Sickle cell disease (SCD) encompasses hemoglobinopathies caused by single-point mutations in hemoglobin (Hb) variants, resulting in morphological abnormalities in red blood cells [1]. Sickle cell anemia (SCA), the most common and severe form, arises from homozygosity for Hb S [2]. SCD is the most prevalent monogenic disorder worldwide, with the highest burden in sub-Saharan Africa. In these regions, limited newborn screening contributes to 50–90 % mortality among affected children, whereas in high-income countries with universal screening and care, childhood mortality is rare and survival extends into midlife [3]. Between 2000 and 2021 the incidence of sickle cell disease (SCD) remained stable, but the number of affected newborns rose by 13.7 % to 515,000, mainly in sub-Saharan Africa and the Caribbean. During the same period, the global SCD population increased by 41.4 % to 7.74 million. In 2021, SCD was responsible for 376,000 deaths, including 81,100 in children under five. According to Global Burden of Disease estimates, it now ranks as the 12th leading cause of death worldwide among children under five years old [4]. SCD occurs due to genetic mutations affecting the β-globin subunits of Hb, resulting in abnormal Hb and the development of sickle-shaped red blood cells [5]. SCD is a hereditary hemolytic anemia characterized by accelerated hemolysis, which precipitates oxidative stress, free radical formation, and chronic pain. These processes drive the wide spectrum of systemic complications and end-organ damage associated with the disease. The resulting by-products of hemolysis are especially worrisome due to their high toxicity to vital organs [6].

Individuals with SCD face a variety of acute and chronic complications, many of which can be life-threatening. However, except for certain cases, like stroke in children, most complications are unpredictable [7]. The clinical presentation of SCD is highly variable; while some patients are nearly asymptomatic, others may experience severe complications resulting from vaso-occlusion, red cell damage leading to hemolysis, and inflammation caused by factors such as intravascular hemolysis and ischemia-reperfusion injury [8]. The most common symptoms and complications are acute chest syndrome, stroke, pulmonary hypertension, priapism, pain crises, and severe hemolytic anemia [9,10]. Infants may initially be asymptomatic, but over time, they generally have complications that intensify with age [11]. Environmental and lifestyle factors, including infections, dehydration, physical stress, and hypoxia, also contribute to the complications [12].

To inherit SCA, an individual must receive two copies of the sickle cell gene, one from each parent. Those who inherit only one gene have sickle cell trait (SCT), which usually does not cause symptoms [13]. Traditional markers of hemolysis, along with newer indicators like red blood cell microparticles, are effective predictors of the clinical course in patients with SCD [14]. However, none of these markers can fully account for the clinical variability observed in SCA. As a result, the search for potential disease markers continues.

Haptoglobin (Hp), an acute-phase reactant protein, is determined by two primary codominant alleles, HP1 and HP2. This results in three possible genotypes (HP1–1, HP1–2, and HP2–2) all associated with the HP gene located on chromosome 16q22 [15]. This protein, found in both mammals and humans, is synthesized by hepatocytes. It has strong antioxidant properties and demonstrates varying levels of antioxidant activity based on its different genotypes [16]. The interaction of Hp with extracellular Hb helps mitigate the oxidative stress caused by cell-free Hb on tissues. This suggests that the severity of major SCD phenotypes may be influenced by specific HP genotypes [17]. Consequently, specific HP genotypes are associated with increased susceptibility to certain diseases and may serve as determinants of clinical prognosis [18].

To date, the determinants of the association between the HP genotype and the clinical and biological manifestations of SCD are unclear [19]. Therefore, it is interesting to know the influence of HP gene polymorphisms on the severity of SCD. There have been several case-control studies published on HP gene polymorphisms and severity of SCD, but the results are conflicting [20–22]. Thus, additional validation is necessary to establish definitive findings.

This meta-analysis was performed to investigate the association of HP genetic variants on the severity of SCD. Additionally, Gene Set Enrichment Analysis (GSEA) was employed to identify biologically significant pathways linked to these polymorphisms, providing a more robust and comprehensive evaluation of their potential role in SCD severity.

This study was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. A comprehensive literature search was performed based on predefined inclusion and exclusion criteria. The study protocol was registered with PROSPERO (ID: CRD42024577211) to ensure methodological transparency and reliability.

Literature search

An extensive literature search was conducted using the following databases: Scopus, Embase, Google Scholar, Web of science, and PubMed. Articles published over a 15-year period from 2010 to 2025 were analyzed. The search employed specific keywords like “Haptoglobin”, “Sickle Cell Anaemia”, “Sickle Cell Anemia”, “Sickle Cell Disease”, “gene polymorphism”, “case vs. control”, “HP gene”, and “Haemoglobin S”, combined with the Boolean operators (OR/AND). Both American and British spellings of key terms were included to ensure comprehensive coverage. The database was systematically updated, and duplicates were removed by manual screening. Reference lists of all included articles, relevant reviews, and meta-analyses were also screened to identify additional eligible studies not captured by the database search. No restrictions were placed on geographic origin, ethnicity, or publication status, provided the study met the inclusion criteria. The search strategy and study selection process followed the PRISMA guidelines to ensure methodological rigor and transparency (Figure 1).

Fig. 1.

PRISMA flow diagram of the literature search and study selection process for HP gene polymorphisms and sickle cell disease (SCD) severity.

The present meta-analysis and gene set enrichment analysis provide the first comprehensive evaluation of the association between HP gene polymorphisms and SCD. A significant association was observed under the dominant genetic model (OR = 0.76; 95 % CI: 0.61–0.94; p-value <0.01), suggesting a potential protective role of certain HP genotypes in respect to the severity of SCD. In contrast, no significant associations were found under the allelic, recessive, or overdominant models. Subgroup analysis by ethnicity revealed a significant association in African and South American populations, however no association was observed in Asian populations. These regional differences may reflect genetic diversity and population-specific variations rather than differences in SCD.

Hp binds free Hb released during hemolysis, preventing kidney damage and oxidative stress [31]. The HP gene has three major genotypes (HP1–1, HP2–1, and HP2–2), which differ in Hb-binding capacity and may influence SCD manifestations [32]. Beyond Hb binding, Hp functions as an antioxidant, exhibits antibacterial activity, and regulates the acute phase response [17]. Among the variants, the HP1–1 phenotype exhibits the greatest activity, followed by HP2–1, while HP2–2 demonstrates the weakest activity [33]. Due to this genetic variation, the HP gene is a key candidate to explore potential associations with the pathophysiology of SCD and its subtypes [34]. Because of genetic diversity, it is proposed that multiple SNPs play a significant role in the clinical variability of SCD, therefore, identifying polymorphic genetic sites could assist in understanding the genetic complexity involved [35].

The results of the present study indicate that HP polymorphisms may influence oxidative stress and inflammation in SCD. This aligns with prior findings showing that an imbalance of the glutathione metabolism exacerbates oxidative stress in SCD [36]. and that HP2–2 carriers have higher iron-induced oxidative stress compared to some individuals with thalassemia [37]. The sensitivity analysis demonstrates that no single study has a major effect on the overall results. This meta-analysis confirms that variations of the HP gene conform to the HWE.

Funnel plots and Egger’s test were employed to identify publication bias and show its absence. The validity of the findings was confirmed by evaluating the methodological quality using the RoB 2 tool, demonstrating decreased risk of bias of the studies. Thus, statistical evidence strongly supports proposed perspectives. Rigorous protocols were followed for data extraction and analysis. The gene enrichment analysis of HP gene polymorphisms and the meta-analysis of SCD risk provide crucial insights into the genetic factors influencing SCD severity. This analysis helps identify key genetic markers and pathways thereby opening the path for the creation of tailored treatments and focused interventions meant to reduce severity and improve patient outcomes [38]. The STRING PPI network analysis plays a pivotal role in elucidating the complex web of interactions between proteins involved in various biological processes. This analysis helps reveal key protein interactions, signaling cascades, and molecular mechanisms, facilitating a deeper understanding of disease etiology, drug targets, and therapeutic strategies [39]. The PPI analysis results identified Hp as a central hub protein, linking Hb subunits with plasma and acute-phase proteins. This network suggests that Hp, together with the HPX, CD163, and Hb subunits, plays a coordinated role in Hb binding, heme clearance, and modulation of inflammation in SCD.

Recent studies have underscored the significant role of HP gene polymorphisms in the pathophysiology of SCD, particularly concerning inflammation and iron metabolism. The study conducted by Tuono et al. demonstrated that SCD patients with the HP2–2 genotype exhibited higher levels of C-reactive protein (CRP), ferritin, and interleukin-6 (IL-6), indicating a pronounced inflammatory response and disrupted iron homeostasis [23]. Similarly, research from West Cameroon highlighted a higher frequency of the HP1–1 genotype among SCD patients, suggesting regional variations in HP genotype distributions [24]. These findings reinforce the idea that the biological impact of HP polymorphisms is context-dependent, shaped by ethnic background, environmental exposure, and genetic interactions. Due to this variability, it is not currently appropriate to generalize HP polymorphisms as a universal predictive biomarker for SCD.

HP1–1 has the strongest Hb-binding and antioxidant activity and HP2–2, the weakest. Knowing a patient's HP genotype helps predict their ability to handle the ‘hemolytic stress’ in SCD. Identifying patients with the HP2–2 genotype (who have higher CRP and ferritin levels) could allow clinicians to provide earlier, more aggressive interventions for those at higher risk of iron-induced oxidative damage. The applicability of such biomarkers requires validation in larger, ethnically diverse cohorts using standardized protocols. This limitation is crucial to address before considering clinical translation.

Genetic heterogeneity across human populations involves variations in allelic architecture that significantly influence gene frequencies and disease severity. Such diversity, which varies across genomic regions and traits, results in distinct population-specific genetic structures [40]. This diversity can manifest in differences in linkage disequilibrium patterns and allele frequencies, which influence how genetic variants relate to traits or diseases across populations [41]. Genetic heterogeneity has critical implications for the discovery, development, and validation of biomarkers. Specifically, a marker identified in one population may lack universal validity due to inter-population differences in allele frequencies and underlying genetic architecture [42]. Although the findings support an association between HP gene polymorphisms and severity of SCD, the genetic heterogeneity among global populations may influence the predictive value of this biomarker. While HP genetic variants may serve as a viable biomarker in specific populations (particularly those of South American and African descent) its universal applicability as a predictive marker for SCD clinical outcomes remains to be established. Further studies are needed in genetically diverse cohorts to assess the broader relevance of HP polymorphisms as a global prognostic biomarker.

This study has some limitations. It did not account for potential impacts from gene-environment interactions or other demographic factors. Additionally, the small sample size may have influenced the findings. Since the research was conducted in a limited population, definitive conclusions cannot be drawn. Therefore, a larger and more diverse sample size, involving various ethnic groups, is necessary. Further investigation with various demographics is recommended to improve the relevance and applicability of the findings. This study did not include Trial Sequential Analysis (TSA), due to the limited number of available studies, making TSA application impractical. This should be addressed in future research with larger datasets.

This meta-analysis demonstrates a significant association between HP gene polymorphisms and the severity of SCD phenotypes under the dominant model, particularly within African and South American cohorts. While no significant correlation was found in Asian populations, these findings suggest that HP variants serve as ethnic-specific modifiers of disease progression rather than primary risk factors. The correlation between increased severity of disease due to HP gene polymorphisms may not be the same in different cultural settings, and genetic correlations might differ greatly across ethnic groups. Such investigations into genetic polymorphisms are vital for improving risk stratification and informing the development of personalized treatment approaches. This study underscores the need for interdisciplinary research that integrates genetic modifiers into the broader understanding of the SCD pathology.

The authors would like to thank the management of Chettinad Academy of Research and Education (Deemed to be University) for providing facilities to perform this study.

This study does not involve experiments with animals or human subjects.

Not applicable.

Data and materials used/analyzed during the current study are available from the corresponding author upon reasonable request.

Not Applicable.

SV conducted literature searches, collected data, developed the methodology, contributed to writing, and performed data analysis. NG, KG and PG performed data analysis and created tables and figures. LK has done validation and data curation; GK conducted investigations, provided editing assistance, supervised, and conceptualized. All authors have thoroughly reviewed and approved the manuscript.