A 2016 quantitative study (Denzler et al., Molecular Cell) measured the 'derepression threshold' (DRT) — the number of miRNA-binding sites a competitor transcript must accumulate before other targets of the same microRNA are measurably derepressed — and found it far above the abundance of typical endogenous transcripts, leaving open whether any endogenous architecture crosses it in vivo. The study's Discussion left unresolved the prevalence of endogenous TDMD (target-directed miRNA degradation) trigger sites; how often highly expressed transcripts with cooperatively spaced sites reduce DRTs in tissues; the lack of a locus-specific, non-overexpression genetic test of ceRNA regulation in vivo; and the aggregate contribution of weak noncanonical and non-3′UTR sites — which the study's modeling showed to be necessary for accurately simulating experimental thresholds — to quantitative threshold prediction. The TDMD branch is now resolved in the literature — endogenous triggers are physiologically widespread and act through the ZSWIM8/CUL3 ubiquitin-ligase mechanism — while the competition-based core remains open: no study has measured how often endogenous linear or circular transcripts combine expression, site spacing, and miRNA-family coexpression sufficiently to reach DRTs in tissues, no precise genome-editing mutation of endogenous target sites at native loci has demonstrated (or refuted) competition-based ceRNA regulation in vivo, and a complete context-specific affinity catalog of all effective sites is still lacking.

Background

MicroRNAs (miRNAs) are ~22-nucleotide regulatory RNAs that load into Argonaute (AGO) proteins and pair with miRNA-responsive elements (MREs), mostly in the 3′ untranslated regions (3′ UTRs) of mRNAs, to repress transcript stability and translation. The competing endogenous RNA (ceRNA) hypothesis proposes that transcripts sharing MREs for the same miRNA family sequester limiting miRNA molecules and thereby derepress one another, creating an RNA–RNA regulatory layer. Two quantitative studies from the Bartel/Stoffel labs tested how much competitor RNA this requires. Bosson, Zamudio and Sharp (2014, Molecular Cell) showed that endogenous miRNA-to-target concentration ratios make potential ceRNA competition unlikely for most miRNAs, and Denzler et al. (2014, Molecular Cell) found that adenoviral overexpression of single-site competitors in mouse liver derepressed miR-122 targets only at competitor levels far above physiological. Denzler, McGeary, Title, Agarwal, Bartel and Stoffel (2016, Molecular Cell) combined single-cell reporter assays in mouse embryonic stem cells, adenoviral competitor titrations in primary hepatocytes, and a mathematical model to define the derepression threshold (DRT): the number of competing MREs (of the order of 10^4–10^5 per cell for typical single sites, depending on cell type) needed before canonical miRNA targets are measurably derepressed. It further showed that extensively paired sites — bulged or near-perfect sites complementing the miRNA 3′ region — reduce miRNA activity mainly by inducing target-directed miRNA degradation (TDMD), the destruction of the miRNA itself, a phenomenon first described for artificial and viral RNAs (Ameres et al. 2010) and in neurons (de la Mata et al. 2015), rather than by reversible competition; and that two sites spaced ~58 nt apart bind miRNAs cooperatively, lowering the DRT only about twofold. Because most endogenous transcripts are expressed orders of magnitude below their DRTs, the authors concluded that physiological ceRNA regulation would require exceptional architectures — very high expression plus multiple cooperatively spaced sites, with the circular RNA CDR1as/ciRS-7, which carries 63 conserved binding sites for miR-7 (Memczak et al. 2013), and the muscle-specific long noncoding RNA linc-MD1 (Cesana et al. 2011) as rare candidate exceptions — or extensively paired sites acting through TDMD, whose endogenous prevalence was then unknown. They closed with four open issues raised in the Discussion: (1) whether endogenous transcripts bearing TDMD-inducing complementarity exist and matter in vivo; (2) how often endogenous linear or circular transcripts are expressed highly enough, with suitably spaced cooperative sites and coexpressed miRNA families, to substantially reduce DRTs in tissues; (3) the need for precise CRISPR-mediated mutation of endogenous MREs at their genomic loci, without overexpression, as the first convincing in-vivo test of ceRNA regulation; and (4) the need to catalog and parameterize transcriptome-wide effective target-site abundance, including weak noncanonical sites and sites outside annotated 3′ UTRs, whose aggregate contribution the modeling showed to be necessary for reproducing experimental thresholds. This first issue is now resolved in the literature: Ghini et al. (2018) identified endogenous mRNAs (e.g., Serpine1) that drive decay of miR-30b/c in mouse fibroblasts; Kleaveland et al. (2018) showed the long noncoding RNA Cyrano destroys miR-7 in mouse brain through an extensively paired site, using gene-edited mice; Shi et al. (2020) and Han et al. (2020) identified the CUL3–ZSWIM8 ubiquitin-ligase pathway that ubiquitinates AGO on TDMD-conformations and showed its loss deregulates many miRNAs; Kingston et al. (2022) and Shi et al. (2023) established that endogenous TDMD sculpt miRNA levels during Drosophila and mouse development in vivo; and Farnung et al. (2026) determined the structural mechanism, with reviews by Buhagiar and Kleaveland (2024) and Hiers et al. (2024) synthesizing this progress. By contrast, the competition-based branch is unchanged: reviews through 2024 (Grüll and Massé 2019; Miotto et al. 2019) still describe physiological ceRNA crosstalk as weak and quantitatively unresolved, and a replication study of the classic PTEN ceRNA result failed (eLife 2020), underscoring that overexpression-based evidence does not settle the question.

Problem Statement

Determine whether competition-based ceRNA regulation — reversible sequestration of microRNAs by endogenous transcripts, as distinct from target-directed miRNA degradation — occurs under physiological conditions in vivo, and how often. Specifically: (a) measure, across tissues, the distribution of endogenous linear and circular transcripts relative to their derepression thresholds (DRTs, the number of competing miRNA-binding sites a transcript must accumulate before shared miRNA targets are measurably derepressed), quantifying how frequently a transcript combines expression level, number and spacing of miRNA-binding sites (including cooperative spacing), and coexpression of the relevant miRNA families sufficiently to reduce its DRT into the physiological abundance range, thereby substantially derepressing shared miRNA targets; (b) provide the definitive locus-specific genetic test that Denzler et al. called for: precise genome editing (for example CRISPR-mediated mutation) of the miRNA-responsive elements of a candidate endogenous transcript at its native locus, without overexpression, demonstrating or refuting that a single-gene perturbation within natural expression ranges changes miRNA availability and derepresses other targets in vivo; and (c) bring the full effective-site repertoire into threshold prediction: account for and parameterize, transcriptome-wide, effective target-site abundance, including the weak noncanonical sites and sites outside annotated 3′ UTRs whose aggregate contribution the source's modeling showed to be necessary for reproducing experimental thresholds, so that model-predicted DRTs incorporate the full site repertoire. All three parts — (a), (b), and (c) — are required, and for part (b) a solution must supply a concrete experimental design (the candidate transcript, the precise native-locus editing of its miRNA-responsive elements, the controls, and the measurements to be made) rather than executed in-vivo measurements. The extensively paired TDMD-trigger branch raised in the same source is excluded as resolved: endogenous transcripts with bulged or near-perfect 3′-paired sites do induce miRNA degradation in vivo through the ZSWIM8 pathway, answering that part of the original question affirmatively.

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