A gene that partially complements xeroderma pigmentosum group A cells maps to human chromosome 8

Splice site mutations in a xeroderma pigmentosum group A patient with delayed onset of neurological disease

XP12BE is a commonly studied XP-A cell line that exhibits slightly increased resistance to UV compared with the majority of XP-A cell lines. The elevated UV survival is common to a subset of XP-A cell lines and correlates with delayed onset of the neurological disease in patients. We identified the XPA mutations in XP12BE by single strand conformation polymorphism (SSCP) analyses and nucleotide sequencing. XP12BE is a compound heterozygote and both mutations affect mRNA splicing. One mutation is a G to C transversion within the splice donor site of intron 4 that is common to several cell lines from XP-A patients with delayed onset of neurological disease. The other mutation is a G to T transversion at the same position as a G to C transversion in the splice acceptor site of intron 3 that is common in Japanese XP-A patients. We also demonstrated the persistence of the XP12BE mutations in cell line 2-O-A2 which has been shown to express XPA protein. These results suggest that the intron 4 splice donor mutation likely produces some, at least partially functional, XPA protein that accounts for the increased UV survival of XP-A cell lines derived from patients with delayed onset of neurological disease.

Analysis of cells harboring a putative DNA repair gene reveals a lack of evidence for a second independent xeroderma pigmentosum group A correcting gene

The UV hypersensitivity of xeroderma pigmentosum (XP) complementation group A cells is restored to near-normal by transfection of the XPA gene located on human chromosome 9. However, it has been reported that a cosmid related to a cDNA on chromosome 8 is also able to partially correct the UV sensitivity of XP-A cells. We describe here an investigation of a representative cosmid transfectant, denoted 2-0-A2. Whole cell extracts prepared from 2-0-A2 cells carried out DNA repair synthesis in vitro that was in the normal range, consistent with their UV-resistant phenotype. Immunoblotting indicated that 2-0-A2 cells expressed full-length XPA protein. This was unexpected because the 2-0-A2 cell line was thought to have been isolated by transfection of a cell line derived from patient XP2OS, and a known homozygous mutation in XP2OS prevents expression of XPA gene product. This mutation creates an AlwNI restriction endonuclease cleavage site in XPA and was not present in 2-0-A2. These results prompted an RFLP analysis which revealed that the 2-0-A2 cell line was not derived from XP2OS but from another line that fails to express XPA protein, XP12BE. It appears that the significant UV-resistance and DNA repair capacity of 2-0-A2 can be ascribed to the re-expression of XPA in XP12BE, and it is unnecessary to postulate a second XP-A complementing gene to explain the results.

Mutation and expression of the XPA gene in revertants and hybrids of a xeroderma pigmentosum cell line

A series of ultraviolet (UV)-resistant cell lines have been generated from a UV-sensitive XP group A cell line homozygous for a stop codon (TGA) in the chromosome 9 XPA gene. Three lines generated by chemical mutagenesis acquired the ability to excise (6-4) photoproducts but not cyclobutane dimers from the whole genome; two lines generated by a fusion procedure with hamster cells acquired the ability to excise both (6-4) photoproducts and cyclobutane dimers from the whole genome. A central region of the hamster XPA gene was cloned and sequenced. With the use of species-specific primers in the polymerase chain reaction, we found that the hybrid cell lines do not contain a hamster XPA gene. Sequence analysis showed that all of the UV-resistant cell lines contain reversions of the human stop codon, resulting in missense mutations (glycine or leucine for arginine) or wild-type sequences. The concentration of XPA protein in revertant cell lines was about one-half that in normal cells, which would be expected from heterozygous cells; there was no evidence that the mutant proteins were less stable than the wild-type proteins. These results are consistent with the idea that the XPA protein initiates repair by binding to damaged sites with various affinities, depending on the photoproduct and the transcriptional state of the region. A concentration of XPA protein near 50% is needed before repair can proceed into nontranscribed regions of the genome. The revertant cell lines represent a class of missense mutations in the XPA gene that may have altered specificity and that can be used to understand some of the regulatory differences in repair of photoproducts in various regions of the genome.

Complementation of DNA repair defect in xeroderma pigmentosum cells of group C by the transfer of human chromosome 5

Complementation of DNA excision repair defect in xeroderma pigmentosum cells of group C (XP-C) has been achieved by the transfer of human chromosome 5. Individual human chromosomes tagged with a selectable marker were transferred to XP-C cells by microcell fusion from mouse-human hybrid cell lines each bearing a single different human chromosome. Analysis of the chromosome transfer clones revealed that introduction of chromosome 5 into XP-C cells corrected the DNA repair defect as well as UV-sensitive phenotypes, while chromosomes 2, 6, 7, 9, 13, 15, 17, and 21 failed to complement. The introduced chromosome 5 in complemented UVr clones was distinguished from the parental XP-C chromosomes by polymorphism for dinucleotide (CA)n repeats at two loci, D5S117 and D5S209. In addition, an intact marked chromosome 5 was rescued into mouse cells from a complemented UVr clone by microcell fusion. Five subclones of a complemented clone that had lost the marked chromosome 5 exhibited UV-sensitive and repair-deficient phenotypes identical to parental XP-C cells. Concordant loss of the transferred chromosome and reappearance of XP-C phenotype further confirmed the presence of a DNA repair gene on human chromosome 5.

A xeroderma pigmentosum complementation group A related gene: confirmation using monoclonal antibodies against the cyclobutane dimer and (6-4) photoproduct

Xeroderma pigmentosum complementation group A was partially complemented by a cosmid genomic clone containing a 42-kb human DNA insert selected with a cDNA clone that we obtained through cDNA competition between the repair-proficient and repair-deficient cell line. The relationship between these two clones was confirmed using PCR amplifications. The enhancement in DNA-repair capacity of the transformants was assessed with the monoclonal antibodies specific for cyclobutane dimers and (6-4) photoproducts and partially correct the xeroderma pigmentosum complementation group A defect. Furthermore, the level of the photoproduct-repair capacity is in agreement with the survival enhancement calculated from the D37 values. This gene was mapped to chromosome 8, suggesting that this may represent one of the defective gene(s) in xeroderma pigmentosum complementation group A.