β-Galactosidase Deficiency (β-Galactosidosis): G_M1 Gangliosidosis and Morquio B Disease

ABSTRACT

1. Hereditary deficiency of lysosomal acid β-galactosidase (β-galactosidosis) is expressed clinically as two different diseases, G_M1 gangliosidosis (OMIM 230500) and Morquio B disease (OMIM 253010). The mode of inheritance is autosomal recessive. G_M1 gangliosidosis is a neurosomatic disease occurring mainly in early infancy (infantile form; type 1). Developmental arrest is observed a few months after birth, followed by progressive neurologic deterioration and generalized rigospasticity with sensorimotor and psychointellectual dysfunctions. Macular cherry-red spots, facial dysmorphism, hepatosplenomegaly, and generalized skeletal dysplasia are usually present in infantile cases. Cases of later onset have been described as the late infantile/juvenile form (type 2) or adult/chronic form (type 3). They are observed as progressive neurologic diseases in children or young adults. Dysmorphic changes are less prominent or absent in these clinical forms, although vertebral dysplasia is often detected by radiographic studies. No specific neurologic manifestations are known for late infantile/juvenile patients with G_M1 gangliosidosis. Extrapyramidal signs of protracted course, mainly presenting as dystonia, are the major neurologic manifestation in adults with G_M1 gangliosidosis.

2. Morquio B disease is a clinically mild phenotype of Morquio A disease. It is expressed as generalized skeletal dysplasia with corneal clouding, resulting in short stature, pectus carinatum (sternal protrusion), platyspondylia, odontoid hypoplasia, kyphoscoliosis, and genu valgum. There is no central nervous system involvement, although spinal cord compression may occur at the late stage of the disease. Intelligence is normal, and hepatosplenomegaly is not present. X-ray changes are of pathognomonic significance.

3. There is diffuse atrophy of the brain in patients with early-onset G_M1 gangliosidosis. Neurons are filled with numerous membranous cytoplasmic bodies (MCBs), and inclusions of other types are observed in glial cells: pleomorphic lipid bodies, membranovesicular bodies, or large, compact oval deposits. There are histiocytes with distended cytoplasm in visceral organs. Cytoplasmic inclusions observed under electron microscopy are different from MCBs in neurons. They are vacuoles filled with fine granular, tubular, or amorphous osmiophilic material. These changes are less prominent in cases of mild phenotypic expression.

4. Glycoconjugates with terminal β-linked galactose are increased in tissues and urine from patients with G_M1 gangliosidosis and Morquio B disease. Ganglioside G_M1 and its asialo derivative G_A1 accumulate in the G_M1 gangliosidosis brain. High amounts of oligosaccharides derived from keratan sulfate and glycoproteins have been reported in visceral organs and urine from G_M1 gangliosidosis or Morquio B disease patients. Undersulfated keratan sulfate also has been described.

5. Two lysosomal enzymes are known for hydrolysis of terminal β-linked galactose at acidic pH in various glycoconjugates. One is an enzyme usually called β-galactosidase (EC 3.2.1.23), catabolizing ganglioside G_M1, galactose-containing oligosaccharides, keratan sulfate, and other β-galactose-containing glycoconjugates (G_M1 β-galactosidase). The enzyme activity is markedly reduced or almost completely deficient in cells and body fluids from patients with β-galactosidosis. Heterogeneous kinetic or physicochemical properties have been found in the mutant enzymes. The degree of substrate storage and residual enzyme activity ...