Beta-mannosidosis in a domestic cat associated with a missense variant in MANBA

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.

Angiokeratoma corporis diffusum in human β-mannosidosis: Report of a new case and a novel mutation

BACKGROUND

Human beta-mannosidosis, a rare disorder of oligosaccharide catabolism, results from a deficiency of beta-mannosidase activity. So far, mutational analysis has been performed in only seven families and revealed 11 mutations in the MANBA gene which encodes the enzyme beta-mannosidase.

OBJECTIVES

We report here a 36-year-old Arab female with beta-mannosidosis who presented with mental retardation and multiple angiokeratomas. We describe in this patient a novel null mutation and review the previously reported MANBA gene mutations and their clinical correlations.

METHODS

Histopathology, ultrastructural analysis, and enzyme assays were performed. Sequencing of cDNA and genomic DNA analysis was conducted in a search for a mutation in the MANBA gene.

RESULTS

Histopathology of a skin biopsy specimen from the patient showed the characteristic findings of angiokeratoma. Electron microscopy showed cytoplasmic vacuolation. Enzymatic activity of beta-mannosidase in the patient's serum, leukocytes, and fibroblasts was less than 1% of control values. Sequencing of the MANBA cDNA revealed a G-->A transition in exon 6 at nucleotide position c.693, resulting in the formation of a stop codon (W231X).

LIMITATIONS

Only one family was studied.

CONCLUSIONS

A new case of human beta-mannosidosis is presented and the first MANBA gene mutation from Arab ancestry is reported. Reviewing the reported MANBA gene mutations does not reveal a clear genotype-phenotype correlation. The importance of angiokeratoma corporis diffusum as the clue to the diagnosis of beta-mannosidosis and other lysosomal storage diseases is emphasized.