PGD — preimplantation genetic diagnosis
Preimplantation genetic diagnosis is a method of early testing for abnormalities in embryos to prevent the birth of children with genetic diseases. PDG is used to screen eggs, sperm and embryos for chromosomal abnormalities and individual genetic lesions.
Blastocyst selection
Infertility treatment focuses on embryo selection at the blastocyst stage. Fertilized eggs unable to develop into blastocysts are not transferred.
Eighteen to twenty hours after the addition of sperm or the use of ICSI (1st day), two pronuclei are formed during normal fertilization. They are the precursors of the nuclei of future blastomeres, into which the fertilized egg begins to divide. With proper fertilization, both pronuclei are clearly distinguishable.
Embryos with four blastomeres are considered promising for the second day of culturing.
Blastomere aspiration for analysis
Every healthy embryo contains 46 chromosomes, 23 of which originate from the egg and 23 from the sperm. Thus, an embryo containing 46 normal chromosomes is called an euploid embryo. Chromosomes are made up of genes that act as chemical messages, informing the cell how to grow and function in the various processes that occur in the human body. There are over 30,000 different genes, made up of DNA arranged in a specific sequence and containing the «code» for each individual gene and its function. Disruption of the normal structure of the code, as well as abnormalities in the number of genes or chromosomes, can have serious consequences.
Aneuploidy is the most common chromosomal abnormality and represents the presence of an extra chromosome (trisomy) or the absence of a chromosome (monosomy). In the presence or absence of extra chromosomes, the probability of embryo implantation in the uterus decreases and the incidence of spontaneous abortion increases.
If chromosomes 13, 18, 21, X or Y are altered, implantation can occur and the pregnancy can progress, but the resulting baby may be born with medical conditions. The most common trisomy is trisomy 21 or Down syndrome. Other abnormalities include Πatay syndrome (trisomy 13), Edward syndrome (trisomy 18), Klinefelter syndrome (47 XXXl), and Turner syndrome (45 X, missing sex chromosome). In spontaneous abortions, trisomies 15, 16, 21, and 22 are the most common. The most common aneuploidy in 3-day embryos is 15, 16.
The likelihood of aneuplody increases with maternal age. Because a woman’s fertility is limited, it is thought that eggs from women after age 40 are more likely to make division errors, which increases the percentage of eggs with missing or extra chromosomes. Studies show that embryos from women aged 35-39 have aneuploidy of more than 20% and embryos from women over 40 have 40-60% aneuploidy.
Although preimplantation genetic diagnosis for aneuploidy significantly reduces the risk of having a child with trisomy or monosomy, it is still not possible to test all chromosomes. The following chromosomes are mainly examined: 13, 15, 16, 17, 18, 21, 22, X and Y chromosomes. The accuracy of ΠGD for the diagnosis of aneuploidy is more than 90%.
PGD is also used to detect translocations, which are changes in the structure of chromosomes. Typical of these patients is that they have no disease, although some have reduced fertility.
An «unbalanced» translocation has excess or missing chromosomal material. The probability of implantation of an embryo with an unbalanced translocation is low, and the probability of miscarriage is high. But if a pregnancy does occur, the baby may have physical or mental disabilities.
Reciprocal translocations affect about 1 in 625 women. This type of translocation means an abnormality in two different chromosomes. Approximately 1 in 900 people has a Robertson translocation involving chromosomes 13, 14, 15, 21 or 22, which may be fused together. The risk of recurrent miscarriage or of having a sick child depends on the chromosome involved and the size of the exchanged fragment.
Using PGD, a gene defect can be detected. There are two categories of gene defects:
- recessive — two defective copies of the gene, one from each parent, are required for the disease to manifest;
- dominant — only one copy of the defective gene is needed to manifest the disease.
Pre-screening is recommended for some of the most common single gene defects, such as cystic fibrosis (MB) and Tay Sachs disease.
The advantages of preimplantation genetic diagnosis are:
- selection and transfer of embryos without chromosomal abnormalities;
- reduction of spontaneous abortions;
- reduction of multiple pregnancies;
- increased rate of embryo implantation;
- increasing the percentage of healthy children born.
The risk of PGD includes:
- embryo damage (<1%);
- incomplete diagnosis (<10%);
- error in identifying abnormal changes in the embryo, normal (<3%);
- error in embryo transfer when 100% of abnormal changes are detected (up to 20%).
This method combines assisted reproductive technologies, embryology and genetics. PGD is the earliest preimplantation diagnostic test for patients who will undergo IVF/ICSI, and it also detects embryos with incorrect genetic code. Thus, at the onset of pregnancy, the risk of congenital genetic diseases of the developing fetus is significantly reduced, as well as the probability of a healthy pregnancy and the birth of a healthy child increases.
The main indications for preimplantation genetic diagnosis are:
- age of a woman over 34 years old;
- male age over 39 years;
- if there is an increased risk of having a child with congenital anomalies — genetic diagnosis is performed for nine chromosomes;
- two or more miscarriages
- two or more failed IVF procedures;
- men with severe disorders of spermatogenesis.
The preparation and the treatment cycle of IVF with PGD is practically the same as conventional IVF:
- hormonal medications to stimulate ovulation;
- transvaginal follicle puncture;
- fertilization of the eggs with sperm;
- transfer of embryos into the uterus on day 5-6.