What Is Mosaicism?

Medically Reviewed by Dan Brennan, MD on July 24, 2022
Written by Amber Felton
4 min read

Mosaicism occurs when two or more groups of cells in a person possess a different genetic makeup. As a mosaicism example, a person might possess some cells that have 46 chromosomes while other cells have 47 chromosomes. 

This difference in a person’s chromosomal makeup can contribute to health problems at birth, such as Mosaic Down syndrome, or cause issues later on in life.

There are several disorders that can be caused by mosaicism, and each disorder will have its own set of symptoms. Some of these disorders include: 

  • Mosaic Down syndrome: This condition results in intellectual delays and disabilities and also contributes to weak muscles and a flat facial structure. Heart defects can also be present, as well as issues with the digestive tract and thyroid.
  • Pallister-Killan mosaic syndrome: This condition leads to weak muscles, intellectual delays, thinning hair, areas of irregular skin color, and other defects that occur at birth.
  • SOX2 anophthalmia syndrome: This condition is rare, but when it does occur, it can cause a child to be born without eyes and experience seizures, brain development issues, and delayed growth.
  • Trisomy 18: This condition slows growth in the womb. This can contribute to the child being born with a smaller-than-normal head and defects in the heart and other organs. Children born with this condition often do not live past their first year.

The effect mosaicism has on a person’s health will depend on the level of mosaicism present at birth. Mosaicism has been linked to various pathologies including Turner syndrome, which is a chromosomal abnormality, and to various forms of cancer.

Potential symptoms can also include cancer, particularly cancers of the blood, as well as birth defects, developmental delays, and potential miscarriages.

Mosaicism occurs during pregnancy after the egg has been fertilized and the zygote begins to divide. This occurs when new cells form and the chromosomes replicate themselves in order for the new cells to have the same number of chromosomes as the original cell. During this period, cells can end up having a different number of chromosomes, resulting in mosaicism. 

Sometimes, abnormal cells will die, and mosaicism will not be an issue. When they survive, though, mosaicism occurs. 

There are two levels of mosaicism: high-level mosaicism and low-level mosaicism. High-level mosaicism is the result of the occurrence of these abnormal cells in early development, and 50% or more cells are affected. Low-level mosaicism occurs when the mosaicism happens later on and a smaller number of cells are impacted.

Meanwhile, in the developmental stage, there are two types of Mosaicism: 

  • Somatic mosaicism: When more than one somatic cell lineage is involved but the sperm and oocytes remain unaffected and the somatic mosaicism does not affect the offspring. When somatic mosaicism occurs during preimplantation, the embryo’s somatic and germline cells may be affected.
  • Germline mosaicism: When more than one cell lineage of the germline cells is involved and affects the offspring. 

In the cellular stage, Mosaicism is also classified into two types: 

  • General mosaicism: Occurs when there are two or more cell lines existing in a baby.  
  • Confined mosaicism: Unlike general mosaicism, confined mosaicism occurs when specific body parts or organs are affected instead of the whole baby. For example, the brain, heart, or liver may be affected while other organs are not.  

There’s no way to predict who will end up with mosaicism. However, during in-vitro fertilization, or IVF, doctors will conduct a preimplantation genetic screening, or PGS, prior to placing the fertilized egg into a woman’s uterus to determine if the risk for mosaicism is present.

Mosaicism genetics testing is used to diagnose this condition. These tests can include blood and skin tests and will sometimes require two different tissues to be tested. In order to confirm whether or not mosaicism is present and the type and severity of the mosaicism, tests may need to be repeated.

In pregnant women, embryos up to 8 to 10 weeks old can be tested during prenatal diagnosis, which can help to determine if an embryo is mosaic or not. The procedures used to test whether an embryo is mosaic include amniocentesis and chorionic villus sampling. Neither of these tests requires embryo tissue to be removed. 

Since in-vitro fertilization generates an increased risk of mutation and mosaicism, preimplantation screening is necessary in order to avoid genetic disorders.

Currently, there is no cure or way to definitely prevent mosaicism from occurring. In terms of treatment, the recommended courses of treatment will vary depending on the type of mosaicism present and the severity. In cases where less than half of cells are abnormal, treatment will be less aggressive. More severe cases will require more intense treatment methods. 

The prognosis related to mosaicism will also depend on the severity of the disorder, the type, and which organs and tissues are affected. Those who possess a lower number of irregular cells might not be as affected as those with a higher number of irregular cells replicating themselves. 

It should also be noted that mosaicism influences the monosomies survival rate. While monosomies are fatal in people, mosaic monosomies can survive post-birth. Mosaicism can also lead to malignancy.