The Principles of Clinical Cytogenetics - Extra Materials - Springer

Autosomal Aneuploidy 153
Fig. 10. Tetrasomy 12p female karyotype.
metaphase cells (231). These authors proposed that lymphocytes containing i(12p) might fail to divide
upon PHA stimulation. These observations suggest that tissue-limited mosaicism in Pallister–Killian
syndrome could result from differential selection against cells containing i(12p) in different tissues
and that this selection can occur both in vivo and in vitro.
Many patients die shortly after birth, but survival to adulthood is possible. Clinically, a distinct
pattern of anomalies is observed in these patients. Growth parameters at birth are usually normal.
Profound hypotonia is present in the newborn period, whereas contractures develop later in life.
Sparse scalp hair, especially bitemporally, is observed in infancy, with coarsening of facial features
over time. Craniofacial dysmorphism includes prominent forehead, large malformed ears, hypertelorism,
epicanthal folds, broad flat nasal bridge, short nose, upturned nares, long philtrum, thin
upper lip, and high arched palate. Most patients have a generalized pigmentary dysplasia with areas
of hyperpigmentation and hypopigmentation. Other abnormalities include short neck, macroglossia,
micrognathia progressing to prognathia, accessory nipples, umbilical and inguinal hernias, and urogenital
abnormalities. Severe mental retardation and seizure are seen in those who survive.
All cases are sporadic. The recurrence risk is probably negligible.
Tetrasomy 18p
Tetrasomy 18p [47, XX or XY,+i(18)(p10)] results from the presence of a supernumerary isochromosome
for the entire short arm of chromosome 18. The syndrome was first described by Froland et
al. in 1963 (232), although identification of the marker as an i(18p) was not made until after the
introduction of banding techniques in 1970. Confirmation of the origin of the marker has been possible
in recent years by FISH studies. Of interest is the finding of a loss of approximately 80% of
chromosome 18 α-satellite DNA in the i(18p) in one case (233).
At least 50 cases have been reported (234–238). Most are nonmosaics. The i(18p) is usually readily
detectable in lymphocytes. Its presence in amniocytes (239) and cultured chorionic villus cells (233)
has also been reported.