Differential repair of the two major UV-induced photolesions in trichothiodystrophy fibroblasts.

Defects in nucleotide excision repair have been shown to be associated with the photosensitive form of the disorder trichothiodystrophy (TTD). Most repair-deficient TTD patients are mutated in the XPD gene, a subunit of the transcription factor TFIIH. Knowledge of the kinetics and efficiency of repair of the two major UV-induced photolesions in TTD is critical to understand the role of unrepaired lesions in the process of carcinogenesis and explain the absence of enhanced skin cancer incidence in TTD patients contrarily to the xeroderma pigmentosum D patients. In this study, we used different approaches to quantify repair of UV-induced cyclobutane pyrimidine dimers (CPD) and pyrimidine (6-4) pyrimidone photoproducts (6-4PP) at the gene and the genome overall level. In cells of two TTD patients, repair of CPD and 6-4PP was reduced compared with normal human cells, but the reduction was more severe in confluent cells than in exponentially growing cells. Moreover, the impairment of repair was more drastic for CPD than 6-4PP. Most notably, exponentially growing TTD cells displayed complete repair 6-4PP over a broad dose range, albeit at a reduced rate compared with normal cells. Strand-specific analysis of CPD repair in a transcriptional active gene revealed that TTD cells were capable to perform transcription-coupled repair. Taken together, the data suggest that efficient repair of 6-4PP in dividing TTD cells in concert with transcription-coupled repair might account for the absence of increased skin carcinogenesis in TTD patients.