Journal: Genome Biology
Article Title: Herpes simplex virus type 1 R-loops are targets for APOBEC-mediated mutagenesis
doi: 10.1186/s13059-026-04078-y
Figure Lengend Snippet: Orthogonal APOBEC3 overexpression systems link HSV-1 mutagenesis to R-loops across multiple cell types. Schematic of the two complementary gain-of-function approaches used to validate the APOBEC–R-loop connection: a doxycycline-inducible A3A-HA construct in RPE-1 cells and CMV-driven A3A-tGFP or A3G-tGFP in HEK293-AD cells. A Experimental design: following doxycycline (dox) induction, RPE-1 A3A-HA cells were infected with HSV-1 and subjected to DRIP-seq (R-loops), anti-HA ChIP-seq (A3A binding), and viral mutation analysis. B Western blot with anti-HA antibody showing time-dependent accumulation of A3A-HA after doxycycline addition; the 4-h induction point was used for all NGS-based assays. Total protein staining is shown as a loading control. C Representative immunofluorescence image of doxycycline-treated, HSV-1–infected RPE-1 cells, illustrating robust A3A-HA expression (green), viral antigen staining (magenta), and their overlap in merged channel. D Constitutive A3A-tGFP and A3G-tGFP expression in HSV-1–infected HEK293-AD cells. Schematic of CMV-driven A3A-tGFP and A3G-tGFP constructs used to generate stable HEK293-AD lines; these cells were infected with HSV-1 and processed for DRIP-seq, ChIP-seq with anti-tGFP, and mutation profiling. E Live-cell confocal images showing A3A-tGFP and A3G-tGFP distributions in HEK293-AD cells. F Representative genome-browser view of a 300-bp HSV-1 region illustrating APOBEC-type mutations identified in RPE-1 (blue), HEK293-AD (green), and Jurkat (red) cells. Mutations cluster in discrete hotspots that recur across independent experimental platforms. G APOBEC mutation overlap across cell types. Upper panel: Venn diagram of C → T changes in Jurkat, RPE-1 A3A-HA, and HEK293-AD A3A/A3G-tGFP cells, showing a statistically significant shared subset of hotspots, with the strongest overlap between the two epithelial cell systems. Lower panel: Venn diagram comparing C → T mutations in HEK293-AD A3A-tGFP and A3G-tGFP cells. Nearly all C → T mutations in A3G-tGFP cells coincide with those in A3A-tGFP cells, whereas A3A-tGFP generates additional sites not targeted by A3G-tGFP. H Inter-mutation distance analysis. Histograms depict the distribution of distances between adjacent APOBEC-type mutations (blue) compared with an equal number of randomly positioned mutations (yellow) for RPE-1 A3A-HA (top), HEK293-AD A3A-tGFP (middle), and HEK293-AD A3G-tGFP (bottom) cells. In all cases, observed inter-mutation distances are markedly shorter than random (Wilcoxon test p < 2.2 × 10⁻ 1 ⁶), indicating strong clustering of APOBEC edits. I Colocalization of APOBEC binding with R-loops at C → T mutations. Anchor plots show normalized DRIP-seq signal (R-loops, red) and APOBEC3 ChIP-seq signal (A3A-HA and A3A/A3G-tGFP, blue) centered on C → T mutation sites extended by ± 1.5 kb. In RPE-1 cells, the A3A-HA and R-loop peaks coincide, whereas in HEK293-AD cells the A3A/A3G-tGFP signals form twin peaks flanking the central R-loop summits, consistent with APOBEC binding to ssDNA bordering the RNA–DNA hybrid within the broader R-loop domain. J Substitution mutation spectra in APOBEC ChIP and input samples. Table summarizing counts and frequencies of base-substitution types in HSV-1 genomes recovered from A3A/A3G ChIP and matched input DNA. C → T transitions and C → G transversions are enriched compared with other substitution classes (two-proportion z-test p < 0.00001)
Article Snippet: For ChIP, 8 μg of rabbit polyclonal anti-APOBEC3A (D-23) (Sc-130688) and anti-APOBEC3G (H-63) (Sc-48820) rabbit polyclonal antibodies were used (Jurkat cells), while the anti-HA rabbit polyclonal antibody (Abcam ab9110) was used for RPE-1 p dox A3A-HA cells and anti-tGFP mouse monoclonal turboGFP antibody (Origene, clone OTI2H8) for HEK293-AD p CMV A3A-tGFP and p CMV A3G-tGFP cells.
Techniques: Over Expression, Mutagenesis, Construct, Infection, ChIP-sequencing, Binding Assay, Western Blot, Staining, Control, Immunofluorescence, Expressing