Molecular Studies of β-Thalassemia Heterozygotes with Raised Hb F Levels

Abstract

Hb F levels in β-thalassemia heterozygotes are usually less than 2%, but amongst 1,059 patients studied, 73 (7%) had Hb F levels above 2.5% (2.6–14.0%). To investigate factors that may influence the increase of Hb F levels in these heterozygotes, we characterized the β-thalassemia mutations and their chromosomal background, γ-globin gene promoter variations, and α-globin genotypes. All 73 β-thalassemia heterozygotes carried β-thalassemia point mutations previously observed in the Greek population; gene mapping excluded b gene cluster deletions; only two cases had an additional γ-globin gene (γγγ/γγ). Five α-globin genes (ααα/αα) were detected in 17/73 cases (23%) as compared to a carrier rate of 1.76% in the general population. Molecular, hematological, and biosynthetic findings in these compound heterozygotes indicated that the raised Hb F levels were caused by cell selection due to ineffective erythropoiesis. In the remaining 56 simple β-thalassemia heterozygotes, 11 β-thalassemia mutations were observed, each on the expected haplotype(s), and analysis of the γ gene promoters revealed three known polymorphisms (in linkage disequilibrium), with minimal influence on γ-globin levels. However, the overall distribution of β-thalassemia mutations in the 56 simple β-thalassemia heterozygotes was significantly different (P<0.0002) compared to that in 986 simple β-thalassemia heterozygotes with <2.5% Hb F., implicating an association between β-thalassemia mutations and moderately increased Hb F levels, most notably codon 39 (C→T), IVS-II-1 (G→A), codon 6 (-A), and codon 8 (-AA), which accounted for 41/56 (73%) cases with >2.5% Hb F. In the remaining 15/56 (27%) cases, no common underlying globin genotypes could explain the raised Hb F levels. Overall, this study indicates that the control of Hb F levels in β-thalassemia heterozygotes is heterogeneous and multi-factorial.