Pyruvate kinase deficiency in Abyssinian and Somali cats

©Kohn, B (2008) Clinical course of pyruvate kinase deficiency in Abyssinian and Somali cats. JFMS 10: 145-153

Erythrocytes generate energy almost exclusively through anaerobic glycolysis. Pyruvate kinase (PK) is one of the key regulatory enzymes of this metabolic pathway and catalyses the transformation of phosphoenolpyruvate to pyruvate, the last energy providing step of the anaerobic glycolysis. Pyruvate kinase is also important in glycolysis, converting phosphoenolpyruvate to pyruvate, the input for aerobic (TCA cycle) respiration. Four homotetramer isoenzymes of PK coded by two different genes (PKLR and PKM) are expressed depending on the developmental stage and tissue. Specific isoenzymes are present in the liver (L type), muscles, or brain (M₁), spleen, leukocytes and thrombocytes (M₂), or erythrocytes (R). A deficiency of the R-PK isoenzyme leads to energy deprivation within the red blood cells, resulting in a shortened survival time (haemolysis).

In humans, PK deficiency is one of the most common hereditary forms of haemolytic anaemia caused by enzymopathy. Until now, more than 180 different mutations have been identified. PK deficiency is transmitted as an autosomal recessive trait. PK deficiency has also been described in several canine breeds such as Basenji, West Highland White Terrier, Beagle, Miniature Poodle, Chihuahua, Pug and Daschund. As measurement of the enzyme activity is diagnostically not useful in dogs, a PCR-based PK test was developed for several breeds.

In 1992, the first case of feline PK deficiency was described in the United States in an Abyssinian male cat. Since then, the disease was identified in other Abyssinians, Somalis and a few domestic shorthair cats in the UK, Australia and Europe. Erythrocytic PK activity was severely decreased in affected cats. Asymptomatic carriers expressed approximately half normal PK activity. Several years ago, the molecular defect was identified as a splicing defect at the 3'-end of exon-6 causing a 13-base pair deletion. Thus, a molecular screening test for PK deficiency in Abyssinian and Somali cats could be developed. 

Diagnosis

PK deficiency can be asymptomatic in cats, can cause a chronic anaemia, or haemolytic crises can occur intermittently. Only in 14 of 25 affected cats  did the owners notice signs of disease, usually starting at an age of less than 3 years. Stressful situations, such as infectious diseases, parturition or exhibitions might cause a sudden deterioration of the clinical picture by non-specific activation of the macrophage system, damage of erythrocytic membranes, or suppression of haematopoiesis. The authors of the case study from Australia suspected an infection with M. haemominutum and a cholangiohepatitis as the triggering factors for the haemolytic crisis.

As PK deficiency can be asymptomatic in cats, testing of Abyssinian and Somali cats used for breeding is strongly recommended.

Even though erythrocytic PK deficiency results in an energy deprivation and thus a shortened survival time of RBCs, some cats seem to be able to compensate the accelerated breakdown of red blood cells. In many cats with PK deficiency, there is a mild to severe reticulocytosis. Despite these increased aggregated reticulocyte counts, some are not anaemic, which indicates an increased and efficient turnover of RBCs.

Differential diagnosis

For differential diagnoses of haemolytic anaemia in cats, infectious diseases (e.g. haemoplasmosis and FeLV) or immune-mediated haemolysis (e.g. autoimmune haemolytic anaemia, Chediak-Higashi syndrome) have to be considered. Haemolytic anaemia cause by PK deficiency is often incorrectly  interpreted as immune-mediated haemolysis. The direct Coombs' test can often be negative in these cases. Moreover, haemolysis can be caused by toxins or chemicals or mechanical damage of the RBC. Increased osmotic fragility of RBCs is an important differential diagnosis for PK deficiency, as this disease has been described in Abyssinian and Somali cats and has a similar clinical picture. As both diseases are hereditary, clinical signs caused by chronic haemolytic anaemia or by haemolytic crises may be noted at a young age. Thus far, increased osmotic fragility of RBCs has only been diagnosed in the United States and in one cat from Switzerland. A hereditary membrane defect has been suggested as a possible cause. In contrast to cats with increased osmotic fragility of RBCs, the mean erythrocytic osmotic fragility was within the reference range in most cats suffering from PK deficiency. Therefore, the osmotic fragility test can be useful to differentiate between these two diseases and may also serve to distinguish from immune-mediated haemolysis or haemoplasmosis.

Clinical pathology

Only few data on plasma biochemistry values of cats suffering from PK deficiency are available in the literature. In 40% of diagnosed PK-deficient cats, plasma bilirubin was increased moderately to severely. It was increased slightly in another four cats. Similarly, in two of three American cats with PK deficiency, serum concentrations of bilirubin were only mildly increased, indicating an efficient compensation of the accelerated breakdown of RBCs. The Australian cat displayed severe hyperbilirubinemia and a cholangitis was diagnosed. Moreover, bilirubin cholelithiasis and the possibility of an extrahepatic bile duct obstruction, which is a frequent complication of haemolysis in man, should be investigated if PK-deficient cats present with jaundice.

Hyperglobulinemia was present in 12 of the 15 cats. Increased globulin concentrations can also be found in cats suffering from increased osmotic fragility of RBCs or immune-mediated haemolytic anaemia. Hyperglobulinemia can indicate a chronic stimulation of the immune system, but was not described in humans or dogs with PK deficiency. Six of the 15 cats displayed mildly, one cat moderately increased activities of liver enzymes. Hypoxia of the liver caused by severe anaemia or a circulatory failure can damage hepatocytes. Increased values of liver enzymes have also been described in cats with immune-mediated haemolytic anaemia or increased osmotic fragility of RBCs. Moreover, chronic haemolysis is capable of causing significant iron overload. Liver iron concentration over a critical threshold leads to the formation of radicals. Increased redox-active iron present in hemoproteins and cytosolic iron pool catalyses oxidative damage to lipids, proteins and nucleic acids. Iron-catalysed injury results in damage to cell constituents, including mitochondria, lysosomes, and the sarcolemmal membrane. Thus, chronic haemolysis can lead to hepatocellular damage and finally cirrhosis of the lver. This has been described in humans and dogs with PK deficiency. In the Australian cat in this study, the liver showed histologically extramedullary haematopoiesis and moderate bile duct proliferation, with portal areas showing mild lymphocytic accumulations.

Mild to moderate splenomegaly has been described in cats suffering from PK deficiency. Histopathological examination of the spleen of two affected Abyssinian cats showed distinct extramedullary haematopoiesis and hemosiderosis caused by chronic haemolysis.

Treatment

 Therapeutic options are limited. Experimentally, bone marrow transplantation has been described in dogs. In affected cats, splenectomy can be performed in cases of recurrent haemolytic crises or severe splenic enlargement, which can restrict the expansion of the stomach and cause inappetence. The objective of a splenectomy is to remove one major site of RBC phagocytosis. In one Abyssinian cat, the hematocrit stabilised after splenectomy. This cat survived at least 10 years. A stabilisation of hematocrit  and improvement of the general condition were noted in two other cats after splenectomy. Splenectomy is also a therapeutic option in humans suffering from severe transfusion-dependent PK deficiency, hereditary spherocytosis or immune-mediated haemolytic anaemia.

As PK deficiency is often misinterpreted as immune-mediated haemolysis or haemoplasmosis, the cats are frequently treated with prednisolone or antibiotics. Positive effects of glucocorticoids might be due to a membrane stabilising effect and due to inhibition of the mononuclear phagocytic system, thus preventing or delaying phagocytosis of damaged RBCs. A transient or partial response was found in some cats.

To prevent haemolytic crises, stressful situations should be avoided for PK-deficient cats. Blood transfusions can represent a life saving measure during severe haemolytic crises if the hematocrit value decreases below 0.10 - 0.15 L/L.