What is Genetic Testing?
Genetic testing is a type of medical test that identifies changes in chromosomes, genes or proteins.
…some cells must be removed from the embryos through a procedure called an ‘embryo biopsy’.
We took some time to ask Priyanka, a Scientist working in the largest Genetics Center in the Middle East, to talk us through Genetic Testing. Here is what she has to say:
Why do Genetic Testing?
The role of genetics and the environment in the onset of major genetic variations and diseases is well established. Consequently, genetic testing is gaining recognition for the many advantages it has to offer in the prevention, management and treatment of a genetic condition. Genetic testing is a type of medical test that identifies changes in chromosomes, genes or proteins.
“The results of a genetic test can confirm or rule out a suspected genetic condition or help determining a person’s chance to develop or pass on a genetic disorder. ”
Among their many uses, Preimplantation Genetic testing and Prenatal testing have become very common in the field of In Vitro Fertilization. They allow us to screen IVF embryos, fetus or even a baby.
As you aim to have a healthy baby, you’ll evaluate different screening options to find what is best for you. There are two major Preimplanation screening options:
PGT-A (Preimplantation Genetic Testing for Aneuploidies) and
PGT-M (Preimplantation Genetic Testing for Monogenic disorders).
PGT-A, formerly called Preimplantation Genetic Screening (PGS), determines whether there are chromosomal abnormalities in your embryos, which can impact the chance of successful embryo implantation. Extra or missing chromosomes can lead to miscarriage or a chromosomal syndrome like Down Syndrome. PGT-M analyzes for specific gene mutations that one(or both) parents is known to carry. This is performed to ensure passing on of any inherited genetic condition such as, Cystic fibrosis, Sickle Cell Anemia or hundreds of other conditions.
IVF and Genetic Testing
IVF is mostly used to treat infertility and some genetic problems. During the IVF process, a woman is injected with fertility hormones to stimulate her ovaries so she produces several eggs. When the eggs mature, they are collected using a fine needle and are fertilized in the laboratory with sperm provided by the woman’s partner or a donor. Fertilized eggs (embryos) are grown in an incubator for few days. In order to assess the chromosomes with the IVF embryos, some cells must be removed from the embryos through a procedure called an ‘embryo biopsy’.
Embryo Biopsy: Under a microscope, a laser is used to open the embryo and cells are aspirated out from the embryo to be tested (day 5)
This is when the cells of embryos are sent to the Genetic laboratory for testing. Once the laboratory receives the cells, they undergo a non-PCR based amplification technique called Multiple Displacement Amplification which involves rapid amplification of minute amounts of DNA samples to a reasonable quantity for genomic analysis. For PGT-A, Next Generation Sequencing technology is used to detect all of those abnormalities in the chromosomes. The DNA passes through a protocol for smaller genomes which involves tagmentation of template DNA, PCR(Polymerase Chain Reaction) to add adapters and Cleanup of samples. This is then ready for sequencing and loaded onto Illumina Miseq platform. After 6-7 hours, the results are further analyzed and reported to the Fertility clinics. For PGT-M, the DNA is processed parallelly using Sanger Sequencing technique to rule out if the embryos carry a particular mutation or not. PGT-M is offered to only couples with a possibility of passing on an inherited genetic condition.
Based on these results, one or two normal embryos are transferred into the woman’s uterus using a thin tube inserted into the vagina and cervix. If an embryo successfully implants, the woman becomes pregnant. Any remaining healthy embryos can be frozen and stored for later use, if needed.
Prenatal Testing
Prenatal testing is performed to detect problems with the pregnancy as early as possible. The most important step in the prenatal diagnostic procedures is obtaining fetal material. Amniocentesis (check our full article here) which involve the removal of amniotic fluid surrounding the fetus and Chorionic villus sampling where chorionic villi form placenta is removed, are some of the prenatal tests performed to look for certain types of birth defects, such as Down Syndrome which is a chromosomal abnormality. Product of conception is also analyzed to detect the cause of a miscarriage. DNA is extracted from the samples and undergoes a non-PCR method for further amplification and goes through the process similar to the embryo screening. Using the Next Generation Sequencing technology, the chromosomal analysis is done. If the result is a Normal Female, STR (Short Tandem Repeats) analysis is performed to rule out any contamination from the mother.
Targeted Mutation Screening is also a popular genetic testing which is performed to understand the specific mutation in a gene or genes causing a genetic disease in a family by using Sanger sequencing technology. This testing is also a tool for couples to identify if they are carriers of a heredity disease and verify that accurate testing can be offered before PGT-M is considered. For this type of screening, sample types such as peripheral blood, buffy coat and buccal swab are collected and DNA is extracted. DNA, further undergoes a Polymerase chain reaction followed by the Sanger sequencing protocol.
Technology has advanced over the years and various molecular laboratory techniques are updated with time to bring more accurate and faster results. The various genetic screenings are easier because of Whole Genome Sequencing (WGS) that allows to analyze entire genomes. Advancements in next-generation sequencing technology are poised to revolutionize diagnostic testing by the introduction of WGS.WGS is a powerful tool for detecting known and potential disease-causing variations.
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