Deciphering the molecular function of FMRP

About This Grant

Project Summary Fragile X syndrome (FXS) is the most prevalent form of inherited intellectual disability and the primary genetic cause of autism. FXS is caused by CGG repeat expansions or missense/frameshift mutations in the Fmr1 gene encoding Fragile X Messenger Ribonucleoprotein (FMRP), a protein with RNA-binding activity thought to act as a translational repressor. In addition to intellectual disability, FXS patients present seizures, macroorchidism, irregular physical features, and metabolic symptoms. The prevailing hypothesis of FXS pathogenesis posits FMRP as a promiscuous RNA-binding protein that targets hundreds of brain mRNAs, with excessive translation of these mRNAs being the underlying cause of the synaptic and neural network defects and physiological impairment of FXS. However, clinical efforts targeting the aberrant translational upregulation of FMRP target transcripts have not led to clinical benefit, and recent ribosome profiling studies surprisingly indicated a positive role of FMRP in regulating mRNA translation. Thus, the mode of translational regulation by FMRP remains uncertain, and investigations into new biological functions of FMPR or novel pathogenic mechanisms of FXS are warranted. Translational control exerts immediate influence on the composition and abundance of the proteome, making it particularly important when fast cellular response is desired. Defective translational control is profoundly linked to human diseases. Beyond translation initiation, elongation and termination are also key steps subjected to intricate regulation. During elongation, ribosome slowdown and stalling can occur. Some are functional and facilitate cellular dynamics, such as co-translational protein folding and subcellular targeting. Others are detrimental and can be triggered by mRNA damages or secondary structures, insufficient supply of aminoacyl-tRNAs, or stress. Ribosome slowdown and stalling can result in collision, which is sensed as a proxy for aberrant translation and can trigger ribosome-associated quality control (RQC). Key factors involved in RQC include the ubiquitin ligase ZNF598 that marks collided ribosomes and the ASC-1 complex (ASCC) that dispatches the leading collided ribosome. This then triggers downstream events, including ribosome subunit splitting and recycling, and release of stalled nascent peptide chains for clearance by the proteasome. In Preliminary Studies, we discover that FMRP performs a previously unrecognized role in handling collided ribosomes. We hypothesize that inadequate handling of collided ribosomes on synaptic and autism related FMRP target transcripts is a fundamental cause of FXS. To test this hypothesis, we propose the following Specific Aims: 1) Dissect the biochemical mechanisms of FMRP regulation of collided ribosomes; 2) Identify synaptic transcripts regulated by FMRP at the ribosome collision step. By establishing a novel mechanism of FMRP action in handling ribosome collisions, these studies will address current controversies surrounding the molecular function of FMRP and lay the foundation for future investigations into the role of FMRP in synaptic and neuronal processes underlying the pathogenesis of FXS and related mental disorders, ultimately offering treatments for patients suffering from these devastating diseases.