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MUTATION - October 2005 |
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| PRE-IMPLANTATION GENETIC DIAGNOSIS |
| Dhaval Gorasia (5th sem) Gunjan Badrakiya (5th sem) |
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| Introduction |
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| With recent advances in genetics, there are several inherited disorders which can now be diagnosed at a molecular level. For couples who are carriers or affected by any of these conditions and are at high risk of transmitting it to their offspring, it is currently possible to detect the disorder during pregnancy. This is done by one of the two approaches: amniocentesis or chorionic villus sampling. However the couples have the dilemma of whether or not to terminate the pregnancy if the genetic abnormality is present. In some cases this may also not be a viable option for religious or moral reasons. An alternative would then be to diagnose the condition in embryos before the pregnancy is established. Only the unaffected embryos would then be transferred to the uterus. This technique is referred to as pre-implantation genetic diagnosis (PGD) and would obviate the need for screening during a pregnancy and hence prevent the physical and psychological trauma associated with possible termination. |
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| How can a diagnosis be made in the preimplantation period? |
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| In vitro fertilization (IVF) techniques are used to obtain ova (eggs) from the mother, which are then fertilized in the laboratory with sperm obtained from the father & embryos are generated in vitro. 1-2 embryonic cells (blastomeres) are biopsied from the 6-10 cell stage embryo. The biopsied blastomeres can then be used for the diagnosis. The Polymerase Chain Reaction (PCR) is used for the diagnosis of single gene defects or triplet repeat disorder, and Fluoroscent In Situ Hybridisation (FISH) is used to analyse chromosomes. Embryos unaffected by the disease examined are then transferred in the patient’s uterus and, hopefully, the pregnancy is started knowing that the fetus is free from the disease. |
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| What are the different conditions that can be screened? |
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| The conditions screened for include: |
- Cystic fibrosis
- Tay Sachs disease
- Hemophilia
- Fragile X syndrome
- Barth’s syndrome
- Rett’s syndrome
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| Why does the technique involve IVF? |
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| Currently IVF is the only available method for obtaining an embryo in the very early stages of development. Therefore, although couples with a high risk of transmitting a genetic defect to their offspring may have normal fertility, they would need to go through the IVF procedure to provide embryos for screening. |
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| What Next? |
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| The majority of research into PGD is to develop more sophisticated diagnostic procedures. In the future, the use of DNA chips or arrays may enable multiple genes to be analyzed from a single cell. |
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| What about ethics? |
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| Internationally, there is concern that PGD will develop into diagnosis of characteristics and non-disease genes. PGD for sexing for social reasons is already offered in Jordan and Australia. |
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| Conclusion: |
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| PGD is technically a very challenging procedure and couples have to undergo IVF procedures, even though they may be fertile. However, the procedure is a useful option for some couples at risk of transmitting an inherited disease to their offspring, as an alternative to pre natal diagnosis. |
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For further information please mail on maulin_007@yahoo.com. |
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