Evidence of cis-acting factors in replication-mediated trinucleotide repeat instability in primate cells

Abstract

The mechanism of disease-associated trinucleotide repeat instability involves cis-acting factors (cis-elements) in the vicinity of the repeat, but the nature of these elements is unknown. One cis-element may be the location of the replication origin relative to the repeat. We have used an SV40 DNA replication system to investigate the effect of the loca-

Fidelity of Primate Cell Repair of a Double-strand Break within a (CTG)·(CAG) Tract

“…The process of instability is likely to involve DNA slipped/secondary structures (out-of-register interstrand mispairings at repeat sequences). One hypothesis predicts that expansion events arise from errors at DNA replication forks (10,11). We have shown that cis-elements such as the location of replication initiation and fork direction, relative to a repeat tract, can lead to different types and frequencies of repeat instability (10).…”

Positioned to expand

“…The work by Cleary et al 2 helps to explain the nature of the cis-factors that are potentially responsible for TNR expansion. In so doing, the authors developed the first experimental system for measuring TNR expansions in mammalian cells.…”

“…Two previous observations made in bacteria and yeast were confirmed: only repeats of premutational lengths were unstable, and expansions were only observed in one orientation of a repeat relative to the replication origin. Cleary et al 2 suggest that the formation of hairpins within the (CTG) ncontaining Okazaki fragment is responsible for the repeat's expansion (Fig. 1).…”

“…This could change the position of the repeat within the so-called 'Okazaki initiation zone'-a singlestranded portion of approximately 290 nucleotides of the lagging strand template 14 . Cleary et al 2 speculate that TNRs expand when positioned at the 3′ end and contract when at the 5′ end of the Okazaki initiation zone.…”

“…In families with myotonic dystrophy, all chromosomes carrying expanded (CTG) n repeats also contain an Alu element inserted 5 kb from the TNR 15 . Cleary et al 2 suggest that this insertion might change the distance between the TNR and its replication origin, poising it to expand. As such, their hypothesis might be called 'ori-shift' (Fig.…”