Two American studies recently published in Nature appear to dash hopes that it will soon be possible to use stem cells from adults rather than embryos for treating disease. This would have afforded scientists a huge advantage, sidestepping ethical concerns and providing an immediate source of wholly compatible tissue for every individual. Need a new heart? Just scrape off a few skin cells and change them into cardiac muscle cells.
The apparent setback is that when human cells have “differentiated” into their final end product to form skin or muscles, they carry a genetic memory of that tissue, known as epigenetic memory. The studies suggest that attempts to reprogram skin cells to become brain cells will only be partially successful because some of the skin ancestry will inevitably be retained. By contrast, embryonic stem cells are are “totipotent”, meaning they can become anything, although once they have done so they lose their embryonic innocence, so to speak, and become irrevocably identified with their new role.
But probing one of the studies closely, and taking account of research in more primitive animals and plants, it appears this may be only a temporary setback. The process of converting adult stem cells form one type to another involves first taking the cell back to its immediate ancestor in the body, known as a precursor cell, and then persuading this to become another type of adult cell. Both studies found that after this process the new adult stem cell retained some genetic memory of the old and so was in a sense a hybrid. But one of the studies revealed that after the “translated” adult cell continued dividing it finally lost (almost) all memory of its original form, so that eventually for example a skin cell might become (almost) totally cardiac and capable, in principle, of being used to generate a new heart.
If it is possible to speed up this process of memory erasure so it needs fewer cell divisions, adult stem cell therapy will be very much back on the map. Clues may be obtained from plants and also certain amphibians that have retained the ability to regenerate limbs, hearts and other complete body parts from scratch. In some cases the cells are completely reprogrammed without having to divide, through exposure to the correct molecular signals. The Nature studies do not contradict this idea, but merely remind us these are still early days for stem cell science, and that research has to go on embracing both the embryonic and adult varieties.