Preferential effects of the chemotherapeutic DNA crosslinking agent mitomycin C on inducible gene expression in vivo

Areca nut and smokeless tobacco exposure induces micronucleus, other nuclear abnormalities and cytotoxicity in early chick embryo

Areca nut (AN) and smokeless tobacco (SLT) are indiscriminately consumed among the populations of Southeast and South Asian countries, even by women during the gestational period. This study aimed to investigate the genotoxic and cytotoxic potentials of AN and Sadagura (SG), a unique homemade SLT preparation, alone and in combination in early chick embryos. Fertile white leghorn chicken eggs were randomly divided into five treatment groups: vehicle control, positive control (Mitomycin C, 20 μg/egg), AN, SG, and AN+SG. AN, SG, and AN+SG were given at dosages of 0.125, 0.25, and 0.5 mg/egg. The hen's egg test for micronucleus induction (HET-MN) was performed in chick embryos to evaluate the genotoxic potential of the test agents. Furthermore, the cytotoxic potential was assessed by studying erythroblast cell populations and the polychromatic erythrocytes (PCEs) to normochromatic erythrocytes (NCEs) ratio. Our results indicated a significant increase (p < .001) in MN frequency and other nuclear abnormalities, suggesting the potential of AN and SG to cause genotoxicity. Also, AN and SG exposure alone and in combination considerably altered the erythroblast cell population (%) and the PCE to NCE ratio in all the treatment periods. Our findings established the genotoxic and cytotoxic potential of both AN and SG alone and in combination during early embryonic development in the chick embryo.

Topical Mitomycin C Application Is Effective Even in Esophageal Strictures Resistant to Dilatation Therapy in Children

PURPOSE

Several treatment techniques may be used in the treatment of esophageal strictures. The purpose of this study was to present the effects of topical mitomycin C (TMC) as an useful adjunct to dilatation therapy in esophageal strictures.

MATERIALS AND METHODS

A retrospective analysis of patients who underwent TMC between February 2015 and July 2016 was performed. Dysphagia score, periodic dilatation index, and number of dilatations were compared before and after intervention to investigate the efficacy of TMC.

RESULTS

TMC was performed on 20 patients with a median age of 3.5 years (2 to 17 y). The diagnosis was corrosive esophageal strictures in 14 patients, anastomotic strictures in 5 patients, and congenital esophageal stricture in 1 patient. The length of the stricture was long in 10 patients (50%). The median dysphagia score decreased from 2 (1 to 3) to 0 (0 to 2) after application (P<0.001). The median number of dilatation sessions decreased from 5 (1 to 41) to 1 (0 to 11) after intervention (P<0.001). The median periodic dilatation index decreased from 1 (0.66 to 1.34) to 0 (0 to 1.33) after TMC (P<0.001). Regular esophageal dilatation was not necessary in 16 patients after application (80%). The length of the stricture did not affect the efficacy of TMC. The success of treatment was lower in patients with a long treatment period before TMC (>3 y) (50%). No complications were seen in a median follow-up period of 16 months (7 to 22 mo).

CONCLUSIONS

TMC application has a significant positive effect as an adjunct to dilatation therapy in most of the patients with different types of esophageal strictures. It should be performed as a safe and efficient treatment option even in patients who were resistant to dilatation therapy.

Refining indications for the use of mitomycin C using a randomized controlled trial with an animal model

Objectives

To evaluate the effect of mitomycin on the repair of acquired subglottic stenosis and to define the optimal concentration of mitomycin that would minimize restenosis after repair.

Study Design And Setting

A randomized prospective model was used in which 20 ferrets ( Mustela putorius furo) underwent simulated intubation injury that was then treated with CO2 laser lysis.

Results

Comparison of cross-sectional airway areas, after stenosis repair, showed no significant differences between control and mitomycin treatment groups. Comparison of histologic scores for both inflammation and mucosalization yielded no difference between control and treatment animals.

Conclusions

Mitomycin C appeared to have no benefit when placed after repair of an acquired stenosis.

Significance

This study closely models the injury experienced by children with acquired subglottic stenosis. These data provide clear evidence that mitomycin is limited in its effect on established wounds and help further define its role as an adjuvant for surgery in the aerodigestive tract.

Treatment of Refractory Gastrointestinal Strictures With Mitomycin C: A Systematic Review

BACKGROUND AND AIMS

Refractory benign gastrointestinal (GI) strictures represent a difficult management problem given the limited therapeutic interventions available. We performed a systematic review of all published cases using mitomycin C in the treatment of GI strictures.

METHODS

Searches of MEDLINE and Embase databases were performed to identify studies reporting application of mitomycin C for GI strictures. Review of titles/abstracts, full review of potentially relevant studies, and data abstraction were performed independently by 2 authors.

RESULTS

Of 549 citations, 24 studies with 145 patients (74% pediatric and 26% adult) met inclusion criteria. Esophageal strictures were the most common (79%) site of refractory strictures treated with mitomycin C, with caustic injury the most common underlying etiology. The concentration (range, 0.1 to 2 mg/mL; median, 0.4 mg/mL), number of applications (range, 1 to 12; median, 1), duration of applications (range, 1 to 5; median, 2 min), and technique of application (cotton pledget, spray, injection, special catheters) varied among studies. Ninety-one patients (73%; children: 80%, adults: 59%) had a complete response; 26 (21%) had a partial response. Only 1 (0.7%) adverse event was reported: cutaneous sclerosis attributed to microperforation and mitomycin C extravastion after injection. Mean follow-up was 23 (4 to 60) months.

CONCLUSIONS

Local mitomycin C application seems to be a safe and effective therapy for benign refractory GI strictures of varying etiology in both pediatric and adult populations. Although the results of this systematic review are highly encouraging, it should be considered investigational. Additional randomized trials and larger prospective studies are needed to confirm these results and to better define the optimal dose, concentration, duration and technique of mitomycin C application.

Topical application of mitomycin-C in oesophageal strictures

BACKGROUND

Benign oesophageal strictures may occur as a complication of caustic ingestion or severe gastro-oesophageal reflux or as a sequela of oesophageal surgery and other fibrosing conditions. The traditional initial treatment of oesophageal strictures is intraluminal dilation; however, even if frequent, this occasionally may not provide adequate oesophageal lumen capacity or give significant symptom-free intervals, and restricturing after dilation is difficult and challenging. Topical postdilation application of an antifibrotic agent, mitomycin-C, in the treatment of an oesophageal stricture has been described.

PATIENTS AND METHODS

Eight centres participated, with a total of 16 patients (4 girls), median age 48 (range 0-276) months. The causes of stricture were as follows: caustic (10), post-trachea-oesophageal fistula repair (2), peptic (2), Crohn disease (1), and dystrophic epidermolysis bullosa (1). The median (range) length and diameter of the strictures were as follows: 22 mm (8-50 mm) and 1.5 mm (1-6 mm). Of the 16 patients, 15 had undergone repeated dilations varying from 3 to more than 1000 (daily self-bouginage) before mitomycin-C, and the median interval between dilations was 4 weeks. Mitomycin-C 0.1 mg/mL was applied after dilation for a median time of 3.5 minutes and a median of 3 (1-12) times.

RESULTS

Major success, both endoscopic and clinical improvement or cure, occurred in 10 of 16 patients. In 3 of 16 patients the interval period between dilations increased dramatically. Failure of therapy was considered in 3 of 16. All of the patients remained symptom free for a follow-up time of as long as 5 years.

CONCLUSIONS

Postdilation application of topical mitomycin-C resulted in major success in 62.5% of patients and partial success in 19%, and it may be a useful strategy in oesophageal strictures of differing causes that are refractory to repeated perendoscopic dilation.

The effect of mitomycin on extracellular matrix proteins in a rat wound model

OBJECTIVE

To evaluate the effect of topical and injected mitomycin on the expression of extracellular matrix proteins by fibroblasts in an early surgical wound model.

STUDY DESIGN

A prospective, controlled study in a rat wound model.

METHODS

Six linear incisions were placed on the backs of each of three Sprague-Dawley rats, and polyvinyl alcohol sponges were implanted. Two control wounds were implanted with saline-soaked sponges. The two topical test group wounds were treated with 0.5 mg/mL topical mitomycin for 2 minutes, followed by sponge implantation. The two injection test group wounds were injected with 0.3 mL mitomycin (0.5 mg/mL) before incision and sponge implantation. Each incision was closed uniformly with 3-0 nylon suture. The sponges were harvested on the tenth postoperative day. Fibroblasts that had grown into the sponges were separated, and polymerase chain reaction analysis was used to quantify the expression of messenger RNA for several extracellular matrix proteins.

RESULTS

The expression of mRNA for some extracellular matrix proteins (elastase, hyaluronidase, and procollagen) was downregulated in the mitomycin test groups. The effect was more pronounced in the topical mitomycin test group compared with the injection test group. The wounds in the topical group were prone to dehiscence, and the wounds in the injection group demonstrated poor healing when compared with controls.

CONCLUSIONS

Mitomycin may inhibit wound healing by downregulating the gene expression for extracellular matrix proteins. This effect may be selective and may be more pronounced on inducible genes. Such findings prompt further studies regarding possible "best time" windows and selective gene suppression. The use of mitomycin may be limited in situations where wound integrity is necessary.

Evaluation of potential regulatory elements identified as DNase I hypersensitive sites in the CFTR gene

The cystic fibrosis transmembrane conductance regulator (CFTR) gene shows a complex pattern of expression, with temporal and spatial regulation that is not accounted for by elements in the promoter. One approach to identifying the regulatory elements for CFTR is the mapping of DNase I hypersensitive sites (DHS) within the locus. We previously identified at least 12 clusters of DHS across the CFTR gene and here further evaluate DHS in introns 2, 3, 10, 16, 17a, 18, 20 and 21 to assess their functional importance in regulation of CFTR gene expression. Transient transfections of enhan- cer/reporter constructs containing the DHS regions showed that those in introns 20 and 21 augmented the activity of the CFTR promoter. Structural analysis of the DNA sequence at the DHS suggested that only the one intron 21 might be caused by inherent DNA structures. Cell specificity of the DHS suggested a role for the DHS in introns 2 and 18 in CFTR expression in some pancreatic duct cells. Finally, regulatory elements at the DHS in introns 10 and 18 may contribute to upregulation of CFTR gene transcription by forskolin and mitomycin C, respectively. These data support a model of regulation of expression of the CFTR gene in which multiple elements contribute to tightly co-ordinated expression in vivo.

Preferential binding of human full-length XPA and the minimal DNA binding domain (XPA-MF122) with the mitomycin C-DNA interstrand cross-link

Nucleotide excision repair (NER) is an important cellular mechanism that removes radiation-induced and chemically induced damage from DNA. The XPA protein is involved in the damage recognition step of NER and appears to function by binding damaged DNA and recruiting other proteins to the site. It may also play a role in subsequent steps of NER through interaction with other repair proteins. Interstrand cross-links are of particular interest, since these lesions involve both strands of duplex DNA and present special challenges to the repair machinery. Using 14 and 25 bp duplex oligonucleotides containing a defined, well-characterized single mitomycin C (MMC)-DNA interstrand cross-link, we have shown through gel shift analysis that both XPA and a minimal DNA binding domain of XPA (XPA-MF122) preferentially bind to MMC-cross-linked DNA with a greater specificity and a higher affinity (>2-fold) than to the same undamaged DNA sequence. This preferential binding to MMC-cross-linked DNA occurs in the absence of other proteins from the NER complex. Differences in binding affinity and specificity were observed among the different protein-DNA combinations that were both protein and DNA specific. Defining XPA-MMC-DNA interactions may aid in elucidating the mechanism by which DNA cross-links and other forms of DNA damage are recognized and repaired by the NER machinery in eukaryotic cells.

Differential effects of mitomycin C and doxorubicin on P-glycoprotein expression

Previous studies have demonstrated that mitomycin C (MMC) and other DNA cross-linking agents can suppress MDR1 (multidrug resistance 1) gene expression and subsequent functional P-glycoprotein (Pgp) expression, whereas doxorubicin and other anthracyclines increase MDR1 gene expression. In the present study, with stably transfected Madin-Darby canine kidney C7 epithelial cells expressing a human Pgp tagged with green fluorescent protein under the proximal human MDR1 gene promoter, we demonstrated that MMC and doxorubicin have differential effects on Pgp expression and function. Doxorubicin caused a progressive increase in the cell-surface expression of Pgp and function. In contrast, MMC initially increased plasma membrane expression and function at a time when total cellular Pgp was constant and Pgp mRNA expression had been shown to be suppressed. This was followed by a rapid and sustained decrease in cell-surface expression at later times, presumably as a consequence of the initial decrease in mRNA expression. These studies imply that there are at least two independent chemosensitive steps that can alter Pgp biogenesis: one at the level of mRNA transcription and the other at the level of Pgp trafficking. Understanding the combined consequences of these two mechanisms might lead to novel chemotherapeutic approaches to overcoming drug resistance in human cancers by altering either Pgp mRNA expression or trafficking to the membrane.

Role of small loops in DNA melting

Short melted regions less than 100 base pairs (bp) in length are rarely found in the differential melting curves (DMC) of natural DNAs. Therefore, it is supposed that their characteristics do not affect DNA melting behavior. However, in our previous study, a strong influence of the form of the entropy factor of small loops on melting of cross-linked DNAs was established (D. Y. Lando, A. S. Fridman et al., Journal of Biomolecular Structure and Dynamics, 1997, Vol. 15, pp. 141-150; Journal of Biomolecular Structure and Dynamics, 1998, Vol. 16, pp. 59-67). Quite different dependencies of the melting temperature on the relative concentration of interstrand cross-links were obtained for the loop entropy factors given by the Fixman-Freire (Jacobson-Stockmayer) and Wartell-Benight relations. In the present study, the influence of the entropy factor of small loops on the melting of natural DNAs, cross-linked DNAs and periodical double-stranded polynucleotides is compared using computer simulation. A fast combined computational method for calculating DNA melting curves was developed for this investigation. It allows us to assign an arbitrary dependence of the loop entropy factor on the length of melted regions for the terms corresponding to small loops (less than tau bp in length). These terms are calculated using Poland's approach. The Fixman-Freire approach is used for long loops. Our calculations have shown that the temperature dependence of the average length of interior melted regions (loops) has a maximum at T approximately T(m) (T(m) is the DNA melting temperature) in contrast to the dependence of the total average length of melted regions, which increases almost monotonously. Computer modeling demonstrates that prohibition of formation of loops less than tau base pairs in length does not markedly change the DMC for tau < 150 bp. However, the same prohibition strongly affects the average length of internal melted regions for much smaller tau's. The effect is already noticeable for tau = 1 bp and increases with tau. A tenfold increase in the entropy factor of all loops with length less than tau bp causes a noticeable alteration of the DMC for tau > or = 30 bp. It is shown that DMCs are identical for the Wartell-Benight and for the Fixman-Freire (Jacobson-Stockmayer) form of the loop entropy factor. However, for low degree of denaturation, the average length of internal melted regions is 40% lower for the Wartell-Benight form due to the fluctuational opening of short AT-rich regions less than 10 bp in length. The same calculations carried out for periodical polynucleotides demonstrate a much stronger difference in melting behavior for different forms of entropy factors of short loops. The strongest difference occurs if the length of stable GC-rich and unstable AT-rich stretches is equal to 30 bp. However, the comparison carried out in this work demonstrates that the entropy factor of short loops influences melting behavior of cross-linked DNA much stronger than of unmodified DNA with random or periodical sequences.

The role of reductive enzymes in cancer cell resistance to mitomycin C

Mitomycin C (MMC) is bioreductively activated to DNA binding species via complex chemical pathways involving a common hydroquinone intermediate. A recent publication by Belcourt et al. (1999) has revealed that the bacterial mitomycin C resistance protein (MCRA) acts as a unique hydroquinone oxidase converting this reactive intermediate back to the parent drug in the presence of molecular oxygen, preventing the formation of cytotoxic interstrand DNA crosslinks. It was argued that a mechanism analogous to MCRA may be responsible for the often observed phenomenon of aerobic drug resistance that develops in vitro to MMC in human cancer cell lines. Altered expression of activating reductase enzymes, which usually accompanies aerobic drug resistance, was claimed to be of lesser importance. Therefore, the role of reductases in MMC drug resistance has been reviewed. While it is clear from numerous studies that lowered reductase expression can in certain situations produce drug resistance, simple correlations between a specific enzyme and chemosensitivity generally do not hold due to the complex functional and regulatory interplay that exists among the different activating enzymes and detoxification systems. Copyright 2000 Harcourt Publishers Ltd.

Molecular basis for effects of carcinogenic heavy metals on inducible gene expression

Certain forms of the heavy metals arsenic and chromium are considered human carcinogens, although they are believed to act through very different mechanisms. Chromium(VI) is believed to act as a classic and mutagenic agent, and DNA/chromatin appears to be the principal target for its effects. In contrast, arsenic(III) is considered nongenotoxic, but is able to target specific cellular proteins, principally through sulfhydryl interactions. We had previously shown that various genotoxic chemical carcinogens, including chromium (VI), preferentially altered expression of several inducible genes but had little or no effect on constitutive gene expression. We were therefore interested in whether these carcinogenic heavy metals might target specific but distinct sites within cells, leading to alterations in gene expression that might contribute to the carcinogenic process. Arsenic(III) and chromium(VI) each significantly altered both basal and hormone-inducible expression of a model inducible gene, phosphoenolpyruvate carboxykinase (PEPCK), at nonovertly toxic doses in the chick embryo in vivo and rat hepatoma H411E cells in culture. We have recently developed two parallel cell culture approaches for examining the molecular basis for these effects. First, we are examining the effects of heavy metals on expression and activation of specific transcription factors known to be involved in regulation of susceptible inducible genes, and have recently observed significant but different effects of arsenic(III) and chromium(VI) on nuclear transcription factor binding. Second, we have developed cell lines with stably integrated PEPCK promoter-luciferase reporter gene constructs to examine effects of heavy metals on promoter function, and have also recently seen profound effects induced by both chromium(VI) and arsenic(III) in this system. These model systems should enable us to be able to identify the critical cis (DNA) and trans (protein) cellular targets of heavy metal exposure leading to alterations in expression of specific susceptible genes. It is anticipated that such information will provide valuable insight into the mechanistic basis for these effects as well as provide sensitive molecular biomarkers for evaluating human exposure.

Melting of cross-linked DNA. III. Calculation of differential melting curves

In our previous papers I and II (D. Y. Lando et al, J. Biomol. Struct. Dynam. (1997) v. 15, N1, p. 129-140, p. 141-150), two methods were developed for calculation of melting curves of cross-linked DNA. One of them is based on Poland's and another on the Fixman-Freire approach. In the present communication, III, a new theoretical method is developed for computation of differential melting curves of DNAs cross-linked by anticancer drugs and their inactive analogs. As Poland's approach, the method allows study of the influence of the loop entropy factor, delta(n), on melting behavior (n is the length of a loop in base pairs). However the method is much faster and requires computer time that inherent for the most rapid Fixman-Freire calculation approach. In contrast to the computation procedures described before in communications I and II, the method is suitable for computation of differential melting curves in the case of long DNA chains, arbitrary loop entropy factors of melted regions and arbitrary degree of cross-linking including very low values that occur in vivo after administration of antitumor drugs. The method is also appropriate for DNAs without cross-links. The results of calculation demonstrate that even very low degree of cross-linking alters the DNA differential melting curve. Cross-linking also markedly strengthens the influence of particular function delta(n) upon melting behavior.

The mitomycin bioreductive antitumor agents: cross-linking and alkylation of DNA as the molecular basis of their activity

This review focuses on the chemical and enzymatic aspects of the reductive activation of mitomycin C, its disulfide analogs KW-2149 and BMS-181174, and, in less detail, FR66979 and FR900482, newly discovered antitumor antibiotics related to mitomycins. Furthermore, structural aspects of DNA damage induced by these drugs in vitro and in vivo are described, including the chemical and conformational characteristics of DNA interstrand and intrastrand cross-links and monofunctional alkylation products, with emphasis on DNA adducts of mitomycin C. The DNA sequence specificity of the damage and its mechanism is reviewed. The relationship between the chemical and structural properties of the DNA damage on the one hand, and the antitumor and other biological activities of the mitomycins on the other, is discussed.

Melting of cross-linked DNA: II. Influence of interstrand linking on DNA stability

In the previous paper (D.Y. Lando, J. Biomol. Struct. Dynam, 15, 129-140 (1997)) the melting of cross-linked DNA with N base pairs and omega interstrand cross-links has been considered theoretically. In the present study on the basis of these results, two simple schemes are developed for the computation of melting curves of cross-linked DNA. The investigation of influence of interstrand linking on DNA stability has been carried out by computer simulation. It is shown that the relative concentration of cross-links, CCT = omega/N, their distribution along a DNA molecule, and particular values of the entropy factors of small loops formed by cross-links in melted regions strongly affect the DNA melting temperature, Tm. On the contrary, for DNA without cross-links, a ten-fold increase or decrease in the entropy factors of small loops does not cause the Tm variation. The comparison of the results of calculation with experimental data suggests that the majority of types of cross-link neither maintain ordered parallel orientation of bases in melted regions nor increase considerably the thermostability of cross-linked base pairs. Four different ways of influence of interstrand cross-linking on the DNA double helix stability are considered. It is shown that cross-linking significantly enhances the influence of single strand stiffness in melted regions on DNA melting behavior.

Melting of cross-linked DNA: I. Model and theoretical methods

Covalent and strong coordination binding to DNA of a large number of antitumour drugs and other compounds leads to interstrand cross-link formation. To investigate cross-link influence on double helix stability, two methods are developed for the calculation of melting curves. The first method is based on Poland's approach. It requires computer time proportional to u.N, where u is the average distance (in base pairs) between neighboring cross-links and N is the number of base pairs in the DNA chain. The method is more suitable when u is not large, and small loops formed by interstrand cross-links in melted regions strongly affect DNA melting. The computer time for the second method, based on the Fixman-Freire approach, does not depend on the number of cross-links and is proportional to I.N (I is the number of exponential functions used for a decomposition of the loop entropy factor). It is more appropriate when N and u are large, and therefore particular values of the entropy factors of small loops do not influence DNA melting behavior.

Synthesis and structural characterization of the N2G-mitomycin C-N2G interstrand cross-link in a model synthetic 23 base pair oligonucleotide DNA duplex

Mitomycin C (MMC) is a genotoxic cancer chemotherapeutic agent that reacts principally at the N2 position of guanine to form one of two predominant monoadducts, or a G-G interstrand cross-link at CpG sites, or a G-G intrastrand cross-link at GpG sites. Previous studies of MMC adduction have principally used very short duplex oligonucleotides (5-15 bp) or very long native duplex DNAs. We examined the formation and structural features of the MMC CpG interstrand cross-link on a model 23 bp synthetic oligonucleotide duplex having the (upper strand) sequence 5'-ATAAATACGTATTTATTTATAAA-3'. MMC was reacted with the duplex oligonucleotide in the presence of sodium dithionite at ratios of 6 mM dithionite: 1.5 mM MMC:0.03 mM duplex. The yield of cross-link in the reaction was determined to be approximately 4.8% by denaturing gel electrophoresis, which represented approximately 75% of the total bound MMC. The cross-linked DNA was isolated to greater than 97% purity in a single step by high temperature size exclusion column chromatography. Characterization of the purified product confirmed that the complex contained exclusively the N2G-MMC-N2G cross-link at the single central CpG site. CD spectroscopy demonstrated a negative band at approximately 290-320 nm which has previously been shown to be characteristic of the MMC cross-link. The relative intensity of this band compared to those reported for shorter duplexes suggested that the majority of the duplex is in a normal B-DNA helical configuration. Base-specific chemical footprinting techniques also indicated that there were subtle but distinct structural perturbations principally within the central four to six base pairs containing and adjacent to the cross-link.

Selective recognition of the m5CpG dinucleotide sequence in DNA by mitomycin C for alkylation and cross-linking

The clinically used natural antitumor agent mitomycin C (MC) is known to alkylate DNA monofunctionally and bifunctionally, resulting in the cross-linking of DNA. These reactions occur selectively with guanines at the CpG sequence. We show, confirming a previous report (Millard, J. T.; Beachy, T. M. Biochemistry 1993, 32, 12850) that cross-linking in oligonucleotides is further enhanced when the cytosines in CpG.CpG are 5-methylated to m5CpG.m5CpG. It is shown, furthermore, that guanines in m5CpG are monoalkylated two- to three-times faster than in CpG indicating that the m5C-induced rate enhancement occurs at the first, monoalkylation step of the two-step cross-linking process. The same MC-DNA adducts are formed in methylated as in non-methylated DNA. The basepaired but not the 5'-flanking, m5C residue is responsible for the enhanced alkylation of guanine. Enzymatically activated or Na2S2O4-activated MC shows identical rate-enhancement of alkylation at m5CpG. pBR322 DNA methylated by CpG-methylase was cross-linked two- to three-times more efficiently by MC than non-methylated DNA, indicating that the m5C effect is not an artifact of oligonucleotides. An electronic effect of the 5-methyl group of cytosine transmitted via G.C H-bonding to N2 of guanine is suggested as responsible for increased reactivity with MC. CpG is severely depleted in mammalian DNA and it is speculated that this factor attenuates MC cytotoxicity in human cells.