The ramifications of a new type of gene

WHAT’S a gene? You might think biologists had worked that one out by now. But the question is more slippery than may at first appear. The conventional answer is something like, “a piece of DNA that encodes the structure of a particular protein”. Proteins so created run the body. Genes, meanwhile, are passed on in sperm and eggs to carry the whole process to the next generation.

None of this is false. But it is now clear that reality is more complex. Many genes, it transpires, do not encode proteins. Instead, they regulate which proteins are produced. These newly discovered genes are sources of small pieces of RNA, known as micro-RNAs. RNA is a molecule allied to DNA, and is produced when DNA is read by an enzyme called RNA polymerase. If the DNA is a protein-coding gene, the resulting RNA acts as a messenger, taking the protein’s plan to a place where proteins are made. Micro-RNAs regulate this process by binding to the messenger RNA, making it inactive. More micro-RNA means less of the protein in question, and vice versa.

Often, this regulation is in response to environmental stimuli such as stress. And sometimes, the responses acquired in this way seem to be passed down through the generations, in apparent defiance of conventional genetic theory. The best known example in people comes from the Netherlands, which suffered famine in 1944, at the end of the second world war. Children born of starved mothers were, as might be expected, smaller than usual. But the children of those children were also small. Experiments carried out on mice confirm these observations.

Stress city

In the case of mothers, it is now believed that this process, called intergenerational epigenesis, is caused by micro-RNAs from the parent getting into eggs as they form in a developing fetus. That makes sense. Eggs are large cells, with room to accommodate these extra molecules. But intergenerational epigenetic effects can pass down the male line as well. And how paternal micro-RNAs come to be in an egg is a mystery, for the sperm that would have to carry them there are tiny and have no spare room. Work by Jennifer Chan, a graduate student at the University of Pennsylvania, has, however, shed light on the process.

Ms Chan’s solution was described on February 16th by her research supervisor, Tracy Bale of the University of Maryland, at the annual meeting of the American Association for the Advancement of Science (AAAS), in Austin, Texas. The crucial insight behind her study was that micro-RNAs need not actually get inside sperm cells as they form. They could equally well be attached to sperm just before sexual intercourse. Ms Chan therefore concentrated her attentions on part of the male genital tract called the epididymis. This is where sperm mature. Cells lining the epididymis constantly discharge small, fluid-filled, membrane-bound bubbles called vesicles. When Ms Chan, working with mice, looked in detail at these vesicles, she found that they contained lots of micro-RNAs.

That was interesting. But she then went on to do an experiment. Mice are easily stressed. Simply putting new objects into their living space is enough to induce significant changes in their levels of stress hormones. Stress a male in this way and his offspring (of either sex) will react less to stress than do the offspring of unstressed males. That looks like intergenerational epigenesis. It also makes evolutionary sense, since it calibrates a mouse’s stress response to the stressfulness of the environment—which is likely to be the same as that of its father. To prove that this intergenerational effect was caused by epididymal micro-RNAs, Ms Chan collected these molecules and injected them into fertilised mouse eggs. Those eggs, as she had hypothesised they would, grew into less-stress-reactive adults.

This work is all in mice. But Dr Bale has now roped some men into the experiment, too—namely 25 male students who have provided regular semen samples in order that the micro-RNAs therein can be tracked and correlated with such stressful events as sitting exams. The results of this are yet to come in. But, with her mouse work alone, it looks as if Ms Chan has cracked an important part of the puzzle of intergenerational epigenesis.

Response to stress is not, however, the only thing in which micro-RNAs are implicated. They are also suspected of involvement in schizophrenia and bipolar disorder. To investigate this, a second speaker at the AAAS meeting, Paul Kenny, of the Icahn School of Medicine, in New York, also turned to mice.

The root of Dr Kenny’s suspicion was the discovery, post mortem, in the brains of patients who had been suffering from these conditions, of elevated levels of three micro-RNAs, called MiR206, MiR132 and MiR133b. He and his colleague Molly Heyer therefore looked at the role of these micro-RNAs in regulating brain cells called parvalbumin interneurons, which are thought to be involved in schizophrenia.

Picking one, MiR206, for closer examination, the two researchers created a mouse strain in which the gene for MiR206 was switched off in the parvalbumin interneurons. They then performed experiments to study the behaviour of these mice, assuming that switching the gene off might protect them against schizophrenia-like symptoms. Surprisingly, they found the opposite.

Their first experiment was to play the mice a sudden, loud noise. This will startle any creature, mouse or man. If the noise is preceded by a softer one, however, both humans and murines react far less when the loud noise comes. They are expecting it. But people with schizophrenia seem never to learn this expectation. And neither, to the researchers’ surprise, do mice with the MiR206 knockout.

The scary moment

For people, these observations are often explained by the fact that one symptom of schizophrenia is increased fear. And, in a second experiment, Dr Kenny and Dr Heyer showed, again contrary to expectation, that MiR206-knockouts were unusually fearful as well.

The researchers used a box which contained two lights, each positioned above a lever. First, a light would blink on and go off. Then, after a delay, both lights would come on. That was the signal for the mouse to press a lever. If the lever the mouse pressed was the one not under the initial light, the animal received some food. Drs Kenny and Heyer found that the knocked-out mice collected less food than did normal ones. But this was not because they were making mistakes. If they pressed a lever, they picked the correct one as often as a normal mouse would. Instead, they were pressing any lever less often. That was because they spent most of their time hiding in the corners of the box opposite the wall with the lights and levers. Again, they seemed abnormally afraid.

What all this means for the study of schizophrenia is unclear. It is possible that examination of the other two pertinent micro-RNAs may shed more light on the matter. More generally, though, both Dr Kenny’s work and Ms Chan’s are good examples of the fact that there is more to genes than was once believed.