CDKL5 deficiency disorder is characterized by seizures that begin in infancy, followed by significant delays in many aspects of development.

Seizures in CDKL5 deficiency disorder usually begin within the first 3 months of life, and can appear as early as the first week after birth. The types of seizures change with age, and may follow a predictable pattern. The most common types are generalized tonic-clonic seizures, which involve a loss of consciousness, muscle rigidity, and convulsions; tonic seizures, which are characterized by abnormal muscle contractions; and epileptic spasms, which involve short episodes of muscle jerks. Seizures occur daily in most people with CDKL5 deficiency disorder, although they can have periods when they are seizure-free. Seizures in CDKL5 deficiency disorder usually do not get better with treatment.

Development is impaired in children with CDKL5 deficiency disorder. Most have severe intellectual disability and little or no speech. The development of gross motor skills, such as sitting, standing, and walking, is delayed or not achieved. About one-third of affected individuals are able to walk independently. Fine motor skills, such as picking up small objects with the fingers, are also impaired; about half of affected individuals have purposeful use of their hands. Most people with this condition have vision problems (cortical visual impairment).

Other common features of CDKL5 deficiency disorder include repetitive hand movements (stereotypies), such as clapping, hand licking, and hand sucking; teeth grinding (bruxism); disrupted sleep; feeding difficulties; and gastrointestinal problems including constipation and backflow of acidic stomach contents into the esophagus (gastroesophageal reflux). Some affected individuals have episodes of irregular breathing. Distinctive facial features in some people with CDKL5 deficiency disorder include a high and broad forehead, large and deep-set eyes, a well-defined space between the nose and upper lip (philtrum), full lips, widely spaced teeth, and a high roof of the mouth (palate). Other physical differences can also occur, such as an unusually small head size (microcephaly), side-to-side curvature of the spine (scoliosis), and tapered fingers.

CDKL5 deficiency disorder was previously classified as an atypical form of Rett syndrome. These conditions have common features, including seizures, intellectual disability, and other problems with development. However, the signs and symptoms associated with CDKL5 deficiency disorder and its genetic cause are distinct from those of Rett syndrome, and CDKL5 deficiency disorder is now considered a separate condition.

CDKL5 deficiency disorder appears to be a rare condition with an incidence of 1 in 40,000 to 60,000 newborns. About 90 percent of those diagnosed with CDKL5 deficiency disorder are girls.

As its name suggests, CDKL5 deficiency disorder is caused by variants (also known as mutations) in the CDKL5 gene. This gene provides instructions for making a protein that is essential for normal brain development and function.

Variants in the CDKL5 gene reduce the amount of functional CDKL5 protein or alter its activity in nerve cells (neurons). A shortage (deficiency) of CDKL5 or impairment of its function disrupts brain development, but it is unclear how these changes cause the specific features of CDKL5 deficiency disorder.

Normal Function

The CDKL5 gene provides instructions for making a protein that is found in cells and tissues throughout the body. However, it is most active in the brain and is essential for normal brain development and function. There are five versions (isoforms) of the CDKL5 protein. These isoforms vary in length and in the tissues in which they are most abundant.

Studies suggest that the CDKL5 protein is involved in the formation, growth, and movement (migration) of nerve cells (neurons), as well as cell division. It also plays a role in the transmission of chemical signals at the connections (synapses) between neurons.

The CDKL5 protein acts as a kinase, which is an enzyme that changes the activity of other proteins by adding a cluster of oxygen and phosphorus atoms (a phosphate group) at specific positions. It is possible that one of the proteins targeted by the CDKL5 protein is MeCP2, which is produced from the MECP2 gene. The MeCP2 protein plays important roles in the function of neurons and other brain cells and in the maintenance of neuronal synapses. Researchers have not determined which other proteins are targeted by the CDKL5 protein.

Health Conditions Related to Genetic Changes

CDKL5 deficiency disorder

At least 150 mutations in the CDKL5 gene have been found to cause CDKL5 deficiency disorder. This rare condition is characterized by seizures that begin in infancy, followed by significant delays in many aspects of development. Affected individuals have severe intellectual disability and most do not walk independently. About 90 percent of people diagnosed with CDKL5 deficiency disorder are female.

CDKL5 deficiency disorder was previously classified as an atypical form of Rett syndrome. However, CDKL5 deficiency disorder is now considered a separate condition. Rett syndrome, which affects development in girls and women, results from mutations in the MECP2 gene. Because the CDKL5 and MeCP2 proteins may interact in the brain, the two disorders might be caused by a similar mechanism.

Mutations in the CDKL5 gene reduce the amount of functional CDKL5 protein or alter its activity in neurons. A shortage (deficiency) of CDKL5 or impairment of its function disrupts brain development, but it is unclear how these changes cause the specific features of CDKL5 deficiency disorder.

Most CDKL5 gene mutations change single protein building blocks (amino acids) in the CDKL5 protein. This type of mutation occurs most often in a region of the protein called the kinase domain, which is essential for the protein's kinase function. Mutations in the kinase domain disrupt the ability of CDKL5 to add phosphate groups to other proteins. Compared with other types of mutation, these mutations are associated with more severe signs and symptoms of CDKL5 deficiency disorder.

Other CDKL5 gene mutations alter different regions of the CDKL5 protein or lead to the production of an abnormally short version of the protein. Research has shown that mutations affecting parts of the protein other than the kinase domain tend to cause less severe signs and symptoms of CDKL5 deficiency disorder than other types of mutations.

Other Names for This Gene

• CDKL5_HUMAN
• CFAP247
• cyclin-dependent kinase-like 5
• serine/threonine kinase 9
• STK9

Genomic Location

This condition is inherited in an X-linked dominant pattern. The CDKL5 gene is located on the X chromosome, which is one of the two sex chromosomes. In females (who have two X chromosomes), a variant in one of the two copies of the CDKL5 gene in each cell causes the disorder. In males (who have only one X chromosome), a variant in the only copy of the gene causes the disorder. A characteristic of X-linked inheritance is that fathers cannot pass X-linked traits to their sons.

Almost all cases of this condition result from new (de novo) variants in the CDKL5 gene that occur during the formation of reproductive cells (eggs or sperm) or in early embryonic development. These cases occur in people with no history of the disorder in their family.

Researchers suspect that the signs and symptoms of CDKL5 deficiency disorder vary in severity in part because of a process called X-inactivation. Early in embryonic development in females, one of the two X chromosomes is permanently inactivated in somatic cells (cells other than egg and sperm cells). X-inactivation ensures that females, like males, have only one active copy of the X chromosome in each body cell. Usually X-inactivation occurs randomly, such that each X chromosome is active in about half of the body cells. This means that about half of cells have an active X chromosome with a CDKL5 gene variant, and half have an active X chromosome without the variant. However, groups of cells that arise from a single original cell have the same copy of the X chromosome inactivated, so the distribution is not exactly half and half. The proportion of neurons in the brain that have the active X chromosome with the variant helps determine how severe the features of the condition are in a given individual. Females with a higher percentage of neurons with the variant have more severe signs and symptoms than females with a lower percentage of neurons with the variant.

Because males have only one X chromosome in each cell, the altered version of the CDKL5 gene is active in all cells. Affected males have no normal copies of the gene.