Rett syndrome, a genetic disease that causes severe mental retardation, has long been a rare and sad puzzle for scientists—but it’s a puzzle with greater importance than its small patient population (~15,000 in the U.S.) would suggest. The syndrome is marked by a sudden regression in normal development when a child is between the ages of 6 and 18 months; this regression includes strikingly autism-like features such as the loss of speech and social withdrawal. Researchers who study Rett hope that by understanding it, they might also understand more about autism and perhaps other behavior-related disorders. That makes a new report, which appeared online in Nature on March 18, particularly intriguing: Young mice that have the Rett gene defect, and normally develop a Rett-like disorder, were largely protected by a bone-marrow transplant that introduced healthy immune cells into their brains.
“We are still far from the clinic, but in these mice the results from the marrow transplants were amazing,” says Jonathan Kipnis, an associate professor of neuroimmunology at the University of Virginia who is senior author of the report.
Rett syndrome typically is caused by spontaneous mutations, in sperm DNA, of the gene Mecp2, which normally is highly expressed in brain cells. The gene is found on the X chromosome, and boys, who have only one X chromosome, seldom survive long after birth if their Mecp2 gene is disrupted. Girls have two X chrosomomes, only one of which is active in any given cell, and so only about half the brain cells of girls with Rett syndrome have active, functioning copies of Mecp2. These girls develop normally at first, but between the ages of 6 and 18 months they regress, showing autism-like signs along with breathing difficulties, learning difficulties, motor abnormalities, and seizures. These signs stabilize after a few years, and girls with Rett commonly live into their 50s; but they are severely disabled and often can neither speak nor walk—and essentially there is no effective treatment for their condition.
The Mecp2 gene codes for the MECP2 protein, which normally plays a broad “epigenetic” role in keeping some genes switched off and others switched on. But scientists still don’t understand how the loss of MECP2’s function leads to the multiple problems of Rett syndrome. In Rett-model mice (typically made by knocking out both Mecp2 genes) the addition of a healthy version of Mecp2 to mature neurons reverses most disease signs. Yet other recent studies have indicated that the Mecp2 mutation exerts its effects partly through support cells in the brain, such as astrocytes and microglia.
For the new study, Kipnis and his colleagues decided to find out what would happen if they removed only the microglia in Rett mouse brains and replaced them with healthy, non-mutant microglia. To do this, they gave the mice a dose of radiation that kills nearly all immune cells (which are more sensitive to radiation because they are relatively fast-dividing). Then, as doctors often do for leukemia patients, they gave the mice new immune systems by transplanting bone marrow from closely related but healthy mice. Microglia are immune-type cells that originate in bone marrow and then migrate to the brain when they start to mature.
To Kipnis’s surprise, the Rett mice developed almost normally after receiving new microglia this way. Their neurons and astrocytes still contained the Rett mutation, but with healthy, non-mutant microglia, they attained a near-normal lifespan as well as near-normal weight, behavior, and other signs. Even without a transplant, restoring the Mecp2 gene just to the mice’s microglia enabled a partial recovery. By contrast, when the Rett mouse brains were shielded with lead so that their diseased microglia were not killed by the radiation, new microglia from the transplant failed to migrate to the brain in large numbers, and the mice developed their usual Rett-like signs.
“It’s amazing that these mice lived as long as they did,” says Kamal Gadalla, a Rett researcher at the University of Glasgow who was lead author of a recent review on the disease. “It’s quite promising.”
The transplant results, along with similar results from a 2011 study of Mecp2 gene-replacement in astrocytes, “are pretty surprising and challenge the dogma in the field,” says Steven Gray, a Rett gene therapy researcher at the University of North Carolina.
Kipnis’s study was funded by the Rett Syndrome Research Trust in Fairfield, Conn. Monica Coenraads, the Trust’s co-founder and executive director, cited a recent case of a girl with Rett who had recovered some communication skills after a bone marrow transplant for leukemia—although, she says, the girl later died of her leukemia. “That’s the only such anecdote I’m aware of, but now that this paper is out, I’m hoping that we’ll hear of other cases in which a marrow transplant has led to improvements in a Rett patient,” Coenraads says.
Killing the immune cell population with radiation and then restoring it with a marrow transplant is probably too risky a procedure to use in children with Rett syndrome. However, as parents of these children are all too aware, there is no other near-term treatment option. Coenraads hopes that if Kipnis’s results can be confirmed by other labs, researchers will be able to find safer ways of getting marrow-donor microglia into Rett patients’ brains in sufficient numbers. “Bone marrow transplants are now being done on quite a long list of diseases,” she notes.
Alternately, researchers could target Rett children’s existing microglia with gene therapy, as Kipnis’s team did to mice in their study. So far, attempts to repair Rett syndrome with gene therapies that deliver Mecp2 to all brain cells have faced two big challenges. One is that the expression level of Mecp2 needs to be kept in a tight range; a gene therapy that caused abnormally high Mecp2 expression in neurons could be toxic to a patient’s brain. “It’s not really clear yet how little is enough and how much is too much,” says Gray.
The other challenge is that gene therapies, which typically employ viruses to carry therapeutic genes into cells, so far do so with relatively low efficiency. Both challenges might be surmountable by targeting a gene therapy to microglia instead of all brain cells. “I quite like the idea of using the glia instead of neurons, because if anything went wrong with the glia, it would not be as detrimental as it could be for neurons,” Gadalla says. There also is some evidence that gene therapy vectors can deliver their payloads into non-neuronal cells such as microglia more efficiently than into neurons. “Maybe you don’t have to target everything, to get a therapeutic effect with gene therapy,” Gray says.
Other possible strategies include the use of drugs that boost the activity of healthy Mecp2 copies in brain cells.
It isn’t entirely clear why the transplanted microglial cells in Kipnis’s study had such strong effects in protecting against Rett symptoms, but Kipnis suspects that it has to do with microglial cells’ work as gobblers and recyclers of miscellaneous debris within brain tissue. Rett-mutant microglia in the lab dish showed a weakened debris-gobbling (“phagocytosis”) ability; and when Kipnis’s team used a drug to weaken microglial phagocytosis in the brains of the transplant-recipient mice, the mice lost most of the protective effect of the transplant, and developed their usual Rett-like condition. “If the brain in Rett syndrome cannot clear cellular debris, then it has not only sick neurons but also a worsening support environment around those neurons,” Kipnis says. “If we can restore that support, then the sick neurons may be able to function at a higher capacity.” Thus, yet another treatment strategy, which Kipnis now is investigating, is to find drug compounds that can boost the level of phagocytosis in microglia.
Implications for other diseases
Researchers naturally hesitate to conclude much from a small study in mice, but Kipnis’s results add to other evidence that microglia play a significant role in disorders of cognition and behavior. Alzheimer’s researchers have known for some years that microglia help protect against the disease by removing amyloid beta protein aggregates. Scientists at the University of Utah reported in 2010 that the delivery of healthy microglia via a marrow transplant in mice cured a genetic grooming and hair-pulling condition that resembles human obsessive-compulsive disorder. There is also accumulating evidence that brain inflammation—which includes microglial activation—can shut down the production of new hippocampal neurons and contribute to symptoms of depression.
A disordered neural immune system might contribute to some cases of autism too, and Kipnis now hopes to find out whether the loss of phagocytosis is relevant to autism-spectrum disorders. “If we can show in mice that by removing the phagocytotic activity of microglia we mimic one of the autistic syndromes, then I think that could change the whole field,” he says. “We are far from there, but that’s where we are trying to go.”