Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 7 The Erythrocyte: Physiology, Metabolism, and Biochemical Disorders
normal dog RBCs. As expected, the low 2,3DPG concentration
also results in an increased oxygen affinity of Hb in
affected dog RBCs ( Giger and Harvey, 1987 ).
Hematological parameters of affected dogs are similar to
normal dogs at birth, because all newborn dogs have RBC
PFK activities about three times that of normal adult dogs
( Harvey and Reddy, 1989 ). This high PFK activity results
from the presence of the L-type subunit of PFK, which is
negligible in normal adult canine RBCs ( Harvey and Reddy,
1989 ; Mhaskar et al. , 1992 ). Both total PFK activities and
the amounts of L-type subunit present decrease dramatically
during the first 6 to 8 weeks of life. The M-type subunit
is low at birth, but increases as the L-type decreases in
normal dogs. These changes result from the replacement of
RBCs formed in the fetus with those formed after birth.
Deficient dogs generally exhibit less evidence of myopathy
than is observed in PFK-deficient people, probably
because canine skeletal muscle is less dependent on anaerobic
glycolysis than human skeletal muscle, owing to a lack
of the classical fast-twitch glycolytic (type IIB) fibers in
dogs ( Snow et al. , 1982 ). Affected dogs appear to tire more
easily than normal, and in vivo muscle studies of PFK-deficient
dogs indicate altered muscle function in these animals
(Brechue et al. , 1994 ; Giger et al. , 1988b ; McCully et al. ,
1999 ). A severe progressive myopathy with associated abnormal
polysaccharide deposits in skeletal muscle has been recognized
in an aged PFK-deficient dog ( Harvey et al. , 1990a ).
In contrast to PK deficiency, myelofibrosis and liver failure
have not been recognized in dogs with PFK deficiency.
Homozygous affected animals over 3 months of age
can easily be identified by measuring RBC PFK activity.
Heterozygous carrier dogs have approximately one-half
normal enzyme activities in RBCs ( Harvey and Reddy,
1989 ). A DNA test using polymerase chain reaction technology
has been developed that can clearly differentiate
normal, carrier, and affected English springer spaniel dogs
regardless of age ( Giger et al. , 1995 ). This test is also positive
in American cocker spaniel and whippet dogs with PFK
deficiency ( Giger et al. , 1992 ; Hayes et al. , 2007 ).
2 . Pyruvate Kinase Defi ciency in Dogs and Cats
PK deficiency occurs in basenji ( Giger and Noble, 1991 ;
Searcy et al. , 1971, 1979 ), beagle ( Giger et al. , 1991 ;
Harvey et al. , 1977 ; Prasse et al. , 1975 ), West Highland
white terrier ( Chapman and Giger, 1990 ), Cairn terrier
( Schaer et al. , 1992 ), miniature poodle, Chihuahua, pug,
dachshund, and toy American Eskimo dogs ( Giger, 2000 ;
Harvey, 1996 ). PK deficiency is transmitted as an autosomal
recessive trait. Homozygously affected animals have
decreased exercise tolerance, pale mucous membranes,
tachycardia, and splenomegaly.
Affected animals have mild to moderate anemia with
marked reticulocytosis when young ( Harvey, 2006 ).
Myelofibrosis and osteosclerosis develop in the bone marrow,
and hemachromatosis and cirrhosis develop in the liver
as the dogs age ( Searcy et al. , 1979 ; Weiden et al. , 1981 ).
Hematocrit and reticulocyte counts decrease as myelofibrosis
and osteosclerosis become severe ( Whitney and Lothrop,
1995 ). Affected dogs generally die between 1 and 5 years of
age because of bone marrow failure or liver failure ( Giger,
2000 ; Zaucha et al. , 2001 ). It is proposed that the marrow
fibrosis, like the cirrhosis, occurs in response to damage
caused by iron overload ( Zaucha et al. , 2001 ). However, factors
associated with marked erythropoiesis may also contribute
to the development of myelofibrosis ( Bader et al. , 1992 ).
RBCs of affected dogs lack the normal adult R isozyme
of PK but have a persistence of an M 2 isozyme that is
normally present in many fetal and adult tissues, including
erythroid precursor cells, but not in mature RBCs ( Becker
et al. , 1986 ; Black et al. , 1978 ; Whitney et al. , 1994 ).
Consequently, many affected dogs have normal or increased
PK activity, making it difficult to diagnose this defect based
solely on total RBC PK activity. Heterozygous animals
have approximately 50% of normal RBC PK activity.
The enzyme activity in hemolysates of affected dogs is
unstable and decreases rapidly when samples are kept at
room temperature ( Standerfer et al. , 1974 ). If the M 2 -isozyme
is unstable in vivo , as it is in vitro , its rapid loss of activity
would explain the dramatically shortened life span of RBCs
in this disorder ( Dhindsa et al. , 1976 ).
Because the defect in glycolysis occurs below the
diphosphoglycerate shunt, RBCs from PK-deficient dogs
have increased concentrations of 2,3DPG ( Harvey, 2006 ).
As a consequence, the whole blood P 50 is higher than that
of normal dogs ( Dhindsa et al. , 1976 ).
Additional assays (an enzyme heat stability test, measurement
of RBC glycolytic intermediates, electrophoresis
of isozymes, and enzyme immunoprecipitation) may be
used to reach a diagnosis of PK deficiency in dogs in which
the total enzyme activity is not decreased ( Giger and Noble,
1991 ; Harvey et al. , 1990b ; Schaer et al. , 1992 ). The defect
in basenji dogs is the result of a single nucleotide deletion
in the R-type PK gene ( Whitney et al. , 1994 ; Whitney and
Lothrop, 1995 ). Unfortunately, different mutations in the
R-type PK gene have been identified in other dog breeds
( Giger, 2000 ; Skelly et al. , 1999 ). Consequently, different
DNA-based diagnostic assays must be developed or validated
for each affected dog breed. Fortunately, DNA-based
tests for PK deficiency have been developed for several
breeds of dogs ( Giger, 2005 ).
RBC PK deficiency has been characterized in Abyssinian,
Somali, and domestic shorthair cats ( Ford et al. , 1992 ; Giger
et al. , 1997 ; Mansfield and Clark, 2005 ). Affected cats have
intermittent mild to moderate anemia that may be slightly
macrocytic and hypochromic. Reticulocyte counts are
slightly to markedly increased. Splenectomy may reduce the
severity of the anemia in cats. In contrast to dogs in which
the anemia is typically first recognized in young animals,
some cats have not been diagnosed until they were old aged