Long Terminal Repeats of Gammaretroviruses Retain Stable Expression After Integration Retargeting or Knock-In into the Restrictive Chromatin of Lamina-Associated Domains

Retroviruses integrate their genomes into the genomes of infected host cells and form a genetic platform for stable gene expression. Epigenetic silencing can, however, hamper the expression of integrated provirus. As gammaretroviruses (γRVs) preferentially integrate into sites of active promoters and enhancers, the high expression activity of γRVs can be attributed to the integration preference. Long terminal repeats (LTRs) of some γRVs were shown to act as potent promoters for gene expression. Here, we investigate the capacity of different γRV LTRs to drive stable expression inside a non-preferred epigenomic environment using MLV-derived BET-independent (Bin) vectors. We demonstrate that different γRV LTRs are either rapidly silenced or long-term active with active proviral population prevailing under normal and retargeted integration. The K562 cell line was transduced at low multiplicity of infection with MLV-derived vectors expressing GFP under the control of MLV long terminal repeat. The MLV vectors were produced with wt or BET-independent (Bin) integrase; Bin vectors carried integrase with W390A mutation or W390A-mutated integrase fused with CBX chromodomain. The GFP-expressing K562 cells were bulk sorted at three days post infection (3 dpi) and harvested for DNA extraction at 14 dpi. Amplicons with integration sites were prepared separately for each transduced population and samples were pooled prior the sequencing.