A Severe Mouse Model of Alpha-Thalassemia to Study Abnormal Iron Metabolism and Erythropoiesis, Hematopoietic Stem Cell Behavior and Development of a Gene Therapy Approach for Its Treatment

Alpha thalassemia (α-thal) is caused by insufficient production of the α-globin protein because of either deletional or non-deletional inactivation of endogenous α-globin genes. Clinical presentation of deletional α-thal varies from an asymptomatic condition (one inactivated α-globin gene) to a complete knockout (Hb Bart's Hydrops Fetalis). In patients with severe α-thal, a blood transfusion independent state is achievable through allogeneic bone marrow transplantation.

The aims of this study are to develop a novel adult mouse model of α-thal and a gene therapy approach for this disease.

We generated adult animals that do not produce α-globin chains (α-KO) through transplantation of homozygous B6.129S7-Hbatm1Paz/J fetal liver cells (FLC; isolated at E14.5) into WT recipient mice. These animals demonstrate a worsening phenotype, paradoxically showing elevated hematocrit, high reticulocyte count and a high number of red blood cells (RBC) which expressed only β-globin chains (HbH). RBC show aberrant morphology and aggregation of α- globin tetramers on RBC membranes. Due to severe inability of these RBC to deliver oxygen, the mice eventually succumb to anemia, showing splenomegaly and other organ pathologies, including vaso-occlusive events. These animals show iron deposition in the liver and kidney, in agreement with very low levels of hepcidin expression in the liver, and elevated erythropoietin (EPO) in the kidney.

Interestingly, α-KO embryos show lower numbers of FLC compared to WT embryos, lower frequency of engraftable hematopoietic stem cells (HSC; Lin-Sca-1+c-kit+CD48-), decreased clonogenic potential (fewer class 4 CFUs) and elevated erythroferrone. Lethally irradiated mice transplanted with FLC-KO require 5-6x as many cells as those transplanted with FLC-WT for recovery, further suggesting some level of engraftment impairment. Our current hypothesis is that excessive hypoxia in the embryos impairs HSC function and stem cell fitness. Additional assays are in progress to assess the nature of this impairment.

To generate a gene therapy tool to rescue these animals and eventually cure severe human α-thal patients, we screened multiple lentiviral vectors to identify the variant capable of producing the highest human α-globin protein per copy. The selection was conducted in murine erythroleukemia cells and human umbilical cord derived erythroid progenitor (HUDEP) cells, modified by knocking out all the human α-globin genes. We identified ALS20α, a vector where α-globin is under control of the β-globin promoter and its locus control region, as the most efficient vector. One copy of ALS20α produces exogenous α-globin at a level comparable to that produced by one endogenous α-globin gene. These results suggest that a relatively low VCN could result in dramatic therapeutic benefits. Transplantation of ALS20α transduced murine BM-KO results in correction of the disease phenotype in a dose-dependent manner. At VCN<1 we observe a delay in death proportional to the VCN value, while at VCN>1 we observe phenotypic normalization, including Hb, hepcidin and EPO levels.

We tested ALS20α in CD34 cells isolated from four patients with both deletional and non- deletional HbH disease. We measured the change of β/α-globin mRNA ratio (β/αR) and protein level by HPLC in erythroblasts derived from these cultures. For the specimen with mutational HbH, the initial β/αR matches that of healthy controls, as the mutations do not eliminate the ability for the gene to produce aberrant mRNA transcripts, and decreased with increasing VCN. Erythroblasts with deletional HbH have a β/αR approximately 3x higher than normal cells, decreasing in a dose dependent manner with increasing VCN. HPLC detection of HbH (β4), a hallmark of HbH disease, is observed in hemolysis products from all non-transduced α−thal erythroblasts. A ~50% reduction of HbH is detected in the very same specimens upon integration of ALS20α (VCN between 1 and 2).

In conclusion, we generated an adult mouse model of lethal α-thal and, in preliminary experiments, we rescue it with ALS20α. Furthermore, ALS20α successfully improves α-globin levels in patient cells. Further experiments are in progress to establish the consistency of our vector's expression in vivo, as well as to demonstrate its ability to transduce bona fide long-term HSCs.