POSSIBLE FACTORS INFLUENCING THE HEMOGLOBIN AND FETAL HEMOGLOBIN LEVELS IN PATIENTS WITH BETA-THALASSEMIA DUE TO A HOMOZYGOSITY FOR THE IVS-I-6 (T-]C) MUTATION

We have collected haematological, haemoglobin (Hb) and DNA sequence da ta for 29 patients with a homozygosity for the IVS-I-6 (T-->C) mutatio n with the intention of identifying factors contributing to the observ ed variability in the severity of the disease. None of the patients ha d received blood transfusion therapy for at least 6 months prior to th e 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 conco mitant alpha-thalassaemia-2 (alpha-thal-2) heterozygosity. An inverse correlation between the Hb F and Hb A(2) levels was observed. The majo rity of the patients were homozygous for haplotype VI (49/58 chromosom es) but haplotypes IV (2/58) and VII (7/58) were also present. The onl y haplotype IV homozygote had high Hb F levels with high G(gamma) valu es and the C-->T mutation at position -158 in the G(gamma) promoter, w hile both high and low Hb F levels were observed among patients with h aplotypes VI and VII. Analysis of sequence variations in regulatory re gions included the 5'hypersensitive sites (HS) 4, 3 and 2 of the locus control region (LCR), the G(gamma) and A(gamma) 5' flanking regions, the second intervening sequence (IVS-II), and the 5' beta-globin gene region in two patients with high Hb F (one homozygote each for haploty pes VI and IV), and in two patients with low Hb F levels (one homozygo te each for haplotypes VI and VII). Haplotype specific differences wer e observed in the LCR 5' HS-2 and in the G(gamma) and A(gamma) flankin g and IVS-II regions; however, no differences were present between the low and high Hb F-producing haplotype VI chromosomes, suggesting a ma jor role for factors which are not linked to the beta-globin gene clus ter in mediating gamma-globin gene expression in patients with this ty pe of beta-thal.