Sickle cell disease (SCD) is a monogenic disorder with considerable clinical diversity. The presence of sickle hemoglobin was first described in India in 1952 in the tribal populations of the Nilgiri hills in south India . Over the last 60 years, several studies have shown that SCD is a major health burden in India.
The sickle cell gene is predominantly seen among the tribal populations in India, many of whom live in remote hilly regions and are often marginalized from the mainstream society. It is also prevalent among the scheduled castes and other background classes who are economically disadvantaged. Carrier frequencies range from 1 to 35% in many communities among these weaker and often neglected sections of society in different states (Fig. 1). There is wide variability in the prevalence of sickle cell trait (AS) and sickle cell anemia (SS) in population groups within small geographic areas [2–9,10▪]. The sickle cell gene is now also seen in some other communities at low frequencies [11–13].
Way back in 1988, the estimated number of sickle heterozygotes (AS) and homozygotes (SS) were 2 434 170 and 121 375, respectively, in India . Since then many surveys have been done, and with the population of India now reaching 1.21 billion these figures would be much higher. A recent calculation using currently available data indicates that 50% of the total AS and SS neonates are born in only three countries, Nigeria, India and DR Congo. For SS neonates, the estimate for India was 42 016, which corresponds to 88% of the homozygous cases in Asia [14▪]. This emphasizes the need for a national registry for an accurate estimation.
CLINICAL FEATURES, MORBIDITY AND MORTALITY
The clinical presentation of SCD in India is relatively milder than in African patients; however, it is extremely variable in different tribal and non-tribal populations. Vasoocclusive crisis has been most commonly observed and leg ulcers and priapism are rare . In Orissa, high frequencies of alpha thalassemia and high fetal hemoglobin (HbF) levels were observed as well as a higher frequency of splenomegaly, painful events, febrile illness and anemia . In Maharashtra and Gujarat, the rate of painful crisis, hospitalization and infections was significantly higher among non-tribal populations in central India compared with tribal groups in south Gujarat. Vaccination in childhood, adequate intake of fluids and avoiding extreme temperatures have reduced the episodes of crisis .
Several studies have now documented the clinical events, morbidity and mortality in SCD patients from different regions. Seventy-nine deaths were reported among 1800 SCD patients in Orissa over 16 years. The main causes were attacks of pain, hepatic failure, fever, severe anemia and acute splenic sequestration. The fetal hemoglobin levels in these cases varied from less than 5 to 25% .
Perinatal outcomes have been studied in SS and AS individuals compared with control groups in Maharashtra and Orissa. Significant complications during pregnancy included severe anemia, vasoocclusive crises and toxemia. Neonatal outcomes such as preterm births, low birth weight, intrauterine deaths and early neonatal deaths were significantly higher than in the control group. However, successful pregnancies were achieved in 84.4% of women by meticulous antenatal management [18–20]. Managing pregnancies in women with SCD is a challenge and there are no data from India on pregnancy outcomes in different rural areas where facilities would be limited.
A retrospective study evaluated the clinical records of 316 children with sickle cell anemia (SCA) registered at Nagpur, with an average follow up of 5.8 ± 5.7 years. These children had been given penicillin prophylaxis and the clinical events were analyzed until they were started on hydroxyurea or until their last follow up. The median age at presentation was 3.84 years. There were 1725 hospitalizations in 282 patients, most commonly for severe anemia requiring blood transfusions. There were 175 episodes of severe febrile illness requiring hospitalization including 62 episodes of blood culture-proven bacterial septicemia in 42 patients. Splenic sequestration, cerebrovascular accidents and dactylitis were also common. Ten deaths were recorded during this period and they were attributed to severe anemia and hypoxic encephalopathy, severe bacterial infections, sequestration crises, and tuberculosis with AIDS, while in two deaths at home the reason was unknown . This cohort demonstrates that in some children in central India SCD can be almost as severe as in patients with African haplotypes seen in the co-operative study . Another study from Nagpur had also reported 1.25 episodes of sickle cell crises and 1.38 episodes of infections per patient per year among 325 children .
Morbidity in 85 young children with SCD who were hospitalized has been described. In this cohort, acute febrile illness was the most common event (31%) and bacteremia was proved in 20 events despite penicillin prophylaxis, followed by severe anemia requiring transfusions, acute painful events and hand foot syndrome, while splenic sequestration crises, acute chest syndrome and stroke were less common [24▪▪]. The causative organisms for infections were Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa, thus highlighting the difference in the pattern of bacteremia in children with SCD in India compared with Africa and other countries where Streptococcus pneumoniae and Haemophilus influenzae are the common causes for bacteremia [22,25].
Strong linkage of the βs gene with an independent Asian haplotype associated with high HbF levels was first shown in Orissa [26,27]. Subsequent studies in both tribal and non-tribal SCD patients from different states confirmed that this haplotype was predominant throughout India. However, around 10% of βS chromosomes are linked to the Cameroon, Benin, Bantu or other atypical haplotypes [28–34].
HbF is a major modulator of the hematological and clinical presentation of SCD, as it can inhibit the deoxygenation-induced polymerization of sickle hemoglobin . However, it has now been shown that the phenotype of adult patients in Saudi Arabia with the Arab–Indian haplotype is not benign despite high HbF levels [36▪]. Indian patients with this haplotype also have higher HbF levels but a very variable clinical presentation [16,34,37].
Variations in the β globin gene cluster alone cannot explain the differences in HbF levels seen among SCD patients, and there are other transacting factors that influence HbF levels and the number of F cells. Polymorphisms in three major quantitative trait loci (QTLs) (chromosome 11-Xmn1Gγ, chromosome 6q23-HBSIL-MYB and chromosome 2p16-BCL11A) have explained some of the variations seen in HbF levels in SCD [38,39].
In SCD patients with the Arab–Indian haplotype from Saudi Arabia, single nucleotide polymorphisms (SNPs) in the BCL11A and HBS1L-MYB that were earlier associated with HbF in other populations explained only 8.8% of the variation in HbF . Our preliminary findings on three SNPs in the BCL11A gene in SCD patients are showing some association of the A allele of rs 4671393, the C allele of rs 11886868 and the G allele of rs 7557939 with higher HbF levels . It has recently been shown that it is not just the amount of HbF or total number of F cells that are important but rather the ratio of HbF/F cell and the proportion of F cells that have adequate HbF to inhibit polymerization of sickle hemoglobin (HbS) that may predict the severity of the disease .
α-Thalassemia in Indians is most frequently due to heterozygosity or homozygosity for the –α3.7 and –α4.2 deletions. Studies from Orissa had shown that the co-inheritance of α-thalassemia in SCD patients led to higher hemoglobin levels, red blood cell counts and HbA2 levels and lower reticulocyte counts, mean corpuscular volume (MCV), mean corpuscular hemoglobin and HbF levels, whereas the frequency of splenomegaly was not influenced by the α-genotype . In western India, co-inherited α-thalassemia resulted in a milder presentation among tribal patients in Gujarat with fewer episodes of painful crises, acute chest syndrome, infections and the need for hospitalization and blood transfusions. However, splenomegaly was more common in them . Similar findings were reported in another study from New Delhi .
OTHER GENETIC MODIFIERS
It is now well recognized that SNPs in many candidate genes, which are mediators of inflammation, vasoregulation, nitric oxide biology and hyperbilirubinemia, may potentially contribute to different subphenotypes in SCD patients . However, it has been cautioned that although several candidate gene-association studies and genome-wide association studies have been undertaken for many subphenotypes, the functional SNP or the mechanism by which the disease is modified is unknown, as sample sizes were small and the findings were not validated .
Few such studies have been initiated in Indian patients. Nitric oxide plays a critical role in endothelial dysfunction. Three endothelial nitric oxide synthase (eNOS) gene polymorphisms showed significantly higher frequencies of mutant alleles in SCD patients . They also influenced the age of menarche in females with SCD .
A hypercoagulable state in SCD has been well established. The prevalence of factor V Leiden (G1691A), methylene tetrahydrofolate reductase (MTHFR; C677T) and the prothrombin (G20210A) gene polymorphisms showed no significant differences in patients and controls in two of three studies [49–51].
Neutrophil activation plays an important role in the initiation and propagation of vasoocclusive crisis. The genotype frequencies of the H/R genotype of FcγRIIA and the NA1/NA1 genotype of FcγRIIIB were significantly decreased in SCD cases as compared to controls, with a decrease in the expression of CD62L .
Osteonecrosis is a serious complication in SCD. The frequency of one SNP (rs 7170178) (A/G, GG) in the ANXA2 gene was significantly higher in patients than in controls and was associated with sickle cell osteonecrosis . However, no significant association was found between the −538 T/C polymorphism in bone morphogenic protein 4 (BMP 4) and orthopedic complications . There was evidence to show that BMP–SMAD signaling components along with APT were overexpressed, mediated apoptosis and may be important in orthopedic complications in SCD .
INTERACTION OF SICKLE HEMOGLOBIN WITH OTHER HEMOGLOBINOPATHIES
Co-inheritance of the sickle gene with other hemoglobinopathies has led to disorders of variable severity.
There are only a few reports on the clinical and hematological features of HbS–β-thalassemia from India. Seventeen patients from Orissa had the severe β+-thalassemia mutation IVS 1-5 (G → C) with very low levels of adult hemoglobin (3–5%). The clinical features were similar to SCA patients from this region . Among 21 HbS–β-thalassemia patients from western India, the β0-thalassemia mutation CD 15 (G → A) was predominantly seen among tribals and the severe β+-thalassemia mutation IVS 1-5 (G → C) in non-tribals. HbS–β-thalassemia was clinically more severe among the non-tribals .
Sickle hemoglobin–δβ thalassemia
The Gγ(Aγδβ)0 thalassemia mutation is the commonest mutation causing δβ thalassemia in Indians . Three cases of HbS–Gγ(Aγδβ)0 thalassemia were encountered in Orissa in two families. This condition was associated with frequent painful episodes, repeated hospitalization and transfusion requirements in two cases .
Sickle hemoglobin E disease
Sickle hemoglobin E (HbSE) disease is relatively uncommon with only around 30 cases reported so far, the largest series being from the Sultanate of Oman . In India, eight cases of HbSE disease have been reported from Vellore in the south. They all presented in the second decade and had a mild clinical course presenting mainly with joint pains .
There are only occasional reports on HbSDPunjab disease in the literature, although this condition is not uncommon in India where the prevalence of HbDPunjab is 1–3% in the north-western region . Moderate anemia, mild jaundice and hepatosplenomegaly were reported in a 15-year-old Muslim girl . We have seen 15 cases of HbSDPunjab disease over the last 10 years. The majority of them had a severe clinical presentation with painful episodes, severe anemia requiring frequent blood transfusions, febrile illness and avascular necrosis (our unpublished observation). A recent report from Orissa on 42 cases with HbSDPunjab disease showed that the frequency of vasoocclusive crises was similar to sickle homozygous cases. They found a negative correlation between HbF levels and the risk of vasoocclusive crises .
Sickle hemoglobin C disease
HbC is generally not seen in India and only one case of HbC–β0 thalassemia has been described . Only one family with sickle HbC disease has been reported from Orissa, where two siblings with sickle HbC disease had a variable clinical presentation .
Hydroxyurea is a powerful inducer of HbF production and is one of the main therapeutic agents used in the management of SCD. The majority of the studies that have proved the efficacy of hydroxyurea therapy in SCD have been in patients with the African haplotypes with lower baseline HbF levels [67–69], and there are limited data on the efficacy of hydroxyurea in Indian patients with SCD, many of whom have severe clinical manifestations despite higher baseline HbF levels.
The findings of four studies are summarized in Table 1[33,70–72]. In the first study, 77 severe SCD patients were enrolled for hydroxyurea therapy. A significant clinical response was seen in the majority of the patients. There was a significant but variable increase in HbF (Fig. 2) and F cells along with a significant increase in γ mRNA expression. The hemoglobin and MCV levels increased with some reduction in white blood cell and platelet counts, reticulocyte counts and serum bilirubin levels. The reduction in bilirubin levels was higher in patients with the (TA)7/7 repeats in the promoter region of the UGTIAI gene after hydroxyurea therapy. No significant toxicity was seen [33,73]. The maximum tolerated dose of hydroxyurea was not required for a therapeutic effect. Similar findings were shown in another smaller study from Chhattisgarh; however, the dose of hydroxyurea was higher .
Two further studies subsequently showed the efficacy of fixed low dose hydroxyurea in Indian patients. The first was a double-blind randomized controlled study in pediatric cases, mainly non-tribals from Nagpur . A similar study from Orissa also showed an impressive improvement in the clinical and hematological parameters with minimal toxicity .
The recently concluded Pediatric Hydroxyurea Phase III Clinical Trial in infants (9–18 months) has proved the efficacy and safety of hydroxyurea .
The majority of the studies in the United States have used higher doses escalating to the maximum tolerated dose . In India, a low dose would be more acceptable and economically viable as the majority of these patients are from a low-income group.
There is ample evidence from several countries that newborn screening and comprehensive care dramatically reduce morbidity and mortality of SCD patients in the first 5 years of life [76–79].
Newborn screening is in its infancy in India with pilot studies being undertaken in a few states over the last 3–4 years. Using a targeted screening approach, 88 babies with SCA, mainly non-tribals, were identified in central India . They are being followed up regularly and provided comprehensive care. A high incidence of severe anemia and septicemia is being observed in this cohort (our unpublished observations). In Chhattisgarh, a 1-year pilot study identified five babies with SCD .
The feasibility of newborn screening among the tribal populations in rural areas in south Gujarat has also been demonstrated where 45 babies with SCA have been enrolled for comprehensive care. This cohort is showing a milder presentation (our unpublished observations).
Following up all the babies with SCD in these cohorts has been a challenge because of the low literacy in these families, long distances they have to travel to reach the center and seasonal migration for work, particularly among the tribal groups.
These studies will be the first to understand the natural history of SCD in India and need to be extended to other regions of this vast country.
PREVENTION AND CONTROL
In spite of improvements in management, there is considerable morbidity and mortality in Indian patients. With the successful implementation of awareness programs as well as screening and genetic counseling in antenatal clinics, there are a significant number of couples opting for prenatal diagnosis.
Our experience has shown that around 30% of couples come for prenatal diagnosis prospectively and the majority of them opt for termination of pregnancy when the fetus is affected [82,83]. Over 400 couples have undergone prenatal diagnosis of SCD at our Institute. The Indian Council of Medical Research (ICMR) is trying to establish regional centers for awareness, screening and control of sickle cell disorders in different states. Yet, there is a dilemma whether prenatal diagnosis for sickle cell disorders is justified for every pregnancy at risk, as some of these babies would have a mild disorder .
Although considerable work has been done on SCD in India, the majority of this has been done by individual centers and has had limited impact at the national level. Many patients in remote rural areas are not adequately managed and are often unable to reach a hospital. The need for networking and collaborative efforts has now been realized.
Under the National Rural Health Mission (NRHM), different state governments are establishing centers in different districts for the diagnosis and management of SCD patients. Gujarat, Chhattisgarh and Maharashtra have taken the lead in this. The ICMR has also initiated programs on SCD under their Tribal Health Research Forum. These government initiatives will strengthen the services for SCD patients in India. The need for North–South partnerships has been emphasized, as global collaborations will improve our understanding and management of the disease .
The authors would like to thank Mrs Preeti Pradeep for her help in preparing the article.
Conflicts of interest
There are no conflicts of interest.
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