Vulnerabilities of Alternative Lengthening of Telomeres in Cancer Cells Captured by CRISPR Genome Screening
Background
As cell division progresses, telomeres at the ends of chromosomes gradually shorten. Normal cells stop dividing and undergo senescence or death upon reaching this limit, but cancer cells overcome this to proliferate endlessly. Approximately 85-90% of human tumors maintain their ends by reactivating the telomerase enzyme. Conversely, 5–10% of all tumors utilize the Alternative Lengthening of Telomeres (ALT) pathway to extend telomere length without this enzyme. This survival mechanism is commonly found in refractory cancers such as leukemia or gliomas.
Until now, the academic community has focused on finding a single target protein that selectively eliminates ALT cancer cells. The assumption was that inhibiting the homologous recombination-based DNA replication process could prevent cancer cell proliferation. However, efforts to identify a single factor that directly initiates or independently controls ALT have repeatedly encountered obstacles. This is because the complex DNA damage response system and multiple repair mechanisms are intertwined, making it difficult to pinpoint tumor-specific vulnerabilities using traditional biochemical techniques alone.
Key Findings
Researchers utilized CRISPR-based functional genomics technology to comprehensively re-examine the genetic dependencies of ALT-positive cancer cells. The research team cross-analyzed multifaceted strategies, including hypothesis-driven targeted loss-of-function screening, phenotype-based high-content screening, in silico analysis of large-scale public dependency databases, and genome-wide screening in ALT-like states.
As a result, it was revealed that ALT tumors rely on a network that buffers chronic telomere dysfunction rather than depending on a single, dedicated factor. Key factors involved in replication stress buffering, DNA repair, and chromosome end maintenance were repeatedly identified in the screening. Representative examples include Fanconi anemia complementation group M (FANCM), SMARCAL1 (a factor associated with the SWI/SNF complex), and the transcriptional co-activator SUB1.
Chromosome ends in the ALT state are constantly exposed to severe replication stress and abnormal structures. If buffering factors such as FANCM or SMARCAL1 are deficient, the balance of stress control collapses instantly. It has been confirmed that telomere collapse leads to genome-wide replication catastrophe, resulting in cell death. This is the moment when the precarious tightrope walk maintained by tumor cells comes to an end.
Significance and Outlook
This mechanistic clarification fundamentally changes the direction of treatment strategies for ALT-positive tumors. It demonstrates that a synthetic lethality approach—pushing the structural stress already present in cancer cells into an overload state—is much more effective than the past perspective of searching for a single specific factor.
The challenges are also clear. Since genes such as FANCM and SMARCAL1 also contribute to maintaining genome stability in normal cells, the potential toxicity to normal tissues arising from targeting them must be mitigated. It is time for follow-up research to verify the in vivo target selectivity of molecular inhibitors and to track resistance development pathways.
Alternative lengthening of telomeres (ALT) is a telomerase-independent telomere maintenance mechanism that supports the growth of approximately 5-10% of human tumors and represents an attractive but insufficiently explored therapeutic target. CRISPR-based functional genomics has emerged as a powerful approach to systematically interrogate ALT biology and identify genetic dependencies associated with the ALT state. In this Review, we discuss the CRISPR-based strategies applied to ALT, including hypothesis-driven targeted loss-of-function screens, phenotype-driven optical screens, in silico mining of large-scale dependency datasets, and genome-wide screens in ALT-adjacent states. Across these approaches, recurrent dependencies converge on pathways mediating replication stress buffering, DNA repair, and telomere maintenance, including FANCM, SMARCAL1, and SUB1. Collectively, these findings support a model in which ALT-positive cells rely not on a unique ALT-specific factor, but rather on stress-buffering mechanisms that prevent chronic telomeric dysfunction from triggering replication catastrophe and cell death, highlighting these pathways as promising candidates for further therapeutic investigation.
Precision oncology treatment strategies targeting patient groups with high ALT ratios, such as osteosarcoma, soft tissue sarcoma, and glioblastoma, can be established. First, ALT-positive patients can be screened using next-generation sequencing and C-circle detection biomarkers, after which clinical designs can be implemented by administering drug candidates that inhibit FANCM or SMARCAL1 alone or in combination with drugs that amplify replication stress, such as ATR inhibitors. This can present a new therapeutic alternative for sarcoma patients who did not respond to existing standard cytotoxic anticancer drugs.