Hemoglobinopathies are genetic disorders affecting the structure or production of the protein portion of hemoglobin [1]. About 5% of the global population carries an abnormal hemoglobin variant; 300,000 to 400,000 infants are born with severe hemoglobinopathies worldwide, and 80% of these cases occur in developing countries [2].

Among the most widespread hemoglobinopathies throughout the world, sickle cell disease (SCD) is caused by an autosomal recessive inheritance of a single nucleotide base substitution (HBB c.20T $>$ A, HBB p.Glu6Val) in the sixth codon of the $\beta$-globin subunit [3]. This mutation results in amino acid substitution Glu6Val, which characterizes abnormal hemoglobin called HbS [4, 5]. Hemoglobin SS genotype (HbSS) is the most common type of SCD [6]. Homozygous individuals (HbSS) are often affected by severe signs and symptoms of SCD characterized by a painful vaso-occlusive crisis, chronic hemolytic anemia, and an increased risk of infections [7].

Although it is caused by a single amino acid substitution, Glu6Val, SCD is characterized by a wide range of clinical manifestations [8, 9]. The cause of this clinical heterogeneity still needs to be fully understood. However, a relatively higher production of fetal hemoglobin (HbF) in adulthood and the consequent increased number of erythrocytes with HbF are likely to be the major factors that modulate the disease phenotype observed in SCD patients [9, 10]. Therefore, high HbF levels in adults are among the most common factors that can lead to a considerable decrease in disease severity by mitigating polymerization and precipitation of sickle cell hemoglobin in red blood cells [11].

HbF synthesis is generally reduced to very low levels, i.e., less than 0.6% of the total hemoglobin in normal adults [12, 13]. However, the HbF level is generally elevated in patients with SCD, ranging from 1% to 30% of the total hemoglobin [11]. HbF expression is regulated by elements linked to the $\beta$-globin gene cluster (11p15.5) and other quantitative trait loci (QTL) of the HBB transgenes, one of which is BCL11A locus (B-cell lymphoma/leukemia 11A) located on chromosome 2 [14]. The BCL11A gene encoding a zinc finger transcription factor was shown to function as a repressor of $\gamma$-globin expression and HbF production in the erythrocytes of adults [11]. Studies have shown that the inactivation of the BCL11A transcription factor is protective against the complications of SCD in transgenic mice with SCD through the induction of high-level pancellular HbF [15].

A follow-up study involving six SCD patients recently validated the BCLA11 gene as an effective target for HbF induction therapy [16]. Several single nucleotide polymorphisms (SNPs) influencing the HbF level have been described at this QTL in patients with SCD or thalassemia worldwide [15, 17]. These QTLs represent 15–30% of the variation in HbF in sickle cell patients of African origin [11]. Subsequent studies on large populations of patients with thalassemia or SCD showed the distribution of several SNPs, which are significantly associated with high expression of HbF in subjects from various geographical origins [12, 13, 14].

SCD is a major heritable genetic disease that affects Africa. With a population consisting of ‘white Moors’ of Arab descent and Black Africans, Mauritania is also of concern. An earlier study conducted on hemoglobinopathies among 700 blood donors in Nouakchott, the capital city of Mauritania, showed that 16.6% of healthy blood donors were carriers (i.e., heterozygous) of abnormal hemoglobin (21.4% of Mauritanians of Black African origin and 4.5% of white Moors) [18]. Sickle cell trait (HbA/S) was the most common hemoglobin disorder (8.7%), followed by hemoglobin C (HbA/C) trait (3%) and $\beta$-thalassemia trait (2.5%) [18]. The southern region of Mauritania was identified as an area of high incidence, with a prevalence of hemoglobin disorders reaching 52% in the Senegal river basin bordering Senegal [19]. Despite these findings, SCD has received little attention from health authorities, and little is known about its impact on public health in the Mauritanian population. In particular, the influence of the BCL11A gene polymorphism on HbF levels and the clinical severity of the disease have not been studied in Mauritanian patients with SCD.

In the present study, three SNPs, rs4671393 (A$\mathrm{>}$G) and rs11886868 (C$\mathrm{>}$T) in intron 2 and rs1052520 (GA) in the 3${}^{\prime }$UTR regions of the BCL11A locus, were investigated, and the associated variation in HbF levels was assessed in Mauritanian homozygous sickle cell patients.

In SCD, a single $\beta$6 Glu–Val amino acid substitution leads to potentially life-threatening complications. Some patients tolerate the disease and survive for years or even decades without any notable vascular pathology, while others may present severe and fatal clinical signs [21, 22, 23]. This highlights the importance of studying the polymorphisms contributing to the observed clinical heterogeneity [23, 24]. In the present study, the prevalence of HbSS among the highly selected participants was 8.8%. Deyde et al. [18] reported that 8.7% of Mauritanian healthy blood donors were carriers of the sickle cell trait (HbAS), but none had sickle cell disease (HbSS), as expected in healthy populations. Veten et al. [25] reported a slightly lower frequency (5.71%; 60/1050) of sickle cell mutation (HbS) carries among another group of blood donors, including 5 (8.3%) individuals homozygous (HbSS) for the mutant allele. Various hematological and biochemical parameters determined the phenotypic characterization of sickle cell in 50 HbSS study participants. Our results showed normocytic anemia in these 50 individuals with HbSS. These results agree with previously published data [26]. Furthermore, a high average HbF level (15.0 $\pm$ 6.0%) was observed among the study participants despite excluding those under three years of age. This rate is higher than those reported in other studies carried out in children with sickle cell disease in Nigeria, Uganda, and other sub-Saharan African countries where mean ($\pm$ SD) HbF values of 9.9 $\pm$ 6.0%, 9.0 $\pm$ 5.6%, and 8.9 $\pm$ 6.4% were reported, respectively [11, 27, 28].

HbF is known as the principal SCD genetic modifier, and the presence of $\ge$15% HbF in the whole blood of an SCD patient was reported to reduce the clinical severity of the disease. Earlier studies have shown that a 20% HbF level in the whole blood is associated with an amelioration of sickle cell disease symptoms, although a linear trend was not found between HbF levels and morbidity [29, 30]. Several factors, including age, genetics, and environment, could explain this variation. In addition, a significant association was observed between HbF and hemoglobin levels in our study. Our findings were similar to those of Chaouch et al. [30], suggesting the important role of HbF in the modulation of anemia in sickle cell patients.

This is the first study focusing on the genotyping of three SNPs (rs118868686, rs4671393, and rs1052520) in the BCL11A gene and their correlation with HbF levels in Mauritanian subjects with SCD. In a previous study that genotyped 95 SNPs, including 17 at the BCL11A locus, in 1032 African–Americans with sickle cell anemia, Galarneau et al. [31] found that rs4671393 was the genetic marker that was most strongly associated with HbF levels. In that study, they also found two other SNPs in intron 2 of BCL11A, rs7599488 and rs10189857, which were associated with HbF levels. In the present study, these latter SNPs were not included in our molecular analysis.

Our results showed the presence of three different genotypes for rs4671393 and rs11886868, with a slightly higher prevalence of minor alleles than those reported in Cameroonians [32]. This slight difference can be explained by the ethnic diversity of our study population, which includes a high proportion of non-Black African populations and the relatively small number of SCD patients included and analyzed. Regarding rs1052520, a single homozygous genotype (GG) was observed in all individuals with HbSS (n = 50), which suggests that this SNP may be fixed and may, therefore, not be suitable as a genetic marker in Mauritania.

In 2007, Menzel et al. [13] reported that intron 2 of the BCL11A gene has the strongest associations with HbF variation in a 14 kb sequence. In addition, genome-wide association studies identified the rs4671393 SNP as the determinant marker of the HbF level [32]. This SNP alone explained 10% of the variation in HbF levels in patients with sickle cell anemia from northern Brazil [33]. In addition, the minor A allele was also associated with significantly higher HbF levels [32]. It is worth noting that our results revealed a statistically significant correlation between rs4671393 and HbF level in individuals with HbSS. Indeed, the presence of the minor (A) allele was associated with the HbF level increase in our study population. These results are similar to those found in an Angolan pediatric population [34].

The main limitations of the present study are the inclusion of a small number of individuals with SCD and a limited number of SNPs analyzed in the BCL11A locus or reported to modulate HbF levels.

The results of the present study highlight the differential frequencies of three SNPs in the BCL11A locus in Mauritanian individuals with HbSS that were previously reported to be associated with HbF levels. The HbF level is crucial in determining the severity of SCD, and genetic modification of HbF expression may offer clinical benefits in diagnosis and disease monitoring. The findings of the present study have important implications for genetic counseling and clinical management of SCD patients.

However, further research is needed to explore the association between genetic markers and HbF levels to improve the characterization of sickle cell disease and potentially improve the management of this life-threatening condition.

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary materials.

ATB collected data, performed blood and molecular analyses, interpreted the data, wrote the first draft of the manuscript. MT collected data and performed blood analysis. HS performed statistical analysis. SMG performed statistical analysis, interpreted data and supervised laboratory analyses. AM and AOMSB conceived, designed, and supervised the research, wrote, critically reviewed and edited the manuscript. All authors have participated sufficiently in the work to take public responsibility for appropriate portions of the content and agreed to be accountable for all aspects of the work in ensuring that questions related to its accuracy or integrity. All authors read and approved the final manuscript. All authors contributed to editorial changes in the manuscript.

The Institutional review board of the University of Nouakchott reviewed and approved the study protocol (approval no. 00000062/19/UNA). The study was conducted in accordance with the declaration of Helsinki and approved by the ethics Committee of the University of Nouakchott. The objective of the study was explained to all participants in their respective local mother dialect, Hassaniya, Pular, Soninke or Wolof. Written informed consent was obtained from each adult participant or, in the case of children 18 years old, their parents or legal guardians, prior to their enrollment. In addition, adolescents aged between 12 and 18 years old provided their assent.

Not applicable.

This research received no external funding.

The authors declare no conflict of interest.