HTLV-1 antisense transcription is promoted by increased SP1 binding at 3'-LTR G-Quadruplexes

The human T-cell lymphotropic virus type 1 (HTLV-1) is a highly oncogenic delta-retrovirus. It presents 5'- and 3'-long terminal repeats (LTR) that are enriched in putative G-quadruplex (G4)-forming sequences. G4s are non-canonical nucleic acid structures that regulate key biological processes in both human and viral genomes. We here investigated the presence and functional role of G4s within the HTLV-1 3'-LTR, which governs the antisense transcription of the viral bZIP factor (HBZ), the main responsible for T-cell transformation. We identified seven highly conserved sequences that folded into two-layer G4s in both single- and double-stranded DNA in vitro. We demonstrated G4 folding in infected cells by chromatin immunoprecipitation and showed SP1 enrichment at the 3'-LTR G4s. We showed that G4 stabilization with a ligand enhances antisense transcription by promoting recruitment of SP1. Our findings unveil a G4-mediated regulatory mechanism sustaining HTLV-1 antisense transcription and provide new insights into the complex interplay between the HTLV-1 genome and host cellular factors, contributing to our understanding of retroviral replication strategies to be exploited as new therapeutic targets.