Homozygosity for a haplotype in the HBG2-OR51B4 region is exclusive to Arab-Indian haplotype sickle cell anemia

Hb F Levels in β-Thalassemia Carriers and Normal Individuals: Known and Unknown Quantitative Trait Loci in the β-Globin Gene Cluster

In the already identified quantitative trait loci (QTL), modulating Hb F levels are cis-acting haplotypes of the β-globin gene cluster itself, although the single nucleotide polymorphisms (SNPs) accounting more for the association, remain uncertain. In this study, the role in Hb F production of previously reported candidate SNPs within the β-globin gene cluster was reexamined, along with a yet poorly studied variation in the BGLT3 gene. In a sample of β-thalassemia (β-thal) carriers, we succeeded in replicating the significant association between increased Hb F levels and rs7482144 (C>T) (HBG2 XmnI), which is the most well-established variation in the cluster influencing the trait. This SNP was found to be in strong linkage disequilibrium (LD) with a variation in the HBBP1 gene [rs10128556 (G>A)], which consistently revealed a similar association signal. Remarkably, much stronger than the latter associations were those involving both rs968857 (T allele) (3' HBBP1) and rs7924684 (G allele) (BGLT3), two SNPs that were also in strong LD. As the pattern of LD detected in the β-globin gene cluster does not correlate with a tight linkage between markers, complex interactions between SNPs at the cluster seem to modulate Hb F. Seeing that no such associations were detected in normal subjects, the question can be raised on whether, under erythropoiesis stress, epigenetic mechanisms contribute to change the regulation of the entire β-globin gene cluster. In conclusion, we provide statistical evidence for a new player within the β-globin gene cluster, BGLT3, that in cooperation with other regions influences Hb F levels in β-thal carriers.

Protocol for "Genetic composition of sickle cell disease in the Arab population: A systematic review"

Background

Sickle Cell Disease (SCD) is a global health issue in hematology with a progressively increasing prevalence. There are recent advances in the management of SCD, with new drugs being introduced. It is essential to analyze the genetic makeup of SCD regionally to anticipate the effectiveness of management modalities. This systematic review's main objectives are (a) to combine the existing knowledge of the genetic composition of SCD in the Arab population and (b) to analyze the various phenotypes of SCD prevalent in the Arab population.

Methods

We will perform a systematic review and search multiple electronic databases predefined search terms to identify eligible articles. Eligible studies should report findings on the genetic testing of Sickle Cell disease in the 22 Arab countries. Case reports, case series, observational studies with cross‐sectional or prospective research design, case‐control studies, and experimental studies will be included. Study quality will be independently evaluated by two reviewers using the statistical methodology and categories guided by the Cochrane Collaboration Handbook and PRISMA guidelines.

Discussion

This review will explore and integrate the evidence available on the various genotypes and phenotypes of SCD in the Arab population. By acquiring and summarizing data about the genetic and phenotypic variants of the SCD patient population, this study will add to the knowledge and help find more precise treatments.

Systematic review registration

The protocol is registered at the International Prospective Register of Systematic Reviews (PROSPERO; registration number: CRD42020218666).

Targeting fetal hemoglobin expression to treat β hemoglobinopathies

INTRODUCTION

Sickle cell disease and β thalassemia are the principal β hemoglobinopathies. The complex pathophysiology of sickle cell disease is initiated by sickle hemoglobin polymerization. In β thalassemia, insufficient β-globin synthesis results in excessive free α globin, ineffective erythropoiesis and severe anemia. Fetal hemoglobin (HbF) prevents sickle hemoglobin polymerization; in β thalassemia HbF compensates for the deficit of normal hemoglobin. When HbF constitutes about a third of total cell hemoglobin, the complications of sickle cell disease are nearly totally prevented. Similarly, sufficient HbF in β thalassemia diminishes or prevents ineffective erythropoiesis and hemolysis.

AREAS COVERED

This article examines the pathophysiology of β hemoglobinopathies, the physiology of HbF, intracellular distribution and the regulation of HbF expression. Inducing high levels of HbF by targeting its regulatory pathways pharmacologically or with cell-based therapeutics provides major clinical benefit and perhaps a "cure."

EXPERT OPINION

Erythrocytes must contain about 10 pg of HbF to "cure" sickle cell disease. If HbF is the only hemoglobin present, much higher levels are needed to "cure" β thalassemia. These levels of HbF can be obtained by different iterations of gene therapy. Small molecule drugs that can achieve even modest pancellular HbF concentrations are a major unmet need.

Fetal hemoglobin in sickle cell anemia

Fetal hemoglobin (HbF) can blunt the pathophysiology, temper the clinical course, and offer prospects for curative therapy of sickle cell disease. This review focuses on (1) HbF quantitative trait loci and the geography of β-globin gene haplotypes, especially those found in the Middle East; (2) how HbF might differentially impact the pathophysiology and many subphenotypes of sickle cell disease; (3) clinical implications of person-to-person variation in the distribution of HbF among HbF-containing erythrocytes; and (4) reactivation of HbF gene expression using both pharmacologic and cell-based therapeutic approaches. A confluence of detailed understanding of the molecular basis of HbF gene expression, coupled with the ability to precisely target by genomic editing most areas of the genome, is producing important preliminary therapeutic results that could provide new options for cell-based therapeutics with curative intent.

Prevalence and Diversity of Haplotypes of Sickle Cell Disease in the Eastern Province of Saudi Arabia

Hb F modulates sickle cell disease. Five major haplotypes of the β-globin gene cluster are associated with sickle cell disease. In the Eastern Province of Saudi Arabia, the Arab-Indian (AI) is most common. Single nucleotide polymorphism (SNP) genotyping (rs3834466, rs28440105, rs10128556, and rs968857) was carried out by nuclease allelic discrimination assay with target-specific forward and reverse primers, TaqMan probes, labeled with VIC and FAM. In 778 patients with sickle cell disease from the Eastern Province, a haplotype was assigned to 90.9% of all samples; 9.1% were classified as compound heterozygotes for the AI and an atypical haplotype. The distribution of haplotypes for 746 Hb S (HBB: c.20A > T) homozygotes was: 614 AI/AI, nine SEN/SEN (Senegal), 42 SEN/AI, nine CAM/CAM (Cameroon), one CAR (Central African Republic)/BEN (Benin), 71 AI/atypical. In Hb S/β-thalassemia (Hb S/β-thal), the distribution of Hb S haplotypes was: 22 AI/AI, one CAM/CAM, four AI/SEN, five AI/atypical. Mean Hb F in the haplotypes was: AI/AI 16.6 ± 7.5%, CAM/CAM 8.0 ± 4.1%, SEN/SEN 11.0 ± 5.1%, SEN/AI 15.1 ± 4.6%, AI/atypical 16.2 ± 6.5%. The presence of the SEN and CAM haplotypes was unexpected due to the apparent homogeneity of the population of the Eastern Province. We have successfully classified sickle cell disease haplotypes using the relatively inexpensive TaqMan assay for the first time. In addition, we have previously shown that children with AI haplotype have Hb F of 30.0% and mild disease, while in our cohort of adult AI patients, which might be the largest yet reported, Hb F was about 16.6%.

Sickle cell disease in the era of precision medicine: looking to the future

Introduction

Sickle cell anemia is a mendelian disease that is noted for the heterogeneity of its clinical expression. Because of this, providing an accurate prognosis has been a longtime quest.

Areas covered

Reviewed are the benefits and shortcomings of testing for the major modulators of the severity of disease, like fetal hemoglobin and α thalassemia, along with studies that have attempted to link genetic variation with sub-phenotypes of disease in a predictive fashion. Induced pluripotent stem cells driven to differentiate into erythroid precursor cells provide another area for potential patient-specific drug testing.

Expert opinion

Fetal hemoglobin is the strongest modulator of sickle cell anemia but simply measuring its blood levels is an insufficient means of forecasting an individual's prognosis. A more precise method would be to know the distribution of fetal hemoglobin levels across the population of red cells, an assay not yet available. Prognostic measures have been developed using genetic and other signatures, but their predictive value is suboptimal. Widely applicable assays must be developed to allow a tailored approach to using the several new treatments that are likely to be available in the near future.

Fetal hemoglobin in sickle cell anemia: The Arab-Indian haplotype and new therapeutic agents

Fetal hemoglobin (HbF) has well-known tempering effects on the symptoms of sickle cell disease and its levels vary among patients with different haplotypes of the sickle hemoglobin gene. Compared with sickle cell anemia haplotypes found in patients of African descent, HbF levels in Saudi and Indian patients with the Arab-Indian (AI) haplotype exceed that in any other haplotype by nearly twofold. Genetic association studies have identified some loci associated with high HbF in the AI haplotype but these observations require functional confirmation. Saudi patients with the Benin haplotype have HbF levels almost twice as high as African patients with this haplotype but this difference is unexplained. Hydroxyurea is still the only FDA approved drug for HbF induction in sickle cell disease. While most patients treated with hydroxyurea have an increase in HbF and some clinical improvement, 10 to 20% of adults show little response to this agent. We review the genetic basis of HbF regulation focusing on sickle cell anemia in Saudi Arabia and discuss new drugs that can induce increased levels of HbF.

A candidate transacting modulator of fetal hemoglobin gene expression in the Arab-Indian haplotype of sickle cell anemia

Fetal hemoglobin (HbF) levels are higher in the Arab-Indian (AI) β-globin gene haplotype of sickle cell anemia compared with African-origin haplotypes. To study genetic elements that effect HbF expression in the AI haplotype we completed whole genome sequencing in 14 Saudi AI haplotype sickle hemoglobin homozygotes-seven selected for low HbF (8.2% ± 1.3%) and seven selected for high HbF (23.5% ± 2.6%). An intronic single nucleotide polymorphism (SNP) in ANTXR1, an anthrax toxin receptor (chromosome 2p13), was associated with HbF. These results were replicated in two independent Saudi AI haplotype cohorts of 120 and 139 patients, but not in 76 Saudi Benin haplotype, 894 African origin haplotype and 44 AI haplotype patients of Indian origin, suggesting that this association is effective only in the Saudi AI haplotype background. ANTXR1 variants explained 10% of the HbF variability compared with 8% for BCL11A. These two genes had independent, additive effects on HbF and together explained about 15% of HbF variability in Saudi AI sickle cell anemia patients. ANTXR1 was expressed at mRNA and protein levels in erythroid progenitors derived from induced pluripotent stem cells (iPSCs) and CD34⁺ cells. As CD34⁺ cells matured and their HbF decreased ANTXR1 expression increased; as iPSCs differentiated and their HbF increased, ANTXR1 expression decreased. Along with elements in cis to the HbF genes, ANTXR1 contributes to the variation in HbF in Saudi AI haplotype sickle cell anemia and is the first gene in trans to HBB that is associated with HbF only in carriers of the Saudi AI haplotype. Am. J. Hematol. 91:1118-1122, 2016. © 2016 Wiley Periodicals, Inc.