Short CCG repeat in huntingtin gene is an obstacle for replicative DNA polymerases, potentially hampering progression of replication fork

• Published in: Genes to cells : devoted to molecular & cellular mechanisms Vol. 20; no. 10; pp. 817 - 833
• Main Authors: Le, Hang Phuong, Masuda, Yuji, Tsurimoto, Toshiki, Maki, Satoko, Katayama, Tsutomu, Furukohri, Asako, Maki, Hisaji
• Format: Journal Article
• Language: English
• Published: England Wiley Subscription Services, Inc 01.10.2015
• Subjects: Chromatin; Deoxyribonucleic acid; DNA; DNA Polymerase II - metabolism; DNA Polymerase III - metabolism; DNA Replication; DNA-Binding Proteins - metabolism; DNA-Directed DNA Polymerase - metabolism; Escherichia coli - genetics; Escherichia coli - metabolism; Humans; Huntingtin Protein; Multienzyme Complexes - metabolism; Nerve Tissue Proteins - genetics; Trinucleotide Repeats
• ISSN: 1356-9597; 1365-2443
• DOI: 10.1111/gtc.12275

Trinucleotide repeats (TNRs) are highly unstable in genomes, and their expansions are linked to human disorders. DNA replication is reported to be involved in TNR instability, but the current models are insufficient in explaining TNR expansion is induced during replication. Here, we investigated replication fork progression across huntingtin (HTT)‐gene‐derived fragments using an Escherichia coli oriC plasmid DNA replication system. We found most of the forks to travel smoothly across the HTT fragments even when the fragments had a pathological length of CAG/CTG repeats (approximately 120 repeats). A little fork stalling in the fragments was observed, but it occurred within a short 3′‐flanking region downstream of the repeats. This region contains another short TNR, (CCG/CGG)7, and the sense strand containing CCG repeats appeared to impede the replicative DNA polymerase Pol III. Examining the behavior of the human leading and lagging replicative polymerases Pol epsilon (hPolε) and Pol delta (hPolδ) on this sequence, we found hPolδ replicating DNA across the CCG repeats but hPolε stalling at the CCG repeats even if the secondary structure is eliminated by a single‐stranded binding protein. These findings offer insights into the distinct behavior of leading and lagging polymerases at CCG/CGG repeats, which may be important for understanding the process of replication arrest and genome instability at the HTT gene. We investigated replication fork progression across huntingtin‐gene‐derived fragments using an Escherichia coli oriC‐plasmid DNA replication system. We found a little fork stalling at (CCG/CGG)7, not at (CAG/CTG)n repeats. We tested whether human Pol epsilon (hPolε) and Pol delta (hPolδ) stall at trinucleotide repeats (TNR) found in the fragments. hPolε could replicate (CGG)7, (CAG)65 and (CTG)64, but, it completely halted at (CCG)7. hPolδ replicated all TNR carrying templates. These data illuminate the distinct behavior of replicative polymerases at CCG repeats.