Overall, this study has demonstrated a truly effective and feasible approach to
smuggle a lncRNA-targeting agent across the BTBB to take down the multiple drug
resistance defences of TMZ-resistant GBM and meet the urgent clinical need to
improve the effectiveness of TMZ treatment. Indeed, this method of MALAT1
targeting has arguably yielded the best results to this end of any preclinical approach
taken to date and should be prioritised in the design and trial of new and improved
TMZ treatment regimens that might also be able to effectively use lower and less
toxic TMZ doses.
4.3 TUG1
Another recent glioblastoma-focussed study has not only shed light on tumourpromoting epigenetic dysregulation by TUG1 but has also described one of
the most promising and feasible lncRNA-targeting therapeutic strategies to date.
Katsushima and colleagues were interested in better characterising how GSCs
maintain their characteristics [68]. While it had previously been established that
Notch signalling is a hallmark of GSCs [69], it was unclear how Notch signalling
maintains GSC stemness in glioblastoma (GBM). Based on a previous study indicating that lncRNAs may be involved in Notch signalling regulation in leukaemia
[70], expression profiling in GSCs of lncRNAs potentially regulated by Notch
signalling revealed that TUG1 is upregulated in GSCs and contains multiple
RBPJκ binding motifs in its promoter and thus is a bona fide transcriptional target
of Notch signalling [68]. It was then established that TUG1 promotes GSC cell
viability and maintains stem cell characteristics such as neurosphere formation
and expression of GSC stemness-associated transcription factors [71] Sox2 and
MYC [68]. Two distinct mechanisms by which TUG1 maintains GSC stemness
have been suggested. First, TUG1 contains an MRE for miR-145, a repressor of
GSC stemness-associated transcription factors SOX2 and MYC, and is proposed to
antagonise miR-145 to protect SOX2 and MYC transcripts from miR-145-mediated
downregulation. Second, TUG1 binds to both PRC2 and YY1 and is proposed
to scaffold them to maintain transcriptional repression of neuronal differentiation
genes [68].
To establish the clinical relevance of the findings, not only was it demonstrated
that TUG1 is upregulated in a cohort of GBM patient tumours, but it was also
confirmed that TUG1 is highly expressed in most Notch1-positive cells enriched
around perivascular regions [68], in which Notch ligand-positive endothelial cells
are thought to form a niche to promote GSC self-renewal via Notch signalling [72].
Having shown that TUG1 overexpression was a physiologically relevant factor
in GSC maintenance, a strategy to target TUG1 in GSCs was developed. An
ASO against TUG1 was delivered to xenograft mouse GBMs by using a cyclic
Arg-Gly-Asp (cRGD) drug delivery system (DDS). The cRGD DDS has previously
been developed to deliver a parent compound of the chemotherapeutic agent
oxaliplatin across the blood-tumour-brain barrier (BTBB) to GBM by preferentially
416
D. Hanly et al.
smuggle a lncRNA-targeting agent across the BTBB to take down the multiple drug
resistance defences of TMZ-resistant GBM and meet the urgent clinical need to
improve the effectiveness of TMZ treatment. Indeed, this method of MALAT1
targeting has arguably yielded the best results to this end of any preclinical approach
taken to date and should be prioritised in the design and trial of new and improved
TMZ treatment regimens that might also be able to effectively use lower and less
toxic TMZ doses.
4.3 TUG1
Another recent glioblastoma-focussed study has not only shed light on tumourpromoting epigenetic dysregulation by TUG1 but has also described one of
the most promising and feasible lncRNA-targeting therapeutic strategies to date.
Katsushima and colleagues were interested in better characterising how GSCs
maintain their characteristics [68]. While it had previously been established that
Notch signalling is a hallmark of GSCs [69], it was unclear how Notch signalling
maintains GSC stemness in glioblastoma (GBM). Based on a previous study indicating that lncRNAs may be involved in Notch signalling regulation in leukaemia
[70], expression profiling in GSCs of lncRNAs potentially regulated by Notch
signalling revealed that TUG1 is upregulated in GSCs and contains multiple
RBPJκ binding motifs in its promoter and thus is a bona fide transcriptional target
of Notch signalling [68]. It was then established that TUG1 promotes GSC cell
viability and maintains stem cell characteristics such as neurosphere formation
and expression of GSC stemness-associated transcription factors [71] Sox2 and
MYC [68]. Two distinct mechanisms by which TUG1 maintains GSC stemness
have been suggested. First, TUG1 contains an MRE for miR-145, a repressor of
GSC stemness-associated transcription factors SOX2 and MYC, and is proposed to
antagonise miR-145 to protect SOX2 and MYC transcripts from miR-145-mediated
downregulation. Second, TUG1 binds to both PRC2 and YY1 and is proposed
to scaffold them to maintain transcriptional repression of neuronal differentiation
genes [68].
To establish the clinical relevance of the findings, not only was it demonstrated
that TUG1 is upregulated in a cohort of GBM patient tumours, but it was also
confirmed that TUG1 is highly expressed in most Notch1-positive cells enriched
around perivascular regions [68], in which Notch ligand-positive endothelial cells
are thought to form a niche to promote GSC self-renewal via Notch signalling [72].
Having shown that TUG1 overexpression was a physiologically relevant factor
in GSC maintenance, a strategy to target TUG1 in GSCs was developed. An
ASO against TUG1 was delivered to xenograft mouse GBMs by using a cyclic
Arg-Gly-Asp (cRGD) drug delivery system (DDS). The cRGD DDS has previously
been developed to deliver a parent compound of the chemotherapeutic agent
oxaliplatin across the blood-tumour-brain barrier (BTBB) to GBM by preferentially
416
D. Hanly et al.