Activation of the Fanconi Anemia/BRCA Pathway and Recombination Repair in the Cellular Response to Solar Ultraviolet Light

Abstract

Recombination repair plays an important role in the processing of DNA double-strand breaks (DSB) and DNA cross-links, and has been suggested to be mediated by the activation of the Fanconi anemia (FA)/BRCA pathway. Unlike DNA damage generated by ionizing radiation or DNA crosslinking, UV light-induced DNA damage is not commonly thought to require recombination for processing, as UV light does not directly induce DSBs or DNA cross-links. To elucidate the role of recombination repair in the cellular response to UV, we studied the FA/BRCA pathway in primary skin cells exposed to solar-simulated light. UV-induced monoubiquitination of the FANCD2 protein and formation of FANCD2 nuclear foci confirmed the activation of the pathway by UV light. This was only observed when cells were irradiated during S phase and was not caused by directly UV-induced DSBs. UV-exposed cells did not exhibit FANCD2 nuclear foci once they entered mitosis or when growth-arrested. In addition, UV-induced nuclear foci of the recombination proteins, RAD51 and BRCA1, colocalized with FANCD2 foci. We suggest that in response to UV light, when nucleotide excision repair failed to repair, or when translesional DNA synthesis failed to bypass UV-induced DNA photoproducts, the FA/BRCA pathway mediates the recombination repair of replication forks stalled at DNA photoproducts as a third line of defense. (Cancer Res 2006; 66(23): 11140-7)

No Formation of DNA Double-Strand Breaks and No Activation of Recombination Repair with UVA

“…As demonstrated previously (Dunn et al, 2006), physiological doses of SSL (containing 100, 200, or 300 J m À2 UVB) induces time-and dose-dependent monoubiquitination of the FANCD2 protein (Figure 1a Fibroblasts from patients with FANCA or FANCC were found not to be hypersensitive to UVB or to UVA (Supplementary Figure S1 online). As these cells are unable to ubiquitinate FANCD2 and to activate the FA/BRCA pathway in response to any stimulus, this indicates that a lack of FA/BRCA pathway activation does not increase acute toxicity of either UVB or UVA.…”

“…As demonstrated previously (Dunn et al, 2006), a physiological dose of both SSL (containing 200 J m À2 UVB) and IR (10 Gy) induces nuclear foci of g-H2AX, although in a very different time-course and pattern. UVB-induced foci, in contrast to IR-induced ones, are only seen in a subset of cells and only with a delay of 45 minutes.…”

“…We recently reported activation of this pathway by UVB (Dunn et al, 2006). However, we provided evidence that this FA/BRCA pathway activation by UVB was not due to formation of DNA DSBs, but can be attributed to the presence and processing of stalled replication forks (Dunn et al, 2006).…”

“…They are cytotoxic, contributing to genomic instability by promoting formation of deletions, insertions, or more complex mutations, and are repaired through homologous recombination or non-homologous end joining. We recently reported evidence that UVB, in contrast to ionizing radiation (IR), does not induce DNA DSBs, either directly or indirectly as repair intermediates (Dunn et al, 2006). Exposure to UVA generates singlet oxygen and may also generate oxygen radicals, including the hydroxyl radical through a Fenton reaction with nuclear-based metals (Darr and Fridovich, 1994;Petersen et al, 2000), but this has not been unequivocally shown.…”

BRCA-FA Pathway as a Target for Anti-Tumor Drugs

“…For example, it was recently reported that ultraviolet (UV) light, which does not directly introduce DSBs or DNA interstrand cross-links, can activate the FA/BRCA pathway as evidenced by FANCD2 monoubiquitination [20]. In that study, it was suggested that the BRCA-FA pathway may be responsible for recombinational repair of stalled replication forks when nucleotide excision repair or translesion bypass fail.…”