Sublimiting concentration of TFIIH transcription/DNA repair factor causes TTD-A trichothiodystrophy disorder

Abstract

The repair-deficient form of trichothiodystrophy (TTD) most often results from mutations in the genes XPB or XPD, encoding helicases of the transcription/repair factor TFIIH. The genetic defect in a third group, TTD-A, is unknown, but is also caused by dysfunctioning TFIIH. None of the TFIIH subunits carry a mutation and TFIIH from TTD-A cells is active in both transcription and repair. Instead, immunoblot and immunofluorescence analyses reveal a strong reduction in the TFIIH concentration. Thus, the phenotype of TTD-A appears to result from sublimiting amounts of TFIIH, probably due to a mutation in a gene determining the complex stability. The reduction of TFIIH mainly affects its repair function and hardly influences transcription.

A new, tenth subunit of TFIIH is responsible for the DNA repair syndrome trichothiodystrophy group A

“…Moreover, TFIIH isolated from cells of individuals with TTD-A had normal in vitro enzymatic activities and transiently restored the NER defect in these cells, suggesting that TFIIH was qualitatively not or only mildly defective. The total cellular TFIIH content of cells from individuals with TTD-A seemed to be very low 1 . The reduced steady-state levels of TFIIH are probably a result of complex fragility due to a mutation in an unknown gene 15 .…”

“…In a third form of DNA repair-deficient TTD, called group A, none of the nine subunits encoding TFIIH carried mutations; instead, the steady-state level of the entire complex was severely reduced 1 Hereditary mutations in the repair and transcription factor TFIIH are associated with three photo-hypersensitive syndromes: xeroderma pigmentosum, xeroderma pigmentosum combined with Cockayne syndrome (a neurodevelopmental disorder) and the Cockayne syndrome-like brittle-hair disease TTD 3,4 (Table 1). TFIIH has a central role in transcription of RNA polymerase I and II, nucleotide excision repair (NER) and transcription-coupled repair (TCR) 5 .…”

“…Cells from individuals with TTD-A are only mildly UV-sensitive despite having low UV-induced DNA repair synthesis (UDS). We previously found that treatment with highly purified TFIIH corrected the NER defect of cells from individuals with TTD-A, but none of the nine genes encoding TFIIH were mutated in these individuals 1 . Moreover, TFIIH isolated from cells of individuals with TTD-A had normal in vitro enzymatic activities and transiently restored the NER defect in these cells, suggesting that TFIIH was qualitatively not or only mildly defective.…”

TFIIH-dependent MMP-1 overexpression in trichothiodystrophy leads to extracellular matrix alterations in patient skin

“…The genes responsible for the photosensitive form of TTD encode the XPB, XPD, and p8/TTDA subunits of the general transcription factor IIH (TFIIH). All of the mutations associated with TTD result in reduced cellular levels (8,9) and impaired functioning of TFIIH in NER and basal transcription (10)(11)(12). Furthermore, they may interfere with the role of TFIIH in transcription regulation (13)(14)(15)(16).…”

From proteomics to disease

“…Measurements of TFIIH showed low levels of this complex in cells from individuals with TTD-A 13,14 . But exhaustive sequencing of each of the nine known TFIIH components detected no mutations, and expression of each of the purified TFIIH proteins did not correct the cellular defect 13 .…”