Excision repair in u.v. (254 nm) damaged non-dividing human skin fibroblasts: a major biological role for DNA polymerase alpha

Aphidicolin inhibits excision repair of UV-induced pyrimidine photodimers in low serum cultures of mitotic and mitomycin C-induced postmitotic human skin fibroblasts

The rates of formation and excision of UVC light-induced cyclobutane-type pyrimidine photodimers were determined in cultures of foreskin-derived normal human fibroblasts in mitotic (MF) and mitomycin-C (MMC)-induced postmitotic fibroblasts (PMF). Characteristic morphological changes support the notion that MMC accelerates the differentiation pathway from MF to PMF. In cultures treated with aphidicolin, I am able to show that this inhibitor of alpha and/or delta polymerases significantly inhibits the repair of pyrimidine photodimers in foreskin-derived mitotic and MMC-induced postmitotic fibroblasts in low serum cultures (0.5%) following UVC irradiation. Over the concentration range of 0-2 micrograms/ml of aphidicolin, there is a strong concentration-dependent inhibition of repair in cells treated with 10 J/m2 of UVC and incubated with aphidicolin during the post-incubation time (0-24 h). The results demonstrate that pyrimidine photodimers are repaired in low serum cultures by an alpha- and/or delta-polymerase-dependent pathway. These data also imply that the fibroblast differentiation system is a very useful tool to unravel the complex mechanisms of UV-induced DNA damage and repair.

Does genotypic sex have a direct effect on longevity?

Females of the human species live longer than males, and the longevity differential is probably not entirely explained by reasons which are presently obvious. Genotypic sex has long been suspected to affect longevity to the advantage of the female. Several recent findings about the X and Y chromosomes must be reckoned with in considering determinants of longevity which derive from genotypic sex. The advantages of having two X chromosomes are apparent, notwithstanding X-chromosome inactivation. Not only can some cells compensate for biosynthetic deficiencies of others, but also cell selection according to which X chromosome is active can occur during development according to cell viability and proliferative capacity. It has recently been observed that at least some genes on inactive X chromosomes are reactivated late in life. Details of the reactivation process must be studied to determine its significance and the effects of the process on late life survival. The recent mapping of the catalytic polypeptide of DNA-polymerase-alpha to the X chromosome calls attention to a new property of the genotype which could affect the basic ability of cells to proliferate. It is likely that this enzyme, perhaps in concert with DNA-polymerase-delta, is required for DNA replication, suggesting that two alleles for this enzyme and cell selection within the female phenotypic mosaic for DNA replication may provide a sex-linked determinant of cell proliferation which could be advantageous in late life. Much remains to be learned about the Y chromosome, although there are early results consistent with a determinant of longevity on that chromosome which operates to the male disadvantage and probably does not involve sex hormones. The genotype may be a significant determinant of longevity in humans even if it does not appear to be so in non-human animals, because causes of death are different. Determinants of longevity are based on susceptibility or vulnerability to the causes and diseases of mortality, and these differ in different species.