PGD – monogenic diseases
Preimplantation diagnostic diagnosis (PGD) allows to select healthy embryo for couples if there is an evidence of a risk of transmitting a serious genetic disease caused by single gene disorder from parents to offspring.
What is PGD
Preimplantation genetic diagnosis (PGD) is always associated with assisted reproduction and in-vitro fertilization. Healthy embryo is selected before being transferred into mother’s uterus. It is the only method that can prevent the birth of an affected child before the start of pregnancy.
This method allows to avoid possible difficulties associated with potential abortion of affected foetus as are ethical dilemmas, psychological burden and health complications for the woman.
PGD enables to diagnose the developmental disorders or hereditary diseases before the transfer of the embryo into uterus. For families with a genetic load PGD is the only reliable method to eliminate the transmission of serious defects and hereditary diseases to their offspring.
What are monogenic diseases and what are the most common types
There is about 5,000 rare inherited diseases that are potentially detectable using preimplantation diagnosis. Monogenic diseases are caused by malfunction of one particular gene.
We can divide diseases into three basic groups:
- with recessive inheritance
- with dominant inheritance
- inheritance linked to the sex chromosome
These different groups have different risks of disease transmission to offspring and also different demands on the selection of embryos suitable for transfer.
The most common rare diseases are:
- sickle cell anaemia
- cystic fibrosis
- spinal muscular atrophy
- metabolic disorders
- Marfan syndrome
- Huntington’s chorea familial hypercholesterolemia
- fragile X syndrome
- muscular dystrophy syndrome
- furthermore, a genetic predisposition to cancer e.g. BRCA1 and BRCA2.
How does PGD work
PGD is always performed as part of the IVF process. It means that female partner takes hormonal medications to stimulate the ovaries to produce multiple eggs. Eggs are retrieved by transvaginal collection and after that are fertilised by ICSI with partner’s sperm.
Genetic material can be obtained by biopsy from embryos which are 5 or 6 days old. Group of cells is biopsied from 5 or 6 days old embryos and is sent to genetic lab for further testing.
It is also necessary to obtain a blood sample from the future father, mother and one close relative who we know is either suffering from the disease or is healthy. In most cases, this relative could be a child of the couple or their parents. This is called a reference sample.
All embryos are vitrified (cryopreserved) after the biopsy and stored in embryological lab until the results from genetic lab are obtained.
How reliable is the method
We are able to say with certainty approaching 100% that the tested embryo is either healthy or is a carrier of a disease or bears the genetic burden associated with the future development of the disease.
What methods are used for preimplantation genetic diagnosis?
The method use for PGD is called Karyomapping. It is a new technology for the genetic analysis of the embryo, which makes it possible to prevent the transmission of a genetic load to the next generation. This applies to both couples where both partners are just carriers of the disease or for couples where one or both suffer from some hereditary diseases.
There is also another benefit of Karyomapping examination and it is the fact that aneuploidy screening of all chromosomes (PGS) is also automatically performed on the same sample, which will bring all the benefits associated with this examination. Only healthy embryos with the correct number of chromosomes are then selected for the transfer, which increases the chance of pregnancy and also excludes the chance of conceiving a child with other developmental disabilities caused by wrong number of chromosomes (Down syndrome, Edwards syndrome…).