Since 1990 it is possible to genetically test embryos before they are transferred to the uterus! This method is known as Preimplantation Genetic Diagnosis (P.G.D) and is a reproductive technology that offers couples, at high risk of transmitting a genetic disorder to their future offspring, a valuable diagnostic tool.
Genetic information, in other words our genes, is encoded as DNA sequences within our chromosomes. Normal cells have 46 chromosomes (23 pairs). After fertilization, normal embryos have 23 chromosomes contributed by the spermatozoon and 23 chromosomes contributed by the oocyte. If an embryo has a missing or an extra chromosome it is called aneuploid and it is considered abnormal. Alan Handyside and his colleagues introduced Preimplantation Genetic Diagnosis that improved birth rates of healthy babies worldwide.
Preimplantation Genetic Diagnosis can be used to test for:
1. any genetic condition where a specific gene is known to cause that condition and
2. chromosomal abnormalities.
Diagnosis of monogenic disorders has helped couples where both partners are carriers of the defective gene for a specific genetic disorder (e.g. a or b-thalassemia).
Testing for chromosomal abnormalities involves either numerical disorders or structural abnormalities. It is known that the incidence of chromosomal abnormalities (e.g. Down syndrome) increases with maternal age.
However, there are also other factors, than maternal age, contributing to structural chromosome abnormalities (chromosomal inversion or translocation).
It is common to find in couples that one or both partners are carriers of a chromosomal abnormality (not necessarily the same). This abnormality, not expressed in parents, may cause recurrent miscarriages or repeated IVF failure. Preimplantation Genetic Screening (P.G.S) can diagnose chromosomal abnormalities and protect the couple from the emotional distress or somatic burden caused by recurrent miscarriages or repeated IVF failure. Moreover, time and money are saved.
PGD has been used for:
→ Repeat IVF failure
→ Recurrent miscarriage
→ Advanced maternal age
→ Gender selection
→ Mutation carriers
→ HLA matching
→ Chromosomal abnormalities
→ Unexplained infertility
→ Inherited genetic disorders
→ History of chromosomally abnormal child or pregnancy
Techniques used for PGD / PGS
There are two basic techniques used for PGD, fluorescent in situ hybridization (FISH) and Polymerase chain reaction (PCR).
In the last few years, a new molecular cytogenetic technique developed, known as Array Comparative Genomic Hybridization (a-CGH). a-CGH or other approaches of CGH have been applied successfully to substitute FISH and simple PCR, as it can identify structural chromosome errors and also diagnose specific gene mutations.
According to experience with PGD, a successful program involves the interaction between doctors, clinical embryologists, pshycologists and geneticists. It is also very important for the couple to receive proper genetic counseling prior to PGD.
Couples having PGD should go through Assisted Reproductive Technologies, even in the absence of infertility. ART is the only way to produce the embryos to be tested, outside the woman’s body. The early embryo (6 to 8 cells on day 3 of development) is biopsied by microsurgical removal of 1 to 2 cells. Some embryologists wait until the embryo reaches the blastocyst stage (day 5 of development) to undertake this biopsy. At this stage more cells can be removed and analyzed. Once collected, these cells have to be analyzed for detection of a certain medical condition. Their genetic material is the same as of the embryo they originated from. In all these procedures, analysis must be completed soon (1-2 days after biopsy). Embryo selection is based on diagnosis. Only healthy embryos can be successfully transferred, with high safety and success rates.
There are many advantages to PGD testing, such as:
⇒ Screening out embryos with genetic abnormalities before a pregnancy begins, preventing many women from having to decide whether to abort an abnormal fetus. PGD offers advanced diagnosis, prior to prenatal testing (amniocentesis or chorionic villus sampling).
⇒ It is not a cumbersome procedure.
Few of the most frequently diseases diagnosed by PGD are:
1. Cystic fibrosis
3. Fanconi Anaemia
4. Sickle cell disease
5. Spinal muscular atrophy type 1
6. Myotonic dystrophy
7. Huntington’s Disease
8. Fragile X syndrome
9. Duchenne muscular Dystrophy
10. X-linked Disease
11. Single genes disorder
12. Haemophilia A
Restrictions of PGD
Cancellation of embryo transfer
◊ The largest risk of PGD is not having embryos available for transfer, if the results of testing show that all of the embryos are chromosomally abnormal. In these circumstances, the cycle is cancelled.
◊ The risk of getting incomplete or no diagnosis on biopsied cells, due to technical or other reasons, also exists. However, the couple will decide on whether to transfer embryos or not, upon detailed genetic counseling.
Damage during embryo biopsy
There is always a minor risk of traumatizing an embryo during the embryo biopsy procedure, irrespective of embryologists’ experience. This is more possible with embryos of poor quality. However, the incidence of embryo damage and subsequent degradation after embryo biopsy is very low (less than 1%).
All embryonic cells should contain the same number of chromosomes. It is well documented that some embryos have one or more blastomeres with different chromosomal complement. This phenomenon is called chromosomal mosaicism. This means that the blastomere biopsied for PGD may not represent the rest of the embryo and could result in an adverse outcome. For this reason, couples are strongly advised to undergo prenatal testing (amniocentesis or chorionic villus sampling) after PGD in case of achieving a pregnancy.
Risk of miscarriage
PGS is being used to improve the chance of a successful pregnancy in couples with recurrent miscarriages. However, PGS testing is limited to only testing for a few chromosomes, not all 23 pairs of embryonic chromosomes. Thus, miscarriage due to chromosomal abnormalities can still occur even after PGS.