Anaemia of Renal Insufficiency


Richard Bright discovered the relationship of renal failure and anaemia in 1836. Anaemia of renal insufficiency is mainly results from decreased erythropoietin production and impaired supply of iron to erythroid precursors. Anaemia results in fatigue, cognitive dysfunction and cardiovascular consequences and increased cardiovascular mortality. Correction of anaemia decreases LVH, LVH related hospitalizations and mortality of cardiovascular disease. Without erythropoietin supplementation only 2% (Int J Artif Organs 1981; 4:277-9) needing haemodialysis have a hematocrit of >0.4. Availability of erythropoietin and the use of parental iron has reduced the prevalence of anaemia in patients with renal failure.

Pathogenesis of Anaemia of Renal Insufficiency

Anaemia associated with renal insufficiency results from decreased erythropoietin and iron availability.

Erythropoietin, produced by the kidney, is essential for erythropoiesis (see Erythropoietin and Erythropoietin Receptor Signalling). Renal insufficiency results in decreased renal mass and consequently in decreased erythropoietin production. Erythropoietin levels in patients with renal failure are normal or slightly elevated, but they are 10-100 times lower than a normal individuals with similar levels of anaemia. Decreased erythropoietin hampers erythropoiesis contributing to anaemia (no 1 in figure 1). Erythropoietin supplementation corrects anaemia in most but not all patients.

Three other causes of anaemia have been identified in patients with renal insufficiency. These are

  1. Impaired supply of iron
  2. Folate deficiency due to losses in dialysis
  3. Aluminium intoxication resulting in microcytic hypochromic anaemia

Folate deficiency and aluminium intoxication are of historical importance. Current management of dialysis patients includes folate supplementation and measures to prevent aluminium intoxication. Impaired iron supply to erythroid precursors remains the main cause of anaemia that does not respond to erythropoietin.

Iron is a component of heme (see heme synthesis). Patients with renal insufficiency have decreased iron availability from blood loss, impaired iron absorption and inability to use body iron stores (functional iron deficiency)

Causes of blood loss in patients with renal failure include (2 in figure 1)

  1. Gastrointestinal blood loss due to uraemia induced platelet dysfunction
  2. Blood loss due to dialysis
  3. Iatrogenic blood loss due to investigations (Transfus Med 2009; 19;309-14)

Iron supply to the erythroid precursors is also limited by impaired absorption (3 in figure 1) and a functional iron deficiency (4 in figure 1). Hepcidin plays a central role disrupting iron supply in patients of renal insufficiency. It reduces iron absorption in the gut and hampers the release of macrophage iron. The latter results in a phenomena called as functional iron deficiency i.e. diminished iron availability despite adequate iron stores. Hepcidin levels are increased from increased production because of inflammation and decreased elimination as a result of falling GFR. Infections, exposure of leucocytosis to foreign surfaces during dialysis and from the underlying cause of renal insufficiency are the sources of inflammation.

Clinical Features

The degree of anaemia co-relates with the degree of impairment of renal failure. Anaemia is rarely observed in patients with creatine clearance of greater than 45ml/1.72m2. This corresponds to a serum creatinine of 2-2.5mg/dL.  The anaemia is mild to moderate with the haematoctit stabilizing between 15-30%. Anaemia is less severe in polycystic kidney disease and to a lesser extent in hypertension. It may be more severe in diabetics. The presence of severe anaemia is an indication for evaluation for another cause of anaemia.

Signs and symptoms are those of anaemia and the underlying disease causing renal failure.

Diagnosis

A complete haemogram should be performed in all patients with chronic renal insufficiency at diagnosis. The diagnosis is made by the presence of a mild to moderate normochromic normocytic anaemia, presence of renal insufficiency and the absence of other causes of anaemia. Macrocytosis may be seen. The investigations that are usually performed in a patients with suspected anaemia of renal insufficiency include

  1. Complete haemogram with erythrocytes indices to determine severity of anaemia and to exclude co-existing diseases like iron deficiency and aluminium intoxication (microcytosis) or Folic acid/vitamin B12 deficiency (macrocytosis).
  2. The reticulocyte counts are normal but may rise with azotaemia
  3. Serum iron, total iron-binding capacity, percent transferrin saturation and serum ferritin need to be performed to asses the iron stores and iron delivery

Despite the fact that there is a relatively deficiency of erythropoietin, erythropoietin levels are not useful in the diagnosis of anaemia of renal insufficiency. The bone marrow tends to be moderately hypercellular with a slight erythroid hyperplasia.  A bone marrow aspiration is not necessary for diagnosis.

Treatment

The treatment of anaemia of renal insufficiency in transplant ineligible patients consists of erythropoiesis stimulating agent (ESA), iron and blood transfusion. Transplant when indicated cures anaemia in 80% of the patients. Most authorities define anaemia as a haemoglobin of 13-13.5g/dL in men and less than 12g/dL in women.

Iron therapy

All patients with chronic renal insufficiency need iron supplementation. The route of supplementation may be oral or intravenous. Severity of anaemia, patients preference and adherence and previous experience with that route iron therapy are few of the factors determining the choice. In a meta-analysis of oral iron vs. parental iron, parental iron raised the concentration of haemoglobin by >1g/dL more often than oral iron. The risk of hypotension was higher and gastrointestinal side effects lower with parental iron (Am J Kidney Dis. 2016 Nov;68(5):677-690). As the safety of intravenous iron preparation has increased so has their use. Many trials comparing oral with intravenous iron therapy have shown a better response with intravenous iron. However intravenous iron therapy has not been show to be superior in some trials (Clin J Am Soc Nephrol. 2016 Jul 7; 11(7)).

Erythropoiesis Stimulating Agenst (ESA)

ESA are indicated in patients who have adequate iron reserves. These are patients with transferrin saturationis >25 percent and ferritin >200 ng/mL. ESA should be used in patients with iron deficiency (actual or functional) after iron replenishment. Two ESAs available are erythropoietin and darbepoietin. Treatment with ESA results in improved quality of life, improved cognitive function and reduces left ventricular hypertrophy. The target of therapy is to increase the haemoglobin to 11-12g/dL. Higher targets are associated with more side effects. The rise is typically achieved in 6-8 weeks of therapy. Erythropoietin is administered in a dose of 100-150 units every week. Hypertession is seen in about 35% of the patients. Darbepoietin as administered in a dose 0.45 μg/kg/week for patients on dialysis.

Erythropoietin resistance is failure to respond to erythropoietin. The causes of erythropoietin resistance include inadequate iron availability (decreased reserves and/or functional iron deficiency) inadequate dialysis, marrow fibrosis associated with secondary hyperparathyroidism, folate deficiency, aluminium intoxication and co-existing inflammation. Dialysis should reduce blood urea to less than 35% to allow for optimal erythropoietin action.

 

Renal replacement therapy

Renal Allograft: Renal allograft is effective correction of anaemia in 80% of the patients. The rise is seen in 8-10 weeks. About 20% of the patients may get post-transplant erythrocytosis. The causes of failure of haemoglobin to rise after transplant include hemorrhage, intense immunosuppression or graft rejection

Dialysis: Dialysis has a small effect on the anaemia presumably by clearance of inhibitors. Peritoneal dialysis (PD) is more effective than haemodialysis. Reason a higher efficacy of PD is more effective in unknown but more effective removal of middle molecules (500-1500kD) and lesser of inflammation compared to HD may explain the higher haemoglobin in patients with on PD.

 

 

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Oral Iron Therapy


Iron deficiency anaemia is treated by iron supplementation that may be administered orally or parenterally.  Oral iron absorption is inefficient, is interfered by food and is associated with high incidence of gastrointestinal adverse effects. About 17% of the world population is estimated to be suffering from iron deficiency. Iron deficiency is more common the poorer regions of the world. These regions have limited resources allocated for healthcare. Oral iron is inexpensive and convenient to administer making it the modality of choice for initiation of treatment of iron deficiency. The availability of safer parenteral iron preparations that can replete the iron deficit in one dose has reduced the threshold of switching a person intolerant to iron to parenteral iron.

Oral Iron Absorption

Dietary iron may be heme iron or non-heme iron. Heme iron is absorbed after oxidation to hematin. The absorption of heme iron is more efficient. Dietary non-heme iron is present in the ferric state. Ferric iron is insoluble and needs to be reduced to ferrous iron. Gastric acidity aids this conversion. Medicinal iron is most often administered in the ferrous state. Ferrous iron tends to oxidize to ferric iron at physiological pH. Gastric acid lowers the pH and retards oxidation. The absorption of non-heme iron is aided by ascorbate, animal proteins, human milk, keto sugars, organics, amino acids that form soluble chelates and retarded by phytates present in grains and vegetables, dietary fibre, polyphemols present in tea, coffee and wine, phosphates and phosphoproteins present in egg yolk, bovine milk, calcium and zinc. For a detailed discussion on iron absorption see intestinal iron absorption. Preparations containing iron in the ferric state have a 3-4 fold lower bioavailability than ferrous iron preparations. Ferric iron is insoluble in the alkaline medium of the duodenum and needs to be converted to ferrous iron (ScientificWorldJournal. 2012; 2012: 846824).

Oral Iron Preperations

Oral iron preparations may be heme or non-heme. Heme iron is available as heme iron polymer. Non-heme iron may be in the ferrous or ferric form. Carbonyl is pure iron prepared from the decomposition of iron pentacarbonyl. The preparations are listed in the table below.

 

Preparations Iron Content
Ferrous Sulfate, anhydrous 30%
Ferrous Fumarate 33%
Ferrous Sulfate 20%
Ferrous carbonate, anhydrous 48%
Ferrous Gluconate 12%
Ferric Ammonium Citrate 18%
Ferric bisglycinate 20%
Ferric pyrophosphate 12%
Carbonyl iron ~100%
Heme-iron peptide 100%
Polysaccharide iron complex 100%

Indications of Oral Iron Therapy

Oral iron is indicated for prevention and treatment of iron deficiency anaemia. The rate of iron delivery is insufficient to provide iron when erythropoiesis is stimulated by erythrocyte stimulating agents like erythropoietin and darbepoetin. Oral iron should not be used for such patients.

Trial of Oral Iron Therapy

Serum ferritin is an indicator of total body iron. It is also an acute phase reactant. Low ferritin indicates iron deficiency. The traditional cutoff is 12ng/mL. At this cutoff the sensitivity of ferritin for the diagnosis of iron deficiency anaemia is only 25%. The sensitivity can be increased to 92% with a positive predictive value of 83% if a cutoff of 30ng/mL is used. A trial of oral iron may be given if other causes of anaemia are excluded.

Contraindications to Oral Iron Therapy

  1. Primary hemachromatosis: Primary hemochromatosis is a is absolute contraindication
  2. Peptic ulcer, regional enteritis, or ulcerative colitis can be exacerbated by oral iron.
  3. β-Thalassaemia trait is a relative contraindication. Some patients may develop iron overload. Patients should be given iron only if iron deficiency is established by laboratory investigation.

Failure of Oral Iron Therapy

  1. Failure to take prescribed medication: Oral iron therapy causes gastrointestinal adverse effects in a large proportion patients that are severe enough in some to discontinue therapy. A detailed history must be taken to ascertain that the patient has taken the prescribed dose.
  2. Incorrect or incomplete diagnosis: Iron deficiency anaemia is hypochromic microcytic (see Evaluating Anaemia). Other diseases that result in hypochromic microcytic anaemia are thalassaemia and anaemia of chronic disease. Both are common and can both be confused with iron deficiency as well as co-exist with iron deficiency. Thalassemia trait affects about 1.5% of the world population. About 1.4% of the population is estimated to have anaemia due to infection, inflammation or chronic renal disease (https://dx.doi.org/10.1016%2FS0140-6736(15)60692-4). Incidental occurrence of iron deficiency with thalassaemia trait or anaemia of chronic disease will result in an incomplete response to iron supplementation. Some inflammatory conditions e.g. ulcerative colitis cause blood loss. Blood loss may be seen due to use of non-steroidal inflammatory agents in patients with autoimmune arthritis. Anaemia in these patients shows an incomplete response to iron supplementation because part of the anaemia results from chronic inflammation.
  3. Insufficient amount or inappropriately taken oral iron: The ideal dose is 200mg of elemental iron taken 2 hours before or 1 hour after food. Food interferes with iron absorption but also relieves gastrointestinal adverse effects. Gastrointestinal adverse effects are related to dose. Prescriptions of oral iron may have an insufficient dose or may be administered after food to reduce gastrointestinal adverse effects. This results in an inappropriate haemoglobin response.
  4. Iron demand exceeding intake: Iron demand may exceed supply if there is continued blood loss or in patients where erythropoiesis is stimulated with an erythropoiesis stimulating agent like erythropoietin or darbepoietin. In both these situations the rate of oral iron absorption limits iron availability. These patients need intravenous iron.
  5. Malabsorption of iron: Food interferes with oral iron absorption. Ferrous iron is rapidly oxidised to ferric iron at physiological pH. Gastric acid reduces ferric iron to ferrous iron. Proton pump inhibitors, H2 antagonists, antacids and gastrectomy reduce acidity and can interfere with iron absorption. Iron malabsorption may be seen as part of a malabsorption syndrome. Iron deficiency refractory to iron is rare. Iron refractory iron deficiency anaemia is a disorder resulting from mutations in the TMPRSS6. This mutations results in increase hepcidin production which is sensed by the body as an iron repeated state. Iron is not absorbed despite iron deficiency. The patients do not respond to oral iron and shows a partial response to parenteral iron.

 Interactions

  1. Interaction of iron with food: The absorption of non-heme iron is affected by food (See Intestinal Iron Absorption).
    1. Foods enhancing iron absorption: Ascorbate, animal proteins, human milk, keto sugars, organics, amino acids that form soluble chelates with iron enhance absorption of non-heme iron.
    2. Inhibiting iron absorption: `Inhibitors of absorption of non-heme iron include
      1. Phytates present in grains and vegetables
      2. Dietary fibre
      3. Polypohenols present in tea, coffee and wine,
      4. Phosphates and phosphoproteins present in egg yolk, bovine milk
      5. Calcium and zinc.
  2. Drug-Iron interactions
    1. Iron decreases the absorption of ACE inhibitors, bisphosphonates, levodopa, levothyroxine, penicillamine, quinolone and tetracyclines
    2. The absorption of iron is decreased by drugs that reduce gastric pH. These include H2 antagonists(Cimetidine, ranitidine, famotidine), proton pump inhibitors (omeprazole, pantoprqzole, esomeprazole, lansoprazole, rabeprazole, etc), antacids and cholesterol lowering agents (Cholestyramine and Colestipol).

Dose

  1. Children: 3–6 mg/kg daily in 3 divided doses.
  2. Adult: Usual therapeutic dosage: 50–100 mg 3 times daily but a dose of 200mg produces the maximal results. A smaller dosages (e.g. 60–120 mg daily) may be given if patients are intolerant of oral iron, but response in such patients takes a longer time.

Side Effects Of Oral Iron

  1. Gastrointestinal: The commonest adverse effects of iron are gastrointestinal symptoms, including heartburn, nausea, abdominal cramps, diarrhoea or constipation. These may be seen in up to a fifth of the patients. They can be reduced by decreasing the dose or taking iron after food. Taking iron with food can reduce the absorption by about 50%. Enteric coated preparation decrease the side effects by delaying the release of iron. Delaying release may bypass the duodenum that is the site of absorption of iron. The decrease in absorption is particularly marked in patients with aclorhydria as they can not dissolve the enteric coating. The stool of patients taking iron supplements may be discoloured black or green.
  2. Discolouration of teeth: Iron syrups may cause staining to teeth.
  3. Iron overload: Iron overload from oral iron therapy is rare. It has been described in patients with hemachromatosis and chronic haemolytic anaemias.
  4. Iron Poisoning: Iron poisoning usually occurs in children, particularly those younger than 5 years of age, because of accidental ingestion of medicinal iron. Children are likely to ingest these believing them to be candies.

Etiology of Polycythaemia


The terms polycythemia and erythrocytosis though used interchangeably describe different entities. Polycythemia (Gk, polys + kytos, cell, haima, blood) is an absolute increase in the mass of red cells. Erythrocytosis refers to only increase in the concentration of erythrocytes. Erythrocytosis may be due to increase in red cells (polycythemia, absolute erythrocytosis) or decrease in plasma volume (relative erythrocytosis).

Erythropoiesis and It’s Control

Erythrocyte are produced from haemopoietic stem cells (see Morphology of Erythroid Processors). Erythropoietin (EPO) prevents apoptosis of erythroid precurssors and increases erythrocyte production. Hypoxia stimulates erythropoietin production.

Erythropoietin is produced in response to hypoxia. Hypoxia sensing involves

  • von-Hipple Lindau oncogene (VHL) protein
  • Hypoxia inducible factor (HIF) of there are three forms HIF1, HIF2 and HIF3. HIF2 is involved in erythropoietin production in response to hypoxia.
  • Prolyl-4-hydroxylase

Oxygen is critical for cell survival and rapid response to hypoxia is critical. Synthesizing proteins as effectors of response is slow process that may not be appropriate for a stress like hypoxia. Instead of synthesizing HIF in response to hypoxia the cells control the levels HIF by controlling it’s destruction. Cells on one hand synthesize HIF at a steady rate and on the other carry out a proteosomal degradation HIF by oxygen dependent mechanism.

Prolyl hydroxylase (PDH) is a constutively expressed enzyme that is involved in the process of marking HIF for proteosome mediated destruction. Proteosomal destruction of HIF also requires the von Hipple-Lindau oncogene protein. PHD is oxygen sensitive. Hypoxia reduces the PHD activity which in turn increases HIF levels by reducing its degradation. HIF induces the transcription of many genes involved in response to hypoxia including the erythropoietin gene. Increased HIF causes erythrocytosis. Mutations of the PDH and VHL gene have an effect similar to hypoxia. They increase erythrpoietin secretion and cause increased erythrocyte production.

Erythropoietin action is mediated by the erythropoietin receptor (ER). Binding of EPO to ER results in conformational change and the receptor is switched on. Receptors mediate their actions by a cascade of enzymatic actions. One of the most common mechanism of receptors action is activation of receptor tyrosine kinase activity. This allows the receptor to phosphorylate proteins and start a cascade of changes, usually phosphorylations, that lead to target actions. Not all receptors have tyrosine kinase activity. Those without tyrosine kinase activity rely of non-receptor tyrosine kinase for action. The erythropoietin receptor lacks tyrosine kinase activity essential for signal transduction. It associated with the non-receptor tyrosine kinase Janus kinase 2 (JAK2). JAK2 mediates the action of erythropoietin receptor (see ErythropoietinReceptor Signalling for details of erythropoietin production and action).

Table 1 – Causes of Polycythaemia
Primary Polycythaemia (serum erythropoietin low)
Inherited/Congenital Acquired
Familial Erythrocytosis Type I Polycythaemia Vera
Secondary Polycythaemia (serum erythropoietin normal/high)
Inherited/Congenital Acquired
  1. Disorders of Oxygen Sensing
    • Familial Erythrocytosis Type II (VHL Mutations)
    • Familial Erythrocytosis Type III (PHD2 mutations)
    • Familial Erythrocytosis Type II (HIF2A Mutations)
  2. High Affinity Haemoglobins
  3. Inherited Methaemoglobinaemia
  4. Congenital heart disease with right to left shunt
  1. Hypoxia
    • Chronic lung disease
    • Conginital heart disease with reversal of left to right shunt
    • Smokers
    • High altitude
  2. Ectopic erythropoetin seceretion
  3. Paraneoplastic Erythropoietin seceretion

Causes of Polycythemia (See Table 1)

Erythrocytosis may be primary or secondary, inherited or acquired. Primary erythrocytosis is erythropoietin independent. These patinets have low EPO levels. Patients with secondary erythropoiesis have have normal or high erythropoietin. The genetic defects associated with polycythaemia are listed in table 2.

  1. Primary Polycythaemia (Polycythaemia with low serum erythropoietin)
    1. Inherited
      1. Familial erythrocytosis type I: Inherited erythrocytosis is an autosomal dominant disorder resulting from truncating mutation in the erythropoietin receptor that result in deletion of between 57 to 127 amino acids from the cytoplasmic domain of the receptor (see table 2 below). The cytoplasmic domain appears to be necessary for inhibition of receptor action. Truncation results in the loss of the SHP-1 binding site. SHP-1 dephosphorylates JAK2 and attenuates erythropoietin rceptor signaling. Deletion causing familial erythrocytosis type I result in a loss of cytoplasmic portion of the erythropoietin receptor resulting in a receptor that can not interact with SHP-1 and be the turned off. Pathways stimulated by the erythropoietin receptor are continuously activated and polycythaemia is seen. The erythropoietin levels are low.
    2. Accquired
      1. Polycythaemia Vera: Polycythaemia vera is an accquired polycythaemic myeloproliferative disease. Majority (95%) of the patients have the JAK2V617F mutation. Patients not having JAK2V617F mutation have mutations in exon 12. JKAK2 is the mediator of effects of erythropoietin receptor. These mutations result in increased erythropoietin sensitiviy and polycythaemia (see table 2 below). The erythropoietin levels are low.
  2. Secondary Polycythaemia (Polycythaemia with normal or high serum erythropoietin
    1. Inherited
      1. Familial Erythrocytosis type 2 (Chuvash and other Polycythemias): Patients with familial erythrocytosis from the Chuvash republic of the former Soviet Union was found to have an Arg to Trp substitution at the amino acid position 200 (ARG200TRP) in the VHL gene. Other VHL mutations associated with erythrocytosis including VAL130LEU, ASP126TYR, PRO192SER and HIS191SER (see table 2 below). These patients have an abnormality in hypoxia sensing and result in an inappropriately high production for the degree of hypoxia. VHL muttions result in von Hipple-Lindau syndrome that predisposes the individula to renal cell carcinoma. No such predisposition is found in patients with type 2 familial erythrocytosis. Patients with Chuvash polycythaemia develop vertibral angiomas, are predisposed to venous thrombosis and respond abnormally to hypoxia
      2. Familial Erythrocytosis Type 3Familial erythrocytosis type 3 results from mutations of the PDH2 gene resulting in a defect in oxygen sensing (see table 2 below). These patients have inappropriately normal erythropoietin levels. Identified mutations are listed in table above.
      3. Familial Erythrocytosis Type 4: Familial erythrocytosis type 4 is an autosomal dominant disorder resulting from gain of function mutations in the HIF2A gene that codes for the α subunit of hypixia inducible factor 2 (see table 2 below). The mutations cause a defect in oxygen sensing resulting in an inappropriately high production of erythropoietin for the oxygenation.
      4. Haemoglobin Anomalies Causing Polycythaemia: Polycythaemia is caused by high affinity haemoglobins and congenital methaemoglobinaemia (see table 2 below). Mutations resulting in high affinity haemoglobins result in impaired oxygen release. this is sensed as hypoxia, erythropoietin production increased and polycythaemia results. Congenital methaemoglobinca results in suboptimal oxygen delivery erythropoietin secretion and polycythaemia.
    2. Acquired polycythaemia
      1. Accquired secondary polycythaemia: Accquired secondary polycythemia is observed in the following conditions
        1. Conditions associated with hypoxia:
          • Chronic Obstructive pulmonary disease
          • Left to right shunt with eisenmengerization
          • Chronic Smokers
          • High altitude polycythaemia
        2. Conditions associated with ectopic erythropoietin secretion:
          • Solitary renal cysts
          • Polycystic kidney disease
          • Hydronephrosis
        3. Paraneoplastic erythropoietin secretion:
          • Renal cell carcinoma – most common
          • Hepatoma – Less common
          • Occasional associations: Adrenal tumours,  cerebellar hemangioblastoma, hemangiomas, pheochromocytomas, sarcomas, uterine fibroids and Wilms tumor.
Table 2 – Genetic Defects Causing Polycythaemia
Syndrome Gene Mutations
Inherited disorders
Familial Erythrocytosis Type I (Autosomal Dominant) Erythropoietin Receptor
  • TRP439TER
  • 1-BP INS, 5975G
  • ASN487SER
  • 7-BP DEL, NT5980
  • 1-BP INS, 5967T
  • TYR426TER
  • 5968_5975DUP
Familial Erythrocytosis Type II (Autosomal reccssive) VHL Gene
  • ARG200TRP
    (Chuvash Polycythaemia)
  • VAL130LEU
  • ASP126TYR
  • PRO192SER
  • HIS191ASP
Familail Erythrocytosis Type III (Autosomal Dominant, all patinets have been heterozygous)  PDH2 gene
  • PRO317ARG
  • ARG371HIS
  • HIS374ARG;
Familial Erythrocytosis Type IV (Autosomal DOminant) HIF2A gene
  • GLY537TRP
  • GLY537TRP
  • MET535VAL
High Affinity Haemoglobin (Autosomal Dominant) HBA, HBB 93 varients listed at http://globin.bx.psu.edu/hbvar/menu.html accessesd on 11th July 2013
Congenital Methemoglobinaemia Cytochrome b5Cytochrom b5 Reductase, Hemoglobin M The specific mutations are described in the links to the genes in the adjacent columns
2,3 Bisphosphoglucerate Mutase deficiency 2,3-Bisphoglyceromutase ARG89CYS; 1-BP DEL, 205C
Acquired
Polycythaemia Vera JAK2  VAL617ILE (V617F), Exon 12 mutations

Erythropoietin and Erythropoietin Receptor


(Post updated on June 21st 2017)

Erythropoietin (EPO), a 34kD 166 amino acid polypeptide, is the main regulator of erythrocyte production. It acts via the erythropoietin receptor (EPOR). 

Importance of Erythropoietin Signalling

No inactivating mutations of EPO signalling pathway are known.  Mice with deletion of EPO or EPOR gene die of anaemia at a gestational age of 12-13 days. They have some haematopoiesis in the yolk sac but none in in foetal liver. CFU-E do not survive in the absence of erythropoietin. Erythropoietin is also essential for survival of the more mature population of BFU-E. EPO reduces apoptosis. The effects are mediated by STAT5 (signal transducer and activator of transcription 5). STAT5 induces the production of the anti-apoptotic protein bcl- x by binding to it’s promoter. As will be discussed below the non-receptor tyrosine kinase Janus kinase 2 (JAK2) activates STAT. Erythropoietin has effect on a range on non-errythroid tissue. These are beyond the scope of this article and is discussed elsewhere (JEM 2013;210:205).



Control of EPO Production


Erythropoietin is produced in response to hypoxia by the interstitial fibroblasts of the kidney. The induction of erythropoietin secretion by hypoxia involves

  1. Hypoxia inducible factor (HIF) that promotes the transcription of genes induced by hypoxia. There are three HIF, HIF1, HIF2 and HIF3
  2. HIF prolyl hydroxylase (PHD) an enzyme that uses oxygen as a substrate and is inactive in hypoxia. It promoted the degradation of HIF.
  3. pVHL (von Hipple-Lindau tumour suppressor gene product) is involved in degradation of HIF

Protein synthesis needs energy. It may appeal to common sense to synthesise proteins only when needed and conserve energy. Increasing protein synthesis is a slow process making it unsuitable for situations where a rapid response is needed. Hypoxia needs a rapid response. The alternate strategy adopted by the body is to synthesise a protein continuously and control the levels of the protein by controlling degradation. A peptide may be made inactive by breaking just one critical bond. Synthesis needs building tens to hundred bonds. Degradation can be stopped almost instantaneously resulting in a rapid rise of the desired molecule. HIFs are synthesised and degraded continuously. The degradation mechanism is hypoxia sensitive allowing for a rapid rise of HIFs and consequently in the transcription of genes under control of HIFs as the partial pressure of oxygen falls. 

There are three HIFs, HIF-1, HIF-2 and HIF-3. HIF-2 controls erythropoietin production. HIF-2 has two units, α and β. The levels of the β subunit are constant. The levels of the α vary inversely with oxygen availability and determine the HIF 2 concentration activity. The α subunit is continuously being synthesized. Synthesis is matched by degradation by a proteolytic system known as proteasome. Proteasome only degrades peptides marked for destruction. Peptides are marked for destruction by tagging them by a multiple molecules of a protein ubiquitin. Ubiquitin is transferred by an enzyme complex known as ubiquitin E3 ligase. This complex consists of pVHL (product of the von Hipple-Lindau tumour suppressor gene), elongins B and C, cullin 2 and ring box 1 (Rbx1). pVHL identifies targets giving the ubiquitin E3 ligase specificity for HIF-1 and HIF-2. HIF needs to be hydroxylated at proline residues before polyubiquitination. The hydroxylation is brought about at proline residues by HIF prolyl hydroxylase (PHD). Oxygen is one of the substrates for PHD. In hypoxic conditions hydroxylation and consequently polyubiquitination does not take place. Proteasomal destruction stops and the levels of HIF-2α levels rapidly rise. HIF-2 translocates to the nucleus where it combines with HIF-2β. The heterodimer (HIF 2) acts on segments of DNA, known as hypoxia response elements, flanking the erythropoietin gene finally leading to erythropoietin synthesis. HIF 2 along with HIF 1, which is regulated by mechanisms identical to those regulating HIF 2, promotes the expression of multiple genes of proteins involved in response to hypoxia.



Mechanism of Action


EPO acts via the EPOR. Binding of erythropoietin has proliferative and anti-apoptotic effects. Receptor activation results in activation of a cascade of enzymes by phosphorylation. These include STATs (a family of seven proteins), Pi3K and RAS/MAPK/Erk. Phosphorylation can either be brought about by the tyrosine kinase activity of the receptor or, as is the case with the erythropoietin receptor, by a non-receptor tyrosine kinase associated with the receptor. The Janus kinase 2 (JAK2) is the non-receptor tyrosine kinase that associates with the erythropoietin receptor. The EPO receptor is a homodimer (a dimmer made from similar monomers). Each monomer associates with a JAK2 molecule.  Binding of EPO to EPOR brings about a conformational change in the receptor bringing the two JAK2 moleclues in proximity that results in transautophosphorylation and activation of the JAK2 kinases. Activated JAK2 phosphorylates tyrosine residues on the receptor forming docking sites for molecules that activate the following pathways:

  1. Dimerization and translocation of STAT5 to the nucleus where it induces transcription of genes involved in proliferation and cell survival. STAT pathway appears to be the most important pathway for EPO action.
  2. Phophoinositide-3-kinase (PI3-K) mediated induction of several anti-apoptotic proteins e.g. Bcl-2 abd BclX. As mentioned above STAT5 is also involved in induction of bcl-x.
  3. Activation of Ras/extracellular-signal-regulated kinase mitogen-activated protein (RAS/Erk/MAP) kinase pathway that sustains proliferation.

The EPO signalling is short lasting and the activated EPOR signal pathway returns to normal levels in 30-60 minutes. The cytoplasmic portion of the receptor is polyubiquitinated and degraded by proteasome. The extracellular portion bound to EPO is internalized and degraded. Regulators of cytokine activity can inhibit EPOR.


Mutations and Therapeutic Manipulation of EPO Signalling


Acquired (somatic) mutations of the JAK2 kinase are associated with myeloproliferative disease. The JAK2V617F mutation is seen in about 95% of the patients of polycythaemia vera and about 50% the patients with essential thrombocytosis and idiopathic myelofibrosis. JAK2V617F causes the JAK2 molecule to be constitutively (continuously, irrespective of EPO binding to EPOR) active eliminating the need for binding of EPO to EPOR for activation pathways stimulated by EPOR. JAK2 exon 12 mutations are found in patients with JAK2V617F negative polycythemia vera. These patients, unlike those with JAK2V617F positive patients, do not show thrombocytosis or leucocytosis. Inherited (germline) mutations in the EPOR, HIF 2α, VHL gene and the PHD gene have been associated with congenital erythrocytosis. The erythrocytosis seen in patients with EPOR mutations is primary, i.e. with low EPO levels. Other mutations result in secondary erythrocytosis, i.e. with inappropriately high EPO levels. Inhibitor of HIF prolyl hydroxylase FG-2216 and FG-4592 are under evaluation for treatment of anaemia associated with chronic kidney disease.