Page 89 - Annual report 2021-22

Annual Report 2021-22 |

Shantanu Chowdhury

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Shantanu Chowdhury has been studying the importance of G-quadruplex structures in cancer.

G-quadruplex, telomerase and cancer

In more than 80% of human tumors, up-regulation or reactivation of the telomerase enzyme results
in aggressive cancers. With the overarching goal of inhibiting telomerase activity his group has
adopted a three-tier inhibition of telomerase activity by (i) direct interaction with telomerase reverse
transcriptase subunit (hTERT) thereby suppressing telomerase activity, (ii) by directly suppressing the
expression of telomerase catalytic subunit (hTERT) by stabilizing G-quadruplex motifs in the
telomerase promoter using specific small molecules and (iii) by stabilizing G-quadruplex at telomeres
and therefore inhibit association of telomerase at the telomere ends. NME2, a protein known to
interact with quadruplex, suppresses telomerase through direct interaction with the catalytically
active protein subunit of human telomerase (hTERT). A stretch of 11 amino acids of NME2 (140-
VDYKSCAHDWV-150) were involved in the interaction with the reverse transcriptase domain of hTERT.
After testing multiple modes of delivery, liposome encapsulation was determined to be the most
effective mode of delivery of the telomerase-suppressing NME2-derived peptides. The tumor
suppressing potential of these peptides were further established by in vivo experiments using
xenografts developed in mice. Within the stretch of 11 amino acids in NME2, serine 144 appeared
most significant because of its interaction with the reverse transcriptase domain of hTERT.

Two point mutations in the hTERT promoter are clinically prevalent across patients of several cancers.
These mutations destabilize a G-Quadruplex (G4) structure that forms within the wild type hTERT
promoter. This G4 is important for suppression of hTERT expression and its destabilization leads to
elevated hTERT expression. By screening, his group identified 2 G4 ligands- SMH1-4.6 and JD83, to be
most potent in reducing hTERT expression. The NME2 peptide and a G4 binding ligand can therefore
be used in combination to target hTERT at both transcriptional and post-translational stages. In
collaboration with Sagar Sengupta, NII, the role of quadruplex structures in the promoter of the BLM
Helicase gene was also studied.

Using cells where they artificially inserted the hTERT-promoter driven reporter cassette at the CCR5
safe-harbour locus (~40Mb away from the telomeres) with or without clinical mutations, Shantanu’s
lab found that that the TRF2-mediated regulation of hTERT can occur independent of telomere looping
by deposition of the H3K27Me3 suppressor histone mark leading to hTERT repression. hTERT
promoter mutations frequently found in many aggressive cancers including glioblastoma and
melanomas led to destabilization of a G quadruplex structure formed at the hTERT promoter. This
destabilization led to loss of TRF2 occupancy and hTERT reactivation.

TRF2 mediated effects in interleukin signaling and anti-tumour immune response

Molecular mechanisms of how telomere length influences genome-wide TRF2 occupancy, and TRF2-
mediated transcription are being studied. TRF2 can transcriptionally activate IL1R1 (Interleukin 1