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 m̈m 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.