Genetic Modifiers of Sickle Cell Disease: A Genotype-Phenotype Relationship Study in a Cohort of 82 Children on Mayotte Island

Characterization of a cohort of Angolan children with sickle cell anemia treated with hydroxyurea

BACKGROUND

Sickle Cell Anemia (SCA) is a monogenic disease, although its severity and response to treatment are very heterogeneous.

OBJECTIVES

This study aims to characterize a cohort of Angolan children with SCA and evaluate their response to hydroxyurea (HU) treatment and the potential side effects and toxicity.

METHODS

The study enrolled 215 patients between 3 and 12 years old before and after the administration of HU, at a fix dose of 20 mg/kg/day for 12 months.

RESULTS

A total of 157 patients started HU medication and 141 of them completed the 12-month treatment. After initiating HU treatment, the frequency of clinical events decreased (transfusions 53.4 %, hospitalizations 47.1 %). The response to HU medication varied among patients, with some experiencing an increase in fetal hemoglobin (HbF) of <5 %. The mean increase in HbF was 11.9 %, ranging from 1.8 % to 31 %. Responders to HU treatment were 57 %, inadequate responders 38.7 % and non-adherent 4.2 %. No clinical side effects related to HU were reported. Hematological toxicities were transient and reversible. Children naïve to HU and with lower HbF reported higher number of hospitalizations caused by malaria infection. During HU treatment, the frequency of malaria episodes did not appear to be affected by HbF levels.

CONCLUSIONS

the present study provided a valuable contribution to the understanding of the clinical and laboratory profiles of Angolan children with SCA. These findings support the evidence that the implementation of prophylactic measures and treatment with HU is associated with increased survival in children with SCA.

Genetic Variation and Sickle Cell Disease Severity: A Systematic Review and Meta-Analysis

Importance

Sickle cell disease (SCD) is a monogenic disorder, yet clinical outcomes are influenced by additional genetic factors. Despite decades of research, the genetics of SCD remain poorly understood.

Objective

To assess all reported genetic modifiers of SCD, evaluate the design of associated studies, and provide guidelines for future analyses according to modern genetic study recommendations.

Data Sources

PubMed, Web of Science, and Scopus were searched through May 16, 2023, identifying 5290 publications.

Study Selection

At least 2 reviewers identified 571 original, peer-reviewed English-language publications reporting genetic modifiers of human SCD phenotypes, wherein the outcome was not treatment response, and the comparison was not between SCD subtypes or including healthy controls.

Data Extraction and Synthesis

Data relevant to all genetic modifiers of SCD were extracted, evaluated, and presented following STREGA and PRISMA guidelines. Weighted z score meta-analyses and pathway analyses were conducted.

Main Outcomes and Measures

Outcomes were aggregated into 25 categories, grouped as acute complications, chronic conditions, hematologic parameters or biomarkers, and general or mixed measures of SCD severity.

Results

The 571 included studies reported on 29 670 unique individuals (50% ≤ 18 years of age) from 43 countries. Of the 17 757 extracted results (4890 significant) in 1552 genes, 3675 results met the study criteria for meta-analysis: reported phenotype and genotype, association size and direction, variability measure, sample size, and statistical test. Only 173 results for 62 associations could be cross-study combined. The remaining associations could not be aggregated because they were only reported once or methods (eg, study design, reporting practice) and genotype or phenotype definitions were insufficiently harmonized. Gene variants regulating fetal hemoglobin and α-thalassemia (important markers for SCD severity) were frequently identified: 19 single-nucleotide variants in BCL11A, HBS1L-MYB, and HBG2 were significantly associated with fetal hemoglobin (absolute value of Z = 4.00 to 20.66; P = 8.63 × 10-95 to 6.19 × 10-5), and α-thalassemia deletions were significantly associated with increased hemoglobin level and reduced risk of albuminuria, abnormal transcranial Doppler velocity, and stroke (absolute value of Z = 3.43 to 5.16; P = 2.42 × 10-7 to 6.00 × 10-4). However, other associations remain unconfirmed. Pathway analyses of significant genes highlighted the importance of cellular adhesion, inflammation, oxidative and toxic stress, and blood vessel regulation in SCD (23 of the top 25 Gene Ontology pathways involve these processes) and suggested future research areas.

Conclusions and Relevance

The findings of this comprehensive systematic review and meta-analysis of all published genetic modifiers of SCD indicated that implementation of standardized phenotypes, statistical methods, and reporting practices should accelerate discovery and validation of genetic modifiers and development of clinically actionable genetic profiles.

Impact of Genetic Polymorphisms in Modifier Genes in Determining Fetal Hemoglobin Levels in Beta-Thalassemia

Genetic polymorphisms in Quantitative Trait Loci (QTL) genes such as BCL11A, HBS1L-MYB and KLF1 have been reported to influence fetal hemoglobin (HbF) levels. This prospective study was planned to evaluate the role of genetic polymorphisms in QTL genes as determinant of HbF levels in beta thalassemia major patients. The study was carried out on 100 thalassemia major patients. Blood samples were collected in EDTA and plain vials for biochemical and molecular evaluation. The BCL11A, HBS1L-MYB and KLF1 genotypes were determined using a polymerase chain reaction (PCR)-based method. Red Blood Cell (RBC) indices and HbF levels were assessed. In silico analysis was assessed using loss-of-function tool (Lof Tool). Statistical difference and genetic comparisons between groups were evaluated by using SPSS for Windows, version 16.0 (SPSS Inc., Chicago, IL, USA). Comparisons between quantitative variables were carried out after data explored for normality using Kolmogorov–Smirnov test of normality. Logistic regression was used for computation of ORs and 95% CIs (Confidence Interval). We observed association of HbF levels in thalassemia major patients with the polymorphisms in BCL11A (rs11886868 rs7557939; rs1427407 and rs766432) and HBS1L-MYB (rs9399137) gene. The results of this study indicated that the presence of polymorphisms on modifier genes are strongly associated with an increase in HbF levels in thalassemia major patients. Further research with a larger sample size and with other genes of modifier genes is required.

Fetal hemoglobin regulating genetic variants identified in homozygous (HbSS) and heterozygous (HbSA) subjects from South Mexico

Hemoglobin S is caused by a nucleotide change in HBB gene (HBB:c.20A&gt;T, p.Glu6Val), is presented in diverse forms: simple carriers (HbSA), homozygotes (HbSS) also known as sickle cell anemia, and compound heterozygotes with other β-hemoglobinopathies. It is worldwide distributed, in Mexico, is frequently observed in the southern states Guerrero, Oaxaca and Chiapas. Elevated fetal hemoglobin (HbF) is associated with mild phenotype; single-nucleotide variants (SNVs) in modifier genes, such as BCL11A, HBG2, HBBP1 pseudogene and HBS1L-MYB intergenic region, upregulate HbF synthesis. The aim of this study was to identify HbF regulating genetic variants in HbSS and HbSA Mexican subjects. We studied 39 individuals (HbSS = 24, 61%, HbSA = 15, 39%) from Chiapas (67%) and Guerrero (33%), peripheral blood was collected in ethylenediamine tetraacetic acid (EDTA) for molecular and hematological studies, DNA was isolated by salting-out technic and genotyping was performed through allelic discrimination by real time polymerase chain reaction (RT-PCR) using Taqman® probes for 15 SNV (in BCL11A: rs6706648, rs7557939, rs4671393, rs11886868, rs766432, rs7599488, rs1427407; HBS1L-MYB: rs28384513, rs7776054, rs9399137, rs4895441, rs9402686, rs1320963; HBG2: rs7482144; and HBBP1: rs10128556). The obtained data were analyzed using IMB SPSS v.22.0 software. All minor alleles were observed in frequencies over 0.05, the most frequent was rs9402686 (0.82), while the less frequent was rs101028556 (0.08). In HbSS group, the mean fetal hemoglobin was 11.9 ± 5.9% and was significantly elevated in BCL11A rs11886868 wildtype homozygotes and in carriers of HBS1L-MYB intergenic region rs7776054 (p = 0.04 and p = 0.03, respectively). In conclusion, in HbSS Mexican patients, two SNVs were observed related to increased HbF; BCL11A rs11886868 and HBS1L-MYB rs7776054.

The Effect of Five Single Nucleotide Polymorphisms on Hb F Variation of β-Thalassemia Traits and Hematologically Normal Individuals in Southeast Turkey

β-Thalassemia (β-thal) is caused by deficiency of β-globin chain synthesis and leads to the accumulation of unstable globin chain production. This results in a higher Hb F level in order to neutralize the excess α chains. In addition, γ-globin gene expression, due to genetic factors after birth, leads to increased Hb F levels in adulthood [hereditary persistence of fetal hemoglobin (Hb) (HPFH)]. In this study, the relationship between β-thal trait and individuals with suspected HPFH and a control group was investigated in Adıyaman, Turkey. Single nucleotide polymorphism (SNP) analyses were performed in five different polymorphic regions using real-time polymerase chain reaction (qPCR) methods [rs4671393 (G>A), rs766432 (A>C), rs9402686 (G>A), rs28384513 (T>G), rs1609812 (A>G)]. No significant difference was found between the control and β-thal group in the codominant inheritance model in the rs1609812 (A>G) polymorphism region only, while all the other polymorphic regions were found to be statistically significant. It was found that different genotype models increased Hb F levels between 1.6- and 3.06-fold in four studied polymorphic regions [rs4671393 (G>A), rs766432 (A>C), rs9402686 (G>A), rs28384513 (T>G)]. All of the polymorphic regions increased the Hb F levels from 1.86- to 24.76-fold, except rs9402686 (G>A) and rs28384513 (T>G) over dominant and rs1609812 (A>G) codominant inheritance models. The AC and AA genotypes increased Hb F levels in the B-cell CLL/lymphoma 11 A haplotype studies. It was determined that both haplotypes 2 and 4 increased Hb F levels. As a result, SNPs strongly affect the Hb F levels in both healthy individuals and β-thal trait.

High fetal hemoglobin level is associated with increased risk of cerebral vasculopathy in children with sickle cell disease in Mayotte

Background

Understanding the genetics underlying the heritable subphenotypes of sickle cell anemia, specific to each population, would be prognostically useful and could inform personalized therapeutics.The objective of this study was to describe the genetic modulators of sickle cell disease in a cohort of pediatric patients followed up in Mayotte.

Methods

This retrospective cohort study analyzed clinical and biological data, collected between January1^st2007 and December 31^st2017, in children younger than 18 years.

Results

We included 185 children with 72% SS, 16% Sβ0-thalassemia and 12% Sβ + thalassemia. The average age was 9.5 years; 10% of patients were lost to follow up. The Bantu haplotype was associated with an increase in hospitalizations and transfusions. The alpha-thalassemic mutation was associated with a decrease of hemolysis biological parameters (anemia, reticulocytes), and a decrease of cerebral vasculopathy. The Single Nucleotide Polymorphisms BCL11A rs4671393, BCL11A rs11886868, BCL11A rs1427407 and HMIP rs9399137 were associated with the group of children with HbF > 10%. Patients with HbF > 10% presented a significant risk of early onset of cerebral vasculopathy.

Conclusions

The most remarkable result of our study was the association of SNPs with clinically relevant phenotypic groups. BCL11A rs4671393, BCL11A rs11886868, BCL11A rs1427407 and HMIP rs9399137 were correlated with HbF > 10%, a group that has a higher risk of cerebral vasculopathy and should be oriented towards the hemolytic sub-phenotype.

Combined and differential effects of alpha-thalassemia and HbF-quantitative trait loci in Senegalese hydroxyurea-free children with sickle cell anemia

BACKGROUND

Our objective was to investigate the combined and differential effects of alpha-thalassemia -3.7 kb deletion and HbF-promoting quantitative trait loci (HbF-QTL) in Senegalese hydroxyurea (HU)-free children and young adults with sickle cell anemia (SCA).

PROCEDURE

Steady-state biological parameters and vaso-occlusive crises (VOC) requiring emergency admission were recorded over a 2-year period in 301 children with SCA. The age of the first hospitalized VOC was also recorded. These data were correlated with the alpha-globin and HbF-QTL genotypes. For the latter, three different genetic loci were studied (XmnI, rs7482144; BCL11A, rs1427407; and the HBS1L-MYB region, rs28384513) and a composite score was calculated, ranging from zero (none of these three polymorphisms) to six (all three polymorphisms at the homozygous state).

RESULTS

A positive clinical impact of the HbF-QTL score on VOC rate, HbF, leucocytes, and C-reactive protein levels was observed only for patients without alpha-thalassemia deletion. Conversely, combination of homozygous -3.7 kb deletion with three to six HbF-QTL was associated with a higher VOC rate. The age of the first hospitalized VOC was delayed for patients with one or two alpha-thalassemia deletions and at least two HbF-QTL.

CONCLUSION

Alpha-thalassemia -3.7 kb deletion and HbF-QTL are modulating factors of SCA clinical severity that interact with each other. They should be studied and interpreted together and not separately, at least in HU-free children.

Associations of α-thalassemia and BCL11A with stroke in Nigerian, United States, and United Kingdom sickle cell anemia cohorts

Alpha-thalassemia and the BCL11A rs1427407 T allele are commonly observed in sickle cell anemia (SCA) patients and are associated with reduced hemolysis and higher hemoglobin F levels, respectively. We investigated whether a high-risk genetic profile, defined as SCA patients who did not inherit either α-thalassemia or the BCL11A rs1427407 T allele, had stronger associations with clinical and laboratory variables than the individual genetic components in the University of Ibadan cohort (n=249). We then replicated our findings in SCA cohorts from the University of Illinois at Chicago (UIC)(n=260) and Walk-Treatment of Pulmonary Hypertension and Sickle cell disease with Sildenafil Therapy (Walk-PHaSST)(n=387). High-risk was associated with higher reticulocytes (15.0% vs. 7.8%, P=0.08) and stroke history (6% vs. 1%, P=0.02) than standard risk patients and these associations were more significant than the individual genetic components in the University of Ibadan cohort. These findings were replicated in high-risk patients from UIC and Walk-PHaSST for reticulocytes (UIC: 13.5% vs. 11.8%, P=0.03; Walk-PHaSST: 9.6% vs. 8.2%, P=0.0003) and stroke history (UIC: 32% vs. 22%, P=0.07; Walk-PHaSST: 14% vs. 7%, P=0.01). On combined analysis, high-risk had strong associations with increased markers of hemolysis (hemoglobin β= -0.29, 95%CI: -0.50 to -0.09; P=0.006; reticulocyte% β=2.29, 95%CI: 1.31 to 3.25; P=1x10^-5) and stroke history (OR=2.0, 95%CI: 1.3 to 3.0; P=0.0002), but no association with frequent vaso-occlusive crises (≥3/year). A high-risk genetic profile is associated with increased hemolysis and stroke history in three independent cohorts. This profile may help identify patients to prioritize for hydroxyurea and for closer monitoring strategies for stroke.