PGD & PGS
Preimplantation genetic diagnosis (PGD)
PGD is a genetic diagnosis technique that provides information about the gene structure of cells in an embryo.
An embryonic biopsy is used – about 3 to 8 cells from a 5-day embryo (blastocyst stage). The harvested cells are sent to the laboratory for testing. After the biopsy, the embryo is frozen and preserved until transfer and implantation.
The PGD is able to identify approximately 2,000 genetic diseases, it has 98% accuracy and it is used to identify healthy embryos. It is generally useful to couples with a family history of genetic diseases that can be inherited by their children.
Specific markers of certain diseases, such as monogenic disorders, including cystic fibrosis and sickle cell disease, X chromosomal fragility, and Duchenne muscular dystrophy, can be identified by using the preimplantation genetic diagnosis technique.
Preimplantation genetic screening (PGS)
PGS is used to determine whether the cells in an embryo have the normal number of chromosomes (46).
A biopsy from a 5-day embryo (blastocyst stage) is used. The embryonic cells are then sent to a laboratory that uses the technique to identify the number of chromosomes in each cell. Embryos with a normal number of chromosomes are called „euploid” and those with an abnormal number are called „aneuploid”. The aim of PGS is to prevent the transfer of an abnormal embryo to the uterus.
As all women/men will have some eggs/sperm that are chromosomally abnormal, all couples are at risk of having abnormal embryos. This risk significantly increases as a woman gets older.
An abnormal embryo almost always fails to implant or, if it does, it ends with a biochemical pregnancy (hormonal evidence of pregnancy only), miscarriage, foetal death later during pregnancy, or birth of a child with severe health problems.
Couples undergoing IVF (in vitro fertilization) can make use of PGS if they:
- have been diagnosed with severe male infertility,
- had recurrent IVF failures,
- had multiple miscarriages,
- age over 35 years.
Pre- and post-test genetic counseling
Pre-test genetic counseling has to inform the individual:
- What is the test for?
- symptoms, natural history of the disease,
- prospects of prevention, or treatment,
- inheritance pattern, the risk of the disease in the counselee’s situation,
- reliability and limitation of the test
- possible psychological impact and other consequences of the test result.
- Possible uncertainties due to the present lack of knowledge should be declared.
- The counselor should offer assistance in the decision making
A written summary of the discussion should be prepared.
Post-test genetic counseling should give information about:
- Whether the test has to be repeated because of possible uncertainties.
- A follow-up plan should be discussed.
- Depending on the disease being tested a consultation with a psychologist or follow-up contact with the genetic counseling unit should be offered.
- A written summary of the test result should be given to the counselee.
- Written material to help the counselee to spread the information in the family may be offered.
The process of a genetic counselling session
- Collecting genetic and other clinical information of the patient affected by a genetic disorder
- Drawing up a family tree
- Syndrome analysis
- Making the correct diagnosis, differential diagnosis
- Determining the genetic background of the disease and method of inheritance
- Risk calculation (background risk; risk of genetic disorder; risk of genetic testing)
- Organizing the possible ways of genetic prevention and diagnostics
- Giving a written genetic finding
- Giving verbal information for the patient, helping him to make a decision.
Reasons to attend a genetic counselling service
- Family history of a chromosome abnormality
- Family history of a genetic disorder or a malformed family member
- Molecular test for single gene disorder
Special reason for pregnant women and/or couples planning family
- Increased maternal age (>35 years of age)
- Abnormal serum screening results or ultrasound findings
- Consanguinity, or marriage between close relatives
- Recurrent spontanous abortions, infertility
- Teratogenic effect (drugs, radiation exposition)
- Common chronic disorders (hypertension, common cancers, coronary heart disease) 2 patients affected among first degree relatives and 3 patients affected among second and third degree relatives.
𝐆𝐞𝐧𝐞𝐭𝐢𝐜 𝐛𝐚𝐜𝐤𝐠𝐫𝐨𝐮𝐧𝐝 𝐨𝐟 𝐏𝐫𝐚𝐝𝐞𝐫–𝐖𝐢𝐥𝐥𝐢 𝐬𝐲𝐧𝐝𝐫𝐨𝐦𝐞 (𝐏𝐖𝐒) 𝐚𝐧𝐝 𝐀𝐧𝐠𝐞𝐥𝐦𝐚𝐧 𝐬𝐲𝐧𝐝𝐫𝐨𝐦𝐞 (𝐀𝐒)
Genomic imprinting is the monoallelic expression of genes, dependent on the parental origin of the chromosome. It plays a role in growth and development. Imprinting disorders like Prader-Willi syndrome (PWS) and Angelman syndrome (AS) originate from a disturbance in this monoallelic expression by disruption or epimutation of imprinted genes. PWS and AS are distinct neurogenetic disorders, both usually caused by chromosomal deletions on chromosome 15q11 or by uniparental disomy (UPD). In UPD, both copies of a chromosome are inherited from a single parent. These 15q11 chromosomal alterations result in an aberrant expression profile of gene loci that are subject to imprinting. The absence of a paternal allele of chromosome 15q11, due to a chromosomal deletion of (part of) the paternal allele or the presence of two imprinted copies due to maternal UPD, results in PWS. The absence of the maternal copy of the same region or paternal UPD causes AS.