New laboratory studies indicate that the neurological problems caused by Fragile X syndrome, the most common form of inherited intellectual disability, may be due to excess synthesis of certain neuronal proteins that must be present at exactly the right time and place for an individual to learn.
In patients with Fragile X syndrome, which affects approximately one in 5,000 males and half that number of females worldwide, mutations in the gene encoding the Fragile X mental retardation protein (FMRP) can cause cognitive dysfunction and facial abnormalities, among other symptoms. The majority of children with Fragile X syndrome also exhibit behaviors typically associated with autism.
“This work closes a loop between molecular biology and human cognition.”
Robert B. Darnell
Researchers have known that FMRP binds to messenger RNAs (mRNAs), the molecules that shuttle genetic instructions from DNA to the complexes that translate that information into proteins. But until now, the biochemical consequences of FMRP’s interaction with mRNA have been unknown.
In the July 22, 2011, issue of the journal Cell, Howard Hughes Medical Institute investigator Robert Darnell, together with his wife Jennifer Darnell and colleagues at Rockefeller University report that they have identified more than 800 mRNAs that associate with FMRP inside nerve cells. Moreover, they have demonstrated what goes awry when FMRP is missing from those targets, as happens in the brains of children with Fragile X syndrome. Many of the RNAs they found to be regulated by FMRP are also suspected to play a role in autism.
Their study links mRNA regulation to cognition and exquisitely suggests how RNA regulation dictates cellular function. “This work closes a loop between molecular biology and human cognition,” says Darnell, the study’s senior author. “We made the unexpected discovery that cognitive dysfunction associated with Fragile X syndrome is the likely consequence of failure of an RNA-binding protein to block translation of certain key mRNAs critical for learning and memory.”
Additionally, he notes, the prevalence of suspected autism genes among their findings could hold important insights for studying that condition. “Approaching autism from this new genetic/molecular angle of insight into Fragile X syndrome offers insight into a common set of relevant genes and causes underlying both Fragile X syndrome and autism,” he says.
Darnell’s laboratory has focused on the regulation of mRNAs in neurons since his team discovered in the 1990s that autoimmune responses associated with certain cancers promote neurodegeneration by attacking brain-specific RNA-binding proteins. The new FMRP findings culminate a decade of research by Jennifer Darnell, a biochemist in her husband’s lab and leader of the group’s Fragile X team.
To identify the specific mRNAs that interact with FMRP inside nerve cells, the group isolated polyribosomes, complexes of mRNA attached to the ribosomes that catalyze protein synthesis, from the brain tissue of mice. They then employed CLIP, a technique developed in the Darnell lab that allows researchers to identify RNAs that interact with specific protein molecules, and pinpointed 842 distinct mRNAs recognized by FMRP. Approximately 30 percent of the mRNAs encode proteins found at neuronal synapses, the contact points at which neurons communicate with one another. Many of the mRNAs on their list have been previously implicated in autism.
“Only some the 842 mRNAs that we identified are likely to be crucial to the development of symptoms in Fragile X,” says Jennifer Darnell. “One focus now is to examine overlap with autism susceptibility candidate genes. This study will help people in that field winnow down extensive candidate genes to those whose mis-expression underlies autistic symptoms.”
To determine FMRP’s precise contribution to cellular activity, the group consulted co-author Joel Richter, a biochemist at the University of Massachusetts Medical School who helped them develop an assay to measure in a test tube how FMRP affects the translation of mRNAs into proteins. Their analyses showed that normal FMRP is able to halt ribosomes in their tracks as they move along mRNA strands, stalling protein synthesis. In the absence of FMRP -- or in the presence of a rare mutant form of FMRP found in a severely affected Fragile X patient -- translation speeds along unimpeded.
Why might the ability to freeze polyribosomes into a “locked-and-loaded” state be required for cognition? Synapses often form far from the main body of a nerve cell (where the cell’s DNA is housed), on projections called dendrites or on neuronal “feelers” reaching for neighboring neurons, known as growth cones. “In neurons, translation occurs near synapses in dendrites and in growth cones,” says Jennifer Darnell, speculating that FMRP may keep inactive polyribosomes pre-packaged for the trip from the cell body where they form, to distant synapses where they are deployed.
Holding polyribosomes in a poised state immediately adjacent to synapses could also serve a timing function. The ability to reshuffle neuronal connections rapidly is the very essence of learning: proteins that cement or dissolve synapses in response to input possibly need to be manufactured “on site” simply because hauling them from the cell body would take too much time.
To Robert Darnell, a practicing neurologist, the new findings hold clinical promise. “Understanding this mechanism relates back to understanding the clinical disorders of Fragile X syndrome and autism, and opens the door to developing new therapies that might be able to correct the defect in translational control,” he says.
He also finds the results satisfying because they counter the focus on DNA as the main player in human disease. These new FMRP findings add to growing evidence that RNA is actively involved in regulatory processes in the cell, and Darnell is delighted that investigators are rethinking how genetic information is relayed via the nucleic acid still defined by textbooks as an “intermediary.”
“Some evolutionary biologists think that the original life form on Earth was no more than RNA with a membrane around it,” he says. “As life evolved, RNA did not just turn into an “intermediate” but it likely remains at the core of complex functions, and that is particularly apparent in neurons.”