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The Mouse
Challenged queen of the lab
Medicine
“Mus musculus” (the house mouse) has been at the center of biomedical
breakthroughs for the last century. But there are voices that are challenging
its capacity to model human diseases well. Investigation.
NATHANIEL HERZEBERG
……………………………………………….
“Tête-bêche.” In English, “Head to tail.” The title that appeared in the Nature magazine
on November 19, 2014 was meant to be catchy. Just perfect for selling a subject that is really
daunting. The British magazine presented the conclusions of several years of studies conducted
by an international consortium to compare the regulation systems of all the genes, in humans and
mice. Four articles, no less, and an editorial for an answer summarized them as follows: “New
similarities and differences … However, once again, this was only the visible face of a
painstaking task that offered the scientific community twelve articles spread out over five
magazines that week. Go ahead, sound the victory!
The competition had not been waiting around, however. The month before, Science had
celebrated the queen of the lab on its cover, by promising a trip into “the best mouse factory.”
Inside, the big American rival was telling its readers how genetic engineering and the new
transplanting techniques offered new hope to the understanding and treatment of cancer.
Researchers detailed their dream in this article: to genetically modify a mouse or implant a
human tumor on a mouse in order to reproduce the pathology of a specific patient, and identify
the molecules likely to treat that patient.
Between these two articles glorifying rodents, the New England Journal of Medicine
published, at the end of October, a summary of three clinical trials for treating tuberculosis,
which were less flattering for the small animal. Researchers were testing three new drug
cocktails that had passed their tests on mice. The result on humans was a complete failure.
“These trials have cost at least 200 million dollars, [177 million euros], Dr. Clif Barry sighed.
From a certain perspective, it is normal, research costs a lot. Except that this failure had been
predicted. Like all the others that we have been experiencing with mice on tuberculosis for forty
years. But nobody seems to care. Business as usual. ”
The man who attacked the small rodent this way is not a nobody. At age 52, Clifton
Barry III is an expert in his field. Director of the tuberculosis-research program at the National
Institute of Health (NIH), the American doctor is nothing less than number one among the most
cited researchers in the world for this pathology. An enviable position earned, among others, as a
result of dozens of articles on … mice. But for the last three years, Dr. Barry has started to
dislike Mus musculus. “For regular work, toxicology reports, OK. Check that no unexpected
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adverse effect develops in them when we are trying out a new molecule, perfect. But to consider
that a mouse could represent a good model of human diseases, certainly not. ”
Three new drugs for tuberculosis had passed their tests on mice. The result on humans was
a complete failure.
When you listen to him, the reason seems simple: mouse tuberculosis has nothing to do
with ours. The rodent does not cough, it is not contagious. But the difference runs even deeper.
In humans, the famous Koch’s bacilli trigger a specific immunological response when inhaled.
The white cells identify the bacteria, they aggregate and form big balls called granuloma. The
resulting fibrous mass that they form in the lungs does not look reassuring, at first glance. In fact,
it controls the bacilli, instead of killing them. Thus almost 2 billion individuals worldwide are
healthy carriers of the disease. In some of them, the time bomb goes off and they develop a
pathological form of the infection. Most of them spend their lives not knowing about the monster
that they have been sheltering. In mice, on the contrary, there are no healthy carriers or
granuloma. The bacteria attack, develop, and kill.
Clifton Barry has explored this basic difference for a long time, convinced that, by
detailing mouse mechanisms, he would help with the understanding of human pathology.
Switching from one to the other was his daily activity. But three years ago, a South Korean
colleague asked him to test a molecule that had been used in other respiratory pathologies until
then: linezolid. “He assured me that the results on his patients had been spectacular. I did not
believe him. I tested it on mice: nothing. He insisted so much that, in the end, I organized a
human trial. And then it I was astonished.” He took a deep breath and continued: “The previous
treatments had been discovered without using an animal. And forty years later, still without an
animal, and even against it, we found a cure for multiresistant strains. Almost 2 million people
die of this disease each year. I could not continue to treat mice. It is a dead end. I am leaving
that to others.”
Might the mouse be only a trap? Could all the medical research, in all the countries, have
taken the wrong direction and chosen the wrong “horse”? In his office at the Pasteur Institute,
Xavier Montagutelli smiled. Director of the animal units in the building, he invited us to take a
trip back into the past, in order to understand how a 20-gram ball of fur became the queen of the
lab. In the middle of the 19th century, the great Gregor Mendel (1822-1884) had already started
to cross-breed mice. But his bishop had deemed this breeding incompatible with monastic
decency. He then turned to peas. In 1902, in Nancy, Lucien Cuénot resumed his work and
applied it to mice, while in the United States, Clarence Little started to create pure lineages.
“He realized that mice were more tolerant to inbreeding than rabbits or guinea pigs, Xavier
Montagutelli explained. This way, you get identical subjects, which allows you to test any
hypothesis.” Response to an infectious agent, to ionizing radiation, to environmental disruption
or to genetic modification: each new scientific breakthrough is an opportunity for researchers to
take advantage of the benefits offered by the rodent.
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And they are many. Xavier Montagutelly offers a “non-exhaustive” list: “Mice are small.
Mice are not expensive. They reproduce very fast: every three months you have a new
generation. They age fast, which makes them ideal for studying old-age diseases. We know how
to freeze their embryos, their semen, their eggs. And now we know how to handle their genes,
how to add one, to deactivate another, replace a base pair and see its effects. They are an
extraordinary tool.”
A quick look at the list of Nobel prizes in Medicine clearly demonstrates this. Discovery
of the sulfonamides (1939), of penicillin (1945), of the vaccines against yellow fever (1951) and
polio (1954), of the cellular origin of retroviruses (1989), of the HIV virus (2008), or even of
prions (1997): each time, mice are at the forefront. Again, you might say that these are only the
most spectacular breakthroughs, the most directly applicable ones. In reality, since the 1980s,
almost one Nobel prize out of three in Medicine rewards a breakthrough obtained more or less on
a rodent. The latest one, the discovery of the GPS cells that allow for orientation in space, is not
an exception to this rule. “Genetics, cancer, immunity, embryonic system, nervous system,
infectious diseases … Mice are valuable in most fields, Xavier Montagutelli adds. Those who
oppose trials on animals emphasize a few weaknesses, such as septic shock, but these are
exceptions.”
The director of animal units at the Pasteur Institute did not choose this example
randomly. Several dozen meters away, Jean-Marc Cavaillon, head of the Cytokine and
Inflammation Unit, a specialist in septic shock, put it simply: “There is a problem.” Back from
Seoul, where he had just delivered a conference on this issue, he flipped through the charts on his
computer. Here is a monoclonal antibody that saves mice from lymphoma, but sends humans to
the resuscitation unit. There is an anti-inflammatory molecule (ethyl-pyruvate) that has good
effects on rodents, but kills patients in intensive care. “And we can give many other such
examples. Mice are excellent for basic research, for understanding the bigger phenomena, for
deciphering the mechanisms. But whenever we try to model human diseases to find treatments,
we stumble upon the major differences between us and mice,” the researcher confesses.
The biggest blow against the mouse model was dealt from his office. In 2003, an
American pediatrician, professor at the Harvard Medical School, Shaw Warren, came to spend a
sabbatical year at the Pasteur Institute. Together with Jean-Marc Cavaillon, he studied the
behavior of mouse and human cells in a given environment. Back in Boston, together with other
researchers, Warren launched a program aimed at observing gene behavior in the case of
septicemia, this general infection that spreads terror in hospitals. Five years of what he thought
would be an original and relevant work. But upon its publication, the major journals criticized
the absence of any comparison to the reference model: the mouse. To overcome this, Warren
expanded his team, gathered 39 colleagues around him, and operated a systematic comparison of
the genes activated in the case of septicemia, but also in two other inflammatory phenomena:
burns and trauma. The researchers hope to find their “map.” But they are faced with the opposite:
the genetic responses of humans and mice present only “scarce resemblances,” they note. This
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can explain that the failure of 150 anti-septicemia molecules, tested at huge costs, during the last
ten years, due to the fact that sick mice seemed to have a positive response.
Published in the Proceedings of the National Academy of Sciences (PNAS) in February
2013, the article stirred up a storm. Certainly due to its conclusion, likely to “contaminate” all
fields of medicine confronted with inflammations, from cancer to Alzheimer’s disease. But also
because the New York Times disclosed in the process that Nature and Science had rejected the
article. Pandora’s box was now open. Opponents piled up their arguments. They reminded
people that paracetamol and aspirin are fatal to mice or cats, that the infamous thalidomide,
prescribed as an anti-emetic to pregnant women, had been responsible for tens of thousands of
congenital malformations between 1957 and 1960; it had, however, successfully passed the tests
on mice (and also on hamsters and hens). They stated, following the example of Mark Mattson,
head of the Neuroscience Lab at the National Institute on Aging in Baltimore and specialist in
neurodegenerative diseases, that the obese and inactive rodents that can be found in labs can only
model “humans of the same type, couch potatoes, numerous in the United States, it’s true, but
not yet a majority, and certainly not the individuals in good health that we claim to study.” They
also added that 95% of the molecules deemed promising in the case of mice failed during clinical
trials, that the number of new drugs developed had never been so low and their cost never so
high (3 billion euros per molecule launched, according to the current evaluation). Not even to
mention the animal-protection groups that denounce the 15 million mice killed in American labs
each year, and the 7 million ones in Europe. “This might seem like a lot, but it represents 1.6
mice per European who dies,” Xavier Montagutelli put it in perspective.
Fear of these associations weighs a lot in the debates. At the Pasteur Institute, mouse
photos have disappeared from the web site (whereas half of the units are using them), it is no
longer allowed to visit the animal units (“sanitary reasons”), and they “prefer” to no longer
indicate the number of rodents killed. As for their opponents, they soften their criticism. “To
reduce the number of animals used, certainly, but to replace them is both illusory and
dangerous,” says Georges Chapoutier, neurobiologist and philosopher, a big supporter of the
animal cause, however.
“Without animal models, we are facing a catastrophe”
JEAN-MARC CAVAILLON
Researcher at the Pasteur Institute
“All the models are false, but some of them are more useful than others, and mice are one
of them,” admitted toxicologist Pierre-Louis Toutain, professor emeritus at the Veterinary
College of Toulouse, who described, however, how the industry chooses rodents only to defend
the safety of their products. “I have to constantly measure my words, Jean-Marc Cavaillon
admits, because without animal models, even faulty ones, we are facing a catastrophe.”
Two years later, the hurricane has passed and life continues its course. “There is the
weight of history, accumulated skills, production infrastructures that we have to keep using,
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explains Jean-Paul Gaudillière, Science historian and Director of Cermes3 at the School for
Advanced Studies in Social Sciences (EHESS), who has dedicated several articles to the
scientists’ favorite rodent. And there are researchers who need to publish.” With its fast
lifecycle, mice really keep the article machine running. Publish or perish: the old adage is truer
than ever.
However, everyone feels that scientific rigor requires a change in the practice. At the
Mouse Clinical Institute, outside Strasbourg, the director, Yann Hérault, is counting on the
arrival of new lines to diversify the genetic pool of the animals studied. “For decades, we have
made selections to make the analysis as accurate as possible, but we have lost in terms of
scope,” he diagnoses the problem. “We will expand the view.” As for Clifton Barry, he chose to
change the model. He turned to marmosets. Just like his colleague, JoAnn Flynn, from
Pittsburgh, who adopted the crab-eating macaque. “A difficult choice,” she admits. “A monkey
costs a lot [5,000 Euro instead of 15 for a mouse], it takes up a lot of space, it requires
specialized staff, it does not allow for the same genetic modifications, and it has only one baby
per year, not counting the fact that it coughs, so it is contagious. Sanitary precautions are
considerable.” Therefore, should we sacrifice primates? She hesitates. She answers: “It is
painful. They are similar to us and highly evolved. I would like not to have to do this. But,
nowadays, it is the humans who are dying.”
Others, in exchange, dream of the big biotechnological replacement. Cell lineages,
mathematical models, electronic chips including human cells to simulate organ functions …
“The loss of momentum in terms of animal models and the economic crisis have already gotten
countries and pharmaceutical labs moving,” reassures François Busquet, coordinator of the
Research Center for Alternatives to Animal Testing (CAAT) in Brussels.
Jean-Marc Cavaillon and Shaw Warren propose a last alternative. “What if, instead of
relying on the alleged resemblances between humans and mice, we looked for differences? Mice
are 100,000 times more resistant than us to bacteria. It is a known fact. We may be able to find
inspiration in this …” the American says. “Instead of humanizing mice, let us ‘murinize’
humans,” the French man translates. A bright future for the lab mouse.
[photo caption:]
The Mouse Clinical Institute, outside Strasbourg, can house up to 5,000 normal mice and
3,000 transgenic mice per year
PATRICE LATRON/INSERM
File One
Abbie Lathrop aka Mouse Woman
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Lab mice have never seen the countryside. However these selected, purified lineages were born
in a barn, a little over a century ago. Their common mother has a name: Abbie Lathrop (18681918) and a nickname: “Mouse Woman.”
When, in 1900, her weak constitution made her decide her to quit her elementary-school teaching
position in Illinois for the old family farm in Granby (Massachusetts), the 32-year old woman
hoped to start a poultry business, but failed. Her focus then shifted to pets and more specifically
to mice, popular in New England at that time. By choosing varieties and cross-breeding them,
she managed to select the fur color, but also its texture. She was thrilled with her new job, but
there was a looming shadow: the cross-bred animals developed unusual lesions.
She then contacted Leo Loeb (1869-1959), a renowned researcher at the University of
Pennsylvania, who diagnosed the lesions as being cancer tumors. Abbie was crying, but Leo was
smiling. The doctor detected in the rodent a possible model for the study of human pathology.
He asked her to provide him with rodents, but also to participate in his research studies.
Together, they cosigned ten articles. They would emphasize mostly the susceptibility of certain
mice varieties to breast cancer, in other words, the genetic factors of this illness. They would
also show that ovarectomy reduces the incidence of these tumors.
10,000 “snouts”
Abbie would become increasingly famous. Little by little, numerous scientists would buy mice
from the 10,000 “snout” farm now established in Granby. Among them, William Castle (18671962), one of the leaders of reemerging genetics at the beginning of the 20th century. His
Harvard team included young Clarence Little (1888-1971). He was renowned for his research on
rodents, and in 1929 he would establish a specialized, publicly funded facility on an island in the
state of Maine, at Bar Harbor: the Jackson Laboratory.
To this day, “Jackson” is the undisputed leader of research on mice, with no less than 17 Nobel
prizes won. Its mascot: a small, black mouse, the unequivocal star of the lab, the very mouse
that was part of the 2013 space mission, the second mammal to have had its genome sequenced,
in November 2002, a few months after … humans. Commonly called “Black 6,” it also answers
to the kind nickname C57BL/6. C is for Clarence, BL for black, 6 because it was the 6th lineage
created by the biologist in his lab that was eventually selected, and finally, 57 because the
original animal, the one it all started from, would be number 57 on the Mayflower of lab mice:
Abbie Lathrop’s barn. ■
N.H.
FILE TWO
The Other Stars of the Lab Table
According to the latest European Union statistics, 60% of the animals used for scientific research
are mice (almost 20% are rats). However, other living organisms hold a prominent place in labs.
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Mouse-ear cress. A small-size plant, Arabidopsis thaliana is not without virtues: fast lifecycle
(2 months from seed to dispersal), high resistance. Used in plant research since the beginning of
the 20th century, it became popular again at the turn of the century, when its genome was
sequenced, the first plant sequencing. A model organism for geneticists and evolution
specialists, it has unexpected properties. Danish researchers have thus modified one of its genes
to detect land mines: sowed on mined land, the plant flowers have a different color.
Baker’s yeast. Isolated by Dutch brewers in the mid-nineteenth century, Saccharomyces
cerevisiae has become a single-cell model for geneticists, molecular biologists, and
microbiologists. In 1996, it was the first eucaryote whose genome was sequenced. It took the
combined efforts of 96 laboratories around the world. Nowadays it is estimated that humans
share 23% of their genes with the yeast.
“Escherichia coli.” Hundreds of labs around the world puzzle over this bacteria, because it
enters an infant’s digestive tract a few hours after its birth. But also because by reproducing
every twenty minutes (at 37 oC), it multiplies its paths toward the living. A prokaryote organism
model (without a nucleus), its easy genetic engineering has made it critical in the study of
bacteria and their hosts: ourselves.
“Cænorhabditis elegans.” With its 959 cells, 302 of which are neurons, this 1-millimeter
transparent little worm has conquered the labs. It has helped with the understanding of cell aging
mechanisms. Sensitive to smells, capable of becoming alcohol-tolerant, it offers a very good
perspective on the nervous system, but also on life in space. American researchers plan to send
some samples to the International Space Station (ISS), to observe its reactions to microgravity.
In February 2003, C. elegans had shown that it was resistant, in any case: NASA found worms in
the debris of space shuttle Columbia. They were alive.
Fruit fly. Described for the first time in 1830, the fruit fly served as basis for the establishment
of the sexual reproduction genetic principles in 1908. The author, Thomas Morgan (1866-1945)
was awarded a Nobel Prize for them. One century and 100,000 publications later, Drosophila
melanogaster is still a preferred model in genetics and also in embryology, developmental
biology, behavioral science, etc. Especially since the animal has several qualities: it is small and
easy to raise, it does not bite, sting, or cry.
Zebra fish. Here is the rising star, the new celebrity of the lab table. Going from Indian rivers to
amateur fish tanks at the beginning of the 20th century, this striped fish was promoted to model
animal when scientists discovered its capacity to regenerate its organs: fins, skin, but also the
optic nerve or the heart … Since then, it has become the standard in many areas of biology –
embryology, genetics, oncology, toxicology, teratology, neurobiology, environmental sciences,
stem cells, and, obviously, regenerative medicine, and it has its own magazine – Zebrafish. For
five years now, it has been the lab animal with the highest growth rate in both Europe and the
United States. ■
N. H.