title: Expansion of CAG triplet repeats by human DNA polymerases λ and β in vitro, is regulated by flap endonuclease 1 and DNA ligase 1.
abstract: Huntington's disease (HD) is a neurological genetic disorder caused by the expansion of the CAG trinucleotide repeats (TNR) in the N-terminal region of coding sequence of the Huntingtin's (HTT) gene. This results in the addition of a poly-glutamine tract within the Huntingtin protein, resulting in its pathological form. The mechanism by which TRN expansion takes place is not yet fully understood. We have recently shown that DNA polymerase (Pol) β can promote the microhomology-mediated end joining and triplet expansion of a substrate mimicking a double strand break in the TNR region of the HTT gene. Here we show that TNR expansion is dependent on the structure of the DNA substrate, as well as on the two essential Pol β co-factors: flap endonuclease 1 (Fen1) and DNA ligase 1 (Lig1). We found that Fen1 significantly stimulated TNR expansion by Pol β, but not by the related enzyme Pol λ, and subsequent ligation of the DNA products by Lig1. Interestingly, the deletion of N-terminal domains of Pol λ, resulted in an enzyme which displayed properties more similar to Pol β, suggesting a possible evolutionary mechanism. These results may suggest a novel mechanism for somatic TNR expansion in HD.
authors: Institute of Molecular Genetics IGM-CNR, via Abbiategrasso 207, Pavia I-27100, Italy. Electronic address: emmanuelecrespan@gmail.com.
authors: Emmanuele
authors: E
authors: Crespan
authors: Institute of Veterinary Biochemistry and Molecular Biology, University of Zürich-Irchel, Zürich CH-8057, Switzerland.
authors: Ulrich
authors: U
authors: Hübscher
authors: Institute of Molecular Genetics IGM-CNR, via Abbiategrasso 207, Pavia I-27100, Italy.
authors: Giovanni
authors: G
authors: Maga
mesh_terms: D004247:DNA / Q000378:metabolism; D000072481:DNA Ligase ATP; D011088:DNA Ligases / Q000378:metabolism*; D019951:DNA Polymerase beta / Q000378:metabolism*; D004261:DNA Replication; D045585:Flap Endonucleases / Q000378:metabolism*; D006801:Humans; D019680:Trinucleotide Repeat Expansion
chemical_list: C000607892:LIG1 protein, human; D004247:DNA; C415367:DNA polymerase beta2; D019951:DNA Polymerase beta; D045585:Flap Endonucleases; C468317:FEN1 protein, human; D011088:DNA Ligases; D000072481:DNA Ligase ATP
keywords: DNA polymerase β; DNA polymerase λ; Double strand breaks repair; Flap endonuclease 1; Trinucleotide repeat expansion