From Russia with Blood: An Animal Model of Chuvash Polycythemia

Chuvashia is in the center of European Russia with the capital, Cheboksary, located on the Volga River, approximately 400 miles east of Moscow. During the 1970s, Russian scientists, principally Dr. Lydia Polyakova, began characterizing patients from Chuvashia with a familial form of erythrocytosis that differed from classical polycythemia vera. During a 10-year period, approximately 100 patients were identified.¹  In 2002, Prchal and colleagues²  identified a homozygous cytosine to thymidine transition affecting nucleotide 598 (C598T) of the von Hippel-Lindau (VHL) gene in patients with Chuvash polycythemia. Subsequently, homozygosity of C598T was reported in sporadic and familial congenital polycythemia from a number of diverse ethnic groups, and a cluster of Chuvash polycythemia was identified on the Italian island of Ischia (in the Bay of Naples).³  The haplotype of non-Chuvash patients is the same as that of patients from Chuvashia, supporting the concept of a single founder.

How does VHL C598T cause polycythemia? The VHL protein (pVHL) is the substrate recognition component of an ubiquitin ligase complex that, under conditions of normoxia, targets the hydroxylated form of the α-subunit of hypoxia inducible factors (HIFs) for polyubiquination and proteosomal degradation. But under conditions of hypoxia, HIFs are stable because the α-subunit is ineffectively hydroxylated, preventing recognition by pVHL. C598T is a missense mutation that results in substitution of tryptophan for arginine at amino acid 200 (R200W). This amino acid substitution affects one of the two HIF binding sites of pVHL, ultimately resulting in reduced HIF degradation, thereby mimicking the hypoxic state. Support for this concept is provided by studies showing that, under normoxic conditions, cellular expression of HIF-1α is higher in patients with Chuvash polycythemia than in controls.⁴  HIFs are transcription factors. (HIF-1α and HIF-2α are the primary HIF isoforms.) Among the many important genes that are regulated by HIFs are erythropoietin (EPO), vascular endothelial growth factor (VEGF), plasminogen activator inhibitor-1 (PAI-1), glucose transporter-1 (Glut-1), and transferrin receptor. Patients with Chuvash polycythemia have hemoglobin-adjusted serum erythropoietin concentrations approximately 10-fold greater than controls, thereby providing an explanation for the polycythemia component of the phenotype.¹ 

To investigate further the biological consequences of the Chuvash mutation, Hickey and colleagues generated mice homozygous for VHL C598T using homologous recombination. Many of the features of the human disease, including polycythemia and abnormally high serum concentrations of erythropoietin and VEGF, were observed in the murine model (see Table). Differences between the human disease and the murine model are also noteworthy (see Table). For example, in the murine model, the difference in serum erythropoietin concentration between the homozygous VHL C598T mouse and wild type was 1.3-fold versus 10-fold greater than controls in patients with Chuvash polycythemia. The spleens of homozygous VHL C598T mice were remarkable for a strikingly greater number of erythroid progenitors and megakaryocytes and for increased in vitro erythroid differentiation. This latter property is also a feature of the erythroid precursors of Chuvash polycythemia. Whether the splenic phenotype observed in the murine model is a feature of the human disease is unknown, although clinical splenomegaly is uncommon in Chuvash polycythemia. While the Chuvash polycythemia has been attributed to aberrant expression of HIF-1α,⁴  HIF-2α principally mediates the process in the murine model. Whether this difference is truly species-specific or due to differences in experimental design is uncertain, although a recent report supports the hypothesis that the erythropoietin gene is an HIF-2α specific target in humans⁵  (see Table). Further studies using the murine model will provide additional insights into the function of pVHL and into the mechanisms that underlie the remarkable Chuvash polycythemia phenotype (see Table).