TRF2/RAP1 and DNA-PK mediate a double protection against joining at telomeric ends

Area of Science:

• Molecular Biology
• Genetics
• Cell Biology

Background:

• DNA-dependent protein kinase (DNA-PK) is crucial for DNA double-strand break repair.
• KU and DNA-PKcs subunits of DNA-PK are found at mammalian telomeres, raising questions about their role.
• Telomere fusion occurs in cells lacking these DNA-PK subunits.

Purpose of the Study:

• Investigate how DNA-PK is prevented from initiating classical non-homologous end-joining (C-NHEJ) at telomeres.
• Determine how backup end-joining (B-NHEJ) at telomeres is controlled when C-NHEJ is deficient.

Main Methods:

• Utilized plasmid substrates with double-stranded telomeric tracks.
• Employed human cell extracts with varying C-NHEJ or B-NHEJ activity to study end-joining (EJ).

Main Results:

• TRF2/RAP1 complex inhibits C-NHEJ-mediated telomere end fusion at the DNA-PK binding and activation stage.
• DNA-PK actively counteracts a strong LIG4-independent EJ mechanism.
• Telomeres are protected from end-joining by a dual-lock system involving TRF2/RAP1 and DNA-PK.

Conclusions:

• The findings explain telomere protection mechanisms in mammalian cells.
• Alternative, non-classical EJ pathways are critical for preventing telomere fusions.
• This dual-lock system ensures telomere integrity and prevents aberrant end-joining.