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

VI. Inherited Disorders of RBCs
215
( Giger, 2000 ). Total RBC PK activity is markedly reduced in
deficient cats, with no evidence of the persistent M 2 isozyme
reported in dogs ( Ford et al. , 1992 ), and all PK-deficient cats
identified thus far have had the same mutation ( Giger, 2000 ;
Giger et al. , 1997 ; Mansfield and Clark, 2005 ). Also in contrast
to dogs, osteosclerosis has not been recognized in cats
(Giger, 2000 ).
3 . Cytochrome-β 5 Reductase Defi ciency in
Dogs and Cats
Persistent methemoglobinemia associated with RBC Cb 5 R
deficiency has been recognized in chihuahua, borzoi,
English setter, terrier-mix, cockapoo, coonhound, poodle,
corgi, Pomeranian, toy American Eskimo, cocker-toy
American Eskimo, and pit bull-mix dogs and in domestic
shorthair cats ( Atkins et al. , 1981 ; Fine et al. , 1999 ; Giger
et al. , 1999 ; Harvey, 2000 ; Harvey et al. , 1974, 1991, 1994 ,
Letchworth et al. , 1977 ). The deficiency appears to be an
inherited autosomal recessive disorder, as it is in humans
( Harvey, 2000 ). The nature of the enzyme deficiency is
unknown; however, it does not appear to be the result of an
FAD cofactor deficiency in dogs, because RBC glutathione
reductase (another enzyme that requires FAD as a cofactor)
activities were normal in three affected dogs assayed
( Harvey, 2006 ).
Affected animals have cyanotic appearing mucous
membranes and may exhibit lethargy or exercise intolerance
at times, but they usually have no clinical signs of disease.
Blood samples appear dark, suggesting hypoxemia,
but arterial pO 2 values are normal. MetHb content in dogs
with Cb 5 R deficiency varies from 13% to 41%. The MetHb
content in five deficient domestic shorthair cats varied from
44% to 52% ( Harvey, 2006 ). There is an inverse correlation
between RBC enzyme activity and MetHb content in
deficient dogs ( Harvey, 2000 ). The hematocrit is usually
normal in deficient dogs, but usually slightly to moderately
increased in deficient cats, secondary to the chronic
methemoglobinemia and resultant decreased blood oxygen
content. Animals with Cb 5 R deficiency do not require treatment
and have normal life expectancy ( Harvey, 2006 ).
4 . Glucose-6-Phosphate Dehydrogenase Defi ciency in a
Dog and Horse
G6PD deficiency is a very common X-linked inherited
defect of human RBCs, affecting millions of people worldwide
( Beutler, 1994 ). Smith et al. (1976) screened more
than 3000 dogs for G6PD activity and found one male dog
to have approximately 44% of normal activity. The enzyme
was partially purified and characterized, and it was found
to be similar to that of normal dogs. The deficient dog was
not anemic and exhibited no clinical signs; studies were
not done to determine whether his RBCs were more sensitive
to oxidant damage than normal.
In contrast, a persistent hemolytic anemia and hyperbilirubinemia
have been described in an American standardbred
colt with 1% of normal G6PD activity ( Stockham
et al. , 1994 ). Morphological abnormalities of RBCs included
eccentrocytosis, pyknocytosis, increased anisocytosis,
increased Howell-Jolly bodies, and rare Hb crystals. The
presence of eccentrocytes in the absence of exposure to
external oxidants indicated that the deficient RBCs did not
have adequate metabolic capabilities to defend themselves
against endogenous oxidants. Biochemical abnormalities in
RBCs included low GSH, markedly reduced NADPH, and
increased NADP . RBC catalase activity was normal even
though NADPH concentration was 1% of normal. It was
suggested that catalase activity may have been maintained
by the action of NADH. Polymerase chain reaction amplification
of segments of the G6PD gene of the affected colt
revealed a G to A mutation, converting an arginine codon to
a histidine codon ( Nonneman et al. , 1993 ).
5 . RBC FAD Defi ciency and Resultant Enzyme
Defi ciencies in Horses
RBC FAD deficiency has been recognized in an adult
Spanish mustang mare ( Harvey et al. , 2003 ) and in a
7-year-old Kentucky mountain saddle horse gelding
( Harvey, 2006 ). FAD-deficient horses have persistent methemoglobinemia
(25% to 46%), eccentrocytosis, pyknocytosis,
and variable numbers of Hb crystals. No Heinz
bodies were observed in RBCs stained with new methylene
blue. Hematocrits were normal or slightly decreased. The
presence of eccentrocytes and pyknocytes in the absence
of administered or consumed oxidants indicates deficient
metabolic protection against endogenously generated oxidants.
RBC biochemical abnormalities measured include
decreased Cb 5 R activity (about 40% of normal), decreased
GSH concentration (about 60% of normal), and undetectable
GR activity. The GR activity increased to near-normal
values after addition of FAD to the enzyme assay, indicating
a severe deficiency of FAD in RBCs. FAD is a cofactor
for GR and Cb 5 R enzymes; consequently, both RBC
enzyme deficiencies in these horses can be attributed to
decreased RBC FAD concentrations.
The presence of eccentrocytes and pyknocytes were
attributed to inadequate metabolic protection against endogenously
generated oxidants, resulting from a marked deficiency
in GR and resultant decreased GSH concentration
within RBCs. The methemoglobinemia was attributable to
Cb 5 R deficiency.
Following transport into RBCs, riboflavin is first converted
to flavin mononucleotide (FMN) by riboflavin kinase
and then to FAD by FMN adenylyltransferase. Measurements
of RBC flavin concentrations suggest a defect in the riboflavin
kinase reaction. Systemic signs attributable to a generalized
defect in riboflavin metabolism are absent, suggesting
that the defect may be limited to RBCs.