Resetting the developmental clock of human stem cells, scientists have been able to create primordial stem cells (PGCs) in the laboratory. This feat, already achieved in mice and rats, is now possible in humans because scientists discovered how to coax human stem cells to assume a more “naïve” state—a more thoroughly epigenetically “reset” state. To induce this state and go on to create human PGCs, the scientists had to recognize how techniques developed for mice and rats were unsuitable for human cells. Ultimately, the techniques the scientists developed succeeded with both human embryonic stem cells and induced pluripotent stem (iPS) cells.
The creation of human PGCs was accomplished by University of Cambridge researchers led by M. Azim Surani, Ph.D., and Weizmann Institute of Science researchers led by Jacob Hanna., M.D., Ph.D. Together, the researchers compiled their findings and reported them December 24 in the journal Cell, in an article entitled, “SOX17 Is a Critical Specifier of Human Primordial Germ Cell Fate.”
“We demonstrate specification of hPGC-like cells (hPGCLCs) from germline competent pluripotent stem cells,” wrote the authors. “The characteristics of hPGCLCs are consistent with the embryonic hPGCs and a germline seminoma that share a CD38 cell-surface marker, which collectively defines likely progression of the early human germline.”
The authors emphasized that the SOX17 gene is critical for “specification,” the process whereby human stem cells are directed to become PGCs. This was a surprise as the mouse equivalent of this gene is not involved in specification, suggesting a key difference between mouse and human development. SOX17 had previously been shown to be involved in directing stem cells to become endodermal cells, which then develop into cells including those for the lung, gut, and pancreas, but this is the first time it has been seen in PGC specification. Another gene, BLIMP1, represses endodermal and other somatic genes during specification of hPGCLCs.
Unlike mouse embryonic cells, which are easily kept in their stem cell state in the lab, human iPS cells that have been reprogrammed have a strong drive to differentiate. They often retain traces of “priming.” To remove these traces, the researchers created a method for tuning down the genetic pathway for differentiation, effectively creating a new type of iPS cell, which the researchers dubbed “naïve cells.” These naïve cells appeared to rejuvenate iPS cells one step further, closer to the original embryonic state from which they can truly differentiate into any cell type.
Working with naïve human embryonic stem and iPS cells, and applying the techniques that had been successful in the mouse cell experiments, the researchers managed to produce cells that, in both cases, appeared to be identical to human PGCs. The researchers then further tested and refined the method. By adding a glowing red fluorescent marker to the genes for PGCs, they were able to gauge how many of the cells had been programmed. Their results showed that quite a high rate—up to 40%—had become PGCs; this quantity enables easy analysis.
Dr. Hanna points out that PGCs are only the first step in creating human sperm and ova. A number of hurdles remain before labs will be able to complete the chain of events that move an adult cell through the cycle of embryonic stem cell and around to sperm or ova. For one, at some point in the process, these cells must learn to perform the neat trick of dividing their DNA in half before they can become viable reproductive cells. Still, he is confident that those hurdles will one day be overcome, raising the possibility, for example, of enabling women who have undergone chemotherapy or premature menopause to conceive.
“Having the ability to create human PGCs in the petri dish will enable us to investigate the process of differentiation on the molecular level,” explained Dr. Hanna. In addition, as indicated in a press release issued by the Weizmann Institute, further research could provide answers as to the causes of fertility problems, yield insight into the earliest stages of embryonic development, enable the development of new kinds of reproductive technology.
“Germ cells are 'immortal' in the sense that they provide an enduring link between all generations, carrying genetic information from one generation to the next,” added Professor Surani. “The comprehensive erasure of epigenetic information ensures that most, if not all, epigenetic mutations are erased, which promotes 'rejuvenation' of the lineage and allows it to give rise to endless generations. These mechanisms are of wider interest toward understanding age-related diseases, which in part might be due to cumulative epigenetic mutations.”