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Cytogenetics of Mouse Leukemia L1210 I. Association of a Specific Chromosome with Dihydrofolate Reductase Activity in Amethopterin-treated Sublines^1,2,

( Division of Experimental Chemotherapy, Sloan-Kettering Institute for Cancer Research, and Sloan-Kettering Division, Graduate School of Medical Sciences, Cornell University Medical College, New York, New York)

Sublines initiated from small numbers of parental L1210 cells, where the chance of subtelocentric lacking variant cells being present was very small, were treated continually with amethopterin (Methotrexate) to which they became resistant. Chromosomal examinations and assays of dihydrofolate reductase activity were carried out at frequent intervals. In contrast to previous results the subtelocentric chromosomes did not disappear; rather, in 10 out of 18 sublines these bi-armed marker chromosomes were altered in some proportion of the population. Enzyme activity became elevated in all sublines, even where there was no detectable chromosomal alteration.

These data further indicated a relationship between the configuration of the subtelocentric chromosome characteristic of the L1210 cell and the enzyme dihydrofolate reductase. Mutagenic action of amethopterin was suggested.

¹ A preliminary report of these studies was presented at a meeting of the American Association for Cancer Research at Toronto, Canada, on May 23, 1963.

² This investigation was supported in part by grants CA 03192-07S1 and 1 SO FR 05226-01 from the National Institutes of Health, USPHS, and by grant T-107 from the American Cancer Society.

³ USPHS Research Career Development Awardee Grant 1-K3-CA-5275-01.

Received 7/31/64. Revised 10/22/64.