Human xeroderma pigmentosum group D gene encodes a DNA helicase

XERODERMA pigmentosum (XP), a genetically heterogeneous human disease, results from a defect in nucleotide excision repair of ultraviolet-damaged DNA. XP patients are extremely sensitive to sunlight and suffer from a high incidence of skin cancers. Cell fusion studies have identified seven XP complementation groups, A-G^1-3. Group D is of particular interest as mutations in this gene can also cause Cockayne's syndrome and trichothiodystrophy⁴. The XPD gene was initially named ERCC2 (excision repair cross complementing) as it was cloned using human DNA to complement the ultraviolet sensitivity of a rodent cell line⁵. We have purified the XPD protein to near homogeneity and show that it possesses single-stranded DNA-dependent ATPase and DNA helicase activities. We tested whether XPD can substitute for its yeast counterpart RAD3, which is essential for excision repair and for cell viability⁶. Expression of the XPD gene in Saccharomyces cerevisiae can complement the lethality defect of a mutation in the RAD3 gene⁶, suggesting that XPD is an essential gene in humans.