Morquio syndrome

Morquio syndrome's impact on the human skeletal structure.

Morquio syndrome, clinically recognized as Mucopolysaccharidosis Type IV (MPS IV), is a rare and inherited metabolic disorder. Patients with this syndrome lack the necessary enzymes to break down a specific group of sugar molecules termed glycosaminoglycans (often abbreviated as GAGs or mucopolysaccharides). The accumulation of these GAGs, primarily keratan sulfate, leads to multi-organ dysfunction and various associated symptoms.

Genetic Foundation

Morquio syndrome is an autosomal recessive birth defect, implying that a child must inherit two copies of the defective gene (one from each parent) to manifest the disorder. The underlying defect is categorized under lysosomal storage disorders, wherein the body's lysosomes fail to degrade waste molecules, resulting in their accumulation.

Glycosaminoglycans and Their Role

Glycosaminoglycans (GAGs) play a pivotal role in the matrix of many tissues. Under normal circumstances, they undergo constant turnover - synthesis followed by degradation. In Morquio syndrome, the degradation pathway is interrupted due to enzyme deficiencies.

• Keratan sulfate: Particularly important in Morquio syndrome, it's predominantly found in the cornea, cartilage, and bone. The inability to degrade keratan sulfate leads to its systemic buildup.

Symptoms and Organ Systems Affected

Morquio syndrome's clinical presentation varies, resulting from the deposition of GAGs in different body parts:

• Skeletal system: Most individuals exhibit skeletal abnormalities, such as short stature, knock-knees, and abnormalities in the neck vertebrae.
• Respiratory system: Airway obstruction and pulmonary complications are commonly observed.
• Cardiovascular system: Heart valve disease and coronary artery complications can arise.
• Vision: Corneal clouding leading to visual impairment.
• Hearing: Progressive hearing loss can occur due to recurrent ear infections and buildup of GAGs.
• Mobility: Joint issues can restrict movement and cause pain.

Signs and symptoms

Corneal clouding in a 30-year-old male with MPS VI. Morquio syndrome and other MPS disorders may also present with corneal clouding

Patients with Morquio syndrome appear healthy at birth. Types A and B have similar presentations, but Type B generally has milder symptoms. The age of onset is usually between 1 and 3 years of age. Morquio syndrome causes progressive changes to the skeleton of the ribs and chest, which may lead to neurological complications such as nerve compression. Patients may also have hearing loss and clouded corneas. Intelligence is usually normal unless a patient suffers from untreated hydrocephalus.

Physical growth slows and often stops around age 8. Skeletal abnormalities include a bell-shaped chest, a flattening or curvature of the spine, shortened long bones, and dysplasia of the hips, knees, ankles, and wrists. The bones that stabilize the connection between the head and neck can be malformed (odontoid hypoplasia); in these cases, a surgical procedure called spinal cervical bone fusion can be lifesaving. Restricted breathing, joint stiffness, and heart disease are also common. Children with the more severe form of MPS IV may not live beyond their twenties or thirties.

Cause

Morquio syndrome is inherited from an autosomal recessive inherited gene. Every person has two copies of the genes needed to break down keratan sulfate, but only one healthy copy is needed. Both parents pass down one defective copy to their child, resulting in a child with no functional copies of the gene. As such, the body is incapable breaking down keratan sulfate for disposal. The incompletely broken down GAGs remain stored in cells in the body, causing progressive damage. Babies may show little sign of the disease, but as more and more cells become damaged, symptoms start to appear.

Diagnosis

Classification

This syndrome has two forms, A and B, referred to as Morquio A and Morquio B syndrome or MPS IVA and MPS IVB. The two forms are distinguished by the gene product involved; Type A involves a malfunction in the GALNS gene, while Type B involves a malfunction of the GLB1 gene.

Genetics of MPS IV

Morquio syndrome type	Gene	Missing enzyme	Chromosomal region
Type A	GALNS	Galactosamine-6 sulfatase	16q24
Type B	GLB1	Beta-galactosidase	3p22

Treatment

The treatment for Morquio syndrome consists of prenatal identification and of enzyme replacement therapy. On 12 February 2014, the US Food and Drug Administration approved the drug elosulfase alfa (Vimizim) for treating Type A. Currently, there is no treatment for Type B.

Prognosis

The lifespan of patients with Morquio syndrome is variable and depends on the subtype. Type A is generally severe, with a life expectancy in the 20s to 30s.

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