Effective gene therapy of mice with congenital erythropoietic porphyria is facilitated by a survival advantage of corrected erythroid cells.

Achieving long-term expression of a therapeutic gene in a given hematopoietic lineage remains an important goal of gene therapy. Congenital erythropoietic porphyria (CEP) is a severe autosomal-recessive disorder characterized by a deficiency in uroporphyrinogen III synthase (UROS), the fourth enzyme of the heme biosynthetic pathway. We used a recently obtained murine model to check the feasibility of gene therapy in this disease. Lentivirus-mediated transfer of the human UROS cDNA into hematopoietic stem cells (HSCs) from Uros(mut248) mice resulted in a complete and long-term enzymatic, metabolic, and phenotypic correction of the disease, favored by a survival advantage of corrected red blood cells. These results demonstrate that the cure of this mouse model of CEP at a moderate transduction level supports the proof of concept of a gene therapy in this disease by transplantation of genetically modified hematopoietic stem cells.

Modeling of congenital erythropoietic porphyria by RNA interference: a new tool for preclinical gene therapy evaluation.

BACKGROUND: Congenital erythropoietic porphyria (CEP) is a severe autosomal recessive disorder characterized by a deficiency in uroporphyrinogen III synthase (UROS), the fourth enzyme of the heme biosynthetic pathway. We recently demonstrated the definitive cure of a murine model of CEP by lentiviral vector-mediated hematopoietic stem cell (HSC) gene therapy. In the perspective of a gene therapy clinical trial, human cellular models are required to evaluate the therapeutic potential of lentiviral vectors in UROS-deficient cells. However, the rare incidence of the disease makes difficult the availability of HSCs derived from patients. METHODS: RNA interference (RNAi) has been used to develop a new human model of the disease from normal cord blood HSCs. Lentivectors were developed for this purpose. RESULTS: We were able to down-regulate the level of human UROS in human cell lines and primary hematopoietic cells. A 97% reduction of UROS activity led to spontaneous uroporphyrin accumulation in human erythroid bone marrow cells of transplanted immune-deficient mice, recapitulating the phenotype of cells derived from patients. A strong RNAi-induced UROS inhibition allowed us to test the efficiency of different lentiviral vectors with the aim of selecting a safer vector. Restoration of UROS activity in these small hairpin RNA-transduced CD34(+) cord blood cells by therapeutic lentivectors led to a partial correction of the phenotype in vivo. CONCLUSIONS: The RNAi strategy is an interesting new tool for preclinical gene therapy evaluation.

Erythropoietic porphyrias: animal models and update in gene-based therapies.

The inherited porphyrias are inborn errors of haem biosynthesis, each resulting from the deficient activity of a specific enzyme of the haem biosynthetic pathway. Porphyrias are divided into erythropoietic and hepatic according to the predominant porphyrin-accumulating tissue. Three different erythropoietic porphyrias (EP) have been described: erythropoietic protoporphyria (EPP, MIM 177000) the most frequent, congenital erythropoietic porphyria (CEP, MIM 263700), and the very rare hepatoerythropoietic porphyria (HEP, MIM 176100). Bone marrow transplantation is considered as the only curative treatment for severe cases of erythropoietic porphyria (especially CEP), if donors are available. Some EPP patients who undergo liver failure may require hepatic transplantation. Murine models of EPP and CEP have been developed and mimic most of the human disease features. These models allow a better understanding of the pathophysiological mechanisms involved in EP as well as the development of new therapeutic strategies. The restoration of deficient enzymatic activity in the bone marrow compartment following gene therapy has been extensively studied. Murine oncoretroviral, and recently, lentiviral vectors have been successfully used to transduce hematopoietic stem cells, allowing full metabolic and phenotypic correction of both EPP and CEP mice. In CEP, a selective survival advantage of corrected cells was demonstrated in mice, reinforcing the arguments for a gene therapy approach in the human disease. These successful results form the basis for gene therapy clinical trials in severe forms of erythropoietic porphyrias.

Murine retroviral but not human cellular promoters induce in vivo erythroid-specific deregulation that can be partially prevented by insulators.

We are developing lentiviral vectors for gene therapy of red blood cell disorders that co-express a transgene in an erythroid-specific manner and the O(6)-methylguanine-DNA-methyltransferase (MGMT) selective gene in a constitutive way. We report that transduction of murine hematopoietic stem cells (HSCs) with a human phosphoglycerate kinase promoter-based vector at low multiplicity of infection (MOI) does not result in a selective in vivo expansion in the presence of alkylating agents. In contrast, by replacing this cellular promoter with the powerful retroviral-derived myeloproliferative sarcoma virus enhancer, negative control region-deleted, dl587rev primer-binding site substituted promoter, the vector allowed efficient chemoprotection of transduced HSCs at low MOI. However, this promoter interacted with the erythroid HS40/ankyrin enhancer/promoter driving green fluorescent protein, leading to an unexpected loss of erythroid specificity. A partial restoration of tissue-specific expression was obtained by interposition of insulator sequences between the expression units. Alternatively, we found that the strong human cellular elongation factor1-alpha promoter allows similar chemoprotection but without any deregulation of the erythroid-specific promoter in the absence of insulators. These data demonstrate that the level of in vivo deregulation induced by a promoter is not correlated with its transcriptional activity.

[Successful gene therapy of mice with congenital erythropoietic porphyria]. / Succès de la thérapie génique d'un modèle murin de porphyrie érythropoïétique congénitale.

Porphyrias are a group of disorders due to a genetic deficiency in one of the heme biosynthetic pathway enzymes. Congenital erythropoietic porphyria (CEP) is the most severe type characterized by a deficiency in uroporphyrinogen III synthase (UROS) activity. Bone marrow transplantation represents a curative treatment for patients, as long as human leucocyte antigen-compatible donor is available. We used a recently obtained murine model to check the feasibility of gene therapy in this disease. Lentivirus-mediated transfer of the human UROS cDNA into hematopoietic stem cells (HSCs) from Uros(mut 248) mice resulted in a complete and long-term enzymatic, metabolic and phenotypic correction of the disease, favored by a survival advantage of corrected red blood cells. These results demonstrate for the first time that the cure of this mouse model of CEP at moderate transduction level supports the proof of concept of a gene therapy in this disease by transplantation of genetically modified HSCs.

Human cell engraftment after busulfan or irradiation conditioning of NOD/SCID mice.

Human hematopoietic stem cell (HSC) xenotransplantation in NOD/SCID mice requires recipient conditioning, classically achieved by sublethal irradiation. Pretreatment with immunosuppressive and alkylating agents has been reported, but has not been rigorously tested against standard irradiation protocols. Here, we report that treatment of mice with a single dose (35 mg/kg) of Busilvex, an injectable form of busulfan, enables equivalent engraftment compared to 3.5 Gy irradiation. Mice treated with two doses of 25 mg/kg to reduce busulfan toxicity showed increased chimerism. Busulfan conditioning and irradiation resulted in comparable sensitivity of HSC detection as evaluated by limiting dilution analysis.

Clinical-scale cultures of cord blood CD34(+) cells to amplify committed progenitors and maintain stem cell activity.

We developed a clinical-scale cord blood (CB) cell ex vivo procedure to enable an extensive expansion of committed progenitors--colony-forming cells (CFCs) without impairing very primitive hematopoietic stem cells (HSCs). CD34(++) cells, selected from previously cryopreserved and thawed CB units, were cultured in two steps (diluted 1:4 after 6 days) in the presence of stem cell factor (SCF), fms-related tyrosine kinase 3 ligand (Flt-3L), megakaryocyte growth and development factor (MGDF) (100 ng/ml each), granulocyte-colony stimulating factor (G-CSF) (10 ng/ml) in HP01 serum-free medium. HSC activity was evaluated in a serial transplantation assay, by detection of human cells (CD45, CD33, CD19 and CFC of human origin) in bone marrow (BM) of primary and secondary recipient NOD/SCID mice 6-8 weeks after transplantation. A wide amplification of total cells (∼350-fold), CD34(+) cells (∼100-fold), and CFC (∼130-fold) without impairing the HSC activity was obtained. The activity of a particular HSC subpopulation (SRC(CFC)) was even enhanced.Thus, an extensive ex vivo expansion of CFCs is feasible without impairing the activity of HSCs. This result was enabled by associating antioxidant power of medium with an appropriate cytokine cocktail (i.e., mimicking physiologic effects of a weak oxygenation in hematopoietic environment).