In Practice 4: 115-118 (1982)
© 1982 British Veterinary Association

Cancer: New horizons

What is clear is that we all have oncogenes and that these are normally repressed. In cell division, such genes may be switched on to produce small amounts of the protein which does the job. When this is used up, cell division stops. In the forms of cancer described above, the genes get switched on but, in addition, they produce much larger amounts of the transforming protein and, being unable to switch this off, they go on dividing. But since the cancer change is at the gene level, each daughter cell inherits these changed genes and goes on dividing. Thus a cancer arises.

The huge field of research deriving originally from veterinary studies in farm and pet animals has had its major impact in opening the door to the hoped-for discovery of the most basic cell division and cancer mechanisms. But is also shows how to control veterinary diseases. In cats and cattle, methods are already available for diagnosing the presence of leukaemia virus infection. These studies explain why it may be years before a tumour appears in an infected animal. Although enzootic bovine leukaemia virus was introduced to Britain in 1972, by 1979 only three cases of leukosis had occurred despite a considerable spread of infection in those seven years. This probably meant that only three virus genomes out of the millions involved in the thousands of animals infected managed to integrate next to an oncogene.

It has recently been found that some tumour viruses can also switch on a host protein which is only ever expressed in early embryonic development. So that it may be that what is seen in the common cancers is a reversion to a mechanism only used at the time when rapid division is needed in undifferentiated cells or in cell populations which are constantly dividing. These need to go back into a similar state and are represented by stem cells of the lung, mammary gland, alimentary tract and bone marrow, the sites of the commonest cancers in man in this country.