In vitro phenotypic correction of hematopoietic progenitors from Fanconi anemia group A knockout mice

Defective endomitosis during megakaryopoiesis leads to thrombocytopenia in Fanca-/- mice

Fanconi anemia (FA) is an inherited chromosomal instability syndrome that is characterized by progressive bone marrow failure. One of the main causes of morbidity and mortality in FA is a bleeding tendency, resulting from low platelet counts. Platelets are the final products of megakaryocyte (MK) maturation. Here, we describe a previously unappreciated role of Fanconi anemia group A protein (Fanca) during the endomitotic process of MK differentiation. Fanca deficiency leads to the accumulation of MKs with low nuclear ploidy and to decreased platelet production. We show, for the first time, that Fanca(-/-) mice are characterized by limited number and proliferative capacity of MK progenitors. Defective megakaryopoiesis of Fanca(-/-) cells is associated with the formation of nucleoplasmic bridges and increased chromosomal instability, indicating that inaccurate endoreplication and karyokinesis occur during MK polyploidization. Sustained DNA damage forces Fanca(-/-) MKs to enter a senescence-like state. Furthermore, inhibition of the Rho-associated kinase, a regulator of cytokinesis, improves the polyploidization of Fanca(-/-) MKs but greatly increases their genomic instability and diminishes their differentiation potential, supporting the notion that accumulation of DNA damage through endomitotic cycles affects MK maturation. Our study indicates that Fanca expression during endomitosis is crucial for normal megakaryopoiesis and platelet production.

In vivo proliferation advantage of genetically corrected hematopoietic stem cells in a mouse model of Fanconi anemia FA-D1

Fanconi anemia (FA) is an inherited recessive DNA repair disorder mainly characterized by bone marrow failure and cancer predisposition. Studies in mosaic FA patients have shown that reversion of one inherited germ-line mutation resulting in a functional allele in one or a few hematopoietic stem cells (HSCs) can lead to the proliferation advantage of corrected cells, thus over time normalizing the hematologic status of the patient. In contrast to these observations, it is still unclear whether ex vivo genetic correction of FA HSCs also provides a similar proliferation advantage to FA HSCs. Using an FA mouse model with a marked hematopoietic phenotype, the FA-D1 (Brca2Δ27/Δ27) mice, we demonstrate that the lentivirus-mediated gene therapy of FA HSCs results in the progressive expansion of genetically corrected clones in mild-conditioned FA-D1 recipients. Consistent with these data, hematopoietic progenitors from FA recipients progressively became mitomycin C resistant and their chromosomal instability was reverted. No evidence of myelodysplasia, leukemias, or abnormal clonal repopulation was observed at multiple time points in primary or secondary recipients. Our results demonstrate that ectopic expression of BRCA2 confers a beneficial in vivo proliferation advantage to FA-D1 HSCs that enables the full hematopoietic repopulation of FA recipients with genetically corrected cells.