June 2008 – OC LIFE MAGAZINE
By Dr. Minoos Hosseinzadeh
Identifying Inheritable Genetic Diseases through PGD
In the “Infertility Treatment Options: Part II: High Tech” article, in vitro fertilization (IVF) as a form of treatment for infertility patients was explored in detail. One of the many advantages of this procedure is that it enables reproductive endocrinologists and other fertility specialists to look at the chromosomal make-up of an embryo or fertilized egg. The use of preimplantation genetic diagnosis (PGD) in these patients can also help identify specific inheritable genetic abnormalities. In this manner, the results are available prior to the embryos being transferred or implanted into the uterus, thereby allowing for the preferred transfer of normal or unaffected embryos. Prior to the advent of PGD, testing was only available after pregnancy had been achieved through chorionic villus sampling (removal, and testing of a small piece of tissue from the placenta) and amniocentesis.
An Introduction To Preimplantation Genetic Diagnosis
In this procedure, the female patient still undergoes ovarian stimulation followed by harvesting and fertilization of the eggs. The embryos are then grown in a laboratory environment immersed in a special culture solution. At Fertility Institute of San Diego, we use the latest cutting-edge modalities in an attempt to provide the best service for our patients. The PGD is comprised of 2 steps:
- The first step is the embryo biopsy. We usually perform the embryo biopsy at the blastocyst stage of development (day 5 embryo). A laser is used to make an opening in the shell of the embryo and a trophectoderm biopsy is performed in which a few placental cells are removed. This is a very delicate procedure that could damage the embryo.
- The second step is the genetic testing. The cells that are removed are sent to our reference cytogenetic lab. The tests typically performed on the biopsied cells are either
- Polymerase Chain Reaction (PCR) in cases where a single gene is being assessed, or
- Comparative Genomic Hybridization (CGH) in which the complete chromosomal composition of the embryo, all 46 chromosomes, are assessed. This is the most comprehensive way of assessing an embryo for aneuploidy.
As it normally takes up to 48 hours to receive these results, after the embryo biopsy is performed, the embryo is immediately frozen at the blastocyst stage. The normal embryos are transferred back in a subsequent frozen embryo transfer.
undergoes the genetic testing that is indicated.
Inheritable Conditions Diagnosed Using PGD
Preimplantation genetic screening has been used in patients who have a higher risk of having embryos that are genetically abnormal, such as in women with advanced maternal age, those with repeated miscarriages and repeated IVF failure. PGD is also indicated for couples at risk of transmitting sex-linked disorders, single gene defects, or chromosomal disorders to their offspring, such as
- Rett syndrome
- Incontinetia pigmenti
- Vitamin D- resistant rickets
- Cystic fibrosis
- Tay-Sachs disease
- Sicke cell anemia
- Huntington disease
- Down syndrome
- BRCA1 and 2 mutation associated with cancer risk
- Spinal muscular dystrophy
- Fanconi anemia
- Gaucher disease
- Fragile x syndrome
- Duchenne muscular dystrophy
- Von Hippel-Lindau disease
- Marfan syndrome
- Osteogenesis imperfecta
- Retinitis pigmentosa
- Lesh-Nyhan syndrome
Although PGD is not without certain pitfalls, it has completely revolutionized the field of reproductive medicine – especially in cases of genetic diseases. Since its inception in 1988, researchers have sought to improve and expand on the procedure. The future of PGD may include the identification of genetic links to more common diseases like diabetes, endometriosis, hypertension, cardiovascular disease and more – thus, exerting control over the transmission of these diseases to the offspring, and potentially eliminating their genetic transmission within an entire family.
The Future Of Genetic Disease Diagnosis
Newer techniques, such as comparative genomic hybridization, have dramatically increased the accuracy of results. The introduction of DNA chip technology allows for even more genes to be screened. This technique suggests a way for fertility specialists to increase the number of diseases that can be diagnosed, and the number of would-be parents that can benefit from PGD.
Happy Doctor’s Day to a fantastic Doctor!
We are so lucky to have met you and we will be forever grateful for your help!