Which are the causes of Pallister-Killian Syndrome / Tetrasomy 12p?

Pallister-Killian syndrome, also known as tetrasomy 12p, is caused by the presence of an abnormal extra chromosome, specifically an isochromosome 12p, in some of the body's cells. This genetic condition occurs sporadically and is not inherited from parents, meaning it typically happens as a random event during early embryonic development.

What causes Pallister-Killian syndrome?

The primary cause of Pallister-Killian syndrome is the presence of an extra copy of the short arm (p arm) of chromosome 12. In affected individuals, cells contain an isochromosome 12p—a mirror-image structure where two copies of the p arm are joined together. This results in four total copies of the genes located on the 12p arm, rather than the usual two, leading to the clinical manifestations of Pallister-Killian syndrome.

Is Pallister-Killian syndrome hereditary?

Pallister-Killian syndrome is not considered a hereditary condition. It is almost always a "de novo" (new) event, meaning it occurs spontaneously in the egg or sperm or during early cell division after fertilization. Because the extra chromosome is often lost in some tissues over time, researchers refer to the condition as tissue-specific mosaicism.

Why is Pallister-Killian syndrome considered mosaic?

The presentation of Pallister-Killian syndrome is unique because the abnormal cells are not distributed evenly throughout the body. The following factors influence this mosaicism:

• Tissue-specific distribution: The extra chromosome 12p is often found in skin cells (fibroblasts) but may be absent or difficult to detect in blood lymphocytes.


• Developmental timing: The "mosaic" nature means that as an individual with Pallister-Killian syndrome grows, the proportion of cells carrying the extra chromosome can change, which is why skin biopsies are often the gold standard for diagnosis.


• Random occurrence: There are no known environmental, dietary, or lifestyle triggers that cause this chromosomal error.

Next steps

• Consult a clinical geneticist to confirm a diagnosis via skin fibroblast karyotyping.


• Connect with the 9 community members at DiseaseMaps.org to share experiences.


• Speak with a genetic counselor to discuss the negligible recurrence risk for future pregnancies.

Medical disclaimer: This information is for educational purposes and should not replace professional medical advice, diagnosis, or treatment from a qualified healthcare provider.

References

• NIH Genetic and Rare Diseases Information Center (GARD): Pallister-Killian syndrome.


• Orphanet: Isochromosome 12p mosaicism.


• OMIM (Online Mendelian Inheritance in Man): Pallister-Killian syndrome entry #601803.