(Image caption: These scans show atrophy of the cerebellum in a boy with Christianson Syndrome. This symptom was observed in some, but not all boys, with the condition. Credit: Eric Morrow/Brown University)

Diagnostic criteria for Christianson Syndrome

Because the severe autism-like condition Christianson Syndrome was only first reported in 1999 and some symptoms take more than a decade to appear, families and doctors urgently need fundamental information about it. A new study that doubles the number of cases now documented in the scientific literature provides the most definitive characterization of CS to date. The authors therefore propose the first diagnostic criteria for the condition.

"We’re hoping that clinicians will use these criteria and that there will be more awareness among clinicians and the community about Christianson Syndrome," said Brown University biology and psychiatry Assistant Professor Dr. Eric Morrow, senior author of the study in press in the Annals of Neurology. “We’re also hoping this study will impart an opportunity for families to predict what to expect for their child and what’s a part of the syndrome.”

In conducting their study, which includes detailed behavioral, medical and genetic observations of 14 boys with CS from 12 families, the team of scientists and physicians worked closely with families of the small but fast-growing Christianson Syndrome Association , including hosting the group’s inaugural conference at Brown’s Alpert Medical School last summer.

In their study, Morrow’s team was able to quantify the most frequent symptoms specific to CS. These include moderate to severe intellectual disability, epilepsy, difficulty or inability walking and talking, attenuated head and brain growth, and hyperactivity. Boys sometimes exhibit other specific symptoms – including autism-like behaviors, low height and weight, acid reflux, and regressions in speech and motor skills after age 10 – that the researchers include as secondary proposed diagnostic criteria. A third of the boys also had potentially neurodegenerative problems such as atrophy of the cerebellum.

What’s still not clear is whether the disease limits the eventual lifespan of patients.

Distinct genetic cause

Many CS traits, including a very happy disposition, appear similar to those of another autism-like condition, Angelman Syndrome, but the study defines important differences.

Among the most important ones is that the two syndromes have distinct genetic underpinnings. In all CS cases, said Morrow who treats autism patients at the E. P. Bradley Hospital in East Providence, boys have a mutation on the SLC9A6 gene on the X chromosome that disables production of a protein called NHE6 that is important for neurological development.

Girls, who have two X chromosomes, can also be carriers of CS mutations, but they appear to be affected differently and less severely or not at all, the study reports.

The connection to the SLC9A6 gene was first discovered in 2008. In analyzing the genomes of each patient and their parents in the new study, lead authors Matthew Pescosolido, a graduate student, and David Stein, a former undergraduate, found that each boy had only one mutation, but there were many different ones across the entire group. More often than not, they determined, the mutation was not inherited, but an unlucky “de novo” change that occurred in the affected boy. In two situations, boys in unrelated families happened to share the same mutation. These recurrent mutations suggest that there may be hotspots in the DNA for mutation at these sites, Morrow said, although further research will be necessary to sort this out.

Morrow said there is evidence that SLC9A6 mutations – and therefore CS – may be a relatively common source of X-linked intellectual disability. One study, for example found that SLC9A6 mutations in two of 200 people suspected of having X-linked ID. Another found that 1 in 19 families with a case of ID exhibited a mutation that truncated the NHE6 protein.

"If we assume that between 1-3 percent of the world’s population is diagnosed with an intellectual disability and approximately 10-20 percent of the causes are due to X-linked genes, then we can estimate that CS may affect between 1 in 16,000 to 100,000 people," Morrow and his co-authors wrote. Worldwide that frequency would add up to more than 70,000 cases.

Relevance to autism, epilepsy

In a paper published last year, Morrow’s research group found that NHE6 is underexpressed in the brains of many children with more general forms of autism. This potential connection suggests that learning about CS can help doctors and scientists learn about autism.

Similarly by studying the regression of walking and verbal skills among Christianson boys, Morrow said researchers could learn more about regressions in autism.

"Christianson syndrome, I hope will be a model," Morrow said. "If we could understand the biological mechanism that leads to that loss, and we can prevent it, by developing a treatment, then these kids will remain further ahead."

Such advances will require much more study, but Morrow said that by uncovering a variety of mutations that all lead to the disease, the study provides a wealth of new information for that work.

"We can now study these different mutations and learn how this protein works by how it gets inactivated," he said. "All the different ways it gets inactivated can actually inform us about the different components of the protein that have an important function."

Stunted neuron branching restored in mice

In a new study in Neuron, Brown University researchers report that mutation of a gene associated with some autism forms in humans can hinder the proper growth and connectivity of brain cells in mice. They also show how that understanding allowed them to restore proper cell growth in the lab.

Brown University researchers have traced a genetic deficiency implicated in autism in humans to specific molecular and cellular consequences that cause clear deficits in mice in how well neurons can grow the intricate branches that allow them to connect to brain circuits. The researchers also show in their study (online Sep. 12, 2013, in Neuron) that they could restore proper neuronal growth by compensating for the errant molecular mechanisms they identified.

The study involves the gene that produces a protein called NHE6. Mutation of the gene is directly associated with a rare and severe autism-related condition known as Christianson syndrome. But scientists, including senior author Dr. Eric Morrow, have also associated the protein with more general autism.

“In generalized autism this protein is downregulated,” said Morrow, assistant professor of biology in the Department of Molecular Biology, Cellular Biology, and Biochemistry at Brown and a psychiatrist who sees autism patients at the Bradley Hospital in East Providence. “That meant to us that downregulation of NHE6 is relevant to a sizeable subset of autism.”

The NHE6 protein helps to regulate acidity in the endosomes of cells. These endosomes are responsible for transporting material around cells and for degrading proteins including ones that signal neurons to grow the elaborately branched axons and dendrites that form neural connections.

In their experiments the researchers measured acidity in the endosomes of brain cells of normal mice and in mice with mutations in the NHE6 gene. They found that the mutant mice had significantly higher endosome acidity. The mutant mice with the higher endosome acidity also had more degradation of a receptor protein, called TrkB, that responds a neurotrophic factor called BDNF. Together they signal axon and dendrite growth and branching.

Did the higher acidity and lower levels of TrkB signaling affect the neurons? Morrow and his colleagues were able to show directly in the mouse brain that the neuronal branching was diminished as were the number and maturity of connections between neurons, called synapses. Further still, working with co-author Julie Kauer, professor of medical science in the Department of Molecular Pharmacology, Physiology, and Biotechnology, they looked at synaptic and circuit function in the mice, and they found deficits corresponding to those anatomical findings.

“One of the overriding problems in disorders like autism, we think, is that it’s a problem of communication between different areas of the brain and neurons communicating with each other in networks,” said Morrow, who is affiliated with the Brown Institute for Brain Science.

Searching for a rescue

Having discovered a specific chain of events by which NHE6 mutations undermine neural branching and connectivity, Morrow and lead authors Qing Ouyang and Sofia Lizarraga sought to find out why and whether they could fix it.

Sometimes acidity in the endosome can activate protein-degrading enzymes called proteases. The team hypothesized that perhaps the acidity resulting from the absence of NHE6 was leading proteases to degrade TrkB, reducing its levels in mutant neurons compared to normal ones. When they treated mutant cells with a protease inhibitor called leupeptin, they found that the TrkB levels and signaling returned to levels close to those found in the normal cells.

Given that TrkB’s job is to bind with BDNF, the researchers also hypothesized that if the problem of NHE6 mutation was a reduction of TrkB, perhaps a suitable end-run around the problem would be to administer BDNF to cells directly. Indeed they found that NHE6 mutant cells, if given extra BDNF, produced axon and dendrite growth and branching that was more like normal neurons.

“In this paper we show that BDNF signaling is attenuated in the mutant mice, but it’s not blocked,” Morrow said. “You can rescue the [neuronal growth] by turning up the signaling.”

There are already drugs developed to deliver doses of chemicals that increase or mimic BDNF in the body, Morrow said, but many more tests beyond this study would have to be done before scientists and doctors could know whether a BDNF-related drug could have a therapeutic effect for patients with Christianson syndrome or any related form of autism.

“We don’t think that this is everything about the condition,” Morrow said. “But if we were able to treat this one mechanism by adding exogenous drug, would it repair enough or some element of it?”

Christianson syndrome and perhaps only a subset of autism appears to relate to deficits in neural branching. Some forms of autism, in fact, may result from too much branch growth. Moreover, doctors have no precise ways to tell whether a child diagnosed with autism has too much or too little neural branching.

But given the study results suggesting that NHE6 may play a role in some autism forms perhaps by hindering neural branching, the new research suggests a target for addressing it.