Downregulation of Cockayne syndrome B protein reduces human 8-oxoguanine DNA glycosylase-1 expression and repair of UV radiation-induced 8-oxo-7,8-dihydro-2'-deoxyguanine.

Human 8-oxoguanine DNA glycosylase-1 (hOGG1) is the key DNA repair enzyme responsible for initiating repair of UV radiation-induced 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG). Previously we have shown that basal cells in human epidermis are particularly sensitive to UVA-mediated DNA damage probably due to low expression of hOGG1. Here we investigate some aspects of the regulatory role of Cockayne syndrome B (CSB) on hOGG1 expression and function. Cockayne syndrome B and hOGG1 genes were knocked down by miRNA technology in the HaCaT human keratinocyte cell line. Loss of the CSB gene decreased hOGG1 mRNA, and loss of hOGG1 increased CSB, indicating that they influence each other's expression. Protein levels were assessed in cells grown into engineered human skin using immunohistochemistry. This confirmed that CSB knockdown with miRNA reduced hOGG1 protein levels, but hOGG1 knockdown did not influence expression of CSB protein. Using comet assay we found that both hOGG1 and CSB knockdown reduced repair of both UVA- and UVB-induced 8-oxo-dG, consistent with CSB downregulation of hOGG1 mRNA and protein. In contrast, CSB but not hOGG1 knockdown reduced repair of UVB- and UVA-induced cyclobutane pyrimidine dimer photolesions. In engineered human skin, repair of UVA-induced 8-oxo-dG was inhibited by both hOGG1 and CSB knockdown, confirming the functional role of both proteins in cells with 3-D cellular contacts. These findings directly indicate that hOGG1 and CSB influence each other's expression. CSB is required for maintaining hOGG1 enzyme levels and function. Cockayne syndrome B could therefore be required for 8-oxo-dG repair due to its regulatory effect on hOGG1 expression. Cockayne syndrome B but not hOGG1 is also required for efficient repair of cyclobutane pyrimidine dimers. Cockayne syndrome B regulation of DNA repair could contribute to the effect of UVA in causing mutations that lead to skin cancer in humans.