Alternative titles; symbolsFAD2FANCONI ANEMIA, COMPLEMENTATION GROUP D; FANCD; FACDFANCONI PANCYTOPENIA, TYPE 4; FA4▼ DescriptionFanconi anemia (FA) is a clinica...
Alternative titles; symbols
Fanconi anemia (FA) is a clinically and genetically heterogeneous disorder that causes genomic instability. Characteristic clinical features include developmental abnormalities in major organ systems, early-onset bone marrow failure, and a high predisposition to cancer. The cellular hallmark of FA is hypersensitivity to DNA crosslinking agents and high frequency of chromosomal aberrations pointing to a defect in DNA repair (summary by Deakyne and Mazin, 2011).
For additional general information and a discussion of genetic heterogeneity of Fanconi anemia, see 227650.
▼ Clinical Features
Using complementation assays and immunoblotting, a consortium of American and European groups assigned 29 patients with Fanconi anemia from 23 families and 4 additional unrelated patients to complementation group FA-D2 (Kalb et al., 2007). This amounts to 3 to 6% of FA-affected patients registered in various data sets. Malformations were frequent in FA-D2 patients, and hematologic manifestations appeared earlier and progressed more rapidly when compared with all other patients combined (FA-non-D2) in the International Fanconi Anemia Registry.
▼ Biochemical Features
Donahue and Campbell (2002) found that fibroblasts from FA patients from complementation groups A, C, D2, and G were hypersensitive to restriction enzyme-induced cell death following electroporation of restriction enzymes. These fibroblasts also showed reduced efficiency in plasmid end-joining activity. Normal sensitivity and activity were restored following retrovirus-mediated expression of the respective FA cDNAs.
▼ Molecular Genetics
Timmers et al. (2001) identified the D2 complementation group of Fanconi anemia by analysis of cell lines (PD20, VU008, and PD733) from 3 unrelated families with FANCD. Retroviral transduction of the cloned FANCD2 cDNA into FANCD2 cells resulted in functional complementation of mitomycin C sensitivity. The authors found, however, that the gene mutated in the FANCD cell lines HSC62 and VU423 is distinct from FANCD2 and does not map to chromosome 3; they designated this gene FANCD1.
Kalb et al. (2007) performed mutation analysis of 33 patients of complementation group FA-D2, which demonstrated the expected number of 66 mutated alleles, 34 of which resulted in aberrant splicing patterns. Many mutations were recurrent and had ethnic associations and shared allelic haplotypes. There were no biallelic null mutations; residual FANCD2 protein of both isotypes was observed in all available patient cell lines. These analyses suggested that, unlike the knockout mouse model, total absence of FANCD2 does not exist in FA-D2 patients, because of constraints on viable combinations of FANCD2 mutations. Although hypomorphic mutations are involved, patients clinically had a relatively severe form of FA.
▼ Animal Model
Liu et al. (2003) demonstrated that Fancd2-deficient zebrafish embryos developed defects similar to those found in children with FA, including shortened body length, microcephaly, and microphthalmia, which were due to extensive cellular apoptosis. The developmental defects and increased apoptosis could be corrected by injection of human FANCD2 or zebrafish Bcl2 (151430) mRNA, or by knockdown of p53 (191170), indicating that in the absence of Fancd2, developing tissues spontaneously underwent p53-dependent apoptosis.
To investigate the in vivo function of the FA pathway, Houghtaling et al. (2003) created mice with a targeted deletion in the distally acting FA gene Fancd2. Similar to human FA patients and other FA mouse models, Fancd2 mutant mice exhibited cellular sensitivity to DNA interstrand crosslinks and germ cell loss. In addition, chromosome mispairing was seen in male meiosis. However, Fancd2 mutant mice also displayed phenotypes not observed in other mice with disruptions of proximal FA genes. These included microphthalmia, perinatal lethality, and epithelial cancers, similar to mice with Brca2/Fancd1 hypomorphic mutations. The phenotypic overlap between Fancd2 null and Brca2/Fancd1 hypomorphic mice was considered consistent with a common function for both proteins in the same pathway, regulating genomic stability.
To investigate the role of the FA pathway in repair of DNA double-strand breaks (DSBs), Houghtaling et al. (2005) generated Fancd2-null/Prkdc (600899) (sc/sc) double-mutant mice. Prkdc(sc/sc) mutant mice have a defect in nonhomologous end-joining (NHEJ) and are sensitive to ionizing radiation (IR)-induced DNA damage. Double-mutant animals and primary cells were more sensitive to IR than either single mutant, suggesting that Fancd2 may operate in a DSB repair pathway distinct from NHEJ. Fancd2-null/Prkdc(sc/sc) double-mutant cells were also more sensitive to DSBs generated by a restriction endonuclease. Houghtaling et al. (2005) suggested that the role of Fancd2 in DSB repair may account for the moderate sensitivity of FA cells to irradiation and FA cells sensitivity to interstrand crosslinks that are repaired via a DSB intermediate.