The role of PHD2 mutations in the pathogenesis of erythrocytosis

Abstract

The transcription of the erythropoietin (EPO) gene is tightly regulated by the hypoxia response pathway to maintain oxygen homeostasis. Elevations in serum EPO level may be reflected in an augmentation in the red cell mass, thereby causing erythrocytosis. Studies on erythrocytosis have provided insights into the function of the oxygen-sensing pathway and the critical proteins involved in the regulation of EPO transcription. The α subunits of the hypoxia-inducible transcription factor are hydroxylated by three prolyl hydroxylase domain (PHD) enzymes, which belong to the iron and 2-oxoglutarate-dependent oxygenase superfamily. Sequence analysis of the genes encoding the PHDs in patients with erythrocytosis has revealed heterozygous germline mutations only occurring in Egl nine homolog 1 (EGLN1, also known as PHD2), the gene that encodes PHD2. To date, 24 different EGLN1 mutations comprising missense, frameshift, and nonsense mutations have been described. The phenotypes associated with the patients carrying these mutations are fairly homogeneous and typically limited to erythrocytosis with normal to elevated EPO. However, exceptions exist; for example, there is one case with development of concurrent paraganglioma (PHD2-H374R). Analysis of the erythrocytosis-associated PHD2 missense mutations has shown heterogeneous results. Structural studies reveal that mutations can affect different domains of PHD2. Some are close to the hypoxia-inducible transcription factor α/2-oxoglutarate or the iron binding sites for PHD2. In silico studies demonstrate that the mutations do not always affect fully conserved residues. In vitro and in cellulo studies showed varying effects of the mutations, ranging from mild effects to severe loss of function. The exact mechanism of a potential tumor-suppressor role for PHD2 still needs to be elucidated. A knockin mouse model expressing the first reported PHD2-P317R mutation recapitulates the phenotype observed in humans (erythrocytosis with inappropriately normal serum EPO levels) and demonstrates that haploinsufficiency and partial deregulation of PHD2 is sufficient to cause erythrocytosis.

Genetic Evidence of a Precisely Tuned Dysregulation in the Hypoxia Signaling Pathway during Oncogenesis

“…This model may be of major importance in our understanding of tumor risk in patients carrying mutations in other genes belonging the HIF pathway. Indeed, patients with germline mutations in PHD2/EGLN1 and HIF2A/EPAS1 can develop pheochromocytomas or paragangliomas (equivalent to extra-adrenal pheochromocytomas) (49–51). A comparative study of PHD2 mutations showed a differential regulation of HIF that perfectly fits with the model described here (52).…”

Human high-altitude adaptation: forward genetics meets the HIF pathway

“…This was initially demonstrated by the identification of human erythrocytosis patients with heterozygous LOF mutations in the PHD2 gene (Percy et al 2006(Percy et al , 2007Al-Sheikh et al 2008;Ladroue et al 2008). These mutations, it should be noted, typically affect residues that reside in the catalytic domain of the protein (Gardie et al 2014). Subsequent studies using global conditional knockout of the Phd2 gene demonstrated that it plays the dominant role, among the Phd paralogs, in the control of renal Epo (Minamishima et al 2008;Takeda et al 2008).…”

Genetic analysis of erythrocytosis reveals possible causative and modifier gene mutations

“…Under normoxic conditions, HIF1A is hydroxylated by PHD2 (prolyl hydroxylase domain‐containing protein 2, also termed EGLN1), and subsequently degraded by the ubiquitine‐proteasome pathway in a von Hippel‐Lindau tumour suppressor (VHL)‐dependent manner. This exquisitely sensitive mechanism is perturbed by hypoxia, which, by inhibiting HIF1A hydroxylation, physiologically stimulates erythropoietin (EPO) synthesis, leading to red cell overproduction (Gardie et al , ).…”