gene, led to an overexpression of MSH3 and thus to a dramatic change in the relative levels of MutS and MutS{beta}. As a rule, MSH2 is primarily complexed with MSH6. MutS is thus relatively abundant in mammalian cell extracts, whereas MutS{beta} levels are generally low. In contrast, in cells that overexpress MSH3, the available MSH2 protein is sequestered predominantly into MutS{beta}. This leads to degradation of the partnerless MSH6 and depletion of MutS. CHO R and HL60R cells therefore lack correction of base/base mispairs, whereas loop repair is maintained by MutS{beta}. Consequently, frameshift mutations in CHO R are rare, whereas transitions and transversions are acquired at a rate two orders of magnitude above background. Our data thus support and extend the findings of Drummond et al. [Drummond, J. T., Genschel, J., Wolf, E. & Modrich, P. (1997) Proc. Natl. Acad. Sci. USA 94, 10144-10149] and demonstrate that mismatch repair deficiency can arise not only through mutation or transcriptional silencing of a mismatch repair gene, but also as a result of imbalance in the relative amounts of the MSH3 and MSH6 proteins.
