A small chromosome identified in the cancer cells of patients with chronic myelogenous leukaemia (CML) was the first genetic defect to be associated with cancer. This chromosome was subsequently found to be the product of a translocation, a breakthrough that led to the identification of chromosome translocations in other cancers and the discovery of many oncogenes.
In a paper presented at a National Academy of Sciences meeting in 1960, University of Pennsylvania researchers Peter Nowell and David Hungerford first reported the presence of a "minute chromosome" that replaced one of the four smallest autosomes in CML cells. They did not observe this defect in other types of leukaemia or lymphoma cells, and many other cells from these patients contained a normal karyotype. This unique chromosome was designated 'The Philadelphia chromosome' after the city in which the discovery was made.
For many years, scientists thought that the Philadelphia chromosome resulted simply from the loss of genetic material. In the early 1970s, the development of techniques such as quinacrine fluorescence and Giemsa banding allowed researchers to identify and track segments of chromosomes. Accelerating the burgeoning field of cytogenetics, Janet Rowley used these technologies to identify a genetic abnormality in CML cells — the addition of extra material to chromosome 9. She then noticed that the amount of additional material was approximately equal to the amount missing from chromosome 22, and proposed that there was a "hitherto undetected translocation between the long arm of 22 and the long arm of 9" that resulted in formation of the Philadelphia chromosome. In the same year, Rowley reported another translocation between chromosomes 8 and 21 in acute myeloblastic leukaemia cells. She went on to discover more than a dozen translocations that were specific to other types of cancer cells, notably t(15;17) in acute promyelocytic leukaemia and t(14;18) in lymphoma, which was also described by Carlo Croce and colleagues. Cytogenetic analysis is still one of the most reliable methods of diagnosis and of determining prognosis in patients with leukaemia or lymphoma.
So, how do these chromosome rearrange-ments cause cancer? In 1982, Annelies de Klein and colleagues reported that the human cell homologue (c-ABL; also known as ABL1) of the transforming sequence of Abelson murine leukaemia virus was translocated from chromosome 9 to chromosome 22q in CML cells. This finding indicated a role for c-ABL in the generation of human leukaemia. At the same time, Rebecca Taub and co-workers, and Riccardo Dalla-Favera and colleagues reported the translocation of c-MYC into the immunoglobulin heavy chain locus, through a translocation between chromosomes 8 and 14. This translocation had been identified by Laura Zech and co-workers, and is frequently observed in Burkitt lymphoma cells. Evidence that the translocation and subsequent deregulation of MYC expression is an oncogenic event was later provided by the E
-Myc mouse model generated in 1985 by Jerry Adams and colleagues.
Cloning of the breakpoints of other cancer-associated translocations would subsequently lead to the discovery of many other oncogenes, such as B-cell lymphoma 2 (BCL2) and tumour suppressor genes.
