The Pallister–Killian syndrome (PKS), also termed tetrasomy 12p mosaicism or the Pallister mosaic aneuploidy syndrome, is an extremely rare and severe genetic disorder. PKS is due to the presence of an extra and abnormal chromosome termed a small supernumerary marker chromosome (sSMC). sSMCs contain copies of genetic material from parts of virtually any other chromosome and, depending on the genetic material they carry, can cause various genetic disorders and neoplasms. The sSMC in PKS consists of multiple copies of the short (i.e. "p") arm of chromosome 12. Consequently, the multiple copies of the genetic material in the sSMC plus the two copies of this genetic material in the two normal chromosome 12s are overexpressed and thereby cause the syndrome. Due to a form of genetic mosaicism, however, individuals with PKS differ in the tissue distributions of their sSMC and therefore show different syndrome-related birth defects and disease severities. For example, individuals with the sSMC in their heart tissue are likely to have cardiac structural abnormalities while those without this sSMC localization have a structurally normal heart.PKS was first described by Philip Pallister in 1977 and further researched by Maria Teschler-Nicola and Wolfgang Killian in 1981. Presentation
Individuals with PKS present prenatally or at birth with multiple birth defects.
(Small amounts of a fetuss DNA escapes through the placenta to circulate in the mothers blood.) A genome-wide association study done in China used genome-wide cfDNA analyses to diagnose various chromosome-related disorders including PKS. The study scanned the DNA in the blood of 29,007 pregnant women and found three cases with abnormal amount of DNA originating from the entire p-arm of chromosome 12. All three cases were confirmed to have a fetus with PKS. Two of these cases were missed by chromosomal microarray analysis of the placenta and chorionic villi. However, the study did not define the rate of false negative cases, i.e. negative results in women actually carrying a fetus with PKS. While further studies are required, this method may turn out to be a critically useful addition for the prenatal detection of PKS, particularly during early pregnancies for the purpose of pregnancy options counseling. Postnatal diagnosis
The postnatal diagnosis of PKS is strongly suggested or indicated in most cases based on finding the key defects of PKS in an individual on physical examination and various radiography, ultrasound, and related methods. However, some individuals with this syndrome do not have a sufficient number of these defects or have a set of defects that are also compatible with other birth defect disorders such as Fryns syndrome, trisomy 12p, and Sifrim-Hitz-Weiss syndrome (also termed CHD4 Neurodevelopmental Disorder). The diagnosis can be confirmed in these cases as well as in all cases of PKS by detecting its sSMC using special methods.