By applying an approach that is more commonly used in fruit flies and worms to mice, researchers have identified several new pathways that are disturbed in the autism spectrum disorder Rett syndrome. One of those pathways turned out to be cholesterol biosynthesis, and statins improved the symptoms of mice with Rett syndrome.
The team identified the pathway through use of a so-called suppressor screen, a technique in which, essentially, two wrongs make a right. Animals with a known mutation are screened for the presence of a second mutation that can fix, at least to a degree, what is wrong with the first one.
One advantage of the technique, Monica Justice told BioWorld Today, is that it is unbiased. "We didn't have any a priori guesses as to what might be affected – we just let the mice tell us what was important." Justice, who is at the Baylor College of Medicine, is the senior author of the paper describing the new findings, which appeared in the July 28, 2013, online edition of Nature Genetics.
In the case of Rett syndrome, the first known mutation is in the MeCP2 gene, which is mutated in individuals with Rett syndrome. The MeCP2 gene is on the X chromosome, and males with Rett syndrome almost always die during embryonic development. Girls, on the other hand, because they still have one normal copy of the gene, do survive, but they have severe health issues, including autism-like behaviors, lack of motor control and breathing problems. The technique is only beginning to be used in mice, partly because of the amount of breeding and partly because of the amount of sequencing required to identify a rescue mutation's location once one is identified. But, Justice said, "we thought that with next-generation sequencing we could now do this in the mouse. And it worked very well."
In the studies now published in Nature Genetics, Justice and her colleagues identified five mutations that could rescue various symptoms of Rett syndrome.
Justice and her colleagues focused on one mutation, in the gene Sqle, because that gene is involved in a well-known – and already pharmacologically targeted – process: cholesterol synthesis.
That suggested to Justice and her team that the mutation's effects could be "very treatable using drugs that could be repurposed – not only statins, but also other drugs that modulate lipid synthesis."
The team showed that, ironically, an increase in cholesterol synthesis means that the brain ultimately ends up with too little cholesterol. Because excess cholesterol is toxic to the brain, its response is "to shut down the whole pathway," Justice explained. "But that's bad for it, too," leading to some of the neurological symptoms of Rett syndrome.
Justice and her team next tested whether statins – which inhibit cholesterol biosynthesis, though they do not directly affect the enzyme that is encoded by the Sqle gene – could affect the symptoms of Rett syndrome in mice. They found that statins, though the effect was dependent on the exact dose, could "massively" improve motor symptoms. The statins did not affect either breathing or certain forms of neuronal signal processing in Rett syndrome mice, at least not at the specific times and doses that the team tested.
One big question is whether targeting cholesterol synthesis pathways will have any effect on learning and cognition. But mouse learning tasks, Justice noted, depend heavily on the animals' being able to move, which means that separating the motor effects her team has demonstrated from any additional effects on learning and cognition is challenging. And generally, she said, cognitive questions "might better be studied in humans. . . . That's what I think, anyway."
Whatever the effect of targeting cholesterol synthesis turns out to be, Justice pointed out that "each of the modifiers we found rescued different symptoms," suggesting that combination therapy will be necessary to treat girls with the disorder.
Justice and her team are continuing to search for mutations that could give hints for other processes to target. She estimated that her team is about 10 percent to 15 percent through the genome at this point. More generally, she said, the advances in sequencing mean that the suppressor screens can now be applied to looking at mouse models of a number of other diseases, too, such as Huntington's disease, muscular dystrophy and spinal muscular atrophy – anything, really, with a good mouse model.