Pyruvate Kinase Deficiency 

Pyruvate kinase deficiency (PKD) is one of the most common enzymatic defects of the erythrocyte. This disorder manifests clinically as a hemolytic anemia, but surprisingly, the symptomatology is less severe than hematologic indices indicate. Presumably, this is due to enhanced oxygen delivery as a result of the defect. The clinical severity of this disorder varies widely, ranging from a mildly compensated anemia to severe anemia of childhood.^{[1, 2, 3]}

Most affected individuals do not require treatment. In the most severe cases, death may occur in utero as a result of anemia, or blood transfusions or splenectomy may be required, but most of the symptomatology is limited to early life and to times of physiologic stress or infection.

Pyruvate kinase deficiency (PKD) is an erythrocyte enzymopathy involving the Embden-Meyerhof pathway of anaerobic glycolysis. Erythrocytes have evolved without oxidative phosphorylation to form adenosine triphosphate (ATP), the compound essential for providing the erythrocyte energy.^[4]

Pyruvate kinase (PK) catalyzes the conversion of phosphoenolpyruvate to pyruvate. This is 1 of 2 glycolytic reactions in the erythrocyte that result in the production of ATP. A discrepancy between erythrocyte energy requirements and ATP-generating capacity produces irreversible membrane injury, resulting in cellular distortion, rigidity, and dehydration. This leads to premature erythrocyte destruction by the spleen and liver. Low ATP levels are responsible for erythrocyte intracellular electrolyte concentration disruption, resulting from failure of the adenosine triphosphatase cation pump.

The hexose monophosphate shunt and glutathione synthetic pathway protect the erythrocyte against destruction from free radicals and oxidative stress. Loss of adequate ATP diminishes their function.

Young reticulocytes retain mitochondria that produce ATP through oxidative phosphorylation. However, this comes at a price: a sixfold to sevenfold higher oxygen requirement. Paradoxically, this can lead to the demise of any reticulocyte, because its journey through the spleen, an environment deficient in glucose and oxygen, is lengthened by its adhesive tendency. In such an environment, the reticulocyte is at an increased risk of metabolic failure.

Important intermediates proximal to the PK defect influence erythrocyte function. Twofold to threefold increases of 2,3-diphosphoglycerol levels result in a significant rightward shift in the hemoglobin-oxygen dissociation curve. Physiologically, the hemoglobin of affected individuals has an increased capacity to release oxygen into the tissues, thereby enhancing oxygen delivery. Thus, for a comparative hemoglobin and hematocrit level, an individual with PKD has an enhanced exercise capacity and fewer symptoms. This is particularly advantageous during pregnancy, because it enhances transfer of oxygen to the fetal blood. This most likely adds to the particularly benign course of this disease in many affected individuals. Women with PKD typically do not require transfusions during pregnancy.

Isoenzymes

PK exists as 4 isoenzymes. Two isoenzymes (PKM1 and PKM2) are encoded by a genetic locus on band 15q22 and are found in striated muscle, brain, fetal tissue, leukocytes, platelets, lungs, spleen, kidneys, and adipose tissue. The other 2 isoenzymes (PKL and PKR) are encoded by a genetic locus on band 1q21 and are found in liver, normoblasts, reticulocytes, and erythrocytes.^{[5, 6, 7]} The liver and erythroid precursors are capable of activating PK-M2 activity, but this is not the enzyme used under normal conditions.^[8]

In persons with PKD, band 1q21 is defective, resulting in deficient liver and red blood cell isoenzymes. The liver can compensate for the genetic defect in 2 ways. First, because the enzyme deficiency results in a less efficient enzyme rather than a nonfunctioning enzyme, a greater quantity of enzyme can be produced. In addition, the liver can use residual PK-M2 activity. Early in maturation, erythroid precursors use the PK-M2 isoenzyme. However, as the cell matures, the PK-R isoenzyme replaces the PK-M2 enzyme. Because the erythrocyte cannot produce any new protein, it cannot compensate by increasing the quantity of isoenzyme or by using residual PK-M2 isoenzyme.

Enzyme defects that have been described include decreased substrate affinity, increased product inhibition, decreased response to activator, and thermal instability. Mutations that strongly perturb enzyme kinetics and thermostability are associated with severe PKD.^[9] One severe form of PKD, PK Beppu, is associated with persistence of the PK-M2 isoenzyme.

Pyruvate kinase deficiency (PKD) and glucose-6-phosphate deficiency (G6PD) are the most common erythrocyte enzymopathies. PKD is the most common enzymopathy of anaerobic glycolysis. The prevalence rate of a heterozygous carrier of 1 deficient PK gene is believed to be approximately 1%. Screening a US population for the 4 most common gene mutations demonstrates an estimated prevalence of 51 cases per million persons in the white population.^[10] This is 50 times higher than the number of individuals who were diagnosed with PKD at a major pyruvate kinase assay laboratory in the United States over a 25-year period, suggesting that this disorder is frequently underdiagnosed.

PKD is particularly common among the Pennsylvania Amish, in whom the disorder can be traced to a single immigrant couple. Affected individuals can have severe, life-threatening manifestations and can require long-term transfusion therapy and splenectomy in early childhood.^[11]

Although (PKD occurs worldwide, most cases have been reported in northern Europe and Japan, as well as in the United States. The prevalence rate of having 1 deficient PK gene has been estimated in Germany at 1% and in Hong Kong at 3%. The prevalence of diagnosed cases in the northern health region of the United Kingdom is 3.2 cases per million population.^[12] This is not based on genetic diagnosis or a gene screening survey.

The particular mutation responsible for the deficiency differs between populations. For example, the 1529A mutation is believed to cause at least 1 gene mutation in most affected white persons from the United States and Europe. Other individuals, such as Portuguese, Spanish, and Italian persons, are affected by the 1456T mutation. Asian persons in the United States appear to be affected by the 1468T mutation.

The age of onset for inherited pyruvate kinase deficiency (PKD) correlates with severity. Persons with severe disease usually have onset in the neonatal period or infancy. In most affected persons, PKD is detected during childhood, but in individuals who are mildly affected, PKD may not be detected until late adulthood.

Acquired PKD is usually secondary to a particular disease. In such cases, the age of onset varies with the primary disease.

Pyruvate kinase deficiency (PKD) is associated with a wide range of morbidities. Some patients present with a mild, compensated, chronic hemolytic anemia that does not require medical intervention; others present with a severe hemolytic anemia that, in most cases, requires only transfusions during childhood.

Morbidity and mortality correlate with disease severity and usually depend on complications. Mild and moderate forms of the disease are associated with an excellent prognosis. Severe forms of the disease are mostly symptomatic during early childhood. Following early childhood, the disorder is much better tolerated. Hydrops fetalis has been reported in a severely affected fetus.^[13]

Birth history of pyruvate kinase deficiency (PKD) includes severe anemia, severe jaundice,^[14] kernicterus, and a history of exchange transfusion. Family history is consistent with autosomal recessive inheritance. Patients may become symptomatic during times of physiologic stress, including acute illness, particularly viral, and pregnancy.

The following are evident in cases of PKD:

• Mild to severe anemia
• Symmetrical growth delay
• Failure to thrive
• Cholecystolithiasis, usually after the first decade of life, but possibly in childhood
• Frontal bossing
• Icteric sclera
• Mild to moderate splenomegaly
• Upper-right-quadrant tenderness
• Murphy sign
• Chronic leg ulcers

Complications associated with pyruvate kinase deficiency (PKD) include the following:

• Cholecystolithiasis is common in the first decade of life in children with severe anemia
• Splenectomy increases the risk of sepsis by encapsulated bacteria in children and the risk of thromboembolic disease in adults.^[15]
• Ischemic stroke has been reported in previously undiagnosed young adults with pyruvate kinase deficiency^[16]
• Multiple transfusion therapy can cause iron overload^{[17, 18]}
• Blood transfusions expose a person to the risk of contracting certain infections that are not well detected (eg, HIV disease, hepatitis C)
• Repeated transfusions during pregnancy increase the risk of alloimmunization, which may lead to fetal complications

Medical conditions, such as acute leukemia, preleukemia, and refractory sideroblastic anemia, as well as complications from chemotherapy, can cause an acquired pyruvate kinase deficiency (PKD). This type is more common and milder than the hereditary type.

More than 100 genetic defects of the PK gene have been detected.^{[5, 6]} Most defects are missense mutations, but splicing mutations, insertions, and deletions also have been identified. Although inheritance is clinically autosomal recessive, most affected individuals are compound heterozygous for 2 different mutant alleles. Homozygous individuals are usually the product of consanguineous mating.^[19]

Prenatal enzymatic testing is not optimal, because a large amount of fetal blood is required, and the test has a high rate of false-negative results.

Cell indices

The hematocrit value ranges from 17-37%. Lower values occur in early childhood and during the neonatal period, with a 3- to 9-point rise after early childhood. Erythrocytes are normochromic and macrocytic.

The reticulocyte count may be increased by 5-15%. Paradoxically, a high reticulocyte count, as high as 70%, may occur after splenectomy. Leukocyte and platelet counts range from normal to slightly increased.

Monitor the hematocrit value carefully during times of physiologic stress.

Cell morphology

Morphologic abnormalities are not a prominent finding, but hallmarks of accelerated erythropoiesis, such as polychromatophilia, anisocytosis, poikilocytosis, and nucleated red blood cells, may be present.

Siderocytes, target cells, Pappenheimer bodies, Howell-Jolly bodies, and crenated red blood cells may be observed post splenectomy.

Hemoglobin indices

Concurrent with the hematocrit value, the hemoglobin concentration varies from 6-12 g/dL, with a lower concentration early in life. Hemoglobin electrophoresis reveals normal hemoglobin with normal levels of F and A2 hemoglobins.

Hemolytic anemia tests

Erythrocyte lifespan is moderately to severely reduced, depending on the severity of the anemia. Radiochromium labeling reveals an immediate period of destruction, followed by a shortened lifespan for the remainder of labeled cells. The results of this test can help to determine candidacy for splenectomy, because a high rate of immediate destruction suggests significant splenic activity.

The following are other findings on hemolytic anemia testing:

• Erythrocyte osmotic fragility is normal
• The Coombs test result is negative
• The Ham test result is negative
• The Donath-Landsteiner antibody is absent
• Cold agglutinins are absent
• Incubated Heinz body formation is usually abnormal

Hemoglobin metabolic indices

Indirect hyperbilirubinemia reflects the severity of the hemolytic process. levels of 6 mg/dL are not uncommon, and levels greater than 20 mg/dL have been reported. Haptoglobin is reduced in proportion to disease severity.

Enzyme deficiency testing

The precise diagnosis depends on detecting the deficient enzyme.^[20] The enzyme activity rate in most patients who are deficient is 5-25% of normal.

Simple, specific enzyme testing is available, but false-negative results can occur, especially when the defect is due to a compound heterozygous mutation, because kinetic variables are not measured accurately under such circumstances. Measurement of the intermediates proximal to the enzyme defect, specifically 2,3-diphosphoglycerol and glucose-6-phosphate, help confirm the diagnosis.

Deoxyribonucleic acid (DNA) testing

Because of the large number of gene mutations that result in pyruvate kinase deficiency, DNA analysis is limited. However, some exceptions should be noted. If the defects in the parents are known, prenatal diagnosis using DNA testing is possible.^[21] When the mutation is known, the DNA analysis can be limited to specific mutations. This is also useful in prenatal diagnosis. Mutations have been found to affect specific groups. For example, particular mutations have been identified in highly affected groups such as the Pennsylvania Amish.

In severe anemia, radiographs may demonstrate findings of marrow expansion. Biliary tract obstruction may occur as a consequence of this disorder, requiring imaging of the biliary tree.

Pathologic and histologic findings include normoblastic erythroid hyperplasia of the bone marrow, extramedullary hematopoiesis, splenic and hepatic hemosiderosis and splenic congestion, reticuloendothelial hyperplasia, and erythrophagocytosis.

In general, despite significant deficiencies of the liver isoenzyme of pyruvate kinase, results from liver function testing show hyperbilirubinemia unless the patient has iron overload due to multiple erythrocyte transfusions.^{[17, 18]}

In patients with mild to moderate disease, care is predominantly supportive in nature. High-impact contact sports are contraindicated in patients with significant splenomegaly. Red blood cell transfusion may be necessary if the hemoglobin value falls significantly; this tends to occur in early childhood and during periods when physiologic stress is present, such as when an infection exists or during pregnancy.

In pregnant patients with pyruvate kinase deficiency (PKD), uncomplicated pregnancy, delivery, and birth were reported despite a decline in the hemoglobin value to 6.8 g/dL during pregnancy.^[22] In another study of pregnant patients, significant puerperal jaundice was successfully treated with conservative measures.^[23]

In a 5-year-old boy with severe hemolytic anemia due to PKD and heterozygous hemoglobin E, bone marrow transplantation was performed using ABO-identical and HLA-identical marrow from his sister, and more than 3 years post transplant, the patient was still healthy, without symptomatology of PKD.^[24]

Supplemental folic acid and other B vitamins help to prevent deficiencies from increased erythrocyte production.

Large doses of salicylates should be avoided in patients with severe anemia, because salicylates inhibit oxidative phosphorylation, thereby causing further ATP depletion.

Splenectomy

Splenectomy is indicated only for patients with severe anemia. Prophylactic antibiotics should be administered to young patients postsplenectomy. Splenectomy can reduce anemia, but hemolysis will not be abolished. Splenectomy does not improve mild anemia. Splenectomy should be performed by an experienced surgeon, especially in pediatric patients. Consider the susceptibility to infection following splenectomy, especially in children younger than 5 years.^[25]

After splenectomy, the hemoglobin concentration typically increases by 1-3 g/dL; transfusion requirements typically decrease; the danger of an aplastic crisis with infection is reduced; and growth delay, if present, may be reversed, and catch-up growth may ensue.

Consultations

When necessary, the following should be consulted:

• A hematologist should be consulted for management and treatment
• A surgeon should be consulted if splenectomy is considered
• An anesthesiologist should be consulted for presurgical management if anemia is severe
• A gastroenterologist may be necessary to help evaluate complications of the biliary tree

Patients should be educated to regularly use folic acid and B vitamin supplements and to avoid salicylates.

The risk regarding splenectomy versus that of multiple transfusions should be discussed with the parents of children with severe anemia. If a child has splenomegaly, parents should be instructed to have the child refrain from participating in contact sports.

Because the inheritance is phenotypically autosomal recessive, parents and patients should be educated about the low risk of reoccurrence.