Gene amplification in human neuroblastomas: basic mechanisms and clinical implications

Extrachromosomal double minutes (DM) and homogeneously staining regions (HSR) of metaphase chromosomes have been reported frequently in human neuroblastomas and tumor derived cell lines. We and other investigators have determined that virtually all DM- and HSR-bearing neuroblastoma cell lines have multiple copies of the oncogene N-myc. Circumstantial evidence suggests that the extrachromosomal DM may be the level at which amplification takes place, with subsequent linear integration into HSR. Indeed, DM may represent circular molecules. The global organization and fine structure of the amplified sequences remains to be determined, but the unit of amplification is probably between 2 X 10(5) and 2 X 10(6) base pairs (bp). In some cases, the process of amplification may involve somatic recombination with distant DNA segments. We have determined that N-myc amplification is a common finding in primary neuroblastomas from untreated patients. We identified N-myc amplification, ranging from 3- to 300-fold, in 34 of 89 cases (38%). N-myc amplification is found almost exclusively in tumors from patients with advanced stages of disease (stages III and IV, p less than 0.01). In addition, the presence of amplification is highly correlated with rapid tumor progression (p less than 0.001). Thus, amplification of the N-myc oncogene is a new intrastage prognostic factor, and N-myc amplification appears to play an important role in determining disease progression.