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At an HHMI science meeting in 2000, Bear unexpectedly crossed paths with fragile X. In a prepared lecture, he explained the link between protein synthesis and memory. When he returned to his seat, the stranger next to him leaned over, complimented him on the talk, and offered to send him some fragile X knockout mice, which lack the Fmr1 gene. The stranger was then-HHMI investigator Stephen Warren, the geneticist at Emory University who had discovered the mutation for fragile X.
Bear enthusiastically accepted. The conventional wisdom was that LTD’s synaptic weakening was a result of protein synthesis and that one of those LTD proteins was FMRP. That meant that Warren’s Fmr1 knockout mice would presumably show fewer signs of LTD. In Bear’s lab, postdoc Kimberly Huber, a gifted physiologist, performed experiments comparing hippocampal LTD in the knockouts with that in wild–type mice. The experiments were blinded—that is, she didn’t know at the outset which animals were the knockouts and which were wild type.
When the experiment was completed and the scientists finally genotyped the animals, Bear and Huber were dumbfounded. Contrary to expectations, it was the knockout mice that showed high levels of LTD, not the wild type. “I swear to God, I thought somebody had mixed up the code,” says Bear. They repeated the experiment: same incongruous result.
“Being born with a developmental brain disorder may not be an irrevocable sentence.”
“If you’re doing an experiment, and you’ve worked very hard at it, and you get a bizarre result, chances are 99 out of 100 that the bizarre result is just some kind of fluke,” explains Cooper. “But 1 time in 100 it’s not a fluke. That’s up to the taste, the discretion, the daring of the experimentalist. And Mark is a daring experimentalist.”
After pondering the results for several months, Bear came up with an explanation that turned the conventional wisdom on its head. Simply put, it states that mGluR5 drives protein synthesis to keep up with the demands of the cell. FMRP acts as a brake on protein translation. Without FMRP, mGluR5-triggered protein synthesis goes unchecked, eventually disrupting synaptic function.
In 2002, Bear presented the idea at a conference on fragile X at Cold Spring Harbor Laboratory. “I was the last speaker of the meeting. I laid out this idea. And there was a sort of stunned silence. I felt relieved that I hadn’t been laughed at.”
In 2004, based on this single experiment and an exhaustive literature search on the downstream effects of mGluR5, he published a paper in Trends in Neurosciences boldly titled “The mGluR theory of fragile X mental retardation.” It suggested that a vast array of fragile X symptoms—epilepsy, cognitive impairments, developmental delays, loss of motor coordination, anxiety, autistic behavior, habit formation, sensitivity to touch, even changes in gastrointestinal motility—could be accounted for by runaway effects of mGluR5. Fragile X, Bear wrote, was a disease of excess: excessive sensitivity to environmental change, excessive neural connectivity, excessive protein synthesis, excessive excitability, excessive body growth. Was it possible to undo this cellular chain of events?
Bear and his colleagues at MIT later performed a genetic rescue experiment in mice—“rescuing” normal behavior through DNA manipulation. They crossed mice that were heterozygous for the gene that encodes mGluR5 with Fmr1 knockout mice, which lacked the gene for FMRP that restrains protein synthesis. The offspring had only half of the normal mGluR5 receptors and so produced only half the normal amount of mGluR5 protein. Reducing mGluR5 compensated for the lack of FMRP and eliminated many of the symptoms of fragile X. It was as if the genetic manipulation had compensated for lack of a protein synthesis brake by taking the foot off the gas pedal.
In the genetically engineered mice, seven of eight fragile X phenotypes similar to those in humans were corrected or prevented. (The only phenotype not corrected was abnormally large testes.) The animals didn’t have exaggerated LTD, they didn’t suffer seizures when exposed to a loud noise, they didn’t gain abnormal weight, and their neurons didn’t show the abnormal dendrites seen in the Fmr1 knockout mice.
While the results are encouraging—suggesting global effects of a single medication—“the behavioral manifestations of fragile X are different in a mouse than in a human,” Bear says. “It is hard to know a priori which aspects will be helped and which will not.”