Immunohistochemical detection of double-stranded RNA in formalin-fixed paraffin-embedded tissue

Evaluating the potential of anti-dsRNA antibodies as an alternative viral sensing tool in encephalitides of different species

Although laboratory methods have advanced, the cause of many encephalitides is still unknown. Molecular methods like multiplex PCR and microarrays are considered to be often less sensitive than Next Generation Sequencing, whereas the latter is time-consuming and costly. These analyses require appropriate tissue preparations and are more difficult to perform on formalin-fixed, paraffin-embedded (FFPE) tissues. Anti-double-stranded RNA (dsRNA) antibodies could potentially identify virus infections independently of the viral genome and can be applied to FFPE material. This study examined the applicability of monoclonal anti-dsRNA antibodies by immunohistochemistry to confirm encephalitides caused by different RNA viruses and comparing the findings with those obtained using monoclonal and polyclonal virus-specific antibodies. The viruses studied included negative-sense (Borna disease virus 1, BoDV-1; canine distemper virus, CDV; Rift Valley fever virus, RVFV) and positive-sense single stranded RNA viruses (severe acute respiratory disease syndrome coronavirus 2, SARS-CoV-2; tick-borne encephalitis virus, TBEV; Theiler's murine encephalomyelitis virus, TMEV). Interestingly, dsRNA was detected in both infected and non-infected animals and inconsistently co-localized to BoDV-1, TBEV, and TMEV antigen. Strict co-localization was lacking in CDV, SARS-CoV-2 and RVFV. Despite the co-localization of dsRNA with virus antigen for some RNA viruses, anti-dsRNA antibodies were unreliable as markers for unknown virus infections. Future studies should explore the upstream components of the immune response, including the interferon signaling cascade to assess their potential as effective virus-sensing tool.

Targeting MXD1 sensitises pancreatic cancer to trametinib

BACKGROUND

The resistance of pancreatic ductal adenocarcinoma (PDAC) to trametinib therapy limits its clinical use. However, the molecular mechanisms underlying trametinib resistance in PDAC remain unclear.

OBJECTIVE

We aimed to illustrate the mechanisms of resistance to trametinib in PDAC and identify trametinib resistance-associated druggable targets, thus improving the treatment efficacy of trametinib-resistant PDAC.

DESIGN

We established patient-derived xenograft (PDX) models and primary cell lines to conduct functional experiments. We also applied single-cell RNA sequencing, Assay for Transposase-accessible Chromatin with sequencing and Cleavage Under Targets and Tagmentation sequencing to explore the relevant molecular mechanism.

RESULTS

We have identified a cancer cell subpopulation featured by hyperactivated viral mimicry response in trametinib-resistant PDXs. We have demonstrated that trametinib treatment of PDAC PDXs induces expression of transcription factor MAX dimerisation protein 1 (MXD1), which acts as a cofactor of histone methyltransferase mixed lineage leukaemia 1 to increased H3K4 trimethylation in transposable element (TE) loci, enhancing chromatin accessibility and thus the transcription of TEs. Mechanistically, enhanced transcription of TEs produces excessive double-stranded RNAs, leading to the activation of viral mimicry response and downstream oncogenic interferon-stimulated genes. Inhibiting MXD1 expression can recover the drug vulnerability of trametinib-resistant PDAC cells to trametinib.

CONCLUSIONS

Our study has discovered an important mechanism for trametinib resistance and identified MXD1 as a druggable target in treatment of trametinib-resistant PDAC.

Mitochondrial double-stranded RNA homeostasis depends on cell-cycle progression

Mitochondrial gene expression is a compartmentalised process essential for metabolic function. The replication and transcription of mitochondrial DNA (mtDNA) take place at nucleoids, whereas the subsequent processing and maturation of mitochondrial RNA (mtRNA) and mitoribosome assembly are localised to mitochondrial RNA granules. The bidirectional transcription of circular mtDNA can lead to the hybridisation of polycistronic transcripts and the formation of immunogenic mitochondrial double-stranded RNA (mt-dsRNA). However, the mechanisms that regulate mt-dsRNA localisation and homeostasis are largely unknown. With super-resolution microscopy, we show that mt-dsRNA overlaps with the RNA core and associated proteins of mitochondrial RNA granules but not nucleoids. Mt-dsRNA foci accumulate upon the stimulation of cell proliferation and their abundance depends on mitochondrial ribonucleotide supply by the nucleoside diphosphate kinase, NME6. Consequently, mt-dsRNA foci are profuse in cultured cancer cells and malignant cells of human tumour biopsies. Our results establish a new link between cell proliferation and mitochondrial nucleic acid homeostasis.

Significant Variations in Double-Stranded RNA Levels in Cultured Skin Cells

Endogenous double-stranded RNA has emerged as a potent stimulator of innate immunity. Under physiological conditions, endogenous dsRNA is maintained in the cell nucleus or the mitochondria; however, if protective mechanisms are breached, it leaches into the cytoplasm and triggers immune signaling pathways. Ectopic activation of innate immune pathways is associated with various diseases and senescence and can trigger apoptosis. Hereby, the level of cytoplasmic dsRNA is crucial. We have enriched dsRNA from two melanoma cell lines and primary dermal fibroblasts, including a competing probe, and analyzed the dsRNA transcriptome using RNA sequencing. There was a striking difference in read counts between the cell lines and the primary cells, and the effect was confirmed by northern blotting and immunocytochemistry. Both mitochondria (10-20%) and nuclear transcription (80-90%) contributed significantly to the dsRNA transcriptome. The mitochondrial contribution was lower in the cancer cells compared to fibroblasts. The expression of different transposable element families was comparable, suggesting a general up-regulation of transposable element expression rather than stimulation of a specific sub-family. Sequencing of the input control revealed minor differences in dsRNA processing pathways with an upregulation of oligoadenylate synthase and RNP125 that negatively regulates the dsRNA sensors RIG1 and MDA5. Moreover, RT-qPCR, Western blotting, and immunocytochemistry confirmed the relatively minor adaptations to the hugely different dsRNA levels. As a consequence, these transformed cell lines are potentially less tolerant to interventions that increase the formation of endogenous dsRNA.