HNRNPC, or heterogeneous nuclear ribonucleoprotein C, belongs to a large family of RNA-binding proteins that patrol the nucleus of every human cell. Its core job is deceptively simple: it grabs onto newly made RNA molecules and helps decide their fate. But as the review by Zezhao Ji, Yanbin Peng, Abduxukur Ablimit, Zhiwei Liang, and Lixin Wang makes clear, that simple-sounding task cascades into nearly every layer of gene regulation. Through its RNA recognition motifs—the protein domains that physically clasp onto RNA—HNRNPC influences how pre-messenger RNA transcripts are spliced into their mature forms, how stable those transcripts are once assembled, how efficiently they are translated into proteins, and how they interact with the sprawling world of non-coding RNAs that fine-tune cellular behavior.

Perhaps the most consequential of HNRNPC’s functions involves N6-methyladenosine, or m⁶A, the most abundant chemical tag on messenger RNA in mammalian cells. This epitranscriptomic mark, added and removed by dedicated enzyme systems, acts like a molecular zip code that tells reader proteins how to handle a given transcript. HNRNPC functions as one of those readers, and it does so in an unusual way. Rather than recognizing the m⁶A mark directly, it binds methylated RNA regions indirectly, because the methyl group causes local structural changes in the RNA that expose sites HNRNPC prefers to occupy. The consequence is dramatic: by latching onto these regions, HNRNPC can open up or obscure binding sites for other splicing regulators, effectively rewriting the splicing patterns of hundreds of genes at once. The review emphasizes that HNRNPC can also act entirely independently of m⁶A, giving the protein dual modes of action that complicate but also enrich its biological portfolio.

The downstream consequences of this dual-mode regulation are anything but academic. In tumor cells, HNRNPC behaves as a volume knob for oncogenic programs. The review documents how aberrant HNRNPC expression promotes epithelial-mesenchymal transition, the cellular shape-shifting process through which cancer cells gain the mobility needed to invade surrounding tissue and seed metastases. It also modulates programmed death-ligand 1, or PD-L1, the molecular shield that tumors raise against immune attack, thereby influencing how cancer patients respond to immunotherapy. Levels of HNRNPC additionally correlate with tumor mutational burden, an established predictor of immunotherapy outcomes, suggesting that this RNA-binding protein sits at a junction between RNA metabolism and the immune landscape of cancer. Depending on context and target genes, HNRNPC can act either as an oncogene pushing disease forward or as a protective factor restraining it—a duality the authors stress must be resolved before the protein can be safely exploited in the clinic.

That dual nature reflects a broader theme of the review: HNRNPC is not a simple on-off switch. The protein is subject to an array of post-translational modifications that alter its behavior, and it operates within multiprotein complexes where other heterogeneous nuclear ribonucleoproteins and splicing factors modulate its reach. It also intersects with processes such as alternative polyadenylation, which determines where the tail of a messenger RNA is trimmed and extended, affecting transcript stability and translation efficiency. Through its interactions with long non-coding RNAs, circular RNAs, and microRNAs, HNRNPC participates in competing endogenous RNA networks in which different RNA species sequester one another’s regulators, producing ripple effects throughout the cell’s regulatory circuitry.

What elevates the review from molecular biology into clinical territory is its synthesis of HNRNPC’s role in diseases far beyond cancer. In neuropsychiatric conditions, disrupted HNRNPC function has been linked to aberrant RNA processing in neurons, where precisely calibrated splicing is critical for synaptic function and neural development. In reproductive and metabolic diseases, altered expression of the protein appears to disturb gene programs governing hormone responsiveness and energy homeostasis. The review also details HNRNPC’s entanglement with infectious and inflammatory disease: the protein engages with hepatitis B virus and hantavirus during infection, and it influences inflammatory signaling pathways, including effects on tumor necrosis factor alpha induced protein 6, an effector molecule in inflammatory cascades. These connections suggest that a single RNA-binding protein helps mediate how human cells respond to pathogens and inflammatory stress, opening potential avenues for antiviral and anti-inflammatory intervention.

The translational implications are captured in three distinct roles the authors assign to HNRNPC. As a diagnostic biomarker, abnormal HNRNPC expression profiles in patient tissues or biofluids could help identify disease earlier or classify it more precisely than current methods allow. As a prognostic predictor, the protein’s expression levels correlate with disease progression and poor clinical outcomes across multiple cancer types, meaning a simple measurement could inform how aggressively a tumor is likely to behave and which patients need closer surveillance. As a therapeutic target, HNRNPC is now within reach of emerging drug modalities. Antisense oligonucleotides, short synthetic strands that bind RNA with programmable specificity, could be designed to disrupt the HNRNPC-dependent splicing events that drive disease. Because HNRNPC’s pathogenic effects often run through m⁶A-dependent mechanisms, the growing pharmacopoeia of methyltransferase and demethylase inhibitors offers another route to indirect modulation.

The timing of this synthesis is significant. Epitranscriptomics, the study of chemical modifications to RNA, has moved from a niche curiosity to one of the fastest-growing fields in molecular biology over the past decade. The discovery that m⁶A marks recruit reader proteins such as HNRNPC to reshape RNA processing won foundational recognition and has since spawned a global effort to map, perturb, and therapeutically exploit the RNA modification landscape. HNRNPC sits squarely at the center of that effort, not only as an m⁶A reader but as a demonstrated example of how a modification can exert biological effects indirectly, through structural changes in the RNA rather than direct recognition. Understanding such indirect mechanisms is essential for the rational design of drugs targeting the m⁶A system, because inhibiting the writer enzymes may produce effects that propagate through multiple distinct reader proteins with different, sometimes opposing, consequences.

The authors are candid about the unresolved questions that temper enthusiasm. Context dependency remains the field’s central puzzle: the same protein can be oncogenic in one tissue and protective in another, and the determinants of that switch are only partially mapped. The precise structural basis of HNRNPC’s indirect recognition of methylated RNA regions has been worked out in model systems but not fully characterized across the diversity of human transcripts. The interplay between HNRNPC’s post-translational modifications and its RNA-binding preferences is largely unexplored. And translating laboratory findings into clinical biomarkers will require standardized assays, large patient cohorts, and prospective validation—none of which yet exist at scale. The review frames these gaps not as setbacks but as a research roadmap, identifying where focused investment is most likely to yield diagnostic and therapeutic dividends.

Funded by the Shanghai Municipal Key Clinical Discipline of Traditional Chinese Medicine Interdisciplinary Innovation Oncology and Shanghai Tongji Hospital, the work reflects a growing recognition in oncology research that the post-transcriptional layer of gene regulation is fertile ground for clinical innovation. The authors, drawn from Tongji Hospital, Shanghai Pulmonary Hospital, and Xinjiang Medical University, span pathology, integrated medicine, and basic medical science—an interdisciplinary blend suited to a subject that refuses to stay within any single organ system or disease category. Corresponding author Lixin Wang and colleagues argue that as epitranscriptomics matures, HNRNPC is positioned to become a core molecular target for the precision diagnosis and treatment of multiple diseases, a claim the review supports with a systematic accounting of the protein’s expression profiles, pathogenic mechanisms, and clinical associations across organ systems. For clinicians and drug developers, the message is that the era of targeting RNA-binding proteins is arriving, and HNRNPC is among the most compelling candidates to lead it. For patients, it means that a protein once known only to RNA biologists may soon appear in diagnostic panels and clinical trials that translate RNA biology into measurable survival gains.

At the heart of this discovery lies RNA Polymerase I (Pol I), the enzyme responsible for transcribing ribosomal RNA genes—a vital step in the assembly of ribosomes, the cellular machinery that translates genetic codes into functional proteins. While aberrant ribosome biogenesis has historically been recognized as a hallmark of cancer, this study elucidates a previously unappreciated layer of complexity: the connection between rRNA synthesis and the regulation of RNA splicing, a process that enables a single gene to produce diverse protein variants through selective editing of precursor RNA transcripts.

Led by Dr. Marikki Laiho, an expert in Radiation Oncology and Molecular Radiation Sciences, the team demonstrated that pharmacological inhibition of Pol I instigates a unique cellular stress response that reprograms RNA splicing patterns in cancer cells. This reprogramming selectively impairs tumor growth by altering the production of protein isoforms crucial for cancer cell survival and proliferation. Central to this mechanism are ribosomal proteins RPL22 and its paralog RPL22L1, as well as the MDM4 protein, all of which participate in coordinating the dynamic crosstalk between ribosome biogenesis and splicing modulation.

The study employed BMH-21, a small molecule developed in collaboration with Johns Hopkins pharmacology specialists, to obstruct Pol I activity in a comprehensive panel of over 300 cancer cell lines. Strikingly, cancers harboring mutations in RPL22 or exhibiting elevated levels of RPL22L1 and MDM4 were particularly vulnerable to Pol I inhibition. Notably, these molecular alterations frequently occur in tumors characterized by mismatch repair deficiency (MMRd), a genetic condition involving defects in DNA repair pathways. MMRd leads to an accumulation of genomic mutations and is commonly observed in colorectal, gastric, and uterine cancers, which often show resistance to standard therapies.

Further extending their findings beyond cell culture, the researchers evaluated a novel Pol I inhibitor, BOB-42, in animal tumor models that recapitulate patient-derived malignancies bearing these critical genetic signatures. Treatment with BOB-42 resulted in significant tumor suppression, with reductions in tumor size reaching up to 77% in aggressive melanoma and colorectal cancer models. These preclinical successes highlight the therapeutic potential of targeting the rRNA synthesis-splicing axis in cancers that evade existing treatment modalities.

Beyond its tumor-suppressive effects, the study suggests a compelling link between altered splicing patterns induced by Pol I inhibition and enhanced tumor immunogenicity. By reshaping the protein landscape presented by cancer cells, changes in RNA splicing may unmask novel tumor antigens, potentially improving recognition by the immune system. Consequently, the combination of Pol I inhibitors with immunotherapy agents could synergize to overcome immune evasion, a major hurdle in effective cancer treatment.

Dr. Laiho elaborated on this innovative concept, emphasizing the dual role of the ribosomal protein RPL22. Traditionally viewed as a structural ribosomal component, RPL22 also exerts regulatory control over selective RNA splicing. This dual functionality underscores a deeper level of cellular regulation wherein rRNA synthesis and splicing are intimately coordinated to dictate cancer cell behavior. Such a paradigm shift in understanding ribosome-related oncogenic processes could lead to transformative advances in precision oncology.

The implications of this work extend beyond therapeutic targeting of Pol I. By delineating the molecular underpinnings of cancer cells’ sensitivity to rRNA synthesis inhibition, the study offers insights into the vulnerabilities of mismatch repair-deficient tumors, which are often characterized by high mutation burden and poor prognosis. Therapeutic strategies that exploit these vulnerabilities could fill an urgent need for more effective treatments in this patient population.

Moreover, the discovery paves the way for future investigations into the role of ribosomal proteins in RNA metabolism and how their dysregulation contributes to tumorigenesis. The intersection of ribosome biogenesis with RNA splicing regulation represents a fertile frontier for molecular oncology research, promising new biomarkers and drug targets for a variety of cancers.

This pioneering research involved a multidisciplinary team, including insights from experts in cancer biology, pharmacology, and radiation oncology. Their collaborative efforts, complemented by funding from prominent institutions such as the National Institutes of Health and private foundations, exemplify the concerted push toward unraveling complex cancer vulnerabilities.

Acknowledging the translational potential of their findings, the researchers hold intellectual property rights related to Pol I inhibitors, underscoring the practical ambitions of bringing these discoveries from bench to bedside. Future clinical trials assessing the safety and efficacy of compounds like BMH-21 and BOB-42 will be critical to validate their therapeutic promise in cancer patients.

The study profoundly redefines our understanding of how ribosomal RNA synthesis intricately controls tumor cell physiology, revealing an exploitable Achilles’ heel within cancer’s machinery. By co-opting fundamental processes of RNA production and splicing regulation, this research charts a novel course for developing targeted, mechanism-based cancer therapies that could markedly improve patient outcomes in malignancies refractory to current interventions.

Subject of Research: Cancer Biology, Ribosome Biogenesis, RNA Splicing, Therapeutic Targeting
Article Title: Ribosomal RNA Synthesis and RNA Splicing Interplay as a Novel Tumor-Suppressive Pathway in Mismatch Repair-Deficient Cancers
News Publication Date: June 18, 2024
Web References:

• Johns Hopkins Kimmel Cancer Center: https://www.hopkinsmedicine.org/kimmel-cancer-center
• Department of Radiation Oncology and Molecular Radiation Sciences: https://www.hopkinsmedicine.org/radiation-oncology
• Cell Chemical Biology Journal: https://www.cell.com/cell-chemical-biology/home
  Image Credits: Courtesy of Cell Chemical Biology
  Keywords: Cells, Cancer Stem Cells, Ribosomal RNA, RNA Polymerase I, Mismatch Repair Deficiency, RPL22, RNA Splicing, Tumor Suppression, Immunotherapy, Cancer Therapeutics