NM_000558.5(HBA1):c.96-1G>A

For the HBA1 c.96-1G>A splice acceptor variant, which disrupts normal splicing of exon 2 and causes loss of functional alpha-globin, precision gene editing (adenine base editing or prime editing) in autologous hematopoietic stem cells offers a theoretically curative approach. Adenine base editing (ABE) could directly revert the A-to-G change at the canonical splice acceptor site, restoring normal splicing without double-strand breaks. This is particularly relevant because Hb Bart's hydrops fetalis requires loss of all four alpha-globin gene copies — correcting even one or two copies in sufficient HSCs could convert a lethal phenotype to a manageable carrier or trait state.

Alpha-thalassemia major (Hb Bart's hydrops fetalis) results from deletion or inactivation of all four alpha-globin genes (two HBA1 + two HBA2 copies). It is typically caused by homozygous --SEA or other large deletions, but point mutations like c.96-1G>A at splice acceptor sites can functionally inactivate individual alpha-globin genes. The c.96-1G>A variant disrupts the invariant AG dinucleotide at the intron 1/exon 2 boundary, causing exon skipping or cryptic splice site activation, producing no functional alpha-globin from this allele. Adenine base editors (ABE8e and variants) have demonstrated high-efficiency A-to-G editing at splice sites in HSCs in preclinical models for other hemoglobinopathies. The success of exa-cel (exagamglogene autotemcel, approved for SCD and beta-thalassemia) validates the ex vivo HSC gene editing paradigm: HSCs are harvested, edited, and reinfused after myeloablative conditioning. For alpha-thalassemia, the same platform could be adapted with a guide RNA targeting the c.96-1 position.

Hb Bart's hydrops fetalis is typically diagnosed prenatally and is usually lethal in utero or perinatally, creating an extremely narrow treatment window. Ex vivo HSC editing would require postnatal survival (via intrauterine transfusion) and neonatal stem cell collection, which is technically challenging. The c.96-1G>A variant is a point mutation causing alpha-thal, but most Hb Bart's cases involve large deletions (--SEA/--SEA) where base editing is not applicable — this limits the patient population for this specific approach. No clinical trial for gene editing in alpha-thalassemia major has been registered. Myeloablative conditioning in a severely hydrops neonate carries extreme risk. The approach assumes compound heterozygosity where at least some alleles carry correctable point mutations rather than deletions.