Fidelity of DNA Ligase I is sensitive to physiological Mg2+ level

The pool of free intracellular Mg²⁺ varies among tissues with the highest concentration measured in muscle tissue and the lowest measured in immune cells and in the brain. Here we investigate the impact of free Mg²⁺ on the fidelity of human DNA ligase I (LIG1). LIG1 is the major DNA ligase and is required to complete DNA replication, recombination and repair pathways. Biallelic hypomorphic variants of LIG1 cause immunodeficiency-96 (IMD96). We employed steady-state kinetics to compare fidelity of LIG1 towards a damaged nucleobase at the 3´-OH side of a nicked DNA substrate. The fidelity for discrimination between a damaged and undamaged nick increases by a factor of 21-fold when the free Mg²⁺ concentration is decreased from 1.0 to 0.2 mM. This has important implications for neurodegenerative and immune diseases, because the brain and the immune system are reported to have free Mg²⁺ concentration in the range from 0.2 - 0.4 mM. We examined a recently characterized minor variant of LIG1, K845N, which has a protective effect in Huntington's disease, and found that the fidelity of K845N LIG1 is also enhanced as free Mg²⁺ decreases. This increase in fidelity is mainly due to the increased release of the AMP-DNA intermediate from a pro-mutagenic DNA substrate. A model is proposed whereby the fidelity of DNA transactions is sensitive to the availability of the Mg²⁺ cofactor for DNA ligation and therefore ligation fidelity may vary between tissues.