July 26, 2010 - The establishment of induced pluripotent stem cells (iPSCs) stands as a landmark achievement in the study of cell fate reprogramming, and has engendered tremendous hope for applications in drug discovery and the clinic. The original iPSC recipe, however, made use of a retrovirally-delivered factor, c-Myc, which was shown to be associated with tumorigenicity in mouse, raising important questions about the safety of these cells for medical uses. A number of methods for generating iPSCs without the use of c-Myc have subsequently been reported, but they have been plagued by low efficiencies and poor cell quality, and truly safe techniques for iPSC derivation remain elusive.

All that may change with a new study by Masato Nakagawa at the Kyoto University Center for iPS cell Research and Application (CiRA; Shinya Yamanaka, Director), which shows that the related molecule L-Myc can take c-Myc's place and generate safer iPS cells at higher efficiencies. This work, published in the Proceedings of the National Academy of Sciences, represents an important step closer to the goal of developing clinical applications for iPS cells in regenerative medicine.

Nakagawa began by testing the ability of all three members of the Myc family of molecules - c-Myc, N-Myc, and L-Myc - in iPSC generation, along with the other three conventional factors, Sox2, Oct3/4, and KLf4. In comparison to its other family members, the L-Myc protocol showed higher efficiency in establishing iPSC colonies, and lower tendency to generate colonies of non-pluripotent cells. Human iPSCs generated using L-Myc showed all the hallmarks of pluripotency, including morphology, gene expression, karyotype, and differentiative potency.

Generation of mouse iPSCs using L-Myc resulted in higher ratios of colonies expressing pluripotency markers, and yielded iPSCs with similar behavior, morphology and developmental potential to embryonic stem cells, including the ability to contribute to chimeras. And in perhaps the most important difference from the c-Myc method, iPSCs generated using L-Myc showed no increase in either tumorigenicity or mortality in chimeric progeny. The L-Myc protocol could even be modified to reduce the number of other factors required; Nakagawa found that by using L-Myc, he could eliminate the requirement for Sox2. Further studies using a number of mutant forms of c-Myc revealed that as long the mutant's ability to promote iPSC generation is not necessarily linked to its transformation activity, an important discovery in the quest for safer derivation methods.