Visualization of episomal and integrated Epstein-Barr virus DNA by fiber fluorescence in situ hybridization.

For many Epstein-Barr virus (EBV)-associated malignancies, it is still a matter of controversy whether infected cells harbor episomal or chromosomally integrated EBV genomes or both. It is well established that the expression of EBV genes per se carries oncogenic potential, but the discrimination between episomal and integrated forms is of great relevance because integration events can contribute to the oncogenic properties of EBV, whereas host cells that exclusively harbor viral episomes may not carry the risks mediated by chromosomal integration. This notion prompted us to establish a reliable technique that not only allows to unequivocally discriminate episomal from integrated EBV DNA, but also provides detailed insights into the genomic organization of the virus. Here, we show that dynamic molecular combing of host cell DNA combined with fluorescence in situ hybridization (FISH) using EBV-specific DNA probes facilitate unambiguous discrimination of episomal from integrated viral DNA. Furthermore, the detection of highly elongated internal repeat 1 (IR1) sequences provides evidence that this method permits detection of major genomic alterations within the EBV genome. Thus, fiber FISH may also provide valuable insights into the genomic organization of viral genomes other than EBV.