A disease-associated XPA allele interferes with TFIIH binding and primarily affects transcription-coupled nucleotide...

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Xeroderma pigmentosum (XP) is a genetic disorder characterized by extreme UV sensitivity and elevated skin cancer risk due to mutations in DNA repair genes.
• XPA (xeroderma pigmentosum A) is a crucial scaffold protein coordinating DNA repair pathways: global genome nucleotide excision repair (GG-NER) and transcription-coupled nucleotide excision repair (TC-NER).

Purpose of the Study:

• To investigate the molecular mechanisms and clinical consequences of a novel homozygous H244R substitution in the XPA gene.
• To elucidate the specific impact of the XPA-H244R mutation on GG-NER and TC-NER subpathways and its correlation with patient phenotypes.

Main Methods:

• Analysis of patient-derived fibroblasts and reconstituted knockout cells with the XPA-H244R mutation.
• Assessment of protein-protein interactions, specifically XPA's binding to the transcription factor IIH (TFIIH) complex.
• Quantification of DNA repair efficiency (GG-NER and TC-NER) and unscheduled DNA synthesis following UV irradiation.

Main Results:

• The XPA-H244R mutation severely weakens the interaction between XPA and TFIIH, impairing downstream NER complex assembly.
• Cells with XPA-H244R exhibit intermediate UV sensitivity with ~50% residual GG-NER but are highly sensitive to transcription-blocking DNA damage.
• TC-NER is severely deficient in XPA-H244R cells, with no detectable transcription recovery post-UV and absent TC-NER-associated unscheduled DNA synthesis.

Conclusions:

• The XPA-H244R mutation primarily disrupts the TC-NER pathway due to impaired TFIIH binding, while GG-NER remains partially functional.
• This pathway-specific defect explains the dominant neurological features observed in patients with this mutation.
• The C-terminus of XPA plays a critical role in TC-NER, highlighting its distinct function from GG-NER.