Telomerase activity is tightly controlled in humans because telomere length imposes a limit on the number of times that any given cell can divide. Almost all cancer cells have reactivated telomerase in order to overcome this replicative limit via poorly understood mechanisms. One aim is to elucidate the regulatory mechanisms that confine telomerase expression to a subset of stem cells and germ cells and how those regulatory mechanisms are circumvented by cancer cells. Telomere length and telomerase expression also play a role in aging (telomeres become progressively shorter over time), and so a greater understanding of the telomerase regulatory network may also improve our understanding of the molecular mechanisms of aging and lead to potential anti-aging therapies.
Telomerase regulation in normal cells
Telomerase activity is usually undetectable in most human tissues, except for some proliferating stem-like cells in rapid turnover tissues. Previous reports have shown that mitogen stimulated human T lymphocytes transiently turn on telomerase activity that may reduce the rate of telomere loss during rapid proliferation. However, telomerase activation is transient in T-cells as opposed to cancer cells, and is only maintained for a few days even with continual mitogen stimulation. After approximately 4 days of stimulation, telomerase activity greatly decreases and eventually T-cells stop proliferating. With increased human age, T-lymphocytes show progressive telomere shortening. Almost all cancer cells activate telomerase, it does not turn off, cells achieve unlimited proliferation and telomeres do not further shorten. Currently it is unclear how telomerase activation is regulated in normal cells (such as in T lymphocytes) and how this regulation is hijacked by cancer cells.
Telomerase alternative splicing in normal human lymphocytes
Our lab has demonstrated that hTERT alternative splicing is a potential mechanism for telomerase regulation. In our recent studies, we found that changes of telomerase activity after T lymphocyte stimulation corresponded with the expression shifts of several hTERT splicing variants. By 72 hours, the ratio of catalytically active telomerase was increased compared to the non-functional splice variants (minus alpha and minus beta). This observation further emphasizes the potential role of hTERT alternative splicing in telomerase regulation in normal T lymphocytes. Taking advantage of the T lymphocyte stimulation model, we aim to study telomerase regulatory mechanisms in normal cells. The elucidation of how telomerase is regulated reversibly in primary proliferating transiently amplifying cells may facilitate our understanding of the potential mechanism(s) that cancer cells use to maintain telomerase activation. As part of these studies we will be examining T-cells from various aged individuals including super centenarians.
Telomerase alternative splicing in normal and cancer cells
Telomerase is regulated on many levels—from traditional transcriptional control to epigenetic regulation. Expression of hTERT is also regulated at the level of alternative splicing—with the full-length mRNAs being a minor component. We are determining the identities of all hTERT splice variants, the context in which they are expressed, and their function in normal and cancer cells. Since most splice forms are catalytically inactive and some are even potentially inhibitory, small molecules to increase the fraction of inactive hTERT splicing may be a viable option to produce shortening of telomeres in cancer cells, while increasing the fraction of active hTERT splicing could be used to increase the regenerative potential of stem cells.