Hereditary disorders may affect any blood cells or plasma proteins. For many hereditary blood diseases, the biochemical basis has been elucidated; for some, the specific molecular genetic defect has recently been identified. In fact, erythrocytic pyruvate kinase deficiency was the first biochemical defect characterized in companion animals, and hemophilia B was the first molecular defect determined in domestic animals. On the other hand, the precise cause of cyclic hematopoiesis in Gray Collies still needs to be discovered despite extensive research over the past 30 years. Overall, the study of these hereditary blood disorders has greatly contributed to the better understanding of blood cells and protein functions.

Hereditary blood disorders may be classified into erythrocytic defects, bleeding disorders, and immunodeficiencies, although some overlap exists. Several hereditary disorders not only affect a specific function of the hematopoietic system, but also involve other organs such as bone, muscle, and hair coat, thereby forming characteristic clinical syndromes such as chondrodysplastic Alaskan malamute dwarfs with stomatocytosis. Finally, an emerging group of genetic predispositions to infections, immune-mediated, neoplastic diseases are being recognized. Some primary immunodeficiencies (Leukocyte adhesion deficiency in Irish setters) are well defined, whereas for others only a breed predilection to a particular infection suggests a genetic basis, for instance avian tuberculosis in Bassets and leishmaniasis in Foxhounds. Similarly, breed predispositions have been recognized for certain hematologic immune-mediated diseases (immune-mediated hemolytic anemia in Cocker Spaniels) and hematopoietic cancers ( lymphoma in Golden retrievers and malignant histiocytosis in Bernese Mountain dogs) without knowing the molecular basis.

With few exceptions, hereditary blood disorders are autosomal recessively inherited; hemophilia A and B in many breeds of dogs and cats and severe combined immunodeficiency in Bassets and Corgis represent the only X-chromosomal recessive traits. Each defect occurs relatively rarely, although certain breeding practices, popular sire, and founder effects may result in widespread and common occurrences. The true mutant gene frequency has rarely been determined. to limit the further spread of these hereditary disorders, it is pivotal to not only recognize affected animals, but also carriers that can pass on the mutant gene. Accurate biochemical and molecular genetic screening tests have been developed for many diseases, but their application in companion animals has lagged behind.

Therapeutic options for hereditary blood diseases are limited. Animals with primary immunodeficiencies may benefit from antibiotics, albeit their response may only be partial or transient. Plasma transfusions may supplement bleeding animals with coagulopathies. Experimental allogeneic bone marrow transplantation can correct defective blood cells and gene therapy has been attempted. However, it is much more important to control further spread of the disease causing gene by screening breeding animals when tests are available.

Rather discussing all forms of blood diseases this session will concentrate on a few hereditary disorders that cause anemia in companion animals and have been characterized from the clinical signs to the molecular defect.

Hereditary anemias

Although acquired causes, such as infections, immune disorders, intoxication, blood loss, and chronic organ failure, are the main causes for anemia, hereditary blood disorders leading to anemia are also important in clinical practice. They are particularly germane differential diagnoses in animals with Coombs' negative hemolytic anemias without apparent cause. Several hereditary erythrocytic defects have been reported in companion animals, and much new information has emerged over the past decade. In fact, some anemias have been so extensively characterized that the clinical signs to the molecular basis of the erythrocyte defect are known, thereby offering an opportunity to make a precise diagnosis in clinical practice and to prevent these disorders in future generations.

Inherited erythrocyte defects form a large heterogeneous group of diseases. Each erythrocyte disorder is observed only rarely, although a particular defect may occur frequently within a family or breed. If the same disorder is recognized in several breeds, it is likely caused by different mutations of the same gene. The mode of inheritance is autosomal recessive, with the exception of feline porphyria which is inherited by a dominant trait. These disorders have been classified into four groups: 1) heme defects and hemoglobinopathies, 2) membrane abnormalities, 3) cytosolic enzyme deficiencies, and 4) production and maturation defects (Table 1).

In contrast to the common occurrence of such hemoglobin disorders as thalassemia and sickle cell anemia in humans, no hemoglobinopathies have been documented in dogs and cats. Isolated cases of methemoglobinemia associated with methemoglobin/cytochrome b₅ reductase deficiency were found among dogs of various breeds and domestic shorthair cats, but this deficiency results in polycythemia rather than anemia. Defects of heme synthesis known as porphyrias have been reported in anemic Siamese and domestic shorthair cats with pigmented and pink-fluorescent teeth and bones.

Elliptocytosis and microcytosis resulting from a deficiency of the protein band 4.1, which strengthens the interaction between spectrin and actin in the cytoskeleton, has been characterized at the molecular level in an inbred, nonanemic mongrel dog. Other presumed membrane abnormalities include: stomatocytosis in Alaskan malamutes, a Dutch breed with gastritis, and miniature Schnauzers; nonspherocytic anemia in Beagles; and increased osmotic fragility of erythrocytes in an English Springer spaniel and Abyssinian and Somali cats.

Deficiencies of the two key regulatory glycolytic enzymes result in distinctly different forms of hemolytic anemia. The classic pyruvate kinase (PK) deficiency initially reported in Basenjis is now seen in several other canine breeds and in cats. Phosphofructokinase (PFK) deficiency is frequently reported in English Springer spaniels and has also been observed in a Cocker spaniel and a mixed-breed dog.

Whereas the previous defects described result in shortened erythrocyte survival and regenerative anemias, the production and maturation disorders cause a nonregenerative anemia with changes in other bone marrow-derived cells. Cyclic hematopoiesis of gray collies is the classic example. Selective cobalamin (vitamin B₁₂) malabsorption has been reported in giant schnauzers, Beagles, Border collies and cats. These hematopoietic production and maturation abnormalities will not be further discussed.

CLINICAL SIGNS

The clinical features of hemolytic anemia vary among erythrocyte defects, ranging from severe hemolytic crises to well-compensated hemolysis without clinical signs. Splenomegaly may result from severe extravascular hemolysis and extramedullary hematopoiesis and hemosiderosis. Furthermore, erythrocyte defects may be part of a multisystemic syndrome caused by the pleiotropic effects of a single mutant gene as in the case of, for instance, chondrodysplastic Alaskan malamute dwarfs with stomatocytosis.

Table 1. Hereditary anemias caused by erythrocyte defects in companion animals

N, normal; U, unknown; AR, autosomal recessive; AD, autosomal dominant; OF, osmotic fragility.

LABORATORY DIAGNOSIS

Although the type and severity of the anemia, the signalment involved, and pleiotropic effects observed may provide clues to the identity of an inherited erythrocyte defect, a full laboratory evaluation is essential to validate the diagnosis or to define new inherited disorders. In fact, in a few breeds more than one erythrocyte disorder has been recognized. Routine laboratory tests are used to detect hematologic abnormalities and to rule out acquired anemia. An inherited erythrocyte defect should be considered in animals with a Coombs negative hemolytic anemia, a lack of evidence of toxin exposure or infection, and adequate kidney as well as liver function. A careful examination of a peripheral blood smear is crucial for recognizing poikilocytes, such as spherocytes, elliptocytes and stomatocytes, although most erythrocyte defects cause no change in cell shape. The degree of reticulocytosis is often marked, but due to chronicity and a low grade of hemolysis, signs of hemolysis may be mild. Bilirubinuria and bilirubinemia are generally noted. Some defective erythrocytes appear extremely fragile in vitro, resulting in artificial lysis in blood tubes.

Special laboratory tests used to define the nature of an intrinsic erythrocyte defect can be divided into general screening tests, used to characterize unknown erythrocyte disorders, and specific screening tests for known defects. Both are only performed in specialized laboratories (e.g., Josephine Deubler Genetic Disease Testing Laboratory for Companion Animals at the School of Veterinary Medicine, University of Pennsylvania http://www.vet.upenn.edu/penngen).

Erythrocyte osmotic fragility test, membrane protein electrophoresis, and ion transport studies are used to characterize membrane defects. Various hemoglobin separation methods and analysis of shape changes in the hemoglobin-oxygen dissociation curve may be useful for discovering new hemoglobinopathies. Finally, cytosolic enzyme deficiencies can be identified by demonstrating a decrease in the activity of an enzyme, an absence of immunologic cross-reacting material, abnormal enzyme kinetics, accumulation of enzyme substrates, or a lack of enzyme derived products. [For instance, erythrocyte DPG concentration is decreased in PFK deficiency but increased in PK deficiency in dogs.] Unfortunately, these tests are time consuming as well as technically demanding, and require specific handling and shipping as well as the submission of a control sample from a healthy animal.

More recently, molecular genetic screening tests have become available to identify erythrocyte defects caused by specific mutations. Because these tests are mutation-specific, they are generally also breed specific; that is, the same enzyme deficiency in various breeds may be caused by different mutations. These DNA-based tests are also most valuable to identify carriers (heterozygotes).

THERAPY AND PREVENTION

Hemolysis resulting from erythrocyte defects may be well compensated by marked erythropoiesis, thereby causing no or only minimal clinical signs and allowing the animal to have a normal life expectancy. Furthermore, affected animals may have adapted well to the chronic anemia. In contrast, other disorders are associated with severe hemolytic crises for which animals may need to receive supportive therapy including blood-typed, compatible transfusions. Cats with increased erythrocytic osmotic fragility or PK deficiency have marked splenomegaly and may be helped by splenectomy due to the removal of a major site of erythrocyte destruction. PK- and PFK-deficient dogs do not appear to benefit from this procedure, however. Experimentally, allotransplantation of bone marrow has been shown to correct the above enzymopathies in dogs. Furthermore, hemolytic crises in PFK-deficient dogs may be prevented by avoiding panting, strenuous exercise, and heat. Finally, affected and carrier animals should not be used for breeding to prevent the further spread of these disorders.

Phosphofructokinase (PFK) Deficiency

This glycolytic enzyme deficiency is common in field trial English Springer spaniels in the United States, Great Britain, and Denmark, but has also been reported in bench English Springer spaniels, a Cocker spaniel and mixed-breed dogs. It is caused by a missense mutation of the muscle-type PFK which results in truncation and instability of the enzyme, thereby leading to a complete muscle-type PFK deficiency.

The disorder is characterized by hemolytic crises and exertional myopathy. Sporadic dark pigmenturia resulting from severe hemoglobinuria and bilirubinuria is a key feature and commonly develops after episodes of excessive panting and barking, extensive exercise, and high temperature. Hyperventilation-induced alkalemia results in intravascular lysis of PFK-deficient erythrocytes. During these crises, affected dogs may become severely anemic and icteric, and show fever, lethargy, and anorexia which usually resolve within days. Situations triggering hemolytic crises should be avoided. PFK-deficient dogs may reach a normal life expectancy but have persistent bilirubinuria and reticulocytosis despite a normal hematocrit because of the high hemoglobin oxygen affinity of PFK-deficient erythrocytes. Furthermore, because affected dogs totally lack PFK activity in muscle, they have a metabolic myopathy characterized by exercise intolerance, occasional muscle cramps, and mildly increased serum creatine kinase activity, thus, they will perform poorly as field trial dogs. A simple polymerase chain reaction (PCR)-based DNA test accurately diagnoses PFK-deficient and carrier dogs. English Springer and Cocker spaniels with suspicious signs should be screened for PFK deficiency before field trial training and breeding.

Pyruvate Kinase (PK) Deficiency

Although PK deficiency was first characterized in the Basenji breed, the clinical features and biochemical abnormalities appear very similar in other canine breeds (Beagles, miniature poodles, Eskimo Toy, Dachshund, Chihuahua, Pug). Despite the severity of the anemia, the clinical signs, except for pallor, are mild. The anemia is highly regenerative with numerous circulating metarubricytes and reticulocyte counts up to 90%. An unexplained progressive myelofibrosis and osteosclerosis of the bone marrow and generalized hemosiderosis with associated hepatic failure develop, causing death usually at a few years of age. Erythrocytes completely lack the adult erythrocyte isozyme form of PK known as R-PK. Instead, they express a fetal M-PK form that is also present in the spleen and white blood cells. However, M-PK appears unstable and malfunctions in erythrocytes in vivo as shown by the shortened erythrocyte survival and abnormal erythrocyte metabolite pattern. The molecular genetic basis of PK deficiency has been identified in Basenjis, West Highland White terriers, Beagle, and Dachshund, and PCR-based tests are available for these breeds. Work is in progress to characterize the molecular defects in the other known breeds. Thus, a cumbersome PK-enzyme test with isozyme characterization is required to define PK deficiency in other breeds. Carriers do not express the M-PK form and have half normal PK activity; however, differentiation between carriers and homozygous normal dogs based upon enzyme activity can be difficult.

In cats, PK deficiency causes intermittent anemia with a moderate regenerative response, but cats do not develop an osteosclerosis. Cats have splenomegaly, and splenectomy appears to ameliorate the clinical signs of intermittent anemia, the oldest such cat reaching 11 years of age. Erythrocyte PK activity is severely reduced and there is no M-type PK expression, thereby simplifying the diagnosis. Furthermore, a molecular screening test for PK deficiency in Abyssinian and Somali cats has recently been developed and is currently available.

Recently, a marked osmotic fragility of erythrocytes associated with intermittent anemia, severe splenomegaly, and hyperglobulinemia has been observed in Abyssinian and Somali cats and therefore could be confused with PK deficiency. Although the cause has not been identified, affected cats with marked splenomegaly may benefit from prednisone treatment and splenectomy. However, the osmotic fragility of erythrocytes in vitro does not appear to improve.