WHAT IS ANEUPLOIDY?
Our genetic information is contained in structures within cells called chromosomes. We carry two copies of each chromosome, one inherited from each parent. Healthy embryos have 46 chromosomes: 22 pairs of autosomal chromosomes and 1 pair of sex chromosome (XX for females and XY for males). These healthy cells are called euploid.
An embryo that has extra or missing copies of a chromosome is called aneuploid. Aneuploidy results in high rates of implantation failure and miscarriage, or a baby born with a genetic syndrome. Syndromes detected by Previda® include:
- Patau syndrome (+13)
- Edwards syndrome (+18)
- Down syndrome (+21)
- Klinefelter syndrome (XXY)
- Turner syndrome (XO)
- XYY syndrome (XYY)
Growing embryos are constantly dividing. Aneuploidy is caused by an error in cell division. When such an error occurs during the early development of an embryo, it is possible that only a subpopulation of total cells will be aneuploid, resulting in a mosaic embryo.
MOSAICISM
Mosaicism is a biological phenomenon that results in populations of cells with distinct genetic compositions in the same individual. Mosaicism in embryos is primarily caused by errors in mitosis during the early stages of development. Mitosis is the process of cell division that allows an embryo to grow. Each cell divides into two “daughter” cells that are meant to be genetically identical to the original “parent” cell. Mitotic nondisjunction is a random event where chromosomes fail to distribute equally into daughter cells, resulting in cells with missing or extra chromosomes. When this occurs in a small group of embryonic cells, the embryos will contain a mix of chromosomally normal and abnormal cells. While nondisjunction during mitosis leads to mosaicism, nondisjunction during meiosis – a different cell division process – leads to an embryo with a cell population containing 100% of the same aneuploidy type.
MOSAICISM IN IVF
Mosaicism can occur on several levels: gene, sub-chromosome, and chromosome. Sub-chromosome refers to mosaicism of chromosome segments rather then the entire chromosome. Only the sub-chromosome and chromosome levels are considered in preimplantation genetic screening.
Clinical outcome is variable, although only a minority of mosaic embryos result in healthy pregnancies. Compared to a euploid embryo, a mosaic embryo is much more likely to result in implantation failure, miscarriage, or a newborn affected with a genetic disorder.
The impact of mosaicism in IVF is still in preliminary research stages and should be considered carefully when making a clinical decision. Genetic counseling is highly recommended when considering the transfer mosaic embryos. Studies recommend that patients undergo an additional IVF cycle to obtain a euploid embryo instead of transferring a mosaic embryo.
HOW IS MOSAICISM DETECTED?
Mosaicism can occur on several levels: gene, sub-chromosome, and chromosome. Previda® testing uses next-generation sequencing combined with complex bioinformatic algorithms to count chromosome copy number accurately and precisely. When chromosome copy number deviates from the normal status (2n), but does not reach a full monosomy (n) or trisomy (3n) status, it is classified as mosaic. An embryo is called low level mosaic if 20-40% of the cells are deemed abnormal.Sub-chromosome refers to mosaicism of chromosome segments rather then the entire chromosome. Only the sub-chromosome and chromosome levels are considered in preimplantation genetic screening.
Clinical outcome is variable, although only a minority of mosaic embryos result in healthy pregnancies. Compared to a euploid embryo, a mosaic embryo is much more likely to result in implantation failure, miscarriage, or a newborn affected with a genetic disorder.
The impact of mosaicism in IVF is still in preliminary research stages and should be considered carefully when making a clinical decision. Genetic counseling is highly recommended when considering the transfer mosaic embryos. Studies recommend that patients undergo an additional IVF cycle to obtain a euploid embryo instead of transferring a mosaic embryo.
