Are type C viruses transforming agents?

Abstract

Type C RNA viruses have been considered oncogenic because they are found associated with animal tumors and can induce cancers in several animal species. Those viruses that rapidly cause cancer appear to contain an oncogenic gene which resembles genetic sequences present in normal cells. This gene codes for a transforming protein which may be a normal cellular enzyme or a slightly altered cellular product. Its mechanism for transforming a cell is not yet known. Other oncogenic viruses, such as the chronic leukemia viruses, may not produce an oncogenic protein but may affect, by other means, specific target cells so they become malignant. Recent evidence now suggests that the majority of endogenous type C viruses are not transforming agents but inherited in the host to function in other biologic processes. These viruses do not contain transduced cellular genes which are responsible for cancer. Their role probably depends on their expression of other gene products which aid in normal development. These observations suggest that the ultimate control of human cancer may result from the identification of the oncogenic cellular-like genes transduced by some type C viruses even if a true human oncogenic virus is not isolated.