What is Preimplantation Genetic Diagnosis?
Preimplantation Genetic Diagnosis is an established technique used since the late 1980s to detect genetic abnormalities in embryos created by artificial fertilization prior to implantation in the uterus. Preimplantation genetic diagnosis is an alternative solution to the methods of prenatal diagnosis (either with CVS or amniocentesis) for couples at risk of transmitting a hereditary disease to their children.
In prenatal diagnosis, if it is found that the pregnant fetus is infected with a genetic disease, the couple will have to decide whether they wish to continue pregnancy or to terminate pregnancy. Instead, preimplantation genetic diagnosis combined with IVF allows fertile and sub-fertile couples to acquire a healthy child without having to think about the possibility of stopping a pregnancy.
Which patients benefit from preimplantation genetic diagnosis?
Preimplantation genetic diagnosis can benefit every couple with a known increased risk of inheriting a genetic disorder or condition to their child.
It is also applied to IVF couples undergoing PGS in order to increase the probability of pregnancy, by selecting non-aneuploid embryos (e.g. older women (>35), with a history of miscarriages, repeated implant failures in previous attempts, men with non-obstructive azoospermia or known increased aneuploidy in spermcells).
Which diseases can be diagnosed with pre-implantation genetic diagnosis?
Currently Preimplantation Genetic Diagnosis is applied to detect:
- A monogenic disorder:
Where one or two parents are carriers of a genetic disease, e.g. Mediterranean anemia, hemorrhophilia, familial colon cancer, cystic fibrosis, sickle cell anemia, Huntington's disease.
- A known chromosomal disorder:
In terms of numerical abnormalities (e.g. trisomy 21 known as Down's Syndrome) and structural chromosome alterations.
- A serious sex-linked condition:
Such as Duchenne Muscular Dystrophy, Haemophilia A & B and Fragile X Syndrome.
How is Preimplantation genetic diagnosis applied?
Preimplantation Genetic Diagnosis begins with the normal process of IVF involving ovarian stimulation, egg collection, sperm recovery and fertilization in the laboratory. The embryos are then grown in culture for 3 days until they reach the stage of 6-8 cells. PGD involves the removal of a single cell (embryo biopsy) from the fetus and the genetic diagnosis of this cell.
- Embryo Biopsy
Embryo biopsy is usually performed on the morning of the 3rd embryonic development day, where it has reached the stage of 6 to 8 cells (first stages of the cleavage stage). One or two blastomeres are carefully removed after opening the ovule's transparent zone chemically or mechanically, or using a laser, without causing any damage to the future development of the embryo. At this early stage of embryo development, all cells have the same potential for growth, therefore removal of one or two cells from the embryo is not harmful and the embryo can continue its normal development.
Genetic diagnosis is done using various techniques, based on the disease to be avoided.
- Polymerase Chain Reaction (PCR) is used for molecular diagnosis (diagnosis of monogenic disorders). In each case the test is personalized depending on the gene and the disease.
- Fluorescence in situ hybridization (FISH) is used for cytogenetic diagnosis, i.e. chromosome analysis to avoid numerical or structural anomalies.
In more detail:
In this technique, the removed cell is fixed to a glass plate and subjected to chromosomal analysis for specific chromosomes. Today, the most common type of PGS / PGD involves controlling a group of the most common chromosomal abnormalities (chromosomes 13, 16, 18, 21, 22, X and Y). Subsequently, small pieces of DNA (probes) specifically bind (hybridization) to certain regions of the analyzed chromosomes. Each detector is labeled with a different fluorescent dye. These fluorescent probes adhere to the core of the cells removed by biopsy and are expected to hybridize with specific chromosomes. Using a fluorescence microscope, the geneticist can recognize the fluorescent color signal for each type of chromosome present in the cell (one, two or more signals). A single fluorescent signal represents monosomal, two signaling disomy (normal cell) and three trisomy signals of the particular chromosome. In this way, the diagnosis of aneuploidy of the specific chromosomes is performed.
- The method of Comparative Genomic Hybridization in Oligonucleotide Microarray - Array CGH, identifies chromosomal deficits and doubling (possibly responsible for failure or gestational elimination) by checking all the chromosomes of the genetic material of each embryo. The method excels over FISH because all 23 pairs of chromosomes of the human species (and not just 9) are tested, but also because of every 'internal' chromosome anomaly detection. This is particularly useful in cases of hereditary chromosomal translocations. With FISH, we only controlled the pair of chromosomes that had been translocated. However, it has been discovered that translocations are very often damaged in other chromosomes, demonstrating that Array CGH is the most complete and reliable method of chromosomal genetic control method.
Only the normal embryos are then transferred to the woman's uterus for implantation so that the pregnancy starts with knowing that the fetus is healthy.
Concerns and disadvantages
Preimplantation Genetic Diagnosis may seem to be the ideal choice for patients who have a genetic or chromosomal disease, but realistically, Preimplantation Genetic Diagnosis has some concerns or disadvantages:
- Patients should undergo IVF, and especially for a fertile couple, this can be difficult and stressful.
- Technical problems can occur during biopsy.
- Even with successful IVF and Preimplantation Genetic Diagnosis, pregnancy is not guaranteed after transfer.
- A good number of embryos, ideally around 10, are required for Preimplantation Genetic Diagnosis to be successful so that after Preimplantation Genetic Diagnosis, some good quality healthy embryos are available for transport. In some cases, all embryos may be unhealthy, or there may not be any morphologically good embryos to be transferred.
- Single cell analysis has limitations and an incorrect diagnosis can occur. Therefore, prenatal diagnosis is still recommended to confirm the status of the embryo.
- Unfortunately, all chromosomal or genetic abnormalities cannot be diagnosed with Preimplantation Genetic Diagnosis.