The Effect of Aquaporin 2 Gene Variations on Plasma Levels of von Willebrand Factor and Factor VIII and on the Risk of Venous Thrombosis.

High levels of von Willebrand factor (VWF) and factor VIII (FVIII) are a risk factor for thrombosis. Determinants of high VWF and FVIII levels are poorly understood. Secretion of VWF from endothelial storage pools is regulated by vasopressin type-2 receptor (V2R). Previously, we have shown that a V2R variant, which has increased binding affinity for its ligand vasopressin (AVP), is associated with increased levels of VWF and FVIII¹. Nephrogenic Diabetes Insipidus (NDI) is a disorder characterized by renal insensitivity to AVP, caused by mutations in the genes encoding V2R or the aquaporin 2 water channel (AQP2). AQP2 expression is enhanced by stimulation of V2R. Patients with NDI are unable to concentrate their pre-urine. We hypothesized that carriers of AQP2 mutations compensate excess fluid loss by up-regulating AVP release and V2R expression, resulting in increased VWF and FVIII secretion. To test this hypothesis, we set up the Factor Eight in Nephrogenic Diabetus Insipidus study (FENDI), which includes 13 NDI families: 14 NDI patients (12 V2R- and 2 AQP2-linked), 14 carriers (9 V2R- and 5 AQP2-linked) and 25 unaffected family members, as well as 48 unrelated healthy individuals. In addition, we looked at effects of common AQP2 gene variations in a case-control study on venous thrombosis, the Leiden Thrombophilia Study (LETS), which consists of 474 patients with a first deep vein thrombosis and 474 healthy controls, sex and age matched to the patients. In the FENDI, no differences were observed between NDI patients, carriers and unaffected individuals in markers for fluid homeostasis such as hematocrite, serum osmolality and blood pressure. AVP reached detectable levels in all carriers of AQP2 mutations, compared to 27% and 56% in unrelated and related unaffected individuals, respectively. AVP levels, were, when detectable, elevated in all patients and carriers. VWF propeptide, a measure of the VWF secretion rate, VWF antigen and FVIII activity were also highest in carriers of AQP2 mutations. In the LETS, we sequenced a 6.6 kb long genomic region around the AQP2 gene in 25 selected individuals. We identified 18 single nucleotide polymorphisms (SNPs), of which 16 were genotyped in the entire LETS. Although reliable haplotypes could not be formed, due to recombination, the SNPs were linked within 5 clusters. In three of these clusters, up to 2.5-fold increases in thrombosis risk were observed. In these same clusters we observed associations of the AQP2 SNPs with arterial blood pressure. However, none of the AQP2 SNPs were associated with VWF or FVIII levels in healthy controls of the LETS. In conclusion, increased AVP levels in carriers of NDI-causing AQP2 mutations appear associated with increased VWF secretion. Furthermore, in the LETS, common AQP2 gene variations are associated with the risk of venous thrombosis.