Microhomology-mediated DNA strand annealing and elongation by human DNA polymerases λ and β 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 beta, lambda and mu, to promote the MMEJ of different model templates with terminal microhomology regions. Our results reveal that DNA pol lambda 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 beta, not lambda, 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 lambda on non-repetitive sequences, while it limited triplet expansion by DNA pol beta. We propose a possible novel role of DNA pol beta in MMEJ, promoting (CAG)n triplet repeats instability.