Abstract
Previous studies performed in our laboratory have shown that a minor single-stranded DNA component (ssDNA) isolated from the bulk of double-stranded nuclear DNA (nDNA) of human, murine, or avian cells consisted mainly of active transcription sites. In the present work, ssDNA isolated from human lymphoid cells, either malignant or not, amounted to 1.2-1.4% of the total nDNA. This was labeled with 125I and utilized as a probe in RNA-driven iterative hybridization and cross-hybridization assays. In all cases of neoplasia studied, the same subfraction of ssDNA (equivalent to 10-12% of the total ssDNA) from malignant cells could be hybridized with cytoplasmic RNA from malignant cells, whatever their origin: acute lymphoid leukemia cells collected from blood of leukemic patients, local lymphosarcomas, or cells from two established strains grown in suspension, one Epstein-Barr virus positive (Raji strain), the other Epstein--Barr virus negative (Ramos strain). The majority of these tumor-specific DNA sequences could be reassociated with unique-copy DNA from both normal and malignant cells and were not expressed in normal lymphoid cells, including those of an established strain immortalized by Epstein--Barr virus and multiplied in culture. It is concluded that a great number of new transcription units, in the range of 2500-3000 per cell genome, were activated after spontaneous malignant transformation of lymphoid cells.