Scarce but scary

Zn2+ blocks annealing of complementary single-stranded DNA in a sequence-selective manner

Zinc is the second most abundant trace element essential for all living organisms. In human body, 30–40% of the total zinc ion (Zn²⁺) is localized in the nucleus. Intranuclear free Zn²⁺ sparks caused by reactive oxygen species have been observed in eukaryotic cells, but question if these free Zn²⁺ outrages could have affected annealing of complementary single-stranded (ss) DNA, a crucial step in DNA synthesis, repair and recombination, has never been raised. Here the author reports that Zn²⁺ blocks annealing of complementary ssDNA in a sequence-selective manner under near-physiological conditions as demonstrated in vitro using a low-temperature EDTA-free agarose gel electrophoresis (LTEAGE) procedure. Specifically, it is shown that Zn²⁺ does not block annealing of repetitive DNA sequences lacking CG/GC sites that are the major components of junk DNA. It is also demonstrated that Zn²⁺ blocks end-joining of double-stranded (ds) DNA fragments with 3′ overhangs mimicking double-strand breaks, and prevents renaturation of long stretches (>1 kb) of denatured dsDNA, in which Zn²⁺-tolerant intronic DNA provides annealing protection on otherwise Zn²⁺-sensitive coding DNA. These findings raise a challenging hypothesis that Zn²⁺-ssDNA interaction might be among natural forces driving eukaryotic genomes to maintain the Zn²⁺-tolerant repetitive DNA for adapting to the Zn²⁺-rich nucleus.

Features of trinucleotide repeat instability in vivo

Unstable repeats are associated with various types of cancer and have been implicated in more than 40 neurodegenerative disorders. Trinucleotide repeats are located in non-coding and coding regions of the genome. Studies of bacteria, yeast, mice and man have helped to unravel some features of the mechanism of trinucleotide expansion. Looped DNA structures comprising trinucleotide repeats are processed during replication and/or repair to generate deletions or expansions. Most in vivo data are consistent with a model in which expansion and deletion occur by different mechanisms. In mammals, microsatellite instability is complex and appears to be influenced by genetic, epigenetic and developmental factors.