Comparing two cellular reprogramming methods from one donor’s cells yields good news for iPSCs

In 2012, a mere six years after his discovery of induced pluripotent stem cells (iPSCs), Shinya Yamanaka was awarded the Nobel Prize in Medicine. Many Nobel winners aren’t recognized until decades after their initial groundbreaking studies. That goes to show you the importance of Yamanaka’s technique, which can reprogram a person’s cells, for example skin or blood, into embryonic stem cell-like iPSCs by just adding a small set of reprogramming factors.

These iPSCs are pluripotent, meaning they can be specialized, or differentiated, into virtually any cell type in the body. With these cells in hand, researchers have a powerful tool to study human disease and to develop treatments using human cells directly from patients. And at the same time, this cell source helps avoid the ethical concerns related to embryonic stem cells.

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Induced pluripotent stem cell (iPSC) colonies.
Image Credit: Joseph Wu

Still, there has been lingering uneasiness about how well iPSCs match up to embryonic stem cells (ESCs), considered the gold-standard of pluripotent stem cells. One source of those concerns is that the iPSC method doesn’t completely reprogram cells and they retain memory of their original cell source, in the form of chemical – also called epigenetic – modifications of the cells’ DNA structure. So, if a researcher were to make, say, heart muscle cells from iPSCs that have an epigenetic memory of its skin cell origins, any resulting conclusions about a given disease study or cell therapy could be less accurate than ESC-related results. But a report published yesterday in PNAS should help relieve these worries.

The CIRM-funded study – a collaboration between the labs of Joseph Wu and Michael Synder at Stanford University and Shoukhrat Mitalipov at Oregon Health & Science University – carried out an exhaustive series of experiments that carefully compared the gene activity and cell functions of iPSC-derived cells with cells derived from embryonic stem cells. The teams sought to compare cells generated from the same person to be sure any differences were not the result of genetics. To make this “apples-to-apples” comparison, they generated embryonic stem cells using another reprogramming technique called somatic cell nuclear transfer (SCNT).

With SCNT, a nucleus from an adult cell is transferred to an egg which has its own nucleus removed. The resulting cell becomes reprogrammed back into an embryo from which embryonic stem cells are generated – the researchers call them NT-ESCs for short. In this study, the skin cell sample used for making the iPSCs and the cell nucleus used for making the NT-ESCs came from the same person. In scientific lingo, the iPSCs and SCNT stem cells are considered isogenic.

Now, it turns out the NT-ESC reprogramming process is more complete and eliminates epigenetic memory of the original cell source. So why even bother with iPSCs if you have NT-ESCs? There are big disadvantages with SCNT: it’s a complex technique – only a limited number of labs pull it off – and it requires donated human eggs which carries ethical issues. So, if a direct comparison iPSCs and SNCT stem cells shows little difference then it would be fair to argue that iPSCs can replace NT-ESCs for deriving patient-specific stem cells.

And that’s exactly what the teams found, as Dr. Wu summarized it to me in an interview:

“Direct comparison between differentiated cells derived from iPSCs and SCNT had never been performed because it had been difficult to generate patient-specific ESCs by the SCNT method. Collaborating with Dr. Shoukhrat Mitalipov at Oregon Health & Science University and Dr. Michael Snyder at Stanford University, we compared patient-specific cardiomocytes (heart muscle cells) and endothelial (blood vessel) cells derived by these two reprogramming methods (SCNT and iPSCs) and found they were relatively equivalent regarding molecular and functional features.”

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Blood vessel cells derived by iPSC (left) and SCNT (right) reprogramming methods.
Image credit: Joseph Wu

Because the heart muscle and blood vessel cells were similar regardless of reprogramming method, it suggests that the epigenetic memory that remained in the iPSCs is less of a worry. Dr. Wu explained to me this way:

joewu

Joseph Wu

“If iPSCs carry substantial epigenetic memory of the cell-of-origin, it is unlikely these iPSCs can differentiate to a functional cardiac cell or blood vessel cell. Only the stem cells free of significant epigenetic memory can differentiate into functional cells.”

 

Hopefully these results hold up over time because it will bode well for the countless iPSC-related disease studies as well as the growing number of iPSC-related projects that are nearing clinical trials.

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More than Meets the Eye: Stem Cells Generated using Different Methods Produce Different Types of Cells

What’s the best way to make a fully versatile, ‘pluripotent,’ stem cell? Three different methods each have their pluses and minuses. But now new research has found that the stem cells created by each method, while similar on the surface, show vast differences.

The findings, published online today in the journal Nature, reveal new insights into stem cells’ underlying cellular machinery—which is of utmost importance as researchers transform their discoveries from the lab and into much-needed therapies for patients.

Scanning electron micrograph of cultured human neuron from induced pluripotent stem cell.  [Credit: Mark Ellisman and Thomas Deerinck, National Center for Microscopy and Imaging Research, UC San Diego]

Scanning electron micrograph of cultured human neuron from induced pluripotent stem cell. [Credit: Mark Ellisman and Thomas Deerinck, National Center for Microscopy and Imaging Research, UC San Diego]

Stem cells have held promise for regenerating tissues, or even organs, lost or damaged by injury or disease. This is due to stem cells’ ‘pluripotency’—their ability to transform into virtually any cell in the body. Initially, scientists used stem cells extracted from unused embryos that consenting couples had donated to research. But the use of these so-called embryonic stem cells, or ES cells, has since been limited due to ethical considerations and early limits to federal funding.

So scientists have been on the hunt for an alternative method of creating pluripotent cells. And so far, they have come up with two.

One, called somatic cell nuclear transfer (SCNT) takes the genetic material of an adult cell and transplants it into an unfertilized egg. The second method transforms adult cells, such as skin or blood, back into embryonic-like stem cells—called induced pluripotent stem cells, or iPS cells—by manipulating various genes.

Each of the newer methods has its pluses and minuses—but which produces cells that most closely resemble ES cells, still considered the “Gold Standard” in stem cell biology? Since the success of the SCNT technique is so recent, no one had taken a close look until now. So a collaboration of researchers from the University of California, San Diego (UCSD), The Salk Institute for Biological Sciences and Oregon Health & Science University (OHSU), compared the two methods side by side. And what they found was surprising.

Dr. Louise Laurent, co-senior author from UCSD, explained in today’s news release:

“The nuclear transfer ES cells are much more similar to real ES cells than the iPS cells. They are more completely reprogrammed and have fewer alterations in gene expression and DNA methylation levels that are attributable to the reprogramming process itself.”

iPS cell technology, which was pioneered in 2006 by Shinya Yamanaka, offers a series of advantages over traditional ES cells. As Laurent continued:

“The ability to make personalized iPS cells from a patient that could be transplanted back into that patient has generated excitement because it would eliminate the need for immunosuppression.”

iPS cells have generated so much excitement, in fact, that Yamanaka was awarded the 2012 Nobel Prize in Physiology or Medicine for developing this technique.

The SCNT method was developed more recently by OHSU’s Dr. Shoukhrat Mitalipov. The current researchers generated lines of cells using both methods. After confirming that each line was, in fact, pluripotent, they used advanced genomics techniques to examine the biochemical process called ‘DNA methylation’ in each line.

DNA methylation is a fundamental chemical process within each cell. It’s responsible for switching key genes on and off at precise intervals. In recent years, researchers have discovered that the order and timing of this process is vital for the correct development of the cell. As Dr. Joseph Ecker, co-senior author from the Salk Institute, explained:

“If you believe that gene expression and DNA methylation are important, which we do, the closer you get to the patterns of embryonic cells, the better. Right now, nuclear transfer cells look closer to the embryonic stem cells than do the iPS cells.”

However, while the scientists confirmed that SCNT cells more closely resemble ES cells, the process of producing them is far from ideal. First, the SCNT method is technically difficult. And second, federal funds still cannot be used in this procedure—representing a significant hurdle to being widely adopted.

On the other hand, iPS cell generation is, by comparison, a much easier process technically. So perhaps these findings can spur the development of an improved method, taking the technological ease of iPS cell generation and marrying it with the accuracy of the SCNT method. Laurent argues that this could yield a new and improved approach:

“Our results have shown that widely used iPS cell reprogramming methods make cells that are similar to standard ES cells in broad strokes, but there are important differences when you look really closely. By using the egg cell to do the job, we can get much closer to the real thing. If we can figure out what factors in the egg drive the reprogramming process, maybe we can design a better iPS cell reprogramming method.”

Guest blogger Alan Trounson — April’s stem cell research highlights

Each month CIRM President Alan Trounson gives his perspective on recently published papers he thinks will be valuable in moving the field of stem cell research forward. This month’s report, along with an archive of past reports, is available on the CIRM website.

This month’s report includes an important review of studies using bone marrow stem cells for heart disease that showed some disturbingly sloppy handling of the data. It will be critical for the field to address this issue, particularly given the highly variable and often inclusive results of those studies.

But, since this is my last post as President of CIRM, I want it to address some good news in April’s stem cell literature. Two teams reported replicating the breakthrough success of Oregon’s Shoukhrat Mitalipov in creating human embryonic stem cell lines through somatic cell nuclear transfer (SCNT), also known as therapeutic cloning. Replication of results remains a cornerstone of science and it is reassuring to see two teams independently replicate the results of the Oregon team in less than a year.

SCNT, which requires placing the nucleus of a mature cell into an egg that has had its own nucleus removed, has been relatively easy to accomplish in lower mammals but impossible in humans until the Oregon success. But that work got the donor nucleus from fetal tissue or a recently born baby, so it left open the question of whether the procedure would work with a less malleable cell from an older adult.

One of the teams that repeated the work, based in Korea, created two stem cell lines, one from a 35-year-old and one from a 75-year-old. They used largely the same procedure as the Oregon team but made one key change in the protocol. Instead of waiting just 30 minutes to stimulate the egg to divide after fusing the donor nucleus and the egg, they waited two hours. They speculated that this longer incubation may have helped reprogram the genes of the older cells to behave like younger ones.

The second team, at the New York Stem Cell Foundation used a somewhat different procedure to create embryonic stem cell lines including one from the skin of a 32-year-old with diabetes. That became the first disease-specific cloned stem cell line. They modified a procedure they used to create a variant of SCNT-derived stem cells in 2011. At that time they had hypothesized that there must be something in the human egg genes that is necessary to reprogram the adult nucleus. So they left in one set of the egg genes, which resulted in stem cells that were triploid; they had two sets of genes from the donor nucleus and one from the egg. Those cell lines provided a tool for some interesting research but were never really an option for therapeutic cloning where the goal is to create stem cell that match the genetics of the donor to create repair tissue that will not be rejected by the immune system. This time they changed the way they activated the egg and modified some other steps and were able to get true SCNT stem cell lines.

Both these protocols required too many eggs to be viable for patient care procedures until they are greatly refined. However, both teams have announced that they have created iPS type stem cells from the same patients so that researchers can begin the much-needed comparison of embryonic and iPS cells.

My full report is available online, along with links to my reports from previous months.

A.T.