Less repair of pyrimidine dimers and single-strand breaks in genes by Scidcells

Severe combined immunodeficient (Scid) mice have a mutation in the catalyti c subunit of the DNA binding protein kinase that is involved in repair of d ouble-strand breaks in DNA, To determine if the protein also influences rep air of single-strand breaks, we examined the ability of Scid cells to repai r lesions introduced by ultraviolet light and gamma-ray irradiation. DNA re pair was measured both in total genomic DNA and in specific genes from muri ne Scid and wildtype fibroblast cell lines. The removal of pyrimidine dimer s and repair of strand breaks in genes was measured using quantitative Sout hern blot analyses. After ultraviolet irradiation, there was no significant difference in the repair of photoproducts in bulk DNA between Scid and wil dtype cells, as measured by cellular survival and unscheduled DNA synthesis . However, deficient repair was evident in genes, where Scid cells had 25-5 0% less repair in the c-myc and dihydrofolate reductase genes. After gamma- irradiation, Scid fibroblasts had 20-35% less repair of DNA breaks in immun oglobulin kappa and heavy constant genes than wildtype cells. The data sugg est that intact DNA-PK enzyme is needed for the efficient operation of cell ular repair of pyrimidine dimers and single-strand breaks in genes, as well as in its established role in rejoining double-strand breaks. (C) 1999 Aca demic Press.