In the fast-moving mammal cloning field, creating just one duplicate is a feat from a bygone era. Generating dozens of clones is the new cutting edge.
An international team assembled at the University of Hawaii has developed a technique that turns adult mouse cells into cloning factories, where clones can turn around and produce more clones, creating several generations of cloned mice. The researchers hope their method proves to be a breakthrough for animal breeding and basic scientific research.
"We succeeded both in using a new method and new cell type to clone mice from adult cells and in repeating it to produce clones of clones of clones, essentially identical mice born a generation or more apart," said Teruhiko Wakayama, a Japanese researcher who helped develop the technique.
The cloning method, called the Honolulu technique, used cumulus cells, which surround the developing eggs inside the ovaries of female mice and are shed with the eggs during ovulation. Wakayama and his colleagues took the nucleus out of an adult mouse cell and injected it into the egg of another mouse, which also had its nucleus removed. The researchers grew the clone and then cloned it. The team has created about 50 clones using the method.
Other cloning methods have not used nuclear transfer. Scientists at Scotland's Roslin Institute, who created the cloned sheep Dolly, used electric current to fuse a cell from a sheep's mammary gland with an egg from another adult sheep. The egg's nucleus was removed before fusing. The method yielded only one clone.
In the livestock industry, the ability to produce several clones from the same material raises the possibility that a good milk or beef cow can be reproduced in perpetuity.
Last August, researchers at ABS Global created a clone of a bull calf by using an electrical charge to fuse the contents of a cow fetal cell with the contents of an egg cell. The fused cell became a factory for the cow's genetic material because, through cell division, each subsequent cell carried the same chromosomal information as the original fetal cell.
The Honolulu method might work better for producing genetically engineered clones because it is easier to do, said Ryuzo Yanagimachi, a scientist at the University of Hawaii. He also thinks the method might work on larger animals such as cattle, pigs, and sheep.
Researchers at PPL Theraputics agree. The London-based biotechnology company that collaborated with the Roslin Institute to produce Dolly announced Wednesday it had licensed the Honolulu cloning technology for research on cloning pigs and other animals.
With any cloning method, the question remains whether the clone will inherit the genetic baggage carried by the adult whose cell was fused in the process. By the time an animal becomes an adult, its cells have divided thousands of times, differentiating along the way. The differentiation is cumulative as certain genes over time are turned off, causing cells to age. By using embryonic or fetal cells, researchers hope to clean the adult cell's slate.
"We have had to turn back the clock of an adult cell so that it behaves like a newly fertilized embryo, which would develop into a normal adult," said Anthony Perry, a researcher with the Babraham Institute in Cambridge, England, who was part of the Honolulu team.
The scientists hope the continued success of the mice experiment will allow researchers to determine the biological factors that contribute to successful cloning. They also hope to look at how the donor nucleus from a specialized cell, such as the cumulus, is somehow reprogrammed by the egg cell's cytoplasm, allowing it to behave like a newly fertilized embryo and grow into all the different cell types in the animal body.
Success in cloning any species is relative. Researchers at the Roslin Institute had to endure 228 failures to finally get the pregnancy that produced Dolly. ABS Global needed 18 cow pregnancies to yield its bull calf, Gene, last August. For their part, the scientists in Hawaii report a success rate ranging from 1-in-40 to 1-in-80 survivors for every embryo implanted.
Nevertheless, the researchers are confident enough about the Honolulu technique to deal with commercial production matters. The team has licensed its technology to ProBio America, a venture capital company in Honolulu.
Reuters contributed to this report.
