Pallister-Killian Mosaic Syndrome is a rare chromosomal disorder caused by the presence of four copies of the short arm of chromosome 12 instead of the normal two. Major symptoms may include a coarse face with a high forehead, sparse hair on the scalp, an abnormally wide space between the eyes, a fold of the skin over the inner corner of the eyes, and a broad nasal bridge with a highly arched palate. Mental retardation, loss of muscle tone, and streaks of skin lacking color are often present.
Individuals with Pallister-Killian Mosaic Syndrome typically have low muscle tone at birth (hypotonia), sparse scalp hair, a high forehead, a coarse face, an abnormally wide space between the eyes, a broad nasal bridge, a highly arched palate, a fold of the skin over the inner corner of the eyes, and large ears with lobes that are thick and protrude outward.
Additional features frequently found in affected individuals may include streaks of skin in which there is no color (hypopigmentation); extra nipples; seizures; droopy upper eyelids, crossed eyes (strabismus); joints that will not move (contractures); and delays in perceiving, recognizing, judging, sensing, reasoning or imagining (cognitive delays). Mental retardation and difficulties with speech development often occur as well. In rare cases, affected children may experience hearing loss.
Congenital heart defects, hernias of the diaphragm, a narrowing of the external auditory canal (stenosis) and an abnormal opening in the anus have also been associated with Pallister-Killian Mosaic Syndrome. Some affected individuals may have an underdeveloped (hypoplastic) lung, abnormalities of the genitourinary system, and skeletal malformations.
Pallister-Killian Mosaic Syndrome is caused by the presence of four copies of the short arm of chromosome 12 instead of the normal two. The extra two copies of the short arm of chromosome 12 (12p) usually appear as a single chromosome (isochromosome) and are sometimes present in some but not all cells examined. The chromosome abnormality in Pallister-Killian Mosaic Syndrome is limited to specific cell types. The mechanism and parental origin of the isochromosome 12p can usually not be determined.
Chromosomes, which are present in the nucleus of human cells, carry the genetic information for each individual. Human body cells normally have 46 chromosomes. Pairs of human chromosomes are numbered from 1 through 22 and the sex chromosomes are designated X and Y. Males have one X and one Y chromosome and females have two X chromosomes. Each chromosome has a short arm designated “p” and a long arm designated “q”.
Pallister-Killian Mosaic Syndrome is a very rare disorder that affects males and females in equal numbers. More than 30 cases of this disorder have been reported in the medical literature.
Diagnosis is usually made from a chromosome study of skin cells (fibroblasts) that reveals 47 chromosomes including an extra small chromosome that has two short (p) arms and no long (q) arm (isochromosome). A blood chromosome study usually shows normal chromosomes, but some affected persons have some blood cells (lymphocytes) with an isochromosome 12p. Fluorescent in situ hybridization (FISH) may be used with specific chromosome 12 DNA probes to detect isochromosome 12p.
Pallister-Killian Mosaic Syndrome can be diagnosed before birth (prenatally) by removing a small amount of fluid that is in the womb during pregnancy (amniocentesis) or by removing a small number of cells from outside the sac where the fetus develops (chorionic villous sampling).
There is no specific therapy for individuals with Pallister-Killian Mosaic Syndrome. Affected children may benefit from early intervention programs and special education. Genetic counseling may be of benefit for affected individuals and their families. Other treatment is symptomatic and supportive.
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McKusick VA., ed. Online Mendelian Inheritance in Man (OMIM). Baltimore. MD: The Johns Hopkins University; Entry No:601803; Last Update:3/20/01.