Genetic studies of fetal hemoglobin in the Arab‐Indian haplotype sickle cell‐β<sup>0</sup> thalassemia
Authors: Abdulrahman Alsultan, Duyen Ngo, Harold Bae, Paola Sebastiani, Clinton T. Baldwin, Efthymia Melista, Ahmed M. Suliman, Waleed H. Albuali, Zaki Nasserullah, Hong‐yuan Luo, David H.K. Chui, Martin H. Steinberg, Amein K. Al‐Ali
Journal: American Journal of Hematology (2013)

Independently of its African roots, the HbS gene is also indigenous to the Middle East and India where it can be on an Arab-Indian (AI) HBB haplotype. This haplotype is associated with high levels of fetal hemoglobin (HbF) and increased HbF improves many complications of sickle cell disease [1]. Polymorphisms in BCL11A (chr 2p) and HBS1L-MYB (chr 6q) and in elements linked to HBB (chr 11p) are associated with HbF level. In AI haplotype HbS homozygotes, SNPs in BCL11A and HBS1L-MYB accounted for 8.8% of HbF variance. We studied 61 patients with HbS-β0 thalassemia, aged ≥12 years and not taking hydroxyurea at the time HbF was measured by high performance liquid chromatography (HPLC). HbS-β0 thalassemia was determined by the presence of the HbSF phenotype by HPLC, heterozygosity for the HbS mutation and microcytosis. The β0-thalassemia mutation was not genotyped. The HbS mutation was ascertained using ARMS (amplification refractory mutation system) analysis. The AI haplotype was determined by genotyping the 5′ HBG2 Xmn1 C>T restriction site (rs7482144) and a Hinc2 site 5′ to HBE1 (rs3834466), and confirmed by the presence of a C>T polymorphism 68 bp 5′ to HBD 2. Single nucleotide polymorphism (SNP)s studied are shown in Table 1 and were detected by TaqMan assays (Applied Bio systems) or by Sanger di-deoxy sequencing. Linear regression was performed on HbF for each genetic locus, adjusting for gender. The analysis was performed using an additive genetic model whereby the total number of minor alleles present was counted for each subject. β-Thalassemia is common in Al-Ahsa. The four most prevalent mutations are: codon 39 C-T (25%); IVSII-1 G-A (22.2%); IVSI-5 G-C (13.3%); and IVSI 25 bp deletion (13.0%). α-Thalassemia has little effect on HbF in other haplotypes of sickle cell anemia 3, 4. More than 40% of AI haplotype HbS homozygotes had some form of α-thalassemia. HbS-β0 thalassemia patients had a mean HbF of 17.7%, a level similar to that found in HbS homozygotes and similar to those previously reported in AI haplotype HbS-β0 thalassemia but distinct from HbS-β0 thalassemia with African haplotypes. Their HbF distribution is not significantly different from that in AI haplotype HbS homozygotes (Fig. 1). There was no association of SNPs in BCL11A or the HBS1L-MYB intergenic region with HbF (Table 1). The number of patients we studied was small. To have 80% power to detect associations of SNPs in BCL11A (rs766432) with HbF 140 cases would be needed; 1,640 cased are required to detect an association with rs9399137 in HBS1L-MYB. Given the results in sickle cell anemia and the AI haplotype, the effects of these SNPs on HbF in Saudi patients with HbS-β0 thalassemia are likely to be small. Along with interactions of cis-acting elements of the AI haplotype with trans-acting factors prevalent in the Saudi Arab population that remain to be discovered, reduced or abnormal β-globin mRNA caused by β0 thalassemia and stress erythropoiesis might contribute to increasing HbF expression 5. Less β-globin mRNA might enhance the stability of γ-globin RNA by reducing post-transcriptional competition for the translational machinery 6. Sickle cell trait carriers with the AI haplotype and Saudi Arab heterozygotes with β0 thalassemia had nearly normal HbF levels. The absence of hemolytic anemia or suppression of HBB expression trans to the HbS gene might account for their normal HbF. Abdulrahman Alsultan1* Duyen Ngo2 Harold Bae3 Paola Sebastiani3 Clinton T. Baldwin4 Efthymia Melista4 Ahmed M. Suliman5 Waleed H. Albuali5 Zaki Nasserullah6 Hong-yuan Luo2 David H.K. Chui2 Martin H. Steinberg2 and Amein K. Al-Ali7 1Sickle Cell Disease Research Center and Department of Pediatrics, College of Medicine, King Saud University, Riyadh, Saudi Arabia 2Center of Excellence in Sickle Cell Disease, Department of Medicine, Boston University School of Medicine, Boston, Massachusetts 3Department of Biostatistics, Boston University School of Public Health, Boston, Massachusetts 4Center for Human Genetics, Boston University School of Medicine, Boston, Massachusetts 5King Fahad Hospital, Hofuf, Saudi Arabia 6Maternity & Child Hospital, Dammam, Saudi Arabia 7Al-Omran Scientific Chair, College of Medicine, King Faisal University, Al-Ahsa, Saudi Arabia