Patau syndrome is caused by the presence of an extra copy of chromosome 13 in the cells of the body, a condition known as trisomy 13. Normally, humans have 23 pairs of chromosomes, for a total of 46 chromosomes, but in Patau syndrome, there are three copies of chromosome 13 instead of the usual two. This extra genetic material disrupts normal development and leads to the severe physical and intellectual disabilities characteristic of the syndrome.
The extra chromosome 13 usually arises due to an error during the formation of reproductive cells (eggs or sperm) in a process called nondisjunction. Nondisjunction means that the chromosome pairs fail to separate properly, resulting in a sperm or egg cell with an extra chromosome. When this cell contributes to the formation of an embryo, the embryo ends up with three copies of chromosome 13 in all or some of its cells. This error is typically spontaneous and not inherited from the parents, although advanced maternal age can increase the risk of such chromosomal abnormalities.
In some cases, Patau syndrome can result from a chromosomal translocation, where part of chromosome 13 becomes attached to another chromosome. This can be inherited from a parent who carries a balanced translocation, meaning they have all the genetic material but arranged differently and usually without symptoms. If a parent carries such a translocation, there is an increased risk of having a child with Patau syndrome due to the unbalanced distribution of genetic material during reproduction.
The presence of this extra chromosome 13 affects multiple organ systems and leads to a wide range of developmental problems. These include severe intellectual disability, low birth weight, reduced muscle tone, defects in the scalp skin, abnormal brain development, facial abnormalities, and malformations of the heart, kidneys, and other internal organs. Because of the extensive impact on development, Patau syndrome is considered one of the most severe chromosomal disorders.
Environmental factors do not cause Patau syndrome directly, but certain maternal conditions and exposures can influence the risk of chromosomal errors. For example, advanced maternal age is a well-known risk factor because the likelihood of nondisjunction events increases as a woman ages. Other factors such as poorly controlled diabetes, exposure to certain medications or chemicals, and infections during pregnancy can contribute to birth defects in general, but the primary cause of Patau syndrome remains the chromosomal abnormality.
Prenatal screening tests, such as non-invasive prenatal testing (NIPT) and blood marker tests combined with ultrasound, can detect the likelihood of trisomy 13 early in pregnancy by analyzing fetal DNA circulating in the mother’s blood or measuring specific proteins and hormones. Confirmatory diagnostic tests like amniocentesis or chorionic villus sampling (CVS) can then provide a definitive diagnosis by examining the chromosomes of fetal cells.
In summary, Patau syndrome is caused by an extra chromosome 13 due to errors in cell division, primarily nondisjunction, leading to trisomy 13. This chromosomal abnormality disrupts normal fetal development and results in severe physical and intellectual disabilities. While mostly arising spontaneously, the risk increases with maternal age and can sometimes be linked to inherited chromosomal rearrangements.
