Lnp-Targeting Hematopoietic Stem Cells and Lentiviral Gene Transfer to Generate and Rescue a Novel Mouse Model of Lethal Alpha-Thalassemia

α-Thalassemia (α-thal) is caused by insufficient production of the α-globin protein. Clinical presentation of α-thal varies from an asymptomatic condition (one inactivated α-globin gene) to Bart’s Hydrops Fetalis Syndrome/BHFS (complete knockout). In patients with severe α-thal, allogeneic bone marrow transplantation may be required for survival. Alternatively, patients could be treated through autologous BMT following ex vivo transduction with a human α-globin expressing lentiviral vector. However, a proper model system and vector must be established for the development of such a treatment.

We crossed animals that had deletion of both Hba-a1 and Hba-a2 in heterozygosity (Hba-a1^+/ko/Hba-a2^+/ko) to generate knockout (KO) embryonic mice lacking all the α-globin genes (Hba-a1^ko/ko/Hba-a2^ko/ko). Although these embryos die perinatally, their fetal liver cells (FLC) at 13.5 dpc are still viable and represent a source of engraftable hematopoietic stem cells (HSC). KO-FLC were transplanted into congenic myeloablated WT mice to generate mice carrying Hba-a1^ko/ko/Hba-a2^ko/ko HSC. These animals developed a severe phenotype ~7 weeks post engraftment. As these mice lack α-globin expression, we observed β-tetramer formation (β₄, HbH, Fig1A), resulting in aberrant red blood cell (RBC) morphology, β-globin precipitates, high RBC oxygen binding affinity, elevated erythropoietin, hematocrit and hemoglobin levels, splenomegaly and vaso-occlusive episodes, due to the large number of abnormal RBCs. Furthermore, we observed iron deposition in the liver and kidney, in agreement with very low levels of hepcidin expression in the liver.

However, FLC engraftment is cumbersome and requires extensive breeding and many animals to generate enough FLC for few recipients. To facilitate the generation of α-thal mice, we created conditional knockout (cKO) mice that have been edited to remove Hba-a2, while inserting loxP sites flanking Hba-a1 (Hba-a1^fl/Hba-a2^ko). Homozygous cKO animals (Hba-a1^fl/fl/Hba-a2^ko/ko) demonstrate a mild phenotype. To achieve complete deletion of the α-globin genes (Hba-a1^ko/ko/Hba-a2^ko/ko) in HSC, we used lipid nanoparticles (LNPs), which are one of the most promising delivery systems to package and make mRNA into a useful therapeutic. We developed a novel LNP preparation with an antibody to target CD117 and deliver nucleoside-modified mRNA to HSC, (see abstract Breda et al). We treated HSC from homozygous cKO animals (Hba-a1^fl/fl/Hba-a2^ko/ko) ex vivo with CD117-LNP embedded with Cre mRNAs (Cre-CD117-LNP). Cells showed the expected deletion of the α-globin genes and were injected into myeloablated recipient mice. These chimeras recapitulated the phenotype observed following transplantation of constitutive α-globin-KO FLC, including elevated hematocrit, splenomegaly, and culminating in lethality ~7 weeks post-transplant.

We screened multiple potential erythroid specific lentiviral vectors in both mouse and human cell lines and identified a promising candidate, ALS20αI, which at VCN=1 produced human α-globin protein equivalent to that of a single endogenous α-globin gene. Myeloablated recipient mice transplanted with Cre-CD117-LNP-treated cKO BM die roughly 7 weeks post-transplant with the expected pathological phenotype (Fig1A). In contrast, mice receiving BM treated with Cre-CD117-LNP and ALS20αI (VCN>1) all survived long term (>4-month) with normalization of erythropoiesis and iron metabolism. VCNs were proportional to chimeric hemoglobin (human α/mouse β) levels and concurrently decreased or absent HbH (Fig1B-C-D). Additionally, we tested ALS20αI in erythroid progenitors derived from CD34 cells isolated from patients with deletional and non-deletional HbH disease. Preliminary data demonstrates improvement in α-globin/β-globin mRNA ratio and reduction in formation of HbH by HPLC.

In summary, we have developed new mouse models for α-thal with a lethal and reproducible phenotype. Remarkably, use of LNP technology accelerated the generation of α-thal mice and their characterization. Furthermore, our new vector ALS20αI rescued these animals with stable expression of the human α-globin gene. These data strongly support the use of ALS20αI for the cure of severe forms of α-thal, as 2 copies may be sufficient to rescue patients affected by BHFS, while 1 copy may be sufficient to cure patients affected by HbH disease.

HP, SR corresponding Authors.