Brownback's Chimerical Attempt to Curb Science

Outlawing human/animal chimeras will hurt serious research

Last year, Sen. Sam Brownback (R-Kan) introduced the Human Chimera Prohibition Act. The act is cosponsored by his fellow conservatives Sen. John Ensign (R-Nev), Sen. James Inhofe (R-Okla) and Sen. Richard Santorum (R-Penn). The aim of the act is to ban various types of research in which human cells and genetic material are mixed with animal cells and genes. Why? Because the act claims, "respect for human dignity and the integrity of the human species may be threatened by chimeras." Violations of the Act would be punishable by fines of $1 million or ten years in prison or both.

In his 2006 State of the Union message, President Bush, somewhat confusedly, urged Congress to pass legislation that would ban "creating human-animal hybrids." Why confusedly? Because interspecific hybrids are produced by mating two individuals from different species such as donkeys and horses to produce mules, but most people understood the President to be coming out against human-animal chimeras and in support of Sen. Brownback's legislation.

Before looking at the Act, what are chimeras anyway? In Greek mythology a chimera was a fire-breathing monster that had the head of lion, body of a goat, and tail of a serpent. In modern biotechnology chimeras are creatures composed of cells from two or more species. An example would be the "geep" that was created in 1984 by fusing goat and sheep embryos.

So what kind of heinous research does Sen. Brownback want to criminalize? The Act would ban:

(A) a human embryo into which a non-human cell, or any component part of a non-human cell, has been introduced;
(B) a human embryo that consists of cells derived from more than one human embryo, fetus, or born individual;
(C) a human egg that has been fertilized by a non-human sperm;
(D) a non-human egg that has been fertilized by a human sperm;
(E) a human egg into which a non-human nucleus has been introduced;
(F) a non-human egg into which a human nucleus has been introduced;
(G) a human egg or a non-human egg that otherwise contains haploid sets of chromosomes from both a human and a non-human life form;
(H) a non-human life form engineered such that human gametes develop within the body of a non-human life form; or
(I) a non-human life form engineered such that it contains a human brain or a brain derived wholly or predominantly from human neural tissues.

Let's consider how any of these proposed procedures might threaten respect for human dignity and the integrity of the human species. The first consideration, of course, is the endless argument over the moral status of very early embryos. If one believes, for whatever reasons or revelations, that an embryo consisting of a 100 or so cells has the same moral status as a 30-year old mother that's the end of the matter for them. No experimenting on human embryos, period. But please note, some of the procedures banned by the Act arguably do not produce human embryos. And in any case, if one doesn't believe that embryos are people, are there still good moral reasons to ban some of the procedures listed in the Human Chimera Prohibition Act?

Let's turn provision A on its head—if it's wrong to introduce animals cells into a human embryo, is it also wrong to introduce human cells into an animal embryo? Well, actually researchers have already been doing something similar to this. For example, University of Nevada-Reno researcher Esmail Zanjani has been injecting sheep fetuses with human stem cells that then incorporate themselves throughout their bodies to produce human liver, heart, and other cells. Some of these chimeric sheep have livers that are composed of 40 percent human cells. The hope is that this may become a way to produce transplantable tissues.

In May, Chinese researchers reported that they had injected human cord blood cells into goat fetuses which were then born with human cells spread throughout their bodies. Evidently, waiting until fetal development has already progressed significantly means that the injected human cells will not proliferate to dominate the tissues and organs of the developing chimeric animals which should allay any moral qualms that people may have. Injecting human cells into fetal animals would probably not fall afoul of the Act.

But what about a banning the creation of "a human embryo that consists of cells derived from more than 1 human embryo, fetus, or born individual." One clumsy interpretation of this language might find that it prohibits normal sexual reproduction because after all today each embryo "consists of cells derived from more than 1 … born individual"—namely eggs and sperm. This language might well also be interpreted as outlawing both reproductive (to produce a baby) and therapeutic cloning (to produce transplant tissues) since cell nuclei could be taken from another embryo, fetus or person and combined with an enucleated egg. In June, Harvard researchers announced that they have begun a program to clone human embryos to create transplant tissues. This provision would also outlaw an assisted reproduction technique in which cytoplasm containing mitochondria from a donor egg is used to rejuvenate another woman's egg so that she can bear healthy children. Children born using this technique carry mitochondrial genomes from the donor which means their genetic heritage derives from three people.

The next two prohibitions forbid creating embryonic chimeras by fertilizing a human egg by a non-human sperm or fertilizing a non-human egg by a human sperm. As far as I know this has only been done once in 1977 with a researcher using human sperm in an attempt to fertilize a gibbon egg. The sperm did penetrate the gibbon egg, but it turns out that human sperm bounces off the eggs of non-hominoid monkeys, making it likely that it wouldn't do much for the eggs of cows or pigs either. Of course, scientists could force the matter by using intracytoplasmic sperm injection.

Normally, prolifers such as Brownback draw their line at protecting human embryos. However, any embryos produced by combining human and animal gametes are not human. So Brownback and his supporters must believe that human eggs and sperm have a special moral status before they combine to produce embryos. As Monty Python once suggested: Is every sperm sacred? Brownback must also be taken aback by the recent finding that human ancestors and the ancestors of chimpanzees practiced interspecific miscegenation a few million years ago.

Presumably one goal of Brownback's ban on mixing human and animal gametes is to prevent the birth of a creature that is in some sense a diminished human being. Since combining human and non-hominoid animal gametes will most likely not result in viable hybrids, banning that practice seems superfluous. I don't think that any Institutional Review Board would approve of an experiment that was designed to create a live hybrid by means of fertilizing human eggs by chimpanzee sperm or vice versa. But if banning such experiments would make Brownback happy, let's do it.

The next two provisions propose to ban cloning experiments. The second of the two provisions would ban experiments that are already ongoing. For instance researchers at Harvard are adding human cell nuclei to enucleated rabbit eggs. They hope to use this cloning technique to produce transplantable tissues. They are using rabbit eggs because they are much more plentiful than human eggs. The first of these two provisions is superfluous because human eggs are so hard to come by that it is unlikely anyone would use them to produce transplantable tissues for rabbits. In any case, such "embryos" would again not be human embryos. In fact, because such embryos could not fully develop into living creatures, they would be very much like the genetically impaired embryos that some members of the President's Council on Bioethics believe could morally be used as sources of stem cells.

The next provision would prohibit the creation of "a human egg or a non-human egg that otherwise contains haploid sets of chromosomes from both a human and a non-human life form." I'm not sure, but perhaps this provision differs from the preceding ones that outlaw human/animal hybrid embryos by forbidding the addition of individual chromosomes derived from animals or humans to human and animal eggs.

The next to last provision bans the creation of "a non-human life form engineered such that human gametes develop within the body of a non-human life form." It could happen that injecting human stem cells into mice fetuses could result in those stem cells migrating to the ovary or testes of those fetuses where they develop into egg and sperm forming cells. One can even imagine mating two such mice so that the embryo they produce would be a human embryo. Given size differences, a mouse could never give birth to a man. On the other hand, one can also imagine the same thing happening between reproductively chimeric cows or sheep. Given safety issues and concerns about the future well-being of any children that might be born of domestic livestock, researchers must be careful to make sure that this kind of mating between chimeric animals does not occur.

That being said, it has already been proposed that this technique might be adapted to help some people who cannot produce gametes overcome their infertility. For instance, a fertility specialist could inject bone marrow stem cells from an infertile person into fetal mice in which those human stem cells are transformed into cells that produce human gametes. Such fully human gametes could be harvested from the chimeric mice and used to produce genetically related children by means of conventional in vitro fertilization.

Brownback's final prohibition would forbid the engineering of a non-human life form such that it contains a human brain or a brain derived wholly or predominantly from human neural tissues. Already, Stanford University researcher Irving Weissman has injected human neurons into mouse fetuses producing mice with brains composed of 1 percent human neurons. Weissman next wants to create a strain of mice with brains made almost entirely of human neurons. Such mice would be invaluable for studying human brain diseases and testing medicines to cure those diseases. Mice with brains composed entirely of human brain cells are unlikely to begin contemplating the meaning of life. Why? Among other reasons, because mouse brains weigh just 0.4 grams compared to around 1500 grams for human brains.

Still one can imagine that adding a substantial number of human neurons to fetal primates might end up producing a creature that could be regarded as a diminished human being. However, concerns of this sort should not be allowed to outlaw experiments like that of Yale researcher Gene Redmond. Redmond is trying to find a cure for Parkinson's disease using experiments in which he injects human brain cells into the brains of green vervet monkeys.

This quick review shows that current experiments using chimeric animals and embryos do not threaten our "respect for human dignity and the integrity of the human species." Ultimately, the Human Chimera Prohibition Act is a misbegotten legislative blunderbuss that would criminalize much valuable research aimed at curing human diseases. We can afford to wait until we hear that a Harvard or Stanford institutional review board has approved an experiment to produce a humanzee before Congress needs to act.