A G-quadruplex structure within the 3'-untranslated region of hnRNP K affects its translation by controlling the accessibility of miR-1249-3p to its binding site.

Abstract

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder resulting in motor neuron loss in the brain and spinal cord. Mutations of the DNA/RNA binding proteins Tar-DNA binding protein 43 (TDP-43) and fused in sarcoma/translocated in liposarcoma protein (FUS/TLS) are causative of ALS, and these two RNA-binding proteins are the major protein components of the pathological inclusions observed in over 90% of ALS cases. Moreover, a hexanucleotide GGGGCC (G4C2) expansion in the C9ORF72 gene is a common genetic cause of ALS, being found in at least 8% of sporadic and more than 40% of familial ALS. Thus, it is proposed that alterations to RNA metabolism play a pathogenic role in ALS. This hypothesis is supported by a genome-wide study of microRNAs expression in serum from ALS patients that revealed a significant deregulation of miR-142-3p and miR-1249-3p. Interestingly, miR-1249-3p has been shown to regulate the translation of the heterogenous nuclear protein K (hnRNP K), a DNA/RNA binding protein implicated in transcription and translation regulation, as well as in several diseases, including multiple cancers. In ALS, hnRNP K has been shown to associate with TDP-43 and to bind to the RNA C9ORF72 (G4C2) repeat expansion. In this study we identified and characterized by biophysical methods a G-quadruplex (GQ) structure in the 3’-untranslated region of hnRNP K mRNA, which encompasses the miR-1249-3p binding site. We show that this GQ is recognized by the Fragile X Messenger Ribonucleoprotein 1 (FMRP), suggesting a possible mechanism by which the GQ structure affects miR-1249-3p to its binding site accessibility, therefore, regulating the levels the hnRNP K protein.