Global burden of SCD

The rising incidence of SCD has attracted global attention due to its prevalence and socioeconomic significance. The World Health Organization (WHO) and the United Nations (UN) have acknowledged SCD not only as one of the most important hemoglobinopathies but also as a global public health issue [64]. Moreover, hemoglobinopathies had been added to the Global Burden of Disease, Injuries, and Risk Factor Study (the GBD 2010 study) with the purpose of providing a better and more systematic evidence-based assessment [46]. Additionally, a resolution on the prevention and treatment of birth abnormalities, including SCD, was adopted by the 63rd World Health Assembly in 2010.

An estimated 300,000 new-borns are diagnosed with SCD every year, and three fourths of those babies are anticipated to be born in the Sub-Saharan African region, where the disease has been recognized as endemic [9]. Higher occurrence of SCD has also been observed in India. The United States has about 100,000 individuals affected by SCD, whereas the number in Europe lies between 20,000 and 25,000 [8, 43]. The estimated number of SCD-affected births per year is shown in Fig. 1. This map was generated with the help of ITHANET maps [36]. In Europe, SCD is more prevalent in Southern Italy, the Balkans, and Greece, despite the fact that the disease is gradually spreading over most of the continent (mostly in Northern Europe) as a result of recent migration [8].

Fig. 1

Incidence of sickle cell disease worldwide. This map was created with the help of IthaMaps. It shows the number of SCD-affected new-borns per year. The median value for each country is estimated using statistical model and demographic data

In the sub-Saharan African region, it has been estimated that 50–90% of children with SCA die at a relatively young age [61]. Among them, the majority of deceased children suffer from illnesses like malaria and invasive pneumococcal disease [78]. For the time being, estimating the long-term health and economic consequences of SCD requires collecting more information from SCD-prominent areas. Low- and middle-income countries are now going through an epidemiological transition state. Substantially, it will help to reduce infants' mortality due to screening programs, early diagnosis, and adequate treatments. But, international migration widens the spread of βS allele throughout the world, which will eventually increase the disease's health burden [33]. By 2050, it is anticipated that there will be more than 500,000 neonates per year with SCD [74].

Occurrence of SCD in tribal population

Globally, the highest number of tribal populations has been observed in India. Most of the scheduled tribe population resides in the rural areas of Gujarat, Madhya Pradesh, Rajasthan, Maharashtra, Karnataka, Andhra Pradesh, Chhattisgarh, Odisha, Jharkhand, and West Bengal states [30]. SCA is being found to be more prominent among tribal populations. The occurrence of sickle cell carriers varies from 1 to 40% in different tribal groups [45]. Madhya Pradesh shows the highest occurrence of HbS, where tribal groups like Gonds and Bhils are more prevalent [16]. In Maharashtra, the tribal groups of Bhils, Madias, Pawaras, Pardhans, and Otkars have higher incidences of HbS (20–35%) [32]. A notable prevalence of HbS carriers (18.2–34.1%) has also been shown in Kerala [24]. In Gujrat, a higher prevalence of HbS (13–31%) is shown by the Dhodia, Dubla, Gamit, and Naika tribes, whereas some tribes named Chaudry, Rohit, Kukana, Gamit, and Vasava also show a higher occurrence of HbS (6.3–22.7%) [66]. The most prevalent SCD-affected tribals in India are shown in Fig. 2. Despite recent screening initiatives, there is still a lacuna in our understanding of the prevalence of the HbS gene among the tribal people of India.

Fig. 2

Most prevalent SCD-affected tribals in India. The highest number of tribal populations is observed in rural areas of India, mainly in Gujarat, Madhya Pradesh, Rajasthan, Maharashtra, Karnataka, Andhra Pradesh, Chhattisgarh, Odisha, Jharkhand, and West Bengal. Among them, a higher occurrence of SCD is observed in the tribals of Madhya Pradesh, Maharashtra, and Gujrat. A significant number of SCD-carrier individuals are also observed in Kerala

Pathophysiology of SCD

HbS is formed due to the mutation that leads to changing adenine to thymine and forms a new amino acid, valine, from glutamic acid in the HBB gene. Conversion of HbA to HbS leads to altered electrophoretic mobility of the Hb molecule because of the formation of hydrophobic motif, which further facilitates the HbS polymerization during deoxygenation [63]. As a result, long fibers are formed within RBCs, and the toughening of the fibers leads to changing the shape of RBCs to sickled. This phenomenon eventually leads to vaso-occlusion in the smallest blood vessel of nearly all tissues [76].

Vaso-occlusive crisis (VOC), which can lead to various secondary processes such as inflammation, vasculopathy, anemia, hemolysis, and oxidative stress, is the most frequent and prominent complication of SCD [52]. Patients with SCD have reported experiencing acute painful crisis (or sickle cell crisis) as a result of the vaso-occlusive phenomenon, which also results in inflammation and distal tissue ischemia.

In SCD, the polymerization of HbS in the absence of oxygen is a rapid and reversible process. The red blood cell is deformed by HbS polymerization due to the fibers becoming tougher, which disrupts blood flow in small capillaries (and certain big vessels) and becomes the crucial reason for vaso-occlusion and hemolytic anemia. In addition, sickling of red cells in tissues and unsickling in the lungs account for the transiency of erythrocytes, which causes hemolysis, resulting in chronic hemolytic anemia. Chronic hemolysis and frequent vaso-occlusion cause inflammatory responses that lead to vasculopathy with acute and severe chronic damage to different organs, including bone, spleen, liver, kidney, lung, and brain [44]. Furthermore, HbS polymers exhibit various types of cellular abnormalities which contribute to the overall pathophysiological mechanism of SCD.

SCD is associated with several acute and chronic complications, as summarized in Fig. 3. Various acute complications, including acute anemia, acute chest syndrome, cholecystitis, hepatic crisis, multisystem organ failure, pain, priapism, stroke, and splenic sequestration, are observed, which may have life-threatening consequences that eventually lead to substantial morbidity and early mortality, while SCD is also characterized by several chronic complications, including avascular necrosis, chronic anemia, cerebral infarction, leg ulcers, retinopathy, and damage or failure of a wide range of organs such as the heart, kidney, lung, and liver [54]. Chronic complications have become the primary cause of mortality in older age groups due to the involved clinical manifestations [1].

Fig. 3

Clinical manifestations of sickle cell disease. Sickle cell disease (SCD) patients require rapid medical care when they experience acute consequences; one of the most prevalent acute complications is pain. Chronic complications in older SCD patients result in organ failure that may cause death [22, 33, 60, 69]

Diagnostic techniques and different testing periods

The methods for diagnosing SCD would vary with the age of individuals and overlapping testing phases (preconception, prenatal, new-born, and post-neonatal). Preconception diagnostic techniques are performed to identify asymptomatic prospective parents whose offspring will be at risk for SCD. In this case, different techniques, including high-performance liquid chromatography (HPLC), isoelectric focusing (IEF), sickle solubility testing, and Hb electrophoresis, are used for separating Hb species depending upon their protein structure [33]. The sickle solubility testing method does not distinguish between SCT (Sickle cell trait) and SCD (Sickle cell disease) whereas Hb electrophoresis, HPLC, and IEF provide better results by differentiating SCT from SCD. Currently, advanced technology aids in detecting SCT from DNA through exome sequencing and even helps in direct genotyping of the SNP (single-nucleotide polymorphism) of sickle mutation [10].

Prenatal diagnosis is carried on to those couples who show positive results in preconception screening. This kind of invasive technique is used during early pregnancy and is typically considered safe. Fetal DNA from the chorionic villus must be analyzed at 10 to 12 weeks of gestation in order to perform the prenatal diagnosis [29]. Some non-invasive prenatal diagnosis methods are being developed, but further research is needed. These modern methods, such as pre-implantation genetic diagnosis (PGD), can identify fetal DNA in the mother's bloodstream as early as 4 weeks' gestation. In vitro fertilization with PGD is an alternative for couples who test positive for preconception screening in order to identify at-risk embryos prior to being implanted [75].

Hb protein analysis is performed in new-born screening for SCD. There are two new-born screening programs: one is targeted screening, where selective screening for neonates of high-risk parents has been done, and another is universal screening. A universal screening program has more significance than targeted screening because it helps to identify more neonates with SCD, which further helps to decrease the number of deaths [53]. Early diagnosis, follow-up care, and family awareness can also help to reduce mortality [19].

Post-neonatal testing is required for understanding the SCD status of general populations because it has many serious complications. The importance of post-neonatal testing is increased by a number of factors, including the geographical success of new-born screening programs, older patients' access to neonatal results, and the immigration of at-risk individuals who have not previously undergone testing [48].

Advantages of new-born screening for SCD

In 1975, the New York Program first introduced new-born screening (NBS) for SCD [21]. Subsequently, all the states in the USA, and later on, many other countries where SCD is common, commenced the new-born screening program. NBS provides assistance to identify neonates with SCD at birth or shortly thereafter, within the initial days of life, before they display any signs or symptoms. In order to reduce morbidity and mortality, these neonates can then be routinely monitored while receiving comprehensive care. Due to the likelihood of fatal consequences in pre-symptomatic infants, it has been shown in various countries that early diagnosis and care are crucial for neonates with SCD [3, 71].

According to estimates, India, Nigeria, and Democratic Republic of Congo are the countries that are most severely impacted by SCD (both homozygous and heterozygous sickle cell). Of them, India accounts for 15% of the world's new-borns with sickle cell anemia. Additionally, it has been calculated that thorough comprehensive new-born screening and follow-up care might save nearly 10 million children's lives by 2050 [58, 59]. Hence, new-born screening has great importance in terms of finding and treating babies with SCD. In most cases afflicted children are often discovered when they exhibit symptoms. However, over the past few years, multiple new-born screening programs have been started in some places.

Different genotypes of SCD

Hemoglobin (Hb) is the carrier of oxygen in RBC. It is made up of four globin subunits: adult hemoglobin (HbA) consists of two alpha subunits (chromosome 16p13.3) and two beta subunits (chromosome 11p15.4), whereas fetal hemoglobin (HbF) consists of two alpha and two gamma subunits (chromosome 11p15.4) [34]. Sickled erythrocyte has very low life span (only 10–20 days compared to normal 120 days) due to hemolysis [27]. SCD refers to a group of disorders where at least one mutation causes the generation of the βS allele, and another mutation could be any pathogenic mutations in the HBB (hemoglobin subunit beta) gene. Clinical syndrome is observed in cases with SCD. Heterozygous individuals with one βS allele are known as sickle cell trait (HbAS), and no symptoms are observed in this case.

A variant of Hb is HbC, which is formed due to the missense mutation (β6 GAG > AAG \(\to\) β6 Glu > Lys) in the coding region of the HBB gene, and if it co-inherits with HbS, is then known as HbSC. It is most prevalent in West Africa and South-east Asia. It shows milder anemic form and less complicated condition of SCD [58, 59]. Another condition, the HbS/D-Punjab genotype, is found in the north-western region of India. It is a double heterozygous condition in which HbD is associated with HbS. In this condition, the sickle mutation (6 Glu > Val) is co-inherited with another mutation (121 Glu > Gln), which is the cause of HbD-Punjab disease. In this case, the HbF level is elevated, but the symptoms are milder than HbSS [72]. Another genotype HbS/HPFH is formed when HbS is co-inherited with HPFH (hereditary persistence of fetal hemoglobin) caused by HBB cluster gene deletion or point mutation in HBG promoters. In this condition, the fetal HB elevated up to 10–40% [73]. HbS/E genotype occurs when HbS is co-inherited with HbE (substitution of glutamic acid to lysine at 26th position). It shows mild sickling condition [35].

Various βS allele mutations are associated with β-thalassemia. A major genotype condition (HbS/β0) forms when the βS allele is co-inherited with null HBB (Hbβ0), and there is no synthesis of HbA, causing HbS/β0-thalassemia. It clinically shows microcytosis, sickled RBC, and elevated HbF. The HbS/β+ condition depends on the spectrum of HbA production into types I (HbA, 1–7%), II (HbA, 7–14%), and III (HbA, 14–25%). These are the most prevalent variants in India and the Mediterranean region [25]. HbS is co-inherited with many other Hb variants (HbS/other Hb variant), and symptoms may vary depending on the mutation spectrum of the beta-globin gene. Different genetic forms of SCDs are described in Table 1.

Table 1 Summary of different genetic forms of SCDManagementHydroxyurea (hydroxycarbamide)

The Food and Drug Administration’s (FDA) approved low-toxicity drug hydroxyurea is widely used for treating sickle cell disease (SCD) complications. It is an inhibitor of ribonucleotide reductase and functions by arresting cells at S phase [47]. Hydroxyurea can regulate hemoglobin (Hb) concentration by increasing fetal Hb (HbF) production and preventing the sickling of RBC [20]. Also, it can decrease the leukocytes in the blood and reduce surface adhesion properties, thus improving blood flow. It increased the erythrocyte volume while maintaining its shape, which enhanced the MCV [42]. Mean corpuscular volume is referred to as MCV, which is used to measure the average volume and size of red blood cells [41]. It has been reported in a study that hydroxyurea users can reduce their mortality rate by 40% and benefit from the SCD pain crisis. SCD children with hydroxyurea treatment show an ameliorate effect by a 10% increase in HbF. National Institute of Health (NIH) recommended in their guidelines to use hydroxyurea for all HbSS and HbS/β0 patients above 1 year old [28].

Voxelotor (GBT440)

In the case of SCD, at low oxygen levels, HbS erythrocytes can polymerize and distort their shape, ultimately logging the tissue and leading to hypoxia [79]. Voxelotor inhibits HbS polymerization of RBCs and has the potential to reduce disease-related complications [77]. In 2019, the FDA approved the voxelotor drug based on the positive improvement of clinical data.

L-Glutamine

One of the main pathophysiological events that SCD patients experience is oxidative stress. In order to overcome this phenomenon, antioxidants are useful. Although additional study is still needed to fully understand its effects, L-glutamine, which was licensed by the FDA in 2017 to treat SCD patients, plays a role in the synthesis of antioxidants such as reduced glutathione, NADH, NADPH, and nitric oxide. [65].

Blood transfusion

Blood transfusion is an effective disease-modifying therapy that elevates the hemoglobin level and decreases the sickling of RBCs by lowering the HbS percentage and its synthesis. It also minimizes hemolysis. Although chronic blood transfusions put the SCD patient at risk for iron overload in the heart and liver, which is known to damage organs; therefore, chelation treatment is necessary to prevent this damage, and in this instance, deferasirox, deferoxamine, and deferiprone show promising results [38, 70].

Hemopoietic stem cell transplantation (HSCT) and gene therapy

HSCT therapy is one of the potential curative techniques for treating SCD patients. For this therapy, HLA (human leukocyte antigen)-identical siblings’ donors are mostly preferable. This method enhances quality of life, avoids long-term complications, and has overall survival rates of 95%. However, alternative donors (such as haplo-identical or unrelated HLA-matched donors) are also used in this therapy when the patients do not have any HLA-matched donors [39].

Gene therapy is one of the most promising ways to treat SCD patients. A number of trials are ongoing for treating SCD, but still, thorough understanding is required for making it successful. Genome editing is also used as a therapeutic option. The most common editors for this approach are zinc finger nucleases, TALENS, meganucleases, and CRISPR-Cas [2]. In December 2023, the US Food and Drug Administration (FDA) approved two therapies against SCD, known as Casgevy and Lyfgenia. Casgevy is a novel cell-based CRISPR-Cas9 gene therapy for patients 12 years of age or older. This therapy targets stem cells with abnormal HbS and converts them into HbF by down-regulating the transcription factor BCL11A on chromosome 2 [68]. The sickling phenomenon therefore appears to reduce as the quantity of HbF, a high-affinity oxygen carrier protein, increases. Prior to this therapy, a high dose of chemotherapy (which removes the affected cells from the bone marrow) is required for the patients in order to obtain the Casgevy-treated stem cells [4]. The FDA also approved another gene editing therapy, Lyfgenia, which is a lentiviral vector-based genetic modifier. The Lyfgenia-modified stem cell can produce HbAT87Q hemoglobin, which is an analog of adult hemoglobin (HbA). It helps in lowering the sickling of RBC and minimizing blood flow obstruction [4, 17].

Nutritional supplements

Omega-3 (n − 3) fatty acid works as a potent antioxidant as well as antithrombotic and anti-inflammatory. It has been reported that omega-3 (n − 3) fatty acid works effectively on SCD-related complications, including acute painful crises in children. It can modulate SCD-induced oxidative stress by reducing the expression of the nuclear factor-kappa B (NF-κB) gene and adhesive molecules such as β2-integrin, LFA-1 (lymphocyte function-associated antigen-1), and L- and P-selectin [18]. Folic acid has widely been used to increase erythropoiesis and benefit SCD children. Certain herbal mixtures, such as EvenFlo, which is undergoing clinical study, and Niprisan, have also shown promising results in treating SCD [27].

Genetic counseling

HbAS (heterozygous condition of an individual with sickle cell trait but not having SCD) is a type of benign condition that has many uncommonly intense complications. So, having knowledge about the HbAS status is crucial for family planning and preventing individuals from serious complications [5]. Genetic counseling, therefore, has a significant role in terms of screening and detecting the heterozygous genotypic condition of individuals who wish to have children. It is one of the main reasons for conducting a new-born screening program. Simultaneously, HbAS screening also provides assistance in making informed decisions regarding prenatal diagnosis and preconception counseling [71].

One of the most affected countries like India has taken several measures to counter the SCD occurrence. For instance, the Indian Council of Medical Research (ICMR) has initiated different programs to enable genetic advances under its Tribal Health Research Forum (THRF) and National Rural Health Mission (NRHM) in various states of India to provide proper medical diagnosis and follow-up care to the people, mainly the tribal communities in remote regions. In 2016, the Ministry of Health and Family Welfare (MoHFW) also issued guidelines for a nationwide hemoglobinopathy program that includes neonatal screening and understanding the clinical history of SCD in India [50]. The Indian government recently launched an initiative to “eliminate sickle cell anemia by 2047” in India, focusing on raising awareness and providing universal screening for about 70 million individuals aged 0 to 40 in tribal areas, as well as genetic counseling, through joint efforts between the central and state governments. These guidelines would involve educating the parents about the risk factors if one is a carrier for SCD while the other a carrier of alpha thalassemia, so on and so forth. [