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Lethality in yeast of trichothiodystrophy (TTD) mutations in the human xeroderma pigmentosum group D gene.
S N Guzder1, P Sung, S Prakash
1Sealy Center for Molecular Science, University of Texas Medical Branch, Galveston 77555-1061, USA.
Abstract:
Mutations in the human XPD gene result in a defect in nucleotide excision repair of ultraviolet damaged DNA and cause the cancer-prone syndrome xeroderma pigmentosum (XP). Besides XP, mutations in XPD can cause another seemingly unrelated syndrome, trichothiodystrophy (TTD), characterized by sulfur-deficient brittle hair, ichthyosis, and physical and mental retardation. To ascertain the underlying defect responsible for TTD, we have expressed the TTD mutant proteins in the yeast Saccharomyces cerevisiae and determined if these mutations can rescue the inviability of a rad3 null mutation. RAD3, the S. cerevisiae counterpart of XPD, is required for nucleotide excision repair and also has an essential role in RNA polymerase II transcription. Expression of the wild type XPD protein or the XPD Arg-48 protein carrying a mutation in the DNA helicase domain restores viability to the rad3 null mutation. Interestingly, the XPD variants containing TTD mutations fail to complement the lethality of the rad3 null mutation, strongly suggesting that TTD mutations impair the ability of XPD protein to function normally in RNA polymerase II transcription. From our studies, we conclude that XPD DNA helicase activity is not essential for transcription and infer that TTD mutations in XPD result in a defect in transcription.
Insights
Mutations in the XPD gene cause xeroderma pigmentosum (XP) and trichothiodystrophy (TTD). TTD mutations impair XPD
Area of Science:
- Molecular Biology
- Genetics
- Biochemistry
Background:
- Mutations in the human XPD gene are linked to xeroderma pigmentosum (XP), a cancer-prone syndrome involving DNA repair defects.
- XPD gene mutations also cause trichothiodystrophy (TTD), a syndrome with brittle hair, ichthyosis, and developmental issues.
Purpose of the Study:
- To investigate the molecular basis of TTD by examining the function of TTD-associated XPD mutations.
- To determine if TTD mutations affect the essential roles of the XPD protein in DNA repair and transcription.
Main Methods:
- Expressed human XPD proteins, including wild-type and TTD mutants, in yeast Saccharomyces cerevisiae.
- Assessed the ability of expressed XPD proteins to rescue the lethality of a rad3 null mutation, where RAD3 is the yeast homolog of XPD.
Main Results:
- Wild-type XPD and a DNA helicase domain mutant (Arg-48) restored viability to rad3 null yeast, indicating functional DNA repair or transcription.
- XPD variants with TTD mutations failed to rescue the rad3 null mutation, suggesting a functional defect.
Conclusions:
- TTD mutations in the XPD gene impair its function in RNA polymerase II transcription.
- XPD's DNA helicase activity is not essential for transcription, but TTD-associated mutations disrupt the transcription role.