Trisomy 13, also known as Patau syndrome or trisomy D, is a genetic disorder where the patient has 3 copies of the chromosome number 13 rather the usual 2 copies that most people have. The syndrome has been observed as far back as 1657 (Thomas Bartholin, 1616 – 1680, a Danish Physician, mathematician and theologian), but the chromosomal nature of the syndrome was only ascertained and described in 1960 by the German-born American geneticist, Klaus Patau (1908-1975) in whose honor the syndrome is now named.
Chromosomes are rod like structures made up of DNA[1] threads which, along with its associated proteins, constitute the basic building blocks of life as we know it. Chromosomes play an essential role in the transmission of hereditary characteristics from one generation to another amongst all eukaryotic organisms, i.e. those organisms, man inclusive, whose bodies are composed of cells which have a highly organized nucleus surrounded by a membrane.
The condition, as we have said, occurs when extra chromosome number 13 DNA appear in some or all of the cells that make up the body. There are three possible situations: partial trisomy, where there is a part of an extra chromosome 13 in the cells; trisomy 13 mosaicism, where a third number 13 chromosome is present in some of the cells; and the full blown situation where a third chromosome is present in every cell. The extra chromosomal material causes abnormal fetal development. The extra chromosome or chromosomal matter is a result of a nondisjunction[2] of chromosomes during meiosis[3]. Patau syndrome has an incidence of between 1 in 10,000 and 1 in 21,700.
Generally speaking, the syndrome is not one that is inherited; most cases are a result of random events that take place during the formation of reproductive cells, eggs or spermatozoa. An error in cell division may cause some of the reproductive cells to have abnormal numbers of chromosomes which can result in an offspring who suffers from this condition. The chances of having a second child who has the disease is extremely low; less than 1% (but see next paragraph).
However, there is a hereditary factor in a minority of cases. Some people who do not display any symptoms of the syndrome are, nonetheless, carriers of the disease. Such people have what is known as a balanced translocation, i.e. a rearrangement of genetic material between chromosome number 13 and some other chromosome. This rearrangement of genetic material is balanced because there are no extra copies of any chromosome 13; nonetheless, such people are at much greater risk to have a child who suffers from this syndrome. As is the case for other nondisjunctive conditions, e.g. Down and Edwards syndromes, the chances of having a child with Patau syndrome increase with maternal age.
The presence of an extra number 13 chromosome causes a severe disruption in normal fetal development and organs such as the heart and the kidneys can be seriously affected. Where a fetus survives to be born abnormalities, e.g. mental and motor challenges; eye defects; problems relating to the spinal cord; improper division of the forebrain (a condition known as holoprosencephaly); the presence of additional toes and fingers (polydactyly); cleft palate; abnormal genitalia; etc., abound. The prognosis for such a child is extremely poor; more than 80% of such children die before attaining their first year. However, the small percentage of trisomy 13 children who are able to survive through birth and early childhood can be expected to survive into adulthood and those who suffer from the partial and mosaic forms of the disease usually have a completely different, less daunting prognosis than do those who have the full-blown syndrome.
Most diagnosis today are made prenatally[4] by means of amniocentesis[5]. Where one has a child with Patau syndrome, both parents should be genetically tested to rule them out as translocation carriers. If one or both parents are translocation carriers counseling and monitoring can help avoid having another such child.
Given the nature of Patau syndrome, treatment is by a case-to-case approach as patients display considerable differences in the associated problems and the severity of any such problems. The first few days/weeks of life are the most challenging as many such babies exhibit severe neurological problems and/or complex heart defects. Surgery may be required in order to repair heart defects, cleft lips and palate. If the patient survives this extremely trying early period, then the prognosis improves and as time goes on, physical, occupational and speech therapy can assist patients to achieve their full developmental potential.
[1] Deoxyribonucleic acid.
[2] The failure of one of a pair of corresponding chromosomes to take its proper position.
[3] The cell division process that leads to the formation of the fetus.
[4] 91% of all diagnosis made in 2008/2009 in England and Wales.
[5] Withdrawal (by means of a hollow needle) of some amniotic fluid, i.e. the fluid in which the embryo is suspended, and testing the sample in order to determine whether or not there are abnormalities.
