Morquio syndrome (mucopolysaccharidosis type IV; MPS IV) is a mucopolysaccharide storage disease that exists in two forms (Morquio syndromes A and B) and occurs because of a deficiency of the enzymes N-acetyl-galactosamine-6-sulfatase and beta-galactosidase, respectively. A deficiency of either enzyme leads to the accumulation of mucopolysaccharides in the body, abnormal skeletal development, and additional symptoms. In most cases, individuals with Morquio syndrome have normal intelligence. The clinical features of MPS IV-B are usually fewer and milder than those associated with MPS IV-A. Symptoms may include growth retardation, a prominent lower face, an abnormally short neck, knees that are abnormally close together (knock knees or genu valgum), flat feet, abnormal sideways and front-to-back or side-to-side curvature of the spine (kyphoscoliosis), abnormal development of the growing ends of the long bones (epiphyses), and/or a prominent breast bone (pectus carinatum). Hearing loss, weakness of the legs, and/or additional abnormalities may also occur.
The mucopolysaccharidoses (MPS) are a group of inherited lysosomal storage disorders. Lysosomes function as the primary digestive units within cells. Enzymes within lysosomes break down or digest particular nutrients, such as certain carbohydrates and fats. In individuals with MPS disorders, deficiency or malfunction of specific lysosomal enzymes leads to an abnormal accumulation of certain complex carbohydrates (mucopolysaccharides or glycosaminoglycans) in the arteries, skeleton, eyes, joints, ears, skin, and/or teeth. These accumulations may also be found in the respiratory system, liver, spleen, central nervous system, blood, and bone marrow. This accumulation eventually causes progressive damage to cells, tissues, and various organ systems of the body. There are several different types and subtypes of mucopolysaccharidosis. These disorders, with one exception, are inherited as autosomal recessive traits.
Developmental abnormalities in Morquio syndrome are usually detected late in the first year of life or later and may include deformities of the chest (flaring of the lower ribs or abnormal prominence of the breastbone), loose joints, abnormally short neck, or joint deformities such as knock-knees. Affected children have a characteristic facial appearance that may include an enlarged head, broad mouth, prominent cheekbones, an unusually small nose, widely spaced and thinly enameled teeth, and widely separated eyes with subtle corneal clouding. The liver and spleen may be mildly enlarged. Children with Morquio syndrome show marked growth retardation from early in life. The elbows, wrists, hips, knees and other large joints are abnormally flexible, causing overall instability. Early development and intelligence are typically normal. High frequency hearing impairment is common.
Skeletal X-rays typically show marked flattening of the vertebra. The long bones of the arms and legs are characteristically shorter and thicker than normal. The skull is large for the rest of the body. The connection between the first and second vertebrae in the neck is poorly developed and this abnormality can be life threatening. A trivial injury may cause the two vertebrae to slip on each other and compress the spinal cord. Surgery to stabilize the upper cervical spine, usually by spinal fusion, can be lifesaving but life expectancy is decreased somewhat despite surgery. The deformity of the chest causes a strain on the heart and lungs, which may eventually cause respiratory failure.
Morquio syndrome is an autosomal recessive genetic disorder in which a deficiency of lysosomal N-acetylgalactosamine-6-sulfatase in type A of the disorder, and a deficiency of lysosomal beta-galactosidase in type B, leads to an accumulation of keratan sulfate in the cells and tissues of the body. The two types never occur together in the same family.
Genetic diseases are determined by the combination of genes for a particular trait that are on the chromosomes received from the father and the mother.
Recessive genetic disorders occur when an individual inherits the same abnormal gene for the same trait from each parent. If an individual receives one normal gene and one gene for the disease, the person will be a carrier for the disease, but usually will not show symptoms. The risk for two carrier parents to both pass the defective gene and, therefore, have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents and be genetically normal for that particular trait is 25%. The risk is the same for males and females.
Morquio syndrome affects males and females equally often. Estimates of birth prevalence range from 1/40,000 to 1/200,000 births. Morquio syndrome type A occurs more often than type B.
As a group, lysosomal storage diseases (of which the mucopolysaccharidoses are a subgroup) are believed to have an estimated frequency of about one in every 5,000 live births. Although the individual diseases are rare, the group together affects many people around the world.
Excessive amounts of keratan sulfate in the urine will usually be present. The diagnosis is confirmed with the finding of a deficiency of N-acetylgalactosamine-6-sulfatase (type A) or beta-galactosidase (type B) in blood or skin cells. Prenatal diagnosis of Morquio syndrome is possible through amniocentesis or sampling of tissue from the placenta (chorionic villus sampling).
The treatment of Morquio syndrome is symptomatic and supportive. Surgery to decompress and fuse the bones of the upper neck to the base of the skull can prevent destabilization of the cervical vertebrae and potential damage to the spinal cord.
Genetic counseling may be of benefit for affected individuals and their families.
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McKusick VA, ed. Online Mendelian Inheritance in Man (OMIM). Baltimore, MD: The Johns Hopkins University; Entry No. 253000; Last Update: 2/28/03, Entry No. 253010: Last Update: 5/1/02.