Fragile X syndrome (FXS) is the most common hereditary form of intellectual disability affecting approximately 1 in 4,000 males and 1 in 6,000 females. The syndrome develops from a mutation in fmr1 on the q arm of the X chromosome, resulting in loss of RNA-binding protein FMRP, the fragile X mental retardation protein. The end result is a constellation of physical phenotypes, cognitive dysfunction, autistic behaviors, childhood seizures, and on a molecular leve, abnormal dendritic spines. Previous mouse models aimed to identify the deficits in neuronal plasticity have identified an increase in long, thin dendritic spines with an increased turnover rate and decreased response to input. Rapid protein synthesis is necessary for synaptic plasticity, which relies on the translation of existing mRNAs. Normal synaptic communication is dependent on spine dynamics and plasticity. Varying dysfunctions in circuits have been attributed to the loss of FMRP, however the mechanism by which FMRP affects plasticity, circuits and ultimately behavior was primarily unknown until recently.
Dr. Jennifer Darnell at the Laboratory of Molecular Neuro-Oncology, Rockefeller University is a leading expert on FXS. Her laboratory has recently used a new technique to begin to understand the pathophysiology of FXS and the molecular function of FMRP. High throughput sequencing cross-linking immunoprecipitation (HITS-CLIP) uses ultraviolet irradiation to create covalent bonds between proteins and RNA molecules that are in direct contact. By using this method, Dr. Darnell was able to identify 842 FMRP target mRNAs in mouse brain, which were increased in pre- and postsynaptic proteins: NMDA receptor subunits and metabotropic GluR5 receptor were among the several post synaptic mRNAs that interact with FMRP. This supports previous studies that show the increased turnover rate, increased vesicle recycling and increased vesicle pools in Fmr1 KO mice.
While this determined the binding of FMRP directly to mRNAs, it remained to be shown how this affects translation. As expected, Fmr1 KO mice exhibit increased rates of brain protein synthesis. However, the degree of increased synthesis is far more than can be explained simply by the FMRP target mRNAs. This led to the realization that in addition to the direct increase in protein synthesis, there is a global increase in protein synthesis possibly due to the downstream changes in elongation and initiation factors caused by the loss of FMRP.
But how does the presence of FMRP limit translation? Using a brain polyribosome-programmed in vitro translation system it was demonstrated that there is ribosome stalling that occurs at FMRP target transcripts. Thus, the loss of FMRP results in relief of the ribosome stalling and an increase in translation. Several of the proteins affected have been linked to the phenotypes seen in FXS: NMDA and mGluRs affecting synaptic plasticity, ERK and mTORC1 effecting neuronal translation, cAMP and several GTPases which have been shown to alter spine morphology, and SYNGAP1 which has been linked to non-syndromic mental retardation and autism spectrum disorder. The knowledge of the several pathways affected by the loss of FMRP give way to novel therapeutic approaches including most notable the use of antibiotics such as minocycline which repress translation, in order to alleviate some of the increased translational burden in FXS.
The translational role of FMRP both directly and globally, and the significant clinical phenotypes caused by the Fmr1 mutation, is an example of a minor genetic change causing catastrophic downstream effects. While there is still no cure for FXS, understanding the pathophysiology of the disease has allowed researchers to begin to test possible therapeutic approaches, many of which show promise. Dr. Darnell’s work has been integral to the understanding of not only FXS, but synaptic function, autism spectrum disorder, and molecular biology. To learn more about Dr. Darnell’s work and a list of publications, please visit her website at http://www.rockefeller.edu/research/faculty/researchaffiliates/JenniferDarnell.
Amy Taylor is a third year MDPhD candidate at UCSD in the Schizophrenia Research Program.