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Le gène suppresseur de tumeurs TP53 exprime plusieurs isoformes, dont Δ40p53 (perte du domaine de transactivation) et Δ133p53 (perte du domaine de transactivation et d’une partie du domaine de liaison à l’ADN). Ces isoformes inhibent l’activité suppressive de p53 et seraient sur-exprimées dans les cancers (sein et mélanome). Dans les cancers faiblement associés à une mutation TP53, ces isoformes seraient de bons candidats pour inactiver p53. Il convient de comprendre les mécanismes transcriptionnels qui régulent leurs expressions. Δ133p53 est produite par un promoteur alternatif P3 localisé dans TP53. Nous avons montré que Δ133p53 est un gène cible de p53, qui transactive le promoteur P3 par fixation sur un élément de réponse présent dans l’exon 4. L’expression de Δ133p53 est corrélée à celle d’autres gènes cibles de p53 en réponse à un stress génotoxique. De plus, elle réprime la suppression de la prolifération induite par p53 en inhibant ses capacités de liaison à l’ADN. Δ40p53 est produite par épissage alternatif, dont la rétention de l’intron 2 favorise sa traduction et empêche celle de p53. Nous avons montré que des structures de type G-quadruplexes présentes dans l’intron 3 régulent l’exclusion de l’intron 2. Ces structures comprennent le polymorphisme TP53PIN3 (duplication de 16pb), qui change leur localisation et affecte l’expression des ARNm codant p53 et Δ40p53. De plus, nous avons montré que ce polymorphisme est associé à une accélération de la cancérogenèse dans le syndrome Li-Fraumeni, caractérisé par la présence d’une mutation germinale TP53 (effet modificateur: 19 ans de différence à l’âge moyen du premier diagnostique entre les deux variants). L’expression des isoformes de p53 dépend de mécanismes transcriptionnels différents, indiquant des rôles différents dans la modulation des fonctions suppressives de p53. En plus d’inactiver p53 dans les cancers, ces isoformes pourraient être à l’origine des effets modificateurs des polymorphismes de TP53 sur les mutants p53. / The TP53 tumour suppressor gene expresses several isoforms, of which Δ40p53 (lack of transactivation domain) and Δ133p53 (lack of both transactivation and part of DNA-binding domains). These isoforms inhibit p53 suppressive activity and have been shown to be over-expressed in cancers (breat and melanoma). In cancers associated with low TP53 mutation rate, these isoforms could be great candidates to inactivate p53. It seems important to understand the transcriptional mechanisms that regulate their expression. Δ133p53 is produced by an alternative P3 promoter within TP53. We showed that Δ133p53 is a p53 target gene. p53 transactivates the P3 promoter and interact with a response element within exon 4. Δ133p53 expression is correlated to other p53 target genes in response to genotoxic stress. In addition, Δ133p53 inhibits p53-dependent suppression of proliferation by inhibiting p53 DNA-binding activity. Δ40p53 is produced by alternative splicing: retention of intron 2 favours its translation while it avoid the one of p53. We showed that G-quadruplex structures are formed in intron 3 and regulate retention of intron 2. The TP53PIN3 polymorphism (16 bp duplication) is embedded within these structures and affects their locations leading to variation of mRNA expression of p53 and Δ40p53. In addition, we showed that this polymorphism is associated with acceleration of carcinogenesis in Li-Fraumeni syndrome, characterized by germline TP53 mutation (genetic modifier effect: difference of 19 years in mean age at first diagnosis of cancer between the two variants). The expression of p53 isoforms depends on different transcriptional mechanisms, suggesting different roles in the modulation of p53 suppressive functions. In addition to inactivate p53 in cancers, these isoforms could be the mediators of modifier effects observed for TP53 polymorphisms on mutant p53.

Targeting DNA has the advantage over proteins for cancer remediation because of the fewer copies of the ligands required for the desired therapeutic effect. Traditionally, covalent DNA binders like alkylating agents have been used to induce genetic instability through the formation of DNA lesions and strand breaks, leading to cellular apoptosis. The primary drawback of this treatment is the non-specific binding that affects both cancerous and non-cancerous cells. G-quadruplexes are the DNA secondary structures that are present in abundance near the promoter regions of the oncogenes and are involved in the regulation of their activities. A ligand-mediated stabilization of G-quadruplexes in the promoter regions and down-regulation of the associated oncogenes have been validated. In contrast to alkylating agents, G-quadruplex ligands induce genetic stabilization through non-covalent interactions. They can be designed to interact specifically with G-quadruplex DNA over duplex DNA, which reduce side effects arising from the off-targeting. G-quadruplex ligands invariably have the large planar aromatic moiety to interact with G-quadruplexes through π- π stacking interactions. For determining the size effect of the aromatic moiety on stabilization of G-quadruplexes, a series of ligands were synthesized by conjugating nucleobases or 1,10-phenanthroline with an aminoglycoside, neomycin. The resulting conjugates increased the binding affinity synergistically and enabled us to study the effect of the stacking moiety required for G-quadruplex stabilization. Nucleobase-neomycin conjugates did not show stabilization stabilize of human telomeric G-quadruplex. 1,10-Phenanthroline-neomycin conjugate (7b) on the other hand binds to human telomeric G-quadruplex with a Ka of (8.92.4)×108 M-1 and inhibits telomerase activity at 1.56 µM probably through G-quadruplex stabilization. Moving forward, we further enlarged the aromatic moiety by tethering two 1,10-phenantholine molecules together through a five-atom linker. The resulting molecule (2-Clip-phen) was conjugated with various amino-containing side chains. 2-Clip-phen derivatives showed at least 30 times weaker binding to duplex DNA over G-quadruplex DNA. In addition, compounds showed a preference for the antiparallel G-quadruplex conformation over parallel and hybrid G-quadruplex conformations, as shown in the CD spectroscopy studies. Ligands 11 and 13 induced the formation of an antiparallel G-quadruplex from random coils and stabilize it to 60 oC (Tm) in a salt-free condition. Mass spectrometry study showed the formation of a two-tetrad G-quadruplex with the 2-Clip-phen ligand. Docking study showed that the ligand interacts most favorably with antiparallel G-quadruplex conformation, which is supported further by the larger thermal stabilization effect on antiparallel G-quadruplex compared with other G-quadruplex conformations. Our study suggests that 2-Clip-phen can be used as a scaffold for designing G-quadruplex binding ligands that preferentially bind to antiparallel G-quadruplexes, which has never been reported before.