RNAseq analysis of human monozygotic twins at the blastocyst stage - a case study

Discordant growth is a common complication of monochorionic/diamniotic pregnancies; in approximately 50% of cases, the cause is unknown. The case presented here suggests that discordant growth of monozygotic twins could start during preimplantation development. Two inner cell masses (ICMs) within the same blastocyst may originate in uneven splitting of a single “parental” ICM or the two ICMs may be formed independently de novo. We studied the transcriptomes of two morphologically distinct ICMs within a single blastocyst, using high-resolution RNA sequencing. The data indicated that the two ICM were at different stages of development; one was in the earliest stages of lineage commitment, whilst the other had already differentiated into epiblast and primitive endoderm. IGF1-mediated signalling is likely to have a key role in ICM growth, and to be the major driver behind these differences.The expanded Day 6 blastocyst, donated for research, displayed two distinct inner cell masses (ICM) four hours post-thawing. Single cells and cell borders were clearly visible in ICM1, whereas the cells in ICM2 were more tightly packed and no single cell could be distinguished indicating that ICM2 is either better quality or more advanced than ICM1. To explore further this morphological discrepancy, we separated ICM1 and ICM2 from TE and by NGS evaluated the transcriptome of each of the three fractions.