Carrier screening for genetic diseases

Genetic diseases are the main cause of death among infants and are responsible for 20-30% of the total annual infant mortality rate¹. Everyone carries mutations in their genes but not all mutations are pathogenic, i.e. have an effect on our health and development. Genetic diseases are caused by mutations and can be either dominant – caused by a pathogenic mutation in just one copy of a chromosome, or recessive – caused by pathogenic mutations in both copies. If someone has a recessive mutation in only one copy of the gene, that person is considered a “carrier”, but has not inherited the disease.

Usually, carriers of genetic diseases are not aware of their status as they do not exhibit any symptoms and can pass it down to their children. Carrier screening tests that can inform individuals whether they are carriers of a genetic disease, are crucial to minimize the risk of transmitting the disease to their children. When testing is carried out before pregnancy, it can provide information that can guide reproductive choices and offer more time to take appropriate decisions. In case carrier screening is performed during pregnancy, it can help parents better prepare for the birth of their children. Some genetic diseases that are frequently prevalent and have moderate to severe clinical phenotype, can affect quality of life and can often be managed through early interventions.

The results of a carrier screening test can be either positive, meaning that a pathogenic or likely pathogenic mutation has been identified and that the person is a carrier of the genetic disease; or negative, meaning that a pathogenic or likely pathogenic mutation has not been identified. If a pathogenic autosomal recessive mutation is detected in the same gene in both parents, then there is a 25% risk of the child being born with the genetic disease, and 50% risk of being a carrier like the parents. In case a pathogenic mutation is detected only in one of the parents, then the child has 50% risk of becoming a carrier of the disease².

Screening programs for the detection of carriers of severe genetic diseases have been active around the world for many years, with most of them focusing on β-thalassemia. B-thalassemia is one of the most common hereditary single gene diseases around the world and is the most widespread genetic disease amongst the Cypriot population. Cyprus has the highest percentage of healthy carriers of β-thalassemia worldwide⁴’⁵’⁶. Carrier screening programs for β-thalassemia have been active in Cyprus since 1973. After the implementation of the program, the number of affected babies fell from 51 in 1974 to only 8 in 1979⁴. The number of affected babies kept dropping, with only 5 total affected births between the years of 1991 to 2001⁶. Cyprus’ screening program is one of the most successful programs in the world as far as hereditary hemoglobin diseases are concerned.

According to the American Congress of Obstetricians and Gynecologists (ACOG) and the American College of Medical Genetics and Genomics (ACMG), information about carrier screening testing should be provided to all pregnant women, as well as to those who are planning to start a family. Carrier screening tests can be beneficial for any individual wishing to know more about their carrier profile, couples planning to start their families or who are already expecting, candidate gamete donors for parent-donor compatibility, population groups that are at high risk for specific diseases, and for people who have a known family history of a genetic mutation.

The information obtained with such genetic tests can offer healthcare providers and individuals, couples or prospective parents useful genetic insight into one’s carrier status. Carrier screening tests can inform, guide and empower people on their reproductive choices; help them evaluate their options and take informed decisions regarding prevention and therapy of genetic diseases that may be manageable through early interventions.

1. Berry RJ, Buehler JW, Strauss LT, et al. 1987. Birth weight-specific infant mortality due to congenital abnormalities, 1960 and 1980. Public Health Report 102:171-81
2. National Library of Medicine National Institutes of Health, (2020), https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4758394/
3. The American College of Obstetricians and Gynecologists,(2018), https://www.acog.org/Patients/FAQs/Carrier-Screening
4. Onmed.gr, (2015), https://www.onmed.gr/ygeia/story/326064/ti-simainei-na-eisai-foreas-mias-astheneias
5. National Library of Medicine National Institutes of Health, (2020), https://www.ncbi.nlm.nih.gov/pubmed/19390864
6. Ministry of Health Cyprus, (2018)
7. Cousens, Nicole E et al. “Carrier screening for beta-thalassaemia: a review of international practice.” European Journal of Human Genetics: EJHG vol. 18,10 (2010): 1077-83. doi:10.1038/ejhg.2010.90
8. In-cyprus,(2019),https://in-cyprus.com/more-than-1-in-10-cypriots-carriers-of-%CE%B2-thalassemia-world-thalassaemia-day/
9. Nazareth, S. B., Lazarin, G. A., & Goldberg, J. D. (2015). Changing trends in carrier screening for genetic disease in the United States. Prenatal Diagnosis, 35(10), 931–935.https://doi.org/10.1002/pd.4647