Mice with an aspartylglucosaminuria mutation similar to humans replicate the pathophysiology in patients

Modeling Rare Human Disorders in Mice: The Finnish Disease Heritage

The modification of genes in animal models has evidently and comprehensively improved our knowledge on proteins and signaling pathways in human physiology and pathology. In this review, we discuss almost 40 monogenic rare diseases that are enriched in the Finnish population and defined as the Finnish disease heritage (FDH). We will highlight how gene-modified mouse models have greatly facilitated the understanding of the pathological manifestations of these diseases and how some of the diseases still lack proper models. We urge the establishment of subsequent international consortiums to cooperatively plan and carry out future human disease modeling strategies. Detailed information on disease mechanisms brings along broader understanding of the molecular pathways they act along both parallel and transverse to the proteins affected in rare diseases, therefore also aiding understanding of common disease pathologies.

Pre-clinical Gene Therapy with AAV9/AGA in Aspartylglucosaminuria Mice Provides Evidence for Clinical Translation

Aspartylglucosaminuria (AGU) is an autosomal recessive lysosomal storage disease caused by loss of the enzyme aspartylglucosaminidase (AGA), resulting in AGA substrate accumulation. AGU patients have a slow but progressive neurodegenerative disease course, for which there is no approved disease-modifying treatment. In this study, AAV9/AGA was administered to Aga^−/− mice intravenously (i.v.) or intrathecally (i.t.), at a range of doses, either before or after disease pathology begins. At either treatment age, AAV9/AGA administration led to (1) dose dependently increased and sustained AGA activity in body fluids and tissues; (2) rapid, sustained, and dose-dependent elimination of AGA substrate in body fluids; (3) significantly rescued locomotor activity; (4) dose-dependent preservation of Purkinje neurons in the cerebellum; and (5) significantly reduced gliosis in the brain. Treated mice had no abnormal neurological phenotype and maintained body weight throughout the whole experiment to 18 months old. In summary, these results demonstrate that treatment of Aga^−/− mice with AAV9/AGA is effective and safe, providing strong evidence that AAV9/AGA gene therapy should be considered for human translation. Further, we provide a direct comparison of the efficacy of an i.v. versus i.t. approach using AAV9, which should greatly inform the development of similar treatments for other related lysosomal storage diseases.

Susceptibility-Weighted Imaging Findings in Aspartylglucosaminuria

BACKGROUND AND PURPOSE

Aspartylglucosaminuria is a rare lysosomal storage disorder that causes slowly progressive, childhood-onset intellectual disability and motor deterioration. Previous studies have shown, for example, hypointensity in the thalami in patients with aspartylglucosaminuria on T2WI, especially in the pulvinar nuclei. Susceptibility-weighted imaging is a neuroimaging technique that uses tissue magnetic susceptibility to generate contrast and is able to visualize iron and other mineral deposits in the brain. SWI findings in aspartylglucosaminuria have not been reported previously.

MATERIALS AND METHODS

Twenty-one patients with aspartylglucosaminuria (10 girls; 7.4-15.0 years of age) underwent 3T MR imaging. The protocol included an SWI sequence, and the images were visually evaluated. Thirteen patients (6 girls, 7.4-15.0 years of age) had good-quality SWI. Eight patients had motion artifacts and were excluded from the visual analysis. Thirteen healthy children (8 girls, 7.3-14.1 years of age) were imaged as controls.

RESULTS

We found a considerably uniform distribution of decreased signal intensity in SWI in the thalamic nuclei in 13 patients with aspartylglucosaminuria. The most evident hypointensity was found in the pulvinar nuclei. Patchy hypointensities were also found especially in the medial and anterior thalamic nuclei. Moreover, some hypointensity was noted in globi pallidi and substantia nigra in older patients. The filtered-phase images indicated accumulation of paramagnetic compounds in these areas. No abnormal findings were seen in the SWI of the healthy controls.

CONCLUSIONS

SWI indicates accumulation of paramagnetic compounds in the thalamic nuclei in patients with aspartylglucosaminuria. The finding may raise the suspicion of this rare disease in clinical practice.

White Matter Microstructure and Subcortical Gray Matter Structure Volumes in Aspartylglucosaminuria; a 5-Year Follow-up Brain MRI Study of an Adolescent with Aspartylglucosaminuria and His Healthy Twin Brother

OBJECTIVE

Aspartylglucosaminuria is an inherited, lysosomal storage disease causing progressive decline in cognitive and motor functions. The aim of this study was to evaluate volumes of subcortical gray matter structures and white matter microstructure in aspartylglucosaminuria in adolescence in a longitudinal study for the first time.

METHODS

A boy with aspartylglucosaminuria and his healthy twin brother were imaged twice with a 3.0 T MRI scanner at the ages of 10 and 15 years. Subcortical gray matter structure volumes were measured using an atlas-based automatic method, and diffusion tensor imaging was used to evaluate the white matter microstructure of the corpus callosum and the thalamocortical pulvinar tracts.

RESULTS

The subcortical gray matter structures were smaller at onset and diminished at follow-up in the affected twin, with the exception of the amygdala which was larger and remained the size. The largest difference in volume between the twins was found in the thalami. The total gray and white matter volumes decreased in the affected twin. In diffusion tensor imaging analysis, the fractional anisotropy was decreased at onset in the affected twin compared to the healthy brother in the evaluated tracts. The axial, radial and mean diffusivity values were increased in the affected twin. The difference between the twins increased slightly at follow-up.

INTERPRETATION

The findings suggest that volumetric measurements and diffusion tensor imaging based microstructural analysis may be useful modalities for monitoring disease progression and response to emerging treatment in aspartylglucosaminuria, but further studies with more subjects are necessary to confirm the results.

Identification of Small Molecule Compounds for Pharmacological Chaperone Therapy of Aspartylglucosaminuria

Aspartylglucosaminuria (AGU) is a lysosomal storage disorder that is caused by genetic deficiency of the enzyme aspartylglucosaminidase (AGA) which is involved in glycoprotein degradation. AGU is a progressive disorder that results in severe mental retardation in early adulthood. No curative therapy is currently available for AGU. We have here characterized the consequences of a novel AGU mutation that results in Thr122Lys exchange in AGA, and compared this mutant form to one carrying the worldwide most common AGU mutation, AGU-Fin. We show that T122K mutated AGA is expressed in normal amounts and localized in lysosomes, but exhibits low AGA activity due to impaired processing of the precursor molecule into subunits. Coexpression of T122K with wildtype AGA results in processing of the precursor into subunits, implicating that the mutation causes a local misfolding that prevents the precursor from becoming processed. Similar data were obtained for the AGU-Fin mutant polypeptide. We have here also identified small chemical compounds that function as chemical or pharmacological chaperones for the mutant AGA. Treatment of patient fibroblasts with these compounds results in increased AGA activity and processing, implicating that these substances may be suitable for chaperone mediated therapy for AGU.

Distribution and Severity of Neuropathology in β-Mannosidase-Deficient Mice is Strain Dependent

Neurological dysfunction is common in humans and animals with lysosomal storage diseases. β-Mannosidosis, an autosomal recessive inherited disorder of glycoprotein catabolism caused by deficiency of the lysosomal enzyme β-mannosidase, is characterized by intracellular accumulation of small oligosaccharides in selected cell types. In ruminants, clinical manifestation is severe, and neuropathology includes extensive intracellular vacuolation and dysmyelination. In human cases of β-mannosidosis, the clinical symptoms, including intellectual disability, are variable and can be relatively mild. A β-mannosidosis knockout mouse was previously characterized and showed normal growth, appearance, and lifespan. Neuropathology between 1 and 9 months of age included selective, variable neuronal vacuolation with no hypomyelination. This study characterized distribution of brain pathology in older mutant mice, investigating the effects of two strain backgrounds. Morphological analysis indicated a severe consistent pattern of neuronal vacuolation and disintegrative degeneration in all five 129X1/SvJ mice. However, the mice with a mixed genetic background showed substantial variability in the severity of pathology. In the severely affected animals, neuronal vacuolation was prominent in specific layers of piriform area, retrosplenial area, anterior cingulate area, selected regions of isocortex, and in hippocampus CA3. Silver degeneration reaction product was prominent in regions including specific cortical layers and cerebellar molecular layer. The very consistent pattern of neuropathology suggests metabolic differences among neuronal populations that are not yet understood and will serve as a basis for future comparison with human neuropathological analysis. The variation in severity of pathology in different mouse strains implicates genetic modifiers in the variable phenotypic expression in humans.

Mouse phenotyping with MRI

The field of mouse phenotyping with magnetic resonance imaging (MRI) is rapidly growing, motivated by the need for improved tools for characterizing and evaluating mouse models of human disease. Image results can provide important comparisons of human conditions with mouse disease models, evaluations of treatment, development or disease progression, as well as direction for histological or other investigations. Effective mouse MRI studies require attention to many aspects of experiment design. In this chapter, we provide details and discussion of important practical considerations: hardware requirements, mouse handling for in vivo imaging, specimen preparation for ex vivo imaging, sequence and contrast agent selection, study size, and quantitative image analysis. We focus particularly on anatomical phenotyping, an important and accessible application that has shown a high potential for impact in many mouse models at our imaging center.

Use of nonviral promoters in adenovirus-mediated gene therapy: reduction of lysosomal storage in the aspartylglucosaminuria mouse

BACKGROUND

Aspartylglucosaminuria (AGU) is a lysosomal storage disease with severe neurodegenerative clinical features resulting from the deficiency of lysosomal aspartylglucosaminidase (AGA). The AGU knockout mouse is a good model to test different therapy strategies, as it mimics well the human pathogenesis of the disease exhibiting storage vacuoles in all tissues. In this study we investigated the efficiency of nonviral promoters in adenovirus-mediated gene therapy.

METHODS

The deficient corrective enzyme, AGA, was expressed using two tissue-specific promoters, neuron-specific enolase (NSE), astrocyte-specific (GFAP) and the endogenous AGA promoter. An intrastriatal injection site was chosen due to its wide connections in the central nervous system (CNS). The expression of AGA was analyzed 1 week, 2 weeks, 4 weeks, 2 months and 4 months after the virus injection by lysosomal AGA-specific immunostaining. A correction of the lysosomal storage in the brain of treated mice was also studied using toluidine blue stained thin sections.

RESULTS

The overexpressed AGA enzyme was detected in addition to the injection site, also in the ipsilateral parietal cortex indicating migration of AGA in the brain tissue. Duration of AGA expression was markedly longer with all the viruses used compared to the green fluorescent protein (GFP) expression driven by the viral cytomegalovirus (CMV) promoter. In most animals the storage was decreased by at least 50% as compared to untreated AGU mouse brains. Remarkably, >90% correction of storage at the ipsilateral cortex was found with the NSE promoter at 4 weeks and 2 months after injection. Additionally, partial clearance of storage was demonstrated also in the contralateral side of the brain.

CONCLUSIONS

These data implicate that tissue-specific promoters are especially useful in virus-mediated gene therapy aiming at long-term gene expression.

Aspartylglucosaminidase (AGA) is efficiently produced and endocytosed by glial cells: implication for the therapy of a lysosomal storage disorder

BACKGROUND

Aspartylglucosaminuria (AGU) represents diseases affecting the central nervous system and is caused by a deficiency of a lysosomal enzyme, aspartylglucosaminidase (AGA). AGA, like lysosomal enzymes in general, are good targets for gene therapy since they move from cell to cell using the mannose-6-phosphate receptor. Consequently, only a minority of target cells need to be corrected. Here, we wanted to determine which cell type, neurons or glia would better produce AGA to be transported to adjacent cells for use in possible treatment strategies.

METHODS

Adenoviruses containing tissue-specific glial fibrillary acidic protein (GFAP) promoter and neuron-specific enolase (NSE) promoter were generated to target expression of AGA in Aga-deficient mouse primary glial and neuronal cell cultures. In addition an endogenous AGA promoter was used. The experimental design was planned to measure the enzymatic activities in the cells and media of neurons and glia infected with each specific virus. The endocytosis of AGA was analyzed by incubating neuronal and glial cells with media produced by each virus-cell combination.

RESULTS

AGA promoter was shown to be a very powerful glia promoter producing 32 times higher specific AGA activity in glia than in neurons. GFAP and NSE promoters also produced a clear overexpression of AGA in glia and neurons, respectively. Interestingly, both the NSE and GFAP promoters were not cell-specific in our system. The amount of exocytosed AGA was significantly higher in glial cells than neurons and glial cells were also found to have a greater capacity to endocytose AGA.

CONCLUSIONS

These data indicate the importance of glial cells in the expression and transport of AGA. Subsequently, new approaches can be developed for therapeutic intervention.

The Finnish Disease Heritage III: the individual diseases

This article is the third and last in a series entitled The Finnish Disease Heritage I-III. All the 36 rare hereditary diseases belonging to this entity are described for clinical and molecular genetic purposes, based on the Finnish experience gathered over a period of half a century. In addition, five other diseases are mentioned. They may be included in the list of the "Finnish diseases" after adequate complementary studies.

Morphologic phenotyping with MR microscopy: the visible mouse

A method for rapid morphologic phenotyping is demonstrated by using magnetic resonance microscopy. Whole fixed C57BL/6J mice were imaged at 110-microm isotropic resolution; limited volumes of the intact specimen, at 50-microm isotropic resolution; and isolated organs, at 25-microm isotropic resolution. The three-dimensional imaging technique was applied to uricase knockout mice to demonstrate the method for the evaluation of morphologic phenotype.

Applications of the Morris water maze in the study of learning and memory

The Morris water maze (MWM) was described 20 years ago as a device to investigate spatial learning and memory in laboratory rats. In the meanwhile, it has become one of the most frequently used laboratory tools in behavioral neuroscience. Many methodological variations of the MWM task have been and are being used by research groups in many different applications. However, researchers have become increasingly aware that MWM performance is influenced by factors such as apparatus or training procedure as well as by the characteristics of the experimental animals (sex, species/strain, age, nutritional state, exposure to stress or infection). Lesions in distinct brain regions like hippocampus, striatum, basal forebrain, cerebellum and cerebral cortex were shown to impair MWM performance, but disconnecting rather than destroying brain regions relevant for spatial learning may impair MWM performance as well. Spatial learning in general and MWM performance in particular appear to depend upon the coordinated action of different brain regions and neurotransmitter systems constituting a functionally integrated neural network. Finally, the MWM task has often been used in the validation of rodent models for neurocognitive disorders and the evaluation of possible neurocognitive treatments. Through its many applications, MWM testing gained a position at the very core of contemporary neuroscience research.

Hyperactivity, neuromotor defects, and impaired learning and memory in a mouse model for metachromatic leukodystrophy

Deficiency of arylsulfatase A (ASA) causes the autosomal recessive lipidosis, metachromatic leukodystrophy (MLD). Performance on tests of activity, motor ability and learning/memory was assessed in ASA-deficient mice and normal controls at 3, 6 and 12 months-of-age. ASA-deficient mice showed consistently increased cage activity in all age groups, whereas open field activity was increased only in the 3-month-old group. Motor coordination and equilibrium, as tested in the rotarod test, was impaired in 12-month-old ASA-deficient mice. Passive avoidance learning was tested in the step-through box. Performance on this test was impaired in the 12-month-old group only. Spatial learning and memory abilities were tested in the Morris water maze. Six-month-old ASA-deficient mice displayed slightly impaired hidden-platform acquisition performance. Three-month-old animals, on the other hand, did not show any acquisition or retention defect on this task, notwithstanding significantly reduced swimming velocity. Acquisition training, both in the hidden- and visible-platform conditions of the Morris water maze, and retention performance during the probe trials were impaired in 12-month-old ASA-deficient mice. The hyperactivity, motor incoordination and slowing, and the age-related learning/memory defects, reported here in ASA-deficient mice, may relate to the decline of neuromotor and cognitive functions in MLD patients, and could be used as correlative or outcome measures in the study of MLD pathophysiology and treatment.

Brain studies of mouse models for neurogenetic disorders using in vivo magnetic resonance imaging (MRI)

Magnetic resonance imaging (MRI) is a technique commonly used to detect neural abnormalities in routine clinical practice. It is perhaps less well known that the technique can be adapted to measure various anatomical and physiological features of small laboratory rodents. This review focuses on the potential of the MRI technique to image the brain of (transgenic) mouse models for neurological diseases, and aims to introduce these exciting new technological developments to the non-specialist reader.

Aspartylglycosaminuria: biochemistry and molecular biology

Aspartylglucosaminuria (AGU, McKusick 208400) is an autosomal recessive lysosomal storage disease caused by defective degradation of Asn-linked glycoproteins. AGU mutations occur in the gene (AGA) for glycosylasparaginase, the enzyme necessary for hydrolysis of the protein oligosaccharide linkage in Asn-linked glycoprotein substrates undergoing metabolic turnover. Loss of glycosylasparaginase activity leads to accumulation of the linkage unit Asn-GlcNAc in tissue lysosomes. Storage of this fragment affects the pathophysiology of neuronal cells most severely. The patients notably suffer from decreased cognitive abilities, skeletal abnormalities and facial grotesqueness. The progress of the disease is slower than in many other lysosomal storage diseases. The patients appear normal during infancy and generally live from 25 to 45 years. A specific AGU mutation is concentrated in the Finnish population with over 200 patients. The carrier frequency in Finland has been estimated to be in the range of 2.5-3% of the population. So far there are 20 other rare family AGU alleles that have been characterized at the molecular level in the world's population. Recently, two knockout mouse models for AGU have been developed. In addition, the crystal structure of human leukocyte glycosylasparaginase has been determined and the protein has a unique alphabetabetaalpha sandwich fold shared by a newly recognized family of important enzymes called N-terminal nucleophile (Ntn) hydrolases. The nascent single-chain precursor of glycosylase araginase self-cleaves into its mature alpha- and beta-subunits, a reaction required to activate the enzyme. This interesting biochemical feature is also shared by most of the Ntn-hydrolase family of proteins. Many of the disease-causing mutations prevent proper folding and subsequent activation of the glycosylasparaginase.

Correction of peripheral lysosomal accumulation in mice with aspartylglucosaminuria by bone marrow transplantation

OBJECTIVE

Bone marrow transplantation has been shown to alleviate symptoms outside the CNS in many lysosomal storage diseases depending on the type and stage of the disease, but the effect on neurological symptoms is variable or still unclear. Aspartylglucosaminuria (AGU) is a lysosomal storage disease characterized by mental retardation, recurrent infections in childhood, hepatosplenomegaly and coarse facial features. Vacuolized storage lysosomes are found in all tissues of patients and uncleaved enzyme substrate is excreted in the urine. The recently generated AGU mouse model closely mimicks the human disease and serves as a good model to study the efficiency of bone marrow transplantation in this disease.

METHODS

Eight-week-old AGU mice were lethally irradiated and transplanted with bone marrow from normal donors. The AGA enzyme activity was measured in the liver and the brain and the degree of correction of tissue pathology was analyzed by light and electron microscopy. Reverse bone marrow transplantation (AGU bone marrow to wild-type mice) was also performed.

RESULTS

Six months after transplantation the AGA enzyme activity was 13% of normal in the liver, but only 3% in the brain. Tissue pathology was reversed in the liver and the spleen, but not in the brain and the kidney. The urinary excretion of enzyme substrate was diminished but still detectable. No storage vacuoles were found in the tissues after reverse transplantation, but subtle excretion of uncleaved substrate was detected in the urine.

CONCLUSION

Liver and spleen pathology of AGU was corrected by bone marrow transplantation, but there was no effect on lysosomal accumulation in the CNS and in the kidneys.

Toward understanding the neuronal pathogenesis of aspartylglucosaminuria: expression of aspartylglucosaminidase in brain during development

The deficiency of a lysosomal enzyme, aspartylglucosaminidase, results in a lysosomal storage disorder, aspartylglucosaminuria, manifesting as progressive mental retardation. To understand tissue pathogenesis and disease progression we analyzed the developmental expression of the enzyme, especially in brain, which is the major source of the pathological symptoms. Highest mRNA levels in brain were detected during embryogenesis, the levels decreased neonatally and started to increase again from Day 7 on. In Western analyses, a defective processing of aspartylglucosaminidase was observed in brain as compared to other tissues, resulting in very low levels of the mature, active form of the enzyme. Interestingly immunohistochemical analyses of mouse brain revealed that aspartylglucosaminidase immunoreactivity closely mimicked the myelin basic protein immunostaining pattern. The only evident neuronal staining was observed in the developing Purkinje cells of the cerebellum from Days 3 to 10, reflecting well the mRNA expression. In human infant brain, the immunostaining was also present in myelinated fibers as well as in the Purkinje cells and, additionally, in the soma and extensions of other neurons. In the adult human brain neurons and oligodendrocytes displayed immunoreactivity whereas myelinated fibers were not stained. Our results of aspartylglucosaminidase immunostaining in myelinated fibers of infant brain might imply the involvement of aspartylglucosaminidase in the early myelination process. This is consistent with previous magnetic resonance imaging findings in the brains of aspartylglucosaminuria patients, revealing delayed myelination in childhood.

Functional consequences of amyloidosis mutation for gelsolin polypeptide -- analysis of gelsolin-actin interaction and gelsolin processing in gelsolin knock-out fibroblasts

Gelsolin, an actin-modulating protein, derived from a single gene exists in intracellular and secreted forms. A point mutation at position 187 of both forms of gelsolin causes familial amyloidosis of the Finnish type (FAF). Here, we expressed both isoforms of the wild-type and FAF mutant gelsolin in mouse embryonic gelsolin-null fibroblasts. We demonstrate that the FAF mutation does not interfere with the normal actin-modulating function of intracellular gelsolin, and that aberrant processing of secreted FAF gelsolin to FAF amyloid precursor takes place in the gelsolin-negative background. These results suggest that, in patients with FAF, symptoms are caused by the accumulation in their tissues of amyloid derived from plasma gelsolin and are not due to functional differences in cytoplasmic gelsolin.

Expression and endocytosis of lysosomal aspartylglucosaminidase in mouse primary neurons

Aspartylglucosaminuria (AGU) is a neurodegenerative lysosomal storage disease that is caused by mutations in the gene encoding for a soluble hydrolase, aspartylglucosaminidase (AGA). In this study, we have used our recently developed mouse model for AGU and analyzed processing, intracellular localization, and endocytosis of recombinant AGA in telencephalic AGU mouse neurons in vitro. The processing steps of AGA were found to be similar to the peripheral cells, but both the accumulation of the inactive precursor molecule and delayed lysosomal processing of the enzyme were detected. AGA was distributed to the cell soma and neuronal processes but was not found in the nerve terminals. Endocytotic capability of cultured telencephalic neurons was comparable to that of fibroblasts, and endocytosis of AGA was blocked by free mannose-6-phosphate (M6P), indicating that uptake of the enzyme was mediated by M6P receptors (M6PRs). Uptake of extracellular AGA was also studied in the tumor-derived cell lines rat pheochromocytoma (PC12) and mouse neuroblastoma cells (N18), which both endocytosed AGA poorly as compared with cultured primary neurons. Expression of cation-independent M6PRs (CI-M6PRs) in different cell lines correlated well with the endocytotic capability of these cells. Although a punctate expression pattern of CI-M6PRs was found in fibroblasts and cultured primary neurons, the expression was beyond the detection limit in PC12 and N18 cells. This indicates that PC12 and N18 are not feasible cell lines to describe neuronal uptake of mannose-6-phosphate-tagged proteins. This in vitro data will form an important basis for the brain-targeted therapy of AGU.