Pollard, Laura

Abstract Background Deficiency in the enzyme β‐mannosidase was described over three decades ago. Although rare in occurrence, the presentation of childhood‐onset β‐mannosidase deficiency consists of hypotonia in the newborn period followed by global development delay, behavior problems, and intellectual disability. No effective pharmacologic treatments have been available. Methods We report 2‐year outcomes following the first umbilical cord blood transplant in a 4‐year‐old boy with early childhood‐onset disease. Results We show restoration of leukocyte β‐mannosidase activity which remained normal at 2 years posttransplant, and a simultaneous increase in plasma β‐mannosidase activity and dramatic decrease in urine‐free oligosaccharides were also observed. MRI of the brain remained stable. Neurocognitive evaluation revealed test point gains, although the magnitude of improvement was less than expected for age, causing lower IQ scores that represent a wider developmental gap between the patient and unaffected peers. Conclusion Our findings suggest that hematopoietic cell transplant can correct the biochemical defect in β‐mannosidosis, although preservation of the neurocognitive trajectory may be a challenge.

AbstractBackgroundBeta‐galactosidase‐1 (GLB1) is a lysosomal hydrolase that is responsible for breaking down specific glycoconjugates, particularly GM1 (monosialotetrahexosylganglioside). Pathogenic variants in GLB1 cause two different lysosomal storage disorders: GM1 gangliosidosis and mucopolysaccharidosis type IVB. In GM1 gangliosidosis, decreased β‐galactosidase‐1 enzymatic activity leads to the accumulation of GM1 gangliosides, predominantly within the CNS. We present a 22‐month‐old proband with GM1 gangliosidosis type II (late‐infantile form) in whom a novel homozygous in‐frame deletion (c.1468_1470delAAC, p.Asn490del) in GLB1 was detected.MethodsWe used an experimental protein structure of β‐galactosidase‐1 to generate a model of the p.Asn490del mutant and performed molecular dynamic simulations to determine whether this mutation leads to altered ligand positioning compared to the wild‐type protein. In addition, residual mutant enzyme activity in patient leukocytes was evaluated using a fluorometric assay.ResultsMolecular dynamics simulations showed the deletion to alter the catalytic site leading to misalignment of the catalytic residues and loss of collective motion within the model. We predict this misalignment will lead to impaired catalysis of β‐galactosidase‐1 substrates. Enzyme assays confirmed diminished GLB1 enzymatic activity (~3% of normal activity) in the proband.ConclusionsWe have described a novel, pathogenic in‐frame deletion of GLB1 in a patient with GM1 gangliosidosis type II.