Prenatal Genetic Screening - The Role of Expanded Carrier Screening (ECS)

Expanded carrier screening has become a widely used screening method to help identify the carriers of single-gene disorders. This genetic testing gives couples and individuals that are trying to conceive, the opportunity to screen for any autosomal recessive or X-linked genetic disorders that may pass onto their offspring. Before new genetic testing technologies came about, only select disorders and select ethnic groups were offered carrier screening. This was because many ethnic groups were known to carry certain mutations at higher frequencies than others. Due to technological advancements, couples and individuals who are seeking conception, are now given the ability and options to screen from 5-10 conditions, to as many as several hundred conditions.

Most of the diseases screened for show no symptoms in the carrier individuals. Therefore, preconception screening is highly recommended. The conventional approach to carrier screening is done usually in the prenatal setting. There has been an ongoing evolution and revolution in the ability to identify gene variants; correlate such variants with diseases; and to estimate prognoses for the fetus.  There is great debate regarding prenatal genetic testing among national organizations such as the American College of Obstetricians and Gynecologists, the American college of Medical Genetics and Genomics, and the American Society for Reproductive Medicine. So what’s a person to do and how do we go about it?

First, our suggestion is to get educated. Second, we need to realize that the information and testing strategies are frequently changing. Third, we should understand that genetic testing lowers risk, but cannot eliminate genetic risk. Lastly, understand that genetic testing is ultimately up to you and is optional.

How does one get educated?  Ideally, meet with your OB-GYN prior to pregnancy. If you are pursing fertility treatment, Dr. Moomjy will review whether you have had prenatal genetic testing; what level of testing is currently available; and what are the benefits and limitations of such testing.  The main goal is to reduce the risk of an infant or child with a moderate or severe disease of genetic origin. In order to do this, the approach is testing of the patient, and her partner as needed, for carrier status. We all carry normal genes, and we all carry gene variants.  Some of our gene variants are insignificant; some may be associated with disease. These are called pathological variants, or mutations. If we are found to be a carrier for a certain pathological variant, the next question is whether that disease needs one gene variant or two gene variants in order to result in disease? With one exception, the genetic diseases associated with a moderate or severe negative impact upon infancy and childhood are recessive genetic diseases.  This means that the fetus, or baby, must inherit one abnormal gene from each parent. This also means that the disease often is not present in either family, and can be a shock to a couple with an affected child. Gratefully, these diseases are uncommon or rare, but given their severity, they warrant our attention.   

Most recessive conditions that impact a newborn or child have a similar blueprint for the disease process. They are often labeled inborn errors of metabolism. When a baby has inherited 2 pathological gene variants, it cannot produce a necessary metabolite for growth and development. The related disease can occur due to lack of the functional protein or metabolite, or a buildup of the substrate or building blocks in a manner that is toxic to development.  There are hundreds of rare genetic conditions that can significantly impact development. Many are lethal. Most are so rare, that even an Obstetrician-Gynecologist would have to review the medical details of the condition with the geneticist; and would rely on a genetics counselor to review the results of gene variants with the patients. Gratefully, the chance that a couple would both test positive for the same rare recessive condition is less than 1%. However, for those couples who have that information prior to pregnancy and delivery, the information is priceless. In addition to recessive conditions, it is possible to test for Fragile X, a gene that is largely responsible for mental retardation. This gene is X-linked, which means that males who have one X chromosome are at risk of the severe form of the disease, and females who have two X chromosomes, are at risk of a mild form of the disease,  or may be disease free. 

How does one get tested? Testing strategies have evolved greatly over the past 20 years. Primarily because our understanding of genetics has evolved greatly.  Additionally, technology to more reliably identify relevant gene variants has improved as well. Technological advancements have permitted testing of more gene variants, with greater accuracy, less expense, and more rapid turn around times.  

There are two basic strategies: Basic or Ethnic Based Carrier Screening, or Expanded Carrier Screening.  The first strategy relies on family history and ethnic background to identify a patient who might be at risk of Tay Sachs disease, or Sickle Cell disease, or Thalassemia.  As the population becomes more diverse with each generation, people are more likely to have more than one ethnic background; or may not be aware of their ethnic background.  This Ethnic Based carrier screening focuses on those genetic diseases that have a relatively high frequency in the population. It is a reasonable approach if the patient prefers limited information or if there are very limited resources. The alternative strategy of Expanded Carrier Screening (ECS) is considered to be “pan-ethnic”.  It is a testing strategy that provides that same prenatal genetic screening to all patients regardless of the ethnic background. ECS testing panels currently include between 200 and 300 rare genetic diseases. Given that, many of us will be a carrier for at least one condition and it is usually necessary to have the partner tested to really ascertain whether there is risk to a fetus.  There are genetic labs that provide a genetics counselor phone consult to review results and help individuals and couples absorb the information and incorporate it into their family planning.

The limitations of genetic testing, regardless of the strategy of testing, is that we do not know every gene variant; nor do we know the exact correlation of each gene variant with the related genetic disease.  If you have done genetic testing and there were no carrier conditions identified, this means that you have a lower genetic risk. There is no test that will provide a patient with full reassurance of zero genetic risk.  For those who find that they are carriers, the next step is to test the partner and ascertain a couple’s risk. If donor sperm or donor egg is part of the plan for conception, then the donor could be tested if not already done. If both members of a couple are carriers for the same rare disease, then they should have a consultation with a genetics counselor and with Dr. Moomjy re the role of IVF with preimplantation testing of embryos for that rare disease.  This is a strategy to transfer only unaffected embryos and markedly reduce the risk of the disease in the family.

In conclusion, just know that Genetic testing is optional. It is a personal choice best made after understanding the related concepts as presented above and as discussed in your consultation with Dr. Moomjy. Should you choose prenatal genetic testing, with the exception of some results for the Fragile X gene, you will not find out anything new about your health. The testing is focused on developmental, childhood genetic diseases. Genetic testing, like all medical testing, can be stressful.  It is best to understand the concepts and then decide if you want to manage this type of information. We are here to support you every step of the way.

Author
Maureen O'Brien Moomjy, MD, FACOG

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