The role of MYC in cell reprogramming has been determined by Shinya Yamanaka and Kazutoshi Takahashi over a decade after their groundbreaking work with induced pluripotent stem cells (iPSCs).
Initially, they identified four genes—abbreviated as O, S, K, and M—that cause cells to transform into iPSCs. The genes O, S, and K were known to help the cells become pluripotent. The role of gene M (short for MYC), however, was unclear. They knew that by adding MYC, they could reprogram cells 10% more efficiently.
In new research, they discovered that MYC helps cells get around a significant roadblock in the process. They also found that, in some instances, MYC isn't actually needed for adult cells to successfully transform into iPSCs. Details on their findings were published yesterday in Cell Reports.
To reprogram cells, scientists typically add four genes (O, S, K, and MYC) to a dish containing adult cells. This allows the cells to start multiplying, which is a distinctive feature of stem cells. But after three days, the cells suddenly encounter a roadblock and stop multiplying, or proliferating. Then, on day seven, the cells start multiplying again and go on to become iPSCs.
If the researchers don't add MYC to the dish, the cells go through the same process, but they never overcome the obstacle, so they cannot successfully convert into iPSCs.
"We realized that MYC seems to help cells get around this roadblock, and that this needs to happen for adult cells to turn into iPSCs, but we still didn't quite understand how MYC did that," explained Takahashi. "Interestingly, we were able to figure it out thanks to three discoveries that happened independently in the lab, while people were working on different things."
The first discovery helped them find an early indicator of a cell's potential to finish reprogramming. It also allowed them to easily identify when the roadblock would occur, providing a valuable time reference for the subsequent findings.
The second discovery stemmed from a separate project on a protein called LIN41. The scientists found that if they replaced MYC with LIN41 in the cocktail of genes involved in reprogramming—meaning if they used O, S, K and LIN41—they could convert adult cells into iPSCs with the same efficiency.
"This was strange because it meant that, contrary to what we believed, MYC isn't necessary for cells to reprogram efficiently," said Tim Rand, staff scientist at Gladstone and a first author of the study. "It turns out that adding LIN41 altogether avoids the onset of the roadblock that prevents cells from converting into iPSCs."
The team found that when they use the combination of O, S, K, and LIN41, the adult cells don't stop proliferating after the third day. Instead, they continue to multiply as if nothing happened and successfully complete the reprogramming process. This is because LIN41 blocks another protein, called p21, which causes the roadblock.
The third discovery showed that, in a particular cell line, neither MYC nor LIN41 are needed to enhance reprogramming. The scientists went through the same process using tumor-derived cells that continuously multiply. Then, they removed LIN41, and nothing happened. Puzzled, they tried to remove MYC and, once again, nothing changed.
"That result was very shocking to me," said Rand. "Given everything we thought we knew about MYC and LIN41 at the time, we couldn't comprehend how these genes were so beneficial in somatic cell reprogramming, but absolutely useless in tumor reprogramming. Eventually, when we realized how it fit in, it was such useful information. It made us realize that certain cell types can fortuitously accomplish the role of MYC and LIN41 during reprogramming—to disable the p21 response."
Rand and the rest of the team realized that without p21, there is no roadblock, so LIN41 is not needed to avoid it. They also showed that MYC is mainly useful because it activates LIN41. So, without the p21 roadblock, MYC isn't needed either.