Chemistry Reference
In-Depth Information
Since telomerase activity is elevated in the vast majority of tumours, telomerase
participates in the immortality of cells, thanks to telomere elongation. Since telom-
eres are critically shortened in tumours versus normal tissues, it is thus conceivable
to inhibit telomerase as a strategy for cancer therapy. Inhibition of telomerase has
been performed using oligonucleotides targeting hTR or the mRNA of hTERT, or
nucleoside analogues and nonnucleoside inhibitors targeting the catalytic centre of
hTERT. They have been shown to induce progressive telomere shortening and
senescence or apoptosis of treated cancer cells.
60 - 63
7.4 G - Quadruplex Structures and Small Molecules
It has been recognised that the 3
- G - overhang of the telomere, (TTAGGG)
n
, is able
to fold,
in vitro
, into G-quadruplex structures. The structures consist of G-quartets
that are stacked in a planar arrangement of four guanines associated by Hoogsteen-
type hydrogen bonds. Four consecutive repeats of guanines in the telomeric sequence
are required for its folding in an intramolecular G-quadruplex structure. The mono-
valent cations Na
+
or K
+
are required to stabilise these structures by interacting with
the eight carbonyl oxygens of two adjacent G-quartets
64,65
(Figure 7.4 A). Although
there is no direct proof of G-quadruplexes
in vivo
, recent biological evidence is in
favour of their existence in the ciliate
Stylonychia
66
and in humans.
67
Moreover,
some proteins have been shown to promote their formation
68
and to induce their
unfolding.
69
G-quadruplexes can also be formed outside the telomere region, because
intramolecular G-quadruplex-forming sequence motifs are prevalent in the
genome,
70,71
and particularly enriched in gene promoters.
72
Moreover, it has been
shown that gene expression depending on these promoters could be modulated by
G - quadruplex binders.
73
Crystallographic and NMR studies of human telomeric
quadruplex structures have revealed a high degree of polymorphism in the struc-
tures, depending on the folding of the DNA backbone, the orientation of the strands
(parallel or antiparallel) and the conformation of the TTA loops (reverse, lateral or
transversal). Four main G-quadruplex structures have been described for telomeric
DNA: parallel,
74
antiparallel
75
and two mixed-hybrid structures
76,77
(Figures 7.4 B - E).
Each structure has been determined in the presence of a particular monovalent
cation (Na
+
or K
+
), but it is now clear that these structures coexist in solution, what-
ever the cation.
78 - 81
Since the substrate of telomerase is the linear extremity of the
3
′
-G-overhang of telomeric DNA, it has been shown that its folding in G-quadruplex
structures may impede its recognition by telomerase and may interfere with tel-
omere elongation.
82
Therefore, the stabilization of quadruplex DNA structures by
small molecules has been proposed as a new strategy in order to inhibit telomerase
and interfere with telomere maintenance in tumour cells.
53
Many molecules have been synthesised in order to target G-quadruplexes
and then tested on telomerase activity and cell proliferation. The paradigm for
telomerase inhibitors is that senescence of cancer cells will occur after many
rounds of replication because of progressive telomere shortening. Some of the
G-quadruplex binders have been shown to follow this rule.
83
However, the test used
′
