Trisomy 13, also called Patau syndrome, results from three copies of chromosome 13 instead of the normal two copies. This disorder is relatively infrequent, occurring in approximately 1 in 10,000 live births. As with other human chromosomal disorders, most cases of Trisomy 13 are thought to arise from sporadic non-disjunction events during oogenesis. Advanced maternal age is associated with most cases of chromosomal trisomies. Rarely, chromosomal translocations can give rise to Patau syndrome. Even rarer phenomena include partial trisomy of chromosome 13 and mosaicism, in which only some cells contain an extra copy of the chromosome.
Many congenital abnormalities are associated with Trisomy 13. The most prominent anatomic anomalies include cleft lip or palate; microcephaly (small head); small, sometimes fused eyes; low set ears; micrognathia (small lower jaw); a variety of cardiac defects; umbilical and/or inguinal hernias; syndactyly (fused fingers); polydactyly (extra fingers or toes); other skeletal abnormalities; and crytorchidism (undescended testes). Mental retardation and seizures are especially severe in babies with Patau syndrome.
The reason chromosomal trisomies lead to such pronounced physical abnormalities is not completely understood. One possibility is that the presence of an extra chromosome results in a higher rate of abnormal cell division (sometimes termed mitotic catastrophe). Defective cell division, in turn, results in the formation of fewer functional cells during embryogenesis and fetal life. A shortage of healthy cells ultimately translates into aberrant cell migration, stunted growth, organ system malformations, and a relatively short life span.
Another factor thought to contribute to congenital anomalies is the phenomenon of gene dosage. Human cells contain two copies of most genes (the obvious exception being a single copy of the X and Y chromosomes in males). A third copy of most genes probably leads to excess production of the proteins encoded by those genes. In Trisomy 21, for example, a third copy of the APP (Amyloid Precursor protein) gene results in brain pathology resembling Alzheimer’s disease in almost all Down syndrome patients who live past age 40. Similar processes are no doubt at work in Patau syndrome.
Compared to Down syndrome, babies born with Trisomy 13 have a much lower survival rate, with an estimated 80% mortality in the first month of life alone. Treatment of Patau syndrome largely consists of supportive care. Women who have given birth to a baby with Patau syndrome have a small but elevated risk of a chromosomal trisomy in subsequent pregnancies. Trisomy 13 (as well as other chromosomal abnormalities) can be detected prenatally by karyotype analysis following amniocentesis or chorionic villus sampling.