PGT-A Genetic Screening

In previous lessons, we’ve focused on a specific type of preimplantation genetic testing on the embryo, known as PGT-A or preimplantation genetic testing for aneuploidy.

As we discussed in Lesson 1, this type of testing seeks to determine if the embryo has the correct number of chromosomes only. Any IVF patient receiving care in a region of the world where PGT-A is legal and available may consider this type of genetic testing of their embryo(s).

The focus of this lesson will be on two specific types of genetic testing, known as preimplantation genetic testing for monogenic disorders (PGT-M) and preimplantation genetic testing for structural rearrangements (PGT-SR.)

These tests differ from PGT-A in that they are only necessary in a situation where there is a known risk of passing on a genetic mutation.

This lesson focuses on who this test is appropriate for, what the test entails, and other considerations to keep in mind.

PGT-M is a test to determine if an embryo is affected by or a carrier of a monogenic disorder. A monogenic disorder results from a mutation on one set of genes. The disorders commonly screened for by PGT-M are often severe, can be life-threatening, or may lead to pregnancy loss. Using PGT-M allows patients to screen embryos for disease and only transfer embryos that are not affected.

PGT-SR is similar in that it looks for a specific genetic mutation, but in this case the mutation is known as a structural rearrangement. Structural rearrangements, including translocations and inversions, can lead to infertility, increased risk of miscarriage, and severe disabilities in offspring. Like PGT-M, PGT-SR allows patients to screen embryos and only transfer those that do not have the mutation.

Understanding Monogenic Disorders

Monogenic disorders are diseases caused by mutations in a single gene. These genetic disorders are inherited, meaning they can be passed down through the DNA in the egg and sperm used during conception.Examples of monogenic disorders include cystic fibrosis and sickle cell anemia. These conditions often lead to severe or life-limiting outcomes for those affected.

Most of the monogenic disorders discussed are known as autosomal recessive disorders. With a recessive disorder, a person needs to inherit two copies of the genetic mutation in order to also inherit the disease. This means both the egg and sperm source need to carry the genetic mutation to be at risk of passing on the condition.

In the case of autosomal dominant disorder, a person only needs to inherit one copy of the gene mutation to be affected by the disease, which means only the sperm or the egg source need to carry the disease to be at risk for passing on the condition. Typically, in autosomal dominant disorders, the carrier parent is also affected. Another type of monogenic mutation is an x-linked disorder. With an x-linked disorder, only the egg source needs to be a carrier in order to pass the disorder to offspring. X-linked disorders are most frequently passed from genetic mothers who carry the disorder to male offspring.

Important Terminology: Impact of Gene Mutations on Individuals

A person may be affected by the gene mutation, not affected by the gene mutation, or a carrier of the gene mutation.

In an autosomal recessive disorder, an affected person has two copies of the mutated gene (one from the egg source, and one from the sperm source) and typically exhibits symptoms of the disorder. In an autosomal dominant disorder, an affected person may only have one copy of the mutated gene and typically exhibits symptoms of the disorder.

A carrier of the disorder has one copy of the mutated gene, but does not show signs of having the disorder themselves. Although these individuals may be healthy, there is risk of passing on the disorder to offspring when both the sperm and egg source carry the disorder. Typically, carriers do not exhibit any symptoms of the disorder, but in some conditions they may. Because of this, it’s important to have a conversation with the genetic counselor before utilizing PGT-M.

A nonaffected person does not have a copy of the mutated gene, does not have the disease, and cannot pass on this specific gene mutation.

Risk of Passing on a Monogenic Disorder

If both genetic contributors (egg & sperm) are carriers of the same recessive monogenic disorder, there is a 25% chance with each pregnancy that the child will inherit two copies of the mutated gene and therefore would be affected by the disorder.

There is also a 50% chance the child will only inherit one of the affected genes and be a carrier of the disorder. And there is a 25% chance that the child will inherit two normal genes and not be affected or be a carrier.

In the case of a dominant monogenic disorder, the offspring only needs to inherit one copy of the mutated gene in order to be affected. In this case there is a 50% chance the child will receive the mutated gene and be affected by the disorder.

In the case of an X-linked recessive disorder, there is a 50% of any male offspring being affected by the mutation and a 50% chance of female offspring being carriers of the mutation, and therefore being at risk of passing on the mutation to their own offspring in the future.

Carrier Screening

In order to know if PGT-M is right for you, you need to know if you are at risk for passing on a genetic disorder to your offspring. This involves testing the genetic parents (or whomever the egg source or sperm source may be).

Different clinics vary in their approach to genetic testing. Some will opt for a more narrow approach, and only test for a limited number of disorders that are most common. Other clinics may recommend limited testing based on familial risk factors or ethnic background.

Some clinics use a sequential approach, where just one partner will be tested first. If the results show that the tested person does carry a monogenic disorder then the other partner would be tested to see if they are also a carrier for the same disorder. While this approach can cost less because only one partner is required to be tested initially, it can also delay time to treatment if results indicate that the second partner should be tested as well. It can also be risky to attempt pregnancy while waiting for genetic results for the second partner.

Another downside of a more narrow approach is that if the genetic parents are only tested for the disorders they are at highest risk for, there is a chance that a more rare, but equally serious disorder could be missed. For this reason, some clinics take a broader approach offering expanded carrier testing for a range of disorders to both partners from the beginning of treatment.

For those using third party-reproduction, it’s important that egg and sperm donors be tested in this process as well. For those using an egg or sperm bank, there is typically an option to search for donors that do not carry a mutation. This search option ensures there isn’t genetic mutation in common with the other genetic parent.

In some cases, patients may only be interested in being tested for specific disorders for which there is a known family history. However, expanded carrier screening in which patients are tested for a large array of disorders is becoming more standard.

It’s highly recommended to speak to a genetic counselor before undergoing these tests and to consult with one if any results come back positive.

Choosing to Use PGT-M

Patients who are aware that they are at risk for passing on a genetic disorder may consider IVF with PGT-M to screen the embryos. Unlike carrier screening, which involves testing the person contributing the sperm or egg, PGT-M screens the embryo itself.

In some cases, patients may opt to undergo IVF with PGT-M even if they don’t have fertility concerns in order to avoid passing on a disorder to their offspring.

The Process of PGT-M

The process of PGT-M is very similar to what has been described in previous chapters on PGT-A.

Some cases require “test development” in which the egg source, sperm source, and sometimes additional blood relatives submit DNA samples to the specialty lab in order to clearly identify the gene mutation and create a test specifically designed for that exact mutation.

The patient will then undergo IVF and the embryos will be grown in the lab. Most labs require intracytoplasmic sperm injection (known as ICSI) when PGT-M is performed. From there a small biopsy of the embryo’s trophectoderm is obtained. This process mirrors the one described in Lesson 2. The biopsy sample is then sent to the specialty lab for testing.

After the biopsy, the embryos will be frozen while awaiting test results. The embryos deemed appropriate for transfer can later be thawed and transferred into a uterus in hopes of establishing an ongoing pregnancy.

Understanding PGT-M Results

It’s absolutely critical to discuss the results of the PGT-M test with a genetic counselor.

In most cases the results will come back as either:

• Affected: the embryo carries two copies of the mutated gene in a recessive disorder or one copy in a dominant disorder and resulting offspring would likely be symptomatic of the disorder.

• Carrier: the embryo has one copy of the mutated gene of a recessive disorder, is not at risk for developing the disease but may be at risk for passing on the disease to offspring.

• Non-Affected: the embryo does not have any mutated copies of the gene in question, is not at risk for having the disorder or for passing the disorder to offspring.

It’s important to note that some PGT labs use different terminology to describe these results so it’s always best to ask your clinic for clarification.

Understanding Chromosomal Structural Rearrangements

Chromosomal structural rearrangements are abnormalities in the structure of a gene. These abnormalities can result in losing essential DNA fragments, duplication of DNA fragments or the general rearrangement of DNA fragments.

Some structural rearrangements have minimal effects on the carrier, while others can result in an increased risk of miscarriage, developmental disorders and other significant health complications.

Important Terminology: Classification of Chromosomal Structural Rearrangements

Chromosomal structural rearrangements can be classified into several different categories.

The health impact of the different types of rearrangements can vary greatly.

In a balanced structural rearrangement, the placement of the genetic material on the chromosome is altered, but there is no gain or loss of DNA. In other words,, all the genetic material is there, it’s just arranged differently. Carriers of balanced translocations typically do not experience health problems related to the rearrangement, however they could experience decreased fertility, male factor infertility, and recurrent pregnancy loss.

Unbalanced structural rearrangements differ in that genetic material may be missing or unnecessarily duplicated. Unbalanced rearrangements can lead to health concerns including developmental delays, birth defects, fertility issues and recurrent pregnancy loss.

Understanding Your Structural Rearrangement Status

The effects of having a structural rearrangement vary greatly, and in some cases, people who have balanced rearrangements are perfectly healthy. In fact, many people are unaware that they have a structural rearrangement until they find themselves having difficulty conceiving or experiencing multiple pregnancy losses.

Unlike carrier testing for monogenic disorders (covered above), testing for structural abnormalities is not routinely offered for all patients seeking pregnancy through IVF. Instead, providers often don’t recommend the test unless there is reason to suspect an abnormality, such as a family history of genetic disorders, giving birth to another child with a genetic disorder, suffering multiple miscarriages, and sometimes in cases of a very low sperm count.

The testing for structural rearrangements is called karyotyping. Karyotyping looks at the DNA of the egg source and sperm source for any deletions, duplications, inversions, or translocations. A karyotype involves culturing cells and visualizing the actual chromosomes. If an inversion or duplication is suspected, a chromosomal microarray may be requested as the rearrangement may be below the size detectable with karyotype.

If a structural rearrangement is identified, then PGT-SR can be used to screen embryos with the hopes of selecting an embryo that does not carry the rearrangement, as an unaffected embryo would be less likely to result in a miscarriage or offspring with a genetic disorder than an affected embryo.

It’s important to note that not all identified structural rearrangements put you at risk for reproductive complications, and PGT-SR may not be necessary in some cases. It’s also critical to note that not all labs that provide PGT-SR are able to screen for all types of structural rearrangements. For this reason, it will be imperative to have a discussion with your clinic about your own results to help you determine if PGT-SR, and the lab that is utilized for testing, is right for you.

The Process of PGT-SR

In many ways, the process of PGT-SR mirrors the process of both PGT-A and PGT-M. Embryos are grown in the lab to the blastocyst stage, a small biopsy of the trophectoderm layer is removed, and the cells are sent to a specialty lab for testing.

In most cases, PGT-SR does not require test development ahead of time (discussed above), and in that way, is a less involved process than PGT-M.