In the dystopic Brave New World of Aldous Huxley, he did not add artificial gametes to the bevy of reproductive technologies for replacing natural fertility in his future World State. Nevertheless, they might find a place one day for helping people build their families. I get stares when I ask what others think about artificial eggs and sperm. Perhaps they darkly imagine “Frankensperm” as DNA packages propelled by miniature motors, or “Frankeneggs” with DNA coiled around a silicon chip with a door to admit sperm. But biologists are not that creative!
The technology I have in mind, and which I addressed this week at a meeting in Israel, will only be judged successful if engineered gametes are biologically indistinguishable from “wild types” in normal ovaries and testes. This prospect may seem an odd subject to post from here (pictured), but it will never be a lasting controversy compared to the likes of theology and politics.
I suspect after a bumpy ride through sensationalist headlines, artificial gametes will follow a similar course, and possibly help to damp down the demographic anxiety now sweeping the globe. Back in the 1960s, there was a panic about spiraling population growth (The Population Bomb by Paul Ehrlich), but, although our numbers are still climbing from the momentum of high fertility in recent past, fertility rates are declining almost everywhere. Women have fewer babies and start their families later or not at all. Conception is more a matter of choice than chance, and smaller family sizes are desirable because raising children is expensive.
Such private decisions have public consequences, and nervous governments are encouraging parenthood again. Some countries offer free childcare and longer maternity leave. China has relaxed its one-child policy. Japan is in the vanguard of the demographic implosion, but not alone in having a birth rate below replacement levels. And in all but one Western country polled by The Economist (August 27, 2016), the expected average family size was lower than people aspired to as ideal. This reversal of attitudes to family building encourages services almost everywhere for folk who are involuntarily childless. There are now over 1,000 fertility clinics in India, a country long depicted as the epitome of overpopulation. If depopulation of young, working age people continues, surely we will see even more sympathy for these folks, and a greater welcome for the next revolution in fertility treatment than for the first.
Creation of gametes for those who have none would be such a revolution, and would surely bring joy to those who have no rosy options. At present, they may adopt a child or opt for gamete or embryo donation, but neither is an automatic entitlement and comes with financial and social obstacles. Moreover, the desire to have a genetically-related child is compelling because it is biologically grounded. But creating gametes de novo from patients’ own somatic cells is not a light undertaking as it is a turning back to a competence that was lost hundreds of millions of years ago in our evolutionary history.
The chief difficulty is that eggs and sperm are the end-products of slow, complicated processes that first started in our days as embryonic balls of cells. The germ cell lineage that leads to a mature gamete starts with a special type of cell which is “pluripotential,” meaning it is the stem of every cell in the body—brain, bone, blood and so on. It is like a joker in a card game, because it is not a member of a suit but can join any of them. But once a germ cell sets off on its journey of development it doesn’t look back—it loses the pluripotency of its parent cell and can’t switch to another “suit.” Neither can a nerve cell or red blood cell become a germ cell after they have differentiated. At least, they couldn’t until discovery of the trick of injecting nuclei into egg cells, which have “juices” for turning DNA back into thinking it is pluripotent again. That was the amazing lesson Dolly taught us.
When we engineer somatic cells (say, skin cells) to make clones or artificial gametes, both have reversed developmental time to the pluripotent state, but the goals are entirely different. The purpose of reproductive cloning is to duplicate individuals with genomes that are identical to the parent cell. On the other hand, making artificial gametes requires the parent cell to undergo a reduction division for halving its DNA so that it has a genome complementary to a gamete of the opposite sex with which the full chromosome complement is restored at fertilization. And before that division occurs the cell must go through a process of genetic recombination to generate daughter cells with different combinations of genes. Generating diversity is the game of normal reproduction, while uniformity is the hallmark of cloning. Artificial gametes are welcome in theory, but clones are scary.
Although it is very difficult to reverse a somatic cell into pluripotency and then drive it forward make a gamete (among the most specialized cells), two routes are open, although one is guarded by a traffic stop light.
The first involves injecting a nucleus from one of the patient’s somatic cells (e.g., skin) into an egg, which divides to create a ball of cloned cells looking very much like an embryo. It has to be killed, however, if the pluripotent stem cells are to be extracted for making germ cells and, hence, gametes. The moral uncertainty of these embryonic entities and debate about their rights to protection as “human” may never be resolved, thus I believe embryo stem cells are a route to nowhere.
The more attractive candidates by far are the famous iPS cells (induced pluripotent stem cells). They earned the head of a Kyoto University lab the 2012 Nobel Prize for Physiology and Medicine because iPS cells can be made from almost every type of cell in our body, and they are equally pluripotent as embryo stem cells.
In a nutshell, their story is that when a somatic cell is infused with special bunch of molecular transcription factors that are involved in the ground state of pluripotency, its nucleus will “think” it’s inside an embryo stem cell. Once an iPS cell is created it multiplies over and over to make millions of copies, and these daughter cells can be induced to differentiate outside the body along the developmental pathways of all other cell types in the body, including germ cells. The original molecular cocktail was only four factors, but two of them have oncogenic capacity. Oops! Shakespeare would mutter, The course of true science never did run smooth! Researchers are busy replacing them with less threatening ones, and the prospects are good. But there is another challenge for safeguarding health.
When differentiated cells are turned back to become pluripotent they express a different set of genes, which are said to be “epigenetically” controlled. The switches for gene activity lie in the cloud of proteins and methyl groups surrounding the DNA molecule, but what happens if this reprogramming process is incomplete? Is this why iPS cell development is inefficient and can go awry? Almost certainly it is. More to the point of this post, some iPS cells can’t make germ cells, although there is proof of principle.
Another lab at Kyoto University generated gametes from iPS cells made from fibroblast cells. When they were fertilized in vitro with gametes from healthy animals the embryos were transferred to surrogate mother mice which delivered healthy pups. This gold standard for proof is, however, a very long way from a technology for helping patients to conceive with their own gametes, but it does signal the path of progress, just as pioneering IVF studies with mice in the 1960s laid the groundwork for the first revolution in fertility treatment two decades later. It may take that long to bring iPS cells to clinical practice, as the Japanese scientists warn. There are not only technical hurdles to cross, but potential hazards to negotiate, the known and the known unknown.
And yet, there is already some progress towards making sperm from human iPS cells. Once a male germ cell has reached the stage of halving its chromosome number (to haploid), it is potentially ready for fertilization, which can be accomplished by the sperm injection technique (ICSI) before it becomes a motile cell. It is easier to make a sperm than an egg possessing a complex and voluminous cytoplasm, so the problem of equity between the sexes even exists at the cellular level.
A sensational technology that can revolutionize reproductive care is bound to attract huge publicity and suffer from the temptation of a few to push it ahead of biological understanding. The rewards in science go to the first at the finish line. A few years after Dolly was born, the Raëlian cult claimed to have cloned a human baby called Eve which, of course, was never confirmed. No doubt we will hear more stories of reproductive technology gone feral that were first inspired by Huxley. Such stories create anxieties that can arrest progress, and without the resolute pioneers of IVF technology the fertility treatments we now take for granted would have been delayed. Perhaps there are more justifiable concerns about artificial gametes than I have mentioned. Perhaps they will open the door to germline therapy, which is widely feared. And perhaps Huxley was prescient in anticipating human cloning and growing babies in bottles by the “Bokanovsky process!” To put those alarming thoughts to rest and sleep peacefully at night, I put my faith in future generations to make good decisions that we have no need of making as yet.