Breaks before Synthesis, Dr. Hsueh-Ping Chu’s Finding Published on Nature Communication

The maintenance of telomere length is closely related to the processes of cancers and aging. How do telomeres extend their length? It has been well-known that an enzyme called telomerase can lengthen telomeres. However, some cancers do not rely on telomerase activity to lengthen their telomeres. They utilized “Alternative lengthening of Telomeres (ALT)”, a mechanism that includes a break-induced replication to extend telomeres and is highly conserved in many eukaryotes. Patients with ALT cancers have a higher risk of death compared with those having non-ALT cancers.

How do cells initiate the breaks at telomeres? Dr. Hsueh-Ping (Catherine) Chu’s team in the Molecular and Cellular Biology department at NTU discovered that TERRA R-loops and XPF are the drivers. TERRA is a long non-coding RNA, which contains telomeric repeat sequences and forms DNA:RNA hybrids at telomeres. The DNA:RNA hybrid and a displaced single-stranded DNA form an R-loop structure. TERRA R-loops are specifically enriched at telomeres in cancer cells utilizing the ALT mechanism. The research team uncovered that TERRA R-loops trigger telomere clustering and activate DNA damage response by recruiting XPF. Such DNA damage response at telomeres is required for inducing homologous recombination and telomere synthesis in ALT cancer cells.

The Ph.D. student Hong-Jhih Shen developed an RCas9 system to specifically deplete TERRA RNA without editing telomeric DNA in ALT cells and found that TERRA depletion shortens telomere length in ALT cancer cells. The Ph.D. student Chia-Yu Guh systematically identified TERRA interacting proteins in ALT cells and revealed that TERRA interacts with a large subset of proteins involved in the DNA repair pathway. Interestingly, TERRA interacts with several nucleotide excision repair factors including XPF, an enzyme that cuts DNA. The postdoctoral fellow Liv Weichien Chen and the master student Pei-Chen Chiu showed that TERRA R-loops recruit XPF to telomeres, leading to DNA double-strand breaks to activate break-induced telomere synthesis at ALT telomeres.

Targeting XPF by small interference RNAs inhibits cell growth in ALT cancer cells and reduces telomere lengthening. These findings provide new insights into ALT cancer therapy.

This research was published in Nature communications in October 2022.

Read the journal article: https://www.nature.com/articles/s41467-022-33428-0