Two essential splice lariat branchpoint sequences in one intron in a xeroderma pigmentosum DNA repair gene: mutations result in reduced XPC mRNA levels that correlate with cancer risk

The lariat branch point sequence (BPS) is crucial for splicing of human nuclear pre-mRNA yet BPS mutations have infrequently been reported to cause human disease. Using an inverse RT–PCR technique we mapped two BPS to the adenosine residues at positions −4 and −24 in intron 3 of the human XPC DNA repair gene. We identified homozygous mutations in each of these BPS in two newly diagnosed Turkish families with the autosomal recessive disorder xeroderma pigmentosum (XP). Cells from two severely affected children in family A harbor a homozygous point mutation in XPC intron 3 (−9 T to A), located within the downstream BPS. Using a real-time quantitative reverse transcriptase–polymerase chain reaction (QRT–PCR) assay, these cells expressed no detectable (