Sequestration and metabolism of host nucleic acids is important for preventing the aberrant activation of cytosolic innate immune sensors. Various studies have identified key pathways that prevent mitochondrial DNA (mtDNA) from escaping into the cytoplasm; Nicholas Proudfoot and colleagues now describe key roles for the RNA degradasome components SUV3 and polynucleotide phosphorylase (PNPase, encoded by PNPT1) in preventing the accumulation of mitochondrial double-stranded RNA (mtdsRNA).

Owing to their bacterial origin, mitochondria have many prokaryotic features, including a circular genome. As such, bidirectional transcription of mtDNA can generate overlapping transcripts with the potential to form mtdsRNA, but such molecules have not been characterized in vivo. Using a fluorescently labelled antibody that had previously been used to detect viral dsRNA, Dhir et al. found that a weak fluorescence signal could also be detected in uninfected HeLa cells. The signal colocalized with mitochondria, was sensitive to the dsRNA-specific RNAse III and could not be detected in mitochondria-depleted HeLa cells.

Credit: S. Bradbrook/Springer Nature Limited

Inhibition of mitochondrial transcription led to a rapid loss of mtdsRNA, and in small interfering RNA (siRNA)-mediated depletion experiments, the authors identified key roles for the helicase SUV3 and for PNPase in preventing dsRNA accumulation. Further studies suggested that the unwinding activity of SUV3 and exonuclease activity of PNPase are important for limiting dsRNA levels in cells.

Interestingly, only depletion of PNPase (and not SUV3 depletion) triggered a type I interferon (IFN) response in HeLa cells. Microscopy studies revealed that dsRNA accumulation occurred solely in the mitochondria of SUV3-depleted cells, whereas dsRNA accumulated in both the mitochondria and cytoplasm of PNPase-depleted cells. Further knockdown experiments identified MDA5 as the primary cytosolic sensor of mtdsRNA and suggested that mtdsRNA enters the cytosol through BAX–BAK mitochondrial pores in PNPase-depleted cells.

PNTP1 deficiencies may represent a novel type 1 interferonopathy

Importantly, the authors detected accumulation of dsRNA in fibroblasts from four different patients with hypomorphic mutations in PNTP1. Furthermore, upregulation of IFN-stimulated genes was seen in peripheral blood samples from these patients. The authors suggest that PNTP1 deficiencies may represent a novel type 1 interferonopathy.