Enhanced deoxyribonuclease activity in human transformed cells and in Bloom's syndrome cells

Dominant genetic instability and sensitivity to DNA damaging agents in a mammalian cell line

An SV40 transformed Indian muntjac cell line (SVM) has been shown to be hypersensitive to cell killing by a wide range of DNA damaging agents. Evidence points to defects in DNA replication and DNA recombination resulting in chromosome instability both spontaneously and following exposure to DNA damaging agents. We have generated proliferating hybrids between SVM and a spontaneously transformed Indian muntjac cell line (DM). Study of these hybrids indicates that the SVM phenotype acts in a genetically dominant manner and is associated with the expression of SV40 large T antigen. We propose that transformation and immortalization of Indian muntjac fibroblasts by SV40 virus can lead to a set of persistent changes in gene expression that result in chromosome instability and increased sensitivity to DNA damaging agents. Genes involved in these processes are likely to be of great importance as chromosome instability can play a central role in cancer development.

Activity gel and activity blotting methods for detecting DNA-modifying (repair) enzymes

Zymographical methods (activity gel, overlay gel, activity blot and activity blotting) for detecting DNA-modifying (repair) enzymes are reviewed. Emphasis is put on the novel activity blotting method in which DNA repair enzymes electrophoresed on a gel are blotted and detected on a damaged DNA-fixed nylon membrane. Its practical procedures, including a non-radioactive detection procedure, and representative results are also described.

Mechanism of action of deoxyribonuclease II from human lymphoblasts

Deoxyribonuclease II has been purified through five fractionation steps from the human lymphoblast cell line K562. Isolation included DEAE-cellulose and heparin-agarose chromatography followed by fractionation on Mono-S, Mono-Q and Superose-12 FPLC columns. In an extension of previous studies, deoxyribonuclease II was found to introduce a much higher proportion of single-strand nicks relative to double-strand breaks into supercoiled DNA than has been reported for linear DNA. Application of DNA sequencing techniques has further revealed a unique resistance of 3' termini to hydrolysis by this enzyme. Deoxyribonuclease II cleaves at every available site along the duplexed portion of a paired oligonucleotide substrate with the exception of the last four nucleotides. Consistent with previous results, this deoxyribonuclease II is active at low pH in the absence of Mg2+ and is not inhibited by EDTA, but complete inhibition is observed with 100 microM Fe3+. Likewise we confirmed the presence of 3'-phosphoryl termini on the DNA cleavage products since they failed to function as primers for DNA synthesis catalyzed by Escherichia coli DNA polymerase I.

Enzymatic release of 5'-terminal deoxyribose phosphate residues from damaged DNA in human cells

Activities that catalyze or promote the release of 5'-terminal deoxyribose phosphate residues from DNA abasic sites previously incised by an AP endonuclease have been identified in soluble extracts of several human cell lines and calf thymus. Such excision of base-free sugar phosphate residues from apurinic/apyrimidinic sites is expected to be obligatory prior to repair by gap filling and ligation. The most efficient excision function is due to a DNA deoxyribophosphodiesterase similar to the protein found in Escherichia coli. The human enzyme has been partially purified and freed from detectable exonuclease activity. This DNA deoxyribophosphodiesterase is a Mg(2+)-requiring hydrolytic enzyme with an apparent molecular mass of approximately 47 kDa and is located in the cell nucleus. By comparison, the major nuclear 5'----3' exonuclease, DNase IV, is unable to catalyze the release of 5'-terminal deoxyribose phosphate residues as free sugar phosphates but can liberate them at a slow rate as part of small oligonucleotides. Nonenzymatic removal of 5'-terminal deoxyribose phosphate from DNA by beta-elimination promoted by polyamines and basic proteins is a very slow mechanism of release compared to enzymatic hydrolysis. We conclude that a DNA deoxyribophosphodiesterase acts at an intermediate stage between an AP endonuclease and a DNA polymerase during DNA repair at apurinic/apyrimidinc sites in mammalian cells, but several alternative routes also exist for the excision of deoxyribose phosphate residues.

Molecular and biochemical aspects of Bloom's syndrome

Bloom's syndrome (BS) is an autosomal recessive disorder, characterized by a high incidence of cancer at a young age. Cytogenetically, BS cells exhibit a high frequency of chromosomal damage and sister chromatid exchanges. Thus, BS provides one of the best correlations of a human genetic disorder exhibiting both chromosomal instability and a high incidence of cancer. It is increasingly evident that a spontaneous mutagenic event may be responsible for the inherent chromosomal instability. Oxidative stress is now shown to occur in BS cells and may be responsible for the observed chromosomal instability. Furthermore, treatment with antioxidants decreases the level of sister chromatid exchanges. The combination of a mutagenic event and an elevated rate of recombination could potentially lead to homozygosity of tumor suppressor gene function. Hypomethylation and expression of an activated c-myc gene are now demonstrated in BS lymphoblastoid cells. Identifying the mechanism(s) of the ongoing cellular and DNA damage is important in understanding the etiology of this complex disorder. This article reviews the recent biochemical and molecular advances in the study of BS.