BCL11A enhancer Haplotypes Are Associated with the Distribution of HbF in Arab-Indian and African Haplotype Sickle Cell Anemia but Not the Different Population Levels of HbF

• Published in: Blood 2014-12, Vol.124 (21), p.4066-4066
• Main Authors: Sebastiani, Paola, Farrell, John J., Wang, Shuai, Edward, Heather L, Shappell, Heather M., Bae, Harold T., Baldwin, Clinton T., Al-Rubaish, A.M., Naserullah, Zaki, Alsuliman, Ahmed, Patra, Pradeep K., Farrer, Lindsay A., Chui, David H.K., Alsultan, Abdulrahman, Ngo, Duyen A., Al-Ali, Haifa Kathrin, Steinberg, Martin H.
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood.V124.21.4066.4066

Fetal hemoglobin (HbF) modulates the phenotype of sickle cell anemia. In the Middle East and India the HbS gene is often on an Arab-Indian HBB haplotype that is associated with high HbF levels. HbF is “normally” distributed in this population with a mean ~20%. In African HbS haplotypes, HbF levels are much lower (mean value ~6%) with a highly skewed distribution. BCL11A is an important modulator of γ-globin gene (HBG2 and HBG1) expression and BCL11A is regulated by erythroid specific enhancers in its 2nd intron. The enhancers consist of 3 DNase hypersensitive sites (HS) +62, +58 and +55 kb from the transcription initiation site of this gene. Polymorphisms (SNPs) in these enhancers are associated with HbF. The strongest association with HbF levels in African Americans with sickle cell anemia was with rs1427407 in HS +62 and to a lesser extent, rs7606173 in HS+55. Using the results of whole genome sequencing of 14 AI haplotype patients—half with HbF 20%—6 SNPs in the BCL11A enhancer region, rs1427407, rs7599488, rs6706648, rs6738440, rs7565301, rs7606173 and 2 indels rs3028027 and rs142027584 (CCT, CCTCT and AAAAC respectively), were detected as possibly associated with HbF level. There were no novel polymorphisms detected. We genotyped the 6 SNPs and studied their associated haplotypes in 137 Saudi (HbF18.0±7.0%) and 44 Indian patients (HbF23.0±4.8%) with the Arab-Indian HBB haplotype; 50 African Americans with diverse African haplotypes, including 4 Senegal haplotype heterozygotes, (20 with HbF 17.2±4.6% and 30 with HbF 5.0±2.5%) and imputed genotypes for these SNPs in 847 African Americans with sickle cell anemia and diverse haplotypes (HbF 6.6±5.5%). Four SNPs (rs1427407, rs6706648, rs6738440, and rs7606173) in the HS sites showed consistent associations with HbF levels in all 4 cohorts. Haplotype analysis of these 4 SNPs showed that there were 4 common and 10 rare haplotypes. The most common, GCAG, was found in ~54% of Arab-Indian haplotype carriers (HbF, ~20%) and in ~33% of African origin haplotype carriers (HbF, ~5.5%). Two haplotypes, GTAC and GTGC, were carried by ~40% of African American patients and were associated with lower levels of HbF (3.6%-4%). These same haplotypes were carried by 18% of Arab-Indian haplotype carriers and their average HbF level was 17%. These differences were significant. Haplotype TCAG was present in 20% of Arab-Indian and 25% of African haplotype cases, and carriers had on average higher HbF levels