Oculopharyngeal muscular dystrophy

Oculopharyngeal muscular dystrophy is a genetic condition characterized by muscle weakness that begins in adulthood, typically after age 40. The first symptom in people with this disorder is usually droopy eyelids (ptosis), followed by difficulty swallowing (dysphagia). The swallowing difficulties begin with food, but as the condition progresses, liquids can be difficult to swallow as well. Many people with this condition have weakness and wasting (atrophy) of the tongue. These problems with food intake may cause malnutrition. Some affected individuals also have weakness in other facial muscles.

Individuals with oculopharyngeal muscular dystrophy frequently have weakness in the muscles near the center of the body (proximal muscles), particularly muscles in the upper legs and hips. The weakness progresses slowly over time, and people may need the aid of a cane or a walker. Rarely, affected individuals need wheelchair assistance.

There are two types of oculopharyngeal muscular dystrophy, which are distinguished by their pattern of inheritance. They are known as the autosomal dominant and autosomal recessive types.

In Europe, the prevalence of oculopharyngeal muscular dystrophy is estimated to be 1 in 100,000 people. The autosomal dominant form of this condition is much more common in the French-Canadian population of the Canadian province of Quebec, where it is estimated to affect 1 in 1,000 individuals. Autosomal dominant oculopharyngeal muscular dystrophy is also seen more frequently in the Bukharan (Central Asian) Jewish population of Israel, affecting 1 in 600 people.

The autosomal recessive form of this condition is very rare; only a few cases of autosomal recessive oculopharyngeal muscular dystrophy have been identified.

Mutations in the PABPN1 gene cause oculopharyngeal muscular dystrophy. The PABPN1 gene provides instructions for making a protein that is active (expressed) throughout the body. In cells, the PABPN1 protein plays an important role in processing molecules called messenger RNAs (mRNAs), which serve as genetic blueprints for making proteins. The protein alters a region at the end of the mRNA molecule that protects the mRNA from being broken down and allows it to move within the cell.

The PABPN1 protein contains an area where the protein building block (amino acid) alanine is repeated 10 times. This stretch of alanines is known as a polyalanine tract. The role of the polyalanine tract in normal PABPN1 protein function is unknown.

Mutations in the PABPN1 gene that cause oculopharyngeal muscular dystrophy result in a PABPN1 protein that has an extended polyalanine tract. The extra alanines cause the PABPN1 protein to form clumps within muscle cells that accumulate because they cannot be broken down. These clumps (called intranuclear inclusions) are thought to impair the normal functioning of muscle cells and eventually cause cell death. The progressive loss of muscle cells most likely causes the muscle weakness seen in people with oculopharyngeal muscular dystrophy. It is not known why dysfunctional PABPN1 proteins seem to affect only certain muscle cells.

Read more about the PABPN1 gene.

Most cases of oculopharyngeal muscular dystrophy are inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. People with autosomal dominant oculopharyngeal muscular dystrophy have a mutation resulting in a PABPN1 protein with an expanded polyalanine tract of between 12 and 17 alanines.

Less commonly, oculopharyngeal muscular dystrophy can be inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations. In autosomal recessive oculopharyngeal muscular dystrophy, PABPN1 mutations lead to a polyalanine tract that is 11 alanines long. The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.