Who’s Afraid of Artificial Gametes (Eggs and Sperm)?

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.

City of Jerusalem
Jerusalem by Berthold Werner (Own work) [Public domain], via Wikimedia Commons
Artificial gametes are a huge step towards the total conquest of infertility. They will bypass the deficiency of eggs and sperm in people unlucky to be born sterile, or rendered sterile by surgery and cancer drugs, and many others who are sterilized by age and an early menopause. The first revolutionary treatment for infertility (IVF) was received with a mixture of joy by childless couples and horror by people believing it would harm children and be an affront to “human dignity.” The record has been very positive for millions of IVF babies, and the harshest critics of IVF have mellowed. For who can be so hard to say a child should never have been born?

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.

 

 

The Strange Tale of a Chimera

The Chimera had a serpent’s tail with the head of a lion and a goat on its monstrous body. That was a Greek myth, but real-life chimeras exist. They originate from the blending of cells from two fertilized eggs into a single body.

A chimeric man was identified in California recently after undergoing fertility treatment with his wife. She received intra-uterine insemination (IUI) with his sperm for a gynecological problem: it was successful and they had a son. There was nothing unusual in the case until a routine blood test revealed the boy had a blood type that didn’t match either parent. But how could a third party be involved in his conception? Since there was no doubt that she was his biological mother (no mix-up in the birthing center), the husband opted for a paternity test using a buccal cell sample.

It didn’t match his boy, even after retesting. Had the clinic accidentally mixed up his semen sample with one from another patient going through the service on the same day? Technocuckoldy happens.

The couple’s next stop was the office of a geneticist, before consulting their lawyer.

The geneticist suggested they send saliva samples from both dad and son to a personalized genomics company (23andMe.com). When the data came back the story suddenly changed from alarming to interesting.

The DNA of man and boy was a 25% match, not the 50% expected for normal paternity. Taken at face value, the result suggests the boy was a grandson or a nephew of the man instead of his son. But dad really was his biological parent because when his semen was carefully analyzed 10% of his sperm corresponded exactly to the boy’s DNA. The rest was from an unknown relative! There was a similar mix of origins in the man’s buccal sample, and probably in other parts of his body they didn’t test.

The geneticist deduced that the man was harboring cells originating from a twin brother who had vanished before birth. Dad was a chimera. Cells from his moribund twin had colonized his testicles when they were sharing a womb and became spermatogonia for making sperm after puberty.

Chimerism sounds strange and deeply abnormal, but it is a natural phenomenon. In a few species it is a normal process, even a necessary one. Perhaps the weirdest example is the deep sea angler fish, the one with gaping jaws and a dorsal fin modified like a fishing line that serves as a lure for prey. Those are the females. The males were overlooked for a long time because they are tiny. Instead of mating in a conventional way (perhaps they don’t dare), they become absorbed into a female’s body after fusing with her. It is not a final death for a male because his blood vessels join up with hers, enabling some of his cells to survive, including the all-important sperm cells. The female becomes a chimera after receiving the male fertility transplant, which enables her to be self-fertilized.

Strictly speaking, anyone who receives a transplanted organ or bone marrow from a donor is a chimera. But in mammals most chimeras originate from sharing a placental circulation in pregnancy, and this occurs regularly in marmoset monkeys. In cattle it can have biological consequences.

Freemartin cows were recognized as far back as Ancient Rome because they are sterile, which is bad news for farmers. The 18th Century anatomist John Hunter realized they only occur when a female calf has a twin of opposite sex. Conceived as genetic females, freemartins are affected by hormones carried over from blood circulating in their male sibling. Male fetuses have much higher levels of testosterone and AMH, hormones that masculinize the body and cause the uterus to shrivel.

Cross-circulation rarely causes these effects in other species, although in humans it accounts for some rare intersex abnormalities, and can create nutritional deprivation in a twin with a shared placenta.

We used to make chimeric mice with four genetic parents for tracing cell lineages during development. When an 8-cell embryo from a black x black mated mouse and one from a white x white mating were “unshelled” and fused to make a single large embryo, the pup born after transfer to a surrogate mother was piebald. Chimeric pups were sexually normal with a few exceptions that were either intersexes with a testis and an ovary or had an “ovotestis.” Although fusion was almost 100% efficient in the Petri dish, the shell (zona pellucida) prevents chimerism at early stages of pregnancy in women by covering sticky surfaces that might cause embryos to adhere to each other or dangerously attach to the wall of the fallopian tube during passage to the uterus. Human-animal chimeras are now hot in experimental biology, but that’s another topic.

In Aldous Huxley’s Brave New World, human freemartins created at the Hatchery were not chimeras but made by a purely chemical process. These low caste females were sterile, asexual women with beards. The history of using physical differences to debase or abuse people is old and agonizing, but chimeras have avoided that fate and I have never even heard the name used as an insult. That’s because they generally go unnoticed until a genetic test reveals more than one zygotic origin in the same individual, or a striped pattern of sunburn raises a question of why the skin cells were differentially sensitive to u-v radiation.

Chimeras are more common than we realize, and I even wonder if I am among their ranks. I heard that I had a vanishing twin, although that doesn’t necessarily imply I am like the man at the California clinic who is carrying cells from a deceased fetal twin. Nor is that remote possibility something I worry about, and it wouldn’t give me the creeps even if I had brains cells from a brother or sister fetus. Fetal cell transfer is no more spooky to me than an organ transplant, and a good deal more natural. But the fact of a vanishing twin sometimes causes me to muse how my life would be different if he or she had lived.

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