Last update: August 2011
Contact: Theresia Volhard
Research cloning (also commonly known as "therapeutic cloning" in the public debate) refers to the derivation of embryos purely for research purposes. In contrast to reproductive cloning, in which a complete organism capable of life is created, the embryo produced in research cloning is destroyed in order to obtain embryonic stem cells. In the long term, the goal is to develop therapies for previously incurable diseases with the aid of such stem cells. The following "In Focus" issue provides an overview of cloning methods, the central areas of ethical debate and the various international and national regulations relating to the question of research on human embryos and the derivation of such embryos.
I. Medical and scientific aspects
The term "cloning" covers a variety of methods and techniques. Their point of departure is at the molecular level of DNA (deoxyribonucleic acid), which in chemical terms constitutes the genetic material. All cloning techniques share the common goal of creating a genetically identical duplicate: a DNA fragment or molecule, a cell (see module Cloned Cells for Disease Control), a tissue or indeed an entire organism. In all living organisms - with the exception of bacteria - sexual reproduction takes place through the formation of germ cells (sperm cells and egg cells [oocytes]) with a new combination of paternal and maternal DNA, thereby giving rise to a new genome. Cloning methods, on the other hand, are geared to a form of asexual or vegetative reproduction in which the genome of the organism in question is duplicated. There is no recombination of genes, but instead a genetically identical or virtually identical copy of the original is created. It should, however, be noted that in many lower animals and most plants this is a common form of reproduction alongside sexual reproduction. In principle, the formation of identical multiples also occurs naturally in humans in the form of single-egg twins (monozygotic twinning), although only in the context of sexual reproduction. In the laboratory organisms can be artificially cloned in two ways: by means of embryo splitting (see module Embryo Splitting) or through creation of an embryo by way of cell nuclear transfer. In the case of cloning for research purposes, so-called "therapeutic cloning" (see module The Concept of Therapeutic Cloning), embryos are obtained using the procedure of cell nuclear transfer (see module Cell Nuclear Transfer), i.e. the embryos are created by transferring the cytoblast (cell nucleus) of an adult somatic cell to an enucleated oocyte (i.e. an egg cell from which the nucleus has been previously removed).
The embryo that develops following cell nuclear transfer is virtually identical genetically (see module Genetic Identity) to the donor of the transferred cytoblast. After it has been produced, however, the embryo is not implanted into a woman's uterus in order to initiate a pregnancy; instead, it is destroyed in an early stage of embryonic development (blastocyst stage) so that embryonic stem cells (see module Derivation of Human Embryonic Stem Cells) (ESCs) can be removed. These can differentiate into particular cell types in vitro through the introduction of specific growth factors, and then - so it is hoped - be transferred back to the donor organism for therapeutic purposes. The successful derivation of stem cells from pre-cloned primate embryos (see module Stem Cells From Primate Embryos) was first described in November 2007. In March 2008 the therapeutic use of stem cells from cloned parkinsonian mice embryos (see module Cloned Parkinsonian Mice Embryos) was described. Out of skin tissue of the diseased mice embryos were cloned from which stem cells were extracted which were then differentiated into specific neurones. The neurones were injected into diseased donated mice which then showed significant alleviation the illness symptoms.
In January 2008 an American group of researchers including Andrew French for the time succeeded in gaining human embryos through cloning (see module Human Cloning). For this purpose a nucleus of an adult skin cell was transferred into an enucleated egg cell.
ES cells are of interest for research purposes because they have the ability - under the appropriate conditions - to grow into virtually all types of somatic cells. This capability is normally referred to as pluripotency (see module Pluripotency and Totipotency). It remains contentious if stem cells that are obtained through cloning can become totipotent cells as well. An experimental verification of totipotency is morally impermissible, since it would be necessary to grow a complete organism. The hope is that the insights gained from stem cell research can be applied in the future to produce tissues or entire organs for transplant purposes (see module Transplantation Purposes). In order to preclude from the very outset the possibility of the recipient's immune system rejecting the transplanted tissue, the aim is to use cloning techniques to produce stem cells that are genetically identical to the cytoblasts of the transplant recipient and can therefore be used as starting material for the creation of genetically identical tissue.
In the long run, the goal of research cloning is the production of autogenic stem cells for therapeutic ends. These kinds of stem cells would possess to the greatest possible extent the same genetic features as those of the patients who receive therapy.
Although there would be many positive effects of a successful research, such as a certain guarantee of transplant availability, there might still be some risks associated with the use of cloned stem cells. The use of cloned stem cells for therapeutic purposes can possibly induce the growth of tumors. It still needs to be determined how to minimize or abandon those risks completely.
For more information on stem cell research please see the "In Focus" issue on "Human embryonic stem cell research".