Genetics of Sickle Cell-Associated Cardiovascular Disease: An Expert Review with Lessons Learned in Africa

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, α-globin and γ-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, whole-exome 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.

Introduction

Sickle 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 β-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.

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.

FIG. 1.

Flowchart of the literature review employed in the present expert review.

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). 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; Ohene-Frempong 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 <170 cm/sec) (Adams et al., 1992, 2003). High flow rates in the intercranial carotid arteries can be a result of increased blood volume in the artery and/or a decrease in the arterial diameter due to stenosis, with both factors occurring prevalently in SCD (Adams et al., 2003).

While TCD is considered a useful tool for the identification of candidates for prophylactic transfusion, only 10% of these patients develop stroke within a year of the measurement (Steinberg, 2005). This technique, however, represents a screening method that can be successfully implemented in developing countries, as demonstrated by studies in Kenya (Makani et al., 2009), Cameroon (Ruffieux et al., 2013), Nigeria (Lagunju et al., 2012), and Tanzania (Cox et al., 2014), to identify individuals at risk for development of stroke. The success of TCD in identifying these individuals is hindered due to limited resources and a lack of preventative treatment procedures, such as transfusion therapy, resulting in infrequent use of TCD in routine healthcare procedures and subsequent blood transfusions in those patients found to have an abnormal TAMV result (Makani et al., 2009).

There are relatively few reports on the incidence of stroke among SCD patients in Africa. Clinical examination and CT scans identified a stroke prevalence of 6.7% in Cameroon (Njamnshi et al., 2006); however, this value may be an underestimation. A study of children with SCD in Nigeria found a stroke prevalence of 8.7% (Aliyu et al., 2008). The prevalence of silent cerebral infarcts (SCIs) and cerebral vasculopathies has been shown to be even greater than overt stroke risk: SCI occurs in 27% of this population before their sixth birthday and 37% by their 14th birthday (Kinney et al., 1999). SCI is diagnosed by MRI, but has not been studied in Africa due to limited availability of MRI equipment, while the lack of longitudinally monitored SCD cohorts in Africa weakens incidence and prevalence estimates. In particular, Cameroonian SCD patients have been reported to have a high incidence of stroke and extremely impaired neurocognitive functions (Njamnshi et al., 2006; Ruffieux et al., 2011).

The cognitive performance of Cameroonian SCD children was evaluated using a neuropsychological test battery assessing four domains of cognitive functioning (executive function, attention, memory, and sensory-motor skills). A high prevalence of cognitive deficits was found, increasing with age, and with a specific impairment of executive functions and attention (Ruffieux et al., 2011). Up to 37.5% of the 96 SCD patients aged 6–24 years (M = 13.5, standard deviation = 4.9) had mild to severe cognitive deficits that subsequently tended to increase with age (Ruffieux et al., 2011). However, it is noteworthy that data indicating the SCD-related stroke occurrence in Africa come from few and relatively small studies (Makani et al., 2007).

The natural history of stroke in SCD is well described (Ohene-Frempong, 1991; Serjeant, 2005), whereas the underlying pathophysiology of stroke development in SCD children is poorly understood with even fewer validated, predictive genetic risk factors (Flanagan et al., 2013; Hoppe et al., 2001, 2003). The severity of anemia, a history of recurrent acute chest syndrome, obesity, and elevated blood pressure are all considered independent risk factors for the development of stroke (Akingbola et al., 2014; Ohene-Frempong et al., 1998; Wang et al., 2008). Other risk factors include prior transient ischemic attack, low steady-state hemoglobin levels, hypertension, and high leukocyte counts (Ohene-Frempong et al., 1998). However, there are many risk factors that remain elusive (Adams et al., 2003). Multiple genes, gene–gene interactions, and environmental factors likely influence the etiology of stroke, highlighting the complexity of this multifactorial trait (Hoppe et al., 2004).

Genetics and stroke in SCD

Underlying genetic contributions to stroke risk in SCD patients have been identified through sibling pair, epidemiologic, and animal model studies (Driscoll et al., 1997, 2003). The alpha-thalassemia polymorphism, a 3.7 or 4.2 kb deletion (Flanagan et al., 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 vaso-occlusive 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).

Table 1.

Genetic Variants Associated with Stroke Incidence in SCD

Gene	SNPs	HGVS nomenclature	Effect	References
GOLGB1	rs3732410	NP_004478.3:p.Tyr1212Cys	Protective	Flanagan et al. (2013)
ENPP1	rs1044498	NP_006199.2:p.Lys173Gln	Protective	Flanagan et al. (2013)
PON1	rs662	NP_000437.3:p.Gln192Arg	Risk	Flanagan et al. (2013)
ANXA2	rs11853426	NC_000015.10:g.60317231C>T	Risk	Flanagan et al. (2011)
TEK	rs489347	NC_000009.12:g.27187218C>G	Risk	Flanagan et al. (2011)
TGFBR3	rs284875	NC_000001.11:g.91705151A>G	Risk	Flanagan et al. (2011)
ADCY9	rs2238432	NC_000016.10:g.3964140G>A	Protective	Flanagan et al. (2011)
HBA (3.7 or 4.2 kb Alpha-globin gene deletion)			Protective	Hsu et al. (2003)
HBB gene cluster	rs7482144	NC_000011.9:g.5276169G>A	Protective	Lettre et al. (2008)
HBS1L-MYB	rs28384513	NC_000006.12:g.135055071T>G	Protective	Lettre et al. (2008)
	rs9399137	NC_000006.12:g.135097880T>C	Protective	Lettre et al. (2008)
	rs4895441	NC_000006.12:g.135105435A>G	Protective	Lettre et al. (2008)
BCL11A	rs4671393	NC_000002.12:g.60493816A>G	Protective	Lettre et al. (2008)

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

There is a further association with β-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-Göttl 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 single-nucleotide 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-α 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-β 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 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, whole-exome, 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 predispose 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 β-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 (<25) have greater HO-1 expression, while longer repeats (≥25) result in decreased HO-1 activity (Hirai et al., 2003; Taha et al., 2010; Yamada et al., 2000), which are associated with decreased and increased red blood stasis in transgenic SCD mice, respectively (Belcher et al., 2006).

Longer repeats are further associated with reduced glomerular filtration rates in African-American SCD patients (Saraf et al., 2015). Another SNP in HMOX1, rs743811, has been associated with chronic kidney disease and ESRD in a group of African-American SCD patients (Saraf et al., 2015) (Table 2). HMOX1 therefore plays a functional role in modulating the development of kidney disease in SCD patients through insufficient protection from hemoglobin-mediated toxicity (Bean et al., 2012).

Table 2.

Genetic Variants Associated with the Development of Kidney Disease in SCD

Gene	SNPs	HGVS nomenclature	Effect	References
MYH9	rs5750248	NC_000022.11:g.36306846T>C	Risk	Ashley-Koch et al. (2011)
	rs8141189	NC_000022.11:g.36318665T>A	Risk	Ashley-Koch et al. (2011)
	rs11912763	NC_000022.11:g.36288676G>A	Risk	Ashley-Koch et al. (2011)
	rs16996648	NC_000022.11:g.36296706T>C	Risk	Ashley-Koch et al. (2011)
	rs5756152	NC_000022.11:g.36316427A>G	Risk	Ashley-Koch et al. (2011)
	rs16996672	NC_000022.11:g.36329925C>T	Risk	Ashley-Koch et al. (2011)
	rs1557529	NC_000022.11:g.36309484A>G	Risk	Ashley-Koch et al. (2011)
	rs1005570	NC_000022.11:g.36319229A>G	Risk	Ashley-Koch et al. (2011)
APOL1	rs73885319	NP_663318.1:p.Ser358Gly	Risk	Ashley-Koch et al. (2011); Saraf et al. (2015)
	rs60910145	NP_663318.1:p.Ile400Met	Risk	Saraf et al. (2015)
	rs71785313	NP_663318.1:p.Asn404_Tyr405del	Risk	Saraf et al. (2015)
HMOX1	rs3074372	NC_000022.11:g.35380894_35380895insGT	Protective/risk	Saraf et al. (2015)
	rs743811	NC_000022.11:g.35396981T>C	Risk	Saraf et al. (2015)
HBA (3.7 or 4.2 kb Alpha-globin gene deletion			Protective	Guasch et al. (1999)

The MYH9 and APOL1 genes have been associated with risk for FSGS in African-Americans (Ashley-Koch et al., 2011; Kopp et al., 2008). Eleven SNPs have been identified in MYH9 and APOL1 genes, associated with the development of FSGS and ESRD in African-American populations (Genovese et al., 2010; Kao et al., 2008; Kopp et al., 2008), and subsequently validated in an American SCD population (Ashley-Koch et al., 2011; Saraf et al., 2015). Furthermore, the relative association of APOL1 with renal dysfunction was reduced in the presence of MYH9 SNPs (Ashley-Koch et al., 2011) in contrast to the findings of previous studies (Kao et al., 2008; Kopp et al., 2008). Among the remaining eight variations in the MYH9 gene, rs5750248 and rs11912763 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 HIV-associated 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 hypertension 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).

Table 3.

Genetic Variants Associated with the Development of Hypertension or Pulmonary Hypertension in SCD Patients

Gene	SNPs	HGVS nomenclature	Effect	References
GALNT13	rs799813	NC_000002.12:g.154354666C>G	Risk	Desai et al. (2012)
	rs10497120	NC_000002.12:g.153513398A>C	Risk	Desai et al. (2012)
	rs13407922	NC_000002.12:g.154425285G>A	Risk	Desai et al. (2012)
	rs16833378	NC_000002.12:g.153515514A>G	Risk	Desai et al. (2012)
	rs9808145	NC_000002.12:g.154425689T>A	Risk	Desai et al. (2012)
ADORA2B	rs7208480	NC_000017.11:g.15770470C>T	Risk	Desai et al. (2012)
PRELP	rs2794452	NC_000001.11:g.203468576T>C	Risk	Desai et al. (2012)
DRD2	rs7952106	NC_000011.9:g.113424558G>T	Risk	Bhatnagar et al. (2013)

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 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 genome-wide meta-analysis of SBP in a population of African-American 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 middle-income 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, 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 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.

Footnotes

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, 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.

Abbreviations Used

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