PHENOTYPES OF C-MYC-DEFICIENT RAT FIBROBLASTS ISOLATED BY TARGETED HOMOLOGOUS RECOMBINATION

Rat fibroblast cell lines with targeted disruptions of both c-myc gene copies were constructed, Although c-myc null cells are viable, their growth is significantly impaired. The absence of detectable N-myc or L -myc expression indicates that Myc function is not absolutely essentia l for cell viability. The c-myc null phenotype is stable and can be re verted by introduction of a c-myc transgene. Exponentially growing c-m yc null cells have the same cell size, rRNA, and total protein content as their c-myc +/+ parents, but the rates of RNA and protein accumula tion as well as protein degradation are reduced. Both the G(1) and G(2 ) phases of the cell cycle are significantly lengthened, whereas the d uration of S phase is unaffected, This is the first direct demonstrati on of a requirement for c-myc in G(2). The G(0)-->S transition is sync hronous, but S-phase entry is significantly delayed. The c-myc null ce ll lines reported here are a new experimental system in which to inves tigate the importance of putative c-Myc target genes and to identify n ovel downstream genes involved in cell cycle progression and apoptosis .