Carrier screening

Carrier screening identifies individuals who are heterozygous for pathogenic variants in autosomal recessive or X-linked conditions. Its purpose is to inform reproductive decisions: if both partners in a couple carry pathogenic variants in the same AR gene (or if the female partner carries an X-linked variant), offspring are at risk. Practice has shifted from ethnicity-based panels to pan-ethnic expanded carrier screening (ECS).

• Traditional ethnicity-based screening was built around founder variants known to be common in specific populations: Ashkenazi Jewish (Tay-Sachs, Canavan, Gaucher, familial dysautonomia, others), French-Canadian (tyrosinemia I), African (sickle cell, alpha-thalassemia), Mediterranean (beta-thalassemia), Southeast Asian (alpha-thalassemia).
• Expanded carrier screening (ECS) uses NGS-based panels covering 100–500+ conditions regardless of ancestry. Strengths: admixed populations are covered, previously unrecognized carriers are identified, uniform workflow. Limitations: variant interpretation is harder for rare populations with less frequency data; incidental findings raise counseling burden.

• ACMG (2021 update): recommends offering ECS to all patients considering pregnancy or currently pregnant. Tier 3 panel covers conditions with carrier frequency >1/200 in at least one ancestry.
• ACOG (2017, reaffirmed): supports offering any of three approaches (ethnicity-based, pan-ethnic minimum [CF, SMA, hemoglobinopathies], or ECS) to all patients. Patient and clinician choose.
• Conditions universally recommended regardless of ancestry:
  • Cystic fibrosis (CFTR): carrier frequency about 1/25 in European ancestry.
  • Spinal muscular atrophy (SMN1): carrier frequency about 1/40–1/50 in most populations. SMN1 testing uses copy-number analysis (not sequencing); rare "silent" carriers with 2 copies in cis can be missed.
  • Hemoglobinopathies (sickle cell, thalassemias): CBC + hemoglobin electrophoresis as first step; DNA testing as follow-up.
• Fragile X (FMR1) carrier screening is offered to women with family history of intellectual disability, autism, or premature ovarian failure, and increasingly as part of ECS panels.

1. Pre-test counseling: explain purpose, limitations, residual risk, implications of positive results for the patient and family.
2. Test: typically buccal swab or blood. Sequential (female first, then partner if positive) vs concurrent (both tested at once); concurrent is faster for couples with limited time windows.
3. Interpretation: carrier (heterozygous pathogenic variant), non-carrier (no detected pathogenic variant; note residual risk), or uncertain (variant of uncertain significance, not reported in most clinical settings).
4. Post-test counseling: if both partners carry pathogenic variants in the same AR gene, discuss risk (1 in 4 per pregnancy affected, 1 in 2 carrier, 1 in 4 unaffected) and options (prenatal diagnosis, PGT-M, donor gametes, adoption, or proceeding with awareness).

• A negative carrier screen does not reduce risk to zero. The residual risk depends on the assay's detection rate for that ancestry.
• Example: CF carrier screening with a 23-variant panel has ~94% detection in Ashkenazi Jewish, ~88% in European-ancestry, ~69% in African American, and ~49% in Asian American populations. Residual risk is calculated using Bayesian modification of the prior.
• NGS-based panels have higher detection but still miss structural variants, deep intronic variants, and novel alleles.

• Pseudodeficiency alleles (e.g., GAA, HEXA) produce low enzyme activity in vitro without disease. Important not to misclassify as carrier positive.
• SMN1 silent carriers (2 copies in cis, 0 in trans): standard copy-number testing misses these. Linked SNP analysis or ancestry-specific flanking variants can help.
• Fragile X premutations (55–200 CGG repeats): not clinically silent; they confer risk of FXTAS (males, older) and FXPOI (females). Counseling differs from typical "carrier" of an AR disorder.
• Adults identified as carriers of late-onset conditions (e.g., BRCA1/2, Lynch genes) via reproductive screening have personal health implications beyond reproduction; they must be counseled accordingly.