Preimplantation genetic testing for facioscapulohumeral dystrophy caused by contractions of 4q35 D4Z4 repeats.

RESEARCH QUESTION: Preimplantation genetic testing for monogenic diseases (PGT-M) is based on mutation testing combined with linkage analysis; so, how should PGT-M for patients with facioscapulohumeral dystrophy type 1 (FSHD1) be conducted when direct mutation testing is unavailable and specific linked markers are lacking?

DESIGN

Patients with a definite FSHD1 diagnosis who intended to prevent the transmission of the condition to their children were recruited in the Reproductive and Genetic Hospital of CITIC-Xiangya, China between May 2021 and December 2023. Familial genetic risk was evaluated using Bionano, 4q haplotyping and whole-exome sequencing. Linkage markers near the causative 4q35 D4Z4 repeats were identified using targeted sequencing and MicroSeq. Haplotype analysis was conducted to determine affected and unaffected embryos. Embryo diagnosis rate was evaluated. Prenatal diagnosis through amniocentesis was offered at second trimester.

RESULTS

Six patients with FSHD1 were included. During the pre-examination process of PGT-M, a single nucleotide polymorphism 3.5 kb upstream of the affected region using MicroSeq was identified, which could reduce misdiagnosis caused by recombination. A 4q-haplotype fast-determination method was used for determining 4qA and 4qB haplotypes, which could serve as downstream linkage markers. During PGT-M process, 34 blastocysts were evaluated for six families, with a diagnosis rate of 100%. Prenatal diagnosis using Bionano showed a normal 4q35 genotype in four families. Four healthy babies were born after six embryo transfers for five women.

CONCLUSIONS

Selecting effective linkage markers for PGT-M in FSHD1-affected regions is challenging. Successful PGT-M for FSHD1 is reported, which is of great clinical significance for genetic counselling and reproductive intervention for this disease.