Course / PGT-M & PGT-SR
PGT-M and PGT-SR are specialized forms of preimplantation genetic testing designed to identify specific genetic abnormalities before an embryo transfer takes place.
While PGT-A screens for general chromosomal number errors (aneuploidy), PGT-M and PGT-SR focus on inherited disorders and structural rearrangements that may be passed from one or both parents to their offspring.
For couples or individuals who are known carriers of a genetic condition, these tests can significantly reduce the risk of having a child affected by that disorder.
Both PGT-M and PGT-SR involve biopsying a few cells from the outer layer of a developing embryo (trophectoderm). The DNA is then analyzed in a genetics laboratory using highly precise molecular techniques such as Next-Generation Sequencing (NGS) or Polymerase Chain Reaction (PCR).
PGT-M (Monogenic): Detects mutations in a single gene responsible for a specific inherited condition.
PGT-SR (Structural Rearrangements): Detects unbalanced chromosomal arrangements that can arise when one parent carries a balanced translocation or inversion.
These tests allow fertility specialists to identify embryos that are both genetically normal and free from the specific familial mutation, optimizing the chance of a healthy pregnancy.
PGT-M focuses on detecting single-gene mutations associated with known hereditary diseases.
Common examples include:
Cystic fibrosis
Tay-Sachs disease
Sickle cell anemia
Thalassemia
BRCA1/2 and other hereditary cancer mutations (in specific cases)
How it works:
The intended parents undergo genetic carrier screening to identify known mutations.
A custom testing protocol is designed specifically for their family’s genetic variant.
Embryos created via IVF are biopsied, and the DNA from each is analyzed.
Only embryos free of the targeted mutation (and typically also chromosomally normal) are recommended for transfer.
Clinical benefit: PGT-M helps prevent the inheritance of debilitating or life-threatening genetic diseases and is an essential tool for families with a strong history of inherited disorders.
PGT-SR is designed for individuals or couples in which one partner carries a balanced chromosomal rearrangement—such as a translocation or inversion—that doesn’t cause health issues in the carrier but can result in miscarriages or embryos with unbalanced chromosomes.
How it works:
Embryos are biopsied and analyzed to distinguish between those with balanced, unbalanced, or normal chromosomal structures.
Only embryos with normal or balanced chromosomal configurations are considered suitable for transfer.
Clinical benefit: PGT-SR reduces the risk of recurrent pregnancy loss, implantation failure, and congenital abnormalities linked to unbalanced rearrangements.
This approach is often used for couples who have experienced multiple miscarriages or who have known chromosomal rearrangements identified through karyotype testing.
Once the results of PGT-M or PGT-SR are available, embryos are classified as:
Normal/Unaffected: Free from the targeted mutation or unbalanced rearrangement.
Carrier (for PGT-M cases): Carry one copy of the mutation but are not affected by the disease.
Affected/Unbalanced: Carry two copies of the mutation or possess unbalanced chromosomes.
Clinicians prioritize embryos that are both genetically normal and euploid for transfer.
When multiple embryos are available, additional criteria—such as morphology and developmental stage—are considered alongside genetic results.
While these technologies represent a major advancement in reproductive genetics, they are not without limitations:
Complex test setup: Each case requires a custom assay, which can take several weeks to prepare.
Residual risk: No test is 100% accurate; rare recombination events or technical errors can lead to misdiagnosis.
Cost and logistics: PGT-M and PGT-SR are generally more expensive and time-intensive than standard PGT-A.
Embryo availability: Couples with few embryos may find that testing reduces the pool of embryos available for transfer.
Therefore, genetic counseling is essential before undergoing these procedures to ensure informed consent and realistic expectations.
Ethical and Emotional Aspects:
Families should discuss the ethical implications of genetic selection and embryo disposition with their fertility team.
Integration with PGT-A:
Many clinics now combine PGT-M or PGT-SR with PGT-A, allowing simultaneous screening for both single-gene mutations and chromosomal abnormalities.
Laboratory Expertise:
The accuracy of these tests depends heavily on the experience and validation processes of the genetic laboratory performing the analysis.
Future Implications:
Genetic testing technologies continue to evolve rapidly. Continuous review of emerging evidence and technological improvements helps ensure the best clinical outcomes.
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