Engineering the RNA Code: How G-Quadruplex Motifs Could Enable Precision Control of mRNA Isoforms

1. Research Background

Messenger RNA (mRNA) maturation is a critical step in gene expression, determining not only how much protein is produced but also which isoforms are generated. Among the key post-transcriptional events, alternative polyadenylation (APA) plays a major role in shaping mRNA stability, localization, and translational efficiency. Nearly 70% of human transcripts use multiple polyadenylation sites (PAS), producing mRNA isoforms with variable 3′-untranslated regions (3′-UTRs). These differences can profoundly affect how cells respond to stress, differentiate, or develop disease.

Despite its prevalence, how APA site selection is controlled at the molecular level remains poorly understood. Emerging evidence points to the involvement of RNA secondary structures — particularly G-quadruplexes (rG4s) — guanine-rich motifs that can fold into stable four-stranded helices. These rG4s are known to regulate translation and splicing, but their potential role in polyadenylation has been underexplored.

A new study by Pauline Lejault et al. from the Université de Sherbrooke (Canada), published in Cell’s open-access journal iScience (Nov 2025), provides the first comprehensive evidence that rG4 structures located near 3′-UTRs can direct the selection of polyadenylation sites. Using a combination of transcriptome-wide sequencing and molecular validation, the researchers reveal that rG4 motifs within the Neogenin-1 (NEO1) gene — a receptor implicated in neural development and cancer — act as molecular “switches” for APA site choice.

2. Research Methods

To map APA events at a genome-wide level, the team employed an innovative PolyA Click-Seq (PAC-Seq) technique, which captures and quantifies mRNA 3′-ends without prior enrichment. Human HEK293T cells were treated with RHPS4, a small-molecule ligand known to selectively stabilize G-quadruplex structures. By comparing treated and untreated samples, the authors identified APA events and gene expression changes using Differential PolyA-Clustering (DPAC) and DESeq2 bioinformatics pipelines.

Key steps included:

Ligand perturbation: HEK293T cells were exposed to 1.5 µM RHPS4 for 72 hours — a concentration low enough to avoid cytotoxicity while effectively stabilizing RNA G4s.

Sequencing and analysis: PAC-Seq libraries were generated from triplicate samples, enabling high-resolution identification of polyA clusters (PACs).

Computational screening: Potential G-quadruplex-forming sequences (pG4s) near APA sites were predicted using G4 RNA Screener, integrating multiple scoring algorithms (G4Hunter, cG/cC, G4NN).

Experimental validation: Candidate rG4s were tested using fluorescent N-Methyl Mesoporphyrin IX (NMM) binding, Circular Dichroism, and Reverse Transcriptase Stop (RTS) assays. Mutagenesis of key guanine residues confirmed structural and functional dependencies.

This integrated pipeline allowed the authors to connect rG4 structural formation directly to APA dynamics — a relationship previously hinted at but never demonstrated at scale.

3. Research Results

3.1 Transcriptome-Wide Findings

Analysis of 14,819 genes revealed that RHPS4 treatment caused profound transcriptomic remodeling:

498 genes showed significant expression changes (> 1.5-fold, adjusted p < 0.1), with stress-response and apoptosis pathways being most upregulated.

297 APA events were significantly altered, affecting both the usage and location of polyadenylation sites.

These events were categorized as 86 splicing-linked, 36 3′-UTR shortening, 86 lengthening, and 59 mixed cases.

Remarkably, 83% of APA-related polyA clusters contained at least one predicted rG4 motif within ±100 nucleotides — a striking indication that G-quadruplexes are pervasive regulators of RNA 3′-end processing.

3.2 Focus on Neogenin-1 (NEO1)

Among the genes affected, NEO1 emerged as the most illustrative model. This single-pass transmembrane receptor — part of the immunoglobulin superfamily — is involved in axon guidance, vascular development, and cancer progression.

Bioinformatic screening identified two potential rG4s in NEO1’s 3′-UTR:

a proximal rG4, 32 nt downstream of a proximal PAC, and

a distal rG4, 59 nt upstream of a distal PAC.

Both scored highly for G4-forming potential, but fluorescence assays confirmed only the proximal rG4 folded robustly in vitro under physiological K⁺ conditions. Nonetheless, upon RHPS4 stabilization, both proximal and distal rG4s became functionally relevant, with the distal structure showing increased folding and an 18% increase in distal PAS usage (from 37% to 55% PDUI).

3.3 Functional Validation

To dissect causality, the authors cloned wild-type and mutant NEO1 3′-UTRs downstream of a GFP reporter. Four constructs were generated:
WT, proximal-mutant, distal-mutant, and double-mutant (G→A substitutions preventing rG4 formation).

Northern blot analysis showed:

Wild-type 3′-UTR produced both long (3.2 kb) and short (1.5 kb) isoforms (PDUI = 0.22).

Distal rG4 mutation sharply reduced long-isoform formation (PDUI = 0.11).

Proximal rG4 mutation had minimal effect, and the double mutant mirrored the distal mutant.

These data demonstrate that the distal rG4 acts as a key structural determinant guiding polyA-site selection.

Furthermore, expression analyses of APA-regulating complexes revealed that FIP1L1, a subunit of the CPSF/CFIm complex, was up-regulated under RHPS4 treatment. FIP1L1 is known to favor distal PAS usage and 3′-UTR lengthening, suggesting a cooperative interaction between rG4 stabilization and APA-machinery remodeling.

4. Insights and Implications

This study expands the frontier of RNA biology by establishing a direct mechanistic link between G-quadruplex structures and APA regulation. It provides several important insights:

Structural control of RNA 3′-end formation
rG4s act as spatial modulators influencing whether proximal or distal polyA sites are chosen, adding a previously unrecognized structural layer to RNA processing.

Therapeutic potential
Modulating rG4s with small molecules like RHPS4 could enable precision control of mRNA isoform expression — a powerful concept for RNA therapeutics, including mRNA vaccines and antisense oligonucleotides.

Relevance to disease
Dysregulated APA is linked to cancer and neurodegenerative disorders. Given NEO1’s involvement in tumor progression, targeting its rG4-APA axis could open new diagnostic and therapeutic avenues.

Synthetic biology opportunities
Designing synthetic G-quadruplexes within 3′-UTRs could serve as programmable switches to fine-tune mRNA stability, localization, and translation — paving the way for next-generation RNA design platforms.

Broader transcriptomic implications
The finding that over 80% of APA events co-localize with potential rG4 motifs suggests a widespread, possibly evolutionarily conserved regulatory principle in eukaryotic mRNA maturation.