Learning Objectives:After participating in this continuing professional development activity, the provider should be better able to:
1. Describe ethnicity-based, pan-ethnic, and expanded carrier screening approaches.
2. Identify advantages associated with the use of expanded carrier screening.
3. Summarize pretest counseling points for patients considering expanded carrier screening.
4. Explain ACOG and ACMG recommendations for carrier screening.
Carrier screening is used to screen for autosomal recessive and X-linked single-gene disorders. Single-gene disorders, also known as Mendelian disorders, result from a disease-causing variant (formerly referred to as a mutation) at a single genetic locus. With autosomal recessive disorders, both reproductive partners of an at-risk couple (carrier couples) carry a pathogenic variant in the same gene, whereas in X-linked recessive disorders, only the female reproductive partner is a carrier. Carrier couples have a 25% chance of having a child affected with the autosomal recessive disorder with each pregnancy. Because they are typically healthy and lack a significant family history, carrier couples are usually unaware of their reproductive risk until they have an affected child. The aim of carrier screening is to identify asymptomatic individuals and couples at risk for having a child affected with a genetic disease and provide them with meaningful information that they can use to guide their pregnancy planning based on their personal values.
Carrier screening is optimally performed preconception to enable couples to have the greatest number of reproductive options, which may include preimplantation genetic testing (PGT), adoption, the use of oocyte or sperm donors, or prenatal diagnostic testing. Carrier screening can be performed sequentially in which a patient can undergo screening and then the reproductive partner can be screened if indicated based on the patient's results. When carrier screening is performed during an ongoing pregnancy, it is ideal to perform screening on both partners simultaneously so that the screening results can be obtained in a timely manner.1 Carrier screening should be performed on gamete donors before their use.2
Carrier Screening Approaches
Ethnicity-Based Screening
Carrier screening was initially used to screen for conditions based on a patient's reported race or ethnicity such as screening for Tay-Sachs disease in the Ashkenazi Jewish population, sickle cell disease in Black individuals, and beta thalassemia in the Mediterranean population. Ethnicity-based screening is limited, as many individuals do not have accurate knowledge of their race or ethnic ancestry and an increasing number of couples are of mixed race and ethnicity. A study of a large, diverse cohort of 23,453 individuals referred for routine autosomal recessive carrier screening detected 433 individuals who would not have been identified as disease carriers in accordance with conventional ethnicity-based screening paradigms.3 For example, 40% of the individuals found to be carriers for Tay-Sachs disease were not from the Ashkenazi Jewish population. In addition to the limitations noted earlier, restriction of carrier screening using socially defined ethnic constructs or by self-identified ancestry is inequitable and scientifically flawed.1
Pan-Ethnic Screening
A pan-ethnic screening approach involves offering screening to individuals regardless of ethnicity. Cystic fibrosis and spinal muscular atrophy were the first 2 conditions with recommendations for pan-ethnic screening. This was initially performed using genotyping technology.
Expanded Carrier Screening
Expanded carrier screening (ECS) is pan-ethnic screening performed on a large scale; carrier status for multiple conditions is performed concurrently. Recent advances in next-generation sequencing (NGS) technology enable high throughput simultaneous identification of sequence variants across many genes. This has made it possible to efficiently screen for a large number of conditions for the same cost as 1 or 2 single-gene carrier tests.
ECS was initially performed using genotyping panels that test for a limited number of known pathogenic variants in selected genes. The genotyping approach failed to detect some carriers, particularly carriers among minority populations. This limitation has been overcome using a sequencing approach.
Benefits Associated With Expanded Carrier Screening
ECS is consistent with the ethical principles of justice, autonomy, and beneficence.4 The principle of justice is satisfied as all individuals are offered the same screening panel regardless of race and ethnicity. ECS has the potential to increase reproductive autonomy of individuals and couples by providing them with knowledge that could lead to a broad range of reproductive options, particularly if the screening is performed before conception. Implementation of ECS on a population level has the potential to significantly decrease infant mortality and morbidity. Although the incidences of single-gene disorders are low when considered on an individual level, as a group single-gene disorders account for approximately 20% of infant mortality and 18% of infant hospitalizations in developed countries.5,6
The utility of ECS has been demonstrated in several studies.7-11 A study evaluating NGS-based carrier screening for up to 274 genes in 381,014 individuals reported that a risk for a genetic disorder could be identified in the offspring of 1 in 44 (2.3%) of couples assuming random mating. Rates were higher for most within-race/ethnicity couples.11 A systematic review of 17 studies reported that most carrier couples used their carrier status information to inform their family planning decisions either through the use of in vitro fertilization with PGT or, if pregnant, through prenatal diagnostic testing and subsequently deciding on whether to terminate an affected pregnancy. Couples were more likely to alter their reproductive plans with more severe disorders. Two studies involving a large number of carrier couples who had ECS found that carrier couples at risk of having a child with severe, life-limiting, or debilitating conditions were more likely to alter their reproductive plans compared with carrier couples of milder recessive disorders.7,8 Many infertile and subfertile carrier couples who had carrier screening performed as part of their infertility evaluation readily accepted PGT for both severe and relatively mild, low-penetrant, and treatable disorders.10
Panel Screening Performance and Optimal Panel Size
The percentage of variant carriers and at-risk couples identified depends on the characteristics of the population tested and the screening panel. Currently available ECS panels vary in panel size, with the number of diseases being screening ranging from 41 to 1556.12 Although the incremental cost of adding disorders to an NGS-based carrier screening panel can be small in terms of laboratory expenses, there are substantial downstream costs associated with counseling and further testing. Approximately 64% of individuals screened with the 274-gene panel were positive for 1 or more disorders, with 58% positive for 1 disorder, 29% positive for 2 disorders, 10% for 3 disorders, and 0.7% for 5 or more disorders.11
Guo and Gregg13 analyzed a large exome sequencing database to estimate carrier rates across 6 major ancestries for 415 genes associated with severe recessive conditions and assess the incremental yield associated with carrier screening panels of different sizes. They found that screening a smaller number of genes could identify the majority of the at-risk couples. For example, screening all 415 genes in an Ashkenazi Jewish/Ashkenazi Jewish couple identified 2.52% of couples as being at risk. Screening with a panel that consisted of the 40 genes with a gene carrier rate of more than 1% in any ancestry could identify 2.41% of couples as being at risk, a panel including the 87 genes with a gene carrier rate of more than 0.5% could identify 2.50% of at-risk couples, and a panel that included the 244 genes with a gene carrier rate of more than 0.1% would identify all 2.52% of at-risk couples. This study demonstrated that lowering the threshold for genes that should be screened greatly increases the number of genes that would need to be screened, but results in a modest increase in the cumulative carrier rate and a miniscule increase in the at-risk couple rate.
Potential Limitations of Expanded Carrier Screening
Although ECS has been demonstrated to have clinical utility, there are a number of limitations. These include a lack of consistency in laboratory panels and reporting of results. In addition, genetic counseling and educational resources are needed. Moreover, molecular testing is not the recommended screening approach for some condition.
Although ECS has been demonstrated to have clinical utility, there are a number of limitations. There is a lack of consensus regarding how many and which conditions to include on a panel. As of January, 2017, there were 16 laboratories that provided ECS tests including 13 commercial companies, 2 medical hospitals, and 1 academic diagnostic laboratory. The number of conditions tested for by the laboratories ranged from 41 to 1792, yet only 3 conditions (cystic fibrosis, maple syrup urine disease 1b, and Niemann-Pick disease) were screened for by all providers. In addition to the variation in carrier screening panel content, there is a lack of uniformity in laboratory reports including substantial differences in the mutations screened, variant interpretation, and reporting strategies in cases where the same gene was included by multiple providers.12,14 Best practice for the interpretation and reporting of variants of uncertain significance and variants associated with variable expressivity requires additional attention and future research.
As ECS becomes more commonly used on a larger scale, it is critical to develop educational resources for health care providers and patients. Traditional genetic counseling models can be both time- and labor-intensive. Educational models may include videos, pamphlets, chatbots, computer-based learning, or other methods of providing information to patients and assessing their understanding.1
At this time, it is unclear as to whether NGS screening can replace traditional carrier screening methods for Tay-Sachs disease and hemoglobinopathies, although a recent study reported that NGS-based carrier screening performed better than enzymatic screening in both Ashkenazi Jewish and non-Ashkenazi Jewish populations.15 Additional investigation is needed to confirm this and to assess whether NGS screening approaches can replace assessment of a complete blood count, red blood cell indices, and reflex hemoglobin electrophoresis in screening for hemoglobinopathies.11
Society Guidelines/Statements
American College of Obstetricians and Gynecologists
The American College of Obstetricians and Gynecologists (ACOG)16,17 acknowledges that ethnic-specific, pan-ethnic, and ECS are acceptable strategies for prepregnancy and prenatal carrier screening.17 ACOG recommends that each obstetrician-gynecologist, other health care provider, or practice should establish a standard approach that is consistently offered to and discussed with each patient, ideally preconception. If a patient requests a screening strategy other than the one used by the practice or health care provider, the requested test should be made available to the patient after counseling, which should include a discussion of the limitations, benefits, and alternatives.
Regardless of screening strategy and ethnicity, the ACOG recommends pan-ethnic screening for cystic fibrosis, spinal muscular atrophy, and hemoglobinopathies. Fragile X premutation carrier screening is recommended for women with a family history of fragile X-related disorders or intellectual disability suggestive of fragile X syndrome, or women with a personal history of ovarian insufficiency. ACOG recommends offering additional screening based on ethnicity including screening for a limited number of specific diseases in individuals of Ashkenazi Jewish, African, Southeast Asian, or Mediterranean ancestry. ACOG recommends offering carrier screening for Tay-Sachs disease, Canavan disease, cystic fibrosis, and familial dysautonomia to individuals of Ashkenazi Jewish descent and consideration for a more expanded panel including Bloom syndrome, familial hyperinsulinism, Fanconi anemia, Gaucher disease, glycogen storage disease type I, Joubert syndrome, maple syrup urine disease, mucolipidosis type IV, Niemann-Pick disease, and Usher syndrome. When only one partner is of Ashkenazi Jewish descent, that individual should be screened first. If it is determined that this individual is a carrier, the other partner should be offered screening for that disorder. ACOG recommends that individuals of African, Southeast Asian, and Mediterranean ancestry should have screening for hemoglobinopathies with a complete blood count in combination with a hemoglobin electrophoresis.
ACOG has suggested that conditions included on an ECS have a carrier frequency of 1 in 100 or greater.17 This approach has some limitations, as a 1 in 100 cutoff could be a disadvantage to subpopulations where a particular disorder is more common. In addition, the recommendation does not provide guidance with respect to X-linked disorders.
American College of Medical Genetics and Genomics
The American College of Medical Genetics and Genomics (ACMG)1 published updated guidelines for carrier screening in 2021. Tier 3 carrier screening, which includes 97 autosomal recessive and 16 X-linked genes, is recommended for all pregnant patients and those planning a pregnancy. Tier 3 includes autosomal recessive conditions with a carrier frequency of >= 1/200 and X-linked genes with a 1/40,000 disease prevalence. Tier 4 screening, which includes genes less common than those in tier 3, is recommended when a pregnancy results from a known or possible consanguineous relationship, defined as second cousins or closer, or when it is warranted based on personal and/or family history. Of note, ACMG recommends that the term "expanded carrier screening" be replaced by carrier screening.
Expanded Carrier Screening: Pearls for Clinical Practice
Indications for Referral to a Genetic Counselor Before Carrier Screening
All individuals and couples considering pregnancy should have a basic screen for a family history of genetic disorders, with a pedigree to at least the second prior generation (a 3-generation pedigree).18 Individuals with a personal or family history of a genetic condition should be referred to a genetic counselor who can obtain information on the specific familial disease-causing variant and offer a test panel that includes this variant.
Consanguineous couples (2 individuals who are second cousins or closer in a family relationship) are at increased risk of having a child with an autosomal recessive condition. A genetic counselor can discuss the increased risk and the potential benefits associated with an ECS panel.
Pretest Counseling
Pretest counseling should be provided before ECS by knowledgeable health care professionals. Although it is not feasible to counsel patients regarding the specific conditions on the panel, patients should be provided with education that describes the screening process and gives them an overview of the severity of the disorders on the panel. Table 1 lists points to include in pretest counseling. Patients should be informed that carrier screening does not test for all genetic disorders. Although sequencing has significantly increased detection rates, it is important for patients to understand that there is a residual risk of being a carrier after receiving a negative test result.
Posttest Counseling
Patients who screen positive as a carrier for an autosomal recessive or X-linked recessive condition should be referred for genetic counseling. When a patient tests positive as a carrier for an autosomal recessive condition, their partner should be offered carrier screening.
Reproductive partners who test positive as a carrier for the same autosomal recessive condition (carrier couples) have a 25% risk of having an affected offspring. The patient/couple should be referred for genetic counseling and offered diagnostic testing. Prenatal diagnosis can be performed on cells obtained by chorionic villus sampling and amniocentesis. Carrier couples should be informed of the disease manifestations, range of severity, and available treatment options. Carrier couples identified in the preconception period should be referred to discuss the option of PGT.
If a woman tests positive as a carrier for an X-linked recessive disorder, she has a 50% risk for having an affected son. The patient/couple should be referred to a genetic counselor to discuss the pertinent information noted earlier for carrier couples of autosomal recessive conditions.
Relatives of individuals who are identified as carriers are at risk for carrying the same disease-causing variant. Patients should be encouraged to inform their relatives (particularly first-degree relatives of child-bearing age) of the risk and the availability of carrier screening. The provider should not contact these relatives, as there is no provider-patient relationship with the relatives, and confidentiality must be maintained.16
Asymptomatic individuals identified to have 2 pathogenic variants for an autosomal recessive condition or a pathogenic variant for an X-linked recessive condition should be referred for genetic counseling and appropriate medical management. In general, being identified as a carrier of a disorder has no impact on a patient's health. However, there are some conditions for which positive carrier status can be associated with significant health implications. Female carriers who test positive for X-linked conditions such as ornithine transcarbamylase deficiency and hemophilia may manifest symptoms of the disease due to skewed X-inactivation. Carriers of pathogenic dystrophin gene variants associated with Duchenne muscular dystrophy and carriers for Fabry disease are at increased risk for a cardiomyopathy. FMR1 premutation carriers are at increased risk for developing fragile X-associated tremor ataxia syndrome and neurocognitive deficiency, and female premutation carriers are at increased risk for developing premature ovarian insufficiency. Carriers for Gaucher disease (GBA carriers) have an increased risk for developing Parkinson disease later in life. Women who are carriers for ATM and BRCA2/FANCD variants are at increased risk for developing cancer.
Conclusion
It is important for obstetrics and gynecology providers to be knowledgeable about the advantages, limitations, and pre- and posttest counseling considerations associated with carrier screening to optimize patient care. Although ECS can enhance patients' reproductive autonomy and potentially decrease mortality and morbidity, further advancements in patient and provider education, consensus on gene panel content, and laboratory interpretation and reporting are needed. Additional and ongoing guidance and recommendations from professional societies on the implementation of ECS will be critical, as its use continues to increase.
REFERENCES