Genome-Wide Association for Silent Cerebral Infarction (SCI) in Sickle Cell Disease: The Silent Infarct Transfusion Trial (SIT) Cohort.

Neurovascular injury accounts for a large proportion of the morbidity and mortality among sickle cell disease (SCD) patients. Among these patients, SCI is one of the important and frequent causes of neurologic morbidity, affecting approximately 22% of children with SCD prior to their 16^th birthday. The genetic basis for susceptibility to SCI remains unclear, and the identification of predisposing loci that are causative or predictive of SCI are critical to elucidating the etiology of SCD.

To identify SCI susceptible loci, we performed a genome-wide association study (GWAS) in 570 African-ancestry SCD patients from the SIT cohort. DNA samples were genotyped with the IIIumina HumanHap650Y array, which includes 660,918 SNPs, of which ∼100,000 were selected as tags for populations with African ancestry. Reproducibility, calculated from 13 duplicate pairs, was 99.98%, and concordance with HapMap data, calculated from 24 HapMap controls, was 99.76%. Given the admixed nature of African-American populations, we used principal component analysis (PCA) as implemented in EIGENSTRAT to correct for potential population stratification. Six individuals were identified as genetic outliers (>6 standard deviations on any of the first 10 principal components) and dropped from all subsequent analyses, as were first 33 first degree relatives, and 37 samples with missing phenotype data, leaving 151 cases and 343 controls for analysis. We used logistic regression with the presence or absence of SCI as the dependent trait variable, to test for association with each SNP under an additive genetic model, while adjusting for age, sex and first ten principal components from EIGENSTRAT.

Overall, we saw no inflation of our test statistics (λ=1.018), indicating that population substructure is unlikely to influence the results. While no SNPs met genome-wide significance (P<1.27×10⁻⁷), several SNPs showed suggestive evidence for association with SCI. The strongest associations were observed at the NOM1 (chr7, rs887614, p=2.4×10⁻⁶), FRMD4A (chr10, rs3750882, p=9.11×10⁻⁶) and ADAM10 (chr15, rs4275799, p=3.05×10⁻⁶) loci. NOM1, (nucleolar protein with MIF4G domain1), is thought to be involved in protein translation and targeting protein phosphatase 1 to the nucleolus. The function of FRMD4A (FERM domain containing 4A protein), is not well defined. ADAM 10 (ADAM metallopeptidase domain 10) belongs to metalloproteinase family, which is implicated in shedding of several cell surface proteins, including TNF-alpha and E-cadherins. TNF-alpha is known to alter the adhesion of sickled erythrocytes and modulate the severity of SCD; an association between a promoter polymorphism in the TNF-alpha gene (−308) and stroke in SCD has been described previously (Hoppe et al., 2004).

Though we did not detect any strong genome-wide signal in association with SCI, several SNPs showed marginal significance. These findings suggest that common SNPs of moderate effect size are likely to be important in modifying risk for SCI in SCD. Meta-analysis of larger datasets, including additional samples accrued in the SIT cohort, is also warranted to identify SNPs with smaller effects or with rarer allele frequencies that may contribute to the risk of SCI in sickle cell patients.

Casella:Ikaria and Boehringer Ingelheim: Honoraria, No ongoing relationship exists.