Mosaic trisomy 15
What is mosaic trisomy 15?
In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46 chromosomes. Each pair is numbered 1 through 22 except for the sex chromosomes which are denoted by "X" and "Y." Women have two Xs while men have one X chromosome and a Y chromosome. In complete trisomy 15, ALL of the cells of the body have an extra chromosome 15, which means that there are 47 chromosomes in all the cells. A fetus with an extra chromosome 15 in all its cells will usually miscarry and not make it to term. In MOSAIC trisomy 15, only some of the cells of the body have an extra chromosome 15, which means that there are 47 chromosomes in those cells while the other cells in the body have the typical 46 chromosomes. The cells with the extra chromosome 15s are called "trisomic," as they have a third chromosome 15. The word "mosaic" means that only some cells in the body have the extra chromosome while others do not. An extra chromosome 15 in some of the body's cells causes a range of symptoms, including specific facial features, being small in the womb, and heart problems. Everyone with mosaic trisomy 15 will have a different set of symptoms because they will have a different number of trisomic cells and those cells will be in different parts of the body.
When mosaic trisomy 15 is found before a baby is born, through amniocentesis or chorionic villus sampling, it can be very difficult to predict what health problems will occur. If the test finding mosaic trisomy 15 is a chorionic villus sampling, then there is a chance that all the trisomy 15 cells are only in the placenta and not in the baby (confined placenta mosaicism). In this case, an amniocentesis may be suggested to look at the baby's cells directly. If the cells with extra chromosome 15 are in the baby and the placenta, usually the baby's health is predicted based upon the percent of cells that have the extra chromosome 15, if the extra chromosome is from mom or dad, and the number of health problems or birth defects seen on ultrasound. Medical geneticists and genetic counselors can be very helpful in discussing the possible outcomes based on this information.
Most often infants with many cells that have the extra chromosome 15 are very sick and die early in life; however, individuals with fewer trisomic cells have fewer health issues and may live longer. Rarely, individuals with a very small number of trisomic cells found on prenatal testing can have very few to no medical issues and live a normal life span.