Irradiation induces DNA damage and modulates epigenetic effectors in distant bystander tissue in vivo.

Irradiated cells induce chromosomal instability in unirradiated bystander cells in vitro. Although bystander effects are thought to be linked to radiation-induced secondary cancers, almost no studies have evaluated bystander effects in vivo. Furthermore, it has been proposed that epigenetic changes mediate bystander effects, but few studies have evaluated epigenetic factors in bystander tissues in vivo. Here, we describe studies in which mice were unilaterally exposed to X-irradiation and the levels of DNA damage, DNA methylation and protein expression were evaluated in irradiated and bystander cutaneous tissue. The data show that X-ray exposure to one side of the animal body induces DNA strand breaks and causes an increase in the levels of Rad51 in unexposed bystander tissue. In terms of epigenetic changes, unilateral radiation suppresses global methylation in directly irradiated tissue, but not in bystander tissue at given time-points studied. Intriguingly, however, we observed a significant reduction in the levels of the de novo DNA methyltransferases DNMT3a and 3b and a concurrent increase in the levels of the maintenance DNA methyltransferase DNMT1 in bystander tissues. Furthermore, the levels of two methyl-binding proteins known to be involved in transcriptional silencing, MeCP2 and MBD2, were also increased in bystander tissue. Together, these results show that irradiation induces DNA damage in bystander tissue more than a centimeter away from directly irradiated tissues, and suggests that epigenetic transcriptional regulation may be involved in the etiology of radiation-induced bystander effects.

The S. cerevisiae Mag1 3-methyladenine DNA glycosylase modulates susceptibility to homologous recombination.

DNA glycosylases, such as the Mag1 3-methyladenine (3MeA) DNA glycosylase, initiate the base excision repair (BER) pathway by removing damaged bases to create abasic apurinic/apyrimidinic (AP) sites that are subsequently repaired by downstream BER enzymes. Although unrepaired base damage may be mutagenic or recombinogenic, BER intermediates (e.g. AP sites and strand breaks) may also be problematic. To investigate the molecular basis for methylation-induced homologous recombination events in Saccharomyces cerevisiae, spontaneous and methylation-induced recombination were studied in strains with varied MAG1 expression levels. We show that cells lacking Mag1 have increased susceptibility to methylation-induced recombination, and that disruption of nucleotide excision repair (NER; rad4) in mag1 cells increases cellular susceptibility to these events. Furthermore, expression of Escherichia coli Tag 3MeA DNA glycosylase suppresses recombination events, providing strong evidence that unrepaired 3MeA lesions induce recombination. Disruption of REV3 (required for polymerase zeta (Pol zeta)) in mag1 rad4 cells causes increased susceptibility to methylation-induced toxicity and recombination, suggesting that Pol zeta can replicate past 3MeAs. However, at subtoxic levels of methylation damage, disruption of REV3 suppresses methylation-induced recombination, indicating that the effects of Pol zeta on recombination are highly dose-dependent. We also show that overproduction of Mag1 can increase the levels of spontaneous recombination, presumably due to increased levels of BER intermediates. However, additional APN1 endonuclease expression or disruption of REV3 does not affect MAG1-induced recombination, suggesting that downstream BER intermediates (e.g. single strand breaks) are responsible for MAG1-induced recombination, rather than uncleaved AP sites. Thus, too little Mag1 sensitizes cells to methylation-induced recombination, while too much Mag1 can put cells at risk of recombination induced by single strand breaks formed during BER.

Multiple and long-range participation of benzyl groups in intramolecular C-arylation reactions of benzylated glycosides.

The intrinsic reactivity of furanosides bearing activated O-benzyl substituents (3-methoxybenzyl), in the presence of bidentate Lewis acids such as tin(IV) chloride, was explored. These glycosides were found to exhibit extremely interesting chemical properties. Thus, with three reactive substituents (at O-2,3,5), the corresponding glycosides (1 and 7) underwent a novel internal bis-C-arylation process, which involved successive alkylations of the benzyl groups at O-2 and O-3 ("multiple participation"), leading to the formal replacement of the two C-O bonds at the anomeric center of the glycoside by two C-C bonds. The bis-C-arylated constitution of the resulting polycyclic compounds 4 and 8, and the cis configuration of their fused ring system (a tetrahydro-[2]benzopyrano[3,4-d][2]benzoxepin derivative), were determined on the basis of their n.m.r.-spectral parameters. With two 3-methoxybenzyl substituents (at O-3 and O-5, compound 6), intramolecular alkylation of the benzyl group at O-3 or O-5 occurred when glycoside 6 was reacted with titanium(IV) chloride or tin(IV) chloride, respectively, thereby leading to novel bicyclic internal aryl C-glycosides (9 and 12) as major products ("long-range participation"). The constitution of compounds 9 and 12 was unambiguously established by the reactions of analogs of 6 bearing only one 3-methoxybenzyl substituent at a specific position (at O-3: 15; at O-5: 20). The unexpected divergent behavior of 6 in the presence of titanium(IV) and tin(IV) chloride remains to be explained. The availability of compound 9 made it possible to independently prepare the bis-C-arylated derivative 8 (by way of the reverse sequence of internal C-arylation reactions) and thereby to definitively demonstrate its constitution. These unprecedented reactions extend the scope of the intramolecular C-glycosidation of substituted sugars and provide novel methodologies in synthetic carbohydrate chemistry.

Synthesis of C-glycosylarenes by way of internal reactions of benzylated and benzoylated carbohydrate derivatives.

"Internal" C-glycosylarenes [e.g., (2R,3S,3aS,9bR)-3,3a,5,9b-tetrahydro-3-methoxy- and -3,7-dimethoxy-2-methoxymethyl-2H-furo[3,2-c][2]benzopyran] were prepared by intramolecular reactions of 2-O-benzyl derivatives of methyl 3,5-di-O-methyl-D-xylofuranoside (2) and their conversion into authentic C-glycosylated aromatic systems was investigated. The auxiliary benzylic linkage could not be cleaved by hydrogenolysis; isochroman derivatives (e.g., (3S)-3,4-dihydro-3-[(1R,2R)-2-hydroxy-1,3-dimethoxypropyl]-5-metho xy-1H- 2-benzopyran) were obtained under these conditions. However, oxidation of the primary benzylic position with ruthenium tetraoxide gave the corresponding lactone (a dihydroisocoumarin derivative, e.g., (2R,3S,3aS,9bR)-2,3,3a,9b-tetrahydro-3,7-dimethoxy-2-methoxy - methyl-5H-furo[3,2-c][2]benzopyran-5-one) which could be opened by saponification, thereby leading to a stereochemically unique C-glycosylbenzoic acid derivative. The same type of lactone was obtained directly from a derivative of 2 bearing a sufficiently reactive benzoyl group at O-2 (3,5-dimethoxybenzoyl); this process provides a useful approach to a heterocyclic system present in a variety of natural products. In related studies, the 2-O-phenyl substituent was found to be much less reactive than the 2-O-benzyl group in intramolecular Friedel-Crafts reactions of 2-O-substituted glycofuranosides. The first examples of successful internal C-arylation in the pyranoid series were achieved from 2-O-(3-methoxybenzyl)-D- mannopyranosides; the resulting "internal C-glycosides" [( 2R,3S,4S,4aS,10bS)-2,3,4,4a,6,10b- hexahydro-3,4,8-trimethoxy-2-methoxymethylpyrano[3,2-c][2]benzopyr an and 3,4-bis(benzyloxy)-2-benzyloxymethyl-8-methoxy analog] contain a heterocyclic skeleton closely related to that of the natural product bergenin.