Single-cell imaging reveals unexpected heterogeneity of TERT expression across cancer cell lines

Telomerase is pathologically reactivated in most human cancers, where it maintains chromosomal telomeres and allows immortalization. Because telomerase reverse transcriptase (TERT) is usually the limiting component for telomerase activation, numerous studies have measured TERT mRNA levels in populations of cells or in tissues. However, little is known about TERT expression at the single-cell and single-molecule level. Here we analyzed TERT expression across 10 human cancer lines using single-molecule RNA FISH and made several unexpected findings. First, there was substantial cell-to-cell variation in number of transcription sites and ratio of transcription sites to gene copies. Second, previous classification of lines as having monoallelic or biallelic TERT expression was found to be inadequate for capturing the TERT gene expression patterns. Finally, TERT mRNA had primarily nuclear localization in cancer cells and induced pluripotent stem cells (iPSCs), in stark contrast to the expectation that mature mRNA should be predominantly cytoplasmic. These data reveal unappreciated heterogeneity, complexity, and unconventionality in TERT expression across human cancer cells.