Mucopolysaccharidosis IV (MPS IV)
MPS IV, also referred to as Morquio syndrome, is caused by a deficiency of an
enzyme that is needed to break down the mucopolysaccharides/glycosaminoglycans
(GAGs) keratan sulfate and chondroitin 6-sulfate, which are found in the
cartilage and bone. GAGs are long, repeating chains of complex sugar molecules.
When one of several enzymes is absent, a progressive buildup of the GAG material
occurs in various tissues of the body and leads to many health problems.
Forms of the disorder
MPS IV is traditionally divided into two possible forms, with each type
deficient in a specific enzyme. MPS IVA is caused by a
deficiency of the enzyme N-acetylgalactosamine-6-sulfatase. MPS
IVB is caused by a deficiency of the enzyme beta-galactosidase. The
frequency of Morquio syndrome is estimated at 1 in every 200,000 births.
Clinical features
Individuals with Morquio syndrome may present with a broad spectrum of
clinical features between the ages of 1 to 3 ½ years. These symptoms may include
short stature, kyphosis (curvature of the spine causing a “lump on the back”),
genu valgum (knock-knees), hernias, corneal clouding, and coarse facial features
(flattened bridge of the nose, thick lips and an enlarged tongue). The severity
of the findings varies. Typically, intelligence is not affected. Characteristic
findings of Morquio syndrome are seen on x-rays, which allow the diagnosis to be
suspected by an experienced radiologist.
Genetics of the disorder
MPS IV is inherited as an autosomal recessive trait. Here's how that works:
All individuals have two copies of the specific MPS IV gene and usually both
function normally. However, children with MPS IV have alterations in both copies
of their genes so that neither gene copy works properly. Children receive one
copy of the altered gene from their mother and one copy of the altered gene from
their father. The parents are not affected with the condition because they have
one normal copy of the gene paired with one altered copy of the gene. The normal
gene produces enough of the specific enzyme to prevent the disorder. This is
referred to as being a carrier for the condition. When both parents are
carriers, they have a 25% chance with each pregnancy to have an affected child.
In turn, parents who are carriers have a 75% chance with each pregnancy to have
an unaffected child. More here
about autosomal recessive inheritance.
Treatment
Management of the disorder also involves symptomatic treatment of specific
complications that can arise in a patient with MPS IV. Individuals with MPS IV
are often referred to Children's Memorial Hospital's skeletal dysplasia
program.