“Trisomy 13 is a type of numerical chromosomal abnormality, known as Patau syndrome, which occurs by the presence of an extra copy of chromosome 13. Let’s understand the concept of trisomy and how trisomy 13 occurs.”
The presence of an extra chromosome in a genome is a type of chromosomal alteration known as aneuploidy. Some such conditions are common, for example, trisomies. Humans have 23 pairs and a total of 46 chromosomes in a cell.
A pair of X and Y chromosomes are known as sex chromosomes while the rest of the 22 pairs are autosomes. Alterations in the number of chromosomes, increment or decrement, have known association with various health conditions. And even, sometimes, are lethal too.
Aneuploidy is an important event but we will discuss it in some other article. In this article, we will focus on trisomy 13 only. So when an extra 13th number chromosome is inherited with a pair, it causes trisomy 13.
The present condition often known as Patau syndrome is a serious congenital genetic condition and affects many body systems, thus important to study and discuss. I have expertise in molecular cytogenetics.
During our research period, we have prepared, analyzed and reported hundreds of karyotypes, especially Down syndrome (Trisomy 21) which is the most common. So I’m writing this article to let you understand what Patau syndrome is.
Although I have never reported any trisomy 13, unfortunately; but I know the mechanism behind the occurrence of such conditions. So in this article, I will explain the genetics of Patau syndrome or trisomy 13. We will also discuss symptoms, causes, life expectancy and other related information.
The internet contains thousands of resources on this topic but trust me this will be a very good learning scenario for students.
So stay tuned.
What is Trisomy 13?
Trisomy 13 is a chromosomal alteration. The numerical one!
Alternatively, we can say
“An extra copy of chromosome 13 which causes serious health problems is referred to as trisomy 13 or Patau syndrome also known as trisomy D.”
In 1960, Klaus Patau for the first time in history demonstrated the present condition. However, it was first reported in 1657 by Thomas Bartholin. Because Patau had established the genetic basis of the present abnormality, from his name, the condition is named Patau Syndrome.
Trisomy 13 is also denoted as Bartholin- Patau syndrome or Pataus’s syndrome. The global prevalence of the present condition is 1 in 10,000 to 16,000 newborns. Importantly, an increment in maternal age is the major factor for the occurrence. Women above the age of 31 have a higher risk acquisition.
|Type||Numerical chromosomal abnormality|
|Other names||Bartholin-Patau syndrome, Patau’s syndrome, T13|
|Cause||Even of chromosomal non-disjunction|
|Inheritance||Non-inherited (majorly) only translocations are inherited.|
|Occurrence||Presence of an extra copy of chromosome 13.|
|Prevalence||1 in 10,000-16,000 newb borns, globally|
|Mortality rate||Very high|
|Risk factor||Maternal age|
|Survival rate||Very low|
Definition of trisomy 13:
“A rare form of chromosomal aberration in which an extra copy or some portion of chromosome 13 present in a genome with the pair of chromosome 13 caused by chromosomal nondisjunction, is known as trisomy 13 or Patau syndrome.”
Much like other trisomies, the event known as “nondisjunction” is the major cause of trisomy 13. As it is congenital, it results in serious health issues. So the presence of an extra chromosome is indeed a serious problem for the fetus. Let’s understand the mechanism behind the nondisjunction.
Each parent passes down each haploid set of chromosomes to the fetus. Meaning, one set of 23 chromosomes from a father and one set of 23 chromosomes from a mother is inherited into the fetus. This occurs by the fusion of paternal and material sperm and egg during fertilization.
So the fetus comprises 46 chromosomes in 23 pairs, the present condition is referred to as a diploid.
Meiosis is the process of germ cell division. Particularly, meiosis 1 and meiosis 2 govern the entire process of spermatogenesis or oogenesis. Abnormal distribution of chromosomes during this cell division, by any random cause results in uneven chromosomal distribution.
Nondisjunction is one such random event that causes trisomy. Put simply, it’s just an imbalanced distribution of chromosomes. It results in triplication of the genetic material for chromosome 13 thereby causing copy number variation for the same chromosome.
No clear reason for the event nondisjunction is yet reported. It occurs randomly and in any developing fetus. The consequence of such copy number changes is mostly associated with intellectual and physical traits.
Studies suggest that trisomy 13 causes serious developmental and intellectual disability in newborns.
Types of Trisomy 13:
The discussion so far is just a brief understanding. Now dive deeper into the topic. The presence of an extra copy of 13 isn’t the only genetic condition in this category. This means that there are a few types of Patau syndrome reported.
Translocation trisomy 13, Mosaic trisomy 13, and partial and complete trisomy 13 are several common types. Let’s discuss each one.
Translocation trisomy 13:
Only 20% of total cases belong to translocation trisomy 13. It is a very rare event in which during the meiosis of germ cells a huge part of chromosome 13 detaches and is translocated to any chromosome.
During fetal development, along with the pair this translocated part is also inherited and causes copy number variation for chromosome 13. However, the severity of the signs and symptoms of the condition depends on how much and which portion of chromosome 13 is translocated.
For example, if the translocation comprises the portion rich in genes involved in cognitive development, more serious symptoms for cognition are reported. Notedly, translocation trisomy 13 is less severe than complete trisomy 12.
Translocation trisomy 13 is an inherited condition.
Mosaic trisomy 13:
The mosaic condition is even rarer than the translocation. Here, a fetus contains two different types of cell populations, one with three chromosomes 13 and another with two chromosomes 13.
Again, the severity of the condition depends on the degree of mosaicism. The more mosaic cells, the more severe signs and symptoms are! Generally, the condition has mild to moderate symptoms.
Partial trisomy 13:
Partial trisomies are so rare and technically are neither trisomy nor translocation. In this, only a part/portion or a single arm of chromosome 13 will be inherited with a pair.
Partial trisomies are less studied. The severity depends on the amount of portion inherited.
Complete trisomy 13:
Complete trisomy is the most prevalently found, in almost 80% of total Patau syndrome cases. As aforementioned, the entire extra chromosome 13 will be inherited and triple the genetic material for that chromosome.
Complete trisomy 13 is non-inherited, occurred randomly and comes with serious and problematic symptoms. The probability of recurrence by complete trisomy 13 is negligible.
The full penetrance of the disease is shown in the complete trisomy and highly depends on the amount of extra genetic material inherited. Usually, the present condition causes serious mental, intellectual and developmental abnormalities. I have enlisted some of the common symptoms here.
- Close-set or closely located eyes
- Coloboma- a hole in the iris of the eye
- Low-set ears
- Inguinal and umbilical hernia
- Skeletal abnormalities
- Microcephaly- small head
- Small jaw (especially lower)
- Cryptorchidism- is the condition with an undescended testicle
- Absence of eyeballs
- Abnormal share of the pelvic girdle bone
- Abnormal cranial structure
- Congenital heart defects
- Cleft palate or lip
- Extra toes (polydactyly)
- Decrease muscle tone
- Breathing and feeding problems
- Congenital trigger digits
- Prominent heel
- Overlapping fingers over the thumb
- Kidney defect
- Defective head
- Low birth weight
- Underdeveloped nostrils
T13 is the second most common trisomy after Down syndrome showing serious and complicated health issues. Commonly, an infant with trisomy 13 dies immediately after birth or in a few hours. In some cases, a baby can live up to weeks, however, below 10% of babies with trisomy 13 can live up to a year.
The mortality rate of the fetus with trisomy 13- Patau syndrome is very high like trisomy 18. However, babies with partial or mosaic trisomy 13 can live up to adulthood or above but those cases are very rare.
In the majority of conditions with trisomy 13 patients (baby) die in an hour, day or within a year after birth. So the life expectancy with trisomy 13 is very less.
Pictures of trisomy 13:
As we explained in the previous article two common methods are most trustworthy for encountering trisomy i.e ultrasound and genetic analysis. We will discuss each one by one.
Ultrasound is a primary screening method that provides indications early in pregnancy. However, genetic testing or cytogenetic analysis must be required for validating the ultrasound indications.
Using the cytogenetic technique known as karyotyping, chromosomal aberrations can be encountered using the GTG banding technique.
For cytogenetic analysis, amniotic fluid (the technique is called amniocentesis) or chorionic villi sample is taken and cultured for getting metaphase cells.
Metaphases are cultured, harvested and stained by Giemsa and GTG banding. The chromosome plates are later observed under a microscope to look into chromosomal aberrations.
Another emerging genetic testing method nowadays scientists are using is known as cell-free DNA testing in which circulating fragments of fetal DNA are isolated from the mother’s blood and employed for encountering trisomy.
Though the mortality rate of Patau syndrome is high, individuals with less serious symptoms can survive in some cases if proper treatment is given in the early days. The treatment of Patau syndrome may vary on the basis of the type of symptoms shown in the individual.
For example, speech and mental treatment or therapies are given to improve patients’ psychological and mental health.
Individuals with mosaic or partial conditions can live up to adulthood or more, however, the majority of patients die within a few weeks, and proper treatment can help to improve patients’ health.
Surgeries can help to repair heart defects or cleft lips.
Occupational therapy, physical treatment, psychological counseling, motor skills improvement therapy and other different approaches may help the patient.
The survival rate:
Unlike trisomy 21, the overall mortality rate of other trisomies is very high. Babies with trisomy 13 may die in early life, in a day or in a few weeks due to complex mental, physical and neurological problems.
Notwithstanding, in a few cases with mild to moderate Patau syndrome, a patient can live up to a year or up to 10 years. However, extensive care, management and treatments are required.
Approximately 10 to 13% of babies with trisomy 13 may survive up to 10 years. In comparison with trisomy 21, the survival rate of Patau syndrome is very low.
Even if a patient may survive, he or she has to face so many mental, motor and physical problems, unlike a normal child, their life is not at all normal.
Suggestions for parents:
The burden of genetic or congenital abnormalities increases globally, due to a lack of knowledge and proper guidance, patients, as well as parents, are misled. One should have to contact a genetic counselor for proper guidance.
A genetic counselor can explain to you the situation and what to do next. Although I am not a professional genetic counselor, still as a geneticist I can give some suggestions to parents that will definitely help them to manage the situation.
- First of all, don’t panic if your pregnancy is diagnosed with Patau syndrome or trisomy 13.
- Do not take any decision in a rush, immediately contact the genetic counselor near you.
- The recurrent risk associated with trisomy 13 is very less, so the chance of your second pregnancy being affected by Patau syndrome is very less.
- As I said in the previous article, the event occurs totally randomly, none of the parents is responsible for it so do not blame each other for that.
- Instead of that, understand the situation and behave accordingly, and try to support your partner.
- Yes, it is an emotional trauma for the mother, but think positive, this can happen to anyone.
- If your doctor advises you on abortion, think about it, and don’t be so emotional, ultimately your family has to face mental, social and economical problems.
- If there is a 10% chance that your baby may live, you should go for it but remember, you have to face so many problems, in addition to this, your baby has to face problems too.
- Also, living with the present condition can impact the daily lives of a parent, one of the parents should always take care of the baby. So keep these points in mind and make your decision accordingly.
Trisomies are commonly found, though trisomy 13 or Patau syndrome isn’t very common, it’s a problematic condition. The mortality rate is very high, having serious developmental complications. The condition can be diagnosed early, but can’t be cured or managed. So it is advisable to discontinue the pregnancy.
Conclusively, trisomy 13 is a non-inherited genetic (more precisely) chromosomal aberration that causes serious health problems. Prenatal diagnosis is the only way to prevent Patau syndrome, to date, no medications or treatment for trisomy 13 are available.
Sources: Witters G, Van Robays J, Willekes C, et al. Trisomy 13, 18, 21, Triploidy and Turner syndrome: the 5T’s. Look at the hands. Facts Views Vis Obgyn. 2011;3(1):15–21.