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

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Abstract
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 2 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.