R. Hampson et al., MISMATCH REPAIR DEFECTS AND O-6-METHYLGUANINE-DNA METHYLTRANSFERASE EXPRESSION IN ACQUIRED-RESISTANCE TO METHYLATING AGENTS IN HUMAN-CELLS, The Journal of biological chemistry, 272(45), 1997, pp. 28596-28606
Fifteen variants with greater than or equal to 30-fold resistance to N
-methyl-N-nitrosourea were isolated from the Burkitt's lymphoma Raji c
ell line. Eight had received a single treatment with a highly cytotoxi
c dose. The remainder, including the previously described RajiF12 cell
line, arose following multiple exposures to initially moderate but es
calating doses. Surprisingly, methylation resistance arose in three cl
ones by reactivation of a previously silent O-6-methylguanine-DNA meth
yltransferase gene, Five clones, including RajiF12, displayed the micr
osatellite instability and increased spontaneous mutation rates at the
hypoxanthine-guanine phosphoribosyltransferase locus, consistent with
deficiencies in mismatch repair. Defects in either the hMutS alpha or
hMutL alpha mismatch repair complexes were identified in extracts of
these resistant clones by in vitro complementation using extracts from
colorectal carcinoma cell lines. Defects in hMutL alpha were confirme
d by Western blot analysis. Remarkably, five methylation-resistant clo
nes in which mismatch repair defects were demonstrated by biochemical
assays did not exhibit significant microsatellite instability.