XPC branch-point sequence mutations disrupt U2 snRNP binding, resulting in abnormal pre-mRNA splicing in xeroderma pigmentosum patients

Mutations in two branch-point sequences (BPS) in intron 3 of the XPC DNA repair gene affect pre-mRNA splicing in association with xeroderma pigmentosum (XP) with many skin cancers (XP101TMA) or no skin cancer (XP72TMA), respectively. To investigate the mechanism of these abnormalities we now report that transfection of minigenes with these mutations revealed abnormal XPC pre-mRNA splicing that mimicked pre-mRNA splicing in the patients' cells. DNA oligonucleotide-directed RNase H digestion demonstrated that mutations in these BPS disrupt U2 snRNP-BPS interaction. XP101TMA cells had no detectable XPC protein but XP72TMA had 29% of normal levels. A small amount of XPC protein was detected at sites of localized ultraviolet (UV)-damaged DNA in XP72TMA cells which then recruited other nucleotide excision repair (NER) proteins. In contrast, XP101TMA cells had no detectable recruitment of XPC or other NER proteins. Post-UV survival and photoproduct assays revealed greater reduction in DNA repair in XP101TMA cells than in XP72TMA. Thus mutations in XPC BPS resulted in disruption of U2 snRNP-BPS interaction leading to abnormal pre-mRNA splicing and reduced XPC protein. At the cellular level these changes were associated with features of reduced DNA repair including diminished NER protein recruitment, reduced post-UV survival and impaired photoproduct removal.

Reduced XPC DNA repair gene mRNA levels in clinically normal parents of xeroderma pigmentosum patients

Xeroderma pigmentosum group C (XP-C) is a rare autosomal recessive disorder. Patients with two mutant alleles of the XPC DNA repair gene have sun sensitivity and a 1000-fold increase in skin cancers. Clinically normal parents of XP-C patients have one mutant allele and one normal allele. As a step toward evaluating cancer risk in these XPC heterozygotes we characterized cells from 16 XP families. We identified 15 causative mutations (5 frameshift, 6 nonsense and 4 splicing) in the XPC gene in cells from 16 XP probands. All had premature termination codons (PTC) and absence of normal XPC protein on western blotting. The cell lines from 26 parents were heterozygous for the same mutations. We employed a real-time quantitative reverse transcriptase-PCR assay as a rapid and sensitive method to measure XPC mRNA levels. The mean XPC mRNA levels in the cell lines from the XP-C probands were 24% (P<10(-7)) of that in 10 normal controls. This reduced XPC mRNA level in cells from XP-C patients was caused by the PTC that induces nonsense-mediated mRNA decay. The mean XPC mRNA levels in cell lines from the heterozygous XP-C carriers were intermediate (59%, P=10(-4)) between the values for the XP patients and the normal controls. This study demonstrates reduced XPC mRNA levels in XP-C patients and heterozygotes. Thus, XPC mRNA levels may be evaluated as a marker of cancer susceptibility in carriers of mutations in the XPC gene.

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 (<0.1%) normal XPC message. Instead they expressed an XPC mRNA isoform with deletion of exon 4 that has no DNA repair activity in a host cell reactivation (HCR) assay. In contrast, in cells from three mildly affected siblings in family B, the BPS adenosine located at the -24 position in XPC intron 3 is mutated to a G. Real-time QRT-PCR revealed 3-5% of normal XPC message. These cells from family B had a higher level of HCR than cells from the severely affected siblings in family A, who had multiple skin cancers. Mutations identified in two BPS of the XPC intron 3 resulted in alternative splicing that impaired DNA repair function, thus implicating both of these BPS as essential for normal pre-mRNA splicing. However, a small amount of normal XPC mRNA can provide partial protection against skin cancers.

Noncoordinate regulation of the mRNAs encoding cytochromes P-450BNF/MC-B and P-450ISF/BNF-G

The mRNAs encoding the major polycyclic aromatic hydrocarbon-induced cytochromes P-450 from rat, P-450BNF/MC-B and P-450ISF/BNF-G, were characterized using three classes of recombinant plasmids: those complementary to (a) only P-450BNF/MC-B mRNA, (b) only P-450ISF/BNF-G mRNA, and (c) both mRNAs. These classes were identified by hybridization-selected translation and immunoprecipitation using six monoclonal and polyclonal antibodies and were later sequenced to confirm their identity and specificity. These findings indicated that the mRNAs encoding these two P-450s have regions that are unique, as well as regions that are homologous. Hybridization-selected translation also showed that the primary in vitro translation products of the P-450BNF/MC-B and P-450ISF/BNF-G mRNAs are 55 and 52 kDa, respectively, and have both unique and common structural characteristics that can be distinguished immunologically. By Northern hybridization, the P-450BNF/MC-B mRNA was found to be 2900 bases long, while the P-450ISF/BNF-G mRNA was 2100 bases long. Precursors of 3500 and 5200 bases were detected for P-450BNF/MC-B mRNA, while a 3100-base precursor was detected for P-450ISF/BNF-G mRNA. These two mRNAs were induced by beta-naphthoflavone, isosafrole, and 3-methylcholanthrene, but not by phenobarbital. In untreated rats, the P-450BNF/MC-B mRNA was consistently present at very low levels while the P-450ISF/BNF-G mRNA was present in variable amounts, suggesting that the latter mRNA can be induced by dietary or other environmental factors. The kinetics of induction of the P-450BNF/MC-B and P-450ISF/BNF-G mRNAs were measured by dot blot hybridization. P-450BNF/MC-B mRNA increased rapidly, reaching half-maximum by 4 h after treatment with 3-methylcholanthrene, while the P-450ISF/BNF-G mRNA increased more slowly, reaching half-maximum after 12 h. The levels of both mRNAs peaked at 24 h, but decreased thereafter at different rates; P-450BNF/MC-B mRNA dropped by about 20% during the next 24 h, while P-450ISF/BNF-G mRNA dropped by 50 to 70%. These differences in the kinetics of induction and the apparent stabilities of the P-450BNF/MC-B and P-450ISF/BNF-G mRNAs, in conjunction with the observed differences in their levels in untreated rats, suggested that these two mRNAs were not coordinately regulated even though they were induced by the same compounds.(ABSTRACT TRUNCATED AT 400 WORDS)