Distinct mechanisms underlying extrachromosomal telomere DNA generation in ALT cancers

Area of Science:

• Molecular Biology
• Cancer Biology
• Genetics

Background:

• Alternative lengthening of telomeres (ALT) is a telomere maintenance mechanism in 15% of human cancers, independent of telomerase.
• ALT cancers are characterized by extrachromosomal C-circles (cytosine-rich ssDNA) and less abundant G-circles (guanine-rich ssDNA).
• Excessive Okazaki fragment displacement during lagging-strand synthesis is linked to C-circle generation in ALT.

Purpose of the Study:

• To investigate the distinct characteristics and generation mechanisms of G-circles compared to C-circles in ALT.
• To elucidate the roles of DNA translocases SMARCAL1 and FANCM in ALT-associated ssDNA formation.
• To differentiate the origins and initiation pathways of C-rich and G-rich ssDNAs.

Main Methods:

• Investigated G-circle generation in ALT models with co-deficiency of SMARCAL1 and FANCM.
• Characterized the structure and size of G-rich ssDNAs.
• Analyzed the origins and initiation mechanisms of C-rich and G-rich ssDNAs using molecular biology techniques.

Main Results:

• Co-deficiency of SMARCAL1 and FANCM leads to abundant G-circle/G-rich ssDNA generation.
• G-rich ssDNAs are predominantly linear, ranging from 500 to 3000 nucleotides, differing from C-circles.
• Both C-rich and G-rich ssDNAs originate from BLM/POLD-mediated strand displacement but have distinct initiation pathways.

Conclusions:

• C-rich ssDNAs originate from lagging strands initiated by the CST complex.
• G-rich ssDNAs originate from leading strands via RAD51-dependent G-strand synthesis.
• Two distinct mechanisms generate C- and G-circles, respectively, during ALT-mediated telomere elongation.