Kabuki syndrome is a disorder that affects many parts of the body. It is characterized by distinctive facial features including arched eyebrows; long eyelashes; long openings of the eyelids (long palpebral fissures) with the lower lids turned out (everted) at the outside edges; a flat, broadened tip of the nose; and large protruding earlobes. The name of this disorder comes from the resemblance of its characteristic facial appearance to stage makeup used in traditional Japanese Kabuki theater.
People with Kabuki syndrome have mild to severe developmental delay and intellectual disability. Affected individuals may also have seizures, an unusually small head size (microcephaly), or weak muscle tone (hypotonia). Some have eye problems such as rapid, involuntary eye movements (nystagmus) or eyes that do not look in the same direction (strabismus).
Other characteristic features of Kabuki syndrome include short stature and skeletal abnormalities such as abnormal side-to-side curvature of the spine (scoliosis), short fifth (pinky) fingers, or problems with the hip and knee joints. The roof of the mouth may have an abnormal opening (cleft palate) or be high and arched, and dental problems are common in affected individuals. People with Kabuki syndrome may also have fingerprints with unusual features and fleshy pads at the tips of the fingers. These prominent finger pads are called fetal finger pads because they normally occur in human fetuses; in most people they disappear before birth.
A wide variety of other health problems occur in some people with Kabuki syndrome. Among the most commonly reported are heart abnormalities, frequent ear infections (otitis media), hearing loss, and early puberty.
Kabuki syndrome is caused by mutations in the KMT2D gene (also known as MLL2) or the KDM6A gene.
Between 55 and 80 percent of cases of Kabuki syndrome are caused by mutations in the KMT2D gene. This gene provides instructions for making an enzyme called lysine-specific methyltransferase 2D that is found in many organs and tissues of the body. Lysine-specific methyltransferase 2D functions as a histone methyltransferase. Histone methyltransferases are enzymes that modify proteins called histones. Histones are structural proteins that attach (bind) to DNA and give chromosomes their shape. By adding a molecule called a methyl group to histones (a process called methylation), histone methyltransferases control (regulate) the activity of certain genes. Lysine-specific methyltransferase 2D appears to activate certain genes that are important for development.
Between 2 and 6 percent of cases of Kabuki syndrome are caused by mutations in the KDM6A gene. This gene provides instructions for making an enzyme called lysine-specific demethylase 6A. This enzyme is a histone demethylase, which means that it helps to remove methyl groups from certain histones. Like lysine-specific methyltransferase 2D, lysine-specific demethylase 6A regulates the activity of certain genes, and research suggests that the two enzymes work together to control certain developmental processes.
The KMT2D and KDM6A gene mutations associated with Kabuki syndrome lead to the absence of the corresponding functional enzyme. A lack of the enzymes produced from these genes disrupts normal histone methylation and impairs proper activation of certain genes in many of the body's organs and tissues, resulting in the abnormalities of development and function characteristic of Kabuki syndrome.
Some people with Kabuki syndrome have no identified KMT2D or KDM6A gene mutation. The cause of the disorder in these individuals is unknown.
Kabuki syndrome occurs in approximately 1 in 32,000 newborns.
When Kabuki syndrome is caused by mutations in the KMT2D gene, it is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder.
When Kabuki syndrome is caused by mutations in the KDM6A gene, it is inherited in an X-linked dominant pattern. The KDM6A gene is located on the X chromosome, which is one of the two sex chromosomes. In females (who have two X chromosomes), a mutation in one of the two copies of the gene in each cell is sufficient to cause the disorder. In males (who have only one X chromosome), a mutation in the only copy of the gene in each cell causes the disorder. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.
Most cases of Kabuki syndrome result from a new mutation in one of these genes and occur in people with no history of the disorder in their family. In a few cases, an affected person is believed to have inherited the mutation from one affected parent.