Anemia Due to Pyruvate Kinase Deficiency

Article written and reviewed by Dr Luca Moretti

anemia due to pyruvate kinase deficiency is the most common cause of chronic congenital nonspherocytic hemolytic anemia and is characterized by a congenital deficiency of red-cell pyruvate kinase (PK) . This deficiency impairs glycolytic metabolism, which is essential for ATP production in red blood cells, resulting in chronic hemolytic anemia of variable severity.

Although rare, this disorder is particularly important clinically in countries with high rates of consanguinity and in certain at-risk populations. Clinical expression ranges from mild, compensated anemia to severe transfusion-dependent forms with neonatal onset and multiorgan complications. Understanding its molecular basis, pathophysiology, and diagnostic and therapeutic implications is essential for optimal management because diagnosis may be delayed and multidisciplinary care is often required.

Structure and Function of Red-Cell Pyruvate Kinase

pyruvate kinase is a key glycolytic enzyme located in the cytoplasm of all cells, with particular importance in mature red blood cells. Because red blood cells lack mitochondria, glycolysis is their only pathway for producing ATP, the energy molecule required to maintain cell-membrane structure and function, cell volume, and cytoskeletal protein integrity.

In red blood cells, glycolysis breaks glucose down into two molecules of pyruvate through a series of enzymatic reactions that generate ATP. The reaction catalyzed by pyruvate kinase is the final step of glycolysis, converting phosphoenolpyruvate (PEP) to pyruvate and generating ATP by substrate-level phosphorylation:

Phosphoenolpyruvate + ADP → Pyruvate + ATP

This reaction is particularly important in red blood cells, where ATP synthesis depends almost entirely on glycolysis. Pyruvate kinase deficiency reduces ATP production, impairs ionic homeostasis and deformability, and causes predominantly extravascular red-cell removal, especially in the spleen.

In adult red blood cells, pyruvate kinase is encoded mainly by the PKLR (Pyruvate Kinase, Liver and Red blood cell) gene, located on chromosome 1q22. Other isoforms, PKM1 and PKM2, are expressed in muscle and proliferating tissues, respectively, but do not play a role in the pathogenesis of hereditary hemolytic anemia.

Etiology and Genetics

The etiology of anemia due to pyruvate kinase deficiency is exclusively genetic and involves mutations in the PKLR gene. This gene encodes both the hepatic and erythroid isoforms of the enzyme, but clinical manifestations occur mainly in red blood cells, where ATP requirements are particularly critical.

Disease-causing variants are highly heterogeneous and include:

• Missense mutations (amino-acid substitutions), which alter enzyme stability or catalytic activity;
• Nonsense mutations, which introduce premature stop codons and generate a truncated, nonfunctional protein;
• Splice-site mutations, which impair proper mRNA processing;
• Deletions or insertions, which may cause frameshifts and loss of function;
• Promoter variants, which reduce gene expression.

The distribution of variants differs among populations, with some variants being more common in Central and Northern Europe, such as 1529G>A, and others being typical of Middle Eastern and Asian regions.

Inheritance is autosomal recessive: the disease is clinically expressed when two mutated alleles are present, either homozygous or compound heterozygous. Heterozygous carriers are asymptomatic but can transmit the variant to their offspring.

No environmental or acquired factor is known to directly cause pyruvate kinase deficiency, although other inherited conditions may modify the phenotype, including coexisting thalassemia or membrane disorders such as hereditary spherocytosis. Disease frequency is higher in populations with high consanguinity or founder variants.

PKLR variants cause a quantitative and/or qualitative enzyme deficiency through loss of catalytic function or protein instability. Red blood cells are then unable to produce enough ATP to maintain ionic homeostasis, membrane integrity, and deformability, leading to shortened red-cell survival and predominantly splenic removal.

Pathogenesis and Pathophysiology

The pathogenesis of anemia due to pyruvate kinase deficiency is based on a well-defined biochemical mechanism: enzyme deficiency impairs the final step of erythrocyte glycolysis, drastically reducing ATP production. Because red blood cells lack mitochondria, glycolysis is their only energy-producing pathway. Under physiological conditions, conversion of phosphoenolpyruvate (PEP) to pyruvate generates the final ATP molecules required to maintain ionic balance and membrane potential.

In pyruvate kinase deficiency, decreased ATP triggers a series of pathogenic events:

• Reduced activity of membrane ion pumps especially Na⁺/K⁺-ATPase and Ca²⁺-ATPase, resulting in potassium loss and intracellular calcium accumulation;
• Cellular dehydration caused by potassium and water loss, with increased red-cell density and membrane rigidity;
• Cytoskeletal abnormalities and reduced membrane flexibility, making red blood cells unable to deform while passing through splenic sinusoids;
• Accumulation of glycolytic intermediates upstream of the enzymatic block, such as 2,3-diphosphoglycerate (2,3-DPG), which can affect the hemoglobin oxygen-dissociation curve;

The final result is premature red-cell destruction, occurring predominantly in the spleen. Energy-depleted red blood cells are removed by the reticuloendothelial system, causing chronic hemolytic anemia.

A distinctive pathophysiological feature is increased 2,3-DPG in red blood cells, which causes reduced hemoglobin affinity for oxygen. This facilitates oxygen release to peripheral tissues and partly explains the discrepancy between the severity of anemia and the relative clinical tolerance seen in some patients.

Erythropoietic compensation is another key mechanism: chronic anemia stimulates erythropoietin production and activation of bone marrow erythropoiesis. In severe cases, the marrow expands and extramedullary hematopoiesis may develop, with splenomegaly, hepatomegaly, or paraspinal masses.

The pathophysiology of pyruvate kinase deficiency differs from other hereditary hemolytic anemias in several respects:

• Absence of specific red-cell morphological abnormalities, with no marked spherocytosis but modest anisopoikilocytosis;
• A predominantly extravascular hemolytic pattern, with unconjugated hyperbilirubinemia and reticulocytosis;
• Iron accumulation secondary to chronic hemolysis, which may lead to hemosiderosis even in the absence of transfusions.

These features are fundamental for clinical suspicion and differential diagnosis.

Classification and Epidemiology

red-cell pyruvate kinase deficiency is a clinically heterogeneous disorder that can be classified according to:

• Severity of hemolysis (mild, moderate, severe);
• Age at onset (neonatal, childhood, adolescence, adulthood);
• Need for regular transfusions;
• Presence or absence of complications such as gallstones, splenomegaly, and iron overload.

More than 300 different PKLR variants have been described. Genotype-phenotype correlation is not always linear and can be influenced by modifier polymorphisms or associated conditions.

According to clinical severity, forms may be divided into:

• Severe forms (transfusion-dependent): neonatal onset with severe jaundice, profound anemia, need for frequent transfusions, and early complications.
• Moderate forms: chronic hemolytic anemia with occasional transfusion requirements, for example during infections, pregnancy, or physiological stress.
• Mild forms: mild, compensated anemia, often discovered incidentally or through family screening.

This clinical distinction is important for therapeutic management and long-term prognosis.

Epidemiologically, pyruvate kinase deficiency has an estimated diagnosed prevalence of 3.2 to 8.5 cases per million inhabitants in Western populations, but it may be underdiagnosed because of its broad phenotypic spectrum. Some ethnic clusters have higher prevalence because of founder effects, including Northern European populations, the Pennsylvania Amish, and certain populations in the Middle East and Central Asia.

The frequency of heterozygous carriers is probably higher than currently recognized, making molecular diagnosis and family screening essential, especially in the presence of consanguinity or a family history of unexplained hemolytic anemia.

Access to specialist diagnosis and disease awareness remain challenges, especially in low-income countries, where the disorder may be mistaken for other forms of hemolytic anemia or treated inappropriately.

Clinical Manifestations

clinical manifestations of pyruvate kinase (PK) deficiency reflect the complex interaction among the severity of the enzymatic defect, erythropoietic compensatory capacity, and the body’s adaptive mechanisms to chronic hemolytic anemia. The clinical spectrum is extremely heterogeneous, ranging from very severe neonatal-onset disease to mild phenotypes that may remain unrecognized until adulthood. Understanding this variability is essential for suspecting the disorder in the absence of pathognomonic signs and for avoiding diagnostic delays that can lead to preventable complications.

In the most severe cases, PK deficiency presents within the first days of life with early and severe neonatal jaundice, which may require intensive phototherapy or exchange transfusion. The neonatal presentation may also be dominated by profound hemolytic anemia with signs of multiorgan distress, including feeding difficulty, hypotonia, tachycardia, tachypnea and, in the most severe forms, hydrops fetalis with generalized edema, serous-cavity effusions, and high-output heart failure. Although rare, hydrops fetalis represents an extreme and potentially lethal presentation and suggests a particularly severe enzyme deficiency or additional aggravating genetic factors.

With growth, the clinical picture typically stabilizes into chronic hemolytic anemia of variable severity, with symptoms resulting from persistent hemolysis, tissue hypoxia, and hyperbilirubinemia. Common manifestations include:

• Mucocutaneous pallor that is persistent and often associated with scleral and cutaneous jaundice that is intermittent or sometimes persistent;
• Splenomegaly that may be progressive and sometimes massive, causing abdominal fullness, early satiety, or left upper-quadrant pain;
• Chronic fatigue, reduced exercise tolerance, poorer school performance in children, and limitation of daily activities in adults;
• Impaired linear and weight growth, especially in children with severe inadequately treated anemia;
• Pigment gallstones, often developing early because of chronic hemolysis-related hyperbilirubinemia; gallbladder stones may already be evident in childhood;
• Hepatomegaly and, less commonly, signs of extramedullary hematopoiesis such as paraspinal masses or enlargement of secondary hematopoietic organs.

The clinical picture may worsen during acute events such as intercurrent infections, which can increase hemolysis, or aplastic crises, particularly during Parvovirus B19 infection.

A distinctive clinical feature is relative tolerance of low hemoglobin levels. Because of compensatory elevation of 2,3-diphosphoglycerate (2,3-DPG) in red blood cells, hemoglobin has lower oxygen affinity, facilitating oxygen release to tissues. This helps explain why some patients can have markedly reduced hemoglobin without severe symptoms of tissue hypoxia until additional stressors arise.

In mild or compensated forms, the disorder may remain silent for years. It is often discovered incidentally during routine blood testing, evaluation of splenomegaly or early gallstones, or after physiological stress such as pregnancy, surgery, or severe infection. Intrafamilial variability may be marked: individuals carrying the same variant can have very different clinical phenotypes because of epigenetic or environmental factors or coexistence of other hematologic abnormalities.

Long-term complications resemble those of other chronic hemolytic anemias and include:

• Hemosiderosis and organ damage from iron overload, including in patients who have never been transfused, because of increased intestinal absorption secondary to chronic erythropoietic stimulation;
• Acute hemolytic crises, typically triggered by viral infections or metabolic stress;
• Aplastic crises, rare but potentially severe, particularly in children;
• Splenic enlargement with risk of hypersplenism and associated cytopenias;
• Growth and pubertal-development disorders in severe and persistent anemia;
• Cardiac complications such as high-output failure, osteoarticular complications such as osteopenia, and infectious complications, especially after splenectomy.

Overall, symptoms of pyruvate kinase deficiency anemia should be assessed and monitored over time, considering both the natural course and treatment response, with attention to quality of life and psychosocial aspects, particularly in young people and patients with chronic disease.

Diagnostic Evaluation

The diagnosis of anemia due to pyruvate kinase deficiency requires a structured, sequential approach in which clinical suspicion is guided by medical history, family history, and laboratory evaluation and is then confirmed by biochemical and molecular studies. The diagnostic pathway requires clear distinction from the other causes of chronic hemolytic anemia to avoid errors and unnecessary empirical treatments.

1. Clinical suspicion: PK deficiency should be suspected particularly in the presence of congenital chronic nonimmune hemolysis, with anemia of variable severity or occasionally compensated hemolysis, reticulocytosis, and other markers of hemolysis after common causes have been excluded. Suspicion should be particularly high when there is:

• A history of early neonatal jaundice, severe anemia at birth, or need for transfusions in the first months of life;
• A family history of unexplained hemolytic anemia, early gallstones, recurrent splenomegaly, or episodic hemolytic crises;
• Lack of response to inappropriate empirical treatments such as iron, vitamin B12, or folate when no deficiency is present;
• Complications typical of chronic hemolytic anemias, including growth delay, hemosiderosis, hyperuricemia, ankle ulcers, and hemolytic crises during infections;
• Suspicion of extramedullary hematopoiesis, such as a paraspinal mass or hepatomegaly.

2. First-line tests: The complete blood count generally shows normocytic anemia with reticulocytosis. The peripheral blood smear may show anisopoikilocytosis, polychromasia, occasional echinocytes, and circulating erythroblasts; schistocytes are not a characteristic finding. Spherocytes are not predominant.
The hemolysis profile includes:

• Increased reticulocytosis;
• Increased indirect bilirubin;
• Elevated LDH;
• Reduced or undetectable haptoglobin.

Iron studies may show normal or increased ferritin and serum iron when hemosiderosis is present.

3. Differential diagnosis: An essential step is distinguishing PK deficiency from G6PD deficiency, hereditary spherocytosis, and other enzymopathies such as glucose-6-phosphate isomerase or hexokinase deficiency. Specific tests, including a direct antiglobulin test, osmotic fragility testing, and G6PD analysis, contribute to the differential diagnosis; because of limited sensitivity, osmotic fragility testing alone does not exclude hereditary spherocytosis.

4. Biochemical confirmation: Diagnosis is based on direct demonstration of reduced pyruvate kinase enzyme activity in red blood cells with a specific enzyme assay. Performed in specialized laboratories, the assay measures pyruvate kinase activity through the rate of pyruvate formation coupled to NADH oxidation in a lactate dehydrogenase-mediated reaction. Interpretation must account for red-cell age and major interferences: recent transfusions can mask the deficiency, and marked reticulocytosis, including during hemolysis or after splenectomy, can produce falsely normal values. When possible, the assay should be performed at least 90 days after the last red-cell transfusion because a transfusion within the preceding 90 days can make pyruvate kinase activity results unreliable. In splenectomized patients the test is not contraindicated, but results require particular caution and, when necessary, integration with PKLR analysis.

5. Molecular confirmation: Genetic analysis of PKLR by direct sequencing, PCR, or MLPA can identify causative variants, define heterozygous or homozygous status, and support family counseling. Genetic testing is essential in complex or atypical cases and for prenatal or preimplantation diagnosis in at-risk settings.

6. Assessment of complications and second-line investigations: In patients with chronic anemia it is essential to assess iron overload with ferritin, serum iron, transferrin saturation, and hepatic and cardiac T2* MRI when accumulation is suspected; gallstones with abdominal ultrasound; splenomegaly by ultrasound and clinical examination; and any evidence of extramedullary hematopoiesis. In children and adolescents, growth and pubertal development should be monitored, together with the endocrine profile, to prevent late complications.

Treatment and Prognosis

Management of anemia due to pyruvate kinase deficiency is a complex challenge in both pediatric and adult hematology and requires an individualized approach that considers hemolytic severity, age, comorbidities, risk of complications, and available treatment resources. Treatment aims not only to address anemia and prevent complications but also to preserve quality of life and, when feasible, provide definitive therapy.

In patients with mild or well-compensated forms the main strategy is careful clinical observation, together with nutritional support with folic acid, monitoring for complications such as gallstones, hemosiderosis, and growth disorders, and genetic counseling for the patient and family. In many cases anemia is tolerated without regular transfusions because of metabolic compensation and tissue adaptation.

When anemia becomes clinically significant with marked limitation of daily activities, growth delay, symptoms of heart failure, or recurrent hemolytic crises, individualized transfusion support is required. Red-cell transfusions are indicated for:

• Children with severe symptomatic anemia, especially during the first years of life;
• Acute events such as aplastic crises or severe hemolytic crises;
• Preparation for surgery or during pregnancy in symptomatic women;
• Adults with limited cardiac reserve, hypoxic crises, or deterioration in general condition.

The transfusion regimen should be no more intensive than necessary to reduce iron overload and alloimmunization risk, with clinical and laboratory monitoring favored over rigid hemoglobin targets.

A fundamental component of treatment in patients with iron overload is iron chelation therapy. The indication and choice of chelator depend on documented iron burden assessed by serial ferritin, MRI for organ iron, liver, cardiac and renal function, as well as age and tolerability. Chelation should not be started automatically or simply “early,” but when clinically significant iron overload has been demonstrated.

Splenectomy is a treatment option for patients with severe hypersplenism, increased transfusion requirements, or associated cytopenias. The decision must be carefully weighed against long-term infectious and thrombotic risks; post-splenectomy thrombosis is a documented complication of PK deficiency. Infectious risk requires complete vaccination against pneumococcus, meningococcus, and Haemophilus influenzae and, when indicated, antibiotic prophylaxis. In children, splenectomy should be delayed as long as possible, preferably until after 5 to 6 years of age, to reduce the risk of overwhelming sepsis.

In recent years, innovative therapies have changed the treatment paradigm for pyruvate kinase deficiency. Mitapivat, an allosteric pyruvate kinase modulator, is approved for eligible adults in several jurisdictions and may increase hemoglobin or reduce transfusion requirements. It is not a definitive cure, and pediatric use remains subject to specific assessment and local authorization.

Allogeneic hematopoietic stem cell transplantation (HSCT) may represent a potentially curative therapy in rare severe and refractory forms, particularly in children, but it is not standard treatment for most patients. Success depends on donor-recipient compatibility, patient age, anemia severity, and preexisting organ complications. Transplantation carries significant risks, including graft failure, GVHD, and severe infections, and should be reserved for selected cases in accordance with international guidelines.

Prognosis is extremely variable and depends on multiple factors: the type and severity of the molecular defect, age at onset, access to transfusion and chelation therapy, and timely recognition and treatment of complications. At referral centers, quality of life and life expectancy have improved substantially over recent decades because of better supportive care, innovative therapies, and multidisciplinary follow-up. In resource-limited settings, however, the disease remains associated with a high risk of morbidity and mortality from untreated complications.

In conclusion, optimal management of pyruvate kinase deficiency is based on individualized therapy integrating transfusion support, prevention of iron overload, management of complications, psychosocial support, and, when indicated, access to innovative therapies and transplantation. The goal is not merely survival but the fullest possible physical, social, and relational development.

Complications

complications of anemia due to pyruvate kinase deficiency result from the direct consequences of chronic hemolytic anemia and bone-marrow compensation as well as iron overload, transfusion therapy, and splenectomy. Recognition and prevention are important determinants of quality of life and life expectancy.

Iron-overload complications

Excess iron progressively accumulates because of repeated transfusions or increased intestinal absorption driven by chronic erythropoietic stimulation and is the main cause of long-term morbidity. Clinical manifestations include:

• Liver damage including hemosiderosis, fibrosis, and cirrhosis;
• Cardiac disease including heart failure and arrhythmias from myocardial iron deposition;
• Diabetes mellitus and endocrine disorders including hypogonadism, hypothyroidism, and growth impairment;
• Osteopenia and osteoporosis, which are multifactorial and may be promoted by iron overload and other factors;
• Cutaneous hemosiderosis and skin pigmentation.

Prevention and timely treatment with chelation are essential to avoid irreversible organ damage.

Infectious complications

Splenectomized patients are at high risk of serious infection, particularly from encapsulated bacteria such as Streptococcus pneumoniae, Haemophilus influenzae, and Neisseria meningitidis. Vaccination and, in selected cases, antibiotic prophylaxis are recommended. Catheters and vascular access devices may constitute additional infectious risk factors; chronic anemia and gallstones do not in themselves cause opportunistic infections.

Thrombotic complications after splenectomy

Splenectomy increases thrombotic risk in pyruvate kinase deficiency; in the Natural History Study, post-splenectomy thrombosis was reported in 11% of splenectomized patients. This risk must be considered when selecting candidates and during long-term follow-up.

Hepatobiliary complications

Pigment gallstones are a common and often early complication caused by chronic hemolysis-related hyperbilirubinemia. They may cause biliary colic, acute cholecystitis and, less commonly, pancreatitis. Periodic abdominal ultrasound and timely cholecystectomy are recommended when symptoms or complications occur.

Hematologic and skeletal complications

Persistent ineffective erythropoiesis may cause bone-marrow expansion, skeletal deformities involving the skull and long bones, growth delay and, in severe cases, extramedullary hematopoietic masses at paraspinal or abdominal sites. Splenomegaly may cause hypersplenism and associated cytopenias, including leukopenia and thrombocytopenia, with bleeding and infection risk.

Cardiovascular complications

Chronic anemia may lead to high-output heart failure, especially in young children and patients with limited cardiac reserve. Echocardiographic monitoring and timely treatment of symptomatic anemia are essential to prevent myocardial injury.

Treatment-related complications

Transfusions can cause acute or chronic transfusion reactions, red-cell alloimmunization, and, rarely, transmission of infectious agents. Chelation therapy carries risks of renal, hepatic, ocular, and auditory toxicity and therefore requires careful clinical and laboratory monitoring.

Rare and psychosocial complications

In some cases patients may develop lower-limb ulcers, acute hemolytic crises triggered by infections, aplastic crises especially due to Parvovirus B19, and neurologic complications from paraspinal extramedullary masses. Chronic illness, ongoing treatment requirements, and awareness of complication risk significantly affect quality of life, psychological adjustment, and social integration for patients and families.

In summary, recognition and prevention of complications of pyruvate kinase deficiency anemia are central goals of multidisciplinary follow-up, with attention to the clinical, psychological, and social dimensions of comprehensive care.

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