Fragile X mental retardation 1 (FMR1) full-mutation expansion causes fragile X syndrome. Trans-generational fragile X syndrome transmission can be avoided by preimplantation genetic diagnosis (PGD). We describe a robust PGD strategy that can be applied to virtually any couple at risk of transmitting fragile X syndrome. This novel strategy utilises whole-genome amplification, followed by triplet-primed polymerase chain reaction (TP-PCR) for robust detection of expanded FMR1 alleles, in parallel with linked multi-marker haplotype analysis of 13 highly polymorphic microsatellite markers located within 1 Mb of the FMR1 CGG repeat, and the AMELX/Y dimorphism for gender identification. The assay was optimised and validated on single lymphoblasts isolated from fragile X reference cell lines, and applied to a simulated PGD case and a clinical in vitro fertilisation (IVF)-PGD case. In the simulated PGD case, definitive diagnosis of the expected results was achieved for all ‘embryos’. In the clinical IVF-PGD case, delivery of a healthy baby girl was achieved after transfer of an expansion-negative blastocyst. FMR1 TP-PCR reliably detects presence of expansion mutations and obviates reliance on informative normal alleles for determining expansion status in female embryos. Together with multi-marker haplotyping and gender determination, misdiagnosis and diagnostic ambiguity due to allele dropout is minimised, and couple-specific assay customisation can be avoided.

Current strategies for preimplantation genetic testing for aneuploidy or structural rearrangements (PGT-A/SR) rely mainly on next-generation sequencing (NGS) and microarray platforms, which are robust but require expensive instrumentation. We explored the suitability of third-generation single-molecule sequencing as a PGT-A/SR screening platform for both aneuploidy and segmental imbalance. Single-cell and multicell replicates from aneuploid or segmentally unbalanced cell lines (n = 208) were SurePlex-amplified, randomized, and subjected to (a) Nanopore-based single-molecule sequencing (Oxford Nanopore Technologies) and (b) NGS using a leading commercial PGT-A solution (Illumina VeriSeq PGS). Archival SurePlex-amplified trophectoderm biopsy samples (n = 96) previously analyzed using the commercial kit were blinded and reanalyzed using Nanopore. Nanopore-based PGT-A identified the specific aberration in 95.45% (84/88) and 97.78% (88/90) of single-/multicells with an aneuploidy or segmental imbalance (10–30.5 Mb), respectively. Comparison against the commercial kit’s results revealed concordances of 98.86% (87/88) and 98.89% (89/90) for the aneuploid and segmentally unbalanced (10–30.5 Mb aberration) samples, respectively. Detection sensitivity for smaller segmental imbalances (5–5.8 Mb aberration, n = 30) decreased markedly on both platforms. Nanopore-based PGT-A reanalysis of trophectoderm biopsy samples was 97.92% (94/96) concordant with the commercial kit results. Up to 24 SurePlex-amplified single-cell, multicell, or trophectoderm samples could be sequenced in a single MinION flow-cell for subsequent preimplantation genetic testing for aneuploidy or structural rearrangements (PGT-A/SR) analysis, with results obtainable in ≤3 days and at per-sample costs that are competitive with commercial offerings. Nanopore’s third-generation single-molecule sequencing represents a viable alternative to current commercial NGS-based PGT-A solutions for aneuploidy and segmental imbalance (≥10 Mb) screening of single-/multicell or trophectoderm biopsy samples.

Spinocerebellar ataxia type 3/Machado–Joseph disease (SCA3/MJD) is a neurodegenerative disorder caused by the ATXN3 CAG repeat expansion. Preimplantation genetic testing for monogenic disorders (PGT-M) of SCA3/MJD should include reliable repeat expansion detection coupled with high-risk allele determination using informative linked markers. One couple underwent SCA3/MJD PGT-M combining ATXN3 (CAG)_n triplet-primed PCR (TP-PCR) with customized linkage-based risk allele genotyping on whole-genome-amplified trophectoderm cells. Microsatellites closely linked to ATXN3 were identified and 16 markers were genotyped on 187 anonymous DNAs to verify their polymorphic information content. In the SCA3/MJD PGT-M case, the ATXN3 (CAG)_n TP-PCR and linked marker analysis results concurred completely. Among the three unaffected embryos, a single embryo was transferred and successfully resulted in an unaffected live birth. A total of 139 microsatellites within 1 Mb upstream and downstream of the ATXN3 CAG repeat were identified and 8 polymorphic markers from each side were successfully co-amplified in a single-tube reaction. A PGT-M assay involving ATXN3 (CAG)_n TP-PCR and linkage-based risk allele identification has been developed for SCA3/MJD. A hexadecaplex panel of highly polymorphic microsatellites tightly linked to ATXN3 has been developed for the rapid identification of informative markers in at-risk couples for use in the PGT-M of SCA3/MJD.

Combined preimplantation genetic testing of aneuploidy (PGT-A) and monogenic disease (PGT-M) can be achieved through PCR-based whole genome amplification (WGA) and next-generation sequencing (NGS). However, pathogenic variant detection is usually achieved indirectly through single nucleotide polymorphism haplotyping, as direct detection of pathogenic variants is not always possible. We evaluated whether isothermal WGA was suitable for combined PGT-A and PGT-M that also permitted direct detection of repeat expansions and large deletions, in addition to indirect linkage analysis using microsatellite markers. Five-cell replicates from selected cell lines were subjected to isothermal or PCR-based WGA, followed by NGS-based PGT-A and direct and indirect PGT-M of Huntington’s disease and spinal muscular atrophy. Both WGA methods accurately detected aneuploidy and large (10 Mb) segmental imbalances. However, isothermal WGA produced higher genotyping accuracy compared with PCR-based WGA for all analysed microsatellite markers (93.5% vs. 75.6%), as well as at the HTT CAG repeat locus (100% vs. 7.7%) and the SMN1/2 locus (100% vs. 71.8%). These results demonstrate that isothermal WGA is potentially ideal for combined PGT-A and PGT-M that permits both direct and indirect detection of pathogenic variants including repeat expansions and gene deletions.

Spinocerebellar ataxia type 2 (SCA2) is an autosomal dominant neurodegenerative disorder caused by a pathogenic CAG trinucleotide repeat expansion in the ATXN2 gene. At-risk couples can embark on unaffected pregnancies through preimplantation genetic testing of monogenic disorders (PGT-M) of SCA2, which should involve accurate repeat expansion detection together with risk haplotype tracking using informative linked markers. Two couples underwent SCA2 PGT-M involving analysis of whole genome amplified embryonic trophectoderm cells by ATXN2 (CAG)n triplet-primed PCR (TP-PCR) and linkage-based risk allele genotyping using customized markers. To simplify and expedite the identification of informative markers for future PGT-M cases, putative microsatellite markers closely linked to ATXN2 were initially screened for polymorphism using a small set of anonymous DNA samples obtained from Coriell Cell Repository. Shortlisted markers with high polymorphism likelihood were then multiplexed in a single-tube reaction and genotyped on 190 anonymous DNA samples to determine their polymorphic information content. Across both SCA2 PGT-M clinical cases, the linked marker genotypes corroborated the TP-PCR results, allowing clear differentiation between unaffected and affected embryos. In both cases, transfer of an unaffected embryo led to a successful pregnancy and live birth of a healthy baby. In silico mining, filtering, and curation identified 287 microsatellites located within 1.65 Mb of either side of the ATXN2 CAG repeat. Of these, eight upstream and nine downstream polymorphic markers were successfully co-amplified in a single-tube assay and demonstrated high overall heterozygosity in both Chinese and Caucasian populations. Conclusion: To ensure high diagnostic accuracy for PGT-M of SCA2, we developed a heptadecaplex microsatellite marker panel for haplotype-based linkage analysis to complement TP-PCR-based direct detection of the ATXN2 CAG repeat. The panel can rapidly identify informative markers from virtually any couple, and it works equally well on MDA-amplified DNAs for embryonic haplotype analysis.