DNA ligase I null mouse cells show normal DNA repair activity but altered DNA replication and reduced genome stability

Abstract

DNA ligase I is the key ligase for DNA replication in mammalian cells and has also been reported to be involved in a number of recombination and repair processes. Our previous finding that Lig1 knockout mouse embryos developed normally to mid-term before succumbing to a specific haematopoietic defect was difficult to reconcile with a report that DNA ligase I is essential for the viability of cultured mammalian cells. To address this issue, we generated a second Lig1 targeted allele and found that the phenotypes of our two Lig1 mutant mouse lines are identical. Widely different levels of Lig1 fusion transcripts were detected from the two targeted alleles, but we could not detect any DNA ligase I protein, and we believe both are effective Lig1 null alleles. Using foetal liver cells to repopulate the haematopoietic system of lethally irradiated adult mice, we demonstrate that the haematopoietic defect in DNA-ligase-I-deficient embryos is a quantitative deficiency relating to reduced proliferation rather than a qualitative block in any haematopoietic lineage. DNA ligase I null fibroblasts from Lig1 mutant embryos showed an accumulation of DNA replication intermediates and increased genome instability. In the absence of a demonstrable deficiency in DNA repair we postulate that, unusually, genome instability may result directly from the DNA replication defect. Lig1null mouse cells performed better in the survival and replication assays than a human LIG1 point mutant, and we suggest that the complete absence of DNA ligase I may make it easier for another ligase to compensate for DNA ligase I deficiency.

DNA Ligase I Is an In Vivo Substrate of DNA-Dependent Protein Kinase and Is Activated by Phosphorylation in Response to DNA Double-Strand Breaks,

“…Moreover, DNA ligase I point mutations have been shown to cause replication failure, genomic instability, and increased susceptibility to cancer (24). It has also been reported that DNA ligase I-null mutant cells show normal DNA repair activity, but with altered DNA replication and reduced genomic stability (25). These observations support our proposition that DNA-PK-activated DNA ligase I is likely to have functions other than DNA replication and repair.…”

“…Our results show how DNA-PK may be involved in DSB repair. Recent reports suggest that, besides participating in DNA replication and repair including DSB repair ( , ), DNA ligase I may have other functions such as recombination ( − ) and maintenance of genomic stability ( , ). In HeLa cells, a ∼200 kDa factor, which is dependent on DNA-PK, preferentially increases intermolecular ligation, typical of eukaryotic systems ().…”

Okazaki fragment maturation: nucleases take centre stage

“…Supporting this hypothesis, homozygous knockout mutant mice for genes such as FEN1 and Lig 1 exhibit failure of cell proliferation and early embryonic lethality (Bentley et al, 2002;Kucherlapati et al, 2002;Larsen et al, 2003). In addition, we have recently identified an FEN1 mutation F343A/F344A (FFAA), which disrupts the interaction between FEN1 and PCNA (Zheng et al, 2007a).…”

Ancient mitochondrial diversity reveals population homogeneity in Neolithic Greece and identifies population dynamics along the Danubian expansion axis

“…It is generally of the order of 10 generations, but can go up to 100 generations in central Europe, i.e., roughly between 250 and 2500 years. Such dynamics correspond roughly to the ‘leapfrog’ colonization model 96 , 97 with a rapid colonization of suitable niches for establishing agriculture followed by acculturation and genetic exchange with external non-farmers 98 . Under the most probable scenario, the assimilation rate γ was estimated at 11%.…”