Loss of intragenomic DNA repair heterogeneity with cellular differentiation

The influence of terminal differentiation on UV-induced DNA damage and its repair in transcriptionally active and inactive genomic sequences was investigated using the murine 3T3-T proadipocyte cell culture system. Actively cycling 3T3-T cells terminally differentiate into adipocytes after exposure to media containing platelet-depleted human plasma. Suitable DNA fragments were analyzed from four genes: beta-actin, adenosine deaminase, dihydrofolate reductase, and lipoprotein lipase. As a result of 3T3-T cell differentiation, lipoprotein lipase and beta-actin expression was modified, whereas adenosine deaminase and dihydrofolate reductase expression was not affected. A DNA fragment representing the transcriptionally inactive locus 70-38 was also evaluated. UV-induced cyclobutane pyrimidine dimers, detected as UV-specific endonuclease-sensitive sites, in each fragment increased linearly as a function of UV dose (0-20 J/m2) independently of gene expression or differentiation. Sequence-specific repair of dimers was measured in stem and terminally differentiated 3T3-T cells after UV irradiation (10 J/m2). For undifferentiated stem cells, the rate and extent of dimer repair was higher in the actively transcribed adenosine deaminase and dihydrofolate reductase genes than in the inactive lipoprotein lipase or 70-38 fragments, the greater difference being observed in the first 8 h post-UV irradiation. In contrast, similar dimer repair rates were found for each DNA fragment in terminally differentiated 3T3-T cells. These data suggest that cellular differentiation is accompanied by a loss of heterogeneity in intragenomic DNA repair.