P-348: ADAR1-regulated cytoplasmic dsRNA-immune sensing pathway is a novel mechanism of lenalidomide resistance in multiple myeloma

Introduction: Immunomodulatory drugs (IMiDs) such as lenalidomide and pomalidomide are one of the main cornerstone treatments in multiple myeloma (MM). However, acquired resistance to IMiDs commonly underlies relapse, rendering MM generally incurable. IMiDs bind directly to cereblon (CRBN), the substrate adaptor of the CRL4(CRBN) E3 ubiquitin ligase, and promote the proteasomal degradation of IKZF1 and IKZF3, resulting in MM cell growth inhibition. While resistance to IMiDs has been commonly associated with abnormalities in CRBN and the corresponding factors within the same pathway, the underlying mechanisms in the majority of the cases remain unclear. Adenosine-to-inosine RNA editing, a post-transcriptional modification of dsRNA, catalysed by ADAR1, has shown biological and clinical relevance in MM. ADAR1 is important in the regulation of innate immune response via the dsRNA sensing pathway. Nevertheless, how ADAR1 expression and its aberrant RNA editing could affect cancer immunogenicity and importantly, in regulating IMiDs responses in MM, remain largely unexplored. This study elucidates the role of ADAR1 in mediating MM responses to IMiDs, specifically lenalidomide.

Methods: A combination of public MM patients’ dataset analyses (MMRF CoMMpass, APEX, HOVON, UAMS), generation of isogenic lenalidomide-resistant (LenR) cells and MM models with differential ADAR1 expression were utilised to assess the functional role of ADAR1, its effect in regulating lenalidomide responses in MM and the plausible biological mechanism of actions.

Results: We observed a close association between ADAR1 expression with lenalidomide sensitivity in a panel of MM cells and in CoMMpass patients’ (n=776 len-treated patients of a total of 911), which portends clinical significance. MM cells with ADAR1 knockdown demonstrated enhanced lenalidomide sensitivity, associated with the accumulation of endogenous dsRNAs, activation of the dsRNA-sensing pathways and increased interferon (IFN) responses. Conversely, ADAR1 overexpression reduced lenalidomide sensitivity in an RNA-editing dependent manner, which was observed concomitantly with suppression of the dsRNA-sensing pathway. Assessment of the LenR cells versus len-sensitive revealed higher ADAR1 expression, in association with increased RNA editing frequency, impediment of dsRNAs accumulation and suppression of the dsRNA-sensing pathway. Importantly, we identified the MDA5 as the predominant dsRNA sensor stimulating MM immunogenicity to lenalidomide, with ADAR1 as the upstream regulator. Concordantly, these in vitro findings were supported by our in vivo observations.

Conclusions: In summary, we identified ADAR1-mediated suppression of dsRNA-sensing pathway as a novel mechanism regulating lenalidomide resistance. Our study harbours crucial translational value as ADAR1 is located at chromosome 1q21 which is frequently amplified in the MM patients. Further studies involve investigating the association of ADAR1 with CRBN pathway.