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Preimplantation genetic diagnosis - an approach to healthy family

Dr. Ana Cervero
Wednesday, July 05, 2017, 08:00 Hrs  [IST]

Many children across the globe are affected with disorders that are caused by mutations or certain modifications in their genes. In March 2006, combined research carried out by the March of Dimes Birth Defect Foundation and WHO reported the birth defect pervasiveness in India i. e. 61 to 69.9/1000 as live births. Rao and Ghosh (2005) state that 1 out of 20 newborns admitted to the hospital carries a genetic disease that eventually account for nearly 1 out of 10 infant mortality. In the regions with more consanguineous marriages, congenital abnormalities and genetic disorders are the third most common causes of mortality in newborns. Nevertheless, it is crucial to prevent the birth of a child with genetic disorder thus reducing the risk.

Most of the genetic disorders cannot be cured after the baby is born, but it can surely be prevented before conception. To keep this prevention, safety or contraceptives must be used without any doubt. With the evolution and development in medical sciences, several breakthroughs in technologies have been produced. A major contribution of these advances has been in the areas of prevention of disease. Genetics cuts through all medical subspecialities and has an important role to play in reproductive health. One such prolific advancement in the era of medical technology is Preimplantation Genetic Diagnosis (PGD). The advent of PGD acts as a boon to the medical field in resolving various health issues and unwanted medical upcoming.

PGD has evolved during last two decades as an alternative to pre-natal genetic diagnosis (PND) for those couples at risk of transmitting a genetic condition to their offspring. In the 1968 Edwards and Garner were successful in performing the first PGD on rabbit embryos. Handy Side and colleagues reported the first healthy child born through PGD in London in 1990.  Nowadays, PGD is used to discard affected embryos in carriers of single-gene disorders (SGD) and also to discard embryos having structural chromosome abnormalities in couples having known abnormal karyotype. In addition to this, there is one more variant of PGD, which is applied to the screening of numerical chromosome abnormalities in couples with normal karyotype, but with infertility problems, and it is called preimplantation genetic diagnosis for aneuploidy screening (PGD-AS) or Preimplantation Genetic Screening (PGS).

For patients at risk of having offspring with a type of genetic disorders called single-gene disorders or monogenic disease, either PND or PGD for single-gene disorders (PGD-SGD) can be offered to avoid the birth of affected offspring for the analyzed condition. PGD-SGD is a powerful genetic test that is performed on a small biopsy of embryos obtained by an in vitro fertilization (IVF) treatment and allows in identifying embryos, which are at risk to develop the genetic disorder. It is possible to distinguish between genetically normal or genetically abnormal embryos using PGD-SGD and only genetically normal embryos for the condition analysed will be candidates to be transferred.

PGD is used to diagnose and prevent autosomal recessive conditions (e.g. beta thalassemia or spinal muscular atrophy), autosomal dominant (e.g. Huntington disease or myotonic dystrophy type 1) or X-linked single-gene disorders (e.g. fragile X syndrome or Duchenne/Becker muscular dystrophy). The conditions for which PGD has been applied have been increased yearly and most of the genetically inherited diseases that can be diagnosed in human beings can be also identified in the embryos.

PGD-SGD is a boon for those having single gene disorders - autosomal recessive, autosomal dominant, X-linked disorders. If any one individual in the couple is affected or both are carriers of any single gene disorder then there are high chances of passing the same disorder to the child.

For those patients who have a family history of a late onset genetic disorder, PGD can be an option to avoid passing the disease to their offspring. Being aware of the disorder can help the patients to take precautions before opting parenthood.

Those who have had a previous child with a genetic disorder, if their first child born carries a genetic disorder then parents should take precautionary steps while planning for their second child. Moreover, PGD can be employed to conceive a child who may donate compatible cord blood or haematopoietic stem cells for transplantation to save a previous affected sibling.

Importance of PGD-SGD
PGD-SGD helps in preventing the genetic disorders which can be transmitted to future generations. It ensures a healthy life, as it minimizes the chances of offspring inheriting genetic disorders and it also helps in reducing the expenses which are otherwise caused due to genetic disorders.

What is the need of PGD-SGD?
In developed countries, there is a plethora of patients who prefer PGD-SGD over PND because of the procedure with low risk. The use of PGD-SGD implies costlier and longer treatment than PND, and it also requires the assistance of reproductive techniques. However, PND implies a risk of faetal loss that can vary from 0.5% to 1% and cases in which thefoetus is genetically abnormal, the medical termination of the pregnancy may imply some risk to the mother, both physical and emotional. PGD-SGD needs an initial step of genetic counseling for careful evaluation of the family suffering from the disorder and also, giving best advice with regard to techniques available along with their pros and cons, so that couples can take suitable decisions.

Before patients undergo PGD-SGD, the disorder cause has to be identified.

The goal of PGD testing is to help couples in building healthy families. Top 5 most Common genetic disorders in Indian ethnicity are Beta-Thalassemia, Cystic Fibrosis, Sickle Cell Anaemia, Spinal Muscular Atrophy and Haemophilia A.
1) Beta thalassemia is the most common genetic disorder in India and is 2.04% prevalent in Indian population. It has a carrier frequency of 5–17% with some population groups having even higher carrier rates. It causes weakening and destruction of RBC (Red Blood Cells)

2) 2) Cystic Fibrosis (CF) affects the lungs, pancreas, liver, intestine and affects about 1 in 40,000 to 100,000 live births in India.

3) Sickle Cell Anaemia is a genetic blood disorder that induces red blood cells to grow into a crescent shape, like a sickle, inducing anemia. This causes severe pain and permanent harm to cerebellum, heart, lungs, kidneys and other body organs. Carrier frequencies for sickle cell hemoglobin ranges from 17% to 30% or more in the Indian Population.

Technology used in PGD-SGD
Igenomix uses state-of-the-art technology for PGD.  The usage of multiple detection approach (mainly Polymerase Chain Reaction (PCR) in its diverse methods) ensures the most highly reliable results. PGD testing is 98-99% accurate and can deliver results within 2-8 weeks.

Concerns/cons of PGD-SGD
Along with the benefits/pros of PGD-SGD, we should also be aware of the concerns/cons of the technique and should be well informed to the couples while counseling.

While PGD-SGD helps reduce the chances of conceiving a child with a genetic disorder, it cannot completely eliminate this risk. In some cases, further testing is needed during pregnancy to confirm the findings.


(Author is director, PGD Lab)

 

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