Indications and Testing Process for PGT-SR Chromosome Screening in Hong Kong

Opening: Physician's Decision Logic (Real Clinical Scenario)

Clinical Scenario: A couple, the male is 36 years old with a karyotype analysis result of a balanced translocation carrier — 46,XY,t(11;22)(q23;q11.2), the female has a normal karyotype. They have experienced 2 first-trimester spontaneous miscarriages and 1 second-trimester pregnancy termination due to chromosomal abnormalities. Upon consultation, the female's AMH is 2.1 ng/mL, antral follicle count is 12, and there are no other obvious infertility factors. Core consultation question: Is PGT-SR suitable? What is the testing process? What is the probability of a normal embryo?

1. What Does PGT-SR Detect?

PGT-SR (Preimplantation Genetic Testing for Structural Rearrangements) is a technology specifically targeting chromosomal structural rearrangements within preimplantation genetic testing. Unlike PGT-A, which detects chromosomal numerical abnormalities, PGT-SR focuses on chromosome breakpoints, rejoining patterns, and resulting segment gains or losses.

The detection scope includes:

• Reciprocal translocation: Exchange of segments between two chromosomes; carriers usually have a normal phenotype, but unbalanced rearrangements may occur in gametes.
• Robertsonian translocation: Fusion of two acrocentric chromosomes at the centromere, commonly involving chromosomes 13, 14, 15, 21, and 22.
• Pericentric inversion and Paracentric inversion: Inversion of a segment within a chromosome, potentially leading to duplications/deletions in gametes.
• Insertions and marker chromosomes, among other complex rearrangements.

The mainstream detection platform is NGS (Next-Generation Sequencing), supplemented by SNP array or aCGH. Through embryo trophectoderm biopsy (5-10 cells taken on day 5-6 of blastocyst stage), after whole genome amplification, chromosome copy number and breakpoint information are analyzed.

2. When is PGT-SR Necessary?

PGT-SR is not a routine screening test; it has specific clinical indications. A physician's decision to initiate PGT-SR is usually based on the following conditions:

• Known carrier of chromosomal structural abnormality: One or both partners are confirmed by karyotype analysis to have a balanced translocation, Robertsonian translocation, or inversion.
• Recurrent miscarriage clearly related to structural abnormality: Other causes of miscarriage (e.g., uterine abnormalities, immune factors, endocrine issues) have been ruled out, and karyotype analysis of miscarriage tissue indicates unbalanced rearrangement.
• Previous adverse pregnancy history: Previous birth of a child with chromosomal structural abnormality or mid-term pregnancy termination due to fetal structural abnormality.
• Male factor: The carrier rate of chromosomal structural abnormalities is higher in patients with severe oligoasthenoteratozoospermia; some centers recommend karyotype analysis to assess the need for PGT-SR.

In the clinical scenario above, the male is a balanced translocation carrier with a clear history of adverse pregnancy outcomes, making PGT-SR a strong indication.

When is PGT-SR Not Suitable?

• No clear indication of structural abnormality: For advanced maternal age alone, recurrent implantation failure, or recurrent miscarriage with normal karyotype analysis, PGT-A should be considered first, not PGT-SR.
• Unclear breakpoints or technology cannot cover: Some complex rearrangements or breakpoints located in highly repetitive genomic regions may not yield a definitive diagnosis.
• Severely diminished ovarian reserve: Too few oocytes retrieved (e.g., <3-4) may result in no embryos available for testing; the benefit of testing must be weighed against cycle cost.
• Other contraindications: Such as uncontrolled systemic diseases, severe endometrial pathology, etc.

3. How to Choose Between PGT-SR and PGT-A?

The two technologies are not interchangeable; the choice is based on genetic indications:

In clinical practice, some situations require combined testing: For example, a structural abnormality carrier who is also of advanced age may undergo PGT-A analysis alongside PGT-SR to screen for both numerical and structural abnormalities simultaneously. However, such combined testing imposes higher demands on embryo biopsy quality and testing platforms.

4. Specific Process for PGT-SR in Hong Kong

Reproductive medicine centers in Hong Kong with PGT-SR qualifications typically follow this standardized pathway:

1. Genetic Counseling and Karyotype Confirmation: Both partners undergo peripheral blood karyotype analysis (G-banding + SNP array or FISH verification) to identify the breakpoint location and type of structural abnormality.
2. Fertility Assessment and Ovarian Stimulation: The female undergoes basic assessments including AMH, antral follicle count, FSH, etc., to develop an individualized stimulation protocol.
3. Oocyte Retrieval and In Vitro Fertilization: Conventional oocyte retrieval followed by ICSI fertilization to avoid sperm carrying abnormal chromosomes entering the oocyte during conventional IVF.
4. Blastocyst Culture and Biopsy: Culture to the blastocyst stage (D5-D6); an embryologist creates a hole in the zona pellucida and aspirates 5-10 trophectoderm cells.
5. Whole Genome Amplification and Testing: After whole genome amplification of the biopsied cells, copy number analysis and breakpoint mapping are performed using NGS or SNP array.
6. Embryo Classification and Reporting: The genetics team issues a report classifying embryos as: chromosomally normal, balanced carrier (same structural abnormality as the parent but functionally normal), or unbalanced abnormal.
7. Selection of Transferable Embryos: Priority is given to transferring structurally normal embryos; if no normal embryos are available, balanced carrier embryos may be considered for transfer after thorough genetic counseling.
8. Frozen or Fresh Transfer: The testing cycle takes approximately 2-3 weeks; typically, all embryos are frozen, and frozen-thawed embryo transfer is scheduled after the report is issued.

5. Key Details Often Overlooked in the Process

• ICSI is Mandatory: Even if the male's sperm parameters are normal, ICSI must be used in a PGT-SR cycle to prevent abnormal chromosomes attached to the sperm surface from being carried into the oocyte and affecting the diagnosis.
• Biopsy Timing and Embryo Quality: Only blastocysts that meet the biopsy quality criteria (typically 3BB or higher) can be biopsied; approximately 15-30% of embryos may not be biopsiable due to developmental arrest or insufficient quality.
• Breakpoint Clarity: Some balanced translocation breakpoints are located in complex regions (e.g., near telomeres, highly repetitive sequences), which may make the test result inconclusive. It is advisable to confirm the test feasibility with the genetics laboratory before starting the cycle.
• Mosaicism Issues: When an embryo has a mosaic structural abnormality, the test result may not fully reflect the true situation; comprehensive decision-making incorporating embryo morphological grading is necessary.
• Karyotype Confirmation for Both Partners: Testing only one partner while ignoring the other may miss occult structural abnormalities (e.g., low-level mosaicism).

6. Regional Differences: Hong Kong vs. Mainland China and Overseas

Hong Kong's advantages lie in its comprehensive regulatory system + closed-loop local laboratory, making the testing cycle relatively controllable. However, costs are higher than in Mainland China, and both partners must hold valid travel documents to attend appointments in Hong Kong.

7. Clinical Observations from a Physician's Perspective

In assisted reproduction clinical practice, the application of PGT-SR has several points that are often underestimated by patients and even practitioners:

• The probability of a normal embryo is lower than most people expect: For example, with balanced translocations, the proportion of normal or balanced embryos is not 50%, but depends on the translocation type and breakpoint location. Reciprocal translocations typically range from 1/18 to 1/4, and Robertsonian translocations about 1/6 to 1/3. This means a sufficient number of blastocysts may need to be accumulated to obtain a transferable embryo.
• LH levels are associated with karyotype: Some carriers of structural abnormalities (especially males) have abnormal gonadotropin levels, which may affect ovarian response during stimulation and require advance assessment.
• Psychological preparation is equally important: Some cycles may be cancelled due to no transferable embryos, or only balanced carrier embryos may be available for transfer. Patients need to fully understand these possibilities before starting the cycle.
• Transfer decision for balanced carrier embryos: Balanced carrier embryos are genetically equivalent to the parent and usually do not cause an abnormal phenotype. However, some centers still recommend prioritizing the transfer of completely normal embryos. If only balanced carrier embryos are available, transfer can be considered after detailed genetic counseling, and prenatal diagnosis is recommended after pregnancy.

8. Frequently Asked Questions

9. Risks and Limitations

10. Physician's Recommendations and Next Steps

Returning to the opening clinical case: The male is a balanced translocation carrier, the female has a normal karyotype, and there is a clear history of adverse pregnancy outcomes. The physician's recommended pathway is as follows:

1. Complete karyotype confirmation for both (already done), and check the male's sperm DNA fragmentation rate and the female's ovarian reserve.
2. Conduct a PGT-SR feasibility study: Some Hong Kong centers require using DNA from both partners to verify the breakpoints, ensuring the testing platform can accurately identify this translocation type.
3. Initiate an ovarian stimulation cycle with a target of 10-15 oocytes retrieved, aiming to obtain ≥4 biopsiable blastocysts.
4. Perform PGT-SR testing after embryo biopsy, and simultaneously add PGT-A (since the female is 35 years old, aneuploidy risk needs concurrent exclusion).
5. After the report is issued, prioritize transferring an embryo that is both chromosomally structurally normal and euploid; if no normal embryo is available, discuss the possibility of transferring a balanced carrier embryo with the genetic physician.
6. Pregnancy test 12-14 days after transfer; if pregnancy is confirmed, undergo amniocentesis for karyotype analysis + SNP array verification at 18-20 weeks of gestation.

The entire process from the initial visit to obtaining a transferable embryo typically takes 3-5 months (including feasibility study, stimulation, testing, and transfer preparation).

Risk Reminder: PGT-SR is an invasive embryo testing technique. Although its impact on embryo development is minimal, it cannot guarantee 100% testing accuracy. All test results must be confirmed through genetic counseling and prenatal diagnosis. Reproductive decisions for carriers of chromosomal structural abnormalities should be based on comprehensive medical evaluation and personal wishes, and it is recommended to proceed under the guidance of a qualified reproductive medicine center and genetics team.