Long before Dolly was cloned in 1996, scientists had already established a long history of cloning mammals. The first was a genetically identical mouse produced in 1979. Shortly thereafter the first genetically identical cows, chickens, and sheep were produced. What made Dolly a sensation, however, was the method by which she was cloned. Whereas the mammalian clones before her were produced by splitting an embryo in a test tube and then implanting them in surrogate mothers, Dolly was cloned from an adult cell. To be specific, an udder cell taken from a 6-year-old sheep. The cloning method, called somatic cell nuclear transfer (SCNT), involves taking the genetic material from the adult cell and placing it in the nucleus of an egg that has had its own genetic material removed.After Dolly, scientist have used SCNT to clone other mammals including cat, dog, deer, horse, mule, ox, rabbit and rat. That’s significant progress for a technique that Ian Wilmut had to employ 276 times before finally succeeding in cloning Dolly. But today’s scientists aren’t content to just clone once. For several years now attempts have been made to derive as many clones as possible from that one original piece of genetic material.
But there’s been problems. With each round of SCNT recloning, researchers quickly discovered, success rates dropped. In a study performed in 2000, the authors of the current work were able to clone a mouse to the sixth generation – but just barely. That final generation required more than 1,000 SCNT attempts and the sole pup that was born was promptly cannibalized by its mother. Repeatedly cloning cattle and cats went no further than the third generation.
Frustrated scientists attempted to find out why successive cloning was progressively problematic. They found that the original cell from which the clones were ultimately derived often had ‘epigenetic’ abnormalities. Epigenetic regulation refers to the turning on and off of genes by molecules, not the genes themselves. Any random cell could reasonably be expected to have some epigenetic abnormalities, but when all of the organism’s cells are derived from the same cell, whatever abnormalities that cell has will be magnified. For example, a series of cloned mice were shown to express an RNA molecule that inactivated one of the female’s X chromosomes. When the RNA molecule was removed cloning efficiency of the mice increased nearly nine-fold.Based on previous work, the Japanese researchers sought to improve their cloning efficiency by using a chemical called trichostatin A that inhibits the powerful epigenetic protein histone deacetylase. In an experiment that was begun in 2005, the inhibitor allowed them to produce 581 mice through 25 rounds of SCNT cloning. The mice were healthy and were able to reproduce. What’s more the cloning success rate did not decrease with each generation.
If the inhibitor is equally effective in other animals, the technique opens up the possibility of cloning highly-valued animals such as prized cattle or racehorses, or genetically modified animals used in medical research. As the authors note in the study: “Our results show that repeated iterative recloning is possible and suggest that, with adequately efficient techniques, it may be possible to reclone animals indefinitely.”
That’s good news for those that have already turned to cloning to create a small pack of super sniffing inspector dogs at airports, cows that produced humanized milk, even Olympic horses. Cloning remains a young science and scientists no doubt have a long list of organisms they would like to clone. If the current technique means limitless return on one’s cloning efforts, it could entice more scientists to take the first step, and bring cloning from the fringes of science to the mainstream.