Methionine reduces spontaneous and alkylation-induced mutagenesis in Saccharomyces cerevisiae cells deficient in O6-methylguanine-DNA methyltransferase

Mutations in the S-Adenosylmethionine Synthetase Genes SAM1 and SAM2 Differentially Affect Genome Stability in Saccharomyces cerevisiae

Maintenance of genome integrity is a crucial cellular focus that involves a wide variety of proteins functioning in multiple processes. Defects in many different pathways can result in genome instability, a hallmark of cancer. Utilizing a diploid Saccharomyces cerevisiae model, we previously reported a collection of gene mutations that affect genome stability in a haploinsufficient state. In this work we explore the effect of gene dosage on genome instability for one of these genes and its paralog; SAM1 and SAM2 These genes encode S-Adenosylmethionine (AdoMet) synthetases, responsible for the creation of AdoMet from methionine and ATP. AdoMet is the universal methyl donor for methylation reactions and is essential for cell viability. It is the second most used cellular enzyme substrate and is exceptionally well-conserved through evolution. Mammalian cells express three genes, MAT1A, MAT2A, and MAT2B, with distinct expression profiles and functions. Alterations to these AdoMet synthetase genes, and AdoMet levels, are found in many cancers, making them a popular target for therapeutic intervention. However, significant variance in these alterations are found in different tumor types, with the cellular consequences of the variation still unknown. By studying this pathway in the yeast system, we demonstrate that losses of SAM1 and SAM2 have different effects on genome stability through distinctive effects on gene expression and AdoMet levels, and ultimately separate effects on the methyl cycle. Thus, this study provides insight into the mechanisms by which differential expression of the SAM genes have cellular consequences that affect genome instability.

Mutational-reporter transgenes rescued from mice lacking either Mgmt, or both Mgmt and Msh6 suggest that O6-alkylguanine-induced miscoding does not contribute to the spontaneous mutational spectrum

O6-methylguanine methyltransferase, Mgmt, constitutes the first line of defense against O6-alkylguanine, which can result in G : C to A : T transitions upon DNA replication. Mgmt has been found in organisms as diverse as archaebacteria and mammals. This evolutionary conservation suggests that all organisms may be exposed to either endogenous or environmental alkylating agents. We thus hypothesized that tissues of Mgmt-/- mice would exhibit elevated mutant frequencies. Employing the Big Blue trade mark transgenic system, we evaluated lacI mutants rescued from liver and small intestinal DNA of young Mgmt-/- mice. Interestingly, while there was a small difference between Mgmt-/- mice and controls with respect to lacI mutant frequency, no differences attributable to Mgmt deficiency were apparent in the mutational spectra. Although mutations stemming from O6-guanine alkylations would be predicted to be cumulative, we found no evidence of an Mgmt-dependent alteration in mutation spectrum in DNA samples from 12 month-old mice. To optimize our ability to detect mutations resulting from O6-alkylguanine-induced G : T mismatches, mice with combined deficiencies of Mgmt and the DNA mismatch repair molecule, Msh6, were analysed. In spite of this strategy, we observed no significant differences between Mgmt-/- Msh6-/- and Msh6-/- mouse lacI mutations, except for a trend towards a greater percentage (of total transitions) of G : C to A : T changes in Mgmt-/-Msh6-/- livers. Therefore, despite the striking evolutionary conservation of Mgmt, deficiency of this gene did not significantly impact the spontaneous lacI mutational spectrum in vivo.

Folate deficiency in vitro induces uracil misincorporation and DNA hypomethylation and inhibits DNA excision repair in immortalized normal human colon epithelial cells

Epidemiological studies have indicated that folic acid protects against a variety of cancers, particularly cancer of the colorectum. Folate is essential for efficient DNA synthesis and repair. Moreover, folate can affect cellular S-adenosylmethionine levels, which regulate DNA methylation and control gene expression. We have investigated the mechanisms through which folate affects DNA stability in immortalized normal human colonocytes (HCEC). DNA strand breakage, uracil misincorporation, and DNA repair, in response to oxidative and alkylation damage, were determined in folate-sufficient and folate-deficient colonocytes by single cell gel electrophoresis. In addition, methyl incorporation into genomic DNA was measured using the bacterial enzyme Sss1 methylase. Cultured human colonocyte DNA contained endogenous strand breaks and uracil. Folate deficiency significantly increased strand breakage and uracil misincorporation in these cells. This negative effect on DNA stability was concentration dependent at levels usually found in human plasma (1-10 ng/ml). DNA methylation was decreased in HCEC grown in the absence of folate. Conversely, hypomethylation was not concentration dependent. Folate deficiency impaired the ability of HCEC to repair oxidative and alkylation damage. These results demonstrate that folic acid modulates DNA repair, DNA strand breakage, and uracil misincorporation in immortalized human colonocytes and that folate deficiency substantially decreases DNA stability in these cells.

REV3 is required for spontaneous but not methylation damage-induced mutagenesis of Saccharomyces cerevisiae cells lacking O6-methylguanine DNA methyltransferase

O6-methylguanine (O6-MeG) DNA methyltransferase (MTase) removes the methyl group from a DNA lesion and directly restores DNA structure. It has been shown previously that bacterial and yeast cells lacking such MTase activity are not only sensitive to killing and mutagenesis by DNA methylating agents, but also exhibit an increased spontaneous mutation rate. In order to understand molecular mechanisms of endogenous DNA alkylation damage and its effects on mutagenesis, we determined the spontaneous mutational spectra of the SUP4-o gene in various Saccharomyces cerevisiae strains. To our surprise, the mgt1 mutant deficient in DNA repair MTase activity exhibited a significant increase in G:C-->C:G transversions instead of the expected G:C-->A:T transition. Its mutational distribution strongly resembles that of the rad52 mutant defective in DNA recombinational repair. The rad52 mutational spectrum has been shown to be dependent on a mutagenesis pathway mediated by REV3. We demonstrate here that the mgt1 mutational spectrum is also REV3-dependent and that the rev3 deletion offsets the increase of the spontaneous mutation rate seen in the mgt1 strains. These results indicate that the eukaryotic mutagenesis pathway is directly involved in cellular processing of endogenous DNA alkylation damage possibly by the translesion bypass of lesions at the cost of G:C-->C:G transversion mutations. However, the rev3 deletion does not affect methylation damage-induced killing and mutagenesis of the mgt1 mutant, suggesting that endogenous alkyl lesions may be different from O6-MeG.