Canine fucosidosis: clinical findings

Canine Models of Inherited Musculoskeletal and Neurodegenerative Diseases

Mouse models of human disease remain the bread and butter of modern biology and therapeutic discovery. Nonetheless, more often than not mouse models do not reproduce the pathophysiology of the human conditions they are designed to mimic. Naturally occurring large animal models have predominantly been found in companion animals or livestock because of their emotional or economic value to modern society and, unlike mice, often recapitulate the human disease state. In particular, numerous models have been discovered in dogs and have a fundamental role in bridging proof of concept studies in mice to human clinical trials. The present article is a review that highlights current canine models of human diseases, including Alzheimer's disease, degenerative myelopathy, neuronal ceroid lipofuscinosis, globoid cell leukodystrophy, Duchenne muscular dystrophy, mucopolysaccharidosis, and fucosidosis. The goal of the review is to discuss canine and human neurodegenerative pathophysiologic similarities, introduce the animal models, and shed light on the ability of canine models to facilitate current and future treatment trials.

Associations between neurologic dysfunction and lesions in canine fucosidosis

Canine fucosidosis in English Springer spaniels is the only animal model of the neurovisceral lysosomal storage disease fucosidosis available for preclinical therapeutic trials. For this reason, it is crucial to identify critical time points in disease progression, and if there are particular lesions associated with specific aspects of neurologic dysfunction. Historical records of 53 canine fucosidosis cases from 1979 to 2009 containing a neurologic dysfunction score assessing motor, behavioral and sensory dysfunction were interrogated by statistical analysis. Motor and behavioral dysfunction scores assessing gait deficits and apprehensive behavior first significantly increased at 12-17 months, and increased at each 6-month interval thereafter. Sensory dysfunction scores, assessing hearing loss, balance and vision deterioration, did not significantly increase until 18-23 months, and coincided with a rapid decline in neurologic function. Regression analysis incorporating published neuropathology data, measured by image analysis, identified neuroinflammation and apoptotic cell death as significant informative predictors of increasing neurologic dysfunction. These findings indicate that the level of neuropathology required to induce consistent and conspicuous clinical signs in canine fucosidosis is reached by approximately 12 months of age in the absence of other disease processes. Significant association between neuroinflammation and apoptotic cell death also suggests that specifically targeting these lesions combined with enzyme replacement in future studies may reduce disease burden in fucosidosis. Overall, examining this historical clinical data to identify associations between the extent of neuropathology and degree of clinical dysfunction provides a useful reference tool for monitoring disease and evaluating therapeutic trials conducted in canine fucosidosis.

Oligodendrocyte loss during the disease course in a canine model of the lysosomal storage disease fucosidosis

Hypomyelination is a poorly understood feature of many neurodegenerative lysosomal storage diseases, including fucosidosis in children and animals. To gain insight into hypomyelination in fucosidosis, we investigated lysosomal storage, oligodendrocyte death, and axonal and neuron loss in CNS tissues of fucosidosis-affected dogs aged 3 weeks to 42 months using immunohistochemistry, electron microscopy, and gene expression assays. Vacuole accumulation in fucosidosis oligodendrocytes commenced by 5 weeks of age; all oligodendrocytes were affected by 16 weeks. Despite progressive vacuolation, mature oligodendrocyte loss by apoptosis (caspase-6 positive) in the corpus callosum and cerebellar white matter stabilized by 16 weeks, with no further subsequent loss. Axonal neurofilament loss progressed only in late disease, suggesting that disturbed axon-oligodendrocyte interactions are unlikely to be the primary cause of hypomyelination. A 67% decline in the number of Purkinje cell layer oligodendrocytes coincided with a 67% increase in the number of caspase-6-positive Purkinje cells at 16 weeks, suggesting that early oligodendrocyte loss contributes to Purkinje cell apoptosis. Fucosidosis hypomyelination appeared to follow normal spatiotemporal patterns of myelination, with greater loss of oligodendrocytes and larger downregulation of CNP, MAL, and PLP1 genes at 16 weeks in the cerebellum versus the frontal cortex. These studies suggest that survival of oligodendrocytes in fucosidosis is limited during active myelination, although the mechanisms remain unknown.

Myelin genes are downregulated in canine fucosidosis

The processes regulating the complex neurodegenerative cascade of vacuolation, neuroinflammation, neuronal loss and myelin deficits in fucosidosis, a neurological lysosomal storage disorder, remain unclear. To elucidate these processes the gene expression profile of the cerebral cortex from untreated and intrathecal enzyme replacement therapy treated fucosidosis pups and age-matched unaffected controls were examined. Neuroinflammation and cell death processes were identified to have a major role in fucosidosis pathophysiology with 37% of differentially expressed (DE) genes involved in these processes. Critical, specific, early decreases in expression levels of key genes in myelin assembly were identified by gene expression profiling, including myelin-associated glycoprotein (MAG), myelin and lymphocyte protein (MAL), and oligodendrocyte myelin paranodal and inner loop protein (OPALIN). These gene expression changes may be indicative of early neuronal loss causing reduced electrical impulses required for oligodendrocyte maturation.

Treatment of canine fucosidosis by intracisternal enzyme infusion

The blood brain barrier is the major obstacle to treating lysosomal storage disorders of the central nervous system such as canine fucosidosis. This barrier was overcome by three, monthly injections of recombinant canine α-l-fucosidase enzyme were given intracisternally. In dogs treated from 8 weeks of age enzyme reached all areas of central nervous system as well as the cervical lymph node, bone marrow and liver. Brainstem and spinal cord samples from regions adjacent to the injection site had highest enzyme levels (39-73% of normal). Substantial enzyme activity (8.5-20% of normal controls) was found in the superficial brain compared to deeper regions (2.6-5.5% of normal). Treatment significantly reduced the fucosyl-linked oligosaccharide accumulation in most areas of CNS, liver and lymph node. In the surface and deep areas of lumbar spinal cord, oligosaccharide accumulation was corrected (79-80% reduction) to near normal levels (p<0.05). In the spinal meninges (thoracic and lumbar) enzyme activity (35-39% of normal control) and substrate reduction (58-63% affected vehicle treated samples) reached levels similar to those seen in phenotypically normal carriers (p<0.05).The procedure was safe and well-tolerated, treated (average 16%) dogs gained more weight (p<0.05) and there was no antibody formation or inflammatory reaction in plasma and CSF following treatments. The capacity of early ERT to modify progression of biochemical storage in fucosidosis is promising as this disease is currently only amenable to treatment by bone marrow transplantation which entails unacceptably high risks for many patients.

Canine fucosidosis: a neuroprogressive disorder

The lysosomal storage disease, canine fucosidosis, is caused by the absence of the lysosomal enzyme canine α-L-fucosidase with storage of undegraded fucose-rich material in different organs. Canine fucosidosis is a severe, progressive, fatal neurological disease which results in death or euthanasia and is the only available animal model for this human disease. We analysed the progressive neuropathology from birth to severe clinical disease and related this to the clinical signs. At birth no vacuolation was observed in fucosidosis brain; however, a complex storage presence with vacuolation was well established by 4 months of age, before the clinical signs of motor dysfunction which occurred at 10-12 months of age. Purkinje cell loss, neuronal loss, gliosis, perivascular storage and demyelination accompanied disease progression. Increased vacuolation (15.3-fold increase compared to controls) coincided with advanced motor and mental deterioration in late-stage disease. Significant loss of myelin commenced early, with greatest impact in the cerebellum, and was severe in late disease (1.6- to 1.9-fold decrease) compared to controls (p < 0.05) contributing to clinical signs of motor and mental dysfunction. This detailed description and quantification of the CNS pathology in canine fucosidosis will inform monitoring of the onset, progression and response of this disease to therapy.

Amelioration of clinical disease following bone marrow transplantation in fucosidase-deficient dogs

Canine fucosidosis was studied as an animal model for the treatment of neurovisceral lysosomal storage disease. Following successful bone marrow engraftment, dogs with fucosidosis had increased levels of alpha-L-fucosidase enzyme activity in leukocytes, plasma, and neural and visceral tissues. This widespread increase in enzyme activity was accompanied by a rapid improvement in the peripheral nerve and visceral lesions of fucosidosis and a more gradual improvement in the central nervous system pathology. Long-term engraftment from an early age reduced the severity and slowed the progression of clinical neurological disease. Transplantation after the onset of clinical signs was not effective. These findings suggest that the neurological damage caused by some inherited metabolic disorders, such as fucosidosis, may be preventable but emphasise the need for early diagnosis and treatment.

Reproductive abnormalities in canine fucosidosis

Male English springer spaniel dogs affected with fucosidosis, a lysosomal storage disorder, were found to be infertile while females with the disease reproduced successfully. Ejaculates of semen collected from affected dogs had reduced total sperm output and morphologically abnormal spermatozoa. A high proportion of ejaculated spermatozoa had midpiece droplets, bent tails and poor motility. Severely vacuolated epididymal epithelial cells were observed by light microscopy. Electron microscopic examination revealed membrane-bound vacuoles of variable size containing scanty amounts of granular to fibrillar material in epididymal epithelial cells, smooth muscle, myoid cells and Sertoli cells. Male infertility is believed to result from lysosomal storage of fucosyl-linked substrates in cells of the reproductive system. The extensive lesions in the epididymis may have interfered with maturation and transport of spermatozoa. Also, deficiency of alpha-L-fucosidase activity could have impaired the shedding of cytoplasmic droplets from spermatozoa and altered the surface glycoprotein composition of the sperm during epididymal transit.