Familial Global Developmental Delay Secondary to β-Mannosidosis

β-Mannosidosis is a rare lysosomal storage disorder that is caused by a deficiency of β-mannosidase activity, which is due to mutations of the MANBA gene. Two Indian siblings born out of a third-degree consanguineous marriage presented during late infancy with global developmental delay. On examination, both the siblings had hypotonia; hepatosplenomegaly was present in the first sibling whereas it was absent in the second sibling. Fundus evaluation, hearing assessment, and skeletal survey were normal in both siblings. Enzyme assay showed the absence of the β-mannosidase enzyme. Next-generation sequencing showed a homozygous variation of c.1317 + 1G>A in intron 10 of the MANBA (–) gene in the elder sibling. Sanger sequencing confirmed the same mutation in the homozygous state in both siblings and in the heterozygous state in both parents.