Circular DNA tumor viruses make circular RNAs

Area of Science:

• Virology
• Molecular Biology
• Oncology

Background:

• Epstein-Barr virus (EBV) and Kaposi's sarcoma herpesvirus (KSHV) are gammaherpesviruses linked to a significant proportion of human cancers.
• Previous research has primarily focused on linear viral transcripts, with limited understanding of circular RNA (circRNA) expression in these infections.

Purpose of the Study:

• To investigate the existence and characteristics of circular RNAs (circRNAs) encoded by EBV and KSHV.
• To determine the expression patterns and cellular localization of these viral circRNAs in infected cells and tumor tissues.
• To explore the potential role of viral circRNAs in gammaherpesvirus-associated oncogenesis.

Main Methods:

• Utilized RNase R-resistant RNA sequencing to identify and characterize stable circular RNAs (circRNAs) from EBV and KSHV.
• Analyzed circRNA expression in various EBV and KSHV latency types, including clinical tumor samples and cell lines.
• Employed subcellular fractionation and fluorescence in situ hybridization (FISH) to determine the cellular localization of viral circRNAs.

Main Results:

• Identified multiple, stable circRNAs encoded by both EBV (circBARTs, circLMP2, circBHLF1) and KSHV (circvIRF4, circPANs).
• circBARTs were abundantly expressed in diverse EBV-associated cancers and latency types, originating from the BART locus.
• KSHV exhibited a novel hyperbacksplicing mechanism generating numerous circRNAs from the PAN RNA locus; circvIRF4 was constitutively expressed.
• Viral circRNAs were detected in both nuclear and cytoplasmic fractions, with exon-only circBARTs predominantly in the cytoplasm.
• No viral circRNAs were found in polysome fractions, suggesting limited direct protein translation, though it could not be entirely excluded.

Conclusions:

• Viral circRNAs represent a newly discovered class of transcripts encoded by gammaherpesviruses like EBV and KSHV.
• The stable expression of these viral circRNAs in tumors suggests a potential role in the pathogenesis of gammaherpesvirus-related cancers.
• Further research is warranted to elucidate the precise functions and oncogenic mechanisms of these viral circRNAs.