title: Microhomology-mediated DNA strand annealing and elongation by human DNA polymerases λ and β on normal and repetitive DNA sequences.
abstract: '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, λ and μ, to promote the MMEJ of different model templates with terminal microhomology regions. Our results reveal that DNA pol λ 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 λ, 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 λ 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.
authors.affiliation: Institute of Molecular Genetics IGM-CNR, via Abbiategrasso 207, 27100 Pavia, Italy.
authors.forename: Emmanuele
authors.initials: E
authors.lastname: Crespan
authors.forename: Tibor
authors.initials: T
authors.lastname: Czabany
authors.forename: Giovanni
authors.initials: G
authors.lastname: Maga
authors.forename: Ulrich
authors.initials: U
authors.lastname: Hübscher
mesh_terms: D020134:Catalytic Domain; D018797:Cell Cycle Proteins / Q000378:metabolism; D004247:DNA / Q000096:biosynthesis* / Q000737:chemistry / Q000378:metabolism; D059766:DNA End-Joining Repair; D019951:DNA Polymerase beta / Q000737:chemistry / Q000378:metabolism*; D004277:DNA, Single-Stranded / Q000378:metabolism; D004259:DNA-Directed DNA Polymerase / Q000378:metabolism; D006801:Humans; D000071058:Huntingtin Protein; D009419:Nerve Tissue Proteins / Q000235:genetics; D010710:Phosphates / Q000737:chemistry; D018809:Proliferating Cell Nuclear Antigen / Q000378:metabolism; D012091:Repetitive Sequences, Nucleic Acid; D051716:Replication Protein A / Q000378:metabolism; D012689:Sequence Homology, Nucleic Acid; D013698:Templates, Genetic; D019680:Trinucleotide Repeat Expansion; D018911:Trinucleotide Repeats
chemical_list: D018797:Cell Cycle Proteins; D004277:DNA, Single-Stranded; C086055:HTT protein, human; D000071058:Huntingtin Protein; D009419:Nerve Tissue Proteins; D010710:Phosphates; D018809:Proliferating Cell Nuclear Antigen; D051716:Replication Protein A; C069930:rad9 protein; D004247:DNA; C415367:DNA polymerase beta2; C407743:DNA polymerase mu; D019951:DNA Polymerase beta; D004259:DNA-Directed DNA Polymerase