Molecular mechanisms of oncogenesis.

Cellular oncogenes (c-oncs) have been highly conserved throughout evolution and subserve important roles in growth and development. Both in development and the neoplastic state, c-oncs appear to collaborate rather than function independently. Cellular oncogenes are activated in the neoplastic process by four (nonviral) mechanisms; (a) chromosomal translocations; (b) gene amplifications; (c) point mutations; and (d) DNA rearrangements. The timing of c-onc gene product expression may be as important in oncogenesis as the level of expression. At this writing, mutant oncogenes have not been shown to be inherited. Oncogene amplification, if important in oncogenesis, is more likely to be involved with tumor progression rather than initiation. Chromosomal/molecular aberrations tend to be characteristic for a given type of cancer. These genetic alterations are often situated near heritable fragile sites, tumor-suppressor gene loci and/or oncogene loci. Similar molecular mechanisms involving translocations and inversions may underly the common T and B cell neoplasms. The loss/inactivation of both normal alleles at a locus thought to encode for tumor-suppressing activities (antioncogenes) may represent an event common to many childhood and adult neoplasms. The consistency and cell specificity with which this has been identified is consistent with a role for such genes in cellular differentiation. At this writing, the paradigm for such a controlling locus is 13q14, the site of the retinoblastoma gene. Based on recent studies in familial and sporadic Wilms' tumor which suggest etiological heterogeneity, theoretical modifications of the carcinogenesis model which has been central to understanding retinoblastoma may soon be forthcoming to explain molecular mechanisms operative in other cancer. The role of genomic imprinting in carcinogenesis is only recently being explored. Further study of this process may prove to be a fruitful area of future research.