Epigenetic reprogramming in mammalian species after somatic cell nuclear transfer based cloning

The birth of “Dolly”, the first mammal cloned from an adult mammary epithelial cell, abolished the decades` old scientific dogma implying that a terminally differentiated cell cannot be reprogrammed into a pluripotent embryonic state. The most dramatic epigenetic reprogramming occurs in somatic cell nuclear transfer (SCNT) when the expression profile of a differentiated cell is abolished and a new embryo-specific expression profile, involving 10,000–12,000 genes and thus the majority of the genes of the entire genome, is established which drives embryonic and fetal development. The initial release from somatic cell epigenetic constraints is followed by establishment of post-zygotic expression patterns, X-chromosome inactivation, and adjustment of telomere length. SCNT may be associated with a variety of pathological changes of the fetal and placental phenotype in a proportion of cloned offspring, specifically in ruminants, that are thought to be caused by aberrant epigenetic reprogramming. Improvements in our understanding of this dramatic epigenetic reprogramming event will be instrumental in realizing the great potential of SCNT for basic research and for important agricultural and biomedical applications. Here, I review the current knowledge on epigenetic reprogramming after use of SCNT in livestock, with emphasis on gene specific and global DNA methylation, imprinting, X-chromosome inactivation and telomere length restoration in early development.