Chromosomal modifications of a rat glioblastoma cell line during the acquisition and reversal of doxorubicin resistance.

We have studied the cytogenetic alterations occurring during the development and reversal of doxorubicin resistance in a clonal line of rat glioblastoma cells. We have observed during the acquisition of resistance an increase in the modal number of chromosomes, from 42 to 60, and the occurrence, in 90% of the mitoses, or large metacentric markers(s) which were infrequent in the sensitive line. This was associated with a net increase in total DNA amount per cell, from 5.3 to 8.3 pg. During reversal of resistance by 2 years culture without drug of the most resistant line, we observed a rapid decrease of the chromosome number as well as of the DNA content per cell; however, the large metacentric marker(s) were still present in 40% of the mitoses after 9 months of reversal, when the remaining resistance was only 4-fold. In situ hybridization of the chromosomes with a probe for the mdr gene revealed that the average number of stained chromosomes rose from 7% in the sensitive line to 38% in the most resistant line; however, only 9% of the silver grains were detected on the large metacentric markers. We conclude that important chromosome rearrangements occurred during the acquisition of resistance to doxorubicin, leading to a random distribution of the mdr gene in the genome.