Does summation of HMIP SNPs differentiate between the Hb F of the β°Codon39 and the β+IVS-I-6C thalassaemia heterozygotes?

Abstract

This research was intended to confirm the observations of Daw (2012) regarding the differential effect of summation of SNPs at the HMIP locus on (absolute) Hb F levels between β thalassemia (thal) heterozygotes with, either the β°Codon39T, or the β+IVS-I-6C mutations. The β°Codon39T mutation has been known to result in higher Hb F among heterozygotes and homozygotes. However, with the exception of the confounding effect of the conditional dimorphism (C/T) at -158 in the Gγ promoter, known as the XmnI site, an adequate explanation has not been available. The total dataset was increased from 167 of Daw, to 277 in this study. Furthermore, the thalassaemia heterozygotes with mild anaemia were separated from the rest with normal Hb level. Two cis regulatory loci, the Xmnl site and the (AT)xTy polymorphism as well as two trans regulatory sites, BCL11A and HMIP polymorphism were sequenced. Neither one of the trans or cis- regulatory sites nor the BCL11A were found to have significant effect on Hb F level. However, the Hb F of the 35 β°Codon39T heterozygotes with total Hb ≥ 10 g/dl was significantly different from that of β+IVS-I-6C hetetrozygotes (P < 0.005). In particular, those with HMIP (++++) haplotype had Hb F of (24 ± 9 N=8) in the β°Codon39 thalassaemia and (9 ± 2 N=10) in the β+IVS-I-6C thalassaemia even when all cis and trans heterogeneities tested were accounted for. Any explanation appeared inadequate at this stage, but functional experiments could serve to distinguish between alternative mechanisms including the possibility of a trans-regulator that bound both the HMIP locus and the p globin gene around β°Codon39T competitively or cooperatively. A much larger data set is being sought together with the direct quantification of the absolute HbF by immuno-assay and the enumeration of F-erythrocytes by flow cytometry, in order to put the observation on more solid ground and explore possiable mechanisms.