Genetic variations in human fetal globin gene microsatellites and their functional relevance

Short tandem repeats are abundantly present within the genome. They are commonly used as polymorphic markers but their potential functional role is poorly documented. Several of these microsatellites have been described within the beta-globin locus and some could be involved in controlling gene expression. Our purpose was to investigate the extent and significance of the (TG)n(CG)m dinucleotide repeat polymorphisms in the two gamma-globin gene IVS2s. Two groups of subjects were studied: a group of 63 beta-thalassaemic patients presenting either with a severe Cooley's anaemia (n=50) or with thalassaemia intermedia (TI, n=13), and a control group of 60 unrelated healthy individuals. A high heterogeneity of the polymorphic repeats was demonstrated, extending the range of the published alleles from 13 to 22 and allowing a first attempt at making a phenotype/genotype correlation. One specific allele, (TG)13 in the Agamma-gene, was highly enriched in the TI patients (46.1% vs 2.9% of the Cooley's anaemia cases, P < 0.0002, and 23.3% in the normal controls, P < 0.008) and preferentially observed in TI patients with a high haemoglobin F (Hb F). Transient transfection assays in K562 cells, with the growth hormone gene as a reporter, showed a positive regulatory action mediated by a (TG)13-containing 243 nt IVS2 fragment. Finally, a first set of mobility shift experiments with erythroid (K562 and MEL) and nonerythroid (HeLa) cell lines showed binding of erythroid component(s) in this DNA region and the binding pattern was modified upon induction of MEL cells by DMSO. Thus, our in vivo and in vitro data raise the question of a possible contribution of the gamma-gene IVS2s polymorphic microsatellites to the variable Hb F synthesis in the major haemoglobinopathies: a well known, puzzling and still unanswered question.