Molecular Glue Degraders of HuR Suppress BRAF-Mutant Colorectal Cancer
Key Insights
Researchers developed first-in-class molecular glue degraders (dHuR-1 and dHuR-2 (search)) that target the RNA-binding protein HuR (search) for CRBN (search)-dependent proteasomal degradation.
HuR (search) degradation induces exon 18 skipping in BRAF (search) pre-mRNA, reducing oncogenic BRAF protein levels and suppressing MAPK pathway signaling in BRAF-mutant CRC.
dHuR-2 (search) demonstrated potent anti-proliferative activity selectively in BRAF (search)-mutant CRC cell lines and dose-dependent tumor growth inhibition in Colo205 xenograft models.
A team of researchers has identified and characterized first-in-class molecular glue degraders (MGDs) targeting HuR (search), an RNA-binding protein encoded by ELAVL1, demonstrating potent anti-tumor activity in BRAF-mutant colorectal cancer (search) (CRC). The study, published in Nature, describes how these compounds — designated dHuR-1 and dHuR-2 (search) — leverage the cereblon (CRBN (search)) E3 ubiquitin ligase system to induce selective degradation of HuR, leading to disruption of BRAF (search) splicing and suppression of MAPK pathway signaling.
BRAF (search)-mutant CRC represents approximately 10% of CRC cases and is among the most aggressive and therapeutically challenging subtypes, with a median survival of less than 12 months under current therapies. Unlike BRAF-mutant melanoma, where BRAF inhibitors such as vemurafenib and encorafenib show remarkable efficacy, BRAF-mutant CRC exhibits intrinsic resistance due to EGFR (search)-dependent feedback reactivation of the MAPK pathway. Although the FDA-approved combination of encorafenib plus cetuximab improves response rates to 20–26% versus 5% with BRAF inhibitor alone, most patients derive no benefit, and responders typically relapse within 4–6 months. Consequently, more than 75% of patients with BRAF-mutant CRC lack durable treatment options.
Discovery and Mechanism of Action
The researchers designed a CRBN (search)-based molecular glue library containing more than 10,000 compounds. Approximately 200 representative compounds with diversified scaffolds were screened using a homogeneous time-resolved fluorescence (HTRF) assay to measure CRBN binary binding affinity. Global quantitative proteomics in CRBN wild-type and knockout cells identified HuR (search) as a top candidate, with protein abundance reduced specifically in CRBN wild-type cells.
The optimized analogue dHuR-2 (search) achieved superior CRBN (search)-binding affinity (HTRF IC50 = 0.16 µM) and HuR (search) degradation potency, with a half-maximal degradation concentration (DC50) of 3.8 nM, maximum degradation (Dmax) of 96%, and a protein half-life of 2.45 hours. Proteomic profiling revealed selective degradation of HuR and known CRBN neosubstrates ZFP91 and ZMYM2, while GSPT1 and Hu protein paralogues (HuB, HuC, HuD) were spared.
Cryo-electron microscopy resolved the structure of the DDB1–CRBN (search)–MGD–HuR (search) ternary complex to 3.3 Å. The structure revealed that HuR interacts with CRBN–dHuR-2 (search) through its G-loop, with glycine at position 58 (G58) playing a critical role. Mutagenesis studies confirmed a strict glycine requirement at position 58, mirroring the conserved G-loop mechanism observed in other CRBN neosubstrates. Surface plasmon resonance quantified tighter binding of dHuR-2 (KD = 4.6 µM versus 15.5 µM for dHuR-1), and TR-FRET analysis demonstrated enhanced cooperativity for dHuR-2 (EC50 = 0.13 µM versus 0.71 µM).
Selective Vulnerability in BRAF (search)-Mutant CRC
Analysis of DepMap consortium data revealed a strong positive correlation between HuR (search) and BRAF (search), with BRAF mutation status emerging as the strongest predictor of HuR dependency across cancer cell lines. Proof-of-concept studies across 13 CRC cell lines confirmed that dHuR-2 (search) potently reduced viability in all six BRAF-mutant lines, whereas BRAF wild-type cell lines remained resistant despite efficient HuR degradation. CRISPR–Cas9-mediated HUR knockout recapitulated this phenotype, and introduction of a HUR(G58A) mutation or CRBN (search) depletion completely abrogated dHuR-2-induced anti-proliferation in 3D-spheroid assays.
In a Colo205 xenograft model, dHuR-2 (search) administered via oral gavage at doses of 6.25, 12.5, or 25 mg/kg twice daily for 28 days resulted in dose-dependent tumor growth inhibition. The compound was well tolerated, with no significant changes in body weight or adverse clinical observations.
Splicing-Based Regulation of BRAF (search)
Mechanistic studies revealed that HuR (search) degradation promotes skipping of exon 18 in BRAF (search) pre-mRNA through direct HuR binding to a U-rich element in BRAF intron 17. The exon 18-skipped isoform (BRAF-X2) showed dramatically reduced protein expression and impaired function, failing to activate ERK phosphorylation. Polysome fractionation assays indicated that BRAF-X2 was more enriched in monosome fractions with reduced association with polysomes, indicating significantly lower translational efficiency.
This regulatory mechanism appears human-specific, as mouse Braf (search) lacks the conserved U-rich sequence. When replacing human BRAF intron 17 with mouse Braf intron 17 in a minigene assay, HuR (search) degradation-mediated BRAF exon 18 skipping was abolished.
Overcoming BRAF (search) Inhibitor Resistance
BRAF (search) inhibitor-resistant cell lines generated through dose-escalated dabrafenib treatment retained sensitivity to dHuR-2 (search). While BRAF inhibitors induced upregulation of p-EGFR (search) and enhanced p-ERK signaling via a feedback loop within six hours, dHuR-2 sustained MAPK pathway suppression for at least eight days, concomitant with reduced BRAF and EGFR protein levels.
Combination of dHuR-2 (search) with BRAF (search) inhibitors demonstrated synergistic or additive anti-proliferative effects across multiple BRAF-mutant CRC cell lines. An unbiased CRISPR kinome screen in HT29 cells identified EGFR (search) as the top gene whose depletion sensitized cells to dHuR-2, with MAPK1 (ERK2) also among the top candidates. EGFR inhibitors or MEK inhibitors enhanced colony formation inhibition when combined with dHuR-2.
Clinical Translation
The study also demonstrated that HuR (search) degradation reduces VEGFA expression through decreased pre-mRNA splicing efficiency, impairing tumor angiogenesis as indicated by reduced CD31 staining in Colo205 xenograft tumors. Regarding safety, the tool compounds dHuR-1 through dHuR-3 were determined to be inactive (IC50 > 30 µM) against the hERG ion channel. An exploratory tolerability study with dHuR-3 in both wild-type and humanized Crbn (search) knock-in mice showed minimal body weight reduction despite efficient HuR degradation in multiple organs.
The clinical candidate DEG6498, a HuR (search) MGD from Degron Therapeutics (search), has shown an acceptable toxicity and safety profile and has been approved by regulatory agencies for clinical development in advanced solid tumors, with BRAF (search)-mutant tumors included as an expansion cohort (ClinicalTrials.gov NCT07244835).