Methotrexate-inducfd amplification of the human DHHFR/MSH3 region alters the ratio of mutsd and mutsβ and reduces the efficacy of base-base mismatch repair
We examined the expression and fate of human MSH3 in the promyelocytic leukemia cell lines HI.-60 and HL-60R, where HL-60R is resistant to methotrexate by virtue of a DNA amplification event that includes the dihydrolblate reductase (DHFR) gene. This gene amplification directly impacts post-replication repair because human MSH3 is divergently transcribed from the same promoter region as the DHFR gene, and we find that MSH3 is also overexpressed. Nuclear extracts from HL-60 and HL-60R cells were subjected to an identical, rapid purification protocol that captures both heterodimeric hMutSa (MSH2/MSH6) and hMutSfi (MSH2/MSH3). In HL-60 cells the hMutSa to hMutS ratio is roughly 6:1, while in the methotrexate-resistam HL-60R cells, the ratio reverses to <1:100, where MSH2 is now complexed primarily with MSH3. Purified hMutSa and hMutSβ display partial overlap in mismatch repair specificity in complementation assays, where both participate in the repair of a dinucleotide insertion mismatch but only hMutSoc participates in base-base repair. We propose that treatment of tumor cells with methotrexate is likely to produce an unintended alteration in the mismatch repair profile of these cells in vivo, potentially generating hypermutable derivatives that may become more aggressive or resistant to other drugs by virtue of this repair detect. This work was supported by the Howard Hughes Medical Institute.
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