Hypoxia may cause pulmonary and brain edema, pulmonary hypertension, aberrant metabolism and early mortality. To better understand pathological processes associated with hypoxia, we examined gene expression in Chuvash polycythemia blood mononuclear cells. Chuvash polycythemia is a congenital disorder of up-regulated hypoxic response at normoxia wherein VHLR200W homozygosity leads to elevated hypoxia inducible factor (HIF)-1 and HIF-2 levels, thromboses, pulmonary hypertension, lower systemic blood pressure (SBP) and increased mortality. VHLR200W homozygotes are often treated by phlebotomy resulting in iron deficiency, allowing us to evaluate an interaction of augmented hypoxia sensing with iron deficiency.
Expression profiling of 8 VHLR200W homozygotes and 17 VHL wildtype individuals, matched for normal iron status as reflected in serum ferritin concentration, revealed altered regulation of 3069 genes at false discovery rate <0.05, with 847 up-regulated and 2222 down-regulated in VHLR200W homozygotes. Genes induced by homozygous VHLR200W were enriched in immune response pathways; those repressed in RNA transcription and protein synthesis pathways. Forty-two genes showed a >1.5-fold change in expression level, mostly (74%) an increase. Seven showed a >2-fold increase: CA1 (carbonic anhydrase), SELENBP1 (selenium binding protein 1), IL1B (interleukin 1 beta), SLC4A1 (solute carrier family 4 member 1), HBB (hemoglobin beta), and AHSP (alpha hemoglobin stabilizing protein).
Additional studies including 16 VHLR200W homozygotes with low ferritin indicated that iron deficiency enhanced the induction effect of VHLR200W for 51 of the 847 upregulated genes and suppressed the induction effect for 108 of the upregulated genes. Genes further upregulated by iron deficiency included CA1, CSDA (cold shock domain protein A), BCL2L1 (BCL-2 like 1), BPGM (2,3-bisphosphoglycerate mutase), DCAF12 (DDB1 and CUL4 associated factor 12), FECH (ferrochelatase), SELENBP1 and SLC4A1. Genes for which iron deficiency suppressed the induction included inflammatory and immune pathway genes such as CASP5, CXCL16, IFI30, IFI35, IRF5, LILRB1, NOD2, RELT, TCIRG1 and TNFAIP2.
A number of the genes with altered regulation in VHLR200W homozygotes might modify risk of thrombosis (upregulated: F3, SERPINE1, SERPINB2, SERPING1, PLAUR, THBD; down regulated: SERBP1), elevated systolic pulmonary artery pressure (upregulated: HTR1B, THBS1; downregulated: S1PR1, STIM2), or benign hemangioma (downregulated: CCM2). However, expression of these genes tended not to be influenced by iron status. VEGF was induced in VHLR200W homozygotes and surprisingly this induction was suppressed by iron deficiency. Expression relationships suggested a broad effect of VHLR200W in reducing systolic blood pressure through inducing VEGF.
We demonstrate that many genes have commensurate changes of their expression by both iron deficiency and VHLR200W associated normoxic up-regulation of HIFs, as expected. However, there are genes that are regulated asynchronously. Further research is needed to define the molecular bases of separate regulation of genes by HIFs and iron status and to define relative risks and benefits of therapeutic phlebotomy for polycythemia. The resulting elucidation of the genomic pathways affecting predisposition to thromboses, pulmonary hypertension, lower systolic blood pressure and the interaction of augmented hypoxia sensing with iron deficiency should have broad implications leading to a better understanding of the pathophysiology of many diseases and the development of targeted therapies.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.