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Read terms. Reaffirmed This document reflects emerging clinical and scientific advances as of the date issued and is subject to change.
The information should not be construed as dictating an exclusive course of treatment or procedure to be followed. ABSTRACT: Carrier screening is a term used to describe genetic testing that is performed on an individual who does not have any overt phenotype for a genetic disorder but may have one variant allele within a gene s associated with a diagnosis. Information about carrier screening should be provided to every pregnant woman.
Carrier screening and counseling ideally should be performed before pregnancy because this enables couples to learn about their reproductive risk and consider the most complete range of reproductive options. A patient may decline any or all screening. When an individual is found to be a carrier for a genetic condition, his or her relatives are at risk of carrying the same mutation. The patient should be encouraged to inform his or her relatives of the risk and the availability of carrier screening. If both partners are found to be carriers of a genetic condition, genetic counseling should be offered.
What follows is a detailed discussion of some of the more common genetic conditions for which carrier screening is recommended in at least some segments of the population.
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The American College of Obstetricians and Gynecologists the College makes the following recommendations and conclusions:. Information about genetic carrier screening should be provided to every pregnant woman. After counseling, a patient may decline any or all screening. Concurrent screening of the patient and her partner is suggested if there are time constraints for decisions about prenatal diagnostic evaluation.
Prenatal diagnosis and advanced reproductive technologies to decrease the risk of an affected offspring should be discussed. The obstetrician—gynecologist or other health care provider should not disclose this information without permission from the patient.
It is important to obtain the family history of the patient and, if possible, her partner as a screening tool for inherited risk. The family history should include the ethnic background of family members as well as any known consanguinity. Individuals with a positive family history of a genetic condition should be offered carrier screening for the specific condition and may benefit from genetic counseling. Because of the rapid evolution of genetic testing, additional mutations may be included in newer screening panels.
The decision to rescreen a patient should be undertaken only with the guidance of a genetics professional who can best assess the incremental benefit of repeat testing for additional mutations.
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Prenatal carrier screening does not replace newborn screening, nor does newborn screening replace the potential value of prenatal carrier screening. If a patient requests carrier screening for a particular condition for which testing is readily available and which reasonably would be considered in another screening strategy, the requested test should be offered to her regardless of ethnicity and family history after counseling on the risks, benefits, and limitations of screening.
The cost of carrier screening for an individual condition may be higher than the cost of testing through commercially available expanded carrier screening panels. When selecting a carrier screening approach, the cost of each option to the patient and the health care system should be considered. Screening for spinal muscular atrophy should be offered to all women who are considering pregnancy or are currently pregnant. In patients with a family history of spinal muscular atrophy, molecular testing reports of the affected individual and carrier testing of the related parent should be reviewed, if possible, before testing.
If the reports are not available, SMN1 deletion testing should be recommended for the low-risk partner. Cystic fibrosis carrier screening should be offered to all women who are considering pregnancy or are currently pregnant. For couples in which both partners are unaffected but one or both has a family history of cystic fibrosis, genetic counseling and medical record review should be performed to determine if CFTR mutation analysis in the affected family member is available.
A complete blood count with red blood cell indices should be performed in all women who are currently pregnant to assess not only their risk of anemia but also to allow assessment for risk of a hemoglobinopathy. Ideally, this testing also should be offered to women before pregnancy. A hemoglobin electrophoresis should be performed in addition to a complete blood count if there is suspicion of hemoglobinopathy based on ethnicity African, Mediterranean, Middle Eastern, Southeast Asian, or West Indian descent.
If red blood cell indices indicate a low mean corpuscular hemoglobin or mean corpuscular volume, hemoglobin electrophoresis also should be performed. 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 and who are considering pregnancy or are currently pregnant.
If a woman has unexplained ovarian insufficiency or failure or an elevated follicle-stimulating hormone level before age 40 years, fragile X carrier screening is recommended to determine whether she has an FMR1 premutation. Prenatal diagnostic testing for fragile X syndrome should be offered to known carriers of the fragile X premutation or full mutation. DNA-based molecular analysis eg, Southern blot analysis and polymerase chain reaction is the preferred method of diagnosis of fragile X syndrome and of determining FMR1 triplet repeat eg, premutations.
In rare cases, the size of the triplet repeat and the methylation status do not correlate, which makes it difficult to predict the clinical phenotype. In cases of this discordance, the patient should be referred to a genetics professional. When only one partner is of Ashkenazi Jewish descent, that individual should be offered screening first.
If it is determined that this individual is a carrier, the other partner should be offered screening.
However, the couple should be informed that the carrier frequency and the detection store in non-Jewish individuals are unknown for most of these disorders, except for Tay—Sachs disease and cystic fibrosis. Screening for Tay—Sachs disease should be offered when considering pregnancy or during pregnancy if either member of a couple is of Ashkenazi Jewish, French—Canadian, or Cajun descent.
Those with a family history consistent with Tay—Sachs disease also should be offered screening. When one member of a couple is at high risk ie, of Ashkenazi Jewish, French—Canadian, or Cajun descent or has a family history consistent with Tay—Sachs disease but the other partner is not, the high-risk partner should be offered screening. If the high-risk partner is found to be a carrier, the other partner also should be offered screening.
Enzyme testing in pregnant women and women taking oral contraceptives should be performed using leukocyte testing because serum testing is associated with an increased false-positive rate in these populations. If Tay—Sachs disease screening is performed as adult of pan-ethnic expanded carrier screening, it is important to recognize the limitations of the mutations screened in detecting carriers in the general population.
In the presence of a family history of Tay—Sachs disease, expanded carrier screening panels are not the best approach to screening unless the familial mutation is included on the panel. Referral to an obstetrician—gynecologist or other health care provider with genetics expertise may be helpful in instances of inconclusive enzyme testing or in discussion of carrier testing of an individual with non-Ashkenazi Jewish ethnicity whose France partner is a known carrier of Tay—Sachs disease.
Carrier screening is a term used to describe genetic testing that is performed on an individual who does not have any overt phenotype for a genetic disorder but may have one variant allele within a gene s associated with a diagnosis. Carrier screening and counseling ideally should be performed before pregnancy because this enables couples to learn about their reproductive risk and consider the most complete range of reproductive options, including whether or not to become pregnant and whether to use advanced reproductive technologies such as preimplantation genetic diagnosis or use of donor gametes.
Knowledge during pregnancy allows patients to consider muscular diagnosis and pregnancy management options in the event of an affected fetus.
Ideally, information on the specific mutation will be available to aid testing and counseling. Although several different strategies for screening are available and reviewed in Committee Opinion No. The different sections collect topics that had ly been discussed in separate Committee Opinions to show how the aforementioned general principles are used and reflected in carrier screening for specific genetic conditions. Spinal muscular atrophy, also known as SMA, is an autosomal recessive disease characterized by degeneration of spinal cord motor neurons that le to atrophy of skeletal muscle and overall weakness.
Carrier screening for genetic conditions
The disorder is caused by a mutation in the gene known as the survival motor neuron gene SMN1which is responsible for the production of a protein essential to motor neuron function. Because of the severity and relatively high carrier frequency, there has been increasing interest in carrier screening for spinal muscular atrophy in the general prenatal population 3. The genetics of spinal muscular atrophy are complex and, because of limitations in the molecular diagnostic assays available, precise prediction of the phenotype in affected fetuses may not be possible.
The incidence of spinal muscular atrophy is approximately 1 in 6, to 1 in 10, live births, and the disease is reported to be the leading genetic cause of infant death. Carrier frequencies in most populations are estimated at 1 in 40 to 1 in 60, but carrier frequencies appear to be lower in the Hispanic population 4.
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Carrier frequencies and residual risks are outlined by ethnicity in Table 1. There is no effective treatment for the disease. There are several types of spinal muscular atrophy based on age at symptom onset. Earlier onset is correlated with more severe manifestations.
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The most severe and most common form of the disease, type I Werdnig—Hoffmanhas symptomatic onset before 6 months of age and causes death from respiratory failure within the first 2 years of life. Type II spinal muscular atrophy is of intermediate severity, with typical onset before 2 years of age. Affected children are able to sit, but few are able to stand or walk unaided. Respiratory insufficiency is a frequent cause of death during adolescence; however, the lifespan of patients with spinal muscular atrophy type II varies from age 2 years to the third decade of life.
However, the symptom profile is quite variable. Affected individuals typically reach all major motor milestones, but function ranges from requiring wheelchair assistance in childhood to completely unaided ambulation into adulthood with minor muscular weakness. Many patients have normal life expectancies. Type IV has onset in adulthood. There is an additional Type 0 proposed, which has onset in the prenatal period.
There is generally one copy of SMN1 per chromosome, but occasionally two can be located on the same chromosome. A variable of SMN2 gene copies ranging from zero to three may be present, but the SMN2 gene produces only a small amount of functional survival motor neuron protein. A higher of SMN2 copies correlates with generally milder clinical phenotypes, but accurate prediction of the spinal muscular atrophy phenotype based on SMN2 copy is not possible 5. However, this approach is not sufficient to identify patients who are heterozygous, or carriers, for the SMN1 deletion.