Novel Homozygous VHL Mutation in Exon 2 (413C>T:P138L) Is Associated with Congenital Polycythemia, Elevated Level of RUNX1/AML1 Transcript, but Not with the Cancer.

Abstract 2081

There are two known homozygous VHL gene mutations causing polycythemia that are not associated with a VHL cancer syndrome - an R200W mutation endemic in Chuvashia causing the first recognized disorder of augmented hypoxia sensing in normoxia, and an H191D mutation of Croatian origin. Both are located in the distal exon 3 of the VHL gene on its C-terminal domain. As different positions of loss-of-function mutations of VHL gene are associated with different type of cancers, it has been proposed that only C-terminal domain VHL mutations would cause polycythemia. However, we now contradict this notion. We report a novel homozygous variant of the VHL gene located in the middle of coding region in exon 2; 413C>T:P138L. The propositus is a 15 year old Punjabi female with congenital polycythemia; i.e. elevated erythropoietin (EPO) 40 mIU/mL, no JAK2 mutations, and hemoglobin 19–20 g/dl. Her parents are VHL P138L heterozygotes and no VHL tumors are reported in the extended family, in contrast to the other VHL P138 residue (P138R, P138T) mutations that have been reported in VHL syndrome and renal cancer.

pVHL is a negative regulator of hypoxia inducible transcription factors (HIFs) as it degrades a subunits of HIFs. It has been proposed, that an association between mutated pVHL and suppressor of cytokine signaling 1 (SOCS1) leads to JAK2 up-regulation and enhanced erythropoiesis. We show that the VHL P138L mutation, which lies in the catalytic HIF-1a peptide ligand-binding region, perturbs pHIF-1a pVHL interaction due to a conformational effect on the W117 and S111 residues lying within 2.8 to 4.3 Å distance from the mutated L138. The effect of this single mutation on overall structure is a shift of 1.9 Å RMSD (root mean square deviation) from the wild-type complex structure. Molecular dynamics simulations also found that the VHL P138L mutation can affect the binding of SOCS1 protein to the pVHL groove due to a strong steric effect of T124, H125 and D126 residues located close to the C-terminal pVHL. The accumulation of HIFs and up-regulated transcription of downstream target genes including those for glucose transporter-1 (SLC2A1) and transferrin (TF) were found in the propositus' granulocytes.

We then analyzed the VHL P138L propositus and her heterozygous parents' erythroid progenitors and found them to be hypersensitive to EPO (a feature of primary polycythemias without EPO independent colonies). Because of reports that RUNX1/AML1 transcript levels are specifically upregulated in erythroid progenitors in polycythemia vera (PV) and allegedly responsible for PV EPO hypersensitivity, we analyzed erythroid RUNX1/AML1 transcripts in the propositus and her parents and found it increased.

We demonstrate that a homozygous mutation in exon 2 of the VHL gene can also be associated with a polycythemic phenotype rather than VHL syndrome tumors. Further, we report that increased levels of RUNX1/AML1 transcripts are not specific for PV but can be seen in other primary polycythemias.