Telomerase activation by c-Myc in human mammary epithelial cells requires additional genomic changes

A central question in breast cancer biology is how cancer cells acquire telomerase activity required for unlimited proliferation. According to one model, proliferation of telomerase(-) pre-malignant cells leads to telomere dysfunction and increased genomic instability. Such instability leads in rare cases to reactivation of telomerase and immortalization. The mechanism of telomerase reactivation remains unknown. We have studied immortalization of cultured human mammary epithelial cells by c-Myc, a positive transcriptional regulator of the hTERT gene encoding the catalytic subunit of telomerase. Retrovirally introduced c-Myc cDNA resulted in immortalization of human mammary epithelial cells in which the cyclin dependent kinase inhibitor, p16(INK4A), was inactivated by an shRNA-encoding retrovirus. However, while c-Myc introduction immediately resulted in increased activity of transiently transfected hTERT promoter reporter constructs, endogenous hTERT mRNA levels did not change until about 60 population doublings after c-Myc introduction. Increased endogenous hTERT transcripts and stabilization of telomeric DNA in cells expressing exogenous c-Myc coincided with telomere dysfunction-associated senescence in control cultures. Genome copy number analyses of immortalized cells indicated amplifications of some or all of chromosome 5, where hTERT genes are located. hTERT gene copy number, however, was not increased in one case. The results are consistent with the hypothesis that changes in chromosome 5, while not necessarily increasing hTERT gene copy number, resulted in removal of repressive chromatin structures around hTERT loci, allowing induction of hTERT transcription. These in vitro results model one possible sequence of events leading to immortalization of breast epithelial cells during cancer progression.