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* Edward A. Doisy Department of Biochemistry and Molecular Biology
and Contributed by William S. Sly, December 28, 2000
Mucopolysaccharidosis type VII (MPS VII; Sly syndrome) is an
autosomal recessive lysosomal storage disorder due to an inherited deficiency of Mucopolysaccharidoses (MPS)
are a subgroup of lysosomal storage diseases that result from
deficiencies of specific lysosomal enzymes involved in the stepwise
degradation of glycosaminoglycans (GAGs) (for
review, see reference 1). They are characterized by progressive
intralysosomal accumulation of undegraded GAGs that eventually lead to
cellular and organ dysfunction. MPS type VII (Sly syndrome) results
from deficiency of Although MPS VII was one of the last human MPS disorders identified
(2), it was one of the first whose catalytic defect was ascribed to an
already well characterized enzyme (3, 4), and the first for which a
mouse model was identified (6-8). Murine MPS VII was shown to result
from homozygosity for the autosomal recessive, naturally occurring 1-bp
deletion in exon 10 of the gus structural gene, referred to
as the gusmps mutation (9). The MPS VII
(gusmps/mps) mice have been studied very
extensively and are found to have morphologic, genetic, and biochemical
characteristics which closely mimic those of human MPS VII. Affected
mice have facial dysmorphism, growth retardation, deafness, behavioral
deficits, and shortened lifespan. They show widespread storage of GAGs
in lysosomes of visceral organs, skeleton, and brain. The murine model
of MPS VII has been widely used for evaluating the effectiveness of
bone marrow transplantation (10-14), enzyme replacement (15-22), and gene therapy with retroviral (23-28), adenoviral (29-33), and
adeno-associated viral vectors (34-39).
As valuable as the murine model for MPS VII has been, we felt that
introducing a transgene expressing inactive human GUS might give it
considerable added value if it conferred immune tolerance to human GUS
on the MPS VII mouse. Immune tolerance to human GUS would allow
preclinical trials in mice with the form of the human enzyme (or human
gene) intended for administration to human patients. The goal would be
to prevent formation of antibodies to human GUS that might be expected
to alter the targeting and tissue distribution of infused enzyme, or
inhibit catalytic activity of the therapeutic enzyme. Such antibodies
often blunt or abrogate therapeutic responses to enzyme therapy (40)
and gene therapy (41) and can force cessation of therapy by producing
life-threatening reactions (18).
In prior work, we characterized the structure and active site of human
GUS, and identified residue E540 as the active site nucleophile (42,
43). Expression of the cDNA containing the E540A missense mutation in
COS cells or insect cells produced an inactive protein with normal
turnover and stability (43). These studies suggested that a transgene
expressing human GUS E540A on the MPS VII
(gusmps/mps) background might provide a
murine MPS VII model that retains the MPS VII phenotype, but has the
added desirable feature of being immune tolerant to human GUS. We
reasoned that such a model would be a great asset for evaluating the
benefits of enzyme therapy with human GUS or gene therapy using the
human GUS cDNA.
Construction of the Transgenic Mouse.
The 2.2-kb human GUS E540A cDNA (43, 44) was cloned into a
pBluescript KS+ vector between the 0.54-kb mouse phosphoglycerate kinase (PGK) promoter and a 3' element including the rabbit
Applied Biological Sciences
Active site mutant transgene confers tolerance to human
-glucuronidase without affecting the phenotype of MPS VII mice
,
,
Department of Pathology, Saint Louis University
School of Medicine, St. Louis, MO 63104
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Abstract
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-glucuronidase. A naturally occurring mouse model for
this disease was discovered at The Jackson Laboratory and shown to be
due to homozygosity for a 1-bp deletion in exon 10 of the
gus gene. The murine model MPS VII
(gusmps/mps) has been very well
characterized and used extensively to evaluate experimental strategies
for lysosomal storage diseases, including bone marrow transplantation,
enzyme replacement therapy, and gene therapy. To enhance the value of
this model for enzyme and gene therapy, we produced a transgenic mouse
expressing the human
-glucuronidase cDNA with an amino acid
substitution at the active site nucleophile (E540A) and bred it onto
the MPS VII (gusmps/mps) background. We
demonstrate here that the mutant mice bearing the active site mutant
human transgene retain the clinical, morphological, biochemical, and
histopathological characteristics of the original MPS VII
(gusmps/mps) mouse. However, they are now
tolerant to immune challenge with human
-glucuronidase. This
"tolerant MPS VII mouse model" should be useful for preclinical
trials evaluating the effectiveness of enzyme and/or gene therapy
with the human gene products likely to be administered to human
patients with MPS VII.
![]()
Introduction
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-glucuronidase activity (2).
-Glucuronidase
(
-D-glucuronoside glucuronosohydrolase EC.3.2.1.31),
abbreviated GUS, is a tetrameric glycoprotein acid hydrolase localized
primarily in lysosomes and found in virtually all mammalian cells (for
review, see refs. 3 and 4). It acts in lysosomes as an exoglycosidase
to remove glucuronic acid residues from the nonreducing termini of
GAGs. Many different mutations have been found in the GUS
gene in patients with MPS VII, accounting for the considerable clinical
variability among patients with MPS VII (5).
![]()
Materials and Methods
Top
Abstract
Introduction
Materials and Methods
Results
Discussion
References
-globin intron 1 and the SV40 poly(A) signal. The transgene, including the PGK
promoter, cDNA, and 3' element, was removed by digestion with
Xho and NotI, gel isolated, and injected into
male pronuclei of C57BL/6J eggs as described (45). Of 42 pups born
from injected zygotes, six contained the human transgene identified by
PCR of tail DNA, in 1-4 copies as estimated by Southern blot analysis of tail DNA. Two low copy number male founders were crossed with C57BL/6 females, and progeny tested for tolerance to immunizing human
-glucuronidase by ELISA as described below. One (founder 55)
produced tolerant progeny, and the human GUS E540A transgene was placed onto the B6 MPS VII (gusmps)
background by additional crosses to produce a line that was C57BL/6
Tg hGUS E540A/Tg hGUS E540A,
gusmps/+. The colony was maintained by
brother-sister matings of this genotype, and 20% of the offspring
were gusmps/mps homozygotes that showed the
MPS VII phenotype by 28 days of age.
Lysosomal Enzyme Assays. Lysosomal enzymes were assayed fluorometrically by using 4-methylumbelliferyl substrates, as described (45-47). Tissues were dissected and homogenized immediately (by Brinkmann Polytron homogenizer for 30 sec at 4°C) in 5 vol of homogenization buffer (25 mM Tris·HCl, pH 7.2, 140 mM NaCl, 1 mM PMSF). Total homogenate was diluted appropriately for assay in PBS, the final dilution in an equal volume of citrate phosphate buffer, pH 4.4, containing 0.075 M NaCl, 1.0 mg/ml human serum albumin, and 0.001% Triton X-100. Assays on dilutions of wild-type tissue extracts were for 30 min, and for MPS VII/E540ATg extracts, 24 h. Units were nmol hydrolyzed per hour, and activity was expressed as u/mg protein, determined by microlowry assay (47).
Pathology. Multiple tissues from six MPSVII/E540ATg mice from 3-9 months of age were studied morphologically as previously described (7). Tissues were evaluated for the extent of lysosomal storage and compared with those previously described in the MPS VII (gusmps/mps) model. The skeletons of a transgenic and an unaffected 6-month-old mouse were radiographed as previously described (7).
Immunization Method and Analysis of Sera from Immunized Mice by ELISA. Four MPS VII (gusmps/mps) and four MPS VII/E540ATg mice were immunized with purified human GUS beginning at 2 months of age. Each mouse received 50 µg human GUS in 0.2 ml complete Freund's adjuvant intraperitoneally as an initial challenge, and two subsequent boosts with 50 µg human GUS in 0.2 ml of incomplete Freund's adjuvant intraperitoneally (the first boost at 28 days and the other at 42 days after the initial challenge). Blood was collected by eye bleed to measure antibodies to human GUS by ELISA 12 days after each boost.
Analysis of sera from immunized mice was done by ELISA assay on microtiter aliquots. The wells of 96-well microtiter plates were coated overnight at 4°C with 10 µg/ml purified recombinant human
-glucuronidase in 15 mM
Na2CO3, 35 mM
NaHCO3, 0.02% NaN3, pH
9.6. The wells were washed three times with TBST (10 mM Tris, pH 7.5, 150 mM NaCl, 0.05% Tween 20), then blocked for 1 h at room
temperature with 3% casein in PBS (pH 7.2). After washing three times
with TBST, 100 µl of serial 10-fold dilutions of mouse plasma
(10
2-10
8) in TBST were
added to the wells and incubated at 37°C for 2.5 h. The wells
were washed four times with TBST, then 100 µl of TBST containing a
1:500 dilution of peroxidase conjugated goat anti-mouse IgG was added
to the wells and incubated at room temperature for 1 h. The wells
were washed three times with TBST and two times with TBS (10 mM Tris,
pH 7.5, 150 mM NaCl). Peroxidase substrate (ABTS solution, Roche
Molecular Biochemicals) was added (100 µl per well) and plates were
incubated at room temperature for 10 min. The reaction was stopped with
the addition of 2.5 µl of 20% SDS and the plates read at OD 400 nm
on an automatic ELISA plate reader.
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Results |
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We previously reported (48) that the normal human GUS
cDNA transgene fully corrected the mutant phenotype when bred onto the
MPS VII (gusmps/mps) background. These
transgenic mice produce several times normal levels of human
-glucuronidase, and were naturally tolerant to immune challenge with
the human enzyme. To determine whether the inactive human
GUS E540A cDNA transgene could confer tolerance, the
hGUS/E540A transgene was first introduced into C57BL/6 mice as
described in Materials and Methods, and offspring of
founders were tested for tolerance to immunization with purified human GUS.
A low copy number transgenic line was identified that was tolerant to immunization with human GUS, and the hGUS/E540A transgene in this line was crossed onto the B6 gusmps/+ stock to derive a line homozygous for the chromosome carrying the transgene, and heterozygous for the gusmps allele. The colony of C57BL/6TghGUS/E540A mps/+ mice was maintained by brother-sister matings and genotyped by enzymatic analysis of extracts of tail samples for GUS activity and by PCR analysis of genomic DNA for the wild-type and mutant mouse gusmps alleles and for the hGUS/E540ATg allele. MPS VII/E540ATg offspring from this colony were analyzed for morphologic, biochemical, and histopathologic phenotypes and tested for tolerance to immune challenge with human GUS.
Mutant Phenotype of the hGUS/E540A Transgenic MPS VII Mice. Homozygote MPS VII (gusmps/mps) mice carrying the hGUS/E540A transgene, herein referred to as MPS VII/E540ATg mice, were not distinguishable from gusmps/+ and +/+ littermates at birth without genotyping, but could easily be identified visually by the time of weaning from their shortened face and slightly smaller size. As they aged, their growth retardation, shortened extremities, and facial dysmorphism became more prominent. Figure 1A shows the difference in phenotype of wild-type and mutant mice at age 6 months. By this age, radiographic analysis of the axial and appendicular skeleton of MPS VII/E540ATg mice demonstrated marked dysplasia with shortened, broad, sclerotic long bones, a narrow thorax, and sclerosis of the calvarium (Fig. 1B). Other aspects of the MPS VII mutant phenotype (which include deafness, failure to reproduce, and shortened survival) were also retained. The MPS VII/E540ATg mice had a mean survival of 200 days (n = 27; SD ± 61 days). The longest survivor lived 301 days. The cause of death was unclear. However, typically, the mutant mice became progressively less active, stopped eating, and underwent a sharp drop in body weight in the few days before death. Collectively, these findings indicate that the MPS VII/E540ATg mice retained the complete mutant clinical phenotype described for the original MPS VII (gusmps/mps) mice, which do not carry the transgene (6, 7).
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Biochemical Phenotype of the MPS VII/E540ATg Mice.
Table 1 summarizes data comparing
the tissue levels of
-glucuronidase and
-galactosidase in MPS
VII/E540ATg mice and C57BL/6 control mice.
The MPS VII/E540ATg mice showed the profound
deficiency of
-glucuronidase characteristic of MPS VII
(gusmps/mps) mice (6). They also showed the
secondary elevations in tissue levels of
-galactosidase (10, 48).
This secondary elevation has been shown to be a convenient measure of
lysosomal storage secondary to
-glucuronidase deficiency and
provided a means to follow the biochemical response to therapy.
Reductions in the levels of secondary elevations of
-galactosidase
were shown to be associated with correction of lysosomal storage by
bone marrow replacement (10), enzyme therapy (16), and gene therapy
(23).
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Histopathology of the MPS VII/E540ATg Mouse. Multiple tissues from six transgenic mice from 3-9 months of age were studied morphologically as described (7). Tissues were evaluated for the extent of lysosomal storage and alterations were compared with those described in the MPS VII (gusmps/mps) mouse model (6, 7). Widespread lysosomal storage was seen throughout the fixed tissue macrophage system. The liver (Fig. 2A) and spleen (not shown) had marked lysosomal storage in sinus-lining cells. Renal tubular cells and glomerular visceral epithelial cells were also altered with abundant lysosomal storage (Fig. 2B). The brain had lysosomal storage in cells of the leptomeninges, neurons, and glial cells (data not shown). The eye showed corneal fibrocytes and endothelial cells distended with lysosomal storage (Fig. 2C) and enlarged lysosomes in the retinal pigment epithelial cells (Fig. 2D). Osteoblasts were distended with cytoplasmic vacuoles as were bone marrow sinusoidal cells (Fig. 2E). The epiphyseal plates of limb bones were hypercellular and irregular (Fig. 2F). The joints showed synovial proliferation, articular-synovial synechiae, and vacuolated synovial cells. Similar bone alterations affected the middle ear (not shown). The cardiac valves and endocardial fibroblasts also showed storage. In summary, all tissues examined showed the histopathology characteristic of the MPS VII (gusmps/mps) mouse. In fact, in no tissue from the MPS VII/E540ATg mouse could changes be distinguished from those in the MPS VII (gusmps/mps) mouse.
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Tolerance of the MPS VII/E540ATg Mice to Immune
Challenge with Human
-Glucuronidase.
Having established that the transgene expressing the cDNA for
hGUS/E540A did not alter the phenotype of the MPS VII mouse, we next
tested the hypothesis that the transgene would confer tolerance to
human
-glucuronidase on the MPS VII mouse, which has no
endogenous murine GUS. To provide a maximum immunogenic challenge, we used i.p. injection of human GUS in complete Freund's adjuvant as the initial challenge, followed by two boosts with human
GUS in incomplete Freund's adjuvant at 28 and 42 days. As a control
for the effect of the transgene, we used homozygous B6 MPS VII
(gusmps/mps) mice, which do not carry the
transgene that received the same immunogen on the same schedule. At the
first bleed (12 days after the first boost), all four of the MPS VII
(gusmps/mps) controls, but none of the MPS
VII/E540ATg mice, showed anti-human GUS
antibodies by ELISA (data not shown). Fig.
3 shows the ELISA plate assay on blood
taken 12 days following the second boost (i.e., 54 days after the
initial challenge). All four MPS VII control mice had titers of
105 or greater. By contrast, none of the MPS
VII/E540ATg mice showed any response. These
data demonstrated two important points: (i) The MPS VII
(gusmps/mps) mice that do not carry the
transgene are capable of mounting a strong antibody response to human
GUS when challenged in this manner. (ii) The
hGUS/E540A transgene clearly conferred tolerance to human
GUS, even when provided with this extreme immunogenic challenge.
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Discussion |
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The original MPS VII (gusmps/mps) mouse has been widely used as a model for testing experimental therapies for lysosomal storage disorders (10-39), and the value of the model is amply demonstrated by these studies. Syngeneic bone marrow transplantation in the MPS VII (gusmps/mps) mice, if performed in the newborn period before the clinical manifestations became pronounced, prolonged life, improved hearing and growth, and corrected lysosomal storage in many organs, although it did not correct the lysosomal storage in brain (11). Enzyme replacement therapy with purified murine GUS, if begun in the first few days of life, produced impressive reductions in visceral lysosomal storage, normalized the phenotype, and lengthened the lifespan (16-18). It also improved storage in brain if begun before 14 days of age (20). Several approaches to gene therapy for MPS disorders have been studied in this model. Retroviral (23-28), adenoviral (29-33), and adeno-associated virus vectors (34-39) have all been used. Most gene therapy trials with this model have used the human cDNA to express human GUS in the MPS VII mouse, but none have evaluated the effects of antibodies to human GUS on the duration of expression or the magnitude of response.
The MPS VII (gusmps/mps) mouse was recently reported to have an impaired immune response to foreign proteins (22). Given this observation, one might question whether it would be naturally tolerant to human GUS. Results presented here show this is clearly not the case: the MPS VII (gusmps/mps) mouse developed antibodies to human GUS. However, this paradox can be explained. The immune defect in the MPS VII mouse was attributed to inhibition by the accumulated GAGs of proteases required for antigen processing. Activity of these proteases on foreign proteins is required to provide peptides for antigen presentation. However, the same report (22) showed that providing purified murine GUS in vitro corrected this defect. One would expect, then, that the large dose of human GUS delivered as the antigenic challenge would also correct the immune defect in vivo. This, in turn, would enable the MPS VII (gusmps/mps) control mice to develop an immune response to the corrective human GUS, which would be recognized as foreign. The data presented here argue that this is the case. The MPS VII (gusmps/mps) control mice developed a strong antibody response to human GUS.
On the other hand, the MPS VII mouse carrying a transgene expressing the E540A mutant form of human GUS did not develop antibody. In fact, it was tolerant to an extraordinary challenge with human GUS. From these results, we conclude that the MPS VII/E540ATg mouse should provide a valuable model for preclinical studies of enzyme therapy with purified human GUS and of gene therapy with vectors expressing human GUS, because antibodies to the corrective protein will not complicate the interpretation of the results or abrogate the therapeutic responses to the corrective enzyme.
The approach used here to produce an improved murine model of MPS VII should be generalizable to other enzyme deficiency disease models. The first step involves determination of one or more catalytically essential residues of the human enzyme in question. Next, one determines which essential residue can be replaced by an inactivating mutation, yet still allow expression of a stable, inactive enzyme. The next step involves creating a transgenic mouse expressing the inactive human gene product. Once it has been established that one of the transgenic founders expresses enough inactive human enzyme to confer tolerance on the wild-type mouse background, the tolerance-conferring transgene can be crossed onto the strain carrying the mouse null mutant. Finally, the tolerance of the homozygous null strain carrying the transgene must be confirmed by repeating the immune challenge, as done here with human GUS.
Once established, the tolerant mouse model of the disease of interest can be propagated by conventional means. Given the rapidly growing list of knockout mouse models of human diseases, and the interest in using these models in preclinical trials to evaluate the safety and effectiveness of gene products to evaluate experimental therapies using products that might be administered to humans, there should be many opportunities to use "tolerant mouse models."
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Abbreviations |
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MPS VII, mucopolysaccharidosis type VII;
GAGs, glycosaminoglycans;
GUS,
-glucuronidase.
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Footnotes |
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To whom correspondence should be addressed at: Edward
A. Doisy Department of Biochemistry and Molecular Biology, Saint Louis University School of Medicine, 1402 South Grand Boulevard, St. Louis,
MO 63104. E-mail: slyws{at}slu.edu.
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