In the February issue of Nature Genetics, researchers, led by Dr. Rudolf Jaenisch at the Whitehead Institute for Biomedical Research, report that they have successfully used embryonic stem cells to clone mice with the highest efficiency to date. Their study comparing clones generated by two different donor strains suggests that the genetic make up of donor cells plays a key role in determining the viability of clones. In addition, the scientists show for the first time that it is possible to modify the genetic material in embryonic cells before using these cells to clone new animals. The new technology will improve cloning efficiency by providing researchers the genetic tools needed to understand why so few clones survive to term and grow into healthy adults. Most clones die during gestation or soon after birth, and there are many possible reasons for this, including genetic make up of the donor, the cell-cycle stage of the donor cell, loss of genetic information, or the inability of the egg cell to reprogram the donor cell nucleus.
Somatic cells have a limited lifespan, and they are difficult to manipulate genetically.
As a result, they make poor tools for studying the genetic basis of cloning efficiency. The technology of cloning using embryonic stem cells removes that obstacle, allowing researchers to study the various parameters that affect cloning efficiency, says Dr. Jaenisch. Embryonic stem cells normally direct the development of an entire animal. They can grow indefinitely in culture and can also be genetically manipulated to carry extra genes or knock-out genes under study. Thus they provide researchers the perfect genetic tool to study cloning.