New approaches to genetic predisposition for hemorrhagic stroke in sickle cell disease

Hypertension has not traditionally been considered a significant problem in sickle cell disease (SCD) but that view is changing.1 In the general population hypertension leads, among other things, to small vessel disease of the kidney and brain and to intracerebral hemorrhage, which are important complications of SCD. Although SCD is the most common monogenetic disease, its wide phenotypic variation has, for the most part, eluded satisfactory explanation. Except for coinheritance of alpha thalassemia and certain genes that affect fetal hemoglobin production, genomic studies have not converged on clear genetic risk markers for important complications such as stroke2 although promising candidates have recently been proposed.3 Specific genomic risk indicators for intracranial hemorrhage have not been reported. Although first ischemic stroke has been greatly reduced by regular red cell transfusion initiated on the basis of risk stratification by transcranial Doppler ultrasound4 (STOP Protocol) there has been no corresponding reduction in intracranial hemorrhage inSCD.5 With increased survival and decreased burden of ischemic stroke, intracerebral hemorrhage (ICH) will become an increasingly important problem. Robust predictors could lead to targeted prevention and reduction of this important type of stroke.

Our understanding of risk for hemorrhagic stroke in SCD comes primarily from the Cooperative Study of Sickle Cell Disease.6 This was an important effort to characterize the common phenotypes including stroke (ischemic and hemorrhage) but these data are over 30 years and the hemorrhagic stroke predictive model was based on only 27 events. Transcranial Doppler ultrasound is very effective in guiding transfusion or hydroxyurea to prevent ischemic stroke but does not clearly identify those at risk for hemorrhagic stroke. The existing phenotype‐based risk model from the cooperative study is not specific enough to support clinical trials. There is a critical need to modernize our research strategies to find robust models if we are to reduce hemorrhagic stroke.

We propose a shift in perspective. For example, in patients without SCD, several genes have been linked to intracerebral hemorrhage.7 Ongoing efforts are developing stroke genomics from an African perspective8, 9 including achieving a better understanding of the genomics of predisposing factors such as hypertension.10 The H3Africa Project was established by the US National Institutes of Health and the Wellcome Trust to develop capacity in genomic research in Africa, including specific projects focusing on assessment of perceptions about public health interventions to increase awareness, early detection and prevention of SCD‐related complications11 and the Sickle Pan‐African Research Consortium (SPARCO)12 that aims to develop infrastructure for sickle cell disease research, health care, education, and training in Africa. Instead of looking at these patients as individuals with SCD who typically happen to be black, we believe a better approach is to view them as individuals with predominantly African genetic heritage who also happen to have SCD, which can be seen as a significant stressor in the presence of which otherwise minor variants with little health impact in isolation become important determinants of health. We discuss exemplary domains in which genomic studies have yielded promising results in populations without SCD including (1) APOE, specifically the prevalence of the APOE‐ε4; (2) genes identified from African or African American studies predisposing to hypertension; (3) APOL1 G1/G2 variants; (4) haptoglobin haplotype (H2/2 allelic pattern); and (5) other genes associated with intracerebral hemorrhage including ACE, PMF1/SLC25A44, COL4A2, and MTHFR.13

• Apolipoprotein E (APOE): Several studies have linked the APOE‐ε4 of APOE‐ε4 to ICH.7, 14 The APOE allelle is known to be more prevalent in African population and could play a role in ICH in the presence of SCD. Both the studies in Africa and many of the large epidemiological studies from the United States with substantial African American representation document the significantly higher prevalence of the APOE‐ε4 in African and African Americans. It is clear that the APOE‐ε4 imparts a health and survival disadvantage unrelated to Alzheimer’s disease,15 which might be at play in SCD.

• Hypertension: It was thought that SCD was not attended by hypertension but this view is changing. In a 3‐center study Strouse et al reported that recent transfusions and hypertension were associated with hemorrhagic stroke in patients with SCD.16 It is possible that hypertension, perhaps driven by genomic factors,10 has an exaggerated impact in SCD even if “relative” in terms of blood pressure elevation.

• APOL1 G1/G2: The Apolipoprotein L1 (APOL1) gene was first identified as a risk locus for chronic kidney disease among African Americans.17 There is a close association between cerebral and glomerular small vessel diseases and between brain small vessel disease and hemorrhage. Recently an association of APOL1 rs73885319 with small vessel type stroke in a cohort of indigenous African stroke patients was reported.18 For this reason an association between the APOL1 variants and stroke in SCD populations of African ancestry should be explored.

• Haptoglobin Haplotype: SCD is a hemolytic anemia, and large amounts of free hemoglobin are released from red blood cell lysis. Haptoglobin, which normally functions to lock up extracellular hemoglobin and limit the potential oxidative damage, is of particular interest. There are 2 variants, H‐1 and H‐2. H‐1 is more efficient at presenting free Hb to the macrophage, which takes it out of circulation. Individuals with the H2 allelle (especially the H2‐2 genotype) develop a more powerful immune response to infections.18, 19 The problem is that the heightened immune response may be a distinct disadvantage for patients with a systemic illness. An example of this is evident from cardiovascular research in patients with diabetes mellitus (DM). In those with type 2 DM the H2‐2 has been strongly linked to worse vascular outcomes presumably because of a heightened immune response that adversely affects the endothelium. This negative influence of the haplotype is not noted in those without diabetes.20 SCD may set the stage that allows the allelic variation to manifest negative vascular effects. Given the fundamental role of haptoglobin in free hemoglobin metabolism, and the possible link to exaggerated inflammation, haptoglobin haplotype should be included in research into the assessment of risk in these patients.

• Other genes associated with intracerebral hemorrhage: Even though the following genomic variants are drawn from populations other than African/African American, the relative strength of evidence linking them to hemorrhagic stroke suggests that they should also be considered in patients with SCD: angiotensin‐converting enzyme (ACE), polyamine‐modulated factor 1 (PMF1) and solute carrier family 25, member 44(SLC25A44), alpha‐2 type IV collagen (COL4A2), and methylenetetrahydrofolate reductase (MTHFR).13

We believe that mapping what is being learned from these genomic studies—especially those drawn from African or African American populations—to those with SCD and important complications such as hemorrhagic stroke may lead to improved risk models and new treatments.