The importance of circular DNA in mammalian gene amplification.

Despite a decade of intensive research, the molecular mech anisms of gene amplification have remained elusive. However, recent molecular analyses and cytogenetic investigations sug gest that one frequently used mechanism for generating this important form of chromosome instability begins with the production of acentric, circular, extrachromosomal DNA mol ecules which replicate autonomously. Gene amplification then results from the unequal mitotic segregation and progressive accumulation of these elements under selective conditions. In many cases, the initial circles are too small to be visualized by light microscopy, but they enlarge over time to form heterogeneously sized double minute chromosomes. The molecular structures of the initial circular amplification intermediates, their production coincident with deletion of the corresponding chromosomal region in some cases, and the ability to increase the frequencies of both gene amplification and gene deletion with treatments which perturb DNA synthesis lead to the proposal that recombination within stalled replication inter mediates may be an important mechanism for generating cir cular amplification precursors. Furthermore, recent molecular data demonstrate a molecular chronology for gene amplifica tion in which the initial extrachromosomal elements integrate to generate intrachromosomal regions of gene amplification. A similar model was previously proposed to account for a wealth of cytogenetic observations (12-14). The substantial agreement between the molecular and cytogenetic data in human and rodent systems encompassing a variety of genes and selective conditions encourages the belief that the proposed model can account for many cases of gene amplification. In order to facilitate understanding of the interrelationships between the molecular and cytogenetic data presented later in this article, a model for the proposed chronology of gene amplification is presented in Fig. 1.