Des mutations dans les genes FANC sont responsables de l’anemie de Fanconi (AF), une maladie genetique de phenotype complexe incluant une pancytopenie, des malformations congenitales et une predisposition elevee au cancer. L’augmentation par les agents pontant l’ADN de la frequence des aberrations chromosomiques, une caracteristique de l’AF, est utilisee pour le diagnostic. Parmi les onze genes FANC, neuf sont identifies. Huit de ces genes sont localises sur des autosomes, tandis que FANCB est situe sur le chromosome X. L'un des genes FANC est BRCA2, implique dans la predisposition genetique au cancer du sein et/ou de l’ovaire. Sept des proteines FANC s’associent pour former un complexe a geometrie variable dependant de sa localisation subcellulaire, tandis que FANCD1 (BRCA2), FANCD2, FANCI et FANCJ ne sont pas associees au complexe. La mono-ubiquitinylation de FANCD2, dependante du complexe, jouerait un role important dans la gestion des pontages de l’ADN. Les proteines FANC et BRCA1, etroitement associees, participent, entre autres, avec les proteines ATM, NBS1 et ATR, a un reseau multiproteique implique dans la detection, la signalisation et la reparation des lesions bloquant la replication de l’ADN.

Germ-line mutations of the BRCA1 and BRCA2 genes, when they lead to a truncated protein, confer a high risk of breast and ovarian cancer. However, the role of BRCA1 missense mutations in cancer predisposition is unclear. Functional assays may be very helpful to more clearly define the biological effect of these mutations, and could therefore be useful in clinical practice. A recent study using a Host Cell End-Joining assay showed that a truncating mutation results in impaired fidelity of DSB repair by DNA end-joining. In the present study, we examined the fidelity of DSB repair in four lymphoblastoid cell lines with BRCA1 missense mutations. The fidelity of DNA end-joining was impaired in the four cell lines studied compared to the normal control cell line. The fidelity of end-joining was similar to that of a truncated mutation control cell line for one cell line and slightly higher for the other cell lines.

Heterozygosity for mutations in the BRCA1 gene in humans confers high risk for developing breast cancer, but a biochemical basis for this phenotype has not yet been determined. Evidence has accumulated implicating BRCA1, in the maintenance of genomic integrity and the protection of cells against DNA double strand breaks (DSB). Here we present evidence that human cells heterozygous for BRCA1 mutations exhibit impaired DNA end-joining, which is the major DSB repair pathway in mammalian somatic cells. Using an in vivo host cell end-joining assay, we observed that the fidelity of DNA end-joining is strongly reduced in three BRCA1(+/-) cell lines in comparison to two control cell lines. Moreover, cell-free BRCA1(+/-) extracts are unable to promote accurate DNA end-joining in an in vitro reaction. The steady-state level of the wild type BRCA1 protein was significantly lower than the 50% expected in BRCA1(+/-) cells and thus may underlie the observed end-joining defect. Together, these data strongly suggest that BRCA1 is necessary for faithful rejoining of broken DNA ends and that a single mutated BRCA1 allele is sufficient to impair this process. This defect will compromise genomic stability in BRCA1 germ-line mutation carriers, triggering the genetic changes necessary for the initiation of neoplastic transformation.

Fanconi anemia (FA) is an autosomal recessive disorder characterized by chromosomal instability and abnormalities in the processing of DNA lesions induced by cross-linking agents. We previously reported that after photoaddition of psoralen derivatives the frequency ofHPRT− mutants was significantly lower in FA than in normal human lymphoblasts. The hypomutability in FA cells was shown to be associated with an increased deletion frequency at theHPRT gene level. Further characterization of 70 unrearranged mutants (without detectable changes in restriction enzyme fragment length) according to theHPRT gene expression is reported here. Northern blot hybridization analysis demonstrates considerable differences in mRNA phenotyping between normal and FA cells. In normal cells, the minority of spontaneous (31%) and psoralen-induced mutants (0% and 14% according to treatment) arise from mutations that alter theHPRT gene transcription. In contrast to normal cells, in the majority of mutants isolated from FA cells,HPRT gene expression is found to be affected. Indeed a large proportion of either spontaneous (67%) or psoralen-induced (56% and 46%) mutants did not produce detectable amounts of mRNA. These results suggest that the mutagenic processing of spontaneous and psoralen-photoinduced lesions differs in normal and FA cells.