Despite a greater understanding of the biology of FA, the mechanisms underlying the hematopoietic defects, developmental defects, and bone marrow failure of FA remain elusive. The defective DNA repair capacity of FA stem cells may underlie their hypersensitivity to endogenous DNA damage, such as damage from ROS (15
) or from serum formaldehyde (100
). Such hypersensitivity may lead to decreased hematopoietic cellularity (stem cell loss in early development), similar to phenotypes observed in ATM-deficient or ligase IV–deficient (LIG4-deficient) mice (102
). Therefore, limiting the potential sources of endogenous DNA-damaging agents may protect HSCs in FA. A recent study showed that resveratrol, an activator of SIRT1 with antioxidant effect, partially rescues the hematopoietic defects of Fancd2–/–
), which suggests that increased sensitivity to ROS is a contributing factor in FA. Antioxidants may also delay tumor onset in FA; the antioxidizing small molecule Tempol was shown to delay tumor formation in a Fancd2–/–Trp53+/–
mouse model (105
). Screening of small molecules or natural compounds that rescue the hematopoietic defects in FA, particularly those that reduce oxidative DNA damage, may hold promise for future FA therapies.
Several reports provide evidence of somatic reversion in FA, leading to mosaicism. Mosaicism results from spontaneous reverse mutations that restore functional FA alleles (106
). Reverse mosaicism in FA can provide a selective advantage, and the corrected HSCs can improve clinical outcome. Reverse mosaicisim, often referred to as natural gene therapy, suggests that replenishing even a limited fraction of corrected HSCs can significantly improve the hematopoietic defects in FA. The revertant FA stem cells could, in principle, be isolated, expanded in vitro, and grafted back to the patient as an autologous transplant (Table ). Successful attempts have been made to transplant revertant keratinocytes as a treatment for skin disorders (110
). However, questions remain about the ability of corrected cells to sustain the hematopoietic functionality in the long term. Furthermore, the nonrevertant FA cells are still prone to the development of a malignant clonal abnormality (112
). Spontaneous mutations in non-FA genes might confer a selective growth advantage to the corrected clones in FA. Individuals with FA were identified by acquired clonal selection of cells with downregulated ATR-CHK1 proteins and abrogated G2/M checkpoint (113
). These individuals had mild bone marrow abnormalities and lived to adulthood, which suggests that G2/M checkpoint abrogation has clinical benefits for FA cells. Long-term effectiveness is questionable, however, as some of these patients eventually develop leukemia or myelodysplasia.
Potential future therapies for FA
The bone marrow failure in FA individuals may result from hyperactive checkpoint responses in hematopoietic stem and progenitor cells (HSPCs). Primary bone marrow cells from FA patients have elevated levels of p53 and of its downstream effector protein, p21 (A.D. D’Andrea, unpublished observations). Elevated p53 and p21 appear to account, at least in part, for the enhanced cell cycle arrest and apoptosis of HSPCs in the bone marrow of FA patients. A similar mechanism of p53-mediated bone marrow apoptosis has been observed in another inherited bone marrow failure syndrome, Diamond-Blackfan anemia (114
). Small molecule inhibitors of p53 may, in principle, rescue this HSPC apoptosis and improve hematopoiesis in FA patients, but at the risk of increasing leukemia incidence.
Finally, induced pluripotent stem cell (iPS) technology holds promise for FA therapy (Table ). Direct reprogramming of FA patient somatic cells into pluripotent stem cells, which retain the patient’s unique genetic background, may provide a new source of autologous cells for transplant (115
). The limitations of traditional gene therapy (e.g., difficulty in isolating sufficient HSCs) and the complications associated with bone marrow transplantation may be avoided by transplanting cDNA-corrected iPS cells to FA patients (115
). Several issues must be resolved before successful application to the clinic. As for all iPS approaches, use of proto-oncogenes and viral-mediated gene insertion increases the risk of oncogenesis. A safer and more sequence-specific method of gene introduction will be required. Also, the reprogramming efficiency of FA cells must be improved. Knockdown of the FA pathway leads to loss of self-renewal potency, which suggests that a functional FA pathway is required for induction and maintenance of pluripotency (115
). However, a recent study demonstrated that functional complementation of the FA pathway restores the reprogramming efficiency of FA patient cells, providing a strategy in FA-specific iPS generation (117
). Finally, successful induction of blood progenitors from iPS cells, capable of long-term hematopoietic reconstitution, at a scale compatible for transplantation in FA patients remains a challenge (116
). Despite these challenges, iPS technology offers the promise of generating tissue-compatible transplants for all FA patients.