Microhomology-mediated DNA strand annealing and elongation by human DNA polymerases {lambda} and {beta} on normal and repetitive DNA sequences

‘Classical’ non-homologous end joining (NHEJ), dependent on the Ku70/80 and the DNA ligase IV/XRCC4 complexes, is essential for the repair of DNA double-strand breaks. Eukaryotic cells possess also an alternative microhomology-mediated end-joining (MMEJ) mechanism, which is independent from Ku and DNA ligase 4/XRCC4. The components of the MMEJ machinery are still largely unknown. Family X DNA polymerases (pols) are involved in the classical NHEJ pathway. We have compared in this work, the ability of human family X DNA pols β, and μ, to promote the MMEJ of different model templates with terminal microhomology regions. Our results reveal that DNA pol and DNA ligase I are sufficient to promote efficient MMEJ repair of broken DNA ends in vitro, and this in the absence of auxiliary factors. However, DNA pol β, not , was more efficient in promoting MMEJ of DNA ends containing the (CAG)n triplet repeat sequence of the human Huntingtin gene, leading to triplet expansion. The checkpoint complex Rad9/Hus1/Rad1 promoted end joining by DNA pol on non-repetitive sequences, while it limited triplet expansion by DNA pol β. We propose a possible novel role of DNA pol β in MMEJ, promoting (CAG)n triplet repeats instability.