Organization of α-chain genes among Hb G-Philadelphia heterozygotes in association with Hb S, β-thalassemia, and α-thalassemia-2

Abstract

The percentages of the α-chain variant Hb G-Philadelphia (Hb G) or α2 68 Asn→Lysβ2 were evaluated in 84 adult and 18 newborn heterozygotes. These included members of three families who were studied in more detail by nucleic acid hybridization techniques. The adult heterozygotes fell in two categories, one with a higher proportion of Hb G [46.5±1.0% (SD), N=21] and another with lower values (33.9±3.4%, N=63). Among the newborn heterozygotes, two babies fell in the category with the higher proportion of Hb G while 16 babies gave values between 25 and 34%. Studies of α-chain gene organization on the parents of one neonate with a Hb G level of 27% at birth and 37% at 8 months excluded the presence of chromosomes with triplicated α-chain genes which could lead to the α0αG/ααα genotype. Rather, these studies on five Hb G heterozygotes from three families confirmed the linkage between Hb G and a specific type of α-thalassemia-2 associated with the presence of a 16-kbp Bgl II fragment which most probably carries the αG locus since it has been found in 19 Hb G heterozygotes studied to date. The presence of an α-thal-2 heterozygosity and three α-chain genes (α0αG/αα) was confirmed among Hb G heterozygotes with lower proportions of this variant. It is likely that the even lower values found in some newborn could arise through defective assembly of αG-γ dimers. The presence of an α-thal-2 homozygosity and two active α-chain genes, one on each chromosome (α0αG/α0α), was confirmed among heterozygotes with the higher proportion of Hb G. One of each of these categories was present in each of the three families investigated. This type of variability in the number of active α-chain genes due to a heterozygosity or a homozygosity for α-thalassemia-2 explains the trimodality of Hb S percentages among heterozygotes and the atypical hematological or biosynthetic features among patients with β-thalassemia and sickle-cell syndromes.