OMICS A Journal of Integrative Biology Volume 20, Number 10, 2016 ª Mary Ann Liebert, Inc. DOI: 10.1089/omi.2016.0125

Genetics of Sickle Cell-Associated Cardiovascular Disease: An Expert Review with Lessons Learned in Africa Amy Geard,1 Gift D. Pule,1 David Chelo,2 Valentina Josiane Ngo Bitoungui,3 and Ambroise Wonkam1

Abstract

Sickle cell disease (SCD) vastly impacts the African continent and is associated with cardiovascular diseases. Stroke, kidney disease, and pulmonary hypertension are considered as proxies of severity in SCD with several genomic loci implicated in their heritability. The present expert review examined the current data on epidemiology and genetic risk factors of stroke, pulmonary hypertension, and kidney disease associated with SCD, as indexed in PubMed and Google Scholar. Studies collectively show that stroke and kidney disease each affect *10% of SCD patients, with pulmonary hypertension displaying a higher prevalence of 30% among adults with SCD. There is some evidence that these epidemiology figures may be an underestimate in SCD patients living in Africa. A modest number of publications have identified genetic factors involved in pathways regulating inflammation, coagulation, cell adhesion, heme degradation, a-globin and c-globin production, and others, which contribute to the development risk of targeted cardiovascular phenotypes. However, in most cases, these studies have not been validated across populations. There is therefore an urgent need for large-scale genome-wide association, wholeexome and whole-genome studies, and multiomics research on cardiovascular diseases associated with SCD, particularly in Africa, to allow for proportional investment of global research funding on diseases that greatly impact the African continent. Ultimately, this will cultivate socially responsible research investments and identification of at-risk individuals with improved preventive medicine, which should be a cornerstone of global precision medicine. Keywords: sickle cell disease, stroke, kidney disease, pulmonary hypertension, Africa, global precision medicine

Introduction

S

ickle cell disease (SCD) is a monogenic genetic disorder of public health significance, with high prevalence, mortality, and limited interventions. Sickle cell anemia (SCA) is caused by homozygosity for a substitution mutation in the b-globin gene (Sebastiani et al., 2005). An estimated 305,800 SCA-affected births occur annually worldwide, with *80% of this incidence occurring in Africa (Piel et al., 2013). SCD manifests itself in complex clinically heterogeneous phenotypes ranging from early childhood mortality to an almost unrecognizable condition in which patients can survive to late adulthood (Sebastiani et al., 2005). This phenotypic variation is the result of interactions between multiple genetic factors and the environment (Adams et al., 2003). Cardiovascular phenotypes in SCD include complications involving the heart (e.g., heart failure), brain (e.g., stroke), lungs (e.g., pulmonary hypertension), and kidneys (e.g., proteinuria).

Cardiovascular disease is perhaps the most devastating complication for children with SCD, including overt stroke, transient ischemic attacks, silent infarcts, and neurocognitive dysfunctions. Cardiovascular accidents are a common occurrence in sickle cell patients, and longitudinal cohort data from the United States have shown that between 5% and 10% of patients with SCD will experience a clinically overt stroke during childhood (Ohene-Frempong et al., 1998). Moreover, the prevalence of overt stroke in SCD in Africa may be higher than that reported in these high-income countries. Like stroke, renal failure occurs in 5–18% of SCD patients and is associated with early mortality (Platt et al., 1994). Furthermore, an increased tricuspid regurgitation jet velocity (TRV >2.5 m/sec) and pulmonary hypertension defined by right heart catheterization both independently confer increased mortality in SCD (Gladwin et al., 2004). In this review, we address the current knowledge on the burden and genetic modifiers of these cardiovascular phenotypes in SCD with focus on sub-Saharan Africa.

1 Division of Human Genetics, Departments of Medicine and Pathology, Faculty of Health Sciences, University of Cape Town, Cape Town, South Africa. Departments of 2Paediatrics and 3Hematology, Faculty of Medicine and Biomedical Sciences, University of Yaounde´, Yaounde´, Cameroon.

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582 Methods Article sources

A comprehensive literature search was conducted by the authors covering the subject until July 2016, using PubMed (National Library of Medicine), Medline, and Google Scholar. The keywords included individual use or a combination of the following: ‘‘hypertension,’’ ‘‘stroke,’’ ‘‘kidney disease,’’ ‘‘sickle cell disease,’’ and ‘‘Africa.’’ Additionally, specific expert authors’ names that are active in the field of SCD and genetic modifiers were also used to complement the literature searches. The search was performed between May and July 2016. Selection criteria

Abstracts were reviewed for relevance to the scope of the review, and relevant full articles were retrieved. Non-English articles were included if an English translation of the abstract was available. The inclusion criteria focused on academic and research articles describing the prevalence and risk factors for stroke, kidney disease, and pulmonary hypertension and hypertension in SCD patients. Articles were excluded based on sample size and validation of the relevant findings. To maximize the inclusion of potentially relevant articles, the main search was conducted, separately, by an MSc student and a PhD student in Human Genetics working on SCD (first and second authors) and reviewed successively by a pediatric hematologist, laboratory hematologist, and medical geneticist, with expertise in SCD (third, fourth, and senior authors, respectively). Data analysis

Following the methodology of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA; Moher et al., 2009); the number of records identified, included, and excluded is displayed in Figure 1. However, there was insufficient research in this field to allow for comprehensive meta-analysis, thus qualitative analysis of the articles was performed. Results Stroke in SCD Epidemiology and risk factors. Stroke in SCD comprises a wide spectrum of clinical manifestations, including transient ischemic attacks, overt stroke, silent infarcts, hemorrhagic strokes, and frequent neurodegenerative decline (Flanagan et al., 2013; Ohene-Frempong et al., 1998). Radiographic data have allowed for the subclassification of ischemic stroke into small-vessel (SV) and large-vessel (LV) subtypes. However, it is unclear whether these represent different pathophysiological mechanisms of stroke (Hoppe et al., 2003). The majority of stroke occurrences in SCD patients younger than 20 years are either ischemic or result from infarction (Adams et al., 2003; Hoppe et al., 2003), while the incidence of intracranial hemorrhagic stroke is more common in older SCD patient populations (Powars et al., 1978). Although stroke can result from multiple factors such as hemorrhage, cerebral infarction, or embolism, occlusive vasculopathy remains the most common cause of cardiovascular disease in SCD patients (Stockman et al., 1972).

GEARD ET AL.

Stroke is a catastrophic vascular complication of SCD and 11% of patients under the age of 20 years will experience overt stroke (Adams et al., 2001; Flanagan et al., 2013; Ohene-Frempong et al., 1998), with the peak incidence being between the ages 2–5 years (Adams et al., 2001; OheneFrempong et al., 1998). The risk of stroke is 250 times greater in adolescents with SCD than in nonsickle individuals (Earley et al., 1998), and it is estimated that in addition to children who experience overt stroke, a further 20–30% will suffer silent infarctions, but with no apparent phenotypic consequence (Earley et al., 1998; Kinney et al., 1999; Moser et al., 1996). SCD is the major cause of stroke in childhood (Hoppe et al., 2003), with significant recurrence rates, which can lead to severe complications such as permanent neurological damage, residual motor deficits, cognitive impairment, and death (Flanagan et al., 2013; Sebastiani et al., 2005). Although the incidence of stroke is severe and recurrent, it is also predictable using specific tools. Transcranial doppler (TCD) ultrasonography is currently the most accurate prognostic tool available for the detection of patients at risk for cardiovascular disease and primary stroke prevention (Adams, 2005; DeBaun et al., 1995; Flanagan et al., 2011; Kwiatkowski et al., 2003). TCD is the periodic screening of time-averaged mean velocities (TAMVs) in the distal internal and carotid arteries and middle cerebral arteries (Adams, 2005; Flanagan et al., 2013). Children with abnormal TCD results (TAMV >200 cm/sec) have a far greater risk of stroke development, particularly ischemic stroke, than children with normal TCD results (TAMV T NC_000009.12:g.27187218C>G NC_000001.11:g.91705151A>G NC_000016.10:g.3964140G>A

Protective Protective Risk Risk Risk Risk Protective Protective

Flanagan et al. (2013) Flanagan et al. (2013) Flanagan et al. (2013) Flanagan et al. (2011) Flanagan et al. (2011) Flanagan et al. (2011) Flanagan et al. (2011) Hsu et al. (2003)

rs7482144 rs28384513 rs9399137 rs4895441 rs4671393

NC_000011.9:g.5276169G>A NC_000006.12:g.135055071T>G NC_000006.12:g.135097880T>C NC_000006.12:g.135105435A>G NC_000002.12:g.60493816A>G

Protective Protective Protective Protective Protective

Lettre Lettre Lettre Lettre Lettre

et et et et et

al. al. al. al. al.

(2008) (2008) (2008) (2008) (2008)

SCD, sickle cell disease; SNP, single-nucleotide polymorphism.

2011), is one of the few genetic modifiers with a confirmed protective association with the development of cardiovascular disease (Table 1; Adams et al., 1994; Flanagan et al., 2013; Hsu et al., 2003). There is an indication, however, that alpha-thalassemia may result in more frequent vasoocclusive pain episodes (Steinberg, 2005). Fetal hemoglobin (HbF) is another genetic factor that has been shown to decrease stroke risk in both Nigerian and African-American cohorts (Table 1) and ameliorate the clinical phenotype of SCD (Enosolease et al., 2005; Fatunde and Scott-Emuakpor, 1992; Platt et al., 1991; Yetunde and Anyaegbu, 2001). HbF levels vary between 1% and 30% in SCD patients, and three genetic loci describe *20% of this variation; BCL11A, HBS1L-MYB, and the HBB cluster (Lettre et al., 2008; Wonkam et al., 2014; Xu et al., 2010). There is a further association with b-globin gene haplotypes (Powars et al., 1990) with the Indian-Arab and Senegal haplotypes associated with milder severity in comparison with the Benin, Bantu, and Cameroon haplotypes, which are associated with lower HbF and more severe clinical manifestations (Diagne et al., 2000; Diop et al., 1999). Gene pathways involved in SCD-related vasculopathy such as inflammation, cell adhesion, coagulation, platelet activation, and vasoregulation have formed the key focus of many studies (Adams et al., 2003; Hoppe et al., 2001; Sebastiani et al., 2005). Heritable thrombophilia has been linked with mutations in genes coding for Factor V, methylenetetrahydrofolate reductase (MTHFR), and prothrombin (Nowak-Go¨ttl et al., 1999). The coinheritance of these genes with SCD is predicted to favor a hypercoagulable state and subsequently act as a risk factor for the development of cardiovascular disease (Adams et al., 2003). Several retrospective studies have identified singlenucleotide polymorphisms (SNPs) associated with the risk of stroke. These genes include two alleles of the VCAM1 gene, a cellular adhesion molecule, with polymorphisms in the coding and intronic regions associated with protective and increased risk for SV stroke development, respectively (Hoppe et al., 2004; Taylor et al., 2002). Subclassification of stroke into the LV and SV subtypes, based on magnetic resonance imaging studies (Steinberg, 2005), has resulted in the identification of

variations associated with susceptibility to specific stroke subtypes in children with SCD. Polymorphisms in TNF, IL4R, and ADRB2 genes were independently associated with increased risk for LV stroke, while certain VCAM1 and LDLR NcoI SNPs were associated with increased SV stroke risk (Hoppe et al., 2004). SNPs in TNF-a and ADRB2 were identified as protective polymorphisms against LV stroke (Hoppe et al., 2004). Genes encoding for human leukocyte antigen (HLA) proteins have also been identified as risk factors for vascular disease as they are responsible for regulating inflammation (Hoppe et al., 2001, 2003). Two polymorphisms in the gene encoding for HLA DPB1 (DBP1*0401 and DBP1*1701) were identified in SV stroke patients and associated with risk and protective effects, respectively. Similarly, HLA polymorphisms (A*0102 and A*2612) conferred increased susceptibility to cardiovascular disease in the LV subgroup, whereas A*3301 was protective against stroke. However, there is no known causative effect for the role of HLA in the development of cardiovascular disease in SCD (Hoppe et al., 2003). Although most of the stroke-associated SNPs that have been identified in pediatric SCD patients have not been validated across populations, there are some genetic modifiers with confirmed stroke associations. The GOLGB1 Y1212C substitution mutation has been associated with protection for ischemic stroke (Flanagan et al., 2013). The ENPP1 K173Q mutation has a similar protective effect, whereas the PON1 Q192R mutation is associated with increased ischemic stroke risk in adults (Flanagan et al., 2013). Further studies have also identified 25 SNPs in 11 different genes that are directly associated with stroke, which include ADCY9, ANXA2, TEK, and TGFBR3 (Table 1) (Flanagan et al., 2011; Sebastiani et al., 2005). Polymorphisms in the ANXA2, TEK, and TGFBR3 genes are associated with increased stroke risk, while ADCY9 rs2238432 is correlated with decreased stroke risk (Flanagan et al., 2011). The TGFBR3 variations along with TGFBR2 form part of the TGF-b signaling pathway, which suggests that this pathway may play a role in stroke development (Sebastiani et al., 2005; Steinberg, 2005). SELP variations are also known to be associated with stroke in the general population (Steinberg, 2005). It is important to note

SICKLE CELL AND CARDIOVASCULAR DISEASE GENETICS

that validated SNPs in the afore-discussed genes are all located in intronic regions, thus emphasizing the importance and need for large-scale genome-wide association, wholeexome, and whole-genome studies. Kidney disease as a phenotype of SCD Epidemiology and risk factors. Renal vascular abnormalities, in conjunction with transient vascular occlusions, are considered characteristic clinical manifestations of SCD (Pegelow et al., 1997), with renal failure, occurring in 5–18% of patients, being a major cause of early mortality (Ashley-Koch et al., 2011; Platt et al., 1994). SCD nephropathy includes papillary necrosis, hyposthenuria, impaired renal acidification, proteinuria, ischemia, and supranormal proximal tubular function as a result of the sickled erythrocytes (Pham et al., 2000; Stuart et al., 2004). Elevated levels of cell-free hemoglobin are characteristic of SCD due to the unstable nature of sickle hemoglobin (Nath and Katusic, 2012; Saraf et al., 2015). Subsequent increases in heme iron, a known proinflammatory molecule, are postulated to preclude vascular injury (Kanakiriya et al., 2003; Tracz et al., 2007). Macroalbuminuria is a sensitive primary indicator of glomerulopathy, preceding the development of progressive renal insufficiency (Guasch et al., 1999) and ultimately leading to end-stage renal disease (ESRD) in both children and adults (Becton et al., 2010; Guasch et al., 1999, 2006). Focal segmental glomerulosclerosis (FSGS) and collapsing glomerulosclerosis are subtypes of podocytopathies and podocyte injury, which is proposed to play a key role in the development and progression of kidney disease (Barisoni and Nelson, 2007). Proteinuria is a crucial component of the nephropathy process (Ashley-Koch et al., 2011), with micro- and macroalbuminuria having an incidence of 68% in adult SCD patients (Guasch et al., 2006). This development is not related to the degree of anemia, but rather an increase in nitric oxide synthesis, which indicates the possible role of hemodynamic mediation in sickle cell nephropathy (Pham et al., 2000). The presence of albuminuria is also associated with decreased glomerular filtration and is therefore indicative of glomerular injury (Guasch et al., 1996). Microalbuminuria is the preferred primary indicator and it has been shown to characterize early stages of SCD nephropathy in both adults and children with SCD (Becton et al., 2010; Guasch et al., 1999). SCD not only results in renal functional disturbances but also anatomical alterations, with renal tubular defects progressively developing from 7 years (Pham et al., 2000). This is due to the hypoxic, hyperosmolar, and acidotic environment of the inner medulla promoting the sickling of red blood cells (Pham et al., 2000). This results in ischemia, blood flow impairment, and papillary necrosis, which is thought to contribute to the relative hypotension typical of SCD patients (Pham et al., 2000). This process can be partially treated and prevented through multiple blood transfusions; however, repeated thrombosis due to sludging and necrosis of the inner medulla and papillae progressively reduces the ability to improve renal function (Pham et al., 2000). This phenotype is generally untreatable and irreversible past the age of 15 years (van Eps et al., 1970). In general, individuals of African ancestry suffer from the highest burden of ESRD, even in the absence of coinherited SCD (Kopp et al., 2008); however, the risk factors that pre-

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dispose African populations to ESRD are largely unexplained and unknown (Kopp et al., 2008). Recent studies of African SCD populations, particularly in Nigeria (Adedoyin et al., 2012; Iwalokun et al., 2012), Ghana (Osei-Yeboah and Rodrigues, 2011), and Congo (Pakasa and Sumaili, 2012), provide amassing evidence of a high prevalence of renal disease in these populations. Genetics of kidney disease in SCD. Genetic factors involved in the development of kidney disease in SCD patients are largely unexplored. Alpha-thalassemia and HbF play similar protective roles as with stroke (Steinberg et al., 2003). The coinheritance of alpha-thalassemia deletions is specifically associated with a lower prevalence of macroalbuminuria (Guasch et al., 1999) and is thought to confer protection through reduced erythrocyte hemoglobin concentration, lower mean corpuscular volume, and not associations with the degree of anemia or severity of hemolysis (Guasch et al., 1999). The effect of b-globin gene haplotypes on the severity of the kidney disease remains controversial as some studies have identified no significant relationship between albuminuria and the different haplotypes (Guasch et al., 1999), whereas longitudinal studies of US populations state that the Bantu haplotype is associated with the highest incidence of renal failure in SCD patients (Powars et al., 1991). Heme oxygenase-1 (HO-1), an inducible isoform of HO, plays a vital role in the amelioration of inflammation and vaso-occlusion commonly seen in SCD (Bean et al., 2012; Belcher et al., 2006). Substantial induction of the HMOX1 gene in response to oxidative stress has been demonstrated in the kidneys of transgenic SCD mice (Nath et al., 2001). A highly polymorphic (GT)n dinucleotide repeat (rs3074372) in the promoter region of HMOX1 is suggested to play a functional role in modulating HO-1 expression levels (Yamada et al., 2000). Shorter repeats (C NC_000022.11:g.36318665T>A NC_000022.11:g.36288676G>A NC_000022.11:g.36296706T>C NC_000022.11:g.36316427A>G NC_000022.11:g.36329925C>T NC_000022.11:g.36309484A>G NC_000022.11:g.36319229A>G NP_663318.1:p.Ser358Gly

Risk Risk Risk Risk Risk Risk Risk Risk Risk

HMOX1

rs60910145 rs71785313 rs3074372

NP_663318.1:p.Ile400Met NP_663318.1:p.Asn404_Tyr405del NC_000022.11: g.35380894_35380895insGT NC_000022.11:g.35396981T>C

Risk Risk Protective/ risk Risk Protective

rs743811 HBA (3.7 or 4.2 kb Alpha-globin gene deletion

HGVS nomenclature

had previously been strongly associated with kidney disease among African-American and Hispanic populations (Table 2) (Behar et al., 2010; Nelson et al., 2010). In the SCD population, there is some evidence, in a single report, for the role of MYH9 in the development of proteinuria and renal dysfunction than APOL1 (Ashley-Koch et al., 2011). This differs from recent studies where APOL1 variants were shown to have a greater association with HIVassociated nephropathy and FSGS (Kasembeli et al., 2015a, 2015b), indicating that alleles may play differential roles in the progression of kidney disease, especially between different disease contexts. The causal variants and precise modus operandi are yet to be identified; therefore, further research is required to determine the mechanisms and other genetic factors influencing the development of kidney disease in SCD patients (Ashley-Koch et al., 2011). Pulmonary hypertension in SCD Epidemiology and risk factors. Approximately 30% of patients have an elevated tricuspid regurgitant jet velocity (TRV) (‡25 m/sec), as measured using transthoracic echocardiography, while right heart catheterization (RHC)confirmed pulmonary hypertension occurs in roughly 10% of these patients (Fonseca et al., 2012; Mehari et al., 2012; Parent et al., 2011; Pegelow et al., 1997). Pulmonary arterial hypertension (PAH) is common with a prevalence of 30% in SCD patients and all-cause mortality rates of 40% at 40 months after diagnosis in the United States (Pegelow et al., 1997). Studies in Nigeria indicate that PAH could represent a significant complication of SCD on the African continent (Aliyu et al., 2008). N-terminal (NT) pro-brain natriuretic peptide (proBNP) >160 ng/L has a 78% positive predictive value for PAH, with elevated levels being common and associated with markers of anemia, inflammation, and iron status, as well as with severe functional impairment among SCD patients in Nigeria (Aliyu et al., 2010). The prevalence of elevated tricuspid regurgitant velocity (TRV) measured by echocardiogram that predicts risk for pulmonary hyperten-

Effect

References Ashley-Koch et al. (2011) Ashley-Koch et al. (2011) Ashley-Koch et al. (2011) Ashley-Koch et al. (2011) Ashley-Koch et al. (2011) Ashley-Koch et al. (2011) Ashley-Koch et al. (2011) Ashley-Koch et al. (2011) Ashley-Koch et al. (2011); Saraf et al. (2015) Saraf et al. (2015) Saraf et al. (2015) Saraf et al. (2015) Saraf et al. (2015) Guasch et al. (1999)

sion and death in adult SCD was similar among patients in Tanzania and the United States (Cox et al., 2014). Genetics of pulmonary hypertension in SCD. There are limited data investigating the underlying genetic factors contributing to the risk of hypertension and particularly pulmonary hypertension in SCD. A preliminary genetic association study comparing patients with an elevated versus normal TRV and RHC-confirmed pulmonary hypertension revealed significant association with five SNPs within GALNT13 and a single SNP within PRELP ( p < 0.005) and a quantitative trait locus upstream of the adenosine-A2B receptor gene (ADORA2B) (Desai et al., 2012) (Table 3). Blood pressure in SCD Epidemiology and risk factors. Hypertension is a major risk factor for development of all stroke types; ischemic stroke, intracerebral hemorrhage, aneurysmal subarachnoid hemorrhage (Dubow and Fink, 2011), or cerebral infarction (Kannel et al., 1976) in both sexes and across all ages (Kannel et al., 1970). Hypertension is defined as systolic blood pressure (SBP) >140 mm Hg and diastolic blood pressure (DBP) >90 mm Hg (Chobanian et al., 2003) with increasing values of both parameters correlating with greater stroke risk. Hypertension represents one of the most important risk factors for stroke and is the most treatable (Cressman and Gifford, 1983). Antihypertensive treatments are supported by inferential evidence as contributors to the declining stroke incidence in the United States (Chobanian et al., 2003; Cressman and Gifford, 1983; Dubow and Fink, 2011; Kernan et al., 2014; Meissner et al., 1988). This may contribute to the trend of decreasing mortality due to cerebrovascular disease and stroke in the United States (Cressman and Gifford, 1983; Dubow and Fink, 2011). While the role of antihypertensive agents in prevention of primary stroke is well established, patients who have suffered prior cerebrovascular events are at high risk of recurrence for which there is no substantial

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Table 3. Genetic Variants Associated with the Development of Hypertension or Pulmonary Hypertension in SCD Patients Gene GALNT13

ADORA2B PRELP DRD2

SNPs

HGVS nomenclature

Effect

References

rs799813 rs10497120 rs13407922 rs16833378 rs9808145 rs7208480 rs2794452 rs7952106

NC_000002.12:g.154354666C>G NC_000002.12:g.153513398A>C NC_000002.12:g.154425285G>A NC_000002.12:g.153515514A>G NC_000002.12:g.154425689T>A NC_000017.11:g.15770470C>T NC_000001.11:g.203468576T>C NC_000011.9:g.113424558G>T

Risk Risk Risk Risk Risk Risk Risk Risk

Desai et al. (2012) Desai et al. (2012) Desai et al. (2012) Desai et al. (2012) Desai et al. (2012) Desai et al. (2012) Desai et al. (2012) Bhatnagar et al. (2013)

evidence associating the management of blood pressure and the risk of recurrent stroke (Schrader et al., 1998). The average blood pressure in patients with SCD is lower than that reported for nondiseased individuals. However relative hypertension in SCD sufferers confers an increased risk for stroke, kidney disease, and early mortality (Gladwin et al., 2004; Gordeuk et al., 2008). This is corroborated in a recent study of a Tanzanian cohort, stating that the SBP and DBP were significantly lower in the cases (109 and 64 mm Hg) compared with the non-SCA controls (116 and 68 mm Hg), respectively (Cox et al., 2014). However, studies in Nigeria provide contrasting results: researchers state that while the DBP was significantly lower in cases compared with controls, the mean SBP was comparable between these two groups, with discrepancy arising regarding the relative levels of mean arterial blood pressure in these two studies (Enakpene et al., 2014; Oguanobi et al., 2010). Genetics of blood pressure in SCD. A single genomewide meta-analysis of SBP in a population of AfricanAmerican SCD children identified suggestive candidate loci (rs7952106 in DRD2 and the MIR4301 gene) (Bhatnagar et al., 2013) (Table 3). These risk polymorphisms have not yet been investigated in an African population, thus highlighting the need for future research. Discussion Advocating for studies on genomics of cardiovascular diseases in SCD

Familial studies have provided evidence that predisposition to specific phenotypes of SCD such as stroke can be genetically inherited (Driscoll et al., 2003). However, there is limited evidence on the genetic polymorphisms influencing the vast clinical heterogeneity and disease severity across populations, with very few studies particularly in Africa. The majority of studies conducted in Africa have been clinical descriptions (Diallo and Tchernia, 2002), providing evidence that SCD is the third leading cause of mortality among hospitalized children (Athale and Chintu, 1994; Koko et al., 1998; Thuilliez et al., 1996). This substantiates the need for research focusing on the association between gene polymorphisms and disease phenotypes to facilitate better prediction of the clinical course of SCD in sub-Saharan Africa, the region with the highest burden of disease. More advanced techniques such as genome-

wide association studies (GWAS), whole-exome sequencing (WES), and whole-genome sequencing (WGS) represent a shift from genetics to genomics (Black et al., 2015), which is also required to identify novel loci that may contribute to the predisposition to complex and multifactorial disease-related cardiovascular phenotypes. However, improvement of local infrastructure in low- and middle-income countries is required for successful implementation and use of such medical genomics research (Forero et al., 2016). These techniques also require extensive bioinformatic resources and support, which in addition to long-term financial support, the availability of adequately trained personnel, and the regulation and structure of healthcare frameworks, prove to be potential barriers to the development of genomic medical centers in low- and middleincome countries (Black et al., 2015; Forero et al., 2016). In comparison with African SCD patients who die in early childhood, patients in first-world countries tend to have better health-related quality of life through appropriate medical and psychosocial care. This is, in part, due to the research and therapeutic advancements in countries such as the United States (Diallo and Tchernia, 2002). Long-term studies in Jamaica and the United States have shown that early detection and treatment of acute events (Vichinsky et al., 1990), as well as hydroxyurea treatment (Charache et al., 1995) and blood transfusion therapy (Adams et al., 1998), can reduce the severity of SCD and subsequently improve the quality of life of patients (Makani et al., 2007). The detection and treatment of SCD are fundamental obstacles in low-income and developing countries, particularly in Africa, as most patients have limited access to clinical care due to a myriad of factors, including distance or cost (Diallo and Tchernia, 2002). Advanced research and care in high-income countries have significantly increased early diagnoses and management of patients, decreasing morbidity and mortality in SCD patients. This can be attributed to interventions such as early prenatal diagnosis, prophylactic treatment with oral penicillin, hydroxyurea treatment, and management of vaso-occlusive events (Diallo and Tchernia, 2002). Low-income developing countries still require improvement in infrastructure to offer basic primary medical care and ensure disease awareness (Diallo and Tchernia, 2002; Makani et al., 2007). There is a critical need for a collaborative effort among ministries of health, international agencies, and research organizations to develop an effective strategy to implement cost-effective,

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sustainable screening programs, improvement of health services and relatable education for family and community members about SCD, and various therapeutic agents such as hydroxyurea, transfusions, and hematopoietic stem cell transplantation (Makani et al., 2007; Rumaney et al., 2014). The high incidence of cardiovascular disease in SCD patients, particularly in Africa, highlights the need for improved prognostic techniques for at-risk individuals, specifically children in the case of primary stroke development (Flanagan et al., 2011). Hypertension is a relatively unreported yet significant contributor to the development of cardiovascular phenotypes such as stroke and kidney disease, which may serve as an additional marker for identification of at-risk individuals. The ability to predict the phenotype of a patient using genetic markers and prognostic tools such as TCD before the complication evolves would allow improved individualized treatment and reduce treatment-related complications (Steinberg, 2005). The identification of genetic polymorphisms associated with the clinical phenotype of SCD can be used to create a panel of biomarkers to identify at-risk individuals and better inform treatment approaches. This review on genetics associated with cardiovascular phenotypes of SCD is limited by the inclusion of articles written largely in English as well as the lack of data available for analysis. Furthermore, the specific pathophysiology of each cardiovascular complication and the pathway through which genetic modifiers act are not elucidated in this review. This topic requires further investigation and inclusion in future analysis. In conclusion, limited genetic studies associated with these critical cardiovascular phenotypes in SCD (stroke, pulmonary hypertension, kidney diseases) have been reported in SCD in general and specifically in patients who reside in Africa, despite the high burden, indicating an urgent need to perform these studies that could inform, in a unique way, the global SCD communities about the values that genes and environment interactions play in the pathogenesis and hopefully the care of SCD. Understanding the genetic networks modulating the development of cardiovascular phenotypes may provide additional information regarding the underlying pathogenesis of SCD and subsequently lead to the identification of new therapeutic targets. Future directives

The present review has provided some perspectives regarding the lack of validated genetic modifiers associated with the development of complex cardiovascular phenotypes of SCD. This indicates the urgent need for investigations aimed at identifying or validating specific SNPs associated with stroke, kidney disease, or pulmonary hypertension in various SCD patient populations. Interdisciplinary and personalized solutions pertaining to various affected populations are required to address the current disparity in healthcare in low- and middle-income countries, particularly in Africa. Second, this review highlights the lack of research performed in Africa, despite this region containing the largest number of people affected by the disease, therefore future research should involve investigation of African cohorts. While the identification of SNPs that may assist in identifying at-risk individuals is beneficial to the SCD population, future studies should also aim to investigate the precise

GEARD ET AL.

mechanism whereby these mutations affect phenotype development. This will assist in the understanding of how these specific phenotypes arise in only certain subgroups of the SCD population, therefore informing new, more targeted preventative treatment developments. Expert Commentary

Cardiovascular phenotypes of SCD represent a major cause of morbidity and mortality in patients. The exact mechanism of stroke, kidney disease, or hypertension development is largely unknown and understudied in the sickle population. The vast clinical heterogeneity observed in the various phenotypes of this simple monogenic disorder indicates that genetic factors may play a role in the development risk of a specific cardiovascular manifestation of SCD. This is substantiated by the role that genetic modifiers such as HbF and alpha-thalassemia play in ameliorating certain clinical features of the disease. The current dilemma stems from the lack of preventative medical care available for patients in developing countries. Preventative treatment techniques administered to at-risk patients may help to reduce the high prevalence of cardiovascular disease among the SCD population, particularly in an African setting. This highlights the need for tools with which to identify at-risk patients and the identification of genetic modifiers associated with the development of specific phenotypes, specifically in the different population groups. Key Issues 









This review has analyzed the epidemiology and risk factors associated with three main cardiovascular phenotypes of SCD; stroke, pulmonary hypertension, and kidney disease. Several genetic factors are known to ameliorate the clinical progression of SCD, and multiple SNPs in several genes have been identified that may be associated with the development of specific cardiovascular phenotypes. Very few SNPs that have been validated across populations are available for predictive testing of cardiovascular complications in SCD, highlighting the need for future research. Research in African context is minimal; however, encouraging data on cardiovascular phenotypes of SCD are evolving, particularly from countries such as Nigeria, Tanzania, and Cameroon. Large-scale genome-wide association, whole-exome, and whole-genome studies are required to identify genetic factors that affect the clinical progression of SCD cardiovascular complications.

Acknowledgments

A.W. conceived and designed the review. A.G., G.D.P., and A.W. performed the search. A.G., G.D.P., D.C., V.J.N.B., and A.W. analyzed the data. A.G., G.D.P., and A.W. wrote the article. A.G., G.D.P., D.C., V.J.N.B., and A.W. revised and approved the manuscript. The study was funded by the National Health Laboratory Services (NHLS), South Africa; and the NIH, USA, grant number 1U01HG007459-01; and the Welcome Trust, Developing Excellence in Leadership,

SICKLE CELL AND CARDIOVASCULAR DISEASE GENETICS

Training and Science (DELTAS) Africa, Award 107755Z/15/ Z. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Author Disclosure Statement

The authors declare that no conflicting financial interests exist. References

Adams GT, Snieder H, McKie VC, et al. (2003). Genetic risk factors for cerebrovascular disease in children with sickle cell disease: Design of a case-control association study and genomewide screen. BMC Med Genet 4, 6. Adams RJ. (2005). TCD in sickle cell disease: An important and useful test. Pediatr Radiol 35, 229–234. Adams RJ, Kutlar A, McKie V, et al. (1994). Alpha thalassemia and stroke risk in sickle cell anemia. Am J Hematol 45, 279–282. Adams RJ, McKie VC, Hsu L, et al. (1998). Prevention of a first stroke by transfusions in children with sickle cell anemia and abnormal results on transcranial Doppler ultrasonography. N Engl J Med 339, 5–11. Adams RJ, McKie VC, Nichols F, et al. (1992). The use of transcranial ultrasonography to predict stroke in sickle cell disease. N Engl J Med 326, 605–610. Adams RJ, Ohene-Frempong K, and Wang W. (2001). Sickle cell and the brain. Hematology Am Soc Hematol Educ Program 2001, 31–46. Adedoyin OT, Adesiyun OO, Adegboye OA, Bello OA, and Fatoye OP. (2012). Sickle cell nephropathy in children seen in an African Hospital—Case Report. Niger Postgrad Med J 19, 119–122. Akingbola TS, Tayo BO, Salako B, et al. (2014). Comparison of patients from Nigeria and the USA highlights modifiable risk factors for sickle cell anemia complications. Hemoglobin 38, 236–243. Aliyu ZY, Gordeuk V, Sachdev V, et al. (2008). Prevalence and risk factors for pulmonary artery systolic hypertension among sickle cell disease patients in Nigeria. Am J Hematol 83, 485–490. Aliyu ZY, Suleiman A, Attah E, et al. (2010). NT-proBNP as a marker of cardiopulmonary status in sickle cell anaemia in Africa. Br J Haematol 150, 102–107. Ashley-Koch AE, Okocha EC, Garrett ME, et al. (2011). MYH9 and APOL1 are both associated with sickle cell disease nephropathy. Br J Haematol 155, 386–394. Athale UH, and Chintu C. (1994). Clinical analysis of mortality in hospitalized Zambian children with sickle cell anaemia. East Afr Med J 71, 388–391. Barisoni L, and Nelson PJ. (2007). Collapsing glomerulopathy: An inflammatory podocytopathy? Curr Opin Nephrol Hypertens 16, 192–195. Bean CJ, Boulet SL, Ellingsen D, et al. (2012). Heme oxygenase-1 gene promoter polymorphism is associated with reduced incidence of acute chest syndrome among children with sickle cell disease. Blood 120, 3822–3828. Becton LJ, Kalpatthi RV, Rackoff E, et al. (2010). Prevalence and clinical correlates of microalbuminuria in children with sickle cell disease. Pediatr Nephrol 25, 1505–1511. Behar DM, Rosset S, Tzur S, et al. (2010). African ancestry allelic variation at the MYH9 gene contributes to increased susceptibility to non-diabetic end-stage kidney disease in Hispanic Americans. Hum Mol Genet 19, 1816–1827.

589

Belcher JD, Mahaseth H, Welch TE, Otterbein LE, Hebbel RP, and Vercellotti GM. (2006). Heme oxygenase-1 is a modulator of inflammation and vaso-occlusion in transgenic sickle mice. J Clin Invest 116, 808–816. Bhatnagar P, Barron-Casella E, Bean CJ, et al. (2013). Genomewide meta-analysis of systolic blood pressure in children with sickle cell disease. PLoS One 8, e74193. Black M, Wang W, and Wang W. (2015). Ischemic stroke: From next generation sequencing and GWAS to community genomics? OMICS 19, 451–460. Charache S, Terrin ML, Moore RD, et al. (1995). Effect of hydroxyurea on the frequency of painful crises in sickle cell anemia. Investigators of the Multicenter Study of Hydroxyurea in Sickle Cell Anemia. N Engl J Med 332, 1317–1322. Chobanian AV, Bakris GL, Black HR, et al. (2003). Seventh report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure. Hypertension 42, 1206–1252. Cox SE, Makani J, Soka D, et al. (2014). Haptoglobin, alphathalassaemia and glucose-6-phosphate dehydrogenase polymorphisms and risk of abnormal transcranial Doppler among patients with sickle cell anaemia in Tanzania. Br J Haematol 165, 699–706. Cox SE, Soka D, Kirkham FJ, et al. (2014). Tricuspid regurgitant jet velocity and hospitalization in Tanzanian children with sickle cell anemia. Haematologica 99, e1–e4. Cressman MD, and Gifford RW. (1983). Hypertension and stroke. Am J Cardiol 1, 521–527. DeBaun MR, Glauser TA, Siegel M, Borders J, and Lee B. (1995). Noninvasive central nervous system imaging in sickle cell anemia. A preliminary study comparing transcranial Doppler with magnetic resonance angiography. J Pediatr Hematol Oncol 17, 29–33. Desai AA, Zhou T, Ahmad H, et al. (2012). A novel molecular signature for elevated tricuspid regurgitation velocity in sickle cell disease. Am J Respir Crit Care Med 186, 359–368. Diagne I, Ndiaye O, Moreira C, et al. (2000). Les syndromes dre´panocytaires majeurs en pe´diatrie a‘ Dakar (Se´ne´gal). Arch Pe´diatr 7, 16–24. Diallo D and Tchernia G. (2002). Sickle cell disease in Africa. Curr Opin Hematol 9, 111–116. Diop S, Thiam D, Cisse M, Toure-Fall AO, Fall K, and Diakhate L. (1999). New results in clinical severity of homozygous sickle cell anemia, in Dakar, Senegal. Hematol Cell Ther 41, 217–221. Driscoll MC, Hurlet A, Berman B, Files B, and Byrne J. (1997). Stroke in SIB-pairs with sickle cell disease. Blood 90, 1160. Driscoll MC, Hurlet A, Styles L, et al. (2003). Stroke risk in siblings with sickle cell anemia. Blood 101, 2401–2404. Dubow J, and Fink ME. (2011). Impact of hypertension on stroke. Curr Atheroscler Rep 13, 298–305. Earley CJ, Kittner SJ, Feeser BR, et al. (1998). Stroke in children and sickle-cell disease: Baltimore-Washington Cooperative Young Stroke Study. Neurology 51, 169–176. Enakpene EO, Adebiyi AA, Ogah OS, et al. (2014). Noninvasive estimation of pulmonary artery pressures in patients with sickle cell anaemia in Ibadan, Nigeria: An echocardiographic study. Acta Cardiol 69, 505–511. Enosolease ME, Ejele OA, and Awodu OA. (2005). The influence of foetal haemoglobin on the frequency of vaso-occlusive crisis in sickle cell anaemia patients. Niger Postgrad Med J 12, 102–105. Fatunde OJ, and Scott-Emuakpor AB. (1992). Foetal haemoglobin in Nigerian children with sickle cell anaemia. Ef-

590

fect on haematological parameters and clinical severity. Trop Geogr Med 44, 264–266. Flanagan JM, Frohlich DM, Howard TA, et al. (2011). Genetic predictors for sroke in children with sickle cell anemia. Blood J Am Soc Hematol 117, 6681–6684. Flanagan JM, Sheehan V, Linder H, et al. (2013). Genetic mapping and exome sequencing identify 2 mutations associated with stroke protection in pediatric patients with sickle cell anemia. Blood 121, 3237–3245. Fonseca GH, Souza R, Salemi VM, Jardim C V, and Gualandro SF. (2012). Pulmonary hypertension diagnosed by right heart catheterisation in sickle cell disease. Eur Respir J 39, 112– 118. Forero DA, Wonkam A, Wang W, et al. (2016). Current needs for human and medical genomics research infrastructure in low and middle income countries. J Med Genet 53, 438–40. Genovese G, Friedman DJ, Ross MD, et al. (2010). Association of trypanolytic ApoL1 variants with kidney disease in African Americans. Science 329, 841–845. Gladwin MT, Sachdev V, Jison ML, et al. (2004). Pulmonary hypertension as a risk factor for death in patients with sickle cell disease. N Engl J Med 350, 886–895. Gordeuk VR, Sachdev V, Taylor JG, Gladwin MT, Kato G, and Castro OL. (2008). Relative systemic hypertension in patients with sickle cell disease is associated with risk of pulmonary hypertension and renal insufficiency. Am J Hematol 83, 15–18. Guasch A, Cua M, and Mitch WE. (1996). Early detection and the course of glomerular injury in patients with sickle cell anemia. Kidney Int 49, 786–791. Guasch A, Navarrete J, Nass K, and Zayas CF. (2006). Glomerular involvement in adults with sickle cell hemoglobinopathies: Prevalence and clinical correlates of progressive renal failure. J Am Soc Nephrol 17, 2228–2235. Guasch A, Zayas CF, Eckman JR, Muralidharan K, Zhang W, and Elsas LJ. (1999). Evidence that microdeletions in the alpha globin gene protect against the development of sickle cell glomerulopathy in humans. J Am Soc Nephrol 10, 1014– 1019. Hirai H, Kubo H, Yamaya M, et al. (2003). Brief report Microsatellite polymorphism in heme oxygenase-1 gene promoter is associated with susceptibility to oxidant-induced apoptosis in lymphoblastoid cell lines. Apoptosis 102, 2002– 2004. Hoppe C, Cheng S, Grow M, et al. (2001). A novel multilocus genotyping assay to identify genetic predictors of stroke in sickle cell anaemia. Br J Haematol 114, 718–720. Hoppe C, Klitz W, Cheng S, et al. (2004). Gene interactions and stroke risk in children with sickle cell anemia. Blood 103, 2391–2397. Hoppe C, Klitz W, Noble J, Vigil L, Vichinsky E, and Styles L. (2003). Distinct HLA associations by stroke subtype in children with sickle cell anemia. Blood 101, 2865–2869. Hsu LL, Miller ST, Wright E, et al. (2003). Alpha Thalassemia is associated with decreased risk of abnormal transcranial Doppler ultrasonography in children with sickle cell anemia. J Pediatr Hematol Oncol 25, 622–628. Iwalokun BA, Iwalokun SO, Hodonu SO, Aina OA, and Agomo PU. (2012). Evaluation of microalbuminuria in relation to asymptomatic bacteruria in Nigerian patients with sickle cell anemia. Saudi J Kidney Dis Transpl 23, 1320–1330. Kanakiriya S, Croatt AJ, Haggard JJ, et al. (2003). Heme: A novel inducer of MCP-1 through HO-dependent and HOindependent mechanisms. Am J Physiol - Ren Physiol 284, F546–F554.

GEARD ET AL.

Kannel WB, Dawber TR, Sorlie P and Wolf PA. (1976). Components of blood pressure and risk of atherothrombotic brain infarction: The Framingham study. Stroke 7, 327–331. Kannel WB, Wolf PA, Verter J, and McNamara PM. (1970). Epidemiologic assessment of the role of blood pressure in stroke. The Framingham study. JAMA 214, 301–310. Kao WH, Klag MJ, Meoni LA, et al. (2008). MYH9 is associated with nondiabetic end-stage renal disease in African Americans. Nat Genet 40, 1185–1192. Kasembeli AN, Duarte R, Ramsay M, et al. (2015a). APOL1 risk variants are strongly associated with HIV-associated nephropathy in Black South Africans. J Am Soc Nephrol 26, 2882–90. Kasembeli AN, Duarte R, Ramsay M, and Naicker S. (2015b). African origins and chronic kidney disease susceptibility in the human immunodeficiency virus era. World J Nephrol 4, 295–306. Kernan WN, Ovbiagele B, Black HR, et al. (2014). Guidelines for the prevention of stroke in patients with stroke and transient ischemic attack: A guideline for healthcare professionals from the American Heart Association/American Stroke Association. Stroke 45, 2160–2236. Kinney TR, Sleeper LA, Wang WC, et al. (1999). Silent cerebral infarcts in sickle cell anemia: A risk factor analysis. The Cooperative Study of Sickle Cell Disease. Pediatrics 103, 640–645. Koko J, Dufillot D, M’Ba-Meyo J, Gahouma D, amd Kani F. (1998). Mortality of children with sickle cell disease in a pediatric department in Central Africa. Arch Pediatr 5, 965– 969. Kopp JB, Smith MW, Nelson GW, et al. (2008). MYH9 is a major-effect risk gene for focal segmental glomerulosclerosis. Nat Genet 40, 1175–1184. Kwiatkowski JL, Hunter JV, Smith-Whitley K, Katz ML, Shults J, and Ohene-Frempong K. (2003). Transcranial Doppler Ultrasonography in siblings with sickle cell disease. Br J Haematol 121, 932–937. Lagunju I, Sodeinde O, and Telfer P. (2012). Prevalence of transcranial Doppler abnormalities in Nigerian children with sickle cell disease. Am J Hematol 87, 544–547. Lettre G, Sankaran VG, Bezerra MA, et al. (2008). DNA polymorphisms at the BCL11A, HBS1L-MYB, and betaglobin loci associate with fetal hemoglobin levels and pain crises in sickle cell disease. Proc Natl Acad Sci USA 105, 11869–11874. Makani J, Kirkham FJ, Komba A, et al. (2009). Risk factors for high cerebral blood flow velocity and death in Kenyan children with Sickle Cell Anaemia: Role of haemoglobin oxygen saturation and febrile illness. Br J Haematol 145, 529–532. Makani J, Williams TN, and Marsh K. (2007). Sickle cell disease in Africa: Burden and research priorities. Ann Trop Med Parasitol 101, 3–14. Mehari A, Gladwin MT, Tian X, Machado RF, and Kato GJ. (2012). Mortality in adults with sickle cell disease and pulmonary hypertension. JAMA 307, 1254–1256. Meissner I, Whisnant JP, and Garraway WM. (1988). Hypertension Management and Stroke Recurrence in a Community (Rochester, Minnesota, 1950-1979). Stroke 19, 459–463. Moher D, Liberati A, Tetzlaff J, Altman DG; The PRISMA Group (2009). Preferred reporting items for systematic reviews and meta-analyses: The PRISMA statement. PLoS Med 6, e1000097.

SICKLE CELL AND CARDIOVASCULAR DISEASE GENETICS

Moser FG, Miller ST, Bello JA, et al. (1996). The spectrum of brain MR abnormalities in sickle-cell disease: A report from the Cooperative Study of Sickle Cell Disease. Am J Neuroradiol 17, 965–972. Nath KA, Grande JP, Haggard JJ, et al. (2001). Oxidative stress and induction of heme oxygenase-1 in the kidney in sickle cell disease. Am J Pathol 158, 893–903. Nath KA, and Katusic ZS. (2012). Vasculature and kidney complications in sickle cell disease. J Am Soc Nephrol 23, 781–784. Nelson GW, Freedman BI, Bowden DW, et al. (2010). Dense mapping of MYH9 localizes the strongest kidney disease associations to the region of introns 13 to 15. Hum Mol Genet 19, 1805–1815. Njamnshi AK, Mbong EN, Wonkam A, et al. (2006). The epidemiology of stroke in sickle cell patients in Yaounde, Cameroon. J Neurol Sci 250, 79–84. Nowak-Go¨ttl U, Stra¨ter R, Heinecke A, et al. (1999). Lipoprotein (a) and genetic polymorphisms of clotting factor V, prothrombin, and methylenetetrahydrofolate reductase are risk factors of spontaneous ischemic stroke in childhood. Blood 94, 3678–3682. Oguanobi NI, Onwubere BJC, Ibegbulam OG, et al. (2010). Arterial blood pressure in adult Nigerians with sickle cell anemia. J Cardiol 56, 326–331. Ohene-Frempong K. (1991). Stroke in sickle cell disease: Demographic, clinical, and therapeutic considerations. Semin Hematol 28, 213–219. Ohene-Frempong K, Weiner SJ, Sleeper LA, et al. (1998). Cerebrovascular accidents in sickle cell disease: Rates and risk factors. Blood 91, 288–294. Osei-Yeboah CT, and Rodrigues O. (2011). Renal status of children with sickle cell disease in Accra, Ghana. Ghana Med J 45, 155–160. Pakasa NM, and Sumaili EK. (2012). Pathological peculiarities of chronic kidney disease in patient from sub-Saharan Africa. Review of data from the Democratic Republic of the Congo. Ann Pathol 32, 40–52. Parent F, Bachir D, Inamo J, et al. (2011). A hemodynamic study of pulmonary hypertension in sickle cell disease. N Engl J Med 365, 44–53. Pegelow CH, Colangelo L, Steinberg M, et al. (1997). Natural history of blood pressure in sickle cell disease: Risks for stroke and death associated with relative hypertension in sickle cell anemia. Am J Med 102, 171–177. Pham PT, Pham PT, Wilkinson AH, and Lew SQ. (2000). Renal abnormalities in sickle cell disease. Kidney Int 57, 1–8. Piel FB, Patil AP, Howes RE, et al. (2013). Global epidemiology of sickle haemoglobin in neonates: A contemporary geostatistical model-based map and population estimates. Lancet 381, 142–151. Platt OS, Brambilla DJ, Rosse WF, et al. (1994). Mortality in sickle cell disease. N Engl J Med 330, 1639–16344. Platt OS, Thorington BD, Brambilla DJ, et al. (1991). Pain in sickle cell disease rates and risk factors. N Engl J Med 325, 11–16. Powars D, Wilson B, Imbus C, Pegelow C, and Allen J. (1978). The natural history of stroke in sickle cell disease. Am J Med 65, 461–471. Powars DR, Chan L, and Schroeder WA. (1990). Beta S-genecluster haplotypes in sickle cell anemia: Clinical implications. Am J Pediatr Hematol Oncol 12, 367–374.

591

Powars DR, Elliott-Mills DD, Chan L, et al. (1991). Chronic renal failure in sickle cell disease: Risk factors, clinical course, and mortality. Ann Intern Med 115, 614–620. Ruffieux N, Njamnshi AK, Wonkam A, et al. (2013). Association between biological markers of sickle cell disease and cognitive functioning amongst Cameroonian children. Child Neuropsychol 19, 143–160. Rumaney MB, Ngo Bitoungui VJ, Vorster AA, et al. (2014). The co-inheritance of alpha-thalassemia and sickle cell anemia is associated with better hematological indices and lower consultations rate in Cameroonian patients and could improve their survival. PLoS One 9, 1–10. Saraf SL, Zhang X, Shah B, et al. (2015). Genetic variants and cell-free hemoglobin processing in sickle cell nephropathy. Haematologica 100, 1275–1284. Schrader J, Ro¨themeyer M, Lu¨ders S, and Kollmann K. (1998). Hypertension and stroke—Rationale behind the ACCESS trial. Basic Res Cardiol 93 Suppl 2, 69–78. Sebastiani P, Ramoni MF, Nolan V, Baldwin CT, and Steinberg MH. (2005). Genetic dissection and prognostic modeling of overt stroke in sickle cell anemia. Nat Genet 37, 435–440. Serjeant GR. (2005). Mortality from sickle cell disease in Africa. BMJ 330, 432–433. Steinberg MH. (2005). Predicting clinical severity in sickle cell anaemia. Br J Haematol 129, 465–481. Steinberg MH, Barton F, Castro O, et al. (2003). Effect of hydroxyurea on mortality and morbidity in adult sickle cell anemia: Risks and benefits up to 9 years of treatment. JAMA 289, 1645–1651. Stockman JA, Nigro MA, Mishkin MM, and Oski FA. (1972). Occlusion of large cerebral vessels in sickle-cell anemia. N Engl J Med 287, 846–849. Stuart MJ, Nagel RL, and Jefferson T. (2004). Sickle-cell disease. Lancet 364, 1343–1360. Taha H, Skrzypek K, Guevara I, et al. (2010). Role of heme oxygenase-1 in human endothelial cells: Lesson from the promoter allelic variants. Arterioscler Thromb Vasc Biol 30, 1634–1641. Taylor VI JG, Tang DC, Savage SA, et al. (2002). Variants in the VCAM1 gene and risk for symptomatic stroke in sickle cell disease. Blood 100, 4303–4309. Thuilliez V, Ditsambou V, Mba JR, Mba MS, and Kitengue J. (1996). Aspects actuels de la drepanocytose chez l’enfant au Gabon. Arch Pediatr 3, 668–674. Tracz MJ, Alam J, and Nath KA. (2007). Physiology and pathophysiology of Heme: Implications for kidney disease. J Am Soc Nephrol 18, 414–420. van Eps LW, Schouten H, Romeny-Wachter CC, and PorteWijsman LW. (1970). The relation between age and renal concentrating capacity in sickle cell disease and hemoglobin C disease. Clin Chim Acta 27, 501–511. Vichinsky EP, Earles A, Johnson RA, Hoag MS, Williams A, and Lubin B. (1990). Alloimmunization in sickle cell anemia and transfusion of racially unmatched blood. N Engl J Med 322, 1617–1621. Wang WC, Pavlakis SG, Helton KJ, et al. (2008). MRI abnormalities of the brain in one-year-old children with sickle cell anemia. Pediatr Blood Cancer 51, 643–6. Wonkam A, Ngo Bitoungui VJ, Vorster AA, et al. (2014). Association of variants at BCL11A and HBS1L-MYB with hemoglobin F and hospitalization rates among sickle cell patients in Cameroon. PLoS One 9, e92506.

592

GEARD ET AL.

Xu J, Sankaran VG, Ni M, et al. (2010). Transcriptional silencing of c-globin by BCL11A involves long-range interactions and cooperation with SOX6. Genes Dev 24, 783–789. Yamada N, Yamaya M, Okinaga S, et al. (2000). Microsatellite polymorphism in the heme oxygenase-1 gene promoter is associated with susceptibility to emphysema. Am J Hum Genet 66, 187–195. Yetunde A, and Anyaegbu. CC (2001). Profile of the Nigerian sickle cell anaemia patients above 30 years of age. Cent Afr J Med 47, 108–111.

Address correspondence to: Ambroise Wonkam, MD, DMedSc, PhD Division of Human Genetics Department of Medicine Faculty of Health Sciences University of Cape Town Anzio Road Observatory Cape Town 7925 Republic of South Africa E-mail: [email protected]

CT DBP ESRD FSGS GWAS HLA LV MRI PAH RHC SBP SCA SCD SCI SNP SV TAMV TCD TRV

¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼ ¼

Abbreviations Used computed tomography diastolic blood pressure end-stage renal disease focal segmental glomerulosclerosis genome-wide association studies human leukocyte antigen large-vessel magnetic resonance imaging pulmonary arterial hypertension right heart catheterization systolic blood pressure sickle cell anemia sickle cell disease silent cerebral infarcts single-nucleotide polymorphism small-vessel time-averaged mean velocities transcranial doppler ultrasonography tricuspid regurgitation jet velocity