Mutation Research/Fundamental and Molecular Mechanisms of Mutagenesis
ReviewMouse models of Fanconi anemia
Section snippets
Fanconi anemia
Fanconi anemia is a rare autosomal recessive disease with a complex spectrum of symptoms including congenital skeletal and renal anomalies, growth retardation, pigmentation abnormalities, fertility defects, aplastic anemia, and increased risk of acute myeloid leukemia and epithelial tumors (see “Fanconi Anemia and its Diagnosis” Auerbach, this issue). Progressive bone marrow failure and late-developing myeloid malignancies account for 90% of mortality in FA patients. Bone marrow failure in FA
Fanconi anemia pathway
The 13 FA proteins work as a complex signaling network that facilitates HR-mediated repair of DSBs caused by DNA ICLs and other types of replication stress (see “The Genetic and Molecular Basis of Fanconi Anemia” de Winter and Joenje, this issue, for more detail and a model). FANC A, B, C, E, F, G, L and M interact to form the FA core complex [19]. The FANCL subunit is an E3 ubiquitin ligase that monoubiquitylates FANCD2 and FANCI during S phase, particularly in response to genotoxic stress [48]
FancA−/− mice
Unlike FA patients, FancA−/− mice, created by deletion of exons 4–7, do not spontaneously display congenital anomalies or severe hematological abnormalities [12]. However, FancA−/− mice do have significantly reduced fertility due to hypogonadism [12]. Despite the mild phenotype, mouse embryonic fibroblasts (MEFs), derived from these mice are hypersensitive to mitomycin C (MMC) and accumulate large numbers of chromosomal aberrations in response to MMC [12], hallmark diagnostic criteria of FA
FancC−/− mice
Like the FancA−/− mice, genetic deletion of FancC in the mouse does not lead to skeletal abnormalities or spontaneous peripheral hematological abnormalities [11], [82]. However, FancC−/− mice are born with sub-Mendelian frequency and have a significantly increased incidence of microphthalmia, a congenital abnormality, if they are bred into a C57BL/6J background [6]. FancC−/− mice have impaired fertility [11], [82] due to impaired proliferation of germ cells during embryogenesis [51], similar to
FancG−/− mice
The phenotype of FancG−/− mice is virtually identical to that of FancA−/− and FancC−/− mice [18]. FancG−/− mice lack the characteristic congenital anomalies characteristic of FA and do not spontaneously develop hematological abnormalities or spontaneous cancer in the first year of life [37]. Like FancA−/− and FancC−/− mice, FancG−/− mice have reduced fertility due to impaired gametogenesis [37], [86], a characteristic of FA (see “Fanconi Anemia and its Diagnosis” Auerbach, this issue). Primary
FancD1/Brca2−/− mice
FANCD1 is identical to BRCA2 [34]. FANCD1/BRCA2 interacts with RAD51 [45] and is required for HR-mediated repair of DNA DSBs [49], [78]. In humans, hapoloinsufficiency of BRCA2 leads to a dramatically increased risk of breast, ovarian and pancreatic cancer. Genetic deletion of FancD1/Brca2 in the mouse results in embryonic lethality [70]. Homozygous deletion of exon 27 of FancD1/Brca2 prevents the interaction of FANCD1/BRCA2 with FANCD2 [2]. Hematopoietic cell function is significantly
FancD2−/− mice
FancD2−/− mice are viable [32], indicating that FANCD2 is not required for mammalian development. FancD2−/− mice are born with sub-Mendelian frequency and display pre- and post-natal growth retardation. The severity of the phenotype of the mice is dependent upon their genetic background, with a more severe phenotype emerging in C57BL/6J than in 129S4, indicating the existence of modifying loci. Like FancA−/−, FancC−/− and FancG−/− mice and FA patients, cells from FancD2−/− mice are
Usp1−/− mice
The deubiquitylating enzyme, USP1 (ubiquitin-specific protease 1), was recently demonstrated to regulate the level of monoubiquitylated FANCD2 and FANCI proteins [55], [76]. Inhibition of Usp1 by siRNA knockdown in human cell lines leads to an accumulation of monoubiquitylated isoforms of FANCD2 and FANCI proteins. Unexpectedly, disruption of Usp1 in chicken DT40 cells results in hypersensitivity to DNA interstrand crosslinking agents, similar to that observed in cells with mutations in FA
Genetic studies: double mutant mice
FancA−/−;FancC−/− mice and cells derived from them are phenotypically identical to single mutants [56]. This provides crucial genetic evidence that FANCA and FANCC are epistatic. FancC−/− mice were crossed into a Sod1−/− background to delete Cu/Zn superoxide dismutase and increase endogenous oxidative stress [25]. The double mutant mice display bone marrow hypocellularity due to a loss of committed progenitor cells, resulting in anemia and leucopenia. FancC−/−;Sod1−/− bone marrow progenitor
Interventional studies to elucidate the function of the FA pathway
FancC−/− mice chronically exposed to a sublethal dose of the crosslinking agent MMC develop progressive pancytopenia due to bone marrow failure [8], identical to the spontaneous symptoms of FA. MMC exposure depletes the bone marrow of CD34+ cells, but not CD34− [7], suggesting that early hematopoietic progenitors are particularly vulnerable to crosslink damage. FancC−/− mice chronically treated with MMC offer a good model in which to study therapeutic interventions for FA. Transplantation of
Utilizing the mouse models to discover therapeutic options for Fanconi anemia
Hematological disease and malignancies are the most common cause of death in FA (see “Fanconi Anemia and its Diagnosis” Auerbach, this issue). This can be prevented by bone marrow transplantation, indicating that FA is a prime candidate disease for gene therapy (see “Finding the Needle in the Hay Stack: Hematopoietic Stem Cells in Fanconi Anemia”, Muller and Williams, this issue, for more information on gene therapy in FA). Indeed, retroviral transduction of FancA−/− hematopoietic stem cells
Summary
In conclusion, FA mouse models now exist, resulting from targeted disruption of the FancA, FancC, FancG, FancD1, FancD2, or Usp1 gene. The mouse models exhibit some but not all of the developmental and hematologic manifestations of human FA patients (Table 1). While FA patients develop spontaneous hematologic failure, most FA mouse models have relatively normal hematologic function, though anemia can be elicited by in vivo exposure to crosslinking agents. Mouse models will be especially useful
Conflict of interest
The authors declare that there are no conflicts of interest.
Acknowledgments
L.J.N. is supported by The Ellison Medical Foundation (AG-NS-0303-05) and NIEHS (R01 ES016114). K.P. and A.D.D. were supported by NIH grant U19AI067751.
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