Cloning nonuniformity

To the editor:

I would like to suggest another dimension to the ongoing discussion concerning developmental defects and high rates of mortality among animals produced by cloning (Nat. Biotechnol. 17, 405 & 733, 1999). This concerns the nature of X inactivation in cloned female animals. One would assume that a nucleus recovered from a cell in the G₀ phase would have one X-inactivated chromosome (X*), and that, during the subsequent manipulations that occur during nuclear transplantation and early embryo development, one of the following would occur: (1) Both Xs are reactivated and then random X inactivation occurs as per normal; (2) both Xs are reactivated, but at a “slower” rate than normal and the subsequent inactivation may be delayed with the result that larger than normal chimeric clones of each X* develop; (3) the reactivation of X* may not occur at all, and the cloned animal may be “imprinted” for one or the other of the Xs as per the original cell from which the nucleus was transplanted—resulting in the same “imprinted” X-chromosome phenomenon as in marsupials; (4) faulty inactivation and reactivation occurs, possibly with some cells in the cloned animal expressing both X alleles.

Each of these scenarios raises some interesting possibilities concerning both the “clonality” and the health of the cloned animal. Thus, even for scenario (1), the animal would obviously not be a clone of the donor, especially if the animal from which it was derived was not inbred. Obviously, this would also be the case for scenario (3). For scenario (4), it is possible that the cloned animal could have some XX-dosage problems that, depending on the extent and tissue locality of the phenomenon, could have serious health consequences (cf. the well-documented problems in human genetics).

Awareness of these potential problems may well enable breeding and manipulation strategies to be developed that avoid the more serious complications that have been reported in some cloning experiments. One would assume that these problems would not occur in male clones.