Deficiency of erythrocyte pyruvate kinase is the most common enzyme deficiency resulting in inherited nonspherocytic hemolytic anemia. In contrast, spherocytic anemia is characterized as an auto-hemolytic disorder that results from molecular defects in erythrocyte (red blood cell, RBC) membrane proteins leading to spherical erythrocytes instead of the normal bi-concave disc shaped cells. Overall, erythrocyte pyruvate kinase deficiency is the second leading cause of hemolytic anemias. Deficiencies in glucose-6-phosphate dehydrogenase represent the leading causes of inherited forms of hemolytic anemia.
Erythrocyte pyruvate kinase deficiency is inherited as an autosomal recessive disorder and mutations in the erythrocyte pyruvate kinase gene occur with a frequency of 1 in 10,000. The disorder is characterized by lifelong chronic hemolysis of variable severity. Red blood cells from heterozygous individuals possess only 40%–60% of the pyruvate kinase activity of normal individuals yet these individuals are almost always clinically normal.
The liver and red blood cell pyruvate kinase gene (symbol PKLR) is located on chromosome 1q21 spanning 9.5 kb and is composed of 13 exons that generate two alternatively spliced mRNAs. The liver (L-PK or PKL) and red blood cell forms (R-PK or PKR) of pyruvate kinase are encoded by the same gene with differences resulting from differential transcriptional initiation. The R-PK promoter is active exclusively in erythrocytes , whereas, the L-PK promoter is active in hepatocytes and pancreatic β-cells. Translation of the R-PK isoform begins in exon 1 of the PKLR gene and translation of the L-PK isoform begins in exon 2. The R-PK protein is 574 amino acids in length and the L-PK protein is 543 amino acids. The pyruvate kinase isoform, originally thought to be muscle specific, is encoded by a separate gene (symbol PKM) located on chromosome 15q22.2–q22.3 composed of 12 exons spanning 32 kb. The PKM gene encodes two different isoforms identified as PKM1 and PKM2 as a result of alternative splicing.
Numerous mutations (130 to date) have been identified in the PKLR gene resulting in erythrocyte PK deficiency with the vast majority of those mutations being single nucleotide changes. In persons of European descent over 50% of individuals with PK deficiency harbor a nucleotide change at position 1529 (encoding amino acid 510) that changes a G to an A residue. This mutation results in an Arg to Gln change in the protein and is identified as the R510Q mutation.
The clinical severity of the anemia resulting from PKLR deficiencies varies from mild and fully compensated hemolysis to severe cases that require transfusions for survival. In the most severe cases an affected fetus will die in utero. Most PKLR deficient patients are diagnosed in infancy or early childhood. Severely affected patients may require multiple transfusions or splenectomy. The requirement for transfusion in many infants will diminish during childhood or following splenectomy. There are many patients with mild PKLR deficiency who do not manifest symptoms until adulthood. In these individuals, the symptoms, that include mild to moderate splenomegaly, mild chronic hemolysis, and jaundice, often manifest incidental to an acute viral infection or during an evaluation in pregnancy.
Clinical evaluation of blood from patients with PKLR deficiency will show normocytic anemia (sometimes macrocytic) which is reflective of the reticulocytosis that is invariably present. When peripheral blood is stained by new methylene blue or brilliant cresyl blue, reticulocytes will be seen as cells containing a fine basophilic network. Reticulocytes are immature red blood cells that have lost their nuclei just prior to entering the circulation. In normal individuals reticulocytes comprise 0.5% to 1.7% of the total erythrocytes. An increase in the percentage of reticulocytes (reticulocytosis) is an important sign in any patient suffering anemia. In PKLR deficiency the level of reticulocytosis results in a reticulocyte level of 4% to 15%. In association with the reticulocytosis there is a reduction in hematocrit (packed cell volume) to around 17% to 37% where it ranges from 41% to 50% in normal individuals. Hemoglobin measurement also shows a decrease to 6–12g/dL where normal ranges from 12–16.5g/dL. Examination of bone marrow from PKLR deficient patients will show normoblastic erythroid hyperplasia.