β-mannosidosis is an ultra-rare lysosomal storage disorder caused by a deficiency of β-mannosidase, which catalyzes the last step of glycoprotein degradation. Owing to the limited number of reported cases, information on the natural history of the disease and brain imaging is scarce. We report 6 new cases and review them together with all the previously reported cases in the literature. We describe the clinical features of 6 unrelated patients with β-mannosidosis, their variants in MANBA, enzyme activity, urine oligosaccharide profiles, brain MRI findings, and longitudinal MRI changes in 2 of them. We also review previously reported patients to broaden the spectrum of clinical features and identify genotype-phenotype correlations. Developmental regression, dysphagia, obsessive-compulsive-like behavior, and erythromelalgia are newly described features associated with β-mannosidosis among the 6 patients from our cohort. Nystagmus in association with β-mannosidosis was also found in 1 patient. Half the patients have abnormal brain MRIs, showing delayed myelination before 2 years of age and diffuse hypomyelination thereafter. A new oligosaccharide was found in the urine of the 2 most severely affected patients. Including our 6 patients, a total of 46 cases from 37 different families have been reported. The mean age of diagnosis is 12.8 years, and the mean age of symptom onset is 2.4 years. Hearing loss was the initial symptom most frequently reported, and intellectual disability was the most frequent symptom overall. Across the cohort, 29 pathogenic MANBA variants were identified, 61.8% were private, and 38.2% have the recurrent c.2158-2A>G variant. Neuroimaging abnormalities were reported in 40% of published cases and included cortical and subcortical atrophy, basal ganglia and white matter calcification, hydrocephalus, periventricular and subcortical white matter hyperintensities, and delayed myelination. Genotype-phenotype correlation in β-mannosidosis remains elusive, likely due to phenotypic variability, disease rarity, and lack of association between the enzyme activity and the clinical severity. Modifier genes in the N-glycan degradation pathway, such as CTBS, may influence disease expression. β-mannosidosis should be considered as a differential diagnosis in patients with syndromic or apparently nonsyndromic hearing loss, unexplained brain hypomyelination, behavioral disturbances, and intellectual disability.

A 6-month-old cat of unknown ancestry presented for a neurologic evaluation due to progressive motor impairment. Complete physical and neurologic examinations suggested the disorder was likely to be hereditary, although the signs were not consistent with any previously described inherited disorders in cats. Due to the progression of disease signs including severely impaired motor function and cognitive decline, the cat was euthanized at approximately 10.5 months of age. Whole genome sequence analysis identified a homozygous missense variant c.2506G > A in MANBA that predicts a p.Gly836Arg alteration in the encoded lysosomal enzyme β -mannosidase. This variant was not present in the whole genome or whole exome sequences of any of the 424 cats represented in the 99 Lives Cat Genome dataset. β -Mannosidase enzyme activity was undetectable in brain tissue homogenates from the affected cat, whereas α-mannosidase enzyme activities were elevated compared to an unaffected cat. Postmortem examination of brain and retinal tissues revealed massive accumulations of vacuolar inclusions in most cells, similar to those reported in animals of other species with hereditary β -mannosidosis. Based on these findings, the cat likely suffered from β -mannosidosis due to the abolition of β -mannosidase activity associated with the p.Gly836Arg amino acid substitution. p.Gly836 is located in the C-terminal region of the protein and was not previously known to be involved in modulating enzyme activity. In addition to the vacuolar inclusions, some cells in the brain of the affected cat contained inclusions that exhibited lipofuscin-like autofluorescence. Electron microscopic examinations suggested these inclusions formed via an autophagy-like process.

Oligosaccharidoses, sphingolipidoses and mucolipidoses are lysosomal storage disorders (LSDs) in which defective breakdown of glycan-side chains of glycosylated proteins and glycolipids leads to the accumulation of incompletely degraded oligosaccharides within lysosomes. In metabolic laboratories, these disorders are commonly diagnosed by thin-layer chromatography (TLC) but more recently also mass spectrometry-based approaches have been published. To expand the possibilities to screen for these diseases, we developed an ultra-high-performance liquid chromatography (UHPLC) with a high-resolution accurate mass (HRAM) mass spectrometry (MS) screening platform, together with an open-source iterative bioinformatics pipeline. This pipeline generates comprehensive biomarker profiles and allows for extensive quality control (QC) monitoring. Using this platform, we were able to identify α-mannosidosis, β-mannosidosis, α-N-acetylgalactosaminidase deficiency, sialidosis, galactosialidosis, fucosidosis, aspartylglucosaminuria, GM1 gangliosidosis, GM2 gangliosidosis (M. Sandhoff) and mucolipidosis II/III in patient samples. Aberrant urinary oligosaccharide excretions were also detected for other disorders, including NGLY1 congenital disorder of deglycosylation, sialic acid storage disease, MPS type IV B and GSD II (Pompe disease). For the latter disorder, we identified heptahexose (Hex7), as a potential urinary biomarker, in addition to glucose tetrasaccharide (Glc4), for the diagnosis and monitoring of young onset cases of Pompe disease. Occasionally, so-called "neonate" biomarker profiles were observed in young patients, which were probably due to nutrition. Our UHPLC/HRAM-MS screening platform can easily be adopted in biochemical laboratories and allows for simple and robust screening and straightforward interpretation of the screening results to detect disorders in which aberrant oligosaccharides accumulate.