Modification of myc gene amplification in human somatic cell hybrids

In order to study whether cell fusion would modify the DNA copy number of an amplified oncogene, somatic cell hybrids were made between the human neuroepithelioma cell line MCIXC and HeLaCOT human adenocarcinoma cells. MCIXC contains approximately 21 copies of the c-myc oncogene and HeLaCOT contains approximately 5 copies relative to the control. All hybrid clones investigated displayed a marked decrease in the number of copies of c-myc DNA (an average of 5 copies), while the level of c-myc RNA in the hybrids was similar to that found in both parents. All eight hybrid clones were found to be completely nontumorigenic even though both parent cells formed tumors in 100% of the nude mice treated by injection. This loss of oncogene amplification in the hybrids was shown not to be due to either heterogeneity of c-myc amplification in the MCIXC parent or segregation of a copy of the chromosome 22 from the hybrids. This loss most likely resulted from the breakdown of a homogeneously staining region (containing the amplified gene copies) into double minutes, which were subsequently lost from the cells. The HeLaCOT cell line was also fused to the human neuroblastoma BE(2)C, which contains approximately 123 copies of the N-myc oncogene relative to control. Ten hybrid clones were found to contain an average of 47 copies of N-myc DNA, significantly less than the 91 copies predicted had no loss occurred. These BE(2)C x HeLaCOT hybrids expressed on average about 15% the N-myc RNA seen in the BE(2)C parent and, as with the MCIXC x HeLaCOT hybrids, were found to be completely nontumorigenic. However, upon passage in culture, one BE(2)C x HeLaCOT hybrid eventually became tumorigenic. This hybrid also displayed reduced copies of N-myc DNA in comparison to its parent hybrid but surprisingly showed a 2-fold increase in N-myc RNA. Thus, the expression of N-myc, but not the amplification state of either myc gene, appears to correlate with the tumorigenicity of the cells.