Erythrocytosis due to PHD2 Mutations: A Review of Clinical Presentation, Diagnosis, and Genetics

The association of mutations in the PHD2 protein of the hypoxia-sensing pathway and erythrocytosis has only been established in the last decade. Here we report the case of a novel PHD2 gene mutation in a patient with erythrocytosis and summarize all reported cases to date. Case Report. A 55-year-old man presented with dyspnea and a previous diagnosis of idiopathic erythrocytosis. PHD gene sequencing revealed a mutation on exon 2. The mutation was recognized as p.(Trp334⁎) (c. 1001G>A) resulting in a truncation of a highly conserved amino acid residue in catalytic domain. A diagnosis of erythrocytosis secondary to mutant PHD2 gene was made. Conclusions. Our findings indicate that with PHD2 mutations there is moderate erythrocytosis and erythropoietin (Epo) levels are generally low to normal. Two patients with PHD2 substitution mutations were found to have paraganglioma and one of these patients had a concurrent pheochromocytoma. In addition, one mutation was associated with sagittal sinus thrombosis. Given the severity of some of the clinical features of these mutations, we conclude that clinical guidelines should include the PHD2 mutation in the idiopathic erythrocytosis workup.


Introduction
The human body is intricately adapted to respond to low oxygen states. Hypoxia activates the hypoxia-inducible factor (HIF) pathway resulting in an increased production of erythropoietin (EPO) which, in turn, activates red blood cell production by binding receptors on erythroid progenitor cells. The HIF pathway includes the HIF-transcription factor, the prolyl hydroxylase domain (PHD) protein family, and von Hippel-Lindau tumor suppressor protein (pVHL). There are three major isoforms of the PHD protein that are capable of binding HIF-, but PHD2 has been shown to be the most critical protein taking part in the oxygensensing pathway [1]. Under conditions of normal oxygen tension, the PHD proteins hydroxylate HIF-allowing for the binding of the pVHL protein. Upon binding HIF-, pVHL forms an E3 ubiquitin ligase complex which allows for HIF-ubiquitination and subsequent degradation by a proteasome. However, under hypoxic conditions, PHD2 reduces hydroxylation of HIF-, allowing HIF-to escape ubiquitin mediated degradation. Subsequently, HIF-acts as a transcription factor and upregulates expression of EPO gene resulting in increased EPO production. EPO is released into the bloodstream, binds to the Epo receptor (EpoR), and promotes growth and differentiation of erythroid precursors in the bone marrow. This causes an increase in red blood cell mass to compensate for the hypoxic condition.
The association of hereditary erythrocytosis with mutations of the PHD2 gene (also known as the EGLN1 gene) has been previously reported [2]. The erythrocytosis occurs by the decreased binding ability of the PHD2 protein and subsequent increased activity of HIF- [3]. Upregulation of HIF-increases red cell mass and may contribute to vascular proliferation and cell growth [4]. This can lead to complications such as pulmonary hypertension and thrombophlebitis [5]. There are also likely decreased interactions of the PHD2 protein with other binding proteins that may contribute to erythropoiesis [6].
We report a patient with a novel PHD2 gene mutation, p.(Trp334 * ), and review the literature with described PHD2 gene mutations.   Percy et al. [2] Case Reports in Hematology 5  Our patient presented to our clinic in 2014 at the age of 55 with recent onset of mild dyspnea and a diagnosis of erythrocytosis since 2009. His medical history included hypertension, hyperlipidemia, sleep apnea, atrial fibrillation, and type 2 diabetes mellitus. His medications included furosemide, enalapril, simvastatin, warfarin, aspirin, digoxin, and metformin. He was a nonsmoker. He was treated with phlebotomies in the past and his hemoglobin values had fluctuated from 180 g/L to 215 g/L. His sister had hemoglobin of 162 g/L. Otherwise family history was unremarkable.
Physical exam included temperature of 36.9 ∘ C, blood pressure of 127/79 mmHg, pulse rate of 60 beats per minute, and respiratory rate of 12 breaths per minute. Findings included a mildly ruddy complexion, lungs clear to auscultation bilaterally, and irregular heart tones. There was neither hepatosplenomegaly nor masses. Extremities revealed no edema and lymph node exam was unremarkable. Neurological exam was normal.
Bone marrow aspirate showed normal maturation and iron stores were present. Cytogenetic analysis did not reveal any abnormal karyotype.
2 V617F mutation was not identified on exon 14; neither were JAK2 mutations found for exons 8 or 12. HIF-2 (EPAS1) and EPOR gene sequencing did not reveal any mutations. However, PHD gene sequencing revealed a mutation on exon 2. The mutation was recognized as p.(Trp334 * ) (c. 1001G>A). A diagnosis of erythrocytosis secondary to mutant PHD2 gene was made.
Currently, the patient remains asymptomatic.

Discussion
Patients included in this review had erythrocytosis and mutations of the PHD2 gene resulting in loss of function of PHD2 proteins. Familial cases suggest autosomal dominant inheritance whereas the isolated cases are likely sporadic germline mutations. There are twenty-five different mutations reported of the PHD2 gene: fifteen substitutions, five nonsense mutations, and five frameshift mutations. Most mutations involve evolutionarily conserved residues that influence protein catalytic activity (see Table 1).
In the HIF pathway, erythrocytosis associated with HIF and VHL proteins is associated with high EPO levels [7]. However, our findings indicate that, with PHD2 gene mutations, EPO levels are generally low to normal. In our review, twenty-one patients had normal EPO levels, four patients had unexplained elevated levels, two had a low level, and the rest were unknown. The low-normal EPO level in the face of erythrocytosis in PHD2 mutants likely involves heightened expression and sensitivity of EPO receptor and HIF regulation [8]. The heightened EpoR sensitivity and expression are presumably a result of PHD2's regulation of HIF and PHD2's interaction with other binding proteins besides HIF [6].
The association of tumorigenesis and germline mutations of proteins in hypoxia-sensing pathway has already been well established [9,10]. The most well-known syndrome to associate erythrocytosis and tumorigenesis is von Hippel Lindau disease, but mutations in PHD1 and HIF2a (EPAS1) are also implicated in tumorigenesis [8,11]. In our review, we found two patients with PHD2 substitution mutations who had paragangliomas and one of these patients had a concurrent pheochromocytoma [8,12]. Furthermore, PHD2 germline mutations can have oncogenic potential in vitro [13] and analysis of paraganglioma cells suggests PHD2 may have tumor suppressor activity since the tumor cells showed loss of heterozygosity of wild-type PHD2 allele [6]. Suppression of tumor growth may be related to the PHD2 gene's regulation of various cytokines [14]. These observations suggest that therapeutic guidelines regarding the diagnostic workup of idiopathic erythrocytosis need to consider the possibility of these autosomal dominant mutations and may benefit the family members by providing early detection of tumors. In addition, the association of a PHD2 substitution mutation and sagittal sinus thrombosis warrants search for thrombotic risks of such mutations [3].