Epigenetic Drift and the Generational Limit of Serial Somatic Cell Nuclear Transfer in Pigs

Somatic cell nuclear transfer (SCNT) in pigs has been a widely used technique for producing gene-edited pigs for biomedical research, yet its wide-spread application through serial cloning remains markedly limited. Unlike in mice, where the serial cloning can be sustained across numerous generations, in pigs it is usually limited to only a few rounds. Specifically, porcine serial cloning has not been reported beyond three consecutive generations in live-born offspring, with blastocyst development rates declining from approximately 4.4% in G1 to 1-5% in G2-G3, and live-birth cloning efficiency (offspring/recipient) dropping sharply with each successive round. Compelling evidence suggests that cumulative epigenetic instability, incomplete embryo genome activation, DNA methylation reprogramming, persistent donor-cell memory, and imprinting disruption collectively erode transcriptional integrity across generations. Although several manipulations, including epigenetic modifiers, transiently improved the early development, they failed to sustain the reprogramming across several generations. Here, we synthesize comparative advances in serial cloning across species and propose that species-specific differences in chromatin plasticity and cytoplasmic reprogramming capacity define a porcine "reprogramming ceiling". Deciphering and overcoming this barrier will be critical for advancing sustainable livestock engineering, xenotransplantation and translational medicine biotechnology.