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Biomedical subjects

L Samson

Publications and source records attributed to L Samson.

At least 19 recordsLinked to original sources
PubMed1998

Crystal structure of a human alkylbase-DNA repair enzyme complexed to DNA: mechanisms for nucleotide flipping and base excision.

DNA N-glycosylases are base excision-repair proteins that locate and cleave damaged bases from DNA as the first step in restoring the genetic blueprint. The human enzyme 3-methyladenine DNA glycosylase removes a diverse group of damaged bases from DNA, including cytotoxic and mutagenic alkylation adducts of purines. We report the crystal structure of human 3-methyladenine DNA glycosylase complexed to a mechanism-based pyrrolidine inhibitor. The enzyme has intercalated into the minor groove of DNA, causing the abasic pyrrolidine nucleotide to flip into the enzyme active site, where a bound water is poised for nucleophilic attack. The structure shows an elegant means of exposing a nucleotide for base excision as well as a network of residues that could catalyze the in-line displacement of a damaged base from the phosphodeoxyribose backbone.

Alkylation↗
PubMed1998

Evidence-based guidelines for universal counselling and offering of HIV testing in pregnancy in Canada.

OBJECTIVE: To provide Canadian health care workers with evidence-based guidelines for universal counselling about HIV testing and the offering of such testing to all pregnant women. OPTIONS: Universal counselling and offering of HIV testing to all pregnant women versus targeted testing of only pregnant women at high risk for HIV infection. Antiretroviral treatment protocols for HIV-positive mothers and their infants are discussed as the intervention to reduce mother-to-child transmission rates. OUTCOMES: Main outcomes are mother-to-child HIV transmission rates and consequences of HIV testing on the mother and infant. EVIDENCE: Articles published from January 1985 to March 1997 identified through a MEDLINE search; articles published in pertinent medical journals in 1996 and 1997 identified through a manual search; and abstracts presented at international HIV/AIDS conferences. BENEFITS, HARMS AND COSTS: Early diagnosis of HIV infection in a pregnant woman optimizes her medical and psychosocial care, decreases the incidence of mother-to-child transmission and decreases the risk of horizontal transmission to sexual partners. New, third-generation HIV tests have reduced false-positive rates and thus diminished the harm of screening. RECOMMENDATIONS: A screening strategy consisting of universal counselling and offering of HIV testing is recommended for all pregnant women in Canada (grade B recommendation). Targeted testing of only pregnant women at high risk for HIV infection fails to identify a substantial proportion of HIV-positive pregnant women and is therefore not recommended (grade D recommendation). Women who identify themselves as being at high risk and whose initial HIV test result is negative should be counselled about the reduction of high-risk behaviours and retested in 6 months (grade B recommendation). Treatment of seropositive women and infants with zidovudine to prevent mother-to-child transmission is recommended (grade A or B recommendation depending on gestational age and CD4 count). VALIDATION: These guidelines are endorsed by the Canadian Pediatric AIDS Research Group and are in agreement with the recommendations of the Canadian Paediatric Society and the US Public Health Service Task Force.

AIDS Serodiagnosis↗
PubMed1998

Hypermutation of immunoglobulin genes in memory B cells of DNA repair-deficient mice.

To investigate the possible involvement of DNA repair in the process of somatic hypermutation of rearranged immunoglobulin variable (V) region genes, we have analyzed the occurrence, frequency, distribution, and pattern of mutations in rearranged Vlambda1 light chain genes from naive and memory B cells in DNA repair-deficient mutant mouse strains. Hypermutation was found unaffected in mice carrying mutations in either of the following DNA repair genes: xeroderma pigmentosum complementation group (XP)A and XPD, Cockayne syndrome complementation group B (CSB), mutS homologue 2 (MSH2), radiation sensitivity 54 (RAD54), poly (ADP-ribose) polymerase (PARP), and 3-alkyladenine DNA-glycosylase (AAG). These results indicate that both subpathways of nucleotide excision repair, global genome repair, and transcription-coupled repair are not required for somatic hypermutation. This appears also to be true for mismatch repair, RAD54-dependent double-strand-break repair, and AAG-mediated base excision repair.

Animals↗
PubMed1998

O6-alkylguanine DNA lesions trigger apoptosis.

It is unclear whether alkylating agents induce apoptosis because they damage DNA, or because they damage other cellular targets. Isogenic Chinese hamster ovary (CHO) cell lines varying in the repair of O6-alkylguanine (O6AlkG) were examined for their propensity to undergo alkylation-induced apoptosis. Robust O6AlkG repair virtually eliminated the apoptogenic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 1,3-bis-(2-chloroethyl)-1-nitrosourea (BCNU, carmustine), as did the expression of BCL-2. O6AlkG repair had no effect on apoptosis induced by tumor necrosis factor alpha or by gamma-irradiation. We conclude that alkylating agents induce apoptosis by virtue of their ability to modify DNA bases and, more specifically, that O6AlkG lesions can trigger such programmed cell death.

Animals↗
PubMed1997

Mammalian DNA repair methyltransferases shield O4MeT from nucleotide excision repair.

O6-Methylguanine (O6MeG) and O4-methylthymine (O4MeT) are potentially mutagenic DNA lesions that cause G:C-->A:T and A:T-->G:C transition mutations by mispairing during DNA replication, and the repair of O6MeG and O4MeT by DNA repair methyltransferases (MTases) is therefore expected to prevent methylation-induced transitions. The efficiency of O6MeG and O4MeT repair by different MTases can vary by several hundred-fold and the aim of this study was to establish the biological consequences of such differences in the efficiency of repair. The ability of three microbial and two mammalian MTases to prevent methylation-induced G:C-->A:T and A:T-->G:C transitions is taken as a measure of their ability to repair O6MeG and O4MeT in vivo respectively. All five MTases give complete protection against G:C-->A:T transitions. However, while the microbial MTases give complete protection against A:T-->G:C transitions, the mammalian MTases actually sensitize cells to A:T-->G:C transitions. We hypothesize that the mammalian MTases bind O4MeT lesions in vivo but that, because they are extremely slow at subsequent methyl transfer, binding shields O4MeT from repair by the nucleotide excision repair pathway. Results are presented to support this hypothesis.

Animals↗
PubMed1996

Cloning and characterization of a cDNA encoding a 3-methyladenine DNA glycosylase from the fission yeast Schizosaccharomyces pombe.

We have begun to develop the fission yeast, Schizosaccharomyces pombe, as a eukaryotic model for cellular defenses against alkylating agents. Here we describe the cloning and characterization of a cDNA, designated mag1, encoding a S. pombe 3-methyladenine (3MeA) DNA glycosylase. 3MeA DNA glycosylases in Escherichia coli are encoded by alkA and tag. S. pombe mag1 was cloned by its ability to reverse the alkylation-sensitive phenotype of an alkA tag E. coli double mutant. The expression of S. pombe mag1 in E. coli confers partial resistance to alkylating agents that produce methyl, ethyl and propyl lesions, and Mag1 production produces 3MeA DNA glycosylase activity. In contrast to the E. coli alkA and Saccharomyces cerevisiae MAG genes, expression of S. pombe mag1 was not appreciably induced by alkylating agents. The mag1 cDNA encodes a protein of 228 amino acids (aa) that shares similarity with 3MeA DNA glycosylases from E. coli (AlkA), Bacillus subtilis (BsAlkA) and S. cerevisiae (MAG). A consensus sequence of 9 aa common to these microbial 3MeA DNA glycosylases is discussed.

Alkylation↗
PubMed1996

Repair-deficient 3-methyladenine DNA glycosylase homozygous mutant mouse cells have increased sensitivity to alkylation-induced chromosome damage and cell killing.

In Escherichia coli, the repair of 3-methyladenine (3MeA) DNA lesions prevents alkylation-induced cell death because unrepaired 3MeA blocks DNA replication. Whether this lesion is cytotoxic to mammalian cells has been difficult to establish in the absence of 3MeA repair-deficient cell lines. We previously isolated and characterized a mouse 3MeA DNA glycosylase cDNA (Aag) that provides resistance to killing by alkylating agents in E. coli. To determine the in vivo role of Aag, we cloned a large fragment of the Aag gene and used it to create Aag-deficient mouse cells by targeted homologous recombination. Aag null cells have no detectable Aag transcripts or 3MeA DNA glycosylase activity. The loss of Aag renders cells significantly more sensitive to methyl methanesulfonate-induced chromosome damage, and to cell killing induced by two methylating agents, one of which produces almost exclusively 3MeAs. Aag null embryonic stem cells become sensitive to two cancer chemotherapeutic alkylating agents, namely 1,3-bis(2-chloroethyl)-1-nitrosourea and mitomycin C, indicating that Aag status is an important determinant of cellular resistance to these agents. We conclude that this mammalian 3MeA DNA glycosylase plays a pivotal role in preventing alkylation-induced chromosome damage and cytotoxicity.

Alkylating Agents↗
PubMed1996

Increasing DNA repair methyltransferase levels via bone marrow stem cell transduction rescues mice from the toxic effects of 1,3-bis(2-chloroethyl)-1-nitrosourea, a chemotherapeutic alkylating agent.

The chloroethylnitrosourea (CNU) alkylating agents are commonly used for cancer chemotherapy, but their usefulness is limited by severe bone marrow toxicity that causes the cumulative depletion of all hematopoietic lineages (pancytopenia). Bone marrow CNU sensitivity is probably due to the inefficient repair of CNU-induced DNA damage; relative to other tissues, bone marrow cells express extremely low levels of the O6-methylguanine DNA methyltransferase (MGMT) protein that repairs cytotoxic O6-chloroethylguanine DNA lesions. Using a simplified recombinant retroviral vector expressing the human MGMT gene under control of the phosphoglycerate kinase promoter (PGK-MGMT) we increased the capacity of murine bone marrow-derived cells to repair CNU-induced DNA damage. Stable reconstitution of mouse bone marrow with genetically modified, MGMT-expressing hematopoietic stem cells conferred considerable resistance to the cytotoxic effects of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), a CNU commonly used for chemotherapy. Bone marrow harvested from mice transplanted with PGK-MGMT-transduced cells showed extensive in vitro BCNU resistance. Moreover, MGMT expression in mouse bone marrow conferred in vivo resistance to BCNU-induced pancytopenia and significantly reduced BCNU-induced mortality due to bone marrow hypoplasia. These data demonstrate that increased DNA alkylation repair in primitive hematopoietic stem cells confers multilineage protection from the myelosuppressive effects of BCNU and suggest a possible approach to protecting cancer patients from CNU chemotherapy-related toxicity.

Animals↗
PubMed1996

The Escherichia coli MutS DNA mismatch binding protein specifically binds O(6)-methylguanine DNA lesions.

DNA mismatch repair defects in certain cell types confer resistance to the cytotoxic effects of alkylating agents, suggesting that a normally functioning DNA mismatch repair pathway can actually mediate alkylation-induced cell death. In eukaryotic cells this phenomenon is only observed in cells lacking adequate DNA methyltransferase for the repair of O6-methylguanine (O6MeG) DNA lesions. It has been proposed that O6MeG may act as a substrate for DNA mismatch repair when paired with cytosine and when mispaired with thymine and that repeated futile DNA mismatch repair at O6MeG DNA lesions is cytotoxic. Here we show that the Escherichia coli MutS DNA mismatch repair binding protein does indeed bind specifically to O6MeG DNA lesions. In contrast, MutS does not bind DNA containing another O-alkylated base, namely O4-methylthymine, or another kind of modified guanine, namely 8-oxoguanine. These results provide direct biochemical evidence for the involvement of DNA mismatch repair in specifically processing O6MeG DNA lesions.

Adenosine Triphosphatases↗
PubMed1996

DNA repair functions in heterologous cells.

Our genetic information is constantly challenged by exposure to endogenous and exogenous DNA-damaging agents, by DNA polymerase errors, and thereby inherent instability of the DNA molecule itself. The integrity of our genetic information is maintained by numerous DNA repair pathways, and the importance of these pathways is underscored by their remarkable structural and functional conservation across the evolutionary spectrum. Because of the highly conserved nature of DNA repair, the enzymes involved in this crucial function are often able to function in heterologous cells; as an example, the E. coli Ada DNA repair methyltransferase functions efficiently in yeast, in cultured rodent and human cells, in transgenic mice, and in ex vivo-modified mouse bone marrow cells. The heterologous expression of DNA repair functions has not only been used as a powerful cloning strategy, but also for the exploration of the biological and biochemical features of numerous enzymes involved in DNA repair pathways. In this review we highlight examples where the expression of DNA repair enzymes in heterologous cells was used to address fundamental questions about DNA repair processes in many different organisms.

Animals↗
PubMed1996

[Urinary excretion of free and total deoxypyridinoline during secondary hyperparathyroidism in the elderly. Comparison of chromatographic (HPLC) and immunoenzymatic (Pyrilinks-D) methods].

The measurement of urinary deoxypyridinoline (DPD) constitutes a specific and sensitive marker of bone resorption. Total and free forms of DPD are determined by chromatographic method (HPLC) after or without hydrolysis of urine, respectively. Pyrilinks-D, a new immunoassay, allows to assess directly the free forms and needs an appropriate hydrolysis step for measuring the total form. We have compared the values of free (F), total (T) and conjugated (NF) forms of DPD determined by HPLC and Pyrilinks-D, in elderly women (n = 21, mean age: 83.5 +/- 1.5 years) with vitamin D insufficiency (25 OH D < 6 ng/mL) and Ca insufficiency responsible for a secondary hyperparathyroidism (iPTH = 45.3 +/- 22.7 pg/mL) and in healthy elderly women (n = 25, mean age: 76.6 +/- 3.1 years) with a normal vit D status (25 OH D > 10 ng/mL) as control group. We have also measured DPD during the course of vit D and Ca supplementation. At baseline, the HPLC and Pyrilinks-D values of DPD/Cr are highly correlated (DPD-T: r = 0.92, p < 0.001 and DPD-F: r = 0.76, p < 0.001), DPD-F and -NF values are correlated with those of DPD-T, while DPD-F and -NF are not correlated between themselves. In elderly with vit D insufficiency, the values obtained with Pyrilinks-D as compared to control subjects, show a significant increase of urinary excretion of DPD-F (8.5 +/- 3.1 vs 5.7 +/- 1.9 nmol/mmol, Cr, p < 0.0001), DPD-T (16.8 +/- 10.2 vs 9.9 +/- 3.5 nmol/mmol, Cr; p < 0.001) and DPD-NF (8.3 +/- 9.0 vs 4.5 +/- 3.3 nmol/mmol, Cr, p < 0.05). The administration of 800 IU of vit D and 1 g of elemental Ca during a course of 6 months normalize the iPTH values (24.4 +/- 11.8 and 30.9 +/- 14.6 pg/mL at 3 and 6 months). Simultaneously, the urinary excretion at 3 and 6 months of DPD-T (12.9 +/- 6.0 and 13.6 +/- 6.5 nmol/mmol, Cr) and of DPD-NF (4.5 +/- 3.3 and 5.5 +/- 4.8 nmol/mmol Cr) assessed by Pyrilinks-D as well as by HPLC decreased significantly, while no change was seen with DPD-F assessed by both methods. The decreases expressed as percent of baseline values were about 20% for DPD-T and more than 30% for DPD-NF, while DPD-F levels remain unchanged. We conclude that the Pyrilinks-D immunoassay presents reliable characteristics and allows to assess either free or total forms of DPD, like the HPLC technique. It constitutes an excellent reflection of bone resorption in elderly with vit D insufficiency. However its application to monitor therapy like vit D and Ca supplementation, needs a hydrolysis step to determine DPD-T which appears in this study more sensitive to the treatment than DPD-F.

Aged↗
PubMed1995

Retrovirus-mediated expression of a DNA repair protein in bone marrow protects hematopoietic cells from nitrosourea-induced toxicity in vitro and in vivo.

Severe and delayed myelosuppression is a major side effect encountered with the clinical use of nitrosourea-type chemotherapeutic drugs. The DNA repair protein O6-methylguanine DNA methyltransferase (MGMT) has been shown to repair nitrosourea-induced DNA damage. We therefore investigated the effect of expressing MGMT in hematopoietic cells (via retrovirus-mediated gene transfer) on nitrosourea-induced toxicity. A retroviral vector (N2/ZipPGK-MGMT) expressing the human MGMT cDNA from the phosphoglycerate kinase promoter was constructed. Infection of murine bone marrow with the N2/ZipPGK-MGMT retrovirus significantly increased the survival of murine bone marrow-committed progenitor cells following in vitro exposure to N-N'-bis(2-chloroethyl)-N-nitrosourea (BCNU, carmustine). MGMT gene transfer also protected murine hematopoietic cells in vivo in a murine model of BCNU-induced myelosuppression. The infusion of 4-6 x 10(6) N2/ZipPGK-MGMT-transduced bone marrow cells into mice every 2 weeks significantly increased peripheral leukocyte counts, platelet counts, and hematocrits compared to infusions of mock-infected bone marrow cells. In addition, bone marrow-committed progenitor cells from some recipient animals demonstrated increased resistance to BCNU in vitro when analyzed 2.5 months after initial treatment. The integration of the N2/ZipPGK-MGMT provirus in the spleen DNA from these animals correlated with committed progenitor cell resistance to BCNU. These data suggest that MGMT expression in hematopoietic progenitor and precursor cells protects against nitrosourea-induced toxicity and that gene transfer may prove useful in attempts to reduce nitrosourea-induced myelosuppression in the clinical setting.

3T3 Cells↗
PubMed1995

Replication protein A binds to regulatory elements in yeast DNA repair and DNA metabolism genes.

Saccharomyces cerevisiae responds to DNA damage by arresting cell cycle progression (thereby preventing the replication and segregation of damaged chromosomes) and by inducing the expression of numerous genes, some of which are involved in DNA repair, DNA replication, and DNA metabolism. Induction of the S. cerevisiae 3-methyladenine DNA glycosylase repair gene (MAG) by DNA-damaging agents requires one upstream activating sequence (UAS) and two upstream repressing sequences (URS1 and URS2) in the MAG promoter. Sequences similar to the MAG URS elements are present in at least 11 other S. cerevisiae DNA repair and metabolism genes. Replication protein A (Rpa) is known as a single-stranded-DNA-binding protein that is involved in the initiation and elongation steps of DNA replication, nucleotide excision repair, and homologous recombination. We now show that the MAG URS1 and URS2 elements form similar double-stranded, sequence-specific, DNA-protein complexes and that both complexes contain Rpa. Moreover, Rpa appears to bind the MAG URS1-like elements found upstream of 11 other DNA repair and DNA metabolism genes. These results lead us to hypothesize that Rpa may be involved in the regulation of a number of DNA repair and DNA metabolism genes.

Base Sequence↗
PubMed1995

Multiple molecular forms of pyridinolines cross-links excreted in human urine evaluated by chromatographic and immunoassay methods.

The measurement of the collagen cross-links, hydroxylysylpyridinoline (HP) and lysylpyridinoline (LP), excreted in urine either in free or peptide-bound forms represents the most extensively investigated biochemical marker of bone collagen degradation. We studied the urinary molecular forms of pyridinolines after separation in free and peptide-linked fractions by chromatography and serial dialysis. The pyridinoline amounts of molecular species (free, < 1000 D, 1000-3500 D, 3500-10,000 D, and > 10,000 D) were evaluated by high performance liquid chromatography (HPLC) as well as with the two newly introduced enzyme-linked immunosorbent assay (ELISA) methods for determination of free pyridinolines (collagen Pyrilinks and collagen Pyrilinks-D). The variability of urinary pyridinoline forms were studied in healthy adult control subjects (n = 10, 38.4 +/- 7.5) years), in adolescents (n = 10, 16 +/- 3.3 years), and in elderly subjects with vitamin D insufficiency (n = 10, 87.3 +/- 4.3 years). Free and peptide-conjugated pyridinolines with MW < 1000 D constitute the major part of urinary cross-links in all groups, with a significantly lesser excretion in elderly patients than in adolescent groups. Expressed as a percent of total cross-links, urinary free pyridinolines assessed by direct HPLC are less in elderly subjects (HP = 34.2 +/- 6.2%, LP = 32.7 +/- 7.6%) than in adolescents (HP = 45.8 +/- 10.8%, p = 0.0065 and LP = 47.8 +/- 12.1%, p = 0.012) and in healthy adults (HP = 39.3 +/- 11.5%, NS and LP = 38.1 +/- 9.3%, NS).(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent↗
PubMed1995

Suppression of Escherichia coli alkB mutants by Saccharomyces cerevisiae genes.

The alkB gene is one of a group of alkylation-inducible genes in Escherichia coli, and its product protects cells from SN2-type alkylating agents such as methyl methanesulfonate (MMS). However, the precise biochemical function of the AlkB protein remains unknown. Here, we describe the cloning, sequencing, and characterization of three Saccharomyces cerevisiae genes (YFW1, YFW12, and YFW16) that functionally complement E. coli alkB mutant cells. DNA sequence analysis showed that none of the three gene products have any amino acid sequence homology with the AlkB protein. The YFW1 and YFW12 proteins are highly serine and threonine rich, and YFW1 contains a stretch of 28 hydrophobic residues, indicating that it may be a membrane protein. The YFW16 gene turned out to be allelic with the S. cerevisiae STE11 gene. STE11 is a protein kinase known to be involved in pheromone signal transduction in S. cerevisiae; however, the kinase activity is not required for MMS resistance because mutant STE11 proteins lacking kinase activity could still complement E. coli alkB mutants. Despite the fact that YFW1, YFW12, and YFW16/STE11 each confer substantial MMS resistance upon E. coli alkB cells, S. cerevisiae null mutants for each gene were not MMS sensitive. Whether these three genes provide alkylation resistance in E. coli via an alkB-like mechanism remains to be determined, but protection appears to be specific for AlkB-deficient E. coli because none of the genes protect other alkylation-sensitive E. coli strains from killing by MMS.

Alkylation↗