GENE-SPECIFIC DNA-REPAIR IN TERMINALLY DIFFERENTIATING RAT MYOBLASTS

Preferential DNA repair in expressed genes has been well documented in proliferating mammalian cells following ultraviolet irradiation. It was of interest to learn whether excision repair is similarly selective in terminally differentiating cells. We have measured the removal of ultraviolet-induced cyclobutane pyrimidine dimers (detected as T4 endonuclease V-sensitive sites) from various genes in cultured L8 rat skeletal myoblasts. In these cells, the transcription of muscle-specific genes such as the embryonic myosin heavy chain (MHC(emb)) gene can be regulated by inducing cells to differentiate. L8 myoblasts are somewhat more sensitive than Chinese hamster ovary cells to ultraviolet radiation, and they exhibit relatively poor overall DNA-repair rates throughout differentiation. Irradiation severely reduces the rates of transcription and steady-state RNA levels for the genes studied. Although differences in kinetics are seen between the repair of active and inactive genes, repair rates are low relative to those previously measured in proliferating rodent cell lines. Repair efficiency in the MHC(emb) gene increases as it is activated during differentiation and, in fact, approaches 100% within 5 days, while that in the silent GAP43 gene is much lower. While repair efficiencies generally correlate with expression in the genes studied, the overall time course of repair appears to be prolonged in these cells compared to that in proliferating cells. These terminally differentiating cells seem to maintain a DNA damage surveillance and repair capacity for selected genes and/or genomic domains.