Genetic association of fetal-hemoglobin levels in individuals with sickle cell disease in Tanzania maps to conserved regulatory elements within the MYB core enhancer.
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/β(0) 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.
Muhimbili Wellcome Programme, Muhimbili University of Health and Allied Sciences, Dar-es-Salaam, Tanzania. siana.nkya@muhimbili-wellcome.org.|Dar es Salaam University College of Education, Dar es Salaam, Tanzania. siana.nkya@muhimbili-wellcome.org.|King's College London, Molecular Haematology, Division of Cancer Studies, London, UK. siana.nkya@muhimbili-wellcome.org.|Department of Haematology and Blood Transfusion, Muhimbili University of Health and Allied Sciences, PO Box 65001, Dar es Salaam, Tanzania. siana.nkya@muhimbili-wellcome.org.
Siana N
SN
Mtatiro
Muhimbili Wellcome Programme, Muhimbili University of Health and Allied Sciences, Dar-es-Salaam, Tanzania. josephine.mgaya@muhimbili-wellcome.org.
Josephine
J
Mgaya
Department of Human Genetics, Wellcome Trust Sanger Institute, Cambridge, UK. ts14@sanger.ac.uk.
Tarjinder
T
Singh
Muhimbili Wellcome Programme, Muhimbili University of Health and Allied Sciences, Dar-es-Salaam, Tanzania. harvest.mariki@muhimbili-wellcome.org.
Harvest
H
Mariki
King's College London, Molecular Haematology, Division of Cancer Studies, London, UK. helen.rooks@kcl.ac.uk.
Helen
H
Rooks
Muhimbili Wellcome Programme, Muhimbili University of Health and Allied Sciences, Dar-es-Salaam, Tanzania. deogratias.soka@muhimbili-wellcome.org.
Deogratius
D
Soka
Muhimbili Wellcome Programme, Muhimbili University of Health and Allied Sciences, Dar-es-Salaam, Tanzania. bruno.mmbando@muhimbili-wellcome.org.|National Institute for Medical Research, Tanga Centre, Tanga, Tanzania. bruno.mmbando@muhimbili-wellcome.org.
Bruno
B
Mmbando
King's College London, Molecular Haematology, Division of Cancer Studies, London, UK. sl.thein@kcl.ac.uk.
Swee Lay
SL
Thein
Department of Human Genetics, Wellcome Trust Sanger Institute, Cambridge, UK. jb26@sanger.ac.uk.
Jeffrey C
JC
Barrett
Muhimbili Wellcome Programme, Muhimbili University of Health and Allied Sciences, Dar-es-Salaam, Tanzania. julie.makani@muhimbili-wellcome.org.
Julie
J
Makani
Muhimbili Wellcome Programme, Muhimbili University of Health and Allied Sciences, Dar-es-Salaam, Tanzania. sharon.cox@muhimbili-wellcome.org.|London School of Hygiene & Tropical Medicine, London, UK. sharon.cox@muhimbili-wellcome.org.
Sharon E
SE
Cox
King's College London, Molecular Haematology, Division of Cancer Studies, London, UK. stephan.menzel@kcl.ac.uk.|King's College London, he James Black Centre, 125 Coldharbour Lane, London, SE5 9NU, UK. stephan.menzel@kcl.ac.uk.
Stephan
S
Menzel
D000755:Anemia, Sickle Cell / Q000235:genetics* / Q000378:metabolism*; D002648:Child; D002675:Child, Preschool; D002874:Chromosome Mapping; D017124:Conserved Sequence; D004742:Enhancer Elements, Genetic / Q000235:genetics*; D005260:Female; D005319:Fetal Hemoglobin / Q000378:metabolism*; D006239:Haplotypes; D006801:Humans; D008297:Male; D020641:Polymorphism, Single Nucleotide; D020598:Proto-Oncogene Proteins c-myb / Q000235:genetics*; D040641:Quantitative Trait Loci / Q000235:genetics; D013636:Tanzania
D020598:Proto-Oncogene Proteins c-myb; D005319:Fetal Hemoglobin