A frozen mouse (top) and the cloned one (dark)

New Delhi, Nov. 3: Japanese scientists have produced clones of dead mice frozen for 16 years, demonstrating the first successful resurrection of the full genome of a long dead animal stored at the temperature of a supermarket freezer.

The researchers at the Centre for Developmental Biology, Kobe, thawed the bodies of the dead mice from minus 20°C, extracted genetic material from their brain tissues and generated cloned embryos.

Tweaking a conventional cloning method, a team led by biologist Teruhiko Wakayama used these cloned embryos to generate embryonic stem cells, and then used the nuclei from these embryonic stem cells to produce healthy cloned mice. The researchers have announced their feat in today’s issue of the journal Proceedings of the National Academy of Sciences.

In dead animals preserved in a frozen state — in the absence of special chemicals called cryoprotectants — ice crystals damage the cells and their nuclei which hold genetic material, or DNA.

The Japanese team has established a technique to collect nuclei from frozen and destroyed tissue and identified the brain as the best source of nuclei for cloning.

The first clone obtained from a dead mouse kept frozen for 16 years was born in November 2007. “The clones are alive and healthy, with normal reproduction,” Wakayama told The Telegraph today.

The researchers said their technique offered a chance to resurrect extinct animals or preserve endangered species as frozen tissues — although some technical challenges still need to be overcome to achieve this.

Scientists have cloned dead animals in the past — from meat refrigerated for two days and from dead mice stored at minus 80°C for up to a year. But until now, DNA frozen and degraded for so long has not yielded live cloned animals.

“The importance of this work is that the mice cadavers were stored at minus 20°C, a temperature of a standard household freezer,” said George Seidel Jr, professor of animal biotechnology at the Colorado State University.

Scientists caution that for logistical and ethical reasons, the research is unlikely to have any bearing on the cloning of humans. Reproductive cloning of humans is currently banned worldwide.

“It’s a red line that can’t be crossed,” said Azim Surani, professor of physiology and reproduction at the University of Cambridge, UK, who was not associated with the study.

“Besides ethical considerations, all evidence from animals shows that clones have numerous and unpredictable abnormalities. Different things go wrong,” Surani added.

Wakayama’s team transferred nuclei from embryonic stem cells into mice egg cells, allowing them to grow into tiny embryos implanted in surrogate mice mothers who delivered healthy clones. The colour of the coat, the sex, and a genetic analysis of the cloned mice showed they were derived from the donor — dead — mice.

Not all the clones had happy lives. One died from respiratory failure after birth, and another showed open eyelids at birth but was cannibalised by the foster mother. But two others grew to adulthood.

Scientists have hailed the work as an advance in cloning technology. “We’re now closer to the dream of producing extinct animals,” said Satish Totey, chief scientific officer at Stempeutics Malaysia, an expert on embryo science.

But researchers also caution that cloning still remains a highly inefficient process and any such effort will face technical challenges. “We’ve been able to get embryonic stem cells only from rodents, monkeys and humans. We don’t yet have embryonic stem cells from other species,” said Surani.

Seidal said suitable species would need to be identified to function as the source of egg cells that will play host to the genetic material of extinct animals and develop into cloned embryos. “The Indian elephant may be useful in a future effort to clone the (extinct) mammoth,” he said.

“But we also need to understand much better how a degraded genome can be used for cloning,” said Deepa Bhartiya, a scientist at the National Institute of Research in Reproductive Health, Mumbai.