Translational regulation of the human achaete-scute homologue-1 by fragile X mental retardation protein

Fragile X syndrome is a common inherited cause of mental retardation that results from loss or mutation of the fragile X mental retardation protein (FMRP). In this study, we identified the mRNA of the basic helix-loop-helix transcription factor human achaete-scute homologue-1 (hASH1 or ASCL1), which is required for normal development of the nervous system and has been implicated in the formation of neuroendocrine tumors, as a new FMRP target. Using a double-immunofluorescent staining technique we detected an overlapping pattern of both proteins in the hippocampus, temporal cortex, subventricular zone, and cerebellum of newborn rats. Forced expression of FMRP and gene silencing by small interference RNA transfection revealed a positive correlation between the cellular protein levels of FMRP and hASH1. A luciferase reporter construct containing the 5'-untranslated region of hASH1 mRNA was activated by the full-length FMRP, but not by naturally occurring truncated FMR proteins, in transient co-transfections. The responsible cis-element was mapped by UV-cross-linking experiments and reporter mutagenesis assays to a (U)(10) sequence located in the 5'-untranslated region of the hASH1 mRNA. Sucrose density gradient centrifugation revealed that hASH1 transcripts were translocated into a translationally active polysomal fraction upon transient transfection of HEK293 cells with FMRP, thus indicating translational activation of hASH1 mRNA. In conclusion, we identified hASH1 as a novel downstream target of FMRP. Improved translation efficiency of hASH1 mRNA by FMRP may represent an important regulatory switch in neuronal differentiation.