The xeroderma pigmentosum group D (XPD) gene: one gene, two functions, three diseases

DNA, the genetic material in all living organisms, is continually exposed to agents that cause damage to its structure, resulting in the loss of vital genetic information. To counteract the potentially devastating effects of such damage, all organisms have evolved a series of different repair processes, with which many kinds of damage to the DNA can be corrected. The importance of DNA repair is shown by the existence of several human genetic disorders that are caused by defects in one of these repair processes. Thus, for example, most individuals with xeroderma pigmentosum (XP) are unable to repair damage generated in DNA by ultraviolet (UV) light from the sun, whereas patients with hereditary nonpolyposis colon carcinoma are defective in the repair of mismatched bases.

XP was the first DNA-repair disorder to be identified. It is a rare autosomal recessive genetic disorder characterized by numerous skin abnormalities ranging from excessive freckling to multiple skin cancers (Fig. 1a) (Bootsma et al. 1998). The incidence of skin cancer is about 2000-fold greater than in normal individuals. All skin abnormalities result from exposure to sunlight and are caused by inability to repair DNA damage induced in the skin by sunlight. The more severely affected patients have neurological abnormalities caused by premature neuronal death. Cells from XP donors are hypersensitive to killing by UV irradiation, and this is caused, in the majority of cases, by defects in nucleotide excision repair (NER), the process with which UV-induced photoproducts in the DNA are removed and replaced (Friedberg et al. 1995). XP is genetically heterogeneous. There are eight complementation groups designated XP-A through G and XP-variant. The XPD gene, defective in XP individuals assigned to the XP-D complementation group, is the topic of this review.