Book of Abstracts: Albany 2005
Mutations By Error-Prone Repair Of Slipped-Strand DNAs Formed By Disease-Associated CTG/CAG Repeats
CTG/CAG instability is the cause of >14 diseases including myotonic dystrophy (DM1) and Huntingtons disease (HD). In humans repeat instability can occur in proliferating and quiescent cells. Depending upon the disease, the tissue or developmental stage the instability manifests as either an expansion-bias, deletion-bias or stable maintenance of the repeat tract.
The majority of proposed mechanisms for repeat instability involve slipped-strand DNA structures formed by the repeats either in non-replicating DNA (at strand breaks) or at replication forks. In both instances, slip-outs of the nicked or continuous strands represent intermediates of expansion or deletion events, respectively. We have biophysically characterized slipped-DNAs. Processing of these mutagenic intermediates may be one of the underlying mechanisms responsible for the various kinds of repeat instability (or stability).
Indeed repeat instability can be perturbed in model organisms that are deficient in various repair processes. However, DM1 and HD patients that display repeat instabilities are not known to be deficient in any repair activity. Thus, repeats from these patients can be expanded, deleted or stably maintained in cells that are repair-proficient. To date, there is no direct experimental evidence of how repair, in a repair-proficient state may contribute to the various kinds of repeat instability.
We established an in vitro assay to determine the fidelity of slipped-DNA processing by human cell extracts that are functional in many DNA metabolic processes. As substrates, we used structurally defined slipped-DNAs containing an excess of CTG or CAG repeats and a single nick. Our results indicate that a variety of repair outcomes can arise. Importantly, the choice of repair path depends critically upon the biophysical features of the slipped-DNAs. The formation of specific DNA intermediates and their distinct repair outcomes may explain the different mutation patterns occurring at each of the various genetic loci or between tissues. The roles of specific repair proteins will be discussed.
?Respect the personality of DNA?
Commandment VIII from Arthur Kornberg?s Ten Commandments of DNA
Gagan B. Panigrahi1
1Genetics & Genomic Biology