SummaryHepatitis B viruses (HBVs), which are enveloped viruses with reverse-transcribed DNA genomes, constitute the family Hepadnaviridae. An outstanding feature of HBVs is their streamlined genome organization with extensive gene overlap. Remarkably, the ∼1,100 bp open reading frame (ORF) encoding the envelope proteins is fully nested within the ORF of the viral replicase P. Here, we report the discovery of a diversified family of fish viruses, designated nackednaviruses, which lack the envelope protein gene, but otherwise exhibit key characteristics of HBVs including genome replication via protein-primed reverse-transcription and utilization of structurally related capsids. Phylogenetic reconstruction indicates that these two virus families separated more than 400 million years ago before the rise of tetrapods. We show that HBVs are of ancient origin, descending from non-enveloped progenitors in fishes. Their envelope protein gene emerged de novo, leading to a major transition in viral lifestyle, followed by co-evolution with their hosts over geologic eras.

Interferon (IFN) induction of IFN‐stimulated genes (ISGs) creates a formidable protective antiviral state. However, loss of appropriate control mechanisms can result in constitutive pathogenic ISG upregulation. Here, we used genome‐scale loss‐of‐function screening to establish genes critical for IFN‐induced transcription, identifying all expected members of the JAK‐STAT signaling pathway and a previously unappreciated epigenetic reader, bromodomain‐containing protein 9 (BRD9), the defining subunit of non‐canonical BAF (ncBAF) chromatin‐remodeling complexes. Genetic knockout or small‐molecule‐mediated degradation of BRD9 limits IFN‐induced expression of a subset of ISGs in multiple cell types and prevents IFN from exerting full antiviral activity against several RNA and DNA viruses, including influenza virus, human immunodeficiency virus (HIV1), and herpes simplex virus (HSV1). Mechanistically, BRD9 acts at the level of transcription, and its IFN‐triggered proximal association with the ISG transcriptional activator, STAT2, suggests a functional localization at selected ISG promoters. Furthermore, BRD9 relies on its intact acetyl‐binding bromodomain and unique ncBAF scaffolding interaction with GLTSCR1/1L to promote IFN action. Given its druggability, BRD9 is an attractive target for dampening ISG expression under certain autoinflammatory conditions.

Transient interferon (IFN) induction of IFN-stimulated genes (ISGs) creates a formidable protective antiviral state. However, loss of appropriate control mechanisms can result in constitutive pathogenic ISG upregulation. Here, we used genome-wide loss-of-function screening to establish genes critical for IFN signaling, identifying all expected members of the JAK-STAT pathway and the previously unappreciated bromodomain-containing protein 9 (BRD9), a defining subunit of non-canonical BAF (ncBAF) chromatin remodeling complexes. Genetic knock-out or small-molecule mediated degradation of BRD9 limited IFN-induced expression of a subset of ISGs in multiple cell-types, and prevented IFN from exerting full antiviral activity against several RNA and DNA viruses. Mechanistically, BRD9 acts at the level of ISG transcription, exhibits a proximal association with STAT2 following IFN stimulation, and relies on its intact acetyl-binding bromodomain and unique ncBAF scaffolding function for activity. Given its druggability, BRD9 may be an attractive target for dampening constitutive ISG expression under certain pathogenic autoinflammatory conditions.

No abstract