Fanconi anemia C gene product plays a role in the fidelity of blunt DNA end-joining

Abstract

Mutations in genes controlling the correct functioning of the replicative, repair and recombination machineries may lead to genomic instability. A high level of spontaneous chromosomal aberrations amplified by treatment with DNA cross-linking agents is the hallmark of Fanconi anemia (FA), an inherited chromosomal instability syndrome associated with cancer proneness. Two of the eight FA genes have been cloned ( FAA and FAC), but their function has not yet been defined. The lack of homology with known genes suggests the involvement of FA genes in a novel pathway specific to vertebrates. Using a DNA end-joining assay in cultured cells, we studied the processing of both blunt and cohesive-ended double strand breaks (DSB) in normal and FA cells. The results show that: (i) the overall ligation efficiency is normal in FA lymphoblasts; (ii) in FA-C, error-free processing of blunt-ended DSB is markedly decreased, resulting in a higher deletion frequency and larger deletion size; (iii) the fidelity of processing of blunt-DSB is completely restored in FACC cells (complemented with wild-type FAC gene) and the deletion size shifted to values similar to that observed in normal cells; (iv) the fidelity of cohesive end-joining is not affected in FA cells; (v) activities and/or expression of known factors involved in DSB processing, such as the components of the DNA-PK complex and XRCC4, are normal in FA cells. Our results provide strong evidence that the lack of a functional FAC gene results in loss of fidelity of end-joining, which likely accounts for the FA-C phenotype of chromosome instability. We conclude that FAC, and perhaps all FA gene products, are likely to play a role in the fidelity of end-joining of specific DSB.