Inter-Species Therapeutic Cloning: The Looming Problem of Mitochondrial DNA and Two Possible Solutions

95 THE CONCEPT of therapeutic cloning, proposed by Gurdon and Colman,1 entails the synthesis of tissues or organs for transplantation into a patient starting from embryonic stem (ES) cells that are genetically identical to that patient, due to having been derived from an oocyte whose nucleus was replaced by one taken from the patient (a process termed somatic cell nuclear transfer, SCNT). It remains the leading proposal for exploiting stem cell technology in the clinic in a manner that avoids the severe drawback of immune rejection. However, it presently faces three formidable hurdles. One, our ability to manipulate the differentiation of ES cells along a desired lineage, is progressively being overcome. The others, unfortunately, are more sociological than technical and are thus, perhaps, even greater. These are (a) the profound ethical concerns voiced by highly influential policy-makers and (b) the inadequate supply of human oocytes to permit synthesis of autologous tissues and organs at a rate comparable with demand. Two recent papers2,3 would seem to cast doubt on the feasibility of the only currently available proposal for escaping both these latter problems. In this perspective, I will explain that proposal, the problem with it that is highlighted by these papers, and two possible ways to avoid that problem. First it is appropriate to mention two other proposals, each of which avoids one of the sociological issues mentioned above but not the other. The supply problem might in principle be met by isolating large numbers of oocytes from females who have died in infancy, since the infant ovary contains many times more oocytes than in adulthood. However, this seems likely to encounter even more ardent ethical opposition than the use of oocytes donated by consenting adults. Conversely, the ethical problem may in principle be met by our recently developed ability to differentiate ES cells into oocytes,4 but the limited availability and variety of the cell lines presently authorised for federally funded work is potentially a barrier to high-volume production of such oocytes. The proposal that meets both objections is to use oocytes from non-human species. The idea here is simple. A non-human oocyte is made of non-human proteins, but when its nucleus is replaced by a human one and development is begun, those non-human proteins are progressively diluted and degraded, so that the ES cells that are the source of the eventual tissue or organ are entirely human in composition. Well, almost entirely—but not, in this protocol, quite. The problem is that not all our proteins are encoded in the nucleus: a paltry 13 of them are encoded in the mitochondrial DNA. Paltry in number but not in function: these proteins are essential subunits of the respiratory chain, without which the cell cannot use oxy