Similar defects in DNA repair and replication in the pigmented xerodermoid and the xeroderma pigmentosum variants

The "pigmented xerodermoid" was previously defined on the basis of mild clinical symptoms that suggested it might be similar to but distinct from xeroderma pigmentosum (XP). XP and pigmented xerodermoid cell cultures were irradiated with ultraviolet light and unscheduled DNA synthesis, strand breakage during repair, chain growth during semiconservative DNA replication with or without caffeine, and the recovery of DNA replication were determined. It is concluded that a pigmented xerodermoid cell culture is indistinguishable from the XP variant and the former term is therefore redundant. The defect common to these cell types appears to be the loss of a gene product that permits normal cells to replicate DNA without interruption at damaged sites (u.v.-induced pyrimidine dimers). The consequence of this loss is that replication forks are blocked more frequently and at lower doses in XP variant cells. The correlation between this defect and high levels of actinic carcinogenesis in these patients points to an important role for perturbations in DNA replication in human carcinogenesis.