Telomeres are protective structures at the ends of chromosomes. They consist of tandem repeats of a DNA sequence, which in all vertebrates is TTAGGG, bound by a six-protein complex known as shelterin that exerts the protective function. Telomeres shorten every time a cell divides, and when they become critically short, this induces a persistent DNA damage response (DDR) at chromosome ends which can jeopardize cell viability.
In the context of the organism, telomeres shorten as our cells divide to regenerate tissues until critical telomere shortening impairs tissue regeneration and contributes to aging-associated pathologies. Mutations in genes involved in telomere maintenance are associated with a group of diseases known as “telomere syndromes,” including aplastic anemia and idiopathic pulmonary fibrosis. The main mechanism for telomere maintenance is the enzyme telomerase. Telomerase elongates telomeres by de novo addition of TTAGGG repeats onto chromosome ends. Telomerase is highly active at the pluripotent stage during embryonic development but its expression is largely silenced after birth with the exception of some adult stem cell compartments. Telomeres shorten with aging in all cells and tissues including stem cell compartments. In contrast, cancer cells reactivate telomerase to avoid critical telomere shortening and to sustain indefinite cell division.
Our group has investigated the possibility of using the enzyme telomerase to counteract telomere shortening with aging and in this manner to cure or to stop the progression of age-associated diseases as well as the so-called telomere syndromes characterized by extremely short telomeres. To this end, we have developed a telomerase gene therapy strategy which is based in using adeno-associated viral vectors carrying the telomerase gene (AAV9-Tert) to activate telomerase in the adult organism. We demonstrated that treatment with AAV9-Tert improved healthspan and increased both median and maximum longevity in wild-type mice (Bernades de Jesus et al., EMBO Mol. Med., 2012). In addition, AAV9-Tert treatment showed important therapeutic effects in mouse models of miocardial infarction (Bär et al., Nat Comms, 2014), aplastic anemia (Bär et al., Blood, 2016), and pulmonary fibrosis (Povedano et al., e-Life, 2018).
It is important to point out, that telomerase reactivation using this strategy will allow only for a temporary telomerase expression, thus decreasing a potential risk of telomerase gene therapy in promoting tumorigenesis. In particular, AAV9 vectors do not integrate into the host genome, therefore telomerase will be only activated for a few cell divisions before the vector is diluted out. Thus, the cancer risk associated with the activation of telomerase is minimized. In agreement with this, we never observed that AAV9-mediated telomerase activation resulted in increased incidence of spontaneous cancer in any of the above models (Bernades de Jesus et al., EMBO Mol. Med., 2012; Bar et al., Nat Comms, 2014; Bär et al., Blood, 2016; Povedano et al., e-Life, 2018)., However, it remains unknown, whether telomerase gene therapy could be tumorigenic in the context of an oncogenic insult.
In our recent study entitled AAV9-mediated telomerase activation does not accelerate tumorigenesis in the context of oncogenic K-Ras-induced lung cancer, published in Plos Genetics (August 2018), we set to study the potential oncogenic effects of AAV9-mediated Tert overexpression in the context of an oncogenic Kras-driven lung carcinogenesis mouse model. This is a very tumor-prone scenario in which mice develop a type of lung cancer that is very similar to the human form (Guerra et al., Cancer Cell, 2003). As a control, we also tested the effect of AAV9 vectors carrying a catalytically inactive form of Tert (Tert-DN), known to inhibit endogenous telomerase activity. We found that overexpression of wild-type Tert does not accelerate the onset or progression of lung tumors despite effectively increasing Tert expression and telomere length in healthy lung tissues and in the tumors. Interestingly, catalytically inactive Tert overexpression effectively impaired lung tumorigenesis, reducing tumor proliferation and increasing telomeric DNA damage.
We concluded that telomerase gene therapy does not favor tumorigenesis even in a cancer-prone context. Thus, our results strongly highlight the safety of AAV9-mediated telomerase activation in the treatment of age-associated diseases and of “telomere syndromes.” Our results also suggest the potential use of AAV9-Tert-DN gene therapy vectors as an anti-cancer treatment.
These findings are described in the article entitled AAV9-mediated telomerase activation does not accelerate tumorigenesis in the context of oncogenic K-Ras-induced lung cancer, recently published in the journal PLOS Genetics. This work was conducted by Miguel A. Muñoz-Lorente, Paula Martínez, Águeda Tejera, Kurt Whittemore, Ana Carolina Moisés-Silva, and Maria A. Blasco from the Spanish National Cancer Centre (CNIO), and Fàtima Bosch from the Universitat Autònoma de Barcelona.