OMIM ID:
Morquio Syndrome (MPS IVB)
Alternate Names
Defective Genes
Clinical Characteristics
Ocular Features
Corneal clouding may not be seen until 10 years of age and is sometimes associated with photophobia. The stroma has fine dust-like particles most dense centrally. Penetrating keratoplasty is rarely indicated. There is little retinal degeneration unlike that often seen in other mucopolysaccharidoses but the corneal clouding often precludes detailed examination.
Systemic Features
This form of mucopolysaccharidosis is characterized by the urinary excretion of keratin sulfate. Age of onset is highly variable but most children are diagnosed by 6 years of age. It is a milder disease than the somewhat similar but genetically distinct Morquio type A (253000) disorder. Intelligence is normal and there is no central nervous system involvement. Hip joints are dysplastic and frequently painful. Vertebral malformations lead to kyphoscoliosis and short trunk dwarfism. Odontoid hypoplasia can cause cervical instability and increases the risk of myelopathy with secondary bowel and bladder dysfunction. Coxa valgum, and narrow phalanges are common. Many individuals have a characteristic gait secondary to genu valgum. Patients with MPS IVB initially do not have the coarse facies seen in some other forms of MPS. Further accumulation of cellular keratin sulfate may lead to some coarsening of facial features, increased corneal clouding, and hepatomegaly. Some form of hearing loss is common.
Genetics
Inheritance
This is an autosomal recessive lysosomal storage disease caused by a mutation in the GLB1 gene (3p21.33) encoding beta-galactosidase. It is allelic to GM1 gangliosidosis (230500). Type A Morquio syndrome (253000) is a separate disorder secondary to a mutation in a different gene.
Pedigree
Autosomal recessive
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.
In order for autosomal recessive disorders to be expressed, offspring generally must inherit two mutations, one from each carrier parent. Carriers with only one mutation, such as the parents, do not have clinical disease. Note that carrier parents can expect that 1 in 4 children (25%) will inherit both mutations and have the disorder, 2 in 4 children (50%) will be carriers like their parents, while 1 in 4 children (25%) inherit neither mutation.