Xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy: do the genes explain the diseases?

Xeroderma pigmentosum, Cockayne syndrome and trichothiodystrophy are three distinct human syndromes associated with sensitivity to ultraviolet radiation. We review evidence that these syndromes overlap with each other and arise from mutations in genes involved in nucleotide-excision repair and RNA transcription. Attempts have been made to explain the syndromes in terms of defects in repair and transcription. These two biochemical pathways do not easily account for all the features of the syndromes. Therefore, we propose a third pathway, in which the syndromes are due, in part, to defects in a demethylation mechanism involving the excision of methylated cytosine. Perturbation of demethylation could affect the developmentally regulated expression of some genes.