Activated Telomerase Targeting as Approach for Cancer Killing

Abstract

Telomerase abnormal re-expression and activation is a near-universal event in cancer development, but no telomerase-targeting therapy specifically directed to cancer development has reached the clinic yet. In 2013, a new opportunity has emerged: telomerase promoter mutations (TPMs) that upregulate telomerase expression, found at high frequencies in different cancer types, such as bladder cancer, gliomas and thyroid cancer. In shed of this recurrent mechanism, the development of a first-in-class precision therapy was initiated, specifically targeting this telomerase reactivation event. The two most recurrent TPMs were found at -124bp and -146bp of the ATG start-site of the telomerase promoter and generate de novo transcription factor (TF) ETS binding sites. GABP is, in mutant promoters, the primary ETS transcription factor recruited to these new binding sites, where its different isoforms are capable of enhancing TERT expression. Therefore, the DNA-GABP interaction appears to be a particularly attractive point for therapeutic intervention to target TPM-driven telomerase reactivation. To this end, a dual modality strategy was explored: (i) anti-gene oligonucleotides (AgONs) designed to invade double-stranded DNA and destabilise the topography at the ETS binding sites generated by TPMs, and (ii) VPC-18005, a small molecule inhibitor previously designed to block the DNA-binding domain of the ETS transcription factor ERG, repurposed for GABPα, given the high conservation of the DNA-binding domains within the ETS family of TFs. WLNA124, amongst different AgONs designs tested, was selected as the lead candidate, since it offered the best balance between binding affinity and specificity. Albeit not statistically significant, this AgON was capable of inhibiting telomerase promoter activity, mainly in the context of the mutated promoter, when tested in cellula. VPC-18005 showed a dose-dependent inhibition, particularly on the mutant TERT promoter. Then, it was studied the combined effect of the two strategies in order to achieve a more robust decrease in telomerase expression in mutation-harbouring cells. This synergy, despite not statistically significant, exhibited a trend to a higher inhibition in the mutated promoter. In conclusion, although at an early stage, this work offers the first evidence that targeting DNA-TF interaction through AgONs, small molecules or both may inhibit TPM-derived telomerase activation. The transference of an effective telomerase precision therapy to society would increase survival rates of several cancer types where TPMs are very prevalent and associated with guarded prognosis.