Possible factors influencing the haemoglobin and fetal haemoglobin levels in patients with β-thalassaemia due to a homozygosity for the IVS-I-6 (T→C) mutation

Summary. We have collected haematological, haemoglobin (Hb) and DNA sequence data for 29 patients with a homozygosity for the IVS-I-6 (TC) mutation with the intention of identifying factors contributing to the observed variability in the severity of the disease. None of the patients had received blood transfusion therapy for at least 6 months prior to the study. Hb levels varied from 5·0 to 9·9 g/dl. Patients with high Hb F (more than 1·5 g/dl or <20%) had high total Hb levels (7·5–9·7 g/dl) but some with low Hb F also had high total Hb levels; two had a concomitant α-thalassaemia-2 (α-thal-2) heterozygosity. An inverse correlation between the Hb F and Hb A₂ levels was observed. The majority of the patients were homozygous for haplotype VI (49/58 chromosomes) but haplotypes IV (2/58) and VII (7/58) were also present. The only haplotype IV homozygote had high Hb F levels with high ^Gγ values and the CT mutation at position – 158 in the ^Gγ promoter, while both high and low Hb F levels were observed among patients with haplotypes VI and VII. Analysis of sequence variations in regulatory regions included the 5 hypersensitive sites (HS) 4, 3 and 2 of the locus control region (LCR), the ^Gγ and ^Aγ 5 flanking regions, the second intervening sequence (IVS-II), and the 5 β-globin gene region in two patients with high Hb F (one homozygote each for haplotypes VI and IV), and in two patients with low Hb F levels (one homozygote each for haplotypes VI and VII). Haplotype specific differences were observed in the LCR 5 HS-2 and in the ^Gγ and ^Aγ flanking and IVS-II regions; however, no differences were present between the low and high Hb F-producing haplotype VI chromosomes, suggesting a major role for factors which are not linked to the β-globin gene cluster in mediating γ-globin gene expression in patients with this type of β-thal.