Human endogenous retroviruses as a key of reprogramming and differentiation potential
A CiRA research team has established that one type of human endogenous retrovirus, HERV-H, which was integrated into the human genome during the evolutionary process, is intimately involved in the iPS cell generation process and in the manifestation of differentiation potential in iPS cells. The team was led by researcher Mari Ohnuki, currently of Ludwig-Maximilian University Munich, CiRA Director/Professor Shinya Yamanaka, and CiRA Senior Lecturer Kazutoshi Takahashi.
By inducing overexpression of four reprogramming genes, OCT3/4, SOX2, KLF4 and c-MYC, somatic cells can be reprogrammed into iPS cells. However, some of these iPS cells have low differentiation potential, which resist efforts to make them differentiate into other cell types (differentiation-defective iPS cells), while some are unable to properly maintain the pluripotent character of an iPS cell. Up till now, scientists have not well understood the mechanism which brings such cells into being.
Dr. Takahashi's research team discovered that the LTR7 regulatory sequence of HERV-H, which has high levels of expression specifically in differentiation-defective iPS cells, is transiently activated by OCT3/4, SOX2, and KLF4 during the reprogramming process. The team also found that, when the activity of LTR7 is suppressed at an early stage in reprogramming, the efficiency of iPS cell generation drops markedly. Activation of LTR7 was found to boost the expression of HERV-H, but when reprogramming was complete, the expression fell to the same level as in ES cells. In differentiation-defective iPS cells, however, LTR7 remained activated, but suppressing the activity of LTR7 allowed differentiation potential to be restored.
This research demonstrates that, for human somatic cells to be reprogrammed and acquire differentiation potential, it is important for the endogenous retrovirus to be transiently hyperactivated. It is hoped that this elucidation of one aspect of the mechanisms that are important in cellular reprogramming technology will contribute to more efficient generation of high-quality iPS cells.
Dr. Takahashi said, "These findings make a highly significant contribution to elucidating the mechanisms of reprogramming and iPS cell differentiation resistance. In our future research, we aim to further deepen our understanding of iPS cells."
The research findings were published online in the US scientific journal Proceedings of the National Academy of Sciences (PNAS) in the week from Monday August 4, 2014 (US Eastern time).
Figure. Transient expression of HERV-H during the reprogramming process
HERV-H expression was examined by qRT-PCR during the reprogramming process and found to be transiently hyperactive.
Intermediates (d): Days after transduction of reprogramming factors,
N: Normal iPS cells
Title of paper
"Dynamic regulation of human endogenous retroviruses mediates factor-induced reprogramming and differentiation potential"
Mari Ohnuki, Koji Tanabe, Kenta Sutou, Ito Teramoto, Yuka Sawamura, Megumi Narita, Michiko Nakamura, Yumie Tokunaga, Masahiro Nakamura, Akira Watanabe, Shinya Yamanaka, and Kazutoshi Takahashi