Pre-implantation Genetic Diagnosis
PGD (Preimplantation Genetic Diagnosis) for single gene disorders is a genetic test that may be performed during assisted reproduction treatment to screen embryos that are at risk to develop a serious genetic disorder.
PGD is performed on a small embryo biopsy and identifies which embryos are not at increased risk of developing the disease.
Some genetic diseases only affect one sex rather than the other. In this case, the embryo is tested to find out its sex and only embryos of the non-affected sex are transferred to the mother’s womb.
PGD is recommended for
- couples at risk of transmitting chromosomal alterations or monogenic diseases,
- couples with a medical history of repeated miscarriages,
- couples with several cycles of IVF that have not achieved pregnancy,
- when are abnormalities in spermatozoa maturation and
- women of advanced age.
The main steps in an IVF/PGD cycle include:
Fertility drugs are administered to a woman
These may be given orally, or by injection over a 2 to 4-week period. The fertility drugs act to stimulate more than one ovarian follicle to mature and provide a number of eggs for fertilisation.
Blood and ultrasound tests
These are used to check the woman’s follicle development so that egg collection can be timed. These tests are also to avoid overstimulating the ovaries.
The woman is placed under a light anaesthetic or sedation (and occasionally a general anaesthetic) so that eggs may be collected with the help of a vaginal ultrasound probe. This will be performed by a gynaecologist.
After egg collection, the eggs will be fertilised with sperm in the laboratory through IVF or ICSI. Sperm may have been freshly collected or previously collected and frozen. The fertilised eggs are matured or ‘cultured’ until 3 or 5 days of age.
Biopsy (removal of cells for testing)
Cell removal is carried out on embryos at 3 days of age when an embryo will usually have between 6 to 8 cells.
A laser is used to open the outside shell of the embryo, and the cell/s are carefully separated from the remaining embryonic cells. In most cases, the ‘hole’ or deficit will close over, and the embryo will develop as it should.
However, a very small percentage of embryos will not continue to develop following embryo biopsy.
The biopsied cells are then prepared and tested for the specific gene or translocation.
Polymerase chain reaction (PCR) is the most common laboratory method used for PGD.
Freezing of embryos
When embryos are tested on day 3, it may be possible to transfer the embryos after biopsy/PGD as the PGD results are usually back within 2 days. When day 5 (blastocyst) embryos are tested, they will require freezing following the biopsy in order to await results.
If the PGD results show that an embryo is suitable for transfer, the embryo are transferred using a catheter that has been guided into the woman’s uterus. Any remaining embryos that are suitable for transfer can be frozen and stored for later use.
Unfortunately, there is always a chance that no embryos created from an IVF cycle will be suitable for transfer.
PGD on D3 embryo
PGD on D3 embryo
PGD and Sex Selection
Some genetic diseases only affect one sex rather than the other. Examples include Duchenne Muscular Dystrophy, which affects boys but not girls (girls may still ‘carry’ the gene for the disease but they will not suffer from it). In these sorts of cases, the embryo is tested to find out its sex and only embryos of the non-affected sex are transferred to the womb. This means that as well as selecting all female embryos, you may also be able to select male embryos that do not carry the gene for the disease.
Greek law prohibits sex selection during assisted reproduction, unless it is necessary to prevent a genetic abnormality or disease.
PGD Genetic Conditions Tested
PGD can be used to test for virtually any genetic condition where a specific gene is known to cause that condition. It is currently approved to screen for over 250 genetic conditions.
List of genetic diseases tested
- Alpers Syndrome
- Anderson Fabry Disease
- Androgen Insensitivity Syndrome
- Autosomal Recessive non-Syndromic Sensorineural Deafness
- Beckers Muscular Dystrophy
- Beta Hydroxyisobuyryl CoA Hydrolase Deficiency (Methacryic Aciduria)
- Beta Thalassaemia
- Breast Cancer, BRCA 1
- Bruton Agammaglobulinemia Tyrosine Kinase (BTK)
- Cardiac Valvular Dysplasia
- Charcot Marie Tooth Disease
- Chronic Granulomatous Disease
- Coffin-Lowry Syndrome
- Congenital Adrenal Hyperplasia
- Congenital Fibrosis of the Extraocular Muscles
- Crouzon Syndrome
- Cystic Fibrosis
- Duchenne Muscular Dystrophy
- Early Onset Alzheimers Disease
- Ectodermal Dysplasia
- Facioscapulohumeral Dystrophy
- Familial Adenomatous Polyposis Coli (FAP)
- Fanconi’s Anaemia A
- Fanconi’s Anaemia C
- Fragile X Syndrome
- Gonadal Mosaicism
- Haemophilia A
- Haemophilia B
- Hallipeau-Siemens Recessive Dystrophic Epidermolysis Bullosa
- Homozygous Familial Hypercholesterolaemia
- Hunters Syndrome
- Huntington’s Disease (Huntington’s Chorea)
- Hydroxyisobuyryl CoA Hydrolase Deficiency
- Incontinentia Pigmenti
- Infantile Battens Disease
- Juvenile Retinoschisis
- Lenz Syndrome
- Lesch Nyan Syndrome
- Li-Fraumeni Syndrome
- Lymphoproliferative Syndrome Marfan Syndrome
- Marfan Syndrome
- Metachromatic Leukodystrophy
- Multiple Endocrine Neoplasia Type1
- Myotonic Dystrophy
- Myotubular Myopathy
- Myotubular Atrophy
- Neurofibromatosis Type I
- Neurofibromatosis Type II
- Niemann Pick Disease Type C
- Ornithine TransCarbamylase (OTC) Deficiency
- Ostheopathia Striata with Cranial Sclerosis (OSCS)
- Otopalatodigital Syndrome Type 2
- Partial/Familial/Type 2 Lipodystrophy
- Pelizaeus Merzbacher Disease
- Prader Willi Syndrome
- Severe Aplastic Anaemia
- Severe Combined Immune Deficiency
- Sickle Cell Anaemia
- Spinal Muscular Atrophy
- Tay Sachs Disease
- Treacher Collins Syndrome
- Von Hippel Lindau (VHL) Syndrome
- Wiscott-Aldrich Syndrome
- X-linked Hereditary Motor and Sensory Neuropathies
- X-linked Hydrocephalus
- X-linked Immunodeficiencies
- X-linked Mental Retardation
- X-linked Spastic Paraplegia
- Chromosomal Rearrangements (PGD is also available for some chromosomal rearrangements)
Most of the risks involved in PGD treatment are similar to those for conventional IVF.
The process of IVF and PGD poses some risks to the woman and the embryo. These may include:
- Health risks of the IVF cycle.
- An embryo may not develop after fertilisation of an egg.
- Not all embryos will be suitable for biopsy.
- The embryo may not develop after biopsy.
- There may not be any unaffected embryos available for transfer.
- Test results may be inconclusive.
- Results are not 100% accurate.
- No pregnancy is achieved, even after the transfer of an unaffected embryo.
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