A Cisplatin Derivative Tetra-Pt(bpy) as an Oncotherapeutic Agent for Targeting ALT Cancer

In approximately 15% of human cancers, telomere length is maintained independently of telomerase by the homologous recombination (HR)-mediated alternative lengthening of telomeres (ALT) pathway. Whether the ALT pathway can be exploited for therapeutic treatment remains unknown. The purpose of this study is to develop oncotherapeutic agent to target ALT cancers.

Surface plasmon resonance assay, antibody to G-quadruplex, and fluorescence in situ hybridization (FISH) were used to discover Tetra-Pt(bpy), a cisplatin derivative that specifically targets telomeric G-quadruplex. We used immunofluorescence, FISH, C-circle assay, and chromosome orientation FISH to evaluate the inhibitory effect of Tetra-Pt(bpy) on ALT activity in human ALT cancers. The shortening of telomere length induced by Tetra-Pt(bpy) was determined by telomere restriction fragment or Q-FISH. Cell destination after Tetra-Pt(bpy) treatment was determined by β-gal staining or apoptosis assay. Nude mice (n = 4 per group) were injected with U2OS cells to evaluate the effects of Tetra-Pt(bpy) on tumor growth. All statistical tests were two-sided.

Tetra-Pt(bpy) inhibits the strand invasion/annealing step of telomeric homologous recombination by selectively converting telomeric ssDNA to a G-quadruplex. ALT-cells treated with Tetra-Pt(bpy) show fewer ALT-associated promyelocytic leukemia bodies (untreated: mean±SD = 5.9±0.2 vs treated: mean±SD = 3.1±0.1, P < .001), fewer extrachromosomal C-circles (untreated: mean±SD = 100.5±1.6 vs treated: mean±SD = 18.0±1.7, P < .001), and reduced telomere sister chromatin exchanges (untreated: mean±SD = 25.2%±1.5% vs treated: mean±SD = 13.1%±1.9%, P < .001). Consequently, critically short telomeres accumulate after multiple population doublings (untreated: mean±SD = 18.9%±1.7% vs treated: mean±SD = 57.4%±2.2%, P < .001), resulting in cell death by apoptosis or senescence. In vivo, Tetra-Pt(bpy) severely inhibits the growth of ALT-cell xenograft tumors in mice (untreated: mean±SD = 57.1±3.7 mm 3 vs treated: mean±SD = 19.0±3.2 mm 3 , P < .001). Importantly, Tetra-Pt(bpy) exhibits no adverse effects on proliferation, gene expression, or telomere metabolism in normal cells.

These results reveal the potential of Tetra-Pt(bpy) as a novel oncotherapeutic agent for targeting ALT cancer cells.