Reduced DNA repair during differentiation of a myogenic cell line.

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Abstract
Repair synthesis induced by 4-nitroquinoline-1-oxide (4NQO) in rat myoblasts before and after cellular fusion was measured by [3H]thymidine incorporation into unreplicated DNA. The level of repair synthesis was reduced after the cells had fused into myotubes. The terminal addition of radioactive nucleotides into DNA strands occurred only to a minor extent, and the dilution of [3H]thymidine by intracellular nucleotide pools was not responsible for the observed difference in repair synthesis. Both the initial rate and the overall incorporation of [3H]thymidine were 50% lower in the myotubes. 4NQO treatment of myoblasts and myotubes induced modifications in the DNA which were observed as single-strand breaks during alkaline sucrose sedimentation. After the myoblasts were allowed a post-treatment incubation, most of the single-strand breaks were no longer apparent. In contrast, a post-treatment incubation of myotubes did not change the extent of single-strand breakage seen. Both myoblasts and myotubes were equally effective in repairing single-strand breaks induced by X radiation. When myoblasts fuse, a repair enzyme activity is lost, probably an endonuclease that recognizes 1 of the 4 NQO modifications of DNA. A partial loss of repair synthetic ability and a complete loss of ability to remove the modification that appears as a single-strand break in alkali resulted.