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Triploidy and Mosaic Triploidy

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Table of Contents

What are Triploidy and Mosaic Triploidy and what causes them?
 
Tests and Diagnosis
 
Signs of Triploidy and Mosaic Triploidy
 
Life Expectancy
 
Quality of Life
 
Recurrance

Written by Alyson Young for Nova Scotia Public Health Services

What are Triploidy and Mosaic Triploidy and What Causes them?

 

The actual cause of triploidy is unknown although it does occur in 1-2% of all clinically recognized conceptions.  Some researchers believe that it may occur when two sperm fertilize one egg (dispermy).  This would have normally divided into twins but instead one fetus has taken on an extra 23 chromosomes, totaling 69.  People normally have 46 chromosomes in a thread of 23 pairs.  The picture above is a normal chromosome thread, imagine there being three blocks on the lines instead of just two.  That is what a full triploidy would resemble.

 

Why mosaic triploidy occurs is in even more debate because there is not a full set of extra chromosomes.  Most geneticists, doctors and researchers believe that some of the chromosomes have the ability to repair themselves and so it leaves a mosaic pattern in the chromosome thread.  Others believe that it’s just the way it happens.  Again, in the picture above, imagine there being a third block on only every second, third or fourth line.  That is what mosaic triploidy would resemble.

 

In mosaic triploidy, the fetus’ chromosomes may only have a certain percentage that the third chromosome had attached.  They could have any number of percentages in any part of the body.  For example, the skin may have 6% of the chromosomes that are in triplicate while the brain may have 80%.    

 

Triploidy also seems to occur more in males than females.  Studies confirming this show that 51-69% of triploidy cases are male.  This is also dependant upon the distribution of the sex chromosomes.

           

Researchers also believe that although the extra chromosomes are mainly paternal (coming from the father), there are some cases where there is a maternal donation (coming from the mother).  These cases are usually present in IVF (In-Vitro Fertilization).  Researchers believe the chromosomes are maternal in origin because an egg is fertilized in a lab with a single sperm.  This is usually done through ICSI (intracytoplasmic sperm injection). 

           

There are some laboratories that have the capability to determine the origins of the extra chromosome.  Most geneticists and doctors do not recommend having this done mainly for emotional reasons of the parents.  Since it seems that triploidy and mosaic triploidy occur as a “fluke”, knowing the origins of the extra chromosomes may only lead to either placing blame on the partner or a parent blaming themselves.  In these cases, it is better to not know the answer.

           

Maternal age or diabetes does not have any effect on whether triploidy or mosaic triploidy will occur and IVF/ICSI also does not put you at a greater risk. 

           

One studied showed that 50% of women that have had a child with triploidy or mosaic triploidy were exposed to some sort of abdominal radiation preconceptionally.  It has also been linked to delayed conception due a woman having long periods or to a woman getting pregnant while using or coming off of the birth control pill. 

 

Tests and Diagnosis

           

The most common ways to test for triploidy and mosaic triploidy are through CVS (chorionic villus sampling) and amniocentesis.  CVS is a when a sample of the placental tissue is taken and analyzed while amniocentesis is a sample of the amniotic fluid.  CVS testing can be done earlier during pregnancy the amniocentesis which is usually done between 18-20 weeks gestation. 

           

If CVS testing shows triploidy, there is still hope.  It may be recommended that you also get an amniocentesis just to be sure.  The reason for this is that triploidy can be present in the placenta and have no effect on the fetus.  The cause for this is unknown.

           

Amniocentesis is the most positive way to test for triploidy and mosaic triploidy because it uses the skin cells of the fetus.  Since either form of triploidy cannot be detected through blood, it is better to test amniotic fluid or skin cells if the test is being done after birth.  Triploidy is found in approximately 8 of 10,000 CVS tests and 4 of 10,000 amniocentesis.

 

Signs of Triploidy and Mosaic Triploidy

           

Since triploidy and mosaic triploidy are so rare, they are usually discovered when looking for something else.  In my personal case, there were enlarged ventricles in the baby’s brain and a “double bubble” or blockage in the intestines (duodenal artresia).  These are markers that are linked to Down’s syndrome but mosaic triploidy was discovered through amniocentesis. 

           

Other anomalies include:

      fetal growth restriction

      partial hydatidiform mole (the placenta and fetus are partially comprised of vesicular villous structures resembling grapes)

      macrocephaly (abnormally large head)

      hydrocephaly (an abnormal condition in which cerebrospinal fluid collects in the ventricles of the brain)

      holoprosencephaly (condition where the fetal brain does not grow forward and divide)

      micrognathia (abnormal smallness of the jaw)

      microphthalmia (abnormal smallness of the eye)

      bulbous nose

      small mouth

      malformed and low set ears

      coloboma of the eye (a defect of the iris caused by failure of tissue of the eyeball to fuse properly)

      cataracts

      cleft lip and/or palate

      webbing (syndactyly) of the third and fourth digits of the hands or feet

      simian crease of the hand (a single crease on the palm of the hand where people normally have three)

      rocker bottom feet

      ventricular septal defects (one or more holes in the muscular wall that separate the right and left ventricles of the heart)

      atrial septal defects (a hole in the two upper chambers of the heart)

      pulmonary hypoplasia (incomplete development of lung tissue)

      diaphragmatic hernia (when part of the stomach protrudes through the diaphragm)

      intestinal malrotation (mechanical obstruction caused by abnormal intestinal development)

      cystic kidney (fluid filled sacs in the kidneys)

      adrenal hypoplasia (the adrenal gland does not produce enough adrenaline)

      ovarian hypoplasia (absence of ovaries in females)

      abnormal male genitalia including hypospadias (when the urethra opens under the surface of the penis), micropenis and undescended testicles

 

Neural tube defects may also be found in 25% and abdominal wall defects in 10-18% of triploidy cases.

           

Mosaic triploidy cases tend to not be as severe as full triploidy but does share some of the same signs.  Not all signs are present in triploidy and there may actually be an absence of any sign until birth. 

 

Life Expectancy

           

Life expectancy for a child born with full triploidy is very short.  Most babies die within a few days and the longest reported survival is 10.5 months. 

 

Babies that are born with mocaisism tend live a longer.   The longest case found while researching this paper was seven years but the child was both physically and mentally handicapped.  Researchers believe that children with mocaisism live longer because the effects of the mocaisism aren’t as severe as full triploidy. 

           

Children that have triploidy or mosaic triploidy usually do not survive through pregnancy. The live birthrate of triploidy is 1/10,000.

 

Quality of Life

           

The quality of life for a child born with full triploidy is obvious considering the effects it has and the actual life expectancy.  On the other hand trying to determine the quality of life for a child with mosaic triploidy is difficult. 

 

Since there are so few cases that are diagnosed and reported, researchers are unsure if there are thousands of people walking around with mocaisism that have no idea and are leading productive lives, or if it really is as rare as they think.  The main reason that geneticists and researchers don’t know if mosaic triploidy is more common is that it cannot be detected through blood samples, it is detected through skin cells or other tissues.  Since the number of extra chromosomes can vary in different tissues of the body, it is impossible to know the full extent of the effect of the mocaisism even after birth.

 

Most studies report that 82-100% of children diagnosed prenatally with triploidy are electively terminated and is reported to be the cause of up to 13% of miscarriages. 

 

Recurrence

           

The chances of having another pregnancy where either forms of triploidy are present are approximately 1%.

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Helping families with a loss from Triploidy

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