Salt deserts

How cells survive, replicate and maintain genome integrity in high NaCl concentrations has long puzzled scientists. A group of US scientists has conducted a series of experiments to determine how this occurs and discovered that cells are actually protected from these conditions by the ‘gene deserts’, the 40% of the genome that are thought to serve no purpose.1

High concentrations of NaCl can lead to the breakdown of DNA within a genome. The breaks remain elevated for as long as NaCl concentration remains high and are rapidly repaired when the concentration is lowered. Repair of the breaks once NaCl is reduced is accompanied by formation of foci containing phosphorylated H2AX (γH2AX), which occurs around DNA double‑strand breaks and assists in their repair.

To investigate exactly what happens when a cell is exposed to high NaCl conditions, the team used a comet assay and pulsed field electrophoresis and confirmed that cells adapted to high NaCl have increased levels of double-strand breaks. γH2AX is predominantly localized to regions of the genome devoid of the genes – that is the gene deserts – which indicates that the high NaCl-induced double-strand breaks are located there. This localization to gene deserts helps to explain why the DNA breaks are less harmful than the random breaks induced by genotoxic agents. The team concluded that the presence of high salt levels around animal cells has directly influenced the evolution of the structure of their genomes.

1 Burg et al., Proceedings of the National Academy of Sciences, 108(51), 20796–20801 (2011).

This story originally appeared in The Column. Click here to view that issue.