Correction of Fanconi Anemia Group C Hematopoietic Stem Cells Following Intrafemoral Gene Transfer

Emerging Therapeutic Approaches for Diamond Blackfan Anemia

Diamond Blackfan Anemia (DBA) is an inherited erythroid aplasia with onset in childhood. Patients carry heterozygous mutations in one of 19 Ribosomal Protein (RP) genes, that lead to defective ribosome biogenesis and function. Standard treatments include steroids or blood transfusions but the only definitive cure is allogeneic Hematopoietic Stem Cell Transplantation (HSCT). Although advances in HSCT have greatly improved the success rate over the last years, the risk of adverse events and mor-tality is still significant.

Clinical trials employing gene therapy are now in progress for a variety of monogenic diseases and the development of innovative stem cell-based strategies may open new alternatives for DBA treatment as well. In this review, we summarize the most recent progress toward the implementation of new thera-peutic approaches for this disorder. We present different DNA- and RNA-based technologies as well as new candidate pharmacological treatments and discuss their relevance and potential applicability for the cure of DBA.

Phenotypic correction of Fanconi anemia cells in the murine bone marrow after carrier cell mediated delivery of lentiviral vector

Fanconi anemia (FA) is an autosomal-recessive disorder associated with hematopoietic failure and it is a candidate for hematopoietic stem cell (HSC)-directed gene therapy. However, the characteristically reduced HSC numbers found in FA patients, their ineffective mobilization from the marrow, and re-oxygenation damage during ex vivo manipulation have precluded clinical success using conventional in vitro approaches. We previously demonstrated that lentiviral vector (LV) particles reversibly attach to the cell surface where they gain protection from serum complement neutralization. We reasoned that cellular delivery of LV to the bone marrow niche could avoid detrimental losses during FA HSC mobilization and in vitro modification. Here, we demonstrate that a VSV-G pseudotyped lentivector, carrying the FANCC transgene, can be transmitted from carrier to bystander cells. In cell culture and transplantation models of FA, we further demonstrate that LV carrier cells migrate along SDF-1α gradients and transfer vector particles that stably integrate and phenotypically correct the characteristic DNA alkylator sensitivity in murine and human FA-deficient target bystander cells. Altogether, we demonstrate that cellular homing mechanisms can be harnessed for the functional phenotype correction in murine FA hematopoietic cells.

Hemophilia A gene therapy via intraosseous delivery of factor VIII-lentiviral vectors

Current treatment of hemophilia A (HemA) patients with repeated infusions of factor VIII (FVIII; abbreviated as F8 in constructs) is costly, inconvenient, and incompletely effective. In addition, approximately 25 % of treated patients develop anti-factor VIII immune responses. Gene therapy that can achieve long-term phenotypic correction without the complication of anti-factor VIII antibody formation is highly desired. Lentiviral vector (LV)-mediated gene transfer into hematopoietic stem cells (HSCs) results in stable integration of FVIII gene into the host genome, leading to persistent therapeutic effect. However, ex vivo HSC gene therapy requires pre-conditioning which is highly undesirable for hemophilia patients. The recently developed novel methodology of direct intraosseous (IO) delivery of LVs can efficiently transduce bone marrow cells, generating high levels of transgene expression in HSCs. IO delivery of E-F8-LV utilizing a ubiquitous EF1α promoter generated initially therapeutic levels of FVIII, however, robust anti-FVIII antibody responses ensued neutralized functional FVIII activity in the circulation. In contrast, a single IO delivery of G-FVIII-LV utilizing a megakaryocytic-specific GP1bα promoter achieved platelet-specific FVIII expression, leading to persistent, partial correction of HemA in treated animals. Most interestingly, comparable therapeutic benefit with G-F8-LV was obtained in HemA mice with pre-existing anti-FVIII inhibitors. Platelets is an ideal IO delivery vehicle since FVIII stored in α-granules of platelets is protected from high-titer anti-FVIII antibodies; and that even relatively small numbers of activated platelets that locally excrete FVIII may be sufficient to promote efficient clot formation during bleeding. Additionally, combination of pharmacological agents improved transduction of LVs and persistence of transduced cells and transgene expression. Overall, a single IO infusion of G-F8-LV can generate long-term stable expression of hFVIII in platelets and correct hemophilia phenotype for long term. This approach has high potential to permanently treat FVIII deficiency with and without pre-existing anti-FVIII antibodies.

Intraosseous delivery of lentiviral vectors targeting factor VIII expression in platelets corrects murine hemophilia A

Intraosseous (IO) infusion of lentiviral vectors (LVs) for in situ gene transfer into bone marrow may avoid specific challenges posed by ex vivo gene delivery, including, in particular, the requirement of preconditioning. We utilized IO delivery of LVs encoding a GFP or factor VIII (FVIII) transgene directed by ubiquitous promoters (a MND or EF-1α-short element; M-GFP-LV, E-F8-LV) or a platelet-specific, glycoprotein-1bα promoter (G-GFP-LV, G-F8-LV). A single IO infusion of M-GFP-LV or G-GFP-LV achieved long-term and efficient GFP expression in Lineage(-)Sca1(+)c-Kit(+) hematopoietic stem cells and platelets, respectively. While E-F8-LV produced initially high-level FVIII expression, robust anti-FVIII immune responses eliminated functional FVIII in circulation. In contrast, IO delivery of G-F8-LV achieved long-term platelet-specific expression of FVIII, resulting in partial correction of hemophilia A. Furthermore, similar clinical benefit with G-F8-LV was achieved in animals with pre-existing anti-FVIII inhibitors. These findings further support platelets as an ideal FVIII delivery vehicle, as FVIII, stored in α-granules, is protected from neutralizing antibodies and, during bleeding, activated platelets locally excrete FVIII to promote clot formation. Overall, a single IO infusion of G-F8-LV was sufficient to correct hemophilia phenotype for long term, indicating that this approach may provide an effective means to permanently treat FVIII deficiency.

Disrupted Signaling through the Fanconi Anemia Pathway Leads to Dysfunctional Hematopoietic Stem Cell Biology: Underlying Mechanisms and Potential Therapeutic Strategies

Fanconi anemia (FA) is the most common inherited bone marrow failure syndrome. FA patients suffer to varying degrees from a heterogeneous range of developmental defects and, in addition, have an increased likelihood of developing cancer. Almost all FA patients develop a severe, progressive bone marrow failure syndrome, which impacts upon the production of all hematopoietic lineages and, hence, is thought to be driven by a defect at the level of the hematopoietic stem cell (HSC). This hypothesis would also correlate with the very high incidence of MDS and AML that is observed in FA patients. In this paper, we discuss the evidence that supports the role of dysfunctional HSC biology in driving the etiology of the disease. Furthermore, we consider the different model systems currently available to study the biology of cells defective in the FA signaling pathway and how they are informative in terms of identifying the physiologic mediators of HSC depletion and dissecting their putative mechanism of action. Finally, we ask whether the insights gained using such disease models can be translated into potential novel therapeutic strategies for the treatment of the hematologic disorders in FA patients.