Defective transcription-coupled repair in Cockayne syndrome B mice is associated with skin cancer predisposition

The Cockayne syndrome B protein, involved in transcription-coupled DNA repair, resides in an RNA polymerase II-containing complex

Transcription-coupled repair (TCR), a subpathway of nucleotide excision repair (NER) defective in Cockayne syndrome A and B (CSA and CSB), is responsible for the preferential removal of DNA lesions from the transcribed strand of active genes, permitting rapid resumption of blocked transcription. Here we demonstrate by microinjection of antibodies against CSB and CSA gene products into living primary fibroblasts, that both proteins are required for TCR and for recovery of RNA synthesis after UV damage in vivo but not for basal transcription itself. Furthermore, immunodepletion showed that CSB is not required for in vitro NER or transcription. Its central role in TCR suggests that CSB interacts with other repair and transcription proteins. Gel filtration of repair- and transcription-competent whole cell extracts provided evidence that CSB and CSA are part of large complexes of different sizes. Unexpectedly, there was no detectable association of CSB with several candidate NER and transcription proteins. However, a minor but significant portion (10-15%) of RNA polymerase II was found to be tightly associated with CSB. We conclude that within cell-free extracts, CSB is not stably associated with the majority of core NER or transcription components, but is part of a distinct complex involving RNA polymerase II. These findings suggest that CSB is implicated in, but not essential for, transcription, and support the idea that Cockayne syndrome is due to a combined repair and transcription deficiency.

Cockayne syndrome: defective repair of transcription?

In the past years, it has become increasingly evident that basal metabolic processes within the cell are intimately linked and influenced by one another. One such link that recently has attracted much attention is the close interplay between nucleotide excision DNA repair and transcription. This is illustrated both by the preferential repair of the transcribed strand of active genes (a phenomenon known as transcription-coupled repair, TCR) as well as by the distinct dual involvement of proteins in both processes. The mechanism of TCR in eukaryotes is still largely unknown. It was first discovered in mammals by the pioneering studies of Hanawalt and colleagues, and subsequently identified in yeast and Escherichia coli. In the latter case, one protein, the transcription repair-coupling factor, was found to accomplish this function in vitro, and a plausible model for its activity was proposed. While the E. coli model still functions as a paradigm for TCR in eukaryotes, recent observations prompt us to believe that the situation in eukaryotes is much more complex, involving dual functionality of multiple proteins.

Disruption of mouse RAD54 reduces ionizing radiation resistance and homologous recombination

Double-strand DNA break (DSB) repair by homologous recombination occurs through the RAD52 pathway in Saccharomyces cerevisiae. Its biological importance is underscored by the conservation of many RAD52 pathway genes, including RAD54, from fungi to humans. We have analyzed the phenotype of mouse RAD54-/- (mRAD54-/-) cells. Consistent with a DSB repair defect, these cells are sensitive to ionizing radiation, mitomycin C, and methyl methanesulfonate, but not to ultraviolet light. Gene targeting experiments demonstrate that homologous recombination in mRAD54-/- cells is reduced compared to wild-type cells. These results imply that, besides DNA end-joining mediated by DNA-dependent protein kinase, homologous recombination contributes to the repair of DSBs in mammalian cells. Furthermore, we show that mRAD54-/- mice are viable and exhibit apparently normal V(D)J and immunoglobulin class-switch recombination. Thus, mRAD54 is not required for the recombination processes that generate functional immunoglobulin and T cell receptor genes.