The vast majority of adult cancer cells achieve cellular immortality by activating a telomere maintenance mechanism (TMM). While this is mostly achieved by the de-silencing of hTERT telomerase gene expression, an alternative homologous recombination-based and telomerase-independent mechanism, known as ALT (Alternative Lengthening of Telomeres), is frequently activated in a subset of tumors, including paediatric cancers. Being absent from normal cells, the ALT mechanism offers interesting perspectives for new targeted cancer therapies. To date, however, the development of better translationally applicable tools for ALT detection in tumor sections is still needed. Here, using a newly derived ALT-positive cancer cell mouse xenograft model, we extensively examined how the previously known ALT markers could be used as reliable tools for ALT diagnosis in tumor sections. Authors found that, together with the detection of ultra-bright telomeric signals (UBS), an ALT hallmark, native telomeric FISH, that detects single-stranded C-rich telomeric DNA, provides a very sensitive and robust tool for ALT diagnosis in tissues. They applied these assays to paediatric tumor samples and readily identified three ALT-positive tumors for which the TMM was confirmed by the gold-standard C-circle amplification assay. Although the latter offers a robust assay for ALT detection in the context of research laboratories, it is more difficult to set up in histopathological laboratories and could therefore be conveniently replaced by the combination of UBS detection and native telomeric FISH.

Here, authors addressed the possibility of developing new tools for ALT detection on tumor biopsies that may be compatible with clinical biology routine. To this end, they first developed a new ALT+ tumor xenograft model to re-evaluate a series of previously described ALT detection assays on tumor tissues. In a second step, they developed a new sensitive assay for ALT detection on tissue sections based on a native FISH protocol that detects single-stranded C-rich telomeric DNA (ss-TeloC), one hallmark of ALT+ cells. Finally, authors applied these various tests to a collection of solid paediatric tumor sections and identified ALT+ osteosarcoma and neuroblastoma tumors.

ALT marker detection in LB857/ALT+ and HT1080/TEL+ cancer cell lines

Article Access: https://doi.org/10.1186/s43556-021-00055-y

Website for Molecular Biomedicine: https://www.springer.com/journal/43556

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