Invited lectures
II-25
PREDICTIVE TOXICOGENOMICS FOR CANCER RISK
ASSESSMENT
NAPOVEDNA TOKSIKOGENOMIKA IN OCENA TVEGANJA ZA NASTANEK RAKA
Joost H. M. van Delft, Jos C.S. Kleinjans
Abstract                  Every organism, and therefore also humans, is the product of its genes in combination
with its environment. The activity of genes is strictly regulated and is continuously liable to changes. The response of an organism to changes in its environment, such as the exposure to a toxic substance, leads to altered gene expressions. Analysing modified gene expression profiles as a result of exposure to chemical substances, can therefore aid to determine the hazardous properties of those substances, such as on carcinogenic capacity. This toxicoge-nomics approach also provides the possibility to drastically reduce the traditional animal tests.
Key words            risk assessment; cancer; toxicogenomics
Izvleček                  Vsak organizem, tudi človeški, je produkt delovanja lastnih genov v kombinaciji z dejavni-
ki okolja. Aktivnost genov je strogo uravnavana in nenehno podvržena spremembam. Organizem se na spremembe v svojem okolju, na primer na izpostavljenost toksičnim snovem, odzove s spremembami v izražanju genov. Z analizo sprememb v izražanju genov, ki so posledica izpostavljenosti kemičnim snovem, lahko določamo škodljive učinke teh snovi, kot je npr. stopnja kancerogenosti. S takim toksikogenomskim pristopom lahko bistveno zmanjšamo uporabo tradicionalnih testov na živalih.
Ključne besede    ocena tveganja; rak; toksikogenomika
The central dogma in molecular biology is that DNA     and as a result of that, becomes a regulator of tranencodes for RNA (transcription), RNA encode for pro-     scription and raises the activity of a many genes teins (translation), and these proteins are the construc-     drastically, thereby eventually leading to the devel-tion tools and engines in the cell by which all pro-     opment of cancer.1–3
cesses are regulated and carried out. Most proteins     In the last decade, methods have become available, are enzymes which have a catalytic function in cell,     which enable to simultaneously analyse the activity whereby substances are converted – metabolised –     of all genes, or the quantities of all proteins or of all into another substance. This may also imply that an-     cellular metabolites. For measuring the expression of other protein is modified, for example because a phos-     all genes, DNA microarrays have been developed, phate group is coupled to it, as a result of which this     which eventually results in genome-wide gene expres-protein becomes active or inactive.                                 sion profiles. Changes in these gene expression proControlling the activity of genes, and with that of     files reflect modifications in the activities of all genes.4, proteins such as enzymes and therefore also the me-     5 For proteins and metabolites other methods have tabolites which reside in cell, is very complex and     been developed, but these will not be discussed here. takes place at several levels: 1) from within the cell     Since exposure of humans and animals to chemical itself, like for example the case at programmed cell     substances leads to changed gene activities in the death (apoptosis), 2) by other cells in the body, like     body, analysing these modulations may therefore for example in the immune system, where one im-     have very interesting applications for toxicology. Tox-mune cell may influence the functioning of another     icology namely investigates whether chemical subimmune cell, and 3) influences from outside the body,     stances have hazardous impact on humans or animals, like as a result of exposure to a toxic substance. A     and importantly also, to retrieve the mechanisms of well-known example of the latter is dioxin, a carcino-     action. Establishing these so-called genomic risk pro-genic substance which binds to a sensor in the cell     files has a central role in toxicogenomics.6–9
Corresponding author / Avtor za dopisovanje:
Joost H. M. van Delft, Department of Health Risk Analyses and Toxicology, Faculty of Health, Medicine and Life Sciences, Maastricht University, P.O. Box 616, 6200MD Maastricht, The Netherlands, tel.: +31 (43) 388 10 92, fax: +31 (43) 388 41 46, e-mail: j.vandelft@grat.unimaas.nl
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Zdrav Vestn 2007; 76: SUPPL II
New drugs, chemicals or ingredients of cosmetic products can only be put on the market, if they have been demonstrated to be safe for humans or animals. This means that each new substance is extensively examined on possible hazardous properties. Although already a lot can be achieved by tests on cultured cells, animal tests are however still necessary. It is expected that investigating genomic risk profiles – thus toxicogenomics – drastically reduces the use of test animals, and thereby also provides wealthy information concerning the mechanisms of
action of a substance.10, 11
Cancer can be caused by chemical substances. Carcinogenic potency is therefore an crucial toxic property, which must be examined to assess the safety of a substance. Two important mechanisms ensure that a substance is carcinogenic. First because the substance damages the DNA, which leads to mutations (change of the genetic code), and as a result of this, a disorganised control of the cell growth (e. g. too many cell divisions or too less apoptosis). Examples of this are many cytostatic anti-cancer drugs and also carcinogenic compounds which are present in cigarette smoke. Secondly, the carcinogenic substances do not damage DNA, but cause by means of other – indirect – mechanisms a stimulation of the cell division or a suppression of the apoptosis.12 Dioxin is a well-known example of such a carcinogen.
Predicting toxic properties of a substance by assessing gene expression profiles, requires that first a large database is built for well-known classes of toxic substances (see Figure 1, with carcinogens as example). That database consists of gene expression profiles for many substances per class. For each class, a gene expression profile can be identified by means of mathematical models, by which that class can be distinguished from others. Next, by comparing the gene expression profile of a new substance with those of the different toxic classes, the best fit with a specific class can be determined. Thereby, it can be assessed whether a substance has a specific toxic property.13–16 At the Department of Health Risk Analysis and Toxicology and in (inter)national collaborations (the Netherlands Toxicogenomics Centre [ www.toxicogeno-mics centre.nl ] and the Carcinogenomics project of the European Union [ www.carcinogenomics.org ]) extensive Toxicogenomics studies are conducted. These particularly aim at developing alternative methods for animal tests based on generating genomic risk profiles by microarray technologies, for carcinogenic-ity, for immunotoxicity and for reprotoxicity.
References
1.  Frueh FW, Hayashibara KC, Brown PO, Whitlock JP, Jr. Use of cDNA microarrays to analyze dioxin-induced changes in human liver gene expression. Toxicol Lett 2001; 122: 189–203.
2.  Labruzzo P, Yu XF, Dufresne MJ. Induction of aryl hydrocarbon hydroxylase and demonstration of a specific nuclear receptor
Known agents
Carcinogens Carcinogens      not DNA              non
DNA damaging   damaging           Carcinogens
data
B pZQQS
c mZQH   1UUU1
Suspected toxicant
L»-No match-«—
Toxicant signature
»-   Match •*
Figure 1. Assessing the carcinogenic property of a chemical compound by means of gene expression profiling in exposed cells.
Sl. 1. Ugotavljanje kancerogenosti kemične snovi z ugotavljanjem sprememb v izražanju genov v izpostavljenih celicah.
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