Alternative Splicing Regulation of Human Telomerase Reverse Transcriptase (TERT): Cancer Therapeutic Implications, Discovery of Novel TERT Isoforms, and Impact of Exercise and Aging

Abstract

Aging is associated with an elevated cancer risk due to the cumulative genetic mutations, cellular damage, and loss of immune cell function. Telomeres, the protective caps at the ends of linear chromosomes, naturally shorten with age, making them a key biomarker of the aging process. Higher levels of physical activity and exercise compared to a sedentary lifestyle are consistently linked to enhanced telomere maintenance in immune cells. Longer telomeres in immune cells may lead to greater anti-tumor immunosurveillance which may underpin the reduced risk of cancers observed in more active individuals. Telomeres are maintained by telomerase, the enzyme that can synthesize telomere repeats. Telomerase is predominantly repressed in somatic cells and is re-activated by cancer cells to ensure their immortality. Conversely, overexpression of telomerase extends lifespan of animals and exercise is associated with higher levels of telomerase activity in immune cells and other tissues. Telomerase activity is regulated by the mRNA and protein expression of TERT, which is the rate-limiting component of telomerase. A key mechanism of TERT mRNA and protein expression regulation is alternative RNA splicing (AS). However, we do not currently know the transfactors that dictate splicing choice, the impact of exercise on thymus TERT AS, the full catalogue of TERT AS isoforms, or the function of most TERT AS isoforms. Understanding the regulation of TERT and telomerase has implications for both aging and cancer therapies, as upregulation of telomerase activity can slow down telomere shortening, potentially mitigating the aging process, while inhibiting telomerase can offer a strategy to impede cancer growth. Thus, the major objectives of my dissertation were to elucidate new transfactors important to cancer cell TERT AS regulation, examine how exercise impacts TERT AS regulation in the thymus, and determine the catalogue of TERT AS isoforms and their functions. The central premise was that TERT AS exhibits distinct regulation depending on the physiological context, and a more complete understanding of telomerase regulation will result in novel therapies for cancer and aging. We tested this central premise in four separate but related projects as part of my dissertation work. In the first study, we found that a splicing factor, SF3B4, regulated TERT AS in lung cancer cells, and when SF3B4 was reduced, it inhibited telomerase activity and cancer cell growth, indicating a potential novel cancer therapeutic target. In the second study, we identified lung cancer-specific TERT AS regulatory factors (U2AF2 and SRSF2). In the third study, we observed that aging reduced thymus TERT expression and three weeks of voluntary wheel running impacted the AS of TERT in transgenic mice expressing human TERT. In the fourth study, we discovered a novel TERT isoform, Delta 2-4, and determined that it does not function in telomere biology but rather protects lung cancer cells from apoptosis. Collectively, these studies advance our understanding of telomerase regulation, thereby offering potential implications for cancer and aging therapies, and also establishes a fundamental basis for future studies, ultimately leading towards a more complete understanding of telomerase regulation.