Gene diagnosis and establishment of mice model for xeroderma pigmentosum group A

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We have cloned a causative gene for group A xeroderma pigmentosum(XPA gene). The XPA cDNA has a single large open reading frame encoding hydrophilic protein of 273 amino acids with C4 type and H2C2 type zinc finger motifs. In order to examine DNA repair functions of the XPA protein, a recombinant XPA protein was produced in E.coli.DNA binding activities of the XPA protein were examined by gel mobility shift and filter binding assays. The XPA protein bound to DNA and more XPA protein bound to DNA damaged by ultraviolet light(UV), cisplatin, acetyl amino fluorene or osmium teraoxide than non damaged DNA.These results indicate that the XPA protein is involved in the damage-recognition step of nucleotide excision repair processes. On the other hand, we have found different types of mutations of the XPA gene in group A XP patients and executed gene diagnosis of group A XP on the basis of our results. Then, we have established XPA-knocked out mice by the targeting of the XPA gene in mouse ES cells. The primary fibroblasts derived from XPA-homozygous mice were hypersensitive to killing by UV and defective in nucleotide excision repair of DNA damaged by UV, while those from heterozygous mice were resistant as those from wild type mice. The XPA homozygous mice treated with 7, 12-Dimethylbenz(a)anthracene(DMBA), which forms bulky DNA adducts, developed severe skin ulcers one week after the treatment and tumors thereafter at high frequency. These results provide a direct evidence that the XPA protein protects mice from continued DNA damage elicited by a chemical carcinogen, and that the mice is a good animal model which reproduces tumor-proneness in group A XP.