Introduction 

Pyruvate kinase deficiency (PKD) is one of the most common causes of congenital nonspherocytic haemolytic anaemia, a genetic condition causing early destruction of red blood cells (haemolysis). Pyruvate kinase is an essential enzyme involved in glycolysis and plays a role in the generation of adenosine triphosphate (ATP) in red blood cells. Mature red blood cells have adapted their metabolism to rely on glycolysis to maintain their structural and functional integrity. Therefore, a deficiency in pyruvate kinase can lead to loss of membrane plasticity, cellular dehydration and premature destruction of red blood cells. Reticulocytes, or immature red blood cells, are most susceptible to damage due to their higher requirement for ATP.^1,2,3

The diagnosis and management of PKD is challenging due to the wide spectrum of clinical signs and symptoms, with many cases going undiagnosed. The most common symptoms seen are anaemia, iron overload, splenomegaly (enlargement of the spleen) and jaundice. Currently, treatments usually rely on supportive care strategies, including transfusions, splenectomy and chelation therapy to tackle the complications associated with chronic haemolytic anaemia. Recently, disease-modifying agents, including stem cell transplants, pyruvate kinase activators and gene therapy, are being investigated to decrease symptoms associated with haemolysis. This article will highlight the symptoms of PKD and the methods employed to manage them.^4,5

Diagnosis

PKD is diagnosed by assessment of clinical signs and markers of chronic haemolytic anaemia. Clinical signs of PKD include:^2,3,5

• Anaemia 
• Splenomegaly 
• Jaundice
• Gallstones
• Increased reticulocytes
• Mild hyperferritinaemia (high levels of ferritin in the blood)

In addition, pyruvate kinase enzyme activity and PKLR genotyping can be considered to further confirm the presence of PKD:⁵

• Pyruvate kinase enzyme activity is measured in red blood cell lysates by specific assays. A low level of activity indicates an enzyme deficiency^4,5
• PKD is an autosomal recessive condition that results from variants in the PKLR gene. Over 200 different mutations have been detected in people with PKD, most of which are missense substitutions. PKLR molecular (genetic) testing can confirm PKD diagnosis by identifying mutations in this gene^3,5

Symptoms of PKD 

Newborns

Some complications in utero (before birth) include hydrops foetalis, growth retardation (intrauterine growth restriction) and prematurity. In some cases, intrauterine blood transfusions may be required. Hydrops foetalis arises due to fluid buildup in the foetus’s tissues, while growth retardation is primarily due to the reduction of energy levels. At birth, neonatal jaundice is commonly observed with indirect hyperbilirubinaemia. In rare cases, pulmonary hypertension due to haemoglobin toxicity and liver failure by direct hyperbilirubinemia can arise.^1,5,6,7

Children and adults

Haemolytic anaemia and splenomegaly are among the most common symptoms detected in children with PKD. Jaundice and scleral icterus (yellowing of the eyes) are observed due to the increase in bilirubin following haemolysis. Some additional symptoms that have been observed include poor feeding, growth and low energy as well as irritability. Some children have found difficulties in performing in school.^4,5

Symptoms in adults are similar to those seen in children, with jaundice, scleral icterus, splenomegaly and gallstones frequently observed. Fatigue, low energy levels and difficulty concentrating as a result of anaemia significantly impact quality of life, with difficulty maintaining full-time employment or participating in activities seen in some people. It is important to note that haemolysis is also increased by acute stressors, such as infections, stress and pregnancy.^4,5

Supportive care strategies 

Red cell transfusion

Transfusions are normally given to people with PKD to maintain healthy red blood cell counts and address the underlying anaemia caused by premature red blood cell destruction. This procedure is common, with 87% of people having had at least one transfusion before the age of 18. The frequency of transfusions changes from person to person depending on growth, daily symptoms and associated complications. In the setting of increased haemolysis by acute stressors, the intervals of transfusion may increase.^4,5,8

The frequency of transfusions decreases with age, with 36% of children receiving regular transfusions compared to 11% in adults. This is related to less frequent haemolytic events associated with infection or the time of splenectomy. Most people with PKD have a splenectomy after the age of 5 to reduce the need for transfusion. In some cases, where symptoms based on quality of life or complications of haemolysis increase, some adults may opt to reinitiate transfusions.^4,5

Splenectomy 

The spleen is an organ involved in filtering out deformed red blood cells and promoting their destruction. Increased haemolysis leads to splenomegaly, which is seen in 80-85% of PKD cases, and anaemia. Splenectomy is a procedure to remove the spleen and has been shown to partially ameliorate anaemia in the majority of people as well as reduce the transfusion burden. Splenectomy is performed between the ages of 5-18, and can be considered in adults if they are receiving frequent transfusions or experiencing daily symptoms that impact quality of life.^4,5,8

It is important to note that the procedure is only done in children above the age of 5 due to the risk of post-splenectomy sepsis. Additionally, those with severe haemolysis, characterised by low haemoglobin levels, two PKLR mutations, and high bilirubin levels, are least likely to increase their haemoglobin levels after a splenectomy. Haemolysis can also persist following the procedure, contributing to the risk of gallstones. Some cases of sepsis and thrombotic events have also been observed, highlighting the importance of patient selection based on symptom presentation.^4,5

Iron overload management 

Iron overload is a common complication of PKD seen in people of all ages, with the risk of iron loading not changing with age. Iron overload arises primarily through chronic transfusion, but can also be observed in non-transfused people. Iron overload, which is seen in 38% of people with PKD, is defined by ferritin levels greater than 1000 ug/l.⁴

Following 10-14 rounds of transfusion, toxic levels of circulating free iron can be detected. 

In non-transfused people, iron overload is usually caused by increased absorption of iron in the intestines as a result of chronic anaemia and ineffective erythropoiesis (red blood cell generation). Chelation therapy is used to manage iron overload, where medications bind to free iron and promote its removal from the body.^4,5

Monitoring iron levels with magnetic resonance imaging (MRI scan) is recommended, with chelation therapy given in intervals.⁵ 

Disease-modifying treatments

Stem cell transplant

Stem cell transplant is a method that involves replacing a person’s blood-forming stem cells with healthy ones from a donor. This could cure PKD by providing the person with a functional pyruvate kinase enzyme and correcting the severe haemolytic anaemia. However, studies investigating this method have found high rates of morbidity and mortality due to the development of graft-versus-host disease, where the recipient's immune system rejects the donor's stem cells. The high associated risk leads to splenectomy and/or regular transfusion as the recommended intervention.^4,5,8

Pyruvate kinase activators

Mitapivat (AG-348) is a small-molecule oral pyruvate kinase activator that is undergoing clinical trials. Studies have shown that this drug is capable of activating both normal and mutated pyruvate kinase, which increases ATP levels in red cells and enhances the flow of glycolysis. An increase in haemoglobin was also observed in 50% of those treated, with levels sustained over the course of treatment. Additionally, the drug was well-tolerated, with common side effects including headache and nausea, though they were only experienced within the first week of treatment. This highlights the potential of boosting red blood cell machinery to decrease haemolysis.^4,5,8

Gene therapy

PKD arises from defects in a single gene and primarily affects one cell type, making it an appropriate candidate for gene therapy. In vivo studies have shown an increase in enzyme activity, haemoglobin levels, as well as a reduction in reticulocyte count and spleen size following therapy. A correction in the glycolytic pathway and no metabolic disturbances or toxicity led to the enrollment of trials assessing safety in humans, making it an exciting future direction to tackle PKD.^4,5,9

Considerations during pregnancy 

During pregnancy, it has been observed that the degree of haemolysis worsens. This leads to an increase in transfusion needs both during pregnancy and after delivery. Transfusion during pregnancy has been shown to promote normal foetal growth. Folic acid supplementation before conception is also recommended to support erythropoiesis, while vitamins, including iron supplements, should be avoided.^4,5

Summary 

Pyruvate kinase deficiency (PKD) causes chronic hereditary haemolytic anaemia through defects in pyruvate kinase enzyme activity during glycolysis, which leads to premature destruction of red blood cells. PKD is diagnosed by the assessment of clinical signs, pyruvate kinase enzyme activity and PKLR genotyping. Symptoms of PKD commonly include haemolytic anaemia, jaundice, iron overload and splenomegaly. Supportive care strategies to tackle PKD include red blood cell transfusions to maintain healthy red blood cell counts and splenectomy to reduce the transfusion burden and partially ameliorate anaemia. Iron overload is managed through chelation. Disease-modifying agents aim to decrease haemolysis, with potential strategies such as stem cell transplants, pyruvate kinase activators and gene therapy currently under investigation. It is important to note that haemolysis can be exacerbated during acute infections or pregnancy, and transfusion frequency can increase in these situations.