Patau Syndrome, also known as Trisomy 13, is a genetic condition that significantly impacts development. It arises from a chromosomal abnormality, leading to multiple organ defects. This rare disorder occurs in approximately 1 in 10,000 to 20,000 live births.
Chromosomal Basis of Patau Syndrome
Human cells contain 23 pairs of chromosomes, which carry genetic instructions. Patau Syndrome occurs when an individual has three copies of chromosome 13 instead of the usual two. This extra chromosome is known as a “trisomy.”
The cause of this extra chromosome is an error during cell division called nondisjunction. This is the failure of chromosomes to separate properly during meiosis, which creates egg and sperm cells. If a reproductive cell with an extra chromosome 13 is fertilized, the resulting embryo will have three copies of chromosome 13 in every cell. This extra genetic material interferes with normal growth.
Different Forms of Trisomy 13
Patau Syndrome can manifest in several genetic forms. The most common is Full Trisomy 13, where every cell in the body contains an extra copy of chromosome 13. This results from nondisjunction during the formation of the egg or sperm.
Another form is Translocation Trisomy 13, which accounts for a smaller percentage of cases. Here, a portion of chromosome 13 attaches to another chromosome, often chromosome 14. This rearrangement can occur spontaneously or be inherited from a parent who carries a “balanced translocation.” A parent with a balanced translocation has the rearranged genetic material but no extra or missing chromosomal content, so they are unaffected.
Mosaic Trisomy 13 is a less common type where only some cells have the extra chromosome 13, while others have the typical two copies. This pattern arises from an error in cell division after fertilization, early in fetal development. Individuals with mosaic Trisomy 13 may experience milder symptoms depending on the proportion and distribution of affected cells.
Factors Influencing Occurrence
The occurrence of Patau Syndrome is influenced by several factors, though most cases arise from random genetic events. Maternal age is the most significant factor associated with the risk of nondisjunction, which leads to Full Trisomy 13. The likelihood of an egg cell having an incorrect number of chromosomes increases as the mother ages, particularly for women over 35.
Despite this association, most instances of Full Trisomy 13 are sporadic and occur by chance during the formation of reproductive cells. These errors are not inherited from parents. There is no known way to prevent these random chromosomal changes.
However, in cases of Translocation Trisomy 13, a parent can be a balanced carrier of a chromosomal rearrangement involving chromosome 13. While rare, this inherited translocation can increase the risk of their child having the condition. Understanding the specific genetic cause in each case is important.
Implications for Future Pregnancies
Understanding the genetic basis of Patau Syndrome in a current or past pregnancy is important for assessing future risks. Genetic counseling is recommended for families who have experienced Patau Syndrome. A genetic counselor can provide information about recurrence risks and explain available testing options.
The recurrence risk varies depending on the specific type of Trisomy 13. For Full Trisomy 13, the recurrence risk in a subsequent pregnancy is low, estimated around 1% above the mother’s age-related risk. However, for Translocation Trisomy 13, especially if a parent is a balanced carrier, the recurrence risk can be higher, sometimes 10% or more. Parental chromosome analysis (karyotyping) is important to determine if a balanced translocation is present. For Mosaic Trisomy 13, the recurrence risk is considered low.
Prenatal diagnostic tests, such as chorionic villus sampling (CVS) or amniocentesis, are available in future pregnancies to detect chromosomal abnormalities. These tests analyze fetal cells to confirm the chromosome count. Noninvasive prenatal testing (NIPT), a blood test, can also screen for an increased risk of Trisomy 13 by analyzing fetal DNA fragments in the mother’s bloodstream.