Destabilization of the adenosine deaminase gene sequences in rat-rat somatic cell hybrids

Rat hepatoma cells amplified for adenosine deaminase (ADA) gene sequences show the amplified DNA on large, homogeneously staining regions (HSRs). The amplified cells are stable in the absence of selection for 12 mo without loss of ADA activity or gene sequences. However, in hybrids formed between an amplified cell line with a prominent HSR and a nonamplified cell line, rapid loss of ADA activity, as well as gene sequences, occurs. Karyotype analyses of the hybrids indicate that the HSR structures are no longer visible in a large percentage of the hybrid metaphase spreads and appear to have been replaced by DNA structures that resemble double minutes. Our data provide evidence that the extent of the breakdown of the HSR in the hybrids may be affected by the presence of an active adenosine kinase or the level of ATP in the cells and additional unidentified factors are present in the hybrids that affect the integrity of the HSR structure. There is no evidence for a specific trans-acting factor in nonamplified cells that regulates gene amplification.