Shope papilloma virus DNA is extensively methylated in non-virus-producing neoplasms

The extent of viral DNA methylation in non-virus-producing benign and malignant tumors induced by the Shope (rabbit) papilloma virus was investigated with the two isoschizomeric restriction endonucleases MspI and HpaII. The overall extent of methylation is variable in papillomas and uniformly higher in carcinomas. In a transplantable Shope virus-induced carcinoma and in two transplantable carcinoma derived cell lines, the viral DNA is even more highly methylated than in carcinomas induced by the virus directly. However, even in these cells at least two MspI sites remain essentially unmethylated and these sites are also unmethylated in carcinomas induced by the virus directly. At least one additional MspI site is unmethylated in papillomas but is partly methylated in carcinomas. Viral DNA from papillomas, which consists mostly of unit-sized form I and form II DNA and from carcinomas in which large oligomeric forms predominate was separated into fractions containing viral plasmids of different size and conformation. The analysis of the different fractions showed that methylation patterns do not correlate with the size of viral DNA, but they rather correlate with the degree of neoplasia.