Zambezia (1987), XIV (i).CANCEE RESEARCH AND RISK ASSESSMENT:IMPLICATIONS FOR ZIMBABWE*C. J, CHETSANGADepartment of Biochemistry, University of ZimbabweI FEEL MOST honoured by the presence of this body of friends, relatives, academiccolleagues and students. It is the aspiration of all academics at this university andthe world over to be elevated to the status of Full Professor and to occupy theChair of one of the departments of the University. For me this honour comes asone of the most fulfilling awards in my academic career. I want to acknowledgethe part played by many of my students, both in the USA and here at theUniversity of Zimbabwe, my several research collaborators over the years, andmy wife in helping to make this dream come true. Although I had achieved thestatus of Full Professor at the University of Michigan in the USA, it means muchmore to me to have this recognition bestowed on me in my home country.I have chosen the topic of cancer research and risk assessment as the theme ofmy inaugural address. One of the most distinguishing features of the humanpsyche is its curiosity revolving around the two questions: Where did I comefrom? and Where am I going? And perhaps to these two we can add a thirdquestion: How can I be doubly sure that I will get to my destination? The issues inthe late twentieth century have coalesced around this third question. The issues ofpeace and war, food and shelter, and those of ensuring good health, all devolveupon one's need to ensure survival. As this is not the forum for me to address theissues of peace and war, I will focus on the issue of ensuring good health.Provision for the good health of any nation is an extremely elusive objective; eventhe so-called advanced countries are still straggling with this one.In mankind's effort to. maximize food production and the acquisition ofapparel and shelter in order to promote socio-economic development, he has hadto develop industries whose operation has brought with it multiple risk factors.The profit incentive is such that many industries, if unmonitored, have beenknown to discharge large quantities of hazardous materials into the environment.It is the effect of the mutagens and carcinogens present in some of these hazardousmaterials that I would like to now focus on.CAUSES OF CANCERFrom both direct and indirect evidence, there is a clear consensus in support of theview that there are three types of agents that are implicated in the aetiology of* An inaugural lecture delivered before the University of Zimbabwe on 8 August 1985.CANCER RESEARCH AND RISK ASSESSMENTmalignant cell transformation (cancer). These agents are chemicals, radiation,and viruses. It is understood that each of these three agents exerts its effect byaltering the structure of genomic DNA in eukaryotic cells.The chemicals enter our bodies with the food we eat or as a result of our beingexposed to the various forms of industrial waste in our environment. It is bothionizing radiation (X-rays, gamma rays and cosmic rays) and non-ionizingradiation (ultraviolet light) that are implicated in the aetiology of cancer. Onebecomes exposed to these forms of radiation in the workplace, in diagnostic andtherapeutic applications, as well as in the natural environment (cosmic rays).Likewise, humans pick up viruses in water, from the air, and when they establishphysical contact with each other or with animals. Figure 1 presents a schemeillustrating how chemicals, radiation and viruses are thought to exert carcinogeniceffects. Nutritional deficiencies or excesses can also be a cause of cancer.The cancer cell is one whose normal functioning has been altered. As its rateof replication is BO longer regulated, the cancer cell divides continuously.ChemicalsRadiationYitusesTransformed cellsDamaged DNANormal cellsDamaged DNANormal cellsNormal DNATumor cellsDeathFigure 1: PROPOSED SCHEME FOR PROCESSES ANDAGENTS ASSOCIATED WITH CARCINOGENESISC. J. CHETSANGAMetastasis, or spreading of the malignancy, results from the loss of ability ofcancer cells to obey zoning laws; they now spread to other tissues of the body. It isfor these reasons that the cancer cell has been described as a delinquent cell. Thetopological consequence of the continuous division of transformed cells is theformation of lumps or tumours. Another phenotype of transformed cells is theirability to cause tumours when injected into an appropriate animal.There are different types of cancer, most being specific to a tissue. The cancersof different organs can be so different that they can be viewed as different diseases.The prevalence of the different types of malignancies varies with the region of theworld. For example, the three most prevalent types of cancer in North Americaare cancer of the lungs, the intestines (colo-rectal) and female breast. About 24per cent of all deaths from cancer in North America are due to lung cancer, 13 percent to colo-rectal cancer and 9 per cent to breast cancer.HOW CHEMICALS CAUSE CANCERThe mechanism of carcinogenesis is still a phenomenon without a confirmedcandidate. The most widely accepted model is that cancer results from amodification of DNA structure. The first step appears to be the formation ofcovalent adducts between DNA and a chemical carcinogen. Such adiuctsbecome harmful lesions which represent damage in DNA structure. If the damagerepresents a short-lived and transient aberrancy, the affected cell retainsnormalcy. It is my view that the majority of interactions between carcinogens andcells do not lead to cell transformation but result in abortive effects. The reason forthis is that, in addition to the anti-caecer effect of the cell's DNA repairmechanisms, the carcinogenic effects of some chemical carcinogens represent aninitiation event that must next be promoted by the correct agent for malignancy tooccur. Furthermore, if the carcinogenic lesion in DNA is to be 'fixed* into thepermanent register of the genome, the event must be retained during successivecycles of DNA replication.The multifoctorial aetiology of carcinogens is now better appreciated. Somechemical carcinogens may have a role in simply predisposing an organism totumour formation, with the actual triggering of the tumour awaiting the action ofthe promoter which serves as a co-factor. The predisposing (initiation) may beeffected by a virus or a chemical carcinogen. Chemicals initiate carcinogenesis byinducing damage in DNA. The response of the host may be to marshal! itsdefences against the lesion. This involves setting in motion any of the variousmodes of DNA repair.The repair may begin by the excision of the lesion followed by unscheduledDNA synthesis (repair synthesis) and ligation by DNA ligase. Elaboratemechanisms are involved in these repair pathways that restore the structuralintegrity of DNA. There is evidence that errors can be made resulting in4 CANCER RESEARCH AND RISK ASSESSMENTmisrepiication of DNA. Mutagenic lesions can be a result of misrepair ofdamaged DNA. It is these mutagenic lesions that are implicated in triggeringmalignant cell transformation.Chemicals that alkylate DNA appear to be major offenders in inducingcancer. The carcinogenicity of a number of alkylating agents has beendemonstrated in experimental animal models. The acetyiaminofluorene andaflatoxin B; have been shown to cause hepatocellular carcinoma in rats (Kriek,1980); burnt food products and nitroso compounds to cause oesophageal cancer(Laker et al., 1980); smoking to cause lung ameer (Hammond, 1980); and virusesand some types of irradiation to cause leukemias.My laboratory has been studying the mechanisms by which alkylatingchemicals cause cancer. We have focused our activities on the alkylation ofguanines in DNA that leads to a fission of the imidazole ring of guanine (Kohnand Spears, 1967; Chetsanga and Lindahl, 1979). We have also studied theradiogenic cleavage of imidazole rings of non-alkylated adenine and guanine inaqueous solution of DNA treated with ionizing radiation (7-irradiation)(Chetsanga and Grigorian, 1983). Garret and Mehta (1972) have shown that theimidazole ring of non-alkylated adenine can also be opened by treatment of DNAwith 0.8 M potassium hydroxide at 80°C. We have proposed a model of howalkali induce imidazole ring cleavage in alkylated guanine (Chetsanga andMakaroff, 1982).Repair of DNA containing alkylated ring-opened guanineIn 1978, while in Sweden on sabbatical leave from the University of Michigan,I undertook a search for an enzyme that could recognize and excise from DNAthe imidazole ring-opened 7-methylguanine, also known as methylated formamido-pyrimidine (meFAPy). I was fortunate to isolate an enzyme from E.coli thatremoves meFAPy from DNA (Chetsanga and Lindahl, 1979). We gave thisenzyme the Bame formamidopyrimidine-DNA glycosylase (FAPy-DNA glyco-sylase). In 1981, Margison and Pegg were able to identify this enzyme activity inrat-liver DNA.We observed that FAPy-DNA glycosylase is able to remove from DNAthose FAPy residues that were alkylated with either phosphoramide mustard(Chetsanga et al., 1982) and aflatoxin B, (Chetsanga and Frenette, 1983). Thereaction in which AFB5FAPy is removed from DNA by FAPy-DNA glyco-sylase has a higher Km than the reaction in which the enzyme removes meFAPyfrom DNA.Repair of non-alkylated imidazole ring-opened purines in DNAWe have observed that FAPy-DNA glycosylase is unable to remove non-alkylated FAPy from DNA. This was in contrast to the work of Breimer andLindahl (1984) who observed a low-level excision of non-alkylated FAPy byC. J. CHETSANGAFAPy-DNA glycosylase. We have, on the other hand, identified a novel pathwayfor the repair of non-alkylated FAPy in DNA. The repair involves a reciosure ofthe imidazole ring of FAPy catalysed by the enzyme purine imidazole ring (PIR)cyclase (Chetsanga and Grigorian, 1985). The enzyme recloses the openedimidazole rings of both adenine and guanine. This ring reciosure appears to be themajor pathway for the repair of non-alkylated FAPy.The conversion of alkylated guanine to the imidazole ring-opened derivativeoccurs in vivo in rats treated with carcinogens such as dimethylnitrosamine ordimethylhydrazine (Beranek et at, 1982) as well as in rats treated with aflatoxinBt (Chetsanga and Frenette, 1983). The existence of a mechanism for the repair ofthese ring-opened purines suggests that, intracellularly, they are deemedundesirable and deleterious. It has now been demonstrated that the presence ofring-opened alkylated guanine in DNA leads to replication errors at the chain-elongation step (Boiteux and Laval, 1983). It has similarly been shown that thepresence of ring-opened aflatoxin-guanine adducts in the primer-templatepolynucleotide leads to replication errors (Chu and Saffhill, 1983).These observations provide evidence for a physiological significance ofpurine imidazole ring-fission. This ring-cleavage perturbs the integrity of DNA,and leads to discordant consequences to normal cellular metabolism. It is a systemthat deserves more research attention than it has received so far. There remainsthe need to test the hypothesis that it is the ring-cleavage caused by carcinogenicalkylating agents that forms the basis for carcinogenesis.CANCER PREVALENCE DURING THE LAST TWENTY YEARSThe best data available on cancer prevalence come from developed countries.I would like to examine some of the leading forms of cancer and attempt to assessthe putative basis for their aetiology.Lung cancerThis is the most prevalent type of malignancy in the Western world. Since 1960the number of fatalities due to lung cancer has increased by 116 per cent in menand by 200 per cent in women. The evidence shows that lung cancer is caused bysmoking cigarettes. The precipitous rise in its prevalence among women isattributed to the great increase in the number of smokers since 1920. Among thesixteen leading cancers, lung cancer is responsible for the greatest number ofcancer deaths. It appears to be epoxides, acroleins, formaldehyde and acet-aldehyde in smoked cigarettes that are responsible for inducing lung cancer.Stomach cancerStomach cancer is the most frequent type of maligancy world-wide. Fortunately,its incidence is decreasing. Since 1960, there has been a 12 per cent decline in thenumber of mortalities caused by stomach cancer. It remains the leading type of6 CANCER RESEARCH AND RISK ASSESSMENTcancer in Japan. The observed decline in its prevalence seems to be due to changesin lifestyle, particularly changes in diet as certain foodstuffs contain carcinogens(Sugimura, 1984),Hepatocellular carcinomaThis malignancy is the main cause of cancer-related mortalities in Africa, in someAsiatic countries and in most Third World countries situated in the tropics. Mylaboratory has been investigating the aetiology of hepatocellular carcinoma(HCC) during the last six years.There are two schools of thought with regard to the aetiology of HCC: oneschool holds that HCC is caused by aiatoxins, while the other holds the view thatit is induced by hepatitis B virus (HBV). Our working hypothesis is that HCCaetiology entails the combined action of HBV and aflatoxins. In this synergism,HBV probably acts as an initiator while aiatoxins act as co-factor (promotor). Ithas also been proposed that alcohol or cigarette-smoking may also act aspromotors of HCC once it has been induced by HBV (Rigby and Wilkie, 1985).From a number of studies, it has now been concluded that HBV infection andingestion of aflatoxins in contaminated foodstuffs are important risk factorsfor HCC.From a mechanistic consideration, it appears that the entrance of HBV DNAinto hepatocytes somehow provides an element of susceptibility (Fig. 2). It isreasonable to surmise that the subsequent exposure of these cells to aflatoxinsprovides for a chemical interaction between the mycotoxins and the HBV DNAnow lodged in the host cell genome.Hepatitis B virusAflatoxiastHepatocytesin liver**,InitiatedhepatocytesiHepatocellularcarcinomaFigure 2: SCHEME OF INITIATION AND PROMOTION OFHEPATOCELLULAR CARCINOMAC. J. CHETSANGAIn 1985, I received a Z$ 12,000 grant from the Cancer Association ofZimbabwe to investigate the putative Involvement of HBV in the aetiology ofHCC. A current experimental limitation of HBV studies is the lack of a suitablemammalian cell-culture system in which the virus can be propagated. To getround this problem, we have cloned HBV DNA in E.coli and are studying theexpression of viral functions in this bacterial host. We now propose to use thissystem as a source of HBV DNA with which we can proceed to identify HBVvariants in the Zimbabwe population with a view to determining the variant thatis prevalently associated with HCC,Skin cancerThis malignancy, in the form of cancer of the scrotum, was first observed inchimney-sweeps. In Zimbabwe, this cancer is largely found in exposed parts ofthe skin. The ease of its early detection is the reason for its successful clinicalmanagement.Cancer of the cervixThis malignancy is the most prevalent cancer among young women inZimbabwe. The clinical management of cervical cancer is relatively well workedout and relief can be provided as long as early diagnosis has been made. Theaetiology appears to include papilloma virus and a mix of limited opportunitiesfor adequate genital hygiene among the victims, as well as female culturalpractices in vaginal care. Unfortunately, there is no systematic research oncervical cancer in Zimbabwe today. If such research were to lead to afundamental understanding of this malignancy, it may subsequently becomefeasible to design appropriate strategies for its management, and perhapsprevention as wellFUTURE PROGNOSTICATION FOR ZIMBABWEAll indicators suggest that the cancer trends in the developed world will beduplicated sooner or later in Third World countries, including Zimbabwe. Thedata from the Zimbabwe Cancer Registry for 1986 show that cancer of the cervix(cervix uteri) in women is the most predominant of all malignancies in thiscountry (see Table). The other leading forms of cancer in Zimbabwe are those ofthe oesophagus, liver, bladder, breast, stomach, skin, prostate and lung indescending order. The mortalities column shows that HCC is the leading cause ofdeath, followed by oesophageal cancer. Although lung cancer shows an incidenceof 4.8 per cent, it is the fourth leading cause of death in Zimbabwe. Comparedwith the incidence data from the Bulawayo Cancer Registry (Parkin, 1986), thereis clear evidence that many types of cancer are on the increase. Thesedevelopments lend credence to the prediction now being made that there will be acancer epidemic in a majority of the developing countries by the year 2000.CANCER RESEARCH AND RISK ASSESSMENTCANCER INCIDENCE DATA IN HARARE IN 1986*SiteCervixOesophagusLiverBladderBreastStomachSkinProstateLungOthersPercentage of cases24.010.88.27.97.76.66.55.64.817.9Percentage of mortalities8.320.022.09.61.412.01.44.810.010.5"Data are based on sample pool of a total of 1,984 patients.Cancer is now characterized as a disease predominant in old age. Thecontinued increase in life expectancy in developing countries will be ac-companied by increased incidences of cancer. A successful execution of thecurrent campaign against infectious diseases will increase life expectancy and,consequently, the likelihood of an increase in the frequencies of malignancies.CONCLUSIONSince 80-90 per cent of cancers are believed to be preventable upon earlydiagnosis, we can cure significant proportions of existing cancers. We need publicawareness campaigns that can sensitize the general public to the importance ofearly diagnosis. This, in conjuction with the provision of medical specialiststrained in the treatment of the various forms of the predominant cancers, willensure our ability to provide both cure and relief to victims of this dreadfuldisease.With a greater research base, it may be possible for us to understand theaetiology of the most predominant malignancies in the country. This knowledgewill in turn sharpen our capabilities of carcinogen risk assessment and enhancethe success with which we can launch effective preventive measures.ReferencesBERANEK, D. 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