Discovery of oncogenes: The advent of molecular cancer research.

In their classic paper on the identification of the transforming principle of Rous sarcoma virus (RSV) published 1970 in PNAS (1), Peter Duesberg at the University of California, Berkeley, and Peter Vogt, then at the University of Washington, Seattle, drew a seemingly simple yet groundbreaking conclusion. When they analyzed the genomic RNAs of transforming, acutely oncogenic RSV and of transformationdefective (td) mutant derivatives, they found that all transforming virus stocks contained two classes of RNA subunits, a larger one (a) and a smaller one (b), whereas the nontransforming yet replication-competent mutants contained the smaller b subunits only. Duesberg and Vogt concluded that the larger a subunit contained the transforming principle of RSV. Based on this and on subsequent structural comparisons of the a and b subunits of biologically cloned viruses, the transforming principle was defined by the remarkably simple equation a − b = x and was later termed src (for sarcoma). The first biochemical identification of a cancer gene was achieved, initially in a chicken virus. However, the principal proof of a physical underpinning of the cancer gene hypothesis had tremendous impact on a fundamental challenge of medicine, decoding the molecular basis of human carcinogenesis. The genetic and biochemical investigations of the chicken tumor virus RSV and the persistent search for its transforming principle are a classic paradigm in cellular and molecular cancer research (2, 3). In 1911, Peyton Rous at the Rockefeller Institute in New York discovered the first virus—later termed RSV—that could induce solid tumors in infected fowl, demonstrated by experimental transmission of sarcomas using cell-free filtrates of tumor extracts (4). This seminal discovery started the field of tumor virology (2, 3, 5). However, almost half a century had to pass before the first quantitative biological tools were developed to study the biology of RSV and its interaction with infected cells in detail. RSV is capable of transforming primary chicken embryo fibroblasts in culture, and the focus assay developed in 1958 by Howard Temin and Harry Rubin at the California Institute of Technology allowed a quantitative assessment of the virus–cell interaction leading to malignant cell transformation (6). The next crucial steps toward the identification of the underlying principle of RSV oncogenicity were based on classic genetics. The characterization of various viral strains that induced different morphologies of transformed cells suggested that the phenotype of the cancer cell is controlled by the incoming genetic information carried by the viral genome. The isolation of RSV mutants that can transform cells but do not produce infectious progeny, or vice versa, can replicate but have lost cell transforming Fig. 1. Biochemical definition of src, the first oncogene. Panels A and B, above, are from the original PNAS paper by Duesberg and Vogt (1). They show electropherograms of the 60–70S RNAs from two transforming strains of RSV, Schmidt-Ruppin (SR) and B77, before (A) and after (B) heat-dissociation. Insets in A show the final sucrose gradient purification of the RNAs before electrophoretic analysis. The heat-dissociated RNAs were resolved into two subunits with lower (a) and higher (b) electrophoretic mobility. Analyses of biologically cloned viruses revealed that the larger subunit represents the genomic RNA of transforming RSV, whereas the b subunit is the genome of transformation-defective (td) mutants spontaneously segregating from RSV (1, 10). Subsequent mapping studies (10, 11) confirmed that the genomes of RSV and of td mutants share all replicative genes (gag, pol, env) and that the size difference (a − b = x) is caused by the additional src gene at the 3′ end of the RSV genome. Cells infected by RSV become transformed (indicated by rounding) and produce virus progeny (red star symbols), whereas td RSV replicates (green star symbols) but does not transform the host cell. A and B reproduced with permission from ref. 1. Author contributions: K.B. wrote the paper.