Pre-IVF Genetic testing
About Whole Exome Sequencing (WES)
The exome is a subset of the Human genome (about 1.5% of our whole genome) that contains many of the most important DNA sequences: the portions of genes that encode proteins.
Most genes do not encode proteins using a single continuous stretch of DNA. Typically, a gene will encode a protein using multiple distinct stretches of DNA; the information in these separated stretches of DNA is then pieced together to direct production of a protein. The stretches of the gene that are used to direct protein production are called “exons”. Exons are also referred to as the coding region of a gene because they encode the information for a protein.
The stretches of genes that do not encode proteins are called “introns” and are one kind of noncoding DNA sequence. The exons of all our genes make up approximately 1.5% of our genome and are collectively referred to as the “exome”. Whole exome sequencing involves determination of the DNA sequence of most of these protein-encoding exons and may include some DNA regions that encode RNA molecules that are not involved in protein synthesis. Whole exome sequencing offers lower cost analysis than whole genome sequencing
WES should be considered in the clinical diagnostic assessment of a phenotypically affected individual when: (Source American College of Medical Genetics and Genomics (ACMG)
- The phenotype or family history data strongly implicate a genetic etiology, but the phenotype does not correspond with a specific disorder for which a genetic test targeting a specific gene is available on a clinical basis.
- A patient presents with a defined genetic disorder that demonstrates a high degree of genetic heterogeneity, making WES analysis of multiple genes simultaneously a more practical approach.
- A patient presents with a likely genetic disorder but specific genetic tests available for that phenotype have failed to arrive at a diagnosis.
Pre-Implantation Genetic Diagnosis (PGD) is a diagnostic test for preventing the inheritance of single gene disorders. This test is carried out by testing the genotype of embryos of high risk candidate couples during the process of In Vitro Fertilization (IVF), which provides a means for reducing the risk of a couple passing on a specific inherited single gene disorder to their child before an actual pregnancy is established. Thus, maximize the chance of a couple having a successful pregnancy and a healthy child.
It is a highly sophisticated scientific technique were embryos generated through the process of IVF and then one or more cells from the embryo are screened for a specific genetic condition prior to the transfer of the embryo into a woman’s uterus. The screening gives information regarding the specific genetic status of the embryo, enabling selection of unaffected embryos prior to implantation and before pregnancy is established.
Preimplantation genetic diagnosis (PGD) increases the options available for couples who have a known genetically transmittable disease, providing reassurance and a reduced anxiety associated with reproduction. PGD for single gene disorders allows patients to gain information about their embryos prior to pregnancy and thus choose not to be impregnated with an affected embryo. Thus, PGD is considered as a safe, accurate and very early form of prenatal diagnosis for various inherited single gene disorders.
A PGD test can be designed for virtually any genetic disorder whether autosomal recessive disorders, autosomal dominant disorders or X-linked disorders as long as the specific gene fault in the family is known. Examples of most prevalently found single gene disorders in UAE for which PGD is commonly performed are Thalassemia, Cystic Fibrosis (CF), Fragile X Syndrome and Muscular Dystrophy (MD).
PGD can be used to choose embryos that have the capacity to form a healthy baby, so that only these embryos are transferred to the uterus.
When a couple have a known risk (typically 25% or 50%) of having a child with a serious hereditary genetic disorder. Either of the would-be parents are carriers of a known genetic condition or diagnosed with a genetic diseased state. If the couple have previously had an affected child or a family history of a specific genetic disease. Have had several spontaneous pregnancy losses due to a chromosome abnormality or high maternity age.
Prior to the IVF cycle, couples interested in PGD begins with genetic testing for the identification of specific hereditary genetic disorder referred as the Pre-PGD treatment.
Once the couple’s genetic condition and relevant counselling is established, then the IVF cycle is started. In Vitro Fertilization (IVF) techniques are used to create embryos. An embryo is formed when the sperm fertilizes the egg. The IVF techniques used for PGD are like those that are used for the treatment of infertility, although many couples who use PGD are not infertile.
Embryos created using IVF are grown in the laboratory for either three or five to six days. During this time, PGD is performed on embryos that grows into a small ball of with 8-10 cells size.
Each embryo is then biopsied or tested; a process that involves the removal of one or more embryonic cells from each embryo on which genetic testing is performed and the embryo is kept frozen until the completion of genetic analysis results based on the disease are released. Genetic analysis is mainly carried out by Sanger Sequencing for detection of mutation which is causative for the specified genetic disorder.
As all cells in an embryo at this stage of development are usually genetically identical, testing one or a few embryonic cells provides information about the genetic makeup of the rest of the embryo.
Once the results of genetic testing are available, the embryo not susceptible to develop the hereditary genetic condition in question is implanted into the woman’s uterus for healthy pregnancy.
About Preimplantation Genetic Screening
In Vitro Fertilization (IVF) is an assisted reproductive technology (ART) commonly referred to as IVF. IVF is the process of fertilization by extracting eggs, retrieving a sperm sample, and then manually combining an egg and sperm in a laboratory dish. The embryo(s) is then transferred to the uterus.
IVF can be used to treat infertility in the following patients:
Blocked or damaged fallopian tubes
Male factor infertility including decreased sperm count or sperm motility
Women with ovulation disorders, premature ovarian failure, uterine fibroids
Women who have had their fallopian tubes removed
Individuals with a genetic disorder
There are five basic steps in the IVF and embryo transfer process:
Step 1: Fertility medications are prescribed to stimulate egg production. Multiple eggs are desired because some eggs will not develop or fertilize after retrieval. A transvaginal ultrasound is used to examine the ovaries, and blood test samples are taken to check hormone levels.
Step 2: Eggs are retrieved through a minor surgical procedure that uses ultrasound imaging to guide a hollow needle through the pelvic cavity to remove the eggs.
Step 3: The male is asked to produce a sample of sperm, which is prepared for combining with the eggs.
Step 4: In a process called insemination, the sperm and eggs are mixed together and stored in a laboratory dish to encourage fertilization. In some cases where there is a lower probability of fertilization, intracytoplasmic sperm injection (ICSI) may be used. Through this procedure, a single sperm is injected directly into the egg to achieve fertilization. The eggs are monitored to confirm that fertilization and cell division are taking place. Once this occurs, the fertilized eggs are considered embryos.
Step 5: The embryos are usually transferred into the woman’s uterus via a catheter or small tube three to five days following egg retrieval and fertilization. If the procedure is successful, implantation typically occurs around six to ten days following egg retrieval.
Approximately three out of four (75%) embryos created by IVF will not be capable of producing a live born child. Some will fail to implant in the uterus, while others will implant but be unable to carry out early embryonic development. Finally, as in natural pregnancy, approximately 15%-20% of conceptions will be lost as a clinical miscarriage.
While there are many reasons for the failure of an embryo to make a baby, the single most crucial factor is an abnormality of the chromosomes. Similarly, for most couples, a considerable number of the embryos created by IVF will have chromosome abnormalities.
The exact percentage of chromosomally abnormal embryos that each couple produces is related to many factors including maternal age, number of failed IVF cycles, and the type of sperm used.
After embryos are created in the laboratory, they are grown for three days. On day three, the PGD biopsy is done, and one or two cells are removed from the embryo which at this point is referred to as a blastomere.
To obtain results of the biopsy, the one or two cells removed must contain a nucleus, as the nucleus contains the genetic information necessary for testing.