Eastern European (δβ)° thalassemia: Molecular characterization of a novel 9.1-kb deletion resulting in high levels of fetal hemoglobin in the adult

Abstract

A novel deletion in the human β-globin gene cluster associated with increased levels of fetal hemoglobin (HbF) in adult life was molecularly characterized in a member of a family of Eastern European descent. The phenotype of the deletion, documented in five members of the family, shows mild hypochromia and microcytosis (mean corpuscular Hb, 24 to 25.9 pg; mean corpuscular volume, 74 to 78.5 fL) but high production of HbF (13% to 24%) with heterocellular distribution (36% to 86% F cells). Extensive restriction enzyme mapping of the β-globin cluster and sequencing of the region encompassing the breakpoints showed that the deletion starts 1,612 bp upstream of the cap site of the δ-globin gene and terminates within the first intron of the β-globin gene, deleting 9.1 kb of DNA. This length is definitely shorter than the average 12.0 kb of the previously characterized (δβ)° thalassemias. The 5' breakpoint of the new deletion is close to that of the Yugoslavian δβ-thalassemia deletion, whereas the 3' breakpoint is very close to those of the Turkish and the Greek β°-thalassemia deletions. The breakpoints of the deletion occur within a direct repeat containing a tetranucleotide exhibiting homology to a donor-splice site, and is symmetrically flanked by a set of 13- and 14-bp homologous complementary sequences, respectively. It is likely that the deletion may be the result of an 'illegitimate' or 'nonhomologous' recombination event to which these two short sequences may have contributed. It is of interest that the novel deletion (9.1 kb) is comparable to the Italian HPFH-5 deletion (12.9 kb), regarding both the size and the position of the breakpoints. However, the HPFH-5 deletion includes sequences flanking the breakpoints that are preserved in the new deletion. Considering the resulting two discrete phenotypes (ie, δβ-thalassemia v HPFH), it can be hypothesized that the deleted sequences in the Italian HPFH-5 mutation may harbor regulatory elements that exert a negative control on the γ-globin gene expression.