DNA repair deficient photodermatoses.

Photosensitive genodermatoses associated with established defects of DNA repair currently include the autosomal recessive diseases xeroderma pigmentosum (XP), Cockayne's syndrome (CS), trichothiodystrophy (TTD), and Bloom's syndrome (BS). XP is a heterogeneous disorder associated with defective excision repair or daughter strand repair of ultraviolet (UV)-induced DNA damage. It is characterized by cutaneous and ocular abnormalities predominantly on sun-exposed sites and in some cases, neurological features resulting from progressive neuronal loss. Skin involvement includes easy sunburning, pigmentary abnormalities, telangiectasia, dryness, scarring, and susceptibility to multiple benign and malignant neoplasms. In CS, defective repair of actively transcribing DNA is clinically associated with acute photosensitivity, growth retardation, demyelinating neurological abnormalities, and pigmentary retinal degeneration, but without increased cancer susceptibility. TTD is characterized by sulphur-deficient brittle hair, variable growth delay, mental retardation, ichthyosis, and in some cases photosensitivity. Although in some patients there is a deficiency of DNA excision repair identical to that in certain xeroderma pigmentosum patients, no increased cancer risk is present in trichothiodystrophy. In BS, deficient cellular DNA ligase is associated with congenital telangiectasia, photosensitivity, growth retardation, immune deficiency, increased susceptibility to infection, and predominantly internal rather than cutaneous malignancy. Immunological factors may at least determine the varying susceptibility to malignancy of these conditions.