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Meet the Mickey Mouse Mice

Meet the Mickey Mouse Mice

Kat Arney
100 Ideas, Epigenetics, Women in genetics
Mickey Mouse 43.jpg

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As the American scientist and writer Isaac Asimov is often quoted, “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘Eureka’ but ‘That’s funny...’” And this story starts from just such an unexpected observation.

Back in the 1990s, Iranian-born geneticist Minoo Rassoulzadegan noticed something strange when she was working on a project to create mice carrying an altered version of a gene called Kit, which encodes a protein that sits on the surface of cells and transmits signals telling particular genes to be switched on or off. It’s got a range of jobs in the body, from distinguishing the germ cells in the early embryo that will become egg and sperm later in life, to controlling pigment in the skin and hair, and making red and white blood cells.

According to the design of her experiment, every mouse pup in the litter should have one active version of the gene, and one that was switched off. And the way she would spot these mice with a faulty copy? They all have little white fur gloves and socks on their paws - just like Mickey Mouse - with a white tip to their tails. 

But it was when Minoo started breeding these Mickey Mouse mice together that things got strange.

Kit is such an important gene that animals with two faulty copies can’t survive. According to Mendel’s rules of inheritance, breeding two mice that each carry one faulty copy and one normal copy of Kit should lead to a quarter of the embryos failing to develop, due to having got a faulty copy from each parent. Half should have inherited a single faulty Kit, along with the expected white gloves, socks and tail. And the rest should look completely normal and brown, as both their Kit genes are functional.

But this wasn’t the case: weirdly, almost every single mouse had white feet. Yet when Minoo looked at their DNA, only the expected number of pups carried a faulty Kit gene. The rest were completely genetically normal. 

“I thought, this cannot be! It is not possible! Mendel says it is not possible!” she told me, when I went to visit her lab at the University of Nice in the south of France.

Since then, she has dedicated her research to trying to unravel this impossibility.

To try and figure out what’s going on, Minoo kept breeding the mice together, but things just got more and more weird. Crossing the impossible Mickey Mouse mice with two healthy Kit genes with perfectly normal mice, the gloves vanished after a few generations. But if she kept crossing Mickeys together, things got even stranger. With every generation, the animals became more and more blotched with white, and some were even born completely white, even if they had two normal copies of Kit. 

This just shouldn’t happen. So when Minoo began to talk about her unusual findings at scientific conferences, most people dismissed her results as being simply too weird. And in the absence of knowing exactly what was underlying the strange phenomenon, it was little more than a curious observation. 

In search of an answer, Minoo wondered whether more than DNA was being transmitted from parents to pup, which could be somehow switching off Kit even in animals inheriting two normal copies. The most obvious culprit was RNA: a kind of molecular ‘photocopy’ produced when genes are ‘read’ inside cells.

Given that the effect worked whether the original Mickey Mouse parent was male or female, and given that eggs are packed with all kinds of protein and RNA goodies, making it hard to start unpicking what was going on, she went for the easiest option: sperm. Sperm cells are tiny - barely more than a tightly compacted ball of DNA with a tail - so it’s long been assumed that they contribute little more than their genes to the next generation.

So, Minoo took a closer look at the testicles of her Mickey Mouse mice, using an electron microscope to spy on their sperm cells. To her surprise, there was plenty of RNA in there. But what was it doing? And how did it relate to the strange pattern of inheritance?

To find out, she purified RNA from sperm taken from mice containing one faulty copy of Kit. Then, using a tiny glass needle and steady hands, she carefully injected it into genetically normal fertilised egg cells and replaced the manipulated eggs back in a mother’s womb to grow. Pleasingly, they grew into Mickey Mouse pups, complete with white socks, gloves and tails. 

But there was an issue. While injecting RNA from Mickey Mouse Mice always had this effect, very occasionally she’d see white-gloved pups emerging in control experiments using RNA from normal animals. 

“This result led me to think there is something else going on. I wondered if maybe what's happening in these injections from normal mice is that there are some small fragments or degraded RNA made from the Kit gene. And that led us to micro-RNA.”

Micro-RNAs are another hot topic in genetics - tiny fragments of RNA that carry out a seemingly endless number of roles in controlling genes. So what if the faulty version of the Kit gene was actually making some strange micro-RNAs in sperm or eggs that were responsible for exerting the Mickey Mouse effect, which could occasionally be produced by accidentally shredding Kit RNA from normal animals?

To find out, Minoo got hold of some micro-RNAs based on the Kit gene and injected them into normal mouse embryos. Again, she saw the same thing: Mickey Mouse pups with their little socks and gloves. 

But while this is a cool story, with some cute baby mice, it’s still pretty strange and many in the field didn’t believe her results. They started to look more closely, though, when she went and did it again. 

This second result was yet another accidental discovery. As part of her micro-RNA injection experiments, Minoo ordered some random micro-RNAs, which were meant to have no similarity to any known gene, to use as a control. But while the eggs injected with one of these - micro-RNA 24 - didn’t have white gloves or socks, there was something else strange about them. 

“They are like super-mice! Oh my God, they are bigger and they stand up much earlier on their feet. Straightaway you can see it! By two weeks of age you put a normal mouse on a little platform they just walk - fall off - because they don’t understand it. But these big ones… They look - they come back. So they are smarter as well.” 

Minoo took a closer look at the sequence of this mysterious micro-RNA 24, and found it matched up with a gene called Sox9. But unlike the situation with the Mickey Mouse mice, where the micro-RNA switched the Kit gene off, this micro-RNA seemed to be switching Sox9 on. 

And then, she found a third example - injecting micro-RNA matching a gene called CDK9 into genetically healthy fertilised eggs leads to animals with larger hearts than normal, which beat unusually fast, similar to a human condition called cardiac hypertrophy.

Every single week in the UK twelve apparently fit and healthy young people drop dead from an undiagnosed heart condition. Maybe some of these tragic deaths are due to rogue fragments of CDK9 RNA that came along for the ride with a sperm during the earliest moments of life.

There are many researchers who still don’t believe Minoo’s work, but other scientists are starting to look more seriously the the role of RNA in transmitting information down the generations.

Oliver Rando and his team at the University of Massachusetts are investigating the role of RNA in early development, and have found that micro-RNAs coming alongside sperm are essential for early development in mice. And it also looks like daddy mice may pass on the effects of an unhealthy diet to their offspring through tiny RNA fragments known as tRNAs.

Maybe a certain component of complex metabolic diseases like diabetes and obesity, which are influenced by a seemingly impenetrable mix of environmental and genetic causes, might be down to transgenerational transmission of RNA. It’s certainly an intriguing idea that deserves more investigation.

It all comes back to the idea of the embryo as a ‘singularity’ - a unique point in time when an organism exists as just a single cell with one complete set of DNA. Any epigenetic alterations to these cells that can be copied and passed on as the cells divide and grow will create ripples that can last a lifetime, or even further if they don’t get wiped out in the germ cells that will go on to make eggs and sperm.

This is exactly the kind of idea that leads to those ‘Darwin was wrong!’ headlines that get geneticists so worked up. But, in fact, Darwin was right about this too.

Back in the 19th century, before anyone had come up with the concept of genes, Charles Darwin’s personal favourite hypothesis of heredity was the concept of ‘gemmules’ - small particles that travel around in the bloodstream of an animal, gathering information about its characteristics. Then these gemmules combine in the embryo after fertilisation to create all the bits of the body in the combined image of its parents. 

Francis Galton, Darwin’s brilliant but massively racist cousin, set about trying to prove him right by bleeding and breeding hundreds of rabbits in an ultimately fruitless search for these mysterious packages of genetic information. 

Of course, once people started to figure out the true nature of heredity, all this silly gemmule stuff went out of the window. But aren’t these small fragments of RNA in eggs and sperm a bit like modern-day gemmules?

Researchers have even discovered RNA fragments moving around the body in little packets called exosomes, which can be taken up by cells and influence gene activity, so maybe this a way that the rest of our body can ‘talk’ to eggs or sperm, passing messages on to the next generation. 

Although there’s a long way to go to prove this actually happens in animals, let alone humans, if the principle holds true we’re going to need a major rethink of our conventional understanding of genetics and heredity. Watch this space...

Further reading:

“A woman and a Jew? Forget it!”: The story of Salome Gluecksohn-Waelsch

“A woman and a Jew? Forget it!”: The story of Salome Gluecksohn-Waelsch
The seamstress and the scientist: Pauline Gross and Family G

The seamstress and the scientist: Pauline Gross and Family G
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