Pallister-Killian syndrome

Pallister-Killian syndrome (also tetrasomy 12p mosaicism or Pallister mosaic aneuploidy syndrome) is an extremely rare genetic disorder occurring in humans. Pallister-Killian occurs due to tetrasomy of the twelfth chromosome. This leads to the development of isochromosome 12p, made up of the two short legs of the chromosome. Because not all cells have the extra isochromosome, Pallister-Killian is mosaic.

It was first described by Philip Pallister in 1977 and further researched by Maria Teschler-Nicola and W. Killian in 1981.

Symptoms
Symptoms include varying degrees of mental retardation, epilepsy, hypotonia, and both hypopigmentation and hyperpigmentation. Patients also exhibit a distinctive facial structure, characterized by high foreheads, sparse hair on the temple, a wide space between the eyes, a fold of skin over the inner corner of the eyes, and a flat nose. Visual impairments and deafness may occur. Patients may also exhibit congenital heart defects, gastroesophageal reflux, cataracts, and supernumerary nipples. Diaphragm problems seen in newborns can lead to death shortly after birth.

As patients pass into adolescence, the syndrome is characterized by a coarse and flat face, macroglossia prognathia, inverted lower lip, and psychomotor retardation with muscular hypertonia and contractures.

Causes
Pallister-Killian does not appear to be hereditary. Some research has suggested that the presence of the extra chromosome may be linked to premeiotic mitotic errors, both maternally and paternally. Several theories regarding to mechanism of this formation have been introduced.

Diagnosis
The isochromosome can be primarily detected in samples of skin fibroblasts, as well as in chorionic villus and amniotic fluid cell samples. Very rarely, it can also be detected in blood lymphocytes. It is also possible to detect the isochromosome in circulating lymphocytes, as well as other amniotic and placental samples. There is no strict limit as to where the isochromosome can be found. However, it is often unlikely that these samples will be tested when the blood karyotype is normal.

Using an ultrasound, Pallister-Killian may be diagnosed through observation of hypertelorism, broad neck, shorts limbs, abnormal hands or feet, diaphragmatic hernia, and hydramnios. Once born, a child may be diagnosed by observation of the syndrome's distinct facial features.