Extrachromosomal Telomeres Derived from Excessive Strand Displacements.

Area of Science:

• Molecular Biology
• Cancer Research
• Genetics

Background:

• Alternative Lengthening of Telomeres (ALT) is a telomere maintenance mechanism in ~15% of human cancers, characterized by C-circles.
• The generation of extrachromosomal C-rich single-stranded DNAs (ssDNAs), potential C-circle precursors in ALT cells, is not well understood.

Approach:

• Developed a sensitive Strand-Specific Southern-blot for Single-stranded Extrachromosomal Telomeres (4SET) assay.
• Detected linear C-rich ssDNAs (200-1500 nt) in cytoplasm and nucleoplasm, suggesting they precede C-circle formation.
• Investigated the molecular mechanisms underlying C-rich ssDNA generation.

Key Points:

• C-rich ssDNAs and C-circles are abundant in both cytoplasmic and nucleoplasmic fractions.
• C-rich ssDNAs originate from Okazaki fragment processing during lagging strand synthesis.
• Generation requires the CST-PP complex for C-strand priming and DNA Polymerase delta/BLM helicase for strand displacement.

Conclusions:

• Proposes a novel model for C-rich ssDNA and C-circle generation in ALT.
• Identifies key molecular players (CST-PP, Pol delta, BLM) in this process.
• Provides a new method (4SET assay) for studying ssDNA in ALT cells.