Oculopharyngeal muscular dystrophy (OPMD) is a type of muscular dystrophy, a group of genetic diseases characterized by muscle weakness and wasting. The initial symptoms of OPMD are weakness in the muscles of the eyes and throat, but as the disease progresses other muscle groups also may be affected.
The genetic cause of OPMD
OPMD is caused by mutations in the PABPN1 gene, which provides instructions to make a protein involved in protecting messenger RNA (temporary gene copies called mRNA), which are used as templates to make proteins by the cell.
OPMD can be inherited in two distinct patterns: autosomal recessive or autosomal dominant. In normal PABPN1 protein, alanine (an amino acid, or protein building block) is repeated 10 times; in autosomal recessive OPMD, there is a mutation in the PABPN1 gene that results in an additional alanine in the protein sequence (bringing the total to 11 alanines in a row). In autosomal dominant OPMD, the number of alanines is 12 to 17 in a row.
In both types of OPMD, the increase in alanines makes the PABPN1 protein “sticky” forming clumps (aggregates) within muscle cells, causing the symptoms of the disease. The more alanines, the more “sticky” the proteins become, which is thought to be the reason behind the difference between autosomal recessive and autosomal dominant forms of the disease.
How is autosomal recessive OPMD inherited?
Autosomal means the disease-causing gene is situated on an autosomal or non-sex chromosome, meaning that both males and females are equally likely to be affected by the disease. Recessive means that two copies of the disease-causing mutation are required for the disease to develop. People with only one copy of a disease-causing mutation are considered carriers; they generally do not have symptoms of the disease, but do have a 50% chance of transmitting the mutation to their children. Children who inherit two copies of the disease-causing mutation (one from each parent) will develop the disease.
If a parent has the disease (that is, has two copies of the disease-causing mutation) all of his/her children will be carriers. If he/she has children with someone who has a single copy of the disease-causing mutation, their children have a 50% chance of being carriers, and a 50% chance of inheriting two copies of the disease-causing gene and developing the disease.
Because the symptoms of OPMD appear in middle-age (around age 40 or 50) patients should have their children tested for the disease-causing mutation as soon as they are diagnosed. Part of the treatment for OPMD is usually meeting with a genetic counselor who can explain the results of any genetic tests as well as recommending which family members should be tested for the disease.
How is autosomal dominant OPMD inherited?
Autosomal dominant OPMD is the most common form of the disease. For this form of the disease, a single copy of the disease-causing mutation is sufficient to cause the disease. This means that any patient with a single copy of the disease-causing mutation has a 1 in 2 chance of transmitting the mutation to their children, who would also develop the disease. If both parents have a single copy of the disease-causing mutation, their children have a 75% chance of developing the disease, and only a 25% chance of not inheriting OPMD.
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