Nucleic Acids Research, 2001, Vol. 29, No. 11 2234-2243
© 2001 Oxford University Press
Human DNA mismatch repair in vitro operates independently of methylation status at CpG sites
Department of Biology, Indiana University, Bloomington, IN 47405, USA and 1Fox Chase Cancer Center, 7701 Burholme Avenue, Philadelphia, PA 19111, USA
Whereas in Escherichia coli DNA mismatch repair is directed to the newly synthesized strand due to its transient lack of adenine methylation, the molecular determinants of strand discrimination in eukaryotes are presently unknown. In mammalian cells, cytosine methylation within CpG sites may represent an analogous and mechanistically plausible means of targeting mismatch correction. Using HeLa nuclear extracts, we conducted a systematic analysis in vitro to determine whether cytosine methylation participates in human DNA mismatch repair. We prepared a set of A·C heteroduplex molecules that were either unmethylated, hemimethylated or fully methylated at CpG sequences and found that the methylation status persisted under the assay conditions. However, no effect on either the time course or the magnitude of mismatch repair events was evident; only strand discontinuities contributed to strand bias. By western analysis we demonstrated that the HeLa extract contained MED1 protein, which interacts with MLH1 and binds to CpG-methylated DNA; supplementation with purified MED1 protein was without effect. In summary, human DNA mismatch repair operates independently of CpG methylation status, and we found no evidence supporting a role for CpG hemimethylation as a strand discrimination signal.
* To whom correspondence should be addressed. Tel: +1 812 856 4184; Fax: +1 812 855 6705; Email: jdrummon{at}bio.indiana.edu
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