Preimplantation genetic studies
Publication date: 15.07.2021
Preimplantation genetic studies allow a couple to detect a genetic disease at the stage of embryonic development and have a healthy child.
Pre-implantation genetic studies are of several types: genetic testing of aneuploidy – reveals numerical disorders of the embryo’s chromosomes, genetic testing of structural abnormalities of chromosomes – when one of the parents has a structural abnormality of chromosomes and it is diagnosed in the embryo, and genetic diagnosis of genetic diseases – when a specific disease-causing gene is determined in the embryo.
WHAT DOES ANEUPLOIDY MEAN?
The normal human chromosome set consists of 46 chromosomes. 23 chromosomes are maternal and 23 are paternal.
Aneuploidy means a change in the number of chromosomes, one more chromosome is called trisomy, and one less is called monosomy. A female embryo of any age is at risk of aneuploidy. It is known that its risk increases with the age of the mother. If the mother’s age is less than 35 years, it is 30%, and after 35 years, it increases and by 42 years, it is equal to 85%. The normal number of chromosomes of the embryo is an important factor for a successful pregnancy.
INDICATIONS FOR PREIMPLANTATION GENETIC TESTING OF ANEUPLOIDIES ( PGT-A ):
- repeated pregnancy losses;
- Infertility caused by an unknown cause;
- unsuccessful in-vitro fertilization cycle;
- increasing age of the mother;
- male infertility;
- a child with chromosomal aneuploidy;
With preimplantation genetic testing for aneuploidy (PGT-A), it is possible to select an embryo with a normal chromosomal set for embryo transfer. Using the above research allows:
- increase the chance of implantation
- Only one embryo is selected and transferred instead of several, and complications associated with multiple pregnancies are avoided.
- to reduce the risk of spontaneous abortion
- Prevent genetic disease
- Reduce the time to achieve a successful pregnancy
WHAT IS A STRUCTURAL DISORDER OF CHROMOSOMES?
Loss of part of a chromosome, duplication, exchange of areas between two chromosomes, etc.
Robertsonian translocation – when two chromosomes lose their part and the remaining ones are connected to each other by the centromere; In this case, the risk of trisomy and uniparental disomy of the 21st (Down syndrome) and 13th chromosomes (Patau syndrome) of the fetus increases.
Uniparental disomy – usually the fetus receives one of the pair of chromosomes from each parent. In uniparental disomy, both chromosomes or part of them are inherited from only one parent. Examples of uniparental disomy are: Prader-Willi syndrome (overweight, muscle weakness, developmental delay), Angelman syndrome (developmental delay, speech problems, movement disorders), etc.
INDICATIONS FOR PREIMPLANTATION TESTING OF CHROMOSOMAL STRUCTURAL ABNORMALITIES ( PGT-SR ):
- Couples who are carriers of balanced structural abnormalities of chromosomes (repeated pregnancy losses and infertility are noted);
- If one of the parents carries a reciprocal translocation, about 80% of the embryos have a change in genetic material;
WHAT IS A MONOGENIC DISEASE?
A disease caused by a single gene mutation (caused by a change) is monogenic, for example: cystic fibrosis, phenylketonuria, Duchenne muscular dystrophy, etc.
Preimplantation testing of genetic diseases (PGT-M) is a method of genetic diagnosis that allows to determine/exclude the presence of a disease-causing mutation. If infertility is one of the indications for the study of numerical and structural anomalies of the embryo chromosomes, preimplantation genetic testing of monogenic diseases is carried out in fertile couples (couples that do not have infertility problems)!
PREIMPLANTATION TESTING FOR MONOGENIC DISEASES ( PGT-M ) IS INDICATED FOR:
- either parent has an autosomal-dominant inherited disease;
- the presence of a child with a genetic disease in the family;
- carrying an autosomal recessive disease mutation in both parents;
- A woman is a carrier of an X-linked disease-causing mutation;
- to diagnose mitochondrial disease;
- Either parent has a cancer-causing mutation, such as BRCA-1, BRCA-2
- for the purpose of HLA-matching – in the presence of a child in the family with a malignant tumor or other genetic disease (for example, thalassemia) for the purpose of stem cell transplantation;
HOW ARE PREIMPLANTATION GENETIC TESTS PERFORMED AND ARE THEY HARMFUL?
A similar procedure is used for all three types of preimplantation genetic research, the first stage is embryo biopsy, the second stage is sending the obtained material to the laboratory and conducting genetic research.
Embryo biopsy is performed at the blastocyst stage (on the 5th-6th day of embryonic development). The blastocyst contains two types of cells – the trophectoderm – from which the placenta develops and the inner cell mass – from which the fetus develops.
The material for genetic research is taken from the trophectoderm, therefore the cells that give rise to the development of the fetus are not damaged. To date, it has not been described that preimplantation genetic research causes fetal development disorders.
PREIMPLANTATION GENETIC STUDIES ARE AN IMPORTANT PART OF THE IN VITRO FERTILIZATION PROCESS, ITS USE INCREASES THE POSSIBILITY OF A SUCCESSFUL PREGNANCY AND INCREASES THE LIVE BIRTH RATE.
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