In vitro fertilization genetic screening, pre-implantation genetic diagnosis, is a test used in conjunction with in vitro fertilization (IVF) and allows embryos to be tested for chromosomal characteristics and genetic diseases that are passed down through families. In the past, couples with genetic health problems could choose not to have children or to have prenatal testing with amniocentesis at approximately 16-18 weeks of pregnancy or to have chorionic villus sampling at approximately 11-12 weeks to see if the baby was affected. Today, healthy embryos can be identified with genetic screening before the embryo is transferred to the mother's uterus.
Does genetic screening test in IVF diagnose every disease?
Not all diseases can be tested for this way. Single-gene disorders occur through the inheritance of a faulty gene. These disorders are classified as either recessive or dominant. A recessive disease requires two bad copies of the gene to pass the disease on to the baby. With a dominant single-gene disorder, only one copy of the faulty gene is needed to cause the disease.
There are now over 1,000 single-gene disorders that have been identified. Many of these disorders are very rare. However, some are so common in ethnic groups that routine screening is recommended to check if someone is a carrier before becoming pregnant. This is often called carrier genetic testing (or screening).
The most common single gene disorders found in genetic screening in IVF are:
Cystic fibrosis,
Tay-Sachs disease,
Spinal muscular atrophy (SMA),
Hemophilia,
Sickle cell disease,
Thalassemia
However, there are hundreds of genetic diseases that can be tested for with single genes using in vitro fertilization and PGD. A partial list of relatively common single gene diseases is provided below.
Autosomal recessive disorders
Sanhoff disease,
Gaucher disease,
Adenosine Deaminase deficiency,
Fanconi anemia,
Adrenal hyperplasia,
Phenylketonuria (PKU).
Autosomal dominant disorders,
Neurofibromatosis.
Others: Von-Hippel Lindau, myotonic dystrophy, Huntington's disease, Marfan syndrome, early-onset Alzheimer's, polycystic kidney disease, retinitis pigmentosa, familial adenomatous polyposis, achondroplasia, X-linked disorders
How is genetic screening done in IVF?
First, in vitro fertilization treatment, the stages of in vitro fertilization, such as ovarian development, are completed. The egg and sperm cell are combined. Then, a micro sample is taken from the embryo that has formed. This sample is biopsied. The biopsied cells are then sent to the genetics laboratory for evaluation. The genetics laboratory determines that they contain “abnormal” DNA. The test is performed using high-tech genetic technology that analyzes the DNA sequence in the cells for the specific gene in question.
PGD genetic testing for cystic fibrosis
A normal gene combined with an abnormal gene does not cause the disease. That person will simply be a carrier and will not be affected by cystic fibrosis. If both parents are carriers of this recessive genetic condition, the risk of cystic fibrosis disease in each child they have is 4 in 1. Therefore, sometimes a couple who are known carriers of the abnormal cystic fibrosis gene will decide to have in vitro fertilization of the embryos with PGD genetic testing for cystic fibrosis. In such a case, we would expect one of the 4 embryos to have both genes abnormal and a child with cystic fibrosis disease. These abnormal embryos cannot be returned to the mother's uterus. We would expect 2 of the 4 embryos to be carriers for the abnormal gene (one normal copy and one abnormal). We would expect them to have 2 copies of the normal gene (not carriers). The embryos will be selected for transfer to the uterus that do not have 2 copies of the abnormal cystic fibrosis gene, which is shown by genetic testing. Blastomere biopsy on day 3 post fertilization is one way to test for PGD. With blastomere biopsy, a slit is made in the embryo shell on day 3 and a pipette is introduced into the embryo. Then one of the cells is removed from the outside of the embryo, which will be about 8 cells at that time.
If it's an 8-cell embryo and we take one cell out for testing, it's a 7-cell embryo. At this stage of development, not all cells are differentiated. So losing one cell doesn't take anything away from the next child. The cells are all clones of each other at this point and will continue to divide and eventually differentiate into specific cell types.
The biggest problem with day 3 blastomere biopsy is that it has been shown to cause significantly reduced implantation potential after the embryo is transferred to the uterus. Genetic screening in IVF is recommended, especially for couples who are 35 years of age or older, have had recurrent miscarriages, or have genetic diseases.
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