Kennedy’s disease is a recessive genetic disorder that mostly affects adult men in their 20s and 30s. Symptoms include muscle atrophy (degeneration) and endocrine abnormalities. Although Kennedy’s disease has no treatment or cure, research has produced important insights into genetic markers for the condition, which are used in testing and may ultimately lead to new treatments.

Classified as a motor neuron disease, Kennedy’s disease is very rare, affecting less than 1 in 350,000 males. Although some people with the condition lose their ability to walk or swallow at later stages, life expectancy with the condition is generally normal.

Kennedy’s disease is also known as spinal bulbar muscular atrophy (SBMA) and is primarily characterized by a slow degeneration of lower motor neurons in the spinal cord and bulbar region of the brain, which is the brain stem. The disease is one of a group of hereditary spinal muscular atrophy (SMA) conditions that affects motor neurons. Motor neurons are cells in the central nervous system that send signals to muscles. The disease causes muscle weakness and wasting and is particularly apparent in the legs, arms, and muscles of the face and throat.

X-Linked Mutation

Sometimes called X-linked SBMA, Kennedy’s disease is associated with a mutation in the X chromosome. Men are mostly affected by the condition, because they have only one X chromosome. In contrast, women rarely develop Kennedy’s disease because they have two X chromosomes, and the healthy X chromosome will usually mask the mutation. Women would need two mutated X chromosomes to be affected by Kennedy’s disease. Women also have a far lower incidence of the condition because they have less testosterone (the male sex hormone), which is affected by Kennedy’s disease.

Nonetheless, women can be carriers of the disease and risk passing it on. If a woman is a carrier, any daughters will have a 50 percent chance of becoming carriers and any sons will have a 50 percent chance of having Kennedy’s disease. Daughters of men with Kennedy’s disease will always be carriers. Sons of men with the condition will be unaffected, because they will inherit their father’s normal Y chromosome.

History of Kennedy’s Disease

William R. Kennedy, a neurologist at the University of Minnesota, first published a full report on the condition in 1968. The name “Kennedy’s disease” was first used in French medical literature in 1979.

Kennedy’s research built on observations from the 1950s about X-linked muscle weakness in the limbs, face, and neck among males in several families. However, Kennedy was the first to comprehensively describe adult-onset, progressive SBMA.

Kennedy identified specific characterizations of the disease, including:

• Fine-muscle twitches, known as fasciculations, in the face
• Electrophysiological sensory disturbances, which can be asymptomatic or experienced as tingling or numbness in legs or feet
• Gynecomastia, or enlarged breasts
• Diabetes
• Tremors

These characteristics were later added to a fuller definition of the condition.

Genetic Breakthroughs

In the 1990s, researchers discovered the genetic disorder that causes Kennedy’s disease: a mutation of the androgen receptor (AR) gene, which regulates testosterone. People with Kennedy’s disease have an expanded portion of DNA called the trinucleotide repeat. This section is also known as “sticky DNA” because it can clump together and cause abnormalities in the AR gene.

Dysregulation of testosterone causes a condition called androgen insensitivity in people with Kennedy’s disease, resulting in endocrine and metabolic disorders. Along with muscle degeneration and other neurological symptoms, the condition can cause an array of other symptoms and health conditions, including:

• High cholesterol
• Fatty liver disease
• Insulin resistance and a risk of diabetes
• Testicular atrophy and infertility
• Breast overdevelopment

Researchers still do not understand how this genetic disorder damages motor neurons. Much of the current research is focused on making this crucial connection.

Genetic Testing for Kennedy’s Disease

Kennedy’s disease can be accurately diagnosed through a simple blood test that can identify the defective chromosome. The genetic test is especially valuable in accurately distinguishing Kennedy’s disease from other neuromuscular disorders with similar symptoms, such as:

• Amyotrophic lateral sclerosis, also known as ALS or Lou Gehrig’s disease
• Guillain-Barre syndrome
• Multiple sclerosis
• Spinal muscular atrophy

Find a Supportive Community

On mySMAteam, the social network for people with spinal muscular atrophy and their loved ones, members come together to ask questions, give advice, and share their stories with others who understand life with SMA.

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