Increased methotrexate-induced DNA strand breaks and cytotoxicity following mutational loss of thymidine kinase.

The cytotoxicity and DNA lesions induced by methotrexate (MTX) were compared in wild-type, hypoxanthine-guanine phosphoribosyltransferase-deficient (HGPRT) and thymi-dine-kinase-deficient (TK) HL-60 cells. TK- and HGPRT cells were approximately 10 and 3 times more sensitive to MTX than wild-type cells, respectively. Following incubation with 2 mm MTX for 16 hr, TK- cells showed a significantly higher number of DNA strand breaks. Concomitantly, DNA fragmentation at the nucleosomal linker region was detected more prominently in TK- cells. Although MTX tended to decrease TTP pools similarly in all 3 cell types, the initial TTP level in TK- cells was only about one-fifth of that found in the wild type. These results indicate that the thymidine salvage pathway has a pivotal role in mediating MTX-induced toxicity and DNA lesions.