Association of Increased Dihydrofolate Reductase Levels and Chromosome Alteration in Amethopterin-Resistant Sublines of Leukemia L12102

Abstract

Dihydrofolate reductase activity in ascites cells from mice bearing various strains and sublines of Leukemia L1210 was correlated with chromosomal analyses and drug resistance. A parental strain, L1210 (V), which exhibited a subtelocentric chromosome and a specific enzyme activity of 100 to 200, and several drug-resistant sublines selected from it by appropriate treatment were studied. There was no elevation in enzyme activity in a 6-mercaptopurine-resistant subline, L1210/MP (III), which was collaterally sensitive to amethopterin and which retained the subtelocentric chromosome. An amethopterin-resistant subline, L1210/A (IV), and an azaserine-resistant subline, L1210/AZAS (XII), cross-resistant to amethopterin, exhibited eightfold to 12-fold increases in enzyme level and lacked the subtelocentric chromosome. Sublines L1210/A/MP/FU (XVI₂ and XVI₄), selected by simultaneous treatment with amethopterin, 6-mercaptopurine, and 5-fluorouracil and resistant to these drugs, retained the subtelocentric chromosome for many transfer generations and showed no increase in enzyme activity. In two branches of L1210/A/MP/FU (XVI₂) the subtelocentric chromosome disappeared and enzyme activity increased after about 60 generations. Neither change occurred in a third branch within 93 generations. Thus, while disappearance of the subtelocentric chromosome was associated with elevated dihydrofolate reductase activity in several sublines derived from strain L1210 (V), there was no obligatory correlation between amethopterin resistance and increased enzyme. A second parental strain of Leukemia L1210 had no subtelocentric chromosome, though treatment with amethopterin selected resistant sublines with increased enzyme activity. The association between the chromosomal alteration and enzyme activity was, therefore, specific only for certain sublines of L1210 and not a general characteristic of this tumor.