Telomeres are a protective cap that prevents chromosome
ends from
being recognized as double-stranded breaks. In somatic cells, telomeres
shorten with each cell division due to the end replication problem,
which eventually leads to senescence, a checkpoint proposed to prevent
uncontrolled cell growth. Tumor cells avoid telomere shortening by
activating one of two telomere maintenance mechanisms (TMMs): telomerase
reactivation or alternative lengthening of telomeres (ALT). TMMs are
a viable target for cancer treatment as they are not active in normal,
differentiated cells. Whereas there is a telomerase inhibitor currently
undergoing clinical trials, there are no known ALT inhibitors in development,
partially because the complex ALT pathway is still poorly understood.
For cancers such as neuroblastoma and osteosarcoma, the ALT-positive
status is associated with an aggressive phenotype and few therapeutic
options. Thus, methods that characterize the key biological pathways
driving ALT will provide important mechanistic insight. We have developed
a first-in-class phenotypic high-throughput screen to identify small-molecule
inhibitors of ALT. Our screen measures relative C-circle level, an
ALT-specific biomarker, to detect changes in ALT activity induced
by compound treatment. To investigate epigenetic mechanisms that contribute
to ALT, we screened osteosarcoma and neuroblastoma cells against an
epigenetic-targeted compound library. Hits included compounds that
target chromatin-regulating proteins and DNA damage repair pathways.
Overall, the high-throughput C-circle assay will help expand the repertoire
of potential ALT-specific therapeutic targets and increase our understanding
of ALT biology.