Fanconi anemia genes are highly expressed in primitive CD34+ hematopoietic cells

Improved collection of hematopoietic stem cells and progenitors from Fanconi anemia patients for gene therapy purposes

Difficulties in the collection of hematopoietic stem and progenitor cells (HSPCs) from Fanconi anemia (FA) patients have limited the gene therapy in this disease. We have investigated (ClinicalTrials.gov, NCT02931071) the safety and efficacy of filgrastim and plerixafor for mobilization of HSPCs and collection by leukapheresis in FA patients. Nine of eleven enrolled patients mobilized beyond the threshold level of 5 CD34⁺ cells/μL required to initiate apheresis. A median of 21.8 CD34⁺ cells/μL was reached at the peak of mobilization. Significantly, the oldest patients (15 and 16 years old) were the only ones who did not reach that threshold. A median of 4.27 million CD34⁺ cells/kg was collected in 2 or 3 aphereses. These numbers were markedly decreased to 1.1 million CD34⁺ cells/kg after immunoselection, probably because of weak expression of the CD34 antigen. However, these numbers were sufficient to facilitate the engraftment of corrected HSPCs in non-conditioned patients. No procedure-associated serious adverse events were observed. Mobilization of CD34⁺ cells correlated with younger age, higher leukocyte counts and hemoglobin values, lower mean corpuscular volume, and higher proportion of CD34⁺ cells in bone marrow (BM). All these values offer crucial information for the enrollment of FA patients for gene therapy protocols.

Transgenic expression of BRCA1 disturbs hematopoietic stem and progenitor cells quiescence and function

The balance between quiescence and proliferation of HSCs is an important regulator of hematopoiesis. Loss of quiescence frequently results in HSCs exhaustion, which underscores the importance of tight regulation of proliferation in these cells. Studies have indicated that cyclin-dependent kinases are involved in the regulation of quiescence in HSCs. BRCA1 plays an important role in the repair of DNA double-stranded breaks, cell cycle, apoptosis and transcription. BRCA1 is expressed in the bone marrow. However, the function of BRCA1 in HSCs is unknown. In our study, we generated BRCA1 transgenic mice to investigate the effects of BRCA1 on the mechanisms of quiescence and differentiation in HSCs. The results demonstrate that over-expression of BRCA1 in the bone marrow impairs the development of B lymphocytes. Furthermore, BRCA1 induced an increase in the number of LSKs, LT-HSCs, ST-HSCs and MPPs. A competitive transplantation assay found that BRCA1 transgenic mice failed to reconstitute hematopoiesis. Moreover, BRCA1 regulates the expression of p21(waf1)/cip1 and p57(kip2), which results in a loss of quiescence in LSKs. Together, over-expression of BRCA1 in bone marrow disrupted the quiescent of LSKs, induced excessive accumulation of LSKs, and disrupted differentiation of the HSCs, which acts through the down-regulated of p21(waf1)/cip1 and p57(kip2).

Hematopoietic stem cell defects in mice with deficiency of Fancd2 or Usp1

Fanconi anemia (FA) is a human genetic disease characterized by a DNA repair defect and progressive bone marrow failure. Central events in the FA pathway are the monoubiquitination of the Fancd2 protein and the removal of ubiquitin by the deubiquitinating enzyme, Usp1. Here, we have investigated the role of Fancd2 and Usp1 in the maintenance and function of murine hematopoietic stem cells (HSCs). Bone marrow from Fancd2-/- mice and Usp1-/- mice exhibited marked hematopoietic defects. A decreased frequency of the HSC populations including Lin-Sca-1+Kit+ cells and cells enriched for dormant HSCs expressing signaling lymphocyte activation molecule (SLAM) markers, was observed in the bone marrow of Fancd2-deficient mice. In addition, bone marrow from Fancd2-/- mice contained significantly reduced frequencies of late-developing cobblestone area-forming cell activity in vitro compared to the bone marrow from wild-type mice. Furthermore, Fancd2-deficient and Usp1-deficient bone marrow had defective long-term in vivo repopulating ability. Collectively, our data reveal novel functions of Fancd2 and Usp1 in maintaining the bone marrow HSC compartment and suggest that FA pathway disruption may account for bone marrow failure in FA patients.

Knockdown of zebrafish Fancd2 causes developmental abnormalities via p53-dependent apoptosis

Mechanisms underlying the multiple developmental defects observed in Fanconi anemia (FA) patients are not well defined. We have identified the zebrafish homolog of human FANCD2, which encodes a nuclear effector protein that is monoubiquitinated in response to DNA damage, targeting it to nuclear foci where it preserves chromosomal integrity. Fancd2-deficient zebrafish embryos develop defects similar to those found in children with FA, including shortened body length, microcephaly, and microophthalmia, which are due to extensive cellular apoptosis. Developmental defects and increased apoptosis in Fancd2-deficient zebrafish were corrected by injection of human FANCD2 or zebrafish bcl2 mRNA, or by knockdown of p53, indicating that in the absence of Fancd2, developing tissues spontaneously undergo p53-dependent apoptosis. Thus, Fancd2 is essential during embryogenesis to prevent inappropriate apoptosis in neural cells and other tissues undergoing high levels of proliferative expansion, implicating this mechanism in the congenital abnormalities observed in human infants with FA.