Impaired synthesis of both telomere strands and adaptive TERT reduction in RTEL1 ATPase-dead cells

Regulator of Telomere Elongation Helicase 1 (RTEL1) plays a critical role in telomere replication by disassembling DNA secondary structures such as G-quadruplexes and telomeric loops (T-loops). However, its precise mechanism remains unclear. Previously, we generated HeLa cells homozygous for ATPase-dead RTEL1. Here, we found that these cells have a 50-77% reduction in the synthesis of both telomere strands, indicating that RTEL1 ATPase activity is important for both leading-strand and lagging-strand synthesis. Surprisingly, these cells also showed a substantial reduction in Telomerase Reverse Transcriptase (TERT) mRNA levels and an approximately 80% reduction in telomerase activity, without activating alternative lengthening of telomeres (ALT). Forced TERT overexpression suppressed proliferation and caused late S/G2 accumulation, implying that the naturally occurring reduction in TERT provides an adaptive mechanism for cell survival. Expressing the longest RTEL1 isoform at close to physiological levels failed to rescue growth defects, suggesting other isoforms are important for cell survival. These results reveal an unexpected interplay between RTEL1 and telomerase and show how cancer cells compensate for RTEL1 dysfunction by decreasing TERT expression.