The HUSH complex is a gatekeeper of type I interferon through epigenetic regulation of LINE-1s

The Human Silencing Hub (HUSH) complex is necessary for epigenetic repression of LINE-1 elements. We found that depletion of HUSH components in human cell lines and primary fibroblasts leads to induction of interferon-stimulated genes (ISGs) through JAK/STAT signaling. This effect was mainly attributed to MDA5 and RIG-I sensing of double-stranded RNAs and coincided with upregulated LINE-1 RNAs. This included the human-specific family, L1HS, which produced bidirectional RNAs of around 6Kb, as well as ancient primate-conserved LINE-1s. LTRs nearby ISGs were derepressed likely rendering these genes more responsive to interferon. LINE-1 shRNAs could abrogate the HUSH-dependent response, while overexpression of an engineered LINE-1 construct could activate interferon signaling. Finally, we found that the HUSH component, MPP8 is frequently downregulated in diverse cancers and that its depletion leads to DNA damage. These results suggest that LINE-1s may drive physiological or autoinflammatory responses through RNA sensing and gene-regulatory roles and are controlled by the HUSH complex. Overall design: Identification of repetitive elements controlled by the HUSH complex that are capable of inducing a type I interferon response.