Preimplantation Genetic Testing

Preimplantation genetic testing (PGT) tests embryos created by in vitro fertilisation for genetic changes, before an embryo is transferred to the uterus. It allows couples at risk of a serious inherited disease or chromosome problem to select embryos that are unaffected, and it avoids the need to consider termination of an affected pregnancy.

Three types exist. PGT-M tests for a known single gene disorder (monogenic). PGT-SR tests for unbalanced chromosome rearrangements in embryos of carriers of a balanced translocation or inversion. PGT-A tests for abnormal chromosome numbers (aneuploidy), mainly to improve implantation and live birth in selected groups.

The process starts with ovarian stimulation and egg collection, and fertilisation by intracytoplasmic sperm injection (to avoid contamination with extra sperm DNA). Embryos are cultured for 5 to 6 days to the blastocyst stage, and a few cells (5 to 10) are biopsied from the trophectoderm, which forms the placenta. The cells are amplified and tested, and the embryos are frozen (vitrified) while awaiting results. An unaffected embryo is transferred in a later frozen cycle.

PGT-M uses direct testing of the family variant together with linked markers across the gene (haplotyping), to reduce errors. PGT-A and PGT-SR use next generation sequencing or arrays to count chromosomes. The test takes 1 to 3 weeks.

The accuracy is high, with a reported misdiagnosis risk under 1%, though it is not zero. The live birth rate per embryo transfer depends mainly on the age of the woman, and is about 40 to 60% for euploid embryos from women under 35, and lower in older women. The number of embryos available is the main limit.

Indications.

• Couples at risk of a serious single gene disease: cystic fibrosis, spinal muscular atrophy, thalassaemia, sickle cell disease, Huntington disease, Duchenne muscular dystrophy, Fragile X syndrome, hereditary cancer syndromes (for example, BRCA1 and BRCA2), and many others.
• Carriers of balanced chromosomal translocations or inversions, with recurrent miscarriage or infertility.
• Selection of embryos with a compatible tissue type (HLA matching) for a sibling with a serious disease, in places where this is permitted.
• PGT-A in selected women with advanced age, recurrent implantation failure or recurrent miscarriage, though the benefit is debated.
• Sex selection for X-linked disorders when the specific gene variant is not known.

Not indicated for routine use in all in vitro fertilisation cycles, or for trivial or late onset conditions without clear medical grounds, where laws and ethics committees set limits.

Specimen requirements.

• Biopsied trophectoderm cells (5 to 10) from a blastocyst, with embryos frozen until results are available.
• For PGT-M, DNA samples from the couple, and from affected and unaffected relatives, to design the test (family phasing) before the cycle starts. This workup takes 4 to 12 weeks.
• For PGT-SR, karyotype and breakpoint data of the carrier.
• Counselling and written consent covering success rates, risks of the procedure, the possibility of no transferable embryos, the need for confirmatory prenatal testing, and legal and ethical limits.

Normal appearances.

A transferable embryo is unaffected (PGT-M) or has a normal or balanced chromosome complement (PGT-SR, PGT-A), and has good morphology. A normal result lowers risk and does not abolish it.

Interpretation.

• PGT-M: each embryo is reported as unaffected, affected, carrier (for recessive and X-linked conditions) or inconclusive, and unaffected and, where the couple chooses, carrier embryos are transferred.
• PGT-SR: embryos are classed as balanced or normal, or unbalanced. Only balanced or normal embryos are transferred.
• PGT-A: embryos are reported as euploid, aneuploid or mosaic (a mixture of normal and abnormal cells). Euploid embryos are transferred first, and mosaic embryos may be transferred after counselling, since many give healthy babies.
• Failed amplification or an inconclusive result: a second biopsy of the same embryo is possible but rarely done, and these embryos are not transferred without careful counselling.
• After transfer, a pregnancy is confirmed by ultrasound, and prenatal diagnosis (chorionic villus sampling or amniocentesis) is usually offered to confirm the result, because PGT is not completely accurate.
• The couple is told how many embryos were tested, how many are transferable, and the expected chance of a live birth.

Critical values and findings needing immediate action.

• A transfer of an embryo found later to be affected, because of a laboratory error: urgent disclosure, counselling and prenatal testing.
• Severe ovarian hyperstimulation syndrome after stimulation (abdominal distension, rapid weight gain, breathlessness, low urine output): urgent medical care.
• An abnormal prenatal diagnosis result after a PGT pregnancy: prompt counselling.
• No transferable embryos: support and review of options (another cycle, donor gametes, prenatal diagnosis in natural conception).

Limitations and artefacts.

• Whole genome amplification from a few cells causes errors, including allele dropout (failure to amplify one allele), amplification failure, and contamination.
• Mosaicism: the biopsy may not represent the whole embryo, so a normal result can be wrong and an abnormal one may not mean an abnormal baby.
• Recombination and de novo variants can lead to errors in linkage based testing, and the test cannot detect new variants that were not tested.
• Misdiagnosis is rare (under 1%), so prenatal confirmation is usually offered.
• The biopsy itself carries a small risk of damaging the embryo. Whether PGT affects later health is under study, with reassuring findings so far.
• PGT-A has not been shown to improve live birth rates across all groups, and its use has been questioned.
• Success depends on the number and quality of embryos, and many cycles do not give a transferable embryo.
• Costs, access to in vitro fertilisation, emotional strain, and legal and ethical limits vary between countries.

Special groups.

• Women of advanced maternal age: fewer embryos, and lower success rates, with careful counselling on the benefit of testing.
• Couples with a known balanced translocation: PGT-SR reduces miscarriage, although natural conception remains possible.
• Couples at risk of a severe disease who object to termination: PGT offers a way to have an unaffected child.
• Couples with a recessive disease and a carrier partner: PGT-M requires a detailed workup.
• Patients with cancer facing fertility loss: fertility preservation followed by PGT when the time comes.
• Regions: in vitro fertilisation and PGT are available in a few private centres, at high cost, and many countries have laws that restrict their use. For common recessive disorders (thalassaemia, sickle cell disease), carrier screening and prenatal diagnosis are more accessible options. Ethical, religious and legal views differ, and counselling takes these into account.

Follow up tests.

• Prenatal diagnosis (chorionic villus sampling or amniocentesis) to confirm the result.
• Early pregnancy ultrasound, and foetal anomaly scans.
• Non-invasive prenatal testing as an additional screen, which is not diagnostic for single gene disease.
• Further carrier testing and cascade testing of relatives.
• Psychological support and counselling.
• Follow up of children born after PGT.
• Genetic counselling before further cycles.