Panel: how do you know an iPS cell is an iPS cell?
The buzz around induced pluripotent stem (iPS) cells has shifted from how to make them to how to use them. But with more and more laboratories jumping into the field, how can scientists know other scientists (and journal editors) will trust results from the cells they made? In short, what are the minimum criteria for iPS cells?
Five scientists who have reprogrammed cells to pluripotency weighed in on this question. (George Daley from Harvard University; Alex Meissner from the Broad Institute; Kathrin Plath from UCLA; Kazuhiro Sakurada from iZUMI Inc; Shinya Yamanaka of Kyoto University; Junying Yu from the University of Wisconsin-Madison)
Yu answered the question first, citing expression of key antigens and a gene expression profile; Yamanaka said that the retroviral genes still necessary to make the cells needed to be silenced; Meissner thought some sort of genomewide screen examining the epigenetic status would be good. The ability to differentiate into different lineages seemed important.
But what about the traditional test the teratoma? Daley asked. Jun, when prompted, said the “only reliable” in vivo test is to inject cells in a mouse and wait for the formation of a teratoma (a tumor that makes cells representing all the major cells). But that can take two or three months, said Daley, and what does it mean?
Besides, Daley noted, teratomas aren’t all alike; the mix of tissues in those weird lumps varies, and some variation depends on how well the source cells were reprogrammed. Yamanaka showed that cell lines can form teratomas, even if they fail other tests of pluripotency. (To test that mouse cells are pluripotent, scientists use them to make chimeric mice and see if the supposed iPS cells can make sperm or eggs.)
So what test can supply good enough evidence? Plath seemed to speak for the group: “I don’t know,” she said, “but more than a teratoma.” On the other hand, the teratoma assay is accessible for researchers that may not have facilities for sophisticated genomewide screen. Rudy Jaenisch of the Broad Institute told me he thinks the assay is essential. “We cannot call them iPS cells lightly,” he told me. “We need a biological assay.”
It’s not surprising that it’s a matter of debate: it was only June last year that scientists showed definitively that mouse cells could be induced to pluripotency. The reports of human iPS cells followed several months later.
Before the first human iPS cells were produced, the NIH began wrestling with how to define pluripotency in human cells after an order from the Bush administration to replace the word “embryonic” with the word “pluripotent” in the NIH Human Embryonic Stem Cell Registry. ( Scientific definition by political request interviews scientists who later did make human iPS cells, plus several working on alternative strategies; afterward, some prominent scientists, including Yamanaka and Peter Andrews, provided thoughtful responses.)
Now the ISSCR may soon be taking up the question that had been forced on the NIH, only this time the motivation is scientific. When asked who had successfully made iPS cells in their labs, perhaps a dozen scientists in the audience raised their hands. (See Embryonic stem cells 2.0) These scientists need to know whether the “iPS” results from a colleague will apply in their own labs.
To know whether iPS cells flock together and, more importantly whether they can be used in place of embryonic stem cells for research, side-by-side experiments will need to be conducted on embryonic stem cells. These experiments are being done, and small differences between the cells have been identified. The open question is how much ES cells vary, how much iPS cells vary, and how much those variations matter.
PS:
*Last year, a large cooperative study found that different embryonic stem cell lines in culture are very similar. Smaller studies have found differences in how they differentiate into the heart, pancreatic, and neuronal lineages.
*Here are short articles covering the first human reprogramming work by Daley, Plath, Yamanaka and Thomson. The breakthrough in mouse reprogramming is reported here.
*Here are some others of interest: Daley on Lin28
Yamanaka on reprogramming stomach and liver cells
NOTE: In an earlier version of this blog, I'd reported that researchers were looking for criteria that could be used in lieu of teratoma assays. That was my interpretation after attending the session. Since then, a couple people told me they thought the discussion was about what criteria to used in addition to teratomas. I'll let you know what I find out.
