Mutations in XPB and XPD helicases found in xeroderma pigmentosum patientsimpair the transcription function of TFIIH

As part of TFIIH, XPB and XPD helicases have been shown to play a role in n ucleotide excision repair (NER), Mutations in these subunits are associated with three genetic disorders: xeroderma pigmentosum (XP), Cockayne syndrom e (CS) and trichothiodystrophy (TTD), The strong heterogeneous clinical fea tures observed in these patients cannot be explained by defects in NER alon e. We decided to look at the transcriptional activity of TFIIH from cell li nes of XP individuals. We set up an immunopurification procedure to isolate purified TFIIH from patient cell extracts. We demonstrated that mutations in two XP-B/CS patients decrease the transcriptional activity of the corres ponding TFIIH by preventing promoter opening. The defect of XPB in transcri ption can be circumvented by artificial opening of the promoter, Western bl ot analysis and enzymatic assays indicate that XPD mutations affect the sto ichiometric composition of TFIIH due to a weakness in the interaction betwe en XPD-CAK complex and the core TFIIH, resulting in a partial reduction of transcription activity, This work, in addition to clarifying the role of th e various TFIIH subunits, supports the current hypothesis that XP-B/D patie nts are more likely to suffer from transcription repair syndromes rather th an DNA repair disorders alone.