Epigenetics for ecotoxicologists

Do genes define your destiny? The field of epigeneticsexamines how genes and the environment interact to formthe basis of heredity and comes up with some surprisingfindings. We often think of deoxyribonucleic acid (DNA) asthe sole physical basis for heredity, the genetic code thatdetermines everything from eye and hair color to specificpersonality traits. The growing field of epigenetics suggeststhat this traditional paradigm is an oversimplification. Epi-genetics is the study of factors that are heritable, but whichoccur by mechanisms other than changes in the DNA codeitself. C.H. Waddington first used the term epigenetics in theearly 1940s to describe ‘‘the interactions of genes with theirenvironment, which bring the phenotype into being.’’ Epi-genetic marks are susceptible to environmental influences—both chemical and nonchemical—and can be inherited inways that may seem counterintuitive (see sidebar, ExpandingIdeas About Biological Inheritance). Effects of early lifeexperiences, such as parental care or nutrition, can showup later in life and even be passed on to future generations.This emerging area of research has interesting and importantimplications for the field of ecotoxicology—from basicscience to international policy.In classic genetics, genetic material is in the form of DNAand encodes all of the information necessary for life. Thisinformation is copied faithfully and passed down as cellsdivide. Every cell contains a complete copy of the DNAcode, but the pattern of gene expression—that is, whichgenes are turned ‘‘on’’ (expressed) or ‘‘off’’ (nonex-pressed)—determines the cell type and function. A skin cellon your arm contains the same DNA code as a nerve cell inyour brain, but their extreme differences in structure andfunction are due to the particular set of genes that each isexpressing. Epigenetics refers to an annotation in the form ofchemical marks on top of the DNA code; the prefix ‘‘epi’’comesfromaGreekwordmeaning‘‘over’’or‘‘above.’’Thesechemical marks, which are discussed in detail below, affectwhich genes are expressed and at what levels. Epigeneticmarks are highly influenced by the environment and can beinherited along with the genetic code as cells divide mitoti-cally, and in some cases meiotically, from one generation tothe next.Epigenetics is receiving significant attention in the field ofbiomedicine. A PubMed search for ‘‘epigenetic’’ revealedthat more than 1,300 review articles have been written on thetopic, of which more than 50% were published in the lastthree years alone. Epigenetics provides a mechanism for theBarker hypothesis, which postulates that nutrition and otherenvironmentalfactorsearlyindevelopmentcanaltersuscept-ibility to chronic diseases in adulthood. A classic exampleof this is that individuals who were conceived during theDutchWinterHunger(1944–1945)havepersistentepigeneticalterations on a characteristic gestational marker, the IGF2gene, as adults. Furthermore, individuals who experiencedthe Dutch Winter Hunger have higher rates of metabolicdisorders and cardiovascular disease, the epigenetic mecha-nisms of which are the focus of ongoing studies. Nutritionaldeprivation may also have an impact on human longevity;indeed, lifespanisnegativelycorrelated withfoodabundance