Molecular Dissection of Human DNA Excision Repair Genes

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1.Using yeast two hybrid system, we cloned a gene designated XAB2 which encodes new XPA binding protein. The binding of XAB2 to XPA was confirmed by in vitro pull-down assay using GST-XPA and in vitro translated XAB2. Moreover, the XAB2 was co-immunoprecipitated with XPA from HeLa cell extract by anti-XPA antibody, and the XPA was co-immunoprecipitated with XAB2 by anti-XAB2 antibody, confirming the association of XPA and XAB2 in the cells. We also found that the XAB2 associates with CSA and CSB.Since CSA and CSB have a defect in basal transcription or transcription coupled repair, it was strongly suggested that the XAB2 is involved in this mechanism (Tanaka, Wood and Hoeijmakers).2.We established the XPA-or CSB-deficient mice, but they did not show any significant physical abnormalities or pathological alterations by themselves although they are dificient in nucleotide excision repair and sensitive to UV.However, XPA/CSB double knockout mice were small and died in 20 days after birth. This result suggests that XPA and CSB belong to different epistasis groups (Tanaka and Hoeijmakers).3.We found that XPC associates with HHR23B,a human homolog of yeast RAD23 and that the HHR23B and its homolog HHR23A are indispensable to basic process of uncleotide excision repair (Hanaoka, Wood and Hoeijmakers).4.Human and mouse homologs of photoreactivating enzyme gene have been cloned. Gene targeting experiment to establish the mice which are deficient in this gene is in progress (Yasui and Hoeijmakers).