Is fragile X syndrome (FXS) reversible – even if treated after the brain is fully developed? A research team led by Lothar Lindemann, Ph.D., of F. Hoffmann-La Roche, Switzerland, and Mark Bear, Ph.D., of the Massachusetts Institute of Technology, Cambridge, MA, suggests that it is (Michalon et al. 2012). The research team found that a novel compound reverses FXS in mouse models of the disease – even if administration starts when the mice are four to five weeks old and all FXS-related phenotypes are established.
Fragile X syndrome
Fragile X syndrome is the most common cause of inherited intellectual disability. It is due to mutations (usually an expansion of more than 200 unstable CGG repeats) in the fragile X mental retardation 1 (FMR1) gene, a translation regulator on the X chromosome. The mutations cause excessive synaptic protein synthesis. Resulting symptoms range from mild to severe learning disabilities, autism-like behaviors, seizures and physical abnormalities. Males are more frequently and severely affected than females (The Fragile X Foundation).
CTEP and mGlu5
The compound used by the Lindemann/Bear team is 2-chloro-4-((2,5-dimethyl-1-(4-(trifluoromethoxy)phenyl)-1H-imidazol-4-yl)ethynyl)pyridine (CTEP). CTEP is a novel, selective and long-acting inhibitor of metabotropic glutamate receptor 5 (mGlu5). mGlu5 is a glutamate-activated G protein-coupled receptor that is widely expressed in the central nervous system. It is being investigated as a drug target for numerous neurological disorders, including depression, Parkinson's disease and fragile X syndrome (Lindemann et al. 2011). Genetically reducing mGlu5 expression prevents the onset of many fragile X-associated phenotypes in FVB.129P2-Pde6b+ Tyrc-ch Fmr1tm1Cgr/J (004624) mice (Dolen et al. 2007) and acute doses of short-lived mGlu5 inhibitors ameliorate FXS phenotypes in several other model organisms.
CTEP reverses fragile X-associated phenotypes in FXS mouse models
Before the Lindemann/Bear team's study, the ability of chronic pharmacological inhibition of mGlu5 to mitigate fully established FXS had not been tested. The success of such an inhibitor would indicate that fragile X is not irreversible but rather a synaptic signaling imbalance that can be corrected with late-onset drug treatment. To test this hypothesis, the team assessed CTEP's effects on fragile X phenotypes in two strains of Fmr1 knockout (KO) mice: B6.129P2-Fmr1tm1Cgr/J (003025) and FVB.129P2-Pde6b+ Tyrc-ch Fmr1tm1Cgr/J (004624), hereafter referred to respectively as B6 Fmr1 KO and FVB Fmr1 KO mice.
They initiated CTEP treatments in mice that were 4- to 5-weeks old, when the mouse brain is fully developed (though highly plastic) and the key fragile X-associated phenotypes are established. These phenotypes include excessive protein synthesis, defective hippocampal neuronal synaptic function, high dendritic spine density in the brain cortex, memory and learning deficits, elevated startle response to auditory stimuli, high sensitivity to audiogenic seizures and epilepsy, hyperactivity and macroorchidism (abnormally large testes).
The team found that CTEP reduces the excessive protein synthesis in the hippocampus of B6 Fmr1 KO mice to wild-type C57BL/6J (B6, 000664) levels. A single dose administered to 25 to 30-day old B6 Fmr1 KO mice corrects the defects in group 1 (Gp1) mGlu-mediated hippocampal synaptic plasticity. Chronic administration of CTEP to B6 Fmr1 KO mice for 4-17 weeks
- corrects high ERK and mTOR signaling (implicated in coupling mGlu5 to synaptic protein synthesis) in the cerebral cortex ;
- corrects high dendritic spine density in the primary visual cortex;
- strongly suppresses maximum transient synaptic depression (an indicator of Gp1 mGlu activation);
- improves memory in an avoidance task;
- corrects high startle response;
- corrects audiogenic seizure sensitivity (also corrects it in FVB Fmr1 KO mice to wild-type FVB/NJ (001800) levels;
- corrects hyperactivity; and
- partly corrects macroorchidism without affecting progesterone or testosterone levels.
The team found that all the mice tolerate chronic CTEP treatment well. Minor side effects include a slightly lower growth rate, a modest 0.5 oC average decrease in body temperature, and a small but significantly reduced grip strength. CTEP does not noticeably affect rotarod performance or the general fitness of treated mice.
In summary, the Lindemann/Bear team's results strongly suggest that mGlu5 inhibitors can correct a broad range of complex behavioral, cellular, neurological and physical fragile X-associated phenotypes in mammals. Most importantly, they indicate that mGlu5 inhibitors could correct FXS even if administered in early adulthood, after the brain is fully developed.
Another recent breakthrough in fragile X research
"Fragile X – a Faulty Molecular Braking System" Rockefeller University scientists report that the absence of a key protein causes the out-of-control gene over-expression that is largely responsible for fragile X and related neurological disorders.