Alternative Names
chromosome 17p deletion syndrome; deletion 17p syndrome; 17p11.2 monosomy; partial monosomy 17p; SMS; 17p- syndrome
Symptoms & Characteristics
Smith-Magenis syndrome is a genetic condition that affects various parts of the body. Characteristics and symptoms vary greatly from person to person. Some major characteristics and symptoms may include:
- mild to moderate mental retardation/developmental delay
- delayed speech and language skills
- low muscle tone in infancy (hypotonia)
- feeding problems and lethargy in infancy
- sleep disturbances and abnormal sleep patterns
- affectionate, engaging personalities
- abnormal curving of spine (scoliosis)
- behavioral problems (such as tantrums, aggression, impulsiveness, and hyperactivity)
- repetitive self-hugging is a very unique characteristic
- compulsive licking of fingers to turn pages of books is also unique
- vision problems and/or eye abnormalities
- reduced sensitivity to pain and temperature
People with Smith-Magenis syndrome may have self-injurious behaviors such as self-hitting; self-biting; skin picking; head banging; inserting foreign objects into body orifices (called polyembolokoilamania); as well as yanking fingernails and/or toenails (called onychotillomania). In addition, affected people have distinctive facial features, which are more prominent in adulthood such as a broad, flattened, square-shaped face; deep-set eyes; full cheeks; prominent lower jaw; downward turning mouth; and full, outward-curving upper lip.
Treatment
There is currently no cure for Smith-Magenis syndrome. Early diagnosis, routine surveillance and treatment can help to manage some of the symptoms and sometimes prevent related problems.
How Common Is It?
About 1 in 15,000 to 1 in 25,000 people are affected with Smith-Magenis syndrome.
Genetics & Inheritance
In the majority of affected people, Smith-Magenis syndrome is caused by a deletion of genetic material from a specific region of chromosome 17 at the location p11.2. This region contains multiple genes. However, researchers believe that the loss of one particular gene called RAI1 causes most of the characteristic features of this condition. The loss of other genes in this region may contribute to the variability in the features and symptoms among affected people.
- A small percentage of people with Smith-Magenis syndrome have a mutation in the RAI1 gene as opposed to a chromosomal deletion. These affected people tend to be obese and are less likely to have short stature, hearing loss, and heart or kidney abnormalities.
We typically have two normal copies of this chromosome region, one from each parent. In Smith-Magenis syndrome, having one deleted region or one RAI1 mutation is sufficient to cause the disease. This is called autosomal dominant inheritance.
- Although there are no reported cases of people with Smith-Magenis syndrome having affected children, theoretically, an affected person has a 50% chance with each pregnancy to have an affected child.
Almost all affected people are the first one in their family to be affected. Their chromosome deletion occurs sporadically or their RAI1 mutation is a result of a new ("de novo") mutation.
- In rare cases, a chromosome translocation in an unaffected parent leads to the deletion of genetic material in the affected person.
Due to these rare cases, parents of an affected person may want to consider genetic testing for Smith-Magenis syndrome themselves. The benefits and limitations of such testing should be discussed with a qualified health care professional.
- If both parents have negative genetic testing, recurrence risk to have another affected child may still be increased over the general population but less than 1%. This may be due to a complex concept called germline mosaicism in which some of a person's egg or sperm cells may have a disease causing mutation, while the rest of the cells in their body do not.
There are other genetic conditions that have features that overlap with Smith-Magenis syndrome. Some of these conditions include deletion 22q11.2 syndrome, Prader-Willi syndrome and Williams syndrome.
A genetic consultation with a trained genetic professional is important for a complete evaluation, accurate diagnosis, as well as discussion of the benefits and limitations of testing and recurrence risk.
Genetic Testing
Clinical genetic testing for Smith-Magenis syndrome may be available through an in person genetic consultation for people who are considered at risk. Use our find a genetic professional directory to locate a trained genetic professional in your area.
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