In this study we have defined the molecular basis and correlated the clinical phenotype with the α-globin genotype in a large series of patients of Sardinian descent with HbH disease. The most prevalent molecular defect was the deletion of 3 α-globin structural genes most commonly the (– –/–α3.7) genotype (83.6%) and rarely the (– –/– α4.2) genotype (1.4%), followed in decreasing order of incidence by the combination of deletion α°-thalassemia and initiation codon mutation of the α2-gene (– –/αNcoIα = 9.8%), deletion α°-thalassemia and pentanucleotide deletion of IVS-I of the α2-globin gene, (– –/αHphIα = 3.3%) deletion α°-thalassemia and initiation codon mutation of the α1-gene (– –/ααNcoI = 1.3%), a homozygous state for initiation codon mutation of the α2-gene (αNcoα/αNcoIα = 0.7%). Patients with the (– –/αthalα) genotypes showed severer clinical and hematological features as compared to those with the (– –/–α) or those with the (– –/ααthal) genotypes. The single patient with the (αNcoα/αNcoα) genotype had a clinical phenotype intermediate between HbH disease and the α-thalassemia carrier status. This heterogeneity depends on the fact that the α2-globin gene produces 2-3 times α-globin chains than the α1-gene and the single remaining α1-like globin gene in the -α3 7 chromosome has a compensatory increase in the α-globin chain output. α-Globin gene mapping of HbH disease patients may be useful for predicting the clinical outcome and to improve genetic counselling.

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