Hb F Malta I in association with Hb F Sardinia (AyT) and Hb Valletta in heterozygotes : quantification of the six globins suggests developmental control of the XMN-I site and interplay with the (AT)xTy sequence in connection with globin gene switching

Abstract

Although the precise biochemical mechanisms of globin gene switching remain elusive, considerable insight is gained by in vivo expression profiling through quantification of the hemoglobin / globin phenotype of informative heterozygosities and homozygosities / compound heterozygosities in the context of specific regulatory DNA sequence diversity such as the XMN-I or the [(AT)xTy] sequence polymorphisms. The quantification of normal and abnormal globins of Hb F Malta-I (or a2b2, 117(G19)His>Arg) heterozygotes which are in tight linkage disequilibrium with Hb Valletta (or a2b2 287(f3)Thr>Pro) i.e. Gyo, GyFMalta-I, AyI, bV and bA together with extensive haplotyping of homozygotes and heterozygotes including the XMN-I dimorphism in the Gy promoter and the (AT)xTy polymorphism (BP1 binding site) 5′ to the b globin genes had suggested that the XMN-I dimorphism was largely inactive in the normal newborn. In contrast the Hb F levels and the proportion of Gy globin in anemic adult beta-thalassemia homozygotes and compound heterozygotes differed significantly, depending on the XMN-I genotype (TT, TC or CC) Here, we document the occurrence of seven newborn who were heterozygous at three globin loci permitting quantification by reverse phase liquid chromatography of the six globin products; Gyo, GyFMalta-I, AyI, AyT, bV and bA in the context of genotypic variation at the XMN-I and (AT)xTy sequences. The data were compared with those of newborn HbF-Malta-I-Hb-Valletta heterozygotes and anemic adult beta thalassemia homozygotes / compound heterozygotes. The globin quantification together with haplotype data were analysed using the general linear model (two-way ANOVA) by SPSS version 12. The data excluded significant effect of the XMN-I dimorphism alone on relative y/b globin gene expression in the newborn. On the other hand, the (AT)xTy polymorphism with BP1 binding sites of 21 [(AT)7T7], 23 [(AT)9T5], or 25 [(AT)11T3], nucleotides in trans over-ride XMN-I. In contrast, it is the XMN-I dimorphism that over-rides the (AT)xTy diversity in the anemic adult beta thalassemia homozygotes or compound heterozygotes. The GyFMalta-I/Gyo ratio of the newborn heterozygotes with Hb F Malta-I and the AyT/AyI ratio of the newborn heterozygotes with HbF-Malta-I and HbF-Sardinia suggested that the developmental regulation of the XMN-I site may be subject to cis/trans interplay with the (AT)xTy sequences.