Development of a ß-Globin Gene Replacement Strategy as a Therapeutic Approach for ß-Hemoglobinopathies
ß-hemoglobinopathies are the most common genetic disorders worldwide. In sickle cell disease (SCD) a single mutation (E6V) in the b-globin (HBB) gene results in dysfunctional hemoglobin protein, while in ß-thalassemia, over 300 mutations distributed across the HBB gene are known to reduce the production of ß-globin and cause severe anemia. A genetic engineering approach that replaces the whole HBB gene is an ideal strategy to rescue HBB expression for most genotypes but is technically challenging as the insert cannot be homologous to the endogenous gene and codon-optimized, intron-less sequences may not reconstitute adequate HBB levels. Here, we developed a novel approach for a “one-size-fits-all" HBB gene repair strategy using CRISPR-Cas9 which successfully addresses these problems.  First, our DNA donor design avoids sequence homology through a diverged HBB coding sequence and second, incorporates heterologous introns from the fetal g-globin gene, further reducing homology to endogenous HBB while mimicking its intron composition. Screening DNA donors with various heterologous globin introns, polyadenylation signals and truncated sequences, identified a heterologous intron DNA donor that highly expresses ß-globin and rescued ß-globin expression in two in vitro hemoglobinopathy models in hematopoietic stem and progenitor cells (HSPCs). Furthermore, healthy donor HSPCs modified with this HBB gene replacement approach showed successful engraftment in immunodeficient mice at 16 weeks. In summary, we developed a universal HBB gene replacement strategy that results in physiological b-globin production, offering a potential differentiated approach for treating patients with ß-thalassemia, SCD or compound heterozygous individuals. Overall design: HBB gene repair strategy using CRISPR-Cas9 which includes: RNAseq on donors 1125 and 3404; mouse engraftment study, and long read sequencing data.