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

V. Diagnosis
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GROWTH RETARDATION IN PUPS WITH MPS VII
COMPARED TO THEIR NORMAL LITTERMATES
Control Pups (N11)
MPS VII Pups (N10)
0 20 40 60 80
Age in days
FIGURE 24-12 This graph illustrates the growth retardation often seen
in animals with lysosomal storage disorders.
FIGURE 24-13 A normal Siamese cat next to a littermate with MPS
VI. Note the outward manifestations of the skeletal abnormalities: flattened
facies, small size, low posture associated with fusion of the cervical
and lumbar spine.
affected animals, death often occurs at birth or before
weaning.
In humans with LSDs, although frequently no consistent,
specific mutations in some diseases have been
associated with a particular pattern of clinical severity
and progression (genotype-phenotype correlations). Null
mutations that produce little RNA or unstable RNA resulting
in no enzyme protein synthesis (cross-reacting material
[CRM] negative) usually have a severe phenotype.
Although specific mutations have been identified for several
LSDs in animals, there is still not enough information
to be useful in prognosis.
chest, abdomen, CNS, skeleton, and eyes. Laboratory tests
should include a complete blood count with evaluation of
granulocyte and lymphocyte morphology (cytoplasmic inclusions),
skeletal radiographs, and urine screening for abnormal
metabolites, particularly GAGs and oligosaccharides.
Fresh EDTA blood or fresh-frozen serum can be used to
assess lysosomal enzyme activities. Establishing a fibroblast
culture from a skin biopsy and a fresh-frozen liver
biopsy may be helpful for further biochemical analyses.
The disease may progress quickly, and the diagnosis may
only follow a complete postmortem examination.
A pedigree analysis should be performed as part of the
history to determine information about the inbreeding of
the parents and the presence of other family members with
similar clinical signs or that died early. As most LSDs are
inherited as autosomal recessive traits, parents are often
related and are carriers (heterozygotes) but are clinically
(phenotypically) normal. On average, one-fourth of the
offspring of heterozygous parents are affected, two-thirds
of unaffected offspring are carriers, and other relatives may
also be affected ( Fig. 24-14 ).
Abnormal metabolites may be found in urine and their
presence points toward specific metabolic pathways that
warrant further evaluation. A metabolic screen of urine
( Fig. 24-15 ) ( Giger and Jezyk, 1992 ; Jezyk et al. , 1982 ) for
GAGs is a relatively simple and inexpensive approach to
identify the mucopolysaccharidoses and some cases of gangliosidosis
(toluidine blue or MPS spot test; Fig. 24-16 ).
Thin layer chromatography of urinary oligosaccharides is
helpful to identify mannosidosis. Urine samples to be evaluated
should be kept refrigerated or frozen and sent to an
appropriate laboratory.
1
A final diagnosis for LSDs requires the demonstration
of a particular enzyme deficiency by either determining the
lack of enzyme activity or a disease-causing mutation in
an enzyme gene; these tests do not only identify affected
animals but are also helpful in identifying carriers. Enzyme
assays using artificial substrates can usually be performed
on serum, white blood cells, cultured fibroblasts, or liver.
Generally, there is a profound deficiency in activity of the
enzyme, making the diagnosis straightforward. In addition,
the activities of other lysosomal enzymes in the cells or tissues
are frequently higher than normal. The biochemical
status of the clinically normal parents should be evaluated
when possible. In an autosomal recessive disease, heterozygous
parents are expected to have half-normal activity
of the enzyme in question because each parent carries
one normal and one mutant allele. Although in a population,
heterozygotes (carriers) have on average half-normal
V . DIAGNOSIS
The approach to a diagnosis of an LSD includes a complete
history and physical examination with evaluation of the
1
One such laboratory is the Metabolic Screening Laboratory, Section of
Medical Genetics, Veterinary Hospital of the University of Pennsylvania,
3900 Delancey Street, Philadelphia, PA 19104-6010. A complete history
including signalment of the animal should be included with the samples
( http://www.upenn.edu/research/penngen ).