Sickle cell disease (SCD) is associated with pleiotropic clinical outcomes, the severity of which exhibits remarkable inter-individual variability. Investigations of the pathophysiology of SCD have focused on the adverse effects of vaso-occlusion, chronic inflammation, and hemolysis. SCD is also characterized by up-regulation of the hypoxic response to chronic anemia. We postulated that the up-regulated hypoxic response in SCD contributes to altered gene expression that might impact pulmonary hypertension, a complication associated with early mortality.
To identify genes regulated by the hypoxic response and not other effects of chronic anemia, we compared expression variation in peripheral blood mononuclear cells from 13 sickle cell anemia untreated with hydroxyurea and 15 Chuvash polycythemia (CP) patients characterized by homozygous VHLR200W-induced constitutive up-regulation of hypoxia inducible factors in the absence of anemia or hypoxia. Gene expression of both cohorts was profiled on identical Affymetrix exon arrays. The degree and direction of differential gene expression were highly correlated between sickle cell anemia and CP (Spearman’s ρ = 0.73 between regression coefficients of differential gene expression), suggesting that 53% of expression variation in sickle cell anemia is related to hypoxic transcriptional responses. At 5% false discovery rate (FDR), 1040 genes exhibited a >1.15 fold change in both sickle cell anemia and VHLR200W homozygotes, among which 297 were up-regulated and 743 down-regulated. Hypoxia strongly induced inflammatory response pathways but suppressed T-cell activation in sickle cell anemia. MAPK8, encoding a mitogen-activated protein kinase important for stress-induced apoptosis, T-cell differentiation and inflammatory responses, was a hypoxia down-regulated gene and played a central role in hypoxic gene regulation in sickle cell anemia according to gene network analysis.
To assess the genetic contribution to hypoxic transcriptional variation among SCD patients, we mapped expression quantitative trait loci (eQTL) for the 1,040 hypoxia response genes. Association mapping with a focus on local regulatory polymorphisms in 61 SCD patients identified eQTL for 103 of the hypoxia response genes at 5% FDR. We further tested the hypothesis that these hypoxic eQTL potentially underlie heterogeneity in risk of pulmonary hypertension in an additional SCD cohort (University of Illinois cohort). In this cohort, the A allele of an eQTL of MAPK8, rs10857560, was associated with pre-capillary pulmonary hypertension defined as mean pulmonary artery pressure ≥25 and pulmonary capillary wedge pressure ≤15 mm Hg at right heart catheterization (allele frequency=0.66; OR=4.4, P=0.00037, n=238). This association was confirmed in another independent cohort (Walk-PHaSST cohort) (allele frequency=0.65; OR=7.2, P=0.0025, n=519). The homozygous AA genotype of rs10857560, which was associated with decreased MAPK8 expression, was present in all 14 identified pre-capillary pulmonary hypertension cases among the combined 757 SCD patients (P = 6 x 10-6 by the Fisher exact test). Our study demonstrates a prominent hypoxic transcription component in SCD that in part contributes to pre-capillary pulmonary hypertension.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.