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

VI. Therapy
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systems. Liver should be frozen as quickly as possible for
subsequent determination of accumulated substrate, enzyme
activities, and RNA and DNA analyses. Fibroblast cultures
can be established from skin, linea alba, or pericardium (using
sterile technique) for future studies that may require living
cells. Liver, brain, and other tissues should be preserved in
formalin, and samples should be taken for thin section and
electron microscopy (in glutaraldehyde-paraformaldehyde),
particularly from the liver and CNS.
VI . THERAPY
URINE SPOT TEST FOR
GLYCOSAMINOGLYCANS
NORMAL
MPS AFFECTED
FIGURE 24-16 The results of a urine spot test detecting abnormal
amounts of glycosaminoglycans in the urine of animals with MPS.
Enzyme activity
60
50
40
30
20
10
0
10
MEAN SERUM ALPHA-MANNOSIDASE ACTIVITY
±2 S.D. IN A COLONY OF RELATED CATS
Affected Heterozygote Normal
FIGURE 24-17 Serum alpha-mannosidase activity of a colony of cats
illustrating the overlap that exists between normal and heterozygous animals.
Although heterozygote detection is possible in a population, detection
is difficult for an individual. Molecular techniques overcome these
difficulties in carrier detection when the mutation is known.
The combination of secretion of lysosomal enzymes by
cells and uptake of enzymes by diseased cells via the M6P
receptor system forms the basis for the present approaches
to therapy for the LSDs. Providing a source of normal
enzyme to abnormal cells will permit that enzyme to be
taken up by the plasma membrane receptor, resulting in
delivery of the normal enzyme to the lysosome where it
can catabolize stored substrate (except for mucolipidoses).
Fortunately, the amount of enzyme needed in the lysosome
for sufficient function and, thus, phenotypic correction of
an individual cell is only a small proportion of normal. The
three approaches to providing normal enzyme to a patient’s
cells are (1) enzyme replacement therapy (ERT), (2) bone
marrow transplantation (BMT), and (3) gene therapy. In
general, the most difficult target tissue in the LSDs is correction
of the CNS lesions. Approximately 60% of LSDs
have a CNS component, for which systemic therapy is
limited by the blood-brain barrier. Successful treatment
of the neuropathic LSDs will require direct therapy to the
CNS or systemic therapy that crosses the blood-brain barrier.
Animal models have been used extensively to evaluate
these approaches to therapy for LSDs in humans. Although
these novel therapies could be adapted for domestic animals,
supportive care is generally used in clinical practice
and emphasis is placed on prevention of the production of
affected animals in future generations.
A . Enzyme Replacement Therapy (ERT)
The efficacy of the parenteral injection of purified recombinant
enzyme has been tested in various animal models of LSDs,
including MPS VII mice, MPS I dogs and cats, MPS VI
cats, and glycogen storage disease in Japanese quail
(Ellinwood et al. , 2004 ; Haskins et al. , 2002 ). In knockout
mice, enzyme derived from rabbit milk or from Chinese
hamster ovary cells has been shown to be useful ( Ioannou,
2000 ; Kakkis et al. , 1996, 2001 ; Wraith, 2001 ). Today, ERT
by intravenous infusion is the standard therapy for non-neuronopathic
Gaucher disease in human patients and is available
or under evaluation for the treatment of Fabry disease,
Pompe disease, MPS I, MPS II, and MPS VI.
B . Bone Marrow Transplant (BMT)
Heterologous BMT as therapy for LSDs has been performed
for decades (reviewed in Brochstein [1992] ,
Haskins et al. [1991] , Hoogerbrugge and Valerio [1998 ],
Krivit et al. [1999] , and O’Marcaigh and Cowan [1997] ).
This approach provides both normal bone marrow and
bone marrow-derived cells, which are available to release
enzyme continuously for uptake by other deficient cells. In
addition, some monocyte-derived cells can cross the bloodbrain
barrier, becoming microglia and secreting an enzyme
that can be available to neurons. BMT in animal models of
LSDs has been carried out in MPS VII mice, mannosidosis,
and mucolipidoses II cats, GM2 gangliosidosis mice, MPS
VI cats, and the MPS VII dog, among others ( Haskins,
1996 ; Haskins et al. , 1991 ). A combination of neonatal
ERT followed by BMT at 5 weeks of age in MPS VII mice
had positive long-term effects ( Sands et al. , 1997 ).