Pallister-Killian syndrome is a rare chromosomal condition where some cells in the body have an extra copy of genetic material on the twelfth chromosome. Typically cells have 46 chromosomes that are arranged into 23 pairs: 23 from the mother and 23 from the father. Individuals with Pallister-Killian syndrome have some cells with the 46 expected chromosomes plus an extra chromosome that is made of two copies of the short (p) arm from the twelfth chromosome. This means that these cells have four copies of the short (p) arm, rather than the normal two. Pallister-Killian syndrome is called a mosaic syndrome because only some cells contain the extra material. Pallister-Killian syndrome is most often due to a random (de novo) event that occurs during the early development of the egg or sperm.
Symptoms of Pallister-Killian syndrome can vary, although most individuals experience low muscle tone (hypotonia), intellectual disabilities, distinctive facial features, and other birth defects. Sparse hair or bald patches are typical, especially around the temples, but these patches often fill in around middle childhood. Common facial features include a rounded forehead, widely spaced eyes, and a large mouth with a thin upper lip. Less common features include short legs or arms, seizures, or a diaphragmatic hernia (a hole in the muscle that separates the abdomen from the chest cavity). No individual will show all symptoms, so chromosomal analysis is the only way to diagnose the syndrome.
Although there is no cure for Pallister-Killian syndrome, there are many available treatments for the symptoms. Doctors may seek help from various therapists and community services to help care for and support families affected by this condition. Genetic counselors and support groups can be great resources and sources of information and offer a way to connect with other families with the same condition.