Dysmyelinogenesis in caprine beta-mannosidosis: ultrastructural and morphometric studies in fetal optic nerve

Lysosomal storage disease in the brain: mutations of the β-mannosidase gene identified in autosomal dominant nystagmus

PURPOSE

Genetic etiology of congenital/infantile nystagmus remains largely unknown. This study aimed to identify genomic mutations in patients with infantile nystagmus and an associated disease network.

METHODS

Patients with inherited and sporadic infantile nystagmus were recruited for whole-exome and Sanger sequencing. β-Mannosidase activities were measured. Gene expression, protein-protein interaction, and nystagmus-associated lysosomal storage disease (LSD) genes were analyzed.

RESULTS

A novel heterozygous mutation (c.2013G>A; p.R638H) of MANBA, which encodes lysosomal β-mannosidase, was identified in patients with autosomal-dominant nystagmus. An additional mutation (c.2346T>A; p.L749H) in MANBA was found by screening patients with sporadic nystagmus. MANBA was expressed in the pretectal nucleus of the developing midbrain, known to be involved in oculomotor and optokinetic nystagmus. Functional validation of these mutations demonstrated a significant decrease of β-mannosidase activities in the patients as well as in mutant-transfected HEK293T cells. Further analysis revealed that nystagmus is present in at least 24 different LSDs involving the brain.

CONCLUSION

This is the first identification of MANBA mutations in patients with autosomal-dominant nystagmus, suggesting a new clinical entity. Because β-mannosidase activities are required for development of the oculomotor nervous system, our findings shed new light on the role of LSD-associated genes in the pathogenesis of infantile nystagmus.

Beta-mannosidosis mice: a model for the human lysosomal storage disease

Beta-mannosidase, a lysosomal enzyme which acts exclusively at the last step of oligosaccharide catabolism in glycoprotein degradation, functions to cleave the unique beta-linked mannose sugar found in all N-linked oligosaccharides of glycoproteins. Deficiency of this enzyme results in beta-mannosidosis, a lysosomal storage disease characterized by the cellular accumulation of small oligosaccharides. In human beta-mannosidosis, the clinical presentation is variable and can be mild, even when caused by functionally null mutations. In contrast, two existing ruminant animal models have disease that is consistent and severe. To further explore the molecular pathology of this disease and to investigate potential treatment strategies, we produced a beta-mannosidase knockout mouse. Homozygous mutant mice have undetectable beta-mannosidase activity. General appearance and growth of the knockout mice are similar to the wild-type littermates. At >1 year of age, these mice exhibit no dysmorphology or overt neurological problems. The mutant animals have consistent cytoplasmic vacuolation in the central nervous system and minimal vacuolation in most visceral organs. Thin-layer chromatography demonstrated an accumulation of disaccharide in epididymis and brain. This mouse model closely resembles human beta-mannosidosis and provides a useful tool for studying the phenotypic variation in different species and will facilitate the study of potential therapies for lysosomal storage diseases.

Expression of myocilin/TIGR in normal and glaucomatous primate optic nerves

Myocilin/TIGR was the first molecule discovered to be linked with primary open angle glaucoma (POAG), a blinding disease characterized by progressive loss of retinal ganglion cells. Mutations in myocilin/TIGR have been associated with age of disease onset and severity. The function of myocilin/TIGR and its role in glaucoma is unknown. Myocilin/TIGR has been studied in the trabecular meshwork to determine a role in regulation of intraocular pressure. The site of damage to the axons of the retinal ganglion cells is the optic nerve head (ONH). The myocilin/TIGR expression was examined in fetal through adult human optic nerve as well as in POAG. Myocilin/TIGR was expressed in the myelinated optic nerve of children and normal adults but not in the fetal optic nerve before myelination. Also examined was the expression in monkeys with experimental glaucoma. The results demonstrate that optic nerve head astrocytes constitutively express myocilin/TIGR in vivo in primates. Nevertheless, myocilin/TIGR is apparently reduced in glaucomatous ONH. The colocalization of myocilin/TIGR to the myelin suggests a role of myocilin/TIGR in the myelinated optic nerve.

Glycoprotein lysosomal storage disorders: alpha- and beta-mannosidosis, fucosidosis and alpha-N-acetylgalactosaminidase deficiency

Glycoproteinoses belong to the lysosomal storage disorders group. The common feature of these diseases is the deficiency of a lysosomal protein that is part of glycan catabolism. Most of the lysosomal enzymes involved in the hydrolysis of glycoprotein carbohydrate chains are exo-glycosidases, which stepwise remove terminal monosaccharides. Thus, the deficiency of a single enzyme causes the blockage of the entire pathway and induces a storage of incompletely degraded substances inside the lysosome. Different mutations may be observed in a single disease and in all cases account for the nonexpression of lysosomal glycosidase activity. Different clinical phenotypes generally characterize a specific disorder, which rather must be described as a continuum in severity, suggesting that other biochemical or environmental factors influence the course of the disease. This review provides details on clinical features, genotype-phenotype correlations, enzymology and biochemical storage of four human glycoprotein lysosomal storage disorders, respectively alpha- and beta-mannosidosis, fucosidosis and alpha-N-acetylgalactosaminidase deficiency. Moreover, several animal disorders of glycoprotein metabolism have been found and constitute valuable models for the understanding of their human counterparts.

mRNA levels for central nervous system myelin proteins in myelin deficiency of caprine beta-mannosidosis

Caprine beta-mannosidosis is an inherited lysosomal storage disease that leads to a deficiency of oligodendrocytes and hypomyelination. Our previous results demonstrated that low levels of myelin-associated glycoprotein (MAG), 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP) and proteolipid protein (PLP) found in CNS samples correlated with decreased yields of myelin. However, there was a relative preservation of myelin basic protein (MBP) in the spinal cord samples of affected goats. This report shows that the amounts of myelin protein mRNAs in the spinal cords of affected goats relative to control goats are also decreased. The levels of mRNA for MAG, MBP and PLP in affected goat spinal cords compared with those of controls were equally decreased to approximately 50% for the three myelin proteins. This suggests that the relative preservation of MBP protein in the spinal cords is not due to a higher MBP mRNA level, but might be due to a difference in post-transcriptional processes.

Myelin-associated glycoprotein (MAG) in myelin deficiency of caprine beta-mannosidosis

Caprine beta-mannosidosis is an inherited lysosomal storage disorder due to a deficiency of beta-mannosidase which cleaves beta-linked mannose residues from the ends of N-asparagine linked oligosaccharides of glycoproteins. Histological and chemical examination has revealed a deficiency of compact myelin in the brains and spinal cords of affected goats. Since myelin-associated glycoprotein (MAG) is glycosylated and its metabolism could be directly affected in this disease, we investigated the possibility of a differential treatment of MAG in caprine beta-mannosidosis in comparison to non-glycosylated myelin proteins. MAG, myelin basic protein (MBP), 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP), proteolipid protein (PLP) and glial fibrillary acidic protein (GFAP) were quantified by western blot analysis in whole homogenates of spinal cords and hemispheres from affected goats at 1, 3 and 6 days of age and from normal controls. The yields of isolated myelin from the spinal cords of affected goats varied from 37 to 63% of normal and were 7% or less of normal from the hemispheres. In mutant spinal cords, the deficits of MAG, CNP and PLP measured in whole homogenates corresponded reasonably well with the decreased myelin yields, but the levels of MBP were consistently much closer to control levels than those of the other myelin proteins. A greater deficiency of PLP than MBP was also apparent in the myelin fractions purified from the affected spinal cords. In homogenates of mutant hemispheres, MAG, MBP, PLP and CNP were undetectable or at trace levels in comparison to controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Bovine beta-mannosidosis: pathologic and genetic findings in Salers calves

beta-mannosidosis is a recently recognized lysosomal storage disease in newborn Salers calves. Fourteen calves with beta-mannosidase deficiency were examined. Twelve calves were from routine laboratory submissions, and two calves were the result of a breeding trial. Salers calves with beta-mannosidase deficiency were of normal gestational weight, 36 +/- 6 kg, but were affected at birth. The head was moderately domed, and there was mild superior brachygnathism. The calves were recumbent and had a head tremor. There was bilateral renal enlargement, severe hypomyelination in the brain and variable thyroid gland enlargement. Severe cytoplasmic vacuolation was present within neurons, tubule epithelial cells, follicular cells and macrophages of the nervous, renal, thyroid and lymphoid tissues, respectively. Pedigree analysis and breeding trial results were consistent with an autosomal recessive disease. An initial biochemical survey of 1,494 Salers cattle indicated a carrier frequency of 23%.

Dysmyelinogenesis in animal model of GM1 gangliosidosis

Magnetic resonance imaging (MRI), pathologic examinations, and biochemical analyses were performed on 2 different canine mutants with GM1 gangliosidosis (i.e., English Springer Spaniel and Portuguese Water Dog) and on age- and sex-matched controls. Serial MRI studies were also performed on a child with infantile-onset GM1 gangliosidosis. The affected dogs had abnormalities on MRI, including a relative increase in gray matter and an abnormal signal intensity of cerebral and cerebellar white matter observed on T2-weighted MRI. White matter changes on MRI were similar to white matter abnormalities observed in a 15-month-old boy with GM1 gangliosidosis. The weight ratio of white to gray matter from the frontal lobe was markedly reduced. Microscopic examination revealed characteristic ballooned neurons which stained lightly with Luxol-fast blue. The central cerebral and cerebellar folia white matter exhibited pallor and gliosis, while the corpus callosum and fornix stained normally with Luxol-fast blue. Axons appeared intact on Bodian staining. Ultrastructural studies revealed fewer myelinated axons in affected puppies. Total gangliosides in gray matter were elevated. Thin-layer chromatography demonstrated GM1 ganglioside as the predominant ganglioside. The amount of cerebrosides and sulfatides was reduced in the gray and white matter when compared to controls but the ratio in gray and white matter remained unchanged. Immunostaining of neutral glycolipids disclosed increased amounts of stage-specific embryonic antigen-1 glycolipid in gray matter. These findings suggest that canine models for GM1 gangliosidosis are associated with abnormal myelin development which may be similar to the human disease.

Regional central nervous system oligosaccharide storage in caprine beta-mannosidosis

Goats affected with beta-mannosidosis, an autosomal recessive disease of glycoprotein metabolism, have deficient activity of the lysosomal enzyme beta-mannosidase along with tissue storage of oligosaccharides, including a trisaccharide [Man(beta 1-4)GlcNAc(beta 1-4)GlcNAc] and a disaccharide [Man(beta 1-4)GlcNAc]. CNS myelin deficiency, with regional variation in severity, is a major pathological characteristic of affected goats. This study was designed to investigate regional CNS differences in oligosaccharide accumulation to assess the extent of correlation between oligosaccharide accumulation and severity of myelin deficits. The concentrations of accumulated disaccharide and trisaccharide and the activity of beta-mannosidase were determined in cerebral hemisphere gray and white matter and in spinal cord from three affected and two control neonatal goats. In affected goats, the content of trisaccharide and disaccharide in spinal cord (moderate myelin deficiency) was similar to or greater than that in cerebral hemispheres (severe myelin deficiency). Thus, greater oligosaccharide accumulation was not associated with more severe myelin deficiency. Regional beta-mannosidase activity levels in control goats were consistent with the affected goat oligosaccharide accumulation pattern. The similarity of trisaccharide and disaccharide content in cerebral hemisphere gray and white matter suggested that lysosomal storage vacuoles, more numerous in gray matter, may not be the only location of stored CNS oligosaccharides.

Investigation of dysmyelinogenesis in caprine beta-mannosidosis: in vitro characterization of oligodendrocytes

Central nervous system myelin deficiency is a consistent feature of caprine beta-mannosidosis, an autosomal recessive neurovisceral lysosomal storage disease. To investigate the possibility of an intrinsic oligodendrocyte defect in beta-mannosidosis, oligodendrocyte-enriched glial cultures from the cerebral hemisphere white matter of two affected and six control goats were compared with respect to culture yield and morphology. Fewer oligodendrocytes were cultured per gram of white matter from affected animals than from control animals. Galactocerebroside-positive oligodendrocytes from all animals were similar morphologically at all stages of culture by phase contrast and fluorescence microscopy. These findings are consistent with in vivo morphological observations and suggest that differentiated oligodendrocytes from affected animals do not show morphological abnormalities in culture. However, increased numbers of galactocerebroside-negative bipolar cells, which may be glial progenitor cells, were present in cultures from affected animals. This observation suggests the possibility of a defect in differentiation to mature oligodendrocytes, with persistence of the undifferentiated glia during late stages of development.

Caprine beta-mannosidosis: development of glial and myelin abnormalities in optic nerve and corpus callosum

In caprine beta-mannosidosis, an inherited dysmyelinating disorder, the myelin deficit shows substantial variation throughout the nervous system. In this study morphometric analysis of optic nerve and corpus callosum sections at selected developmental stages was conducted in order to investigate development and persistence of myelin sheaths, the population of axons ensheathed, and the extent of myelin deficits and glial cell abnormalities. The results show that the myelin deficit is severe at very early stages of development and persists to about the same extent into postnatal life. The corpus callosum, much more severely involved than the optic nerve, contains a substantially smaller percentage of myelinated axons when compared to control. In both regions, larger axons are preferentially myelinated. In the corpus callosum before myelination begins, many glial cells appear abnormal, suggesting an early cellular defect. In the postnatal, myelin-deficient corpus callosum, there is a substantial decrease in glial cell density as compared to control, with abnormal appearance of many of the remaining cell profiles. These results define developmental characteristics of the dysmyelination in caprine beta-mannosidosis and document both the early appearance and the persistence of glial cell body and myelin abnormalities.

Otic pathology of caprine beta-mannosidosis

Caprine beta-mannosidosis is an autosomal recessive defect of glycoprotein catabolism with a deficiency of tissue and plasma beta-mannosidase activity and tissue accumulation of oligosaccharides within lysosomes. This rapidly fatal genetic disorder of Nubian goats is expressed at birth by a variety of clinical signs including deafness. Affected goats had folded pinnas, and the tympanic cavity was decreased due to multiple, polypoid projections of bone covered by middle ear mucosa which obstructed the view of the cochlear promontory. Numerous cells of the cochlear duct including mesothelial and epithelial cells of Reissner's membrane, mesothelial cells lining the scala tympani, cells of the stria vascularis, numerous supportive cells of the organ of Corti, cochlear hair cells, endothelial cells, perithelial cells, fibroblasts, macrophages, and neurons of the spiral ganglion contained numerous nonstaining intracytoplasmic vacuoles which resulted in distention of affected cells and caused thickening of involved structures. Ultrastructurally, the vacuoles were membrane-bound and consistent with lysosomes. Vacuolated cells were desquamated into the scala vestibuli and scala tympani. This is one of few reports describing light and electron microscopic otic alterations of a storage disease. Goats with beta-mannosidosis appear to be good models of hearing loss in patients with storage disease.