Genetic mapping and expression analysis of the murine DNA ligase I gene

Chronic Replication Problems Impact Cell Morphology and Adhesion of DNA Ligase I Defective Cells

Moderate DNA damage resulting from metabolic activities or sub-lethal doses of exogenous insults may eventually lead to cancer onset. Human 46BR.1G1 cells bear a mutation in replicative DNA ligase I (LigI) which results in low levels of replication-dependent DNA damage. This replication stress elicits a constitutive phosphorylation of the ataxia telangiectasia mutated (ATM) checkpoint kinase that fails to arrest cell cycle progression or to activate apoptosis or cell senescence. Stable transfection of wild type LigI, as in 7A3 cells, prevents DNA damage and ATM activation. Here we show that parental 46BR.1G1 and 7A3 cells differ in important features such as cell morphology, adhesion and migration. Comparison of gene expression profiles in the two cell lines detects Bio-Functional categories consistent with the morphological and migration properties of LigI deficient cells. Interestingly, ATM inhibition makes 46BR.1G1 more similar to 7A3 cells for what concerns morphology, adhesion and expression of cell-cell adhesion receptors. These observations extend the influence of the DNA damage response checkpoint pathways and unveil a role for ATM kinase activity in modulating cell biology parameters relevant to cancer progression.

Tumour suppressor activity of human imprinted gene PEG3 in a glioma cell line

BACKGROUND

Mouse imprinted gene Peg3 encodes a large C2H2 type zinc finger protein with unique characteristics. Peg3 knockout mice were found to show an impairment in maternal behaviour of the adult female. Mouse Peg3 is located on the proximal region of chromosome 7 which is syntenic to the long arm of human chromosome 19. It has been reported that a loss of heterozygosity (LOH) of chromosome 19q occurs frequently in several glioma types.

RESULTS

We isolated human PEG3 cDNA. Both human and mouse PEG3 were strongly expressed in the adult brain and the Peg3 protein was localized in the nuclei of both neurones and glial cells. A significant decrease in PEG3 expression was more commonly observed in glioma cell lines as compared with that in primary cultures of astrocytes. Transfection of PEG3 cDNA in a glioma cell line resulted in a loss of tumorigenicity in nude mice.

CONCLUSIONS

The human PEG3 gene is a paternally expressed imprinted gene. Introduction of PEG3 cDNA into the glioma cells suggests that human PEG3 protein functions as a tumour suppressor. Human PEG3 is located on 19q13.4 and is one of the candidates for tumour suppressor genes that are predicted to be sited in gliomas.

Stability of HPRT marker gene expression at different gene-targeted loci: observing and overcoming a position effect

For sophisticated gene targeting procedures requiring two sequential selective steps to operate efficiently it is essential that the marker genes used are not prone to position effects. The double replacement gene targeting procedure, to produce mice with subtle gene alterations, is based on the use of hypoxanthine phosphoribosyltransferase ( HPRT) minigenes in HPRT-deficient embryonic stem cells. Our standard HPRTminigene, under the control of the mouse phosphoglycerate kinase-1 gene promoter, was stably expressed at five of six target loci examined. At the remaining locus, DNA ligase I (Lig1), expression of this minigene was highly unstable. A different minigene, under the control of the mouse HPRT promoter and embedded in its natural CpG-rich island, overcame this position effect and was stably expressed when targeted to the identical site in the Lig1 locus. The promoter region of the stably expressed minigene remained unmethylated, while the promoter of the unstably expressed minigene rapidly became fully methylated. The difference in the stability of HPRT minigene expression at the same target locus can be explained in the context of the different lengths of their CpG-rich promoter regions with associated transcription factors and a resulting difference in their susceptibility to DNA methylation, rather than by differences in promoter strength.

The role of DNA repair in development

Several pathways of DNA repair are essential for maintaining genomic integrity in mammalian cells. Mismatch repair is the final line of defense against polymerase errors during normal cellular replication. Base excision repair removes endogenous DNA damage resulting from normal cellular metabolism. Nucleotide excision repair removes bulky, transcription blocking, lesions resulting from endogenous and environmental insults to the DNA. The role of DNA repair in mammalian development is not well understood. Nevertheless, clues to the essential nature of these processes are evident in the human DNA repair syndromes, in the nature of the interactions between DNA repair and other proteins, and in the phenotypes of genetically engineered, knockout mice lacking functional repair genes. Questions remain: what is the relative importance of endogenous vs. environmental DNA damage and is repair itself critical for normal development or are transcription-repair interactions more crucial?

DNA ligase I is required for fetal liver erythropoiesis but is not essential for mammalian cell viability

Four distinct DNA ligase activities (I-IV) have been identified within mammalian cells. Evidence has indicated that DNA ligase I is central to DNA replication, as well as being involved in DNA repair processes. A patient with altered DNA ligase I displayed a phenotype similar to Bloom's syndrome, being immunodeficient, growth retarded and predisposed to cancer. Fibroblasts isolated from this patient (46BR) exhibited abnormal lagging strand synthesis and repair deficiency. It has been reported that DNA ligase I is essential for cell viability, but here we show that cells lacking DNA ligase I are in fact viable. Using gene targeting in embryonic stem (ES) cells, we have produced DNA ligase I-deficient mice. Embryos develop normally to mid-term when haematopoiesis usually switches to the fetal liver. Thereupon acute anaemia develops, despite the presence of erythroid-committed progenitor cells in the liver. Thus DNA ligase I is required for normal development, but is not essential for replication. Hence a previously unsuspected redundancy must exist between mammalian DNA ligases.

Zfp60, a mouse zinc finger gene expressed transiently during in vitro muscle differentiation

The complete cDNA coding sequence of the zinc finger gene Zfp60 is reported. The predicted amino acid sequence of the Zfp60 protein has been found to contain 19 zinc finger motives clustered at the C-terminus. At its N-terminus, Zfp60 shares with other members of the zinc finger gene family two additional conserved amino acid modules named Kruppel Associated Boxes (KRAB). The expression patterns of Zfp60, MyoD and MHC mRNAs have been followed during in vitro myogenic differentiation of C2 cells. We show that the bacterial produced Zfp60 protein binds DNA only in presence of zinc ions. Zfp60 locus has been mapped in chromosome 7, where other Zfp loci are localised.