Codominance associated with overexpression of certain XPD mutations

Mutations in the XPD gene are associated with three complex clinical phenot ypes, namely xeroderma pigmentosum (XP), XP in combination with Cockayne sy ndrome (XP-CS), and trichothiodystrophy (TTD). XP is caused by a deficiency in nucleotide excision repair (NER) that results in a high risk of skin ca ncer. TTD is characterized by severe developmental and neurological defects , with hallmark features of brittle hair and scaly skin, and sometimes has defective NER. We used CHO cells as a system to study how specific mutation s alter the dominant/recessive behavior of XPD protein. Previously we ident ified the T46I and R75W mutations in two highly UV-sensitive hamster cell l ines that were reported to have paradoxically high levels of unscheduled DN A synthesis, Here we report that these mutants have greatly reduced XPD hel icase activity and fully defective NER in a cell-extract excision assay. We conclude that the unscheduled DNA synthesis seen in these mutants is cause d by abortive "repair" that does not contribute to cell survival. These mut ations, as well as the K48R canonical helicase-domain mutation, each produc ed codominant negative phenotypes when overexpressed in wild-type CHO cells . The common XP-specific R683W mutation also behaved in a codominant manner when overexpressed, which is consistent with the idea that this mutation m ay affect primarily the enzymatic activity of the protein rather than impai ring protein interactions, which may underlie TTD. A C-terminal mutation un iquely found in TTD (R722W) was overexpressed but not to levels sufficientl y high to rigorously test for a codominant phenotype. Overexpression of mut ant XPD alleles may provide a simple means of producing NER deficiency in o ther cell lines. (C) 2001 Elsevier Science B.V. All rights reserved.