Elucidating the Role of ADAR1 in Regulating Immunotherapeutic Response in Multiple Myeloma

Immunotherapies have changed the treatment paradigm and are the current standard of care in multiple myeloma (MM). The development of immunomodulatory drugs (IMiDs) such as lenalidomide has profound immunostimulatory effects and direct anti-MM activity; however, acquired resistance to IMiDs commonly underlies relapse, rendering MM largely incurable. IMiDs bind to cereblon (CRBN), the substrate adaptor of the CRL4^CRBN E3 ubiquitin ligase, and promote proteasomal degradation of IKZF1 and IKZF3, resulting in MM cell growth inhibition. Whilst CRBN is the primary binding target and has been shown to be pivotal in regulating IMiDs sensitivity, there have been IMiDs-resistant cases that failed to demonstrate any abnormalities in CRBN and its corresponding downstream factors, implying CRBN-independent mechanisms. Adenosine-to-inosine RNA editing, a post-transcriptional modification of dsRNA catalysed by ADAR1 has emerged as a vital contributor to transcriptome remodelling and has shown biological and clinical relevance in MM. Importantly, ADAR1 has been demonstrated to be critical in the regulation of innate immune response via the dsRNA sensing pathway. Nevertheless, how aberrant editing associated with dysregulation of immune sensing pathway and its role in regulating immunotherapeutic responses in MM remain largely elusive. In this study, we elucidated the role of ADAR1 in mediating responses to IMiDs, specifically lenalidomide.

To examine the significance of ADAR1 on modulating responses to lenalidomide, we first conducted a lenalidomide screen in a panel of MM cell lines. Using their IC50, we correlated their responsiveness to lenalidomide with the expression of ADAR1 and observed a positive correlation, where cells with higher ADAR1 expression had a larger IC50 value. We generated isogenic MM models with differential ADAR1 expression and evaluated its effect on their responsiveness to lenalidomide and its regulation on the activity of the dsRNA-sensing pathways. Accordingly, cells with ADAR1 loss demonstrated enhanced sensitivity to lenalidomide which was associated with the accumulation of endogenous dsRNAs, activation of the cytosolic dsRNA-sensing pathways and increased interferon (IFN) responses. Conversely, ADAR1 overexpression reduced sensitivity to lenalidomide with suppression of the dsRNA sensing-mediated response pathway.

To further delineate the mechanism underlying IMiDs resistance, isogenic lenalidomide-resistant (LenaR) cells were also established. Besides the decrease in CRBN expression, we observed that LenaR cells had increased ADAR1 expression with a consequential increase in RNA editing that is critical for preventing dsRNAs accumulation and induction of dsRNA-mediated IFN signalling. In concordance, analyses on MM patients who underwent IMiDs-based regimens revealed that dsRNA sensors expression is correlated with clinical outcomes in patients with differential ADAR1 expression. Importantly, we identified that the MDA5-mediated dsRNA-sensing pathway stimulates immunogenicity of MM cells, with ADAR1 being the upstream regulating factor in the immune sensing pathway contributing to the lenalidomide responsiveness.

In summary, our study identified ADAR1-mediated activation of dsRNA sensing pathway as a novel mechanism regulating lenalidomide sensitivity. The potential role of ADAR1 in modulating immunotherapeutic responses may help unravel potential resistance mechanisms and identify novel therapeutic strategies. Current studies involve elucidating the association of ADAR1 with CRBN pathway.

Chng:Hummingbird: Research Funding; Novartis: Honoraria; Takeda: Honoraria; Abbvie: Honoraria; BMS: Honoraria; Celgene: Honoraria, Research Funding; J&J: Honoraria, Research Funding; Amgen: Honoraria.