Genetic Polymorphisms in NEDD4L Are Associated with Pulmonary Hypertension of Sickle Cell Anemia.

Abstract 2562

Poster Board II-539

Pulmonary hypertension (PH) is present in up to 43% of adults with sickle cell anemia (SCA; homozygosity for HBB, glu6val) when defined by echocardiography. The relatively mild cardiopulmonary hemodynamics, as well as the co-existence of proteinuria, relative systemic hypertension, diastolic dysfunction and an increased hemolytic rate in this population suggests that this echocardiographic finding may be reflective of a more diffuse vasculopathy, the etiology of which, while unclear, may in part result from reduced nitric oxide bioavailability. Importantly, despite mild hemodynamic findings compared with idiopathic pulmonary arterial hypertension, an elevated tricuspid regurgitant jet velocity (TRV) carries with it a 6-10-fold increased mortality risk. We undertook the unbiased approach of a genome-wide association analysis (GWAS) to investigate the hypothesis that single nucleotide polymorphisms (SNPs) in genes affecting the vasculature might be associated with TRV and could play a role in the pathogenesis of PH in SCA. Based upon the associated increased mortality risk, PH cases were defined by a TRV of ≥ 2.5 m/sec. Controls were SCA patients with normal echocardiograms. 166 DNA samples were genotyped (59 PH cases and 107 SCA controls) on the Illumina Human 610-Quad SNP array and the data was analyzed using the software PLINK. Among the genes most significantly associated with a TRV ≥ 2.5 m/sec was NEDD4L where two SNPs were identified (rs559046, p=7.3 × 10⁻⁵, rs1624292, p=2.1 × 10⁻⁴). NEDD4L, on chromosome 18, is an E3 ubiquitin ligase localized to the renal and pulmonary alveolar epithelium. In the kidney, this gene is thought to be important in epithelial sodium channel surface expression and sodium reabsorption. Functional SNPs in this gene have been associated with systemic hypertension and salt sensitivity. Although expressed within the alveolar epithelium, its functional significance in the lung is less well understood. The SNPs we identified in NEDD4L are located within intron 1 similarly to other SNPs associated with systemic hypertension particularly in African Americans. To provide support for our findings, we undertook a candidate gene-focused approach using a phenotype related to but not identical to TRV. We analyzed 121 SNPs in NEDD4L in 139 adult subjects that participated in the Cooperative Study of Sickle Cell Disease (CSSCD). Given the lack of routine echocardiography in this cohort, cases were defined by the presence of an N-terminal pro-brain natriuretic peptide (NT-pro-BNP) level ≥ 160 pg/ml, that was previously associated with increased TRV and mortality in SCA and was therefore suggestive of pulmonary vasculopathy. In the CSSCD population, NT-pro-BNP levels ≥ 160 pg/ml were significantly associated with increased LDH concentrations, one measure of increased intravascular hemolysis (p<0.004). Using a case control approach, we found 4 SNPs in intron 1 of NEDD4L associated with an elevated NT-pro-BNP level (p<0.05) lending credence to the associations we found by GWAS. Although a larger sample size with a similarly ascertained phenotype is necessary to confirm our initial GWAS findings, based on the association of NEDD4L with systemic hypertension and NT-pro-BNP levels, this gene may have a role in the pathogenesis of PH of SCA.