Abstract:
Background: Common genetic variants residing near upstream regulatory elements for MYB, the gene encoding
transcription factor cMYB, promote the persistence of fetal hemoglobin (HbF) into adulthood. While they have no
consequences in healthy individuals, high HbF levels have major clinical benefits in patients with sickle cell disease
(SCD) or β thalassemia. Here, we present our detailed investigation of HBS1L-MYB intergenic polymorphism block 2
(HMIP-2), the central component of the complex quantitative-trait locus upstream of MYB, in 1,022 individuals with
SCD in Tanzania.
Methods: We have looked at 1022 individuals with HbSS or HbS/β0 in Tanzania. In order to achieve a detailed
analysis of HMIP-2, we performed targeted genotyping for a total of 10 SNPs and extracted additional 528 SNPs
information from a genome wide scan involving the same population. Using MACH, we utilized the existing YRI
data from 1000 genomes to impute 54 SNPs situated within HIMP-2.
Results: Seven HbF-increasing, low-frequency variants (β > 0.3, p < 10−5, f ≤ 0.05) were located in two
partially-independent sub-loci, HMIP-2A and HMIP-2B. The spectrum of haplotypes carrying such alleles was
diverse when compared to European and West African reference populations: we detected one such haplotype at
sub-locus HMIP-2A, two at HMIP-2B, and a fourth including high-HbF alleles at both sub-loci (‘Eurasian’ haplotype clade).
In the region of HMIP-2A a putative functional variant (a 3-bp indel) has been described previously, but no such
candidate causative variant exists at HMIP-2B. Extending our dataset through imputation with 1000 Genomes,
whole-genome-sequence data, we have mapped peak association at HMIP-2B to an 11-kb region around rs9494145 and
rs9483788, flanked by two conserved regulatory elements for MYB.
Conclusions: Studies in populations from the African continent provide distinct opportunities for mapping
disease-modifying genetic loci, especially for conditions that are highly prevalent there, such as SCD. Population-genetic
characteristics of our cohort, such as ethnic diversity and the predominance of shorter, African-type haplotypes, can add
to the power of such studies.
