A Review of Carrier Screening in Reproductive Medicine

BACKGROUND:

Professional Organization Screening Guidelines: recommend offering carrier screening for individual conditions based on condition severity, race or ethnicity, prevalence, carrier frequency, detection rates, and residual risk

  • Detection Rate: the proportion of carriers that is identified by the screening test (sensitivity)
  • Disease Prevalence: the proportion of individuals in a population that has a condition
  • Carrier Frequency: the proportion of individuals in a population that has a pathogenic variant for a condition
  • Common – 1 in 50 or greater
  • Uncommon – Between 1 in 50 – 1 in 100
  • Rare – Between 1 in 100 and 1 in 250
  • Residual Risk:
  • To the individual – a numeric measure of the chance of being a carrier after a negative screening test
  • To the fetus – a numeric measure of the chance that a fetus will have a condition after one or both parents test negative for that condition
  • Current Organizations:
  • American College of Obstetricians and Gynecologists
  • American College of Medical Genetics and Genomics
  • National Society of Genetic Counselors
  • All recommend panethnic testing for CF, only American College of Medical Genetics and Genomics recommends panethnic testing for SMA
  • All other conditions are based on race or ethnicity

Expanded carrier screening: More comprehensive screening beyond the currently recommended screening guidelines

  • Include conditions that have a significant variation in their presentations, including variable age of onset
  • Include conditions which testing the general population is not recommended (i.e. Fragile X, Hemochromatosis, and factor V Leiden)
  • Contain rare disorders for which residual risk after a negative screening test may not be possible
  • CONCEPTS:
  • All individuals are offered screening for same set of conditions
  • Most conditions are rare and it is not necessary to fully explain all of the clinical and test characteristics of each condition
  • Education should include a broad description of type of conditions, common features, and limitations of screening
  • Some conditions have less well-defined phenotypes
  • Because most of them are rare, disease prevalence, mutation frequency and detection rates may be imprecise and residual risk may not be reliable
  • Residual risk always remains
  • Screening panels may change over time, but rescreening typically is not offered or recommended
  • Majority of conditions are autosomal recessive with some X-linked or autosomal dominant
  • Screening panels include most conditions recommended in current guidelines but may not be the best method for screening such as:
  • Screening for hemoglobinopathies requires use of MCV and hemoglobin electrophoresis
  • Tay Sachs disease testing with hexosaminidase A enzyme analysis is the best method, sequencing is not recommended
  • For couples identified with a risk of an inherited condition, who have familial variants or who test positive on expanded carrier testing, referral for genetic counseling is recommended as diagnostic testing may be indicated (i.e. SMA, CF, Fragile X etc.)
  • CONSENT:Should be documented in the chart whether counseling and informed consent are achieved or pt declines
  • Carrier screening is voluntary
  • Results are confidential and protected in health insurance and employment
  • Conditions may vary in severity including cognitive impairment, decreased life expectancy, and need for medical or surgical intervention
  • Pregnancy risk assessment depends on accurate knowledge of paternity
  • A negative screen does not eliminate risk to offspring
  • Most of the disorders are autosomal recessive and carriers usually have no clinical consequences. Usually both parents are carriers when offspring are affected.
  • Individuals may learn that they carry one or two pathogenic variants for a condition and learn that they have a condition that could affect their health (i.e. thrombophilia’s).
  • POSTTEST COUNSELING:
  • If both partners are carriers there is a 25% chance of offspring being affected
  • Genetic counseling recommended, including significance of this information for other family members who may also be carriers (i.e., parents or siblings)
  • Refer for diagnostic testing when available
  • If an affected fetus is identified all reproductive options should be discussed:
  • Prenatal Management
  • Delivery planning and coordination of care for the child as well as pregnancy termination or adoption planning
  • Preconception period  review preimplantation genetic diagnosis and use of noncarrier donor gametes as additional options
  • CONSIDERATIONS IN SELECTING AN EXTENDED PANEL TEST:
  • The condition screened for should encompass one or more of the following:
  • Cognitive disability
  • Need for surgical or medical intervention
  • Effect on quality of life
  • Prenatal diagnosis would result in:
  • Prenatal intervention to improve outcomes
  • Delivery management to optimize outcomes
  • Prenatal education of parents regarding special needs
  • Exclude conditions in which:
  • Distinguishing between childhood or adult onset is not possible
  • Variants have high allele frequencies and low penetrance(the extent to which a particular gene or set of genes is expressed in the phenotypes of individuals carrying it, measured by the proportion of carriers showing the characteristic phenotype)
  • The most appropriate approach to screening is something other than molecular testing (i.e. hemoglobin electrophoresis)

Condition Specific Recommendations

Hemoglobinopathies: ACOG Practice Bulletin No. 78, 2007, Reaffirmed 2013

  • 270 million people worldwide are heterozygous carrier of disorders of hemoglobin (1 in 12 African Americans)
  • 300,000 are affected homozygous of compound heterozygotes (two recessive alleles for the same gene, but with those two alleles being different from each other (for example, both alleles might be mutated but at different locations))
  • Each person should have 2 alpha chains and either 2 beta chains (hemoglobin A), 2 gamma chains (hemoglobin F), or 2 delta chains (hemoglobin A2)
  • Sickle cell – abnormal hemoglobin A  hemoglobin S
  • B-thalassemia – decreased/absent levels of hemoglobin A (each person has 2 beta genes)
  • Alpha-thalassemia – decreased/absent levels alpha chains (each person has 4 alpha genes)
  • ONLY MOLECULAR TESTING CAN IDENTIFY THIS CONDITION

  • Prenatal diagnostic testing for sickle cell disease is available
  • Prenatal testing for alpha and beta thalassemia is possible if the mutations and deletions have been previously identified in both parents
  • Preimplantation genetic diagnosis in combination with in vitro fertilization may be a desirable alternative to avoid termination of an affected pregnancy
  • Has been successful for sickle cell disease and most cases of Beta-thalassemia

Conditions prevalent among Ashkenazi Jewish population: ACOG Committee Opinion No. 442, 2009 (Reaffirmed 2014)

  • Screening for Tay-Sachs disease (1 in 30), CF (1 in 24), Canavan (1 in 40) and familial dysautonomia (1/32) should be offered to those with Ashkenazi Jewish ancestry
  • Most screening offers a 95% detection rate in Ashkenazi Jewish descents
  • Additionally, offer Tay Sachs testing to individuals of French Canadian and Cajun descent (ethnic group mainly living in the U.S. state of Louisiana, consisting of the descendants of Acadian exiles (French-speakers from Acadia in what are now The Maritimes of Eastern Canada). Today, the Acadian Creoles make up a significant portion of south Louisiana's population)
  • Carrier screening can be offered for mucolipidosis IV, Niemann-Pick Disease Type A, Fanconi Anemia group C, Bloom syndrome, and Gaucher disease
  • Individuals with a positive family history of one of the above disorders, should be offered carrier screening for the specific disorder and may benefit from genetic counseling
  • Carriers should inform relatives of their risk

Cystic Fibrosis: ACOG Committee Opinion No. 486, 2011 (Reaffirmed 2014)

  • Most common in Ashkenazi Jewish descent (1 in 24) and non-hispanic white (1/25)
  • Detection rates are 94% for Jewish descent, 88% in non-hispanic white, and 49-72% in other ethnicities (other ethnicities therefore have a higher residual risk)
  • Panethnic testing is recommended
  • If a patient has been screened previously for CF, CF screening test should not be repeated
  • Complete analysis of the CFTR gene by DNA sequencing is not appropriate for routine carrier screening
  • For couples in which both partners are unaffected but one or both has a family history of CF, genetic counseling and medical record review should be performed to identify if CFTR mutation analysis in the affected family member is available
  • If a woman’s reproductive partner has CF or apparently isolated congenital bilateral absence of the vas deferens, the couple should be referred to a genetics professional for mutation analysis and consultation

Spinal Muscular Atrophy: ACOG Committee Opinion No. 432, 2009 (Reaffirmed 2014)

  • The incidence of SMA is 1 in 10,000 live births
  • Carrier frequencies are estimated at 1 in 40 to 1 in 60
  • Currently, ACOG recommends testing only when a family history of spinal muscular atrophy is present
  • The American College of Medical Genetics has recently recommended offering carrier testing to all couples regardless of race or ethnicity, however, no pilot studies have been completed in the US that would determine best practices for pretest and posttest education and counseling with specific regard to SMA screening
  • The following should be addressed before panethnic screening for SMA can be recommended:
  • Critical assessment of pilot screening programs
  • Cost effectiveness analysis
  • Development of appropriate education materials for both patients and primary OBGYN’s
  • Development of laboratory assay standards and result reporting

Fragile X Syndrome: ACOG Committee Opinion No. 469, 2010 (Reaffirmed 2014)

  • Most common inherited form of mental retardation
  • Most common known cause of autism or “autisticlike” behaviors
  • CGG repeats in FMR1 gene
  • As the gene expands, the gene is methylated and therefore inactivated
  • 1 in 4,000 males have fragile x syndrome
  • 1 in 8,000 females have fragile x syndrome
  • 1 in 225 women carry a premutation
  • 1 in 45 carry intermediate
  • 1 in 259 for women with no known risk factors (1 in 86 for those with a family history of mental retardation
  • Male premutation carrier frequency is 1/468- 1/813
  • Women with a premutation on one X chromosome, 50% of her children inherit the abnormal gene, 50% inherit the normal gene
  • Men with premutations are not at risk of having children with fragile x as CGG repeats do not usually expand

Mutation in the Fragile X Mental Retardation 1 Gene

Status of Individual / Number of Triplet Repeats
Unaffected / Less than 45
Intermediate / 45-54
Premutation / 55-200
Full mutation / More than 200

Full Mutation Expansion from Maternal Premutation Allele

Maternal Repeat Size / Full Mutation Expansion (%)
55-59 / 4
60-69 / 5
70-79 / 31
80-89 / 58
90-99 / 80
100-200 / 98
  • Gestational age differences in the methylation of specific genes in the chorionic villi may preclude determining the true methylation status of the FMR1 gene, therefore, findings of full mutation FMR1 mutations on fetal DNA analysis on CVS may require a follow up amniocentesis to accurately determine the methylation status of the gene
  • Expansion is influenced by the absence of normally interspersed AGG triplets and the length of total and interrupted CGG repeats
  • The more AGG triplets found, the less likely the mutation will expand to a full mutation in offspring
  • Research is needed to determine the impact, if any, of routinely incorporating AGG testing results into genetic counseling practice

Fragile X associated disorders

  • 20% of women with a premutation have premature ovarian failure by age 40, therefore, all women with a personal or family history of premature ovarian failure should be offered fragile x carrier testing (less than 80 repeats is more highly correlated with premature ovarian failure)
  • 40% of men and as many as 17% of women over age 50 with a fragile x premutation develop Fragile x tremor ataxia syndrome
  • Handful of case reports of children with premutations who have learning and/or behavioral disabilities
  • An increased incidence of hypertension, thyroid disorders, fibromyalgia, tremor, neuropathy, and mental health disorders has been reported among adult women with premutation alleles

Current guidelines:

  • Women with a family history of fragile X-related disorders, unexplained mental retardation or developmental delay, autism, or premature ovarian insufficiency are candidates for genetic counseling and fragile x premutation carrier screening
  • If a women has ovarian insufficiency or failure or elevated FSH before to age 40 with no known cause should be offered fragile x carrier screening
  • Women who request fragile x, regardless of family history, should be offered fragile x carrier screening after genetic counseling
  • All fragile x premutations and mutations should be referred for follow up genetic counseling and fragile x associated syndromes. Prenatal and preimplantation diagnoses and donor eggs should be discussed
  • Prenatal testing with DNA-based molecular analysis (southern blot and PCR) for fragile x should be offered to known carriers of fragile x premutation or full mutation

Clinical Guidelines for testing for heritable thrombophilia:

Siddiqui, T, MD et al. Up to Date. Contraceptive counseling for women with inherited thrombophilia’s. January 2015

Bauer, Kenneth, MD. Up to Date.Screening for Inherited Thrombophilia in asymptomatic individuals.September 2014

Prevalence:

  • Factor V Leiden Heterozygote: 1/20
  • Factor V Leiden Homozygote: 1/5,000
  • Prothrombin: 1/50
  • Protein C deficiency: 1/500
  • Protein S deficiency: 1/500
  • Antithrombin III deficiency: 1/ 2,000-3,000

Population based screening is not recommended due to:

  1. The low frequency of symptomatic condition in general population
  2. Low penetrance of the symptomatic conditions of Factor V Leiden and Prothrombin
  3. Lack of safe, cost effective, long method of prophylaxis if abnormality is found

Facts:

  • VTE is less likely in homozygotes for factor V leiden with negative family history than heterozygous and a positive family history ( Risk 1.3 vs. 4.1)
  • There is a 3-4 fold higher thrombosis risk with Protein C,S, and Antithrombin III deficiencies than factor V Leiden (including those on estrogen therapy as well as postpartum women)
  • Recommendation for screening are as follows:
  • Personal family history of VTE
  • 1st degree relative with VTE associated with antithrombin III, Protein C and S deficiencies
  • Multiple 1 st degree relatives with VTE before the age of 50
  • Women heterozygous for factor V leiden have only a 0.29% per year increased risk of VTE on estrogen/progestin contraceptionvs. 0.5-1% with factor v leiden homozygous
  • Women with known inherited thrombophilia’s are generally advised against estrogen containing therapies, but may continue if given strict precautions

Genetic and acquired risk factors for a first episode of venous thrombosis:

Condition/risk factor(s) / Relative risk / Incidence, percent per year
Normal / 1 / 0.008
Prothrombin gene mutation / 2.8 / 0.02
Oral contraceptives / 4 / 0.03
Factor V Leiden (heterozygous) / 7 / 0.06
Oral contraceptives plus heterozygous factor V Leiden / 35 / 0.29
Factor V Leiden (homozygous) / 80 / 0.5 to 1.0