β-Mannosidosis in Two Brothers with Hearing Loss

β-Mannosidosis is a lysosomal storage disorder caused by a deficiency of the enzyme β-mannosidase (EC 3.2.1.25). This inborn error of glycoprotein catabolism has been described for goats (Jones and Dawson, 1981). Recently, two independent papers have appeared on human β-mannosidase deficiency (Cooper et al., 1986; Wenger et al., 1986). We have diagnosed this disorder in two brothers with hearing problems by analysing urinary oligosaccharides and by measuring the enzyme activity in leukocytes and plasma.