CRISPR editing validation, immunostaining and DNA sequencing of individual fixed bovine embryos

Efficiency of embryo complementation and pluripotency maintenance following multiple passaging of in vitro-derived bovine embryos

Context Current methods to obtain bovine embryos of high genetic merit include approaches that require skilled techniques for low-efficiency cloning strategies. Aims The overall goal herein was to identify the efficacy of alternative methods for producing multiple embryos through blastomere complementation while determining maintenance of cell pluripotency. Methods Bovine oocytes were fertilised in vitro to produce 4-cell embryos from which blastomeres were isolated and cultured as 2-cell aggregates using a well-of-the-well system. Aggregates were returned to incubation up to 7days (Passage 1). A second passage of complement embryos was achieved by splitting 4-cell Passage 1 embryos. Passaged embryos reaching the blastocyst stage were characterised for cell number and cell lineage specification in replicate with non-reconstructed zona-intact embryos. Key results Passage 1 and 2 embryo complements yielded 29% and 25% blastocyst development, respectively. Passage 1 embryos formed blastocysts, but with a reduction in expression of SOX2 and decreased size compared to non-reconstructed zona-intact embryos. Passage 2 embryos had a complete lack of SOX2 expression and a reduction in transcript abundance of SOX2 and SOX17, suggesting loss of pluripotency markers that primarily affected inner cell mass (ICM) and hypoblast formation. Conclusions In vitro fertilised bovine embryos can be reconstructed with multiple passaging to generate genetically identical embryos. Increased passaging drives trophectoderm cell lineage specification while compromising ICM formation. Implications These results may provide an alternative strategy for producing genetically identical bovine embryos through blastomere complementation with applications towards the development of trophoblast and placental models of early development.

One-Step Homology Mediated CRISPR-Cas Editing in Zygotes for Generating Genome Edited Cattle

Selective breeding and genetic modification have been the cornerstone of animal agriculture. However, the current strategy of breeding animals over multiple generations to introgress novel alleles is not practical in addressing global challenges such as climate change, pandemics, and the predicted need to feed a population of 9 billion by 2050. Consequently, genome editing in zygotes to allow for seamless introgression of novel alleles is required, especially in cattle with long generation intervals. We report for the first time the use of CRISPR-Cas genome editors to introduce novel PRNP allelic variants that have been shown to provide resilience towards human prion pandemics. From one round of embryo injections, we have established six pregnancies and birth of seven edited offspring, with two founders showing >90% targeted homology-directed repair modifications. This study lays out the framework for in vitro optimization, unbiased deep-sequencing to identify editing outcomes, and generation of high frequency homology-directed repair–edited calves.