FANCONI-ANEMIA C-GENE-PRODUCT PLAYS A ROLE IN THE FIDELITY OF BLUNT DNA END-JOINING

Mutations in genes controlling the correct functioning of the replicat ive, repair and recombination machineries may lead to genomic instabil ity. A high level of spontaneous chromosomal aberrations amplified by treatment with DNA cross-linking agents is the hallmark of Fanconi ane mia (FA), an inherited chromosomal instability syndrome associated wit h cancer proneness. Two of the eight FA genes have been cloned (FAA an d FAG), but their function has not yet been defined. The lack of homol ogy with known genes suggests the involvement of FA genes in a novel p athway specific to vertebrates. Using a DNA end-joining assay in cultu red cells, we studied the processing of both blunt and cohesive-ended double strand breaks (DSB) in normal and FA cells. The results show th at: (i) the overall Ligation efficiency is normal in FA lymphoblasts; (ii) in FA-C, error-free processing of blunt-ended DSB is markedly dec reased, resulting in a higher deletion frequency and larger deletion s ize; (iii) the fidelity of processing of blunt-DSB is completely resto red in FACC cells (complemented with wild-type FAC gene) and the delet ion 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 proc essing, such as the components of the DNA-PK complex and XRCC4, are no rmal in FA cells. Our results provide strong evidence that the lack of a functional FAC gene results in loss of fidelity of end-joining, whi ch Likely accounts for the FA-C phenotype of chromosome instability. W e conclude that FAG, and perhaps all FA gene products, are likely to p lay a role in the fidelity of end-joining of specific DSB. (C) 1998 Ac ademic Press Limited.