The importance of imprinting in cloning

Abstract

Cloning using nuclear transfer technology is an inefficient process in which most clones die before birth and survivors often display growth abnormalities. This is attributed to genetic imprinting – particular genes in the donor nucleus are permanently turned on or off. Imprinting appears to vary between donor nuclei, hence producing inefficiencies in the cloning process. In an effort to correlate gene expression with survival and foetal overgrowth, David Humphreys et al. (Science, 6 July) examined imprinted gene expression in both mice cloned by nuclear transfer and in the embryonic stem (ES) cell donor populations from which they were derived. Expression in the ES cell genome was found to be extremely unstable, as was the case with most of the cloned mice. What is most astonishing, is that despite widespread gene disregulation, many of the animals survived to adulthood, indicating a robust tolerance in mammalian development. The implication of the work is that even apparently normal cloned animals might have subtle abnormalities in gene expression. DM