Characterisation and locus assignment of two α-globin variants found in the Maltese population: HB St Luke's [α₂95(G2)PRO-->ARGβ₂] and HB SETIF [α₂94(G1)ASP-->TYRβ₂]

Abstract

Our understanding of the molecular regulation of the ζ-α2-α1 globin gene locus lags quite markedly behind that of the γ-δ-β globin genes and is based primarily on the documentation of many molecular types of deletional or non-deletional α-thalassaemias and as many as 191 α-globin variants together with a variety of molecular constructs. Studies on the relative expression of the two types of a.-globin genes are complicated by the highly homologous nature of the α1- and α2-globin genes which result in the production of chemically identical α-globin. The only manner in which the respective protein products can be measured is in the case of heterozygotes or homozygotes who inherit an α-globin variant. This variant is the product of one of the two types of α-globin genes and its quantity therefore reflects the relative expression of that gene, taking into account the stability of the resulting tetramer and the presence, or otherwise, of a coexistent β- or α-thalassaemia. Therefore locus assignment of variants is of crucial importance to the study of the relative expression of α-globin genes. To date the locus of only about one-fifth of the 191 α-variants characterised has been identified. Out of these variants only one-third are located on the α1-globin gene. An α-globin variant is found in 0.2% of the Maltese population, as detected upon routine analysis of blood by isoelectric focusing electrophoresis. Hb St Luke's was first identified in four members of a Maltese family and is present at levels of around 10% in heterozygotes. The substitution of proline by arginine at position 95 of the α-chain results in increased oxygen affinity and dissociation of the haemoglobin tetramer. The Hb St Luke's mutation has already been assigned to the α1-globin locus. The second variant studied here is Hb Setif which has now also been identified in the Maltese population. It has previously been reported in a number of unrelated families from the Mediterranean littoral region. The variant has been found at levels ranging from 12 to 30%. The replacement of aspartic acid by tyrosine at position 94 results in a slight instability probably because the bulky and neutral phenol side chain of the tyrosyl residue abolishes the hydrogen bond between the normal aspartyl residue and asparagine β120. The variant haemoglobin also has a decreased oxygen affinity and cooperativity. In vitro it produces pseudosickling of red cells. Three different and unrelated cases of this variant are described here. In this study we confirm the assignment of the St Luke's mutation to the α1-globin gene locus, we document the assignment of Hb Setif to the α2-globin gene and we compare their levels of expression.