Smith-Kingsmore syndrome is a neurological disorder characterized by a head that is larger than normal (macrocephaly), intellectual disability, and seizures. In some people with this condition, the ability to speak is delayed or never develops. Some children with Smith-Kingsmore syndrome have features of a behavioral condition called attention-deficit/hyperactivity disorder (ADHD) or autism spectrum disorder, which is characterized by impaired communication and social interaction. Structural brain abnormalities may also be present in affected individuals. For example, one or both sides of the brain may be enlarged (hemimegalencephaly or megalencephaly) or have too many ridges on the surface (polymicrogyria), or the fluid-filled spaces near the center of the brain (ventricles) may be bigger than normal (ventriculomegaly).
Many people with Smith-Kingsmore syndrome have unusual facial features, such as a triangular face with a pointed chin, a protruding forehead (frontal bossing), widely spaced eyes (hypertelorism) with outside corners that point downward (downslanting palpebral fissures), a flat nasal bridge, or a long space between the nose and upper lip (long philtrum). However, not everyone with Smith-Kingsmore syndrome has distinctive facial features.
Mutations in a gene called MTOR cause Smith-Kingsmore syndrome. The protein produced from this gene, called mTOR, is a key piece of two groups of proteins, known as mTOR complex 1 (mTORC1) and mTOR complex2 (mTORC2). These two complexes relay signals inside cells that regulate protein production and control several cellular processes, including growth, division, and survival. This mTOR signaling is especially important for growth and development of the brain, and it plays a role in a process called synaptic plasticity, which is the ability of the connections between nerve cells (synapses) to change and adapt over time in response to experience. Synaptic plasticity is critical for learning and memory.
MTOR gene mutations that cause Smith-Kingsmore syndrome increase the activity of the mTOR protein and, consequently, mTOR signaling. As a result, protein production normally regulated by these complexes is uncontrolled, which impacts cell growth and division and other cellular processes. Too much mTOR signaling in brain cells disrupts brain growth and development and synaptic plasticity, leading to macrocephaly, intellectual disability, seizures, and other neurological problems in people with Smith-Kingsmore syndrome. Excessive mTOR signaling in other parts of the body likely underlies the unusual facial features and other less common signs and symptoms of the condition. It is unclear why the brain is particularly affected in people with Smith-Kingsmore syndrome.
Smith-Kingsmore syndrome is a rare condition with an unknown prevalence.
Smith-Kingsmore syndrome follows an autosomal dominant inheritance pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder. Most cases result from new (de novo) mutations in the gene that occur during the formation of reproductive cells (eggs or sperm) in an affected individual’s parent or in early embryonic development. Rarely, people with Smith-Kingsmore syndrome inherit the altered gene from an unaffected parent who has an MTOR gene mutation only in their sperm or egg cells. This phenomenon is called germline mosaicism.
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