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PGD (Preimplantation Genetic Diagnosis)


PGD (Preimplantation Genetic Diagnosis) technology improves the likelihood of a successful pregnancy and birth for two distinctly different groups of patients. Couples with infertility related to recurrent miscarriage or unsuccessful IVF cycles and couples who are at risk for passing on inherited genetic disease to their offspring.

Following ovarian stimulation , egg collection and fertilization, embryos are cultured for another 2 days which they usually consist of 6-8 cells. Each of these cells has complete genetic information and also each cell has the potential to continue growth to establish pregnancy. Therefore, one or two cells can be removed from an 8-cells embryo by using an embryo biopsy procedure and the embryo will continue to develop normally. The removed cells will then be analyzed by using a technique called Fluorescent in situ Hybridization ( FISH ) or Polymerase Chain Reachion ( PCR ). The FISH technique can tell us whether an embryo cell has two X chromosomes ( female ) or one X and one Y chromosome ( male ) and FISH can also be used to detect specific chromosome problems such as Down syndrome. FISH analysis can give us the results within one day and the resulting normal embryos will be transferred back into the uterus.

This technique is now suitable for specific couples such as advanced maternal age who have a very high risk to have a child with Down syndrome , carriers of genetic disease , recurrent abortion and couples who carry X-linked diseases. Haemophilia and Muscular dystrophy are examples of X-linked diseases.

In the future it is likely that genetic testing of embryos will be used more routinely to improve IVF success rates as well as to prevent transmission of genetic disease. With the transfer of genetically normal embryos, a higher percentage of implantation and reduced miscarriage rates can be expected. This sophisticated and technologically advanced testing identifies which embryos are free of abnormalities and more able to achieve the patient's goal of a healthy baby.

PGD is recommended for families with a history of a specific genetic disease. Using polymerase chain reaction, fluorescent PCR and DNA sequencing, the scientists in our PGD laboratory can examine each developing embryo to identify the absence or presence of these specific genetic disorders. As a result, only those embryos free of genetic disease will be transferred to the patientís uterus so as to increase the chance of conception and ultimately a healthy baby.

Single gene disorders are categorized depending upon whether the gene is located on the X chromosome, an autosome or whether the gene is dominant or recessive. These classifications include autosomal recessive, autosomal dominant and X-linked.

For a dominant disorder, one only needs to have the abnormal DNA sequence on one chromosome. If that mutation is passed on to the embryo, the embryo will be affected with that genetic disease. One example of an autosomal dominant disorder is Myotonic dystrophy. Recessive Disorders

Recessive disorders require that the mutation be present on both chromosomes of the chromosome pair. If one only has the mutation on one chromosome, the individual is normal but carries the mutation in his cells and is called a carrier. The fertilization of an egg from carrier parents may result in an embryo having the mutation on both chromosomes of the chromosome pair and the embryo therefore being affected with that genetic disease. For example, Cystic fibrosis (CF) is a common autosomal recessive genetic disorder that primarily affects the lungs of CF patients. The CF mutation affects a protein within the cell that reduces the cell's ability to function properly. This results in a build up of mucous within the lungs, lung dysfunction and possible death.

X-linked disorders are due to mutations of genes on the X chromosome and have different patterns of inheritance due to their transmission on a sex chromosome and whether the embryo is male or female. Examples of X-linked diseases are the Fragile X syndrome and Duchenne muscular dystrophy.
 

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