Megabase-scale analysis of the origin of N-myc amplicons in human neuroblastomas

In order to elucidate the initiation of the N-myc gene amplification, we have analyzed the original structures of the N-myc amplicons among 38 human neuroblastomas. Nineteen DNAs isolated from the N-myc amplicons recognized a continuous stretch totally encompassing a 5.5 megabase region spanning the normal N-myc gene. The co-amplification profiles with these DNAs showed that two of them, which mapped into a 300 kb region flanking the N-myc gene, were commonly amplified in most specimens, while others were differentially amplified among various subsets. These profiles enabled us to divide the N-myc amplicons into several groups and outline their original domains as a continuous stretches, pointing to the existence of 'consensus sites' for the ends of the initial domains in the original region. In one cell line, the domain was found to be several times larger than that of the derivative amplicon; and the rearranged sites identified within the amplicons, which showed no site specificity, were consistent with those deduced from the domain structure. These results lead to a model in which N-myc gene amplification is initiated at some consensus sites by a preferential mechanism and followed by a random loss of the domain structures during subsequent stages.