PGS (pre-implantation genetic screening) involves a biopsy of the Day 5-7 embryos for genetic analysis focused on detecting the number of chromosomes in each embryo. Aneuploidy or presence of abnormal number of chromosomes is frequent in human embryos and usually results in IVF failure. Only a small minority of aneuploid embryos can lead to a live birth with Down syndrome being the most common. The major goal of PGS is to determine which embryos have normal number of chromosomes and are likely to lead to the birth of a healthy baby. PGS allows us to select which embryo to transfer but it does not correct any chromosomal abnormalities and it does not increase the number of normal embryos available for transfer. It also does not detect specific genetic diseases, such as hemophilia (see PGD below), nor does it prevent congenital birth defects such as cleft palate or heart malformation.
The clinical value of PGS depends on the number of embryos available to test: its power to select better embryos increases when a large number of embryos are available for testing. Abnormalities in chromosome number (aneuploidy) increase with advanced maternal age (What is Age Factor?) so older women are the primary candidates for PGS. However, the number of eggs and embryos decreases with advanced age and some patients first accumulate embryos from more than one retrieval before PGS. Abnormal number of chromosomes is also the most common cause of early miscarriages so women with recurrent pregnancy losses also may benefit from this technique.
In testing for aneuploidy the gender of the embryos is also determined. While some couples undergo IVF with PGS for the purpose of gender selection, other patients prefer not to receive this information.
Initially PGS involved biopsy of Day 3 embryos but more recently the biopsy is performed at the blastocyst stage with removal of several cells from the part of the embryo destined to become the placenta (trophectoderm). Blastocyst biopsy appears not to reduce the implantation potential of a well-developed embryo but it does usually require embryo cryopreservation to allow time for genetic testing.
In summary, recent refinements in PGS with blastocyst stage biopsy and micro-array genetic testing of all chromosomes has the potential to improve embryo implantation rates per transfer, to detect embryo gender and to decrease pregnancy losses. PGS adds about $5,000 to the IVF cost.
Blastocyst stage biopsy
PGD (pre-implantation genetic diagnosis) utilizes embryo biopsy for genetic analysis focused on identifying which embryos carry a specific defective gene present in the genetic parents in order to minimize transmission of a serious genetic illness. PGD is usually combined with chromosomal analysis described above as PGS.
Genetic Disorders fall into several distinct classes: autosomal recessive and dominant, X-linked, chromosomal and polygenic. Autosomal recessive conditions require that both genetic parents carry a defective gene and pass it on to the affected child. Carriers are generally healthy so most people do not know their carrier status. If both partners are carriers, about 1 in 4 children will have the disease and half of the children will be carriers. If one partner is a carrier but the other one is not, half of the children will be carriers but none should have the disease. Following are the most common autosomal recessive disorders:
Cystic Fibrosis (CF) is the most common autosomal recessive condition with carrier rate of about 1 in 30. Without any testing, the chance of having an affected child is approximately 1 in 3,600. Only people who have 2 defective genes for CF, one from each genetic parent, are affected and have the actual disease. In order to detect whether an embryo has 2 mutations, it is necessary to know exactly which mutation present in each parent. To date over 900 deleterious mutations for CF have been described so extensive testing is required. While PGD is quite accurate (~95%), it is not as reliable as prenatal diagnosis which tests a much larger number of cells. Therefore, prenatal genetic diagnosis is still performed in pregnancies achieved after PGD in at risk couples.
Tay Sachsis relatively common (1 in 30) in Ashkenazi Jewish, French Canadian and Cajun populations. The enzyme test for Hexosaminidase A has high sensitivity and is widely available. Several other diseases are also common in Ashkenazi Jews and can be tested for.
Autosomal dominant conditions include Huntington’s chorea.
X-linked diseases include hemophilia which is passed on through females but affects only males. Family history, rather than carrier screening, are the basis for early detection and prevention of the autosomal dominant and X-linked conditions.
Fragile X is the most common non-chromosomal cause of mental retardation (Down syndrome is the most common cause of mental retardation) for which detection is available.
Overall, the individual genetic diseases with classic Mendelian inheritance patterns are uncommon. The common diseases with a familial component, such as diabetes and heart disease, involve inheritance of multiple genes as well as complex interaction with the environment and thus cannot be currently detected by PGD.
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