Potential new approaches to the management of the Hb Bart's hydrops fetalis syndrome: The most severe form of a-thalassemia

Abstract

The a-thalassemia trait, associated with deletions removing both a-globin genes from 1 chromosome (genotype z aa/z-), is common throughout Southeast Asia. Consequently, many pregnancies in couples of Southeast Asian origin carry a 1 in 4 risk of producing a fetus inheriting no functional a-globin genes (z-/z-), leading to hemoglobin (Hb) Bart's hydrops fetalis syndrome (BHFS). Expression of the embryonic a-globin genes (z-globin) is normally limited to the early stages of primitive erythropoiesis, and so when the z-globin genes are silenced, at ∼6 weeks of gestation, there should be no a-like globin chains to pair with the fetal g-globin chains of Hb, which consequently form nonfunctional tetramers (g4) known as Hb Bart's. When deletions leave the z-globin gene intact, a low level of z-globin gene expression continues in definitive erythroid cells, producing small amounts of Hb Portland (z2g2), a functional form of Hb that allows the fetus to survive up to the second or third trimester.Untreated, all affected individuals die at these stages of development. Prevention is therefore of paramount importance. With improvements in early diagnosis, intrauterine transfusion, and advanced perinatal care, there are nowa small number of individuals with BHFS who have survived, with variable outcomes. A deeper understanding of the mechanism underlying the switch from z- to a-globin expression could enable persistence or reactivation of embryonic globin synthesis in definitive cells, thereby providing new therapeutic options for such patients.