A study of chromosomal changes associated with amplified dihydrofolate reductase genes in rat hepatoma cells and their dedifferentiated variants.

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Abstract
The karyological consequences of dihydrofolate reductase gene amplification was examined in a series of 6 rat hepatoma cell lines, all derived from the same clone. Cells of 3 of these lines express a series of liver-specific functions whereas those of 3 others fail to express these functions. Cells of each line have been subjected to stepwise selection for methotrexate resistance and, in most cases, resistance is associated with a 40-50-fold amplification of sequences hybridizing to a dihydrofolate reductase cDNA probe. In 1 line no modified chromosome is observed, whereas in 2 others the amplified genes are associated with an expanded chromosomal region. R-banding analysis of these karyotypes showed that few changes have occurred. These observations apply to 2 of the well-differentiated lines, and to a variant able to revert to the differentiated state. In contrast, in the 2 stably dedifferentiated hepatoma cell lines, amplified dihydrofolate reductase genes are found on large chromosomes of variable size, on ring chromosomes and on chromosomes containing terminal, median or multiple centromeres. The nature of the chromosomal changes associated with dihydrofolate reductase gene amplification are the result of differences in cell lines rather than in the protocols employed for selection.