Integrated Epigenetic Reprogramming of Donor Cells and Reconstructed Embryos: A Proposed Framework for Improving Bovine Somatic Cell Nuclear Transfer

Somatic cell nuclear transfer (SCNT) can be used to propagate valuable cattle genotypes and to support genome editing, but its efficiency is still limited by incomplete epigenetic reprogramming. The donor nucleus maintains somatic chromatin and DNA methylation patterns that may interfere with embryonic genome activation, lineage specification and development. This review highlights key barriers to reprogramming and interventions used in donor cells, reconstructed embryos, or both, and discusses the strength and species specificity of the evidence. We propose a molecularly guided two-stage framework: donor cells are primed to overcome epigenetic resistance and then assessed after washout and recovery for viability, genomic integrity and target engagement. Only when a defined molecular defect persists is a transient embryo-stage intervention considered following nuclear transfer. Candidate targets include histone H3 lysine 9 trimethylation (H3K9me3), histone acetylation, DNA methylation, and abnormal X-inactive specific transcript (XIST) expression, although some approaches require further validation in cattle. Factorial experiments should separate the effects of each intervention and their interaction. Blastocyst yield is not the only measure of success; success must also be evaluated through molecular and developmental measures including placental function, pregnancy, live birth, and postnatal health. The integrated framework is not yet validated as a complete bovine protocol but provides testable criteria for improving SCNT efficacy and safety.