Targeting ADAR1 in Hematologic Malignancies: Synthetic Lethality with MDA5 and RIG-I Pathways

Abstract

Adenosine deaminase acting on RNA 1 (ADAR1) is a principal RNA-editing enzyme involved in the editing of adenosine to inosine in double-stranded RNA (dsRNA). The editing prevents the buildup of endogenous dsRNA that otherwise is sensed as foreign by cytosolic receptors like MDA5 and RIG-I. Overexpression of ADAR1 has been identified as a means whereby malignant cells evade innate immune surveillance in hematologic malignancies. By suppressing innate immune sensor activation, ADAR1 supports leukemic and lymphoma cells survival and growth. Current research has discovered that genetic or pharmacologic inhibition of ADAR1 unmasks these cancers to their very own dsRNA signatures and initiates cell death by MDA5 and RIG-I. This phenomenon, referred to as synthetic lethality, opens up new hope for cancer treatment, especially in otherwise resistant cases. This review analyzes the molecular underpinnings of such vulnerability, overviews preclinical and translational data, and discusses preclinical implications for therapeutics aimed at inhibiting the ADAR1–dsRNA sensing pathway in hematologic cancers.

Overview of ADAR1 and RNA Editing

ADAR1 is an RNA-editing enzyme that catalyzes the deamination of adenosine to inosine (A-to-I) in double-stranded RNA molecules. This editing has a protective function in non-malignant cells by altering endogenous RNAs so that they are no longer recognized as foreign or viral. There are two principal isoforms of ADAR1 in mammals: the constitutively expressed and nuclear-localized p110, and the interferon-inducible and primarily cytoplasmic-localized p150. The p150 isoform is particularly recognized to regulate immunity because it edits cytoplasmic dsRNA to suppress aberrant activation of innate immune signaling pathways.

Cells contain pattern recognition receptors (PRRs) such as melanoma differentiation-associated protein 5 (MDA5) and retinoic acid-inducible gene I (RIG-I) that detect long and short dsRNA, respectively. These sensors initiate antiviral immune responses through the induction of type I interferons and pro-inflammatory cytokines. In the absence of sufficient RNA editing by ADAR1, these receptors are triggered by unedited self-RNA, which is a viral RNA mimic. This results in cell death and inflammation, placing an emphasis on the role of ADAR1 in immune homeostasis.

ADAR1 in Normal Hematopoiesis and Immune Regulation

Within the hematopoietic system, ADAR1 is important for lineage development and function. Murine models exhibit that ADAR1 deletion induces early embryonic lethality due to the extensive expression of interferon-stimulated genes (ISGs). Conditional ADAR1 knockout within the hematopoietic compartment induces failure in sustaining hematopoietic stem cell (HSC) maintenance due to immune activation by MDA5 and subsequent apoptosis.

The immune-protective role of ADAR1 in immune cells is tightly linked with its editing of Alu repeats and other repeat elements to produce endogenous dsRNA. These sequences, which are abundant within introns and untranslated regions, are common ligands for RIG-I and MDA5. By editing these elements, ADAR1 suppresses innate immune signaling, facilitating HSCs and progenitor cells to proliferate and differentiate under regulated conditions. This process is particularly important in the context of inflammation or viral infection, where interferon signaling is upregulated and p150 isoform is amplified to protect cells from aberrant immune activation.

ADAR1 Dysregulation in Hematologic Malignancies

Several hematologic malignancies, including acute myeloid leukemia (AML), multiple myeloma (MM), and some lymphomas, have been found to overexpress ADAR1. High expression has been linked in some cases to poor prognosis and drug resistance. Mechanistically, ADAR1 overexpression suppresses the activation of MDA5 and RIG-I and, therefore, enables the malignant cells to escape detection and killing by the innate immune system.

In AML, ADAR1 overexpression facilitates leukemic stem cell (LSC) survival and resistance to differentiating treatment. This resistance is attributable, at least in part, to suppression of endogenous interferon responses, which are central to immune-mediated clearing of rogue cells. In myeloma, there is ADAR1-driven lenalidomide resistance mediated by suppression of interferon signaling pathways that otherwise sensitize myeloma cells to immunomodulatory drugs.

Furthermore, mutations of ADAR1 itself have been discovered in a subset of myeloid malignancies. These mutations, often in the deaminase domain, disrupt editing function and may alter the tumor cells' immunologic phenotype. The bi-dual function of ADAR1—oncogenic when overexpressed and tumor suppressive when mutated—highlights its complex biology in cancer.

Synthetic Lethality Between ADAR1 and dsRNA Sensors

Synthetic lethality refers to the situation where the perturbation of two genes at once is lethal to the cell, but perturbation of either gene alone is compatible with survival. ADAR1-defective cells are reliant on the inhibition of MDA5 and RIG-I for survival. Upon loss or inhibition of ADAR1 function in cells that possess functional intact dsRNA sensors, the sensors detect unedited native RNA and induce a robust interferon response, leading to apoptosis.

This synthetic lethal interaction has been demonstrated in a number of preclinical models. To illustrate, germline deletion of ADAR1 in mice embryonically kills them due to uncontrolled MDA5 activation. Co-deletion of MDA5, nevertheless, rescues this phenotype and allows normal development. In tumor models, ADAR1-deficient tumor cells are selectively killed in the presence of intact MDA5 or RIG-I signaling. This suggests that it is feasible to target ADAR1 in tumors with intact RNA sensors to activate an innate immune-mediated cell death pathway.

The requirement for functional MDA5 or RIG-I also opens the possibility of biomarker in patient stratification. Mutant or deleted tumors in these pathways may not respond to ADAR1 inhibition, but those with intact signaling machinery may be highly susceptible.

Experimental Evidence in Hematologic Malignancies

Recent studies in mouse and human models provided strong evidence in favor of ADAR1 therapeutic targeting of hematologic malignancies. Conditional ADAR1 knockout in a murine AML model led to leukemic blast clearance within hours, with potent induction of interferon-stimulated genes and inflammatory cytokines. Clearance was eliminated in MDA5-deficient animals, confirming that MDA5 activation was the mechanism underlying the effect.

ADAR1 knockdown in myeloma cell lines rendered them sensitive to lenalidomide and interferon-α, showing synergistic cell killing. Such activities were eliminated by co-knockdown of RIG-I or MDA5, further confirming the role of RNA sensor activation in mediating cytotoxic activity. Transcriptome profiling revealed IFN-β, CXCL10, and other ISGs being induced on ADAR1 knockdown.

Apart from genetic models, small molecule ADAR1 inhibitors are also being developed. Small molecules selectively inhibit ADAR1 deaminase activity, allowing accumulation of immunostimulatory dsRNA in the cytoplasm. In preclinical xenografts models, treatment with the ADAR1 inhibitor was extremely effective in lowering tumor burden and increasing survival with little toxicity.

Therapeutic Implications and Drug Development

Blocking ADAR1 offers a novel approach to cancer immunotherapy using the natural antiviral immune response of the body. Unlike traditional immunotherapies based on T-cell stimulation or checkpoint inhibition, ADAR1 inhibition triggers intrinsic immune signaling in tumor cells themselves. This offers several potential advantages, such as reduced dependence on extrinsic immune context, application in immune-desert tumors, and compatibility with existing therapies.

However, some issues still remain. Of concern is the potential for systemic toxicity, especially in type I interferon-sensitive tissues such as bone marrow and liver. To decrease the risk, selective delivery of ADAR1 inhibitors to tumor cells can be performed through the use of nanoparticle-based delivery systems or antibody-drug conjugates. Another approach is to identify malignancy-specific ADAR1 dependencies to enable selective cancer targeting while sparing normal cells.

Besides, combination approaches can synergize the efficacy of ADAR1-based treatments. For example, co-treatment with RIG-I agonists may synergize immune activation, and co-treatment with checkpoint inhibitors may overcome adaptive resistance. Preclinical models prefer such combinations, with increased tumor control and durable responses.

Clinical Translation and Ongoing Trials

A few biopharmaceutical companies are currently advancing ADAR1 inhibitors into the clinic. Initial-stage trials will most likely begin patient enrollment with relapsed or refractory hematologic malignancies, and particularly with increased ADAR1 expression levels and functional MDA5/RIG-I pathways.

Biomarker development will be central to clinical success. Assays for monitoring ADAR1 levels, RNA editing function, and sensor expression are being optimized to guide patient selection. Liquid biopsy approaches may also be used to track therapeutic response and resistance.

Besides, synthetic lethality of ADAR1 has the potential to be extended beyond hematologic malignancies. Solid tumors such as breast, prostate, and melanoma are ADAR1 dependent as well, and hence the idea of this treatment would have more applications in place. Inter-patient studies across various cancers will determine where ADAR1 inhibition is most effective.

Future Directions

With advancing technology, several areas must be investigated in detail. Unraveling the full repertoire of ADAR1 substrates and their immunogenic potential will reveal the mechanisms of dsRNA sensing in cancer. Integrative approaches combining genomics, transcriptomics, and epitranscriptomics will be required to visualize the ADAR1 interactome.

In addition, examination of the roles of MDA5 and RIG-I in different hematologic contexts can potentially reveal lineage-specific vulnerabilities. While both are able to recognize dsRNA, they differ in ligand specificity and downstream signaling. Leverage of these differences could potentially allow for the tuning of immune activation for enhanced therapeutic impact.

Finally, knowledge from viral immunology could inform approaches to modulate ADAR1 activity. Various viruses exploit or repress ADAR1 in an attempt to evade immune detection. Mimicking such viral strategies—or their opposite with synthetic reagents—will yield new cancer therapy approaches.

Conclusion

ADAR1 has proved to be a critical modulator of innate immunity and RNA homeostasis in cancer and normal cells. In hematologic system cancer, overexpression promotes immune evasion by editing endogenous dsRNA and suppressing the activation of cytosolic RNA sensors. Synthetic lethality between ADAR1 and MDA5 or RIG-I is a promising therapeutic approach. Blocking ADAR1 can reveal the immunogenicity of cancer cells and induce their destruction by endogenous immunity. Preclinical data are robust, with early clinical trials on the horizon. By continuous study and careful translation, ADAR1 inhibition may become a corner stone of treatment for hematologic malignancies in the near future.