PHILADELPHIA -- Scientists at the University of Pennsylvania have found a novel way to boost the paltry survival rate of cloned mammals: When two genetically identical cloned mouse embryos are combined, the aggregate embryo is considerably more likely to survive to birth.

A team from Penn's School of Veterinary Medicine reports the results in the Oct. 1 issue of the European Molecular Biology Organization Journal.

"At the blastocyst stage, an early embryonic stage just prior to implantation, mouse clones typically have a much lower than normal number of cells," said corresponding author K. John McLaughlin, assistant professor of animal biology.  "When we combined two clones at the four-cell stage, the embryos showed a remarkable improvement in viability, much greater than expected from the sum of their parts."

Despite the successful cloning of sheep, pigs, cats and most recently rats, mammalian cloning -- in which an ordinary cell's nucleus is transferred to an egg whose nucleus has been removed -- remains remarkably inefficient.  Of every 100 cloned mice, roughly one survives to birth.

The researchers found that when the clone hybrids were transferred back into the uteri of recipient mice, the survival rate jumped to 8 percent.  The researchers even produced a litter of four cloned mouse pups, in stark contrast to the typical single pup born.

Cloning requires the precise genetic reprogramming of the nucleus inserted into an enucleated egg.   This nucleus must abandon its former genetic program and adopt the genetic profile of an embryonic nucleus; failure to do so dooms the embryo.

"The paper provides a new insight into reprogramming following nuclear transfer," said Davor Solter, a developmental biologist at the Max-Planck Institute of Immunobiology who was not involved in this work.  "It confirms indirectly that every cloned embryo is actually different and that reprogramming is random.  It seems that two embryos which are epigenetically different can positively interact and complement each other leading to correct temporal and spatial gene expression.  That this type of interaction can take place was not obvious and it could only be demonstrated by the described approach."

McLaughlin and his colleagues aren't yet sure why the aggregation of cloned embryos boosts survival, although one theory is that the combination of two embryos helps compensate for genetic deficiencies in either.  

"The genetic reprogramming of a cloned embryo never seems to occur with 100 percent accuracy," he said.  "However, the group of genes that fails to reset properly differs in each individual embryo, meaning that each embryo that contributes to an aggregate can help mask the shortcomings of the other.  By combining cloned embryos, you might end up with an embryo that's 99 percent reprogrammed rather than just 90 percent."

When McLaughlin and colleagues cut wild-type mouse embryos in half, they found that the expression of key developmental genes was not affected, suggesting that the developmental deficiencies of cloned embryos are not due to low cell counts alone.  They speculate that cells in a blastocyst may communicate in a way that is compromised in a smaller cloned embryo.

McLaughlin's co-authors on the EMBO Journal paper include Michele Boiani, Sigrid Eckardt, N. Adrian Leu and Hans R. Scholer, all of Penn's Center for Animal Transgenesis and Germ Cell Research. Their work was funded by the Marion Dilley and David George Jones Funds, the Commonwealth and General Assembly of Pennsylvania, the National Institutes of Health, the University of Pennsylvania Research Foundation and the United States Department of Agriculture.