The role of telomerase reverse transcriptase (TERT) in human malignancies : genetic regulation and telomere lengthening-independent oncogenic activities

Sammanfattning: Telomerase is a ribonucleoprotein enzyme maintaining telomere length. Telomerase reverse transcriptase (TERT), which acts as a catalytic unit, is tightly repressed in differentiated human cells while activated in cancer cells for telomere lengthening. Beyond that, TERT has also been shown to contribute to oncogenesis via its telomere lengthening-independent functions. This thesis is designed to define the role of TERT in oncogenesis and the biological implications of the genetic alterations in the TERT gene/promoter. The first paper was focused on the role of TERT in acute myeloid leukemia (AML) therapy targeting mutations of FMS-like tyrosine kinases 3 (FLT3). The internal tandem duplication (ITD) mutation in the juxtamembrane domain in the FLT3 gene has been reported as one of the most frequent mutations in AML, and PKC412 was developed as its specific inhibitor. We observed that in FLT3ITD-harboring primary cells from AML patients and AML cell lines, PKC412 down-regulated TERT expression and telomerase activity in a MYC-dependent manner. Moreover, TERT restored the activity of FLT3 downstream effectors and alternative tyrosine kinase signaling pathways inhibited by PKC412, thereby attenuating PKC412-mediated apoptosis of leukemic cells. Taken together, FLT3ITD regulates TERT expression via a MYC-dependent manner, and TERT down-regulation is required for PKC412-mediated anti-AML efficacy. The recurrent TERT promoter mutations have been demonstrated to stimulate TERT transcription by generating new E26 transformation-specific specific (ETS)-binding sites in different human malignancies. Furthermore, rs2736098 and rs2736100, the two single nucleotide polymorphisms (SNPs) in the TERT locus, have been reported to associate with cancer susceptibility. In paper II, we found that compared to hepatocellular carcinoma (HCC) patients with wild type TERT promoter or healthy controls, a significant difference in these two genotypes was present in patients carrying TERT promoter mutations. We observed a negative association between TERT promoter mutations and rs2736098_TT and rs2736100_CC genotypes. There was no association between TERT promoter mutations and clinico-pathological variables or CTNNB1 mutations. In summary, the germline TERT rs2736098 and rs2736100 polymorphisms may play a role in TERT promoter mutation occurrence in HCC. The dysregulation of DNA methyltransferases (DNMTs) and the aberrant DNA methylation is a cancer hallmark. In paper III, we showed that TERT up-regulated DNA methyltransferase 3B (DNMT3B) expression and thereby contributing to the repression of downstream tumor suppressors as well as the activation of AKT. We found a positive correlation between TERT and DNMT3B expression in both HCC cell lines and primary HCC tumors. Mechanistically, TERT promotes DNMT3B transcription by cooperating with the transcription factor (TF) Sp1. The depletion of TERT expression led to significant demethylation in the tumor suppressor PTEN promoter and a reduced global DNA methylation by down-regulating DNMT3B expression. The restoration of PTEN expression mediated by TERT depletion inhibited AKT activity. Higher levels of TERT and DNMT3B expression predicted a significantly shorter survival in HCC patients according to analysis of The Cancer Genome Atlas (TCGA) dataset. Taken together, we identified the TERT-DNMT3B-PTEN-AKT axis in HCC cells, which promotes HCC progression via aberrant DNA methylation. In conclusion, our studies demonstrated the effect of genetic alterations on TERT/telomerase activation, and the novel telomere lengthening-independent roles of TERT in carcinogenesis, which should be implicated in cancer therapy/precision oncology.