PREIMPLANTATION GENETIC TESTING: Single-cell technologies at the forefront of PGT and embryo research.

While chromosomal mosaicism in the embryo was observed already in the 1990s using both karyotyping and FISH technologies, the full extent of this phenomenon and the overall awareness of the consequences of chromosomal instability on embryo development has only come with the advent of sophisticated single-cell technologies. High-throughput techniques, such as DNA microarrays and massive parallel sequencing, have shifted single-cell genome research from evaluating a few loci at a time to the ability to perform comprehensive screening of all 24 chromosomes. The development of genome-wide single-cell haplotyping methods have also enabled for simultaneous detection of single-gene disorders and aneuploidy using a single universal protocol. Today, three decades later haplotyping-based embryo testing is performed worldwide to reliably detect virtually any Mendelian hereditary disease with a known cause, including autosomal-recessive, autosomal-dominant and X-linked disorders. At the same time, these single-cell assays have also provided unique insight into the complexity of embryo genome dynamics, by elucidating mechanistic origin, nature and developmental fate of embryonic aneuploidy. Understanding the impact of postzygotically acquired genomic aberrations on embryo development is essential to determine the still controversial diagnostic value of aneuploidy screening. For that reason, considerable efforts have been put into linking the genetic constitution of the embryo not only to its morphology and implantation potential, but more importantly to its transcriptome using single-cell RNA sequencing. Collectively, these breakthrough technologies have revolutionized single-cell research and clinical practice in assisted reproduction and led to unique discoveries in early embryogenesis.