Current Gene Therapy - Volume 16, Issue 5, 2016

Fanconi anemia (FA) is a rare genetic syndrome characterized by progressive marrow failure. Gene therapy by infusion of FA-corrected autologous hematopoietic stem cells (HSCs) may offer a potential cure since it is a monogenetic disease with mutations in the FANC genes, coding for DNA repair enzymes [1]. However, the collection of hCD34+-cells in FA patients implies particular challenges because of the reduced numbers of progenitor cells present in their bone marrow (BM) [2] or mobilized peripheral blood [3-5]. In addition, the FA genetic defect fragilizes the HSCs [6]. These particular features might explain why the first clinical trials using murine leukemia virus derived retroviral vectors conducted for FA failed to show engraftment of corrected cells. The gene therapy field is now moving towards the use of lentiviral vectors (LVs) evidenced by recent succesful clinical trials for the treatment of patients suffering from adrenoleukodystrophy (ALD) [7], β-thalassemia [8], metachromatic leukodystrophy [9] and Wiskott-Aldrich syndrome [10]. LV trials for X-linked severe combined immunodificiency and Fanconi anemia (FA) defects were recently initiated [11, 12]. Fifteen years of preclinical studies using different FA mouse models and in vitro research allowed us to find the weak points in the in vitro culture and transduction conditions, which most probably led to the initial failure of FA HSC gene therapy. In this review, we will focus on the different obstacles, unique to FA gene therapy, and how they have been overcome through the development of optimized protocols for FA HSC culture and transduction and the engineering of new gene transfer tools for FA HSCs. These combined advances in the field hopefully will allow the correction of the FA hematological defect in the near future.

Fanconi anemia (FA) is a rare inherited DNA disorder clinically characterized by congenital malformations, progressive bone marrow failure, and cancer susceptibility. Due to a strong survival advantage of spontaneously corrected ‘normal’ hematopoietic stem cells (HSCs) in a few patients, FA is considered a model disorder for genetic correction of autologous stem cells, where genetically corrected stem cells and their progeny have a strong in vivo selective advantage, ultimately leading to normal hematopoiesis. Despite these apparently ideal circumstances, three HSC gene therapy trials with gammaretroviral vectors (stage I) designed to cure the hematological manifestation of FA completely failed to provide long-term clinical benefits for patients, predominantly due to the combination of insufficient gene transfer technologies and incompletely understood FA HSC pathobiology. Currently, FA gene therapy is in stage II where, based on an improved understanding of the cellular defects in FA HSCs, consequently adapted transduction protocols are being used in two phase I/II trials for in vitro genetic correction of FANCA-deficient hematopoietic stem cells. These results are eagerly awaited. Independent from the outcome of these studies, technologies are already available that seem highly attractive for testing in FA. In stage III, this would ultimately include targeted in vivo correction of autologous HSCs by overexpression of nonintegrating lentiviral vectors with scaffold/matrix attachment region elements using specific envelopes as pseudotypes. Although currently still challenging, in a few years in vivo genome editing approaches will be readily available in stage IV, in which the delivery of the editing machinery/ complex is targeted to the autologous FA HSCs by the nonintegrating lentiviral vectors established in stage III. Even low levels of corrected stem cells will then quickly repopulate the entire hematopoiesis of the patient. We therefore are sanguine that in the future, genetic therapy can be used clinically for the correction of FA HSCs in the standard care of FA patients.

Induced pluripotent stem cells (iPSCs) represent an invaluable tool in a chromosomal instability syndrome such as Fanconi anemia (FA), as they can allow to study of the molecular defects underlying this disease. Many other applications, such as its use as a platform to test different methods or compounds, could also be of interest. But the greatest impact of iPSCs may be in bone marrow failure diseases, as iPSCs could represent an unlimited source of autologous cells to apply in advanced treatments such as gene therapy. At the same time, genome editing constitutes the next generation of technology to further develop a safer, personalized, targeted gene therapy. Despite the promising advantages that these two technologies would present in a disease such as FA, the specific characteristics of the disease make both of these processes especially challenging. Efficient and safer FA-hiPSC (human induced pluripotent stem cell) generation methods, robust and reliable differentiation protocols for iPSCs, as well as really efficient delivery methods to perform targeted gene correction should be developed.

Fanconi anemia (FA) is an autosomal recessive, multisystem DNA repair disorder with prominent defects in the hematopoietic stem cell maintenance that result in the progressive attrition and failure in the early school age. Allogeneic stem cell transplantation has proved curative for patients with suitable donors. This, along with the characteristic survival advantage of phenotypically normal over non-corrected FA stem cells underscores the compelling rationale for stem cell gene therapy in the FA. While integrating lentiviral vectors (LV) have become the preferred platform for genetic correction in several hematologic and immunodeficiency disorders, the residual oncogenic potential by these vectors raises concerns in the FA stem cells about insertional mutagenic genetic lesions. On this backdrop, investigators are developing a new generation of non-integrating viral vectors capable of nuclear persistence through serial mitotic cycles and stable under selection to offset the comparatively lower transduction rates. Here, we review the competing approaches to develop such non-integrating lentiviral (NILV) episome vectors that faithfully replicate in the stem cells.

Allogeneic hematopoietic stem cell transplantation is the only curative treatment for patients with the non-malignant bone marrow failure syndrome called Fanconi anemia (FA). However, early and late complications associated with this approach underscore the need for alternative treatments. Gene therapy approaches aiming to correct the genetic defect in the patient’s own hematopoietic stem cells remain the most promising strategy to overcome FA-associated bone marrow failure. Yet, despite more than two decades of clinical research, a therapeutic “success” has not yet been achieved. Here we review the clinical trials conducted to date and highlight the unique features of FA revealed by these studies. These features render FA the “holy grail” of hematopoietic stem cell gene therapy approaches, and identify the future steps required to achieve clinical success in this rare disease.

Background: Elabela (ELA) is a recently identified apelin receptor agonist essential for cardiac development, but its biology and therapeutic potential are unclear. In humans, ELA transcripts are detected in embryonic stem cells, induced pluripotent stem cells, kidney, heart and blood vessels. ELA through the apelin (APJ) receptor promotes angiogenesis in vitro, relaxes murine aortic blood vessels and attenuates high blood pressure in vivo. The APJ receptor when bound to its original ligand, apelin, exerts peripheral vasodilatory and positive inotropic effects, conferring cardioprotection in vivo. Methods: This study initially assessed endogenous ELA expression in normal and diseased rats and then characterized the effects of long-term ELA gene delivery by adeno-associated virus serotype 9 (AAV9) vectors on cardiorenal function in Dahl salt-sensitive rats (DS) on a high-salt diet over 3 months. Results: Endogenous ELA was predominantly expressed in the kidneys, especially in the renal collecting duct cells and was not affected by disease. Rat ELA was overexpressed in the heart via AAV9 vector by a single intravenous injection. ELA-treated animals showed delayed onset of blood pressure elevation. Prior to high-salt diet, a reduction in the fractional sodium and chloride excretion was observed in rats given the AAV9-ELA vector. After three months on a high-salt diet, ELA preserved glomerular architecture, decreased renal fibrosis and suppressed expression of fibrosis-associated genes in the kidneys. Conclusion: ELA is constitutively expressed in renal collecting ducts in rats. Sustained AAV-ELA expression may offer a potential long-term therapy for hypertension and renal remodeling.