What is Smith-Magenis Syndrome?

Overview

Smith-Magenis syndrome is a genetic disability due to a microdeletion or mutation on chromosome 17. The major features of Smith-Magenis Syndrome (SMS) include mild to moderate intellectual disability, delayed speech and language skills, distinctive facial features, sleep disturbances, and behavioural problems.

Characteristics

Individuals with Smith-Magenis Syndrome are characterised by a short flat head, prominent forehead, broad square face, deep-set eyes, underdeveloped midface, broad nasal bridge, short nose, tented upper lip, and a chin that is small in infancy and becomes relatively prominent with age. These facial differences can be subtle in early childhood, but they usually become more distinctive in later childhood and adulthood. Dental abnormalities are also common.

During infancy quite often there are feeding problems, failure to thrive, weak muscle tone, prolonged napping and lethargy. Then later in childhood and adulthood there can be the development of disturbed sleep patterns (difficulty falling asleep, regular waking throughout the night and sleepiness in the day), as well as behavioural problems.

These behaviour problems can be exhibited in a variety of ways, frequent temper tantrums, impulsivity, anxiety, distractibility, aggression and self-injurious behaviours including self-hitting, self-biting, and skin picking.

In addition to the above challenging behaviours, there are stereotypical behaviours including ‘self-hugging’ hand licking and page flipping (“lick and flip”) behaviour, mouthing objects, insertion of hand in mouth, teeth grinding, body rocking, and spinning or twirling objects.

Other signs and symptoms of Smith-Magenis syndrome include short stature, abnormal curvature of the spine (scoliosis), reduced sensitivity to pain and temperature, possible toileting difficulties and a hoarse voice. Some individuals also have ear abnormalities that lead to hearing loss and also possible vision difficulties. Although less common, heart and kidney defects have also been reported in Individuals with Smith-Magenis syndrome.

Scientific Explanation

Smith-Magenis Syndrome (SMS) is a developmental disorder that affects many parts of the body. The major features of this condition include mild to severe learning disability, distinctive facial features, sleep disturbances, and behavioral problems. Smith-Magenis syndrome affects an estimated 1 in 25,000 individuals.

The syndrome is due to an abnormality in the short (p) arm of chromosome 17 and is sometimes called the 17p- syndrome.

Smith-Magenis syndrome is a chromosomal condition related to chromosome 17. Most people with SMS have a deletion of genetic material from a specific region of chromosome 17 (17p11.2). Although this region contains multiple genes, recently researchers discovered that the loss of one particular gene the retinoic acid induced 1 or RAI1 is responsible for most of the characteristic features of this condition. Also, other genes within the chromosome 17 contribute to the variability and severity of the clinical features. The loss of other genes in the deleted region may help explain why the features of Smith-Magenis syndrome vary among affected individuals. A small percentage of people with Smith-Magenis syndrome have a mutation in the RAI1 gene instead of a chromosomal deletion.

These deletions and mutations lead to the production of an abnormal or nonfunctional version of the RAI1 protein. RAI1 is a transcription factor involved in communication messages between DNA and RNA. The groups led by James Lupski (Baylor College of Medicine) and Sarah Elsea (Virginia Commonwealth University) are in the process of studying the exact function of this gene in relation to Smith Magenis Syndrome.

SMS is typically not inherited. This condition usually results from a genetic change that occurs during the formation of reproductive cells (eggs or sperm) or in early fetal development. People with Smith-Magenis syndrome most often have no history of the condition in their family.