The discovery in human leukemic cells of particulate elements encapsulating 70S RNA and RNAdirected DNA polymerase made possible the synthesis of a [3IJDNA probe that could detect leukemia-specific sequences in the DNA of normal and leukemic individuals. In an earlier study of a series of unrelated leukemic patients, we established that the nuclear DNA of their leukemic cells contain particle-related sequences that cannot be detected in leukocytes of normal individuals. This result is inconsistent with the virogene concept that demands the inclusion of one complete copy of oncogenic information in the genome of every normal cell.The present study carries this analysis one step further by showing, with two sets of identical twins, that the leukemic member contains particle-related sequences in the DNA of his leukocytes that cannot be detected in the leukocytes of his healthy identical sibling. This finding implies that the additional leukemia-specific information found in the DNA of the leukemic individuals must have been inserted subsequent to fertilization. This outcome argues against the virogene hypothesis or any other etiologic concept that invokes vertical transmission through the germ line of the particle-related information found uniquely in the DNA of leukemic cells.We have shown by molecular hybridization that human adenocarcinomas of the breast (1), leukemias (2), sarcomas (3), and lymphomas (4) contain RNA molecules possessing a small but significant homology to RNAs of tumor viruses that cause the corresponding malignancies in mice. More telling for a viral involvement was the demonstration with the simultaneous detection test (5) that the RNA detected in these human cancers was 70 S in size and encapsulated with RNA-directed DNA polymerase in a particle possessing a density between 1.16 and 1.19 g/ml (6-9). Furthermore, DNA synthesized endogenously by these RNA-enzyme complexes exhibited evidence of complementarity to RNAs of the analogous murine viruses (6-9).Taken together, these experiments documented the existence in human neoplasias of particulate elements possessing four features diagnostic of animal RNA tumor viruses. The data did not of course establish that the particles identified were causative, but the evidence for their involvement was sufficiently convincing to encourage further exploration of their significance.In our studies of human leukemias we developed the methodology required to separate the particles containing the RNA-dependent DNA polymerase and its 70S RNA template. These particulate elements were then used to generate 3H-labeled DNA probes suitable for detecting corresponding 2629 sequences in genomes of any cells of interest. We found (9) that RNA of human leukemic particles shared sequences with the DNA of normal cells, a feature observed (10) with animal RNA tumor viruses and the normal DNA of their indigenous hosts. Sequences common to both normal DNA and the [3H]-DNA synthesized by leukemic particles were removed by exhaustive hybridization to a vast excess of ...
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