Genetic Analysis of Patients With Sickle Cell Anemia and Stroke Before 4 Years of Age Suggest an Important Role for Apoliprotein E

Abstract

Background - Ischemic stroke is a devastating complication affecting children with sickle cell anemia (SCA). Genetic factors are likely to be important in determining the risk of stroke but are very poorly defined. Methods - We have studied a cohort of 19 children who had an overt ischaemic stroke before 4 years of age. We predicted genetic determinants of stroke would be more prominent in this group. We performed whole exome sequencing on this cohort and applied two hypotheses to our variant filtering. Firstly, we looked for very strong, potentially mono- or oligogenic variants for ischemic stroke and secondly, we considered that more common polygenic variants will be enriched in our cohort. Candidate variants emerging from both strategies were validated in a cohort of 283 patients with SCA and known pediatric cerebrovascular outcomes. We used principal component analysis in this cohort to control for relatedness and population substructure. Results - Our primary finding was that the APOE genotypes ε2/ε4 and ε4/ ε4, defined by the interplay of rs7412 and rs429358 , were associated with increased stroke risk, with an odds ratio of 4.35 (95% CI 1.85-10.0, p=0.0011) for ischemic stroke in the validation cohort. We also found that rs2297518 in NOS2 (OR 2.25 95%CI 1.21-4.19, p=0.014), and rs2230123 in STAT5a (OR=2.60, 95%CI 1.30-5.20 p=0.009) both had increased odds ratios for ischemic stroke, although these two variants were below the threshold for statistical significance after correction for multiple testing. Conclusions - These data identify new loci for future functional investigations into cerebrovascular disease in SCA. Based on African population reference allele frequencies, the APOE genotypes would be present in about 10% of children with SCA and represent a genetic risk factor that is potentially modifiable by both dietary and pharmaceutical manipulation of its dyslipidemic effects.

Precision Medicine and Sickle Cell Disease

“… 28 , 29 Co-inheritance of α-thalassemia is associated with a very wide range of effects in SCD, most of which are beneficial; these include increased hemoglobin, 28 reduced renal impairment, 30 and fewer cerebrovascular complications. 31 α-thalassemia has a more complicated relationship to episodes of acute pain, with many studies suggesting that it is associated with more frequent painful episodes, possibly due to the higher hemoglobin and impaired blood flow in some blood vessels. 32 Overall, the number of functional α-globin genes has a distinct effect on the phenotype of SCA and is an important determinant of severity.…”

“…In general, none of these have identified significant results that have been validated in independent studies. Perhaps most effort has gone in to identifying genetic risk factors for cerebrovascular disease; promising candidate genes for large‐vessel disease include APOE , 31 TNF‐α , 35,36 GOLGB1 , 37 and PON1 37 . Even less is known about the genetics of the more common silent cerebral infarcts, with α‐thalassemia being the only factor which possibly offers some protection 38 .…”

“…The commonest variant causing α‐thalassemia is caused by a 3.7 kb deletion (–α 3.7 ), and about 35% patients are heterozygous for this, with 5% homozygous 28,29 . Co‐inheritance of α‐thalassemia is associated with a very wide range of effects in SCD, most of which are beneficial; these include increased hemoglobin, 28 reduced renal impairment, 30 and fewer cerebrovascular complications 31 . α‐thalassemia has a more complicated relationship to episodes of acute pain, with many studies suggesting that it is associated with more frequent painful episodes, possibly due to the higher hemoglobin and impaired blood flow in some blood vessels 32 .…”

Genome‐wide association study of early ischaemic stroke risk in Brazilian individuals with sickle cell disease implicatesADAMTS2andCDK18and uncovers novel loci

“…The same variant in ENPP1 was confirmed within a Brazilian SCD cohort; however GOLGB1 was not replicated 15 . Recently, APOE variants were also shown to be associated with ischaemic stroke in children with SCD aged four and younger 16 …”

“…15 Recently, APOE variants were also shown to be associated with ischaemic stroke in children with SCD aged four and younger. 16 Investigating genetic risk within non-SCD stroke may provide additional insight into the similarity or differences compared to SCD. Within non-SCD paediatric ischaemic stroke studies, risk variants near the genes ADAMTS2, ADAMTS12, and ADAMTS13 haven been discovered, and both ADAMTS2 and ADAMTS12 have been replicated.…”