Clinical

Clinical Applications

Much of the knowledge and many of the technologic advances gained from the Human Genome Project have clinical applications. Knowing the gene associated with a particular disease and the gene’s DNA sequence, its protein product, and the function of the protein in promoting health or illness provides the basis for diagnostic assays and effective interventions. For example, tumors that appear identical physically can be distinguished by their genetic profiles. This distinction is important for effective therapy because different types of tumors respond to different therapeutic approaches. Not only can such assays be used to definitively reach a diagnosis, they also can be used to detect dysfunction in those without symptoms, which allows interventions to be initiated before the symptoms of a disease become apparent.

Similarly, the information gained has been pivotal in developing diagnostic assays to determine genetic variations in drug-metabolizing enzymes (pharmacogenomics). Each human being has the same basic set of drug-metabolizing enzymes, but the genes and the resulting enzyme functions can vary. One drug may have the intended effects on one person, be ineffective for another, and actually be harmful to a third. The ability to assess an individual’s major drug-metabolizing gene variants helps the physician select the right drug and dosage. Like drugs, food requires enzymatic processes to be digested, absorbed, and used by the body’s cells. The ability to tailor food to the genetic makeup of individuals—the science of nutritional genomics—is expected to be an important application of genetic research, similar in concept to pharmacogenomics.

Genetics and Genomics: Nutritional Genomics, Nutrigenetics, and Nutrigenomics

Genetics is the science of inheritance. Historically genetics focused on identifying the mechanisms by which traits were passed from parent to child, such as eye or hair color, and certain rare diseases that were inherited from generation to generation. Originally, genetic diseases were in a separate category of disease. Today scientists realize that, directly or indirectly, all disease is connected to the information in the genes.

The science of genetics has significantly expanded in scope and includes the whole set of genetic information in an organism—its genome—and the interactions of the various genes and their protein products with each other and the environment. Genomics more accurately describes this complex, interactive situation. Whereas genetics was initially concerned with diseases that arose from a change in a single gene, genomics is more concerned with today’s chronic diseases that result from the interaction between gene variants and factors in the environment. Nutritional genomics focuses on diet- and lifestyle-related disorders that result from these interactions. Nutritional genomics is the field itself, and includes nutrigenetics, nutrigenomics, and epigenetics or epigenomics.

Nutrigenetics concerns how an individual’s particular genetic variations affect function. For example, an individual with a particular variant in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene is likely to require a more bioavailable form of folate for optimal health. Nutrigenomics is the study of the influence of specific environmental factors on changes in the expression of particular genes. Epigenetics and epigenomics provide another influence on outcomes by controlling whether genes can be expressed, which in turn determines whether nutrigenetic or nutrigenomic influences can occur. Diet and other lifestyle choices should be geared toward the particular variants of each individual.

Genetic Basics

Deoxyribonucleic acid (DNA) is the genetic material of all living organisms. The molecule is a double helix consisting of two strands of nucleotide subunits held together by hydrogen bonds. Each subunit contains the sugar deoxyribose, the mineral phosphorus, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C). The nucleotides are arranged side by side, and it is this linear arrangement that determines the particular information encoded in a stretch of DNA.

Approximately 3 billion nucleotides make up the human genome, which is housed in the nucleus of cells. A gene is a sequence of nucleotides that encodes the information for synthesizing a protein. The human genome contains approximately 20,000 to 25,000 genes, which is only 2% of the total genome (Human Genome Project, 2010). Long stretches of nucleotides are often found between one gene and the next along the chromosome. Such sequences are called intervening sequences and compose the majority of the DNA in humans. These sequences do not code for proteins, but they are not “junk DNA.” Instead, they perform structural and regulatory functions, such as controlling when, where, and how much of a protein is produced.

To be useful to the cells, information in the DNA must first be decoded and translated into proteins, which perform the work of the organism at the cellular level. Information decoding occurs in two steps: (1) the process of transcription, during which the enzyme ribonucleic acid polymerase (RNA polymerase) converts DNA into an intermediate molecule (messenger RNA [mRNA]) and (2) a subsequent translation step in which the information encoded within the mRNA directs the assembly of amino acids into the protein molecule according to a universal genetic code. Genes have a common structure, with a promoter region where the binding of the RNA polymerase is controlled, which in turn controls transcription; and a coding (informational) region where the RNA polymerase transcribes the DNA into mRNA. Within the coding region are sequences of nucleotides (exons) that correspond to the order of the amino acids in the gene’s protein product. The coding region also contains introns (sequences that are interspersed between exons and do not code for amino acids).

Following transcription, the mRNA must be processed (posttranscriptional processing) so that the introns are removed before the protein is synthesized. At this point, each set of three nucleotides in the transcribed and processed exon makes up a codon, which in turn specifies a particular amino acid and its position within the protein. Some proteins need further posttranslational processing before they are active, such as occurs with glycoproteins, proenzymes, and prohormones that must be cleaved or enzymatically processed before becoming active.

Prior to (“upstream from”) the promoter region is the regulatory region, where control of transcription takes place. Within this region are response elements, DNA sequences that serve as binding sites for regulatory proteins such as transcription factors and their bound ligands. The binding of transcription factors triggers the recruitment of additional proteins to form a protein complex that in turn changes the expression of that gene by changing the conformation of the promoter region, increasing or decreasing the ability of RNA polymerase to attach and transcribe (express) the gene. The array of response elements within the promoter region can be quite complex, allowing for the binding of multiple transcription factors that in turn fine-tune the control of gene expression. It is through the binding of transcription factors to response elements that environmental factors such as the bioactive components in food essentially “talk” to a gene, conveying information that more or less of its protein product is needed.

The proteins coded for by the genes provide the metabolic machinery for the cells, such as enzymes, receptors, transporters, antibodies, hormones, and communicators. Changes within a gene can alter the amino acid sequence of the DNA protein. Such changes are called mutations, which historically have been associated with the concept of severely impairing the function of that protein and creating dysfunction within the cells and, ultimately, the organism. A single nucleotide change may be all that is needed to cause a debilitating disease. For example, in those with sickle cell disease a single nucleotide change causes a single amino acid change in the hemoglobin molecule, resulting in severe anemia (see Chapter 33).

Changes in the DNA are the basis for evolution; thus clearly not all mutations are harmful. Some changes actually improve function, and many silent mutations have no effect. The effect of the mutation on the functioning of the encoded protein is what determines the outcome, from debilitating disease to no effect at all. All changes to the DNA are technically mutations. However, at this point in the development of genomics, the term mutation tends to be applied to those changes that sufficiently influence function such that a measurable outcome results. In contrast, the term genetic variation (or gene variant) is reserved for those mutations with an effect on function that is not strong enough to lead to a disease or other measurable outcome by itself. Nutritional genomics is primarily concerned with those variations that interact with environmental factors.

Thus a gene can exist in slightly different forms as a result of a seemingly minor change, such as a substitution of a single nucleotide with another (e.g., guanine can replace cytosine). The term for the different forms of a gene is an allele or polymorphism. As a result, genes have protein products with differing amino acid sequences (isoforms) and often different functions. Polymorphism (allelism) is an important concept because it explains why human beings, although 99.9% alike genetically, are distinctively different. The 0.1% difference is sufficient to explain the obvious physical variations among humans. It is also the basis for more subtle differences that may not be readily observable, such as in the functional ability of a key metabolic enzyme to catalyze its characteristic reaction. Such variations are thought to underlie many of the inconsistencies that are observed in therapeutic outcomes and in nutritional intervention research.

The single nucleotide polymorphism (SNP) is the structural variant best studied to date. However, ongoing analysis of the human genome suggests that other structural variations may also play an important role in the genotypic and phenotypic variation among humans (Feuk et al., 2006). Loss or gain of nucleotides, duplication of nucleotide sequences, and copy number variants also have important consequences.

Understanding the prevalence and significance of genetic variation is a primary focus of 21st-century nutrition, which represents a major departure from nutrition research and therapy to date. Each person is susceptible to a different set of diseases, handles environmental toxins differently, metabolizes molecules somewhat differently, and has slightly unique nutritional requirements. These exciting discoveries are revolutionizing the way people think about the clinical aspects of medicine, pharmacology, and nutrition. Personalized therapy using individualized dietary requirements will be the practice of future dietitians.

Modes of Inheritance

Traits are transmitted from one generation to the next in three ways: Mendelian inheritance, mitochondrial inheritance, and epigenetic inheritance.

Epigenetic Inheritance, Genomic Imprinting

Epigenetic inheritance illustrates another mechanism by which genetic information is passed between generations. Epigenetics provides an additional set of instructions beyond that contained in the DNA nucleotide sequence. It affects gene expression but does not change the nucleotide sequence itself (van der Maarel, 2008; Villagra et al., 2010). At least three mechanisms are involved: histone modification, DNA modification, and RNA interference (RNAi).

Histones are proteins associated with DNA. Units of histones form a scaffolding around which DNA is wrapped to create the nucleosome, similar to thread wrapped around a spool. Similar in concept to condensing data on a hard drive, this mechanism helps to fit the large amount of DNA into the small space of the nucleus. When DNA is condensed, it is not available for transcribing into mRNA. The attachment and removal of acetyl groups is an important mechanism for controlling whether DNA is relaxed and available for transcription to proceed or condensed and closed to transcription, respectively.

Similarly, DNA itself can be modified by the covalent attachment and removal of functional groups, such as methyl groups. Methylation takes place at cytosine residues that occur within CpG islands found near a gene’s promoter region. CpG islands (the p refers to the phosphodiester bond between (C) cytosine and (G) guanine nucleotides) are DNA sequences enriched in cytosine and guanine that, when methylated, interfere with transcription and therefore gene expression. In general, methylation silences gene expression and demethylation promotes gene expression.

DNA methylation and histone modification can contribute to genomic imprinting and affect gene expression. Genomic imprinting is an unusual phenomenon in which only one of the two alleles of a gene is expressed, either the allele contributed by the mother or by the father. If each allele contains a different mutation that leads to a measurable phenotype, the individual’s phenotype will differ depending on whether the mother’s or the father’s allele is the one expressed. Prader-Willi syndrome and Angelman syndrome provide examples of genomic imprinting; they involve DNA on chromosome 15. When the father’s allele is expressed, the child develops Prader-Willi syndrome. When the mother’s allele is expressed, the child develops Angelman syndrome. Both syndromes are characterized by intellectual disabilities, but Prader-Willi individuals also experience a lack of perception of satiety, which leads to overeating and morbid obesity. The suspected underlying basis for the phenotypic differences is the different pattern of epigenetic markings (either histone acetylation or DNA methylation) between the two parents rather than differences in the DNA sequence itself. Genomic imprinting has important clinical implications (Butler, 2009; Das et al., 2009).

The third mechanism, RNA interference (RNAi), is a posttranscriptional mechanism whereby short pieces of single-stranded RNA (21-23 nucleotides) attach to DNA or mRNA. Attaching to mRNA interferes with gene expression by preventing translation of the gene into its encoded protein. Attaching to DNA leads to silencing of whole regions of chromosomes, a phenomenon called epigenetic gene silencing, which is the basis for X-inactivation in mammalian females in which one of the two X-chromosomes is silenced. In this way the amount of information contributed by the X-chromosome is equalized between females and males, the latter having only a single X-chromosome (Kloc and Martienssen, 2008; Suzuki and Kelleher, 2009).

Epigenetics is of interest to nutrition professionals because diet has been found to influence at least one epigenetic mechanism, DNA methylation, and the effects can be inherited. The mouse has been used as a mammalian model system for dissecting this complex process. In a landmark study by Waterland and Jirtle (2003), a strain of mice with a mutation in the agouti gene was used. The wild-type (normal) agouti allele causes the mouse’s coat color to be brown. The A^vy mutation (agouti viable yellow allele) causes the coat color to be yellow and, because this allele is dominant, all mice with at least one copy of A^vy have the potential to develop the yellow coat color. The researchers bred genetically identical female mice with brown coats (two copies of the normal agouti allele) with genetically identical males that had two copies of the A^vy mutation and had yellow coats. On a standard mouse chow diet, the coat color of the mothers would be brown, that of the fathers would be yellow, and the coat color of the offspring, who have one agouti allele and one A^vy allele, would be yellow because the A^vy allele is dominant. In this study, half of the females were fed the usual diet and half were fed a methyl-rich diet in which methyl donors such as folate, vitamin B₁₂, choline, and betaine were added to the diet. Most of the unsupplemented mothers had offspring with yellow coats. Most of the offspring from the mothers on the methyl-rich diet, however, had a mottled coat with a mix of brown and yellow (called pseudo-agouti). Clearly, the mother’s diet affected the coat color of the offspring and this effect persisted into adulthood. There was a correlation between mottled coat and degree of methylation of the agouti gene, suggesting that the methyl-rich diet led to epigenetic silencing of the A^vy allele.

Furthermore, this effect of diet could be inherited. Cropley and associates (2006) found that feeding the females of the “grandmother” generation a methyl-rich diet but not enriching the daughter offspring’s diet with methyl donors still produced a number of offspring with mottled brown coats, suggesting that the effect the diet had on coat color could be transmitted between generations. Diet and possibly other environmental factors may have a transgenerational effect through their influence on epigenetic “markings” that affect gene expression without altering the DNA sequence. This type of gene-diet epigenetic mechanism could explain why identical twins, although having the exact same genotype, typically do not have identical phenotypes.

Inheritance and Disease

Changes to the genetic material, whether to the chromosomal DNA, mtDNA, or even a single nucleotide, have the potential to alter one or more proteins that may be critical to the operation of the cells, tissues, and organs of the body. There are important consequences from changes to the genetic material at each of these levels.

Genetic Technologies

Progressing beyond knowing the chromosomal location of a disease trait to associating the disease with a particular mutation and understanding its functional consequences has required the development of sophisticated molecular genetic technologies. One of the most critical technologic advances occurred in the early 1970s with the introduction of recombinant DNA technology, which allowed major progress in terms of studying genes, their functions, and the regulation of their expression. Using bacteria-derived restriction endonucleases (restriction enzymes), researchers could cut the DNA in precise, reproducible locations along the nucleotide chain, isolate the fragments and, using polymerase chain reaction (PCR) technology, make unlimited copies of the DNA for various applications. This basic approach has been the cornerstone of many routine techniques, such as genetic engineering and the production of therapeutic proteins such as insulin and growth hormone as well as new genetic strains of crops and food for animals.

Recombinant DNA technology paved the way for DNA sequencing, which is used to identify the sequence of nucleotides within a gene, pinpoint the exact location of any change, and identify each of the nucleotides in an individual’s genome. A recent improvement to DNA sequencing, whole exome capture, promises to be an efficient way to identify the DNA sequences that constitute genes (Choi et al., 2009). Recombinant DNA is also the basis for detecting variations in DNA sequences that can be used to identify individuals for forensic and paternity purposes and to predict disease susceptibilities. Another important application is gene therapy, by which a corrected gene sequence can be introduced into the cells of an individual with a disease-causing mutation.

One of the outgrowths of these earlier technologies is microarray technology. Microarrays, also called DNA “chips,” are used to determine which genes are expressed at a particular time under particular conditions, such as during the different developmental stages. They can also be used to determine which genes are turned on (or off) in response to environmental factors, such as nutrients. A useful clinical application is the comparison of gene expression between normal and diseased cells, with important implications in cancer.

Another type of genetic technology involves interfering with a gene’s expression to determine the function of that gene and its encoded protein. The concept was originally exploited in model systems involving transgenic animals, particularly the laboratory mouse (“knockout mouse”). Because the mouse and human share many of the same genes, the ability to manipulate the genetic material of the mouse and examine the effect on metabolism and physiologic function has been valuable for understanding humans.

In the knockout mouse, a gene is altered (“knocked out”) so that the normal protein is no longer made. Alternatively, a gene can be altered so that it expresses too much or too little of its product. Regulatory sequences can be altered so that a gene no longer responds appropriately to environmental signals. In these ways the normal function of a gene can be determined, the effects of over-expressing or under-expressing a gene can be studied, and details of the communication process between signals outside the organism and the genetic material inside the organism can be determined. Transgenic mice are particularly valuable for studying gene-diet interactions. A recent application of this concept involves RNAi. Short sequences of RNA bind to mRNA and interfere with translation of the mRNA into protein (“knock down”). By measuring the outcome of a decrease in a particular protein, researchers can gain insight into the role of the protein and its contribution to the organism’s function.

Nutrigenetic Influences on Health and Disease

The interplay between nutrition and genetics varies from being straightforward to being highly complex. The most straightforward is the direct correlation between a faulty gene, a defective protein, a deficient level of a metabolite, and a resultant disease state that is passed on through Mendelian inheritance and is responsive to nutrition therapy. The IEM are good examples of such interactions and have been referred to as genetic diseases. As our understanding of disease at the molecular level has become more sophisticated, this terminology is no longer appropriate. The IEM are characterized as rare mutations that result in protein dysfunction that leads to metabolic disorders. The distinguishing characteristic is the rare occurrence of these particular mutations.

All humans have mutations that result in protein dysfunction that leads to metabolic disease. The human species requires certain amino acids, fatty acids, vitamins, and mineral, and there are mutations that limit the ability to synthesize these important nutrients. The diet must supply them to prevent dysfunction and disease. For example, humans lack the gene for the enzyme gulonolactone oxidase and cannot synthesize vitamin C. If dietary vitamin C intake is below needed levels, individuals are at risk for developing scurvy, which can be fatal.

New is the understanding of the genetic basis for nutrient requirements, the realization that nutrition therapy can circumvent genetic limitations by supplying the missing nutrients, and that each individual may require a different level of nutrient because of his or her particular set of genetic variations. More than 50 metabolic reactions that involve enzymes with decreased affinities for their cofactors and that require high levels of a nutrient to restore function have been identified. Many of the supplementation levels are well in excess of the usual recommended nutrient levels, which highlights the importance of remembering that each individual is genetically unique and has distinct metabolic needs.

Although generalized guidelines for recommended nutrient levels are helpful, individuals may have genetic variations that require them to consume significantly more or less of certain nutrients than the general recommendation. Nutritional genomics has changed the thinking about global dietary recommended intakes, from an age- and sex-related orientation to incorporating nutrigenetic makeup and its influence on protein function (Stover, 2006). Nutrition therapy, then, is a critical tool for compensating for changes in the DNA that can lead to increased risk of disease.

The inborn error of amino acid metabolism, classic homocystinuria, is of interest because it led to the realization that an elevated blood level of homocysteine is an independent risk factor for CVD. A defect in the vitamin B₆–requiring enzyme cystathionine beta-synthase prevents the conversion of homocysteine to cystathionine. Homocysteine accumulates, promotes atherosclerosis, and forms the dipeptide homocystine, which leads to abnormal collagen crosslinking and osteoporosis. Nutrition therapy is multipronged, depending on the specific genetic defect. Some individuals have an enzyme defect that requires a high concentration of the vitamin B₆ cofactor for activity. Others are not responsive to B₆ and need a combination of folate, vitamin B₁₂, choline, and betaine to convert homocysteine to methionine. Others must limit their methionine intake. At least three forms of homocystinuria exist, each requiring a different nutritional approach. The ability to use genetic analysis to distinguish these similar disorders has been a useful technologic advance (see Chapters 6 and 33).

Genetic variation in the MTHFR gene provides an excellent example of nutrigenetics as well as how genetic variation can influence nutrient requirements. This gene codes for the enzyme 5,10-methyltetrahydrofolate reductase that produces the biologically active form of folate (5-methyltetrahydrofolate). Folate is essential for the conversion of homocysteine to S-adenosylmethionine, a critical methyl donor to numerous metabolic reactions, including those involved in synthesizing nucleic acids (see Chapter 33). A common variation in the MTHFR gene is the 677C>T gene variant, which involves substitution of thymine (T) for cytosine (C) at nucleotide position 677 within the coding region of the MTHFR gene. The resultant enzyme has reduced activity, which leads to decreased production of active folate and accumulation of homocysteine. In addition to the increased risk of CVD, elevated serum homocysteine increases the risk of neural tube defects in developing fetuses. As a result of these risks, in the United States cereal grains are now fortified with folic acid to ensure adequate levels in women of childbearing age (see Chapter 16).

Studies indicate that homocysteine levels can be lowered through supplementation with one or more of the B vitamins folate, B₂, B₆, and B₁₂ (Albert et al., 2008; Ebbing et al., 2008; Shidfar et al., 2009; Varela-Moreiras et al., 2009). The genotype of the individual is an important factor in this response, supporting the need for tailoring nutrient recommendations accordingly.

Disease-causing changes can also occur in genes coding for other types of proteins such as transport proteins, structural proteins, membrane receptors, hormones, and transcription factors. Mutations that increase the transport of iron (hereditary hemochromatosis) or copper (Wilson disease) to higher-than-normal levels have nutritional implications (see Chapter 30). Mutations in vitamin D receptors are not only associated with deleterious effects on bone health, but throughout the body because vitamin D is a hormone involved in hundreds of metabolic and regulatory processes. Changes in the gene coding for insulin can result in structural changes in the insulin hormone and lead to dysglycemia, as can mutations in the insulin receptor. Many proteins such as kinases, cytokines, and transcription factors that are involved in critical signaling cascades are subject to mutational changes, altered activities, and health consequences.

Nutrigenomic Influences on Health and Disease

In addition to compensating for metabolic limitations, nutrients and other bioactive components in food can influence gene expression. This ability has long been known from studies with lower organisms, such as is seen with the lac and trp operons of bacteria. In these situations the organism “senses” the presence of a nutrient in its external environment and alters its gene expression accordingly. In the case of lactose, the proteins required to use lactose as an energy source are induced by transcriptional regulation of the genes that code for the lactose transport system and for the enzyme that initially metabolizes lactose. The opposite occurs when tryptophan is present in the environment: the organism inhibits the endogenous biosynthesis of tryptophan by inhibiting transcription of the genes that encode tryptophan biosynthetic proteins. GxE interactions, such as monitoring and responding to environmental signals by changing gene expression, are fundamental processes of living systems, allowing them to use resources efficiently.

Higher organisms such as humans have similar mechanisms by which they monitor the environment that bathes their cells and alter cellular or molecular activities as needed. An example is the response of cells to the presence of glucose. Insulin is secreted and binds to its receptor on the surface of skeletal muscle cells and initiates a stepwise biochemical signaling cascade (signal transduction). Signaling results in the translocation of glucose transporter type 4 (GLUT4), a receptor involved in glucose entry into cells, from the interior of the cell to the cell surface. Exercise also promotes the translocation of GLUT4, which is helpful in controlling blood sugar levels. A drop in blood sugar levels triggers the release of epinephrine and glucagon that, in turn, bind to cell surface receptors in the liver and skeletal muscle and, through signal transduction, stimulate glycogen breakdown to glucose to restore blood sugar levels.

Nutrients and other bioactive food components can also serve as ligands, molecules that bind to specific nucleotide sequences (response elements) within a gene’s regulatory region. Binding results in a change in gene expression through the regulation of transcription. Examples of such food components are the polyunsaturated ω-3 fatty acids. These fats decrease inflammation. They serve as precursors for the synthesis of antiinflammatory eicosanoids and decrease the expression of genes that lead to the production of inflammatory cytokines, such as tumor necrosis factor–alpha and the interleukin-1 genes (Calder, 2009).

The ω-3 and ω-6 fatty acids have also been found to serve as ligands for the peroxisome proliferator-activated receptor (PPAR) family of transcription factors. The PPARs function as lipid sensors and regulate lipid and lipoprotein metabolism, glucose homeostasis, adipocyte proliferation and differentiation, and the formation of foam cells from monocytes during atherogenic plaque formation. They are important components in the sequence of events by which a high-fat diet promotes insulin resistance and obesity (Christodoulides and Vidal-Puig, 2009).

To influence the expression of the genes under its control, a PPAR transcription factor must complex with a second transcription factor, the retinoic X receptor (RXR). Each has its ligand attached—polyunsaturated fatty acid and retinoic acid (vitamin A derivative), respectively. The PPAR-RXR complex can then bind to the appropriate response element within the regulatory region of a gene under its control. Binding results in a conformational change in the structure of the DNA molecule that allows RNA polymerase to bind and transcribe the PPAR-regulated genes, leading to a host of lipogenic and proinflammatory activities. A large number of transcription factors have been identified and the mechanisms of action are under investigation.

The bioactive components that serve as ligands for these transcription factors are either provided by the diet or made endogenously, such as ω-3 and ω-6 fatty acids, cholesterol, steroid hormones, bile acids, xenobiotics (foreign chemicals, or “new-to-nature” molecules), the active form of vitamin D, and numerous phytonutrients, to name just a few (Wise, 2008). In all cases these bioactives must communicate their presence to the DNA sequestered within the nucleus. Depending on their size and lipid solubility, some bioactives can penetrate the various membrane barriers and interact directly with the DNA, as in the fatty acid example discussed previously. Others, including phytochemicals found in the cruciferous vegetables, may not be able to cross the cell membrane and will instead interact with a receptor on the cell surface and set into motion the cascade of signal transduction events that results in a transcription factor being translocated to the nucleus. See Focus On: Phytochemical and Bioactive Food Components.

Identification of the genetic and biochemical mechanisms underlying health and disease provides the basis for developing individualized intervention and prevention strategies. In the case of the ω-3 fatty acids, researchers are actively seeking conditions under which dietary ω-3s can be used to decrease inflammation and increase insulin sensitivity. An understanding of the mechanisms by which gene expression is controlled is also helpful in developing drugs that can target various aspects, including gene expression. For example, the thiazolidinedione class of antidiabetic drugs targets the PPAR mechanism described previously to improve insulin sensitivity.

Identifying bioactive components in fruits, vegetables, and whole grains that are responsible for positive health effects and the mechanisms by which they influence gene expression is of considerable interest. Small-molecular-weight lipophilic molecules can penetrate the cellular and nuclear membranes and serve as ligands for transcription factors that control gene expression. Depending on the gene and the particular bioactive, expression may be turned on or off or increased or decreased in magnitude in keeping with the information received. Examples include resveratrol from purple grape skins; along with a large number of flavonoids such as the catechins found in tea, dark chocolate, and onions and the isoflavones genistein and daidzein from soy.

For bioactive phytochemicals that are too large or too hydrophilic to penetrate the cell’s membrane barriers, communication occurs by means of signal transduction. The bioactive interacts with a receptor protein at the cell surface and initiates a cascade of biochemical reactions that ultimately results in one or more transcription factors interacting with DNA and modulating gene expression. Examples of this type of indirect communication are seen with the organosulfur compounds such as sulforaphane and other glucosinolates from the cabbage family vegetables. As a result of the signaling pathway, transcription factors (e.g., nrf) are activated and increase transcription of the glutathione-S-transferases needed for phase II detoxification, which helps to protect against cancer. Flavonoids such as naringenin found in citrus fruits and quercetin from onions and apples activate signaling pathways, leading to increased apoptosis of cancer cells.

It can be challenging to communicate the specifics of phytochemicals to consumers because they do not think about the bioactive substances in the food they eat. Attempts have been made to simplify the message, such as focusing on thinking of food in terms of its dominant color and understanding that each color contributes different valuable phytochemicals. For example, eating one to two servings from a wide variety of fruits, vegetables, legumes, grains, nuts, and seeds within the red, orange, green, purple, and white color categories daily will supply a variety of healthy phytochemicals. Individuals with particular disease susceptibilities or environmental challenges should increase the number of servings within a particular category to meet their specific health needs. Practitioners can provide a valuable service by translating these research findings into practical food solutions for consumers (Keijr et al., 2010; Kim et al., 2009).

Epigenetic Influences on Health and Disease

Epigenetics, including gene silencing and genomic imprinting, is under active investigation presently (Butler, 2009; Mathers, 2008; Waterland, 2009). Inappropriate gene expression can have serious consequences. During development, for example, specific genes are turned on or off with precise timing. Alterations in that timing can impair fetal development and may cause death. Cancer is another example. Certain genes (oncogenes) promote uncontrolled cell growth; others (tumor suppressor genes) help to put the brakes on such growth. Inappropriate methylation of these genes can result in oncogenes being expressed when they should be silent and tumor suppressor genes being silenced when they would normally be expressed. Either situation can promote uncontrolled cell growth and the development of cancer.

As the significance of epigenetics and the critical nature of epigenetic reprogramming during germ cell development and early embryogenesis is increasingly appreciated, researchers have begun to ask how reproductive technologies such as in vitro fertilization might affect fetal development (Dupont et al., 2009; Grace et al., 2009; Swanson et al., 2009).

Nutritional Genomics and Chronic Disease

Chronic disorders (e.g., CVD, cancer, diabetes, osteoporosis, inflammatory disorders) are typically more complex than single-gene disorders in which the change in the DNA is known, the abnormal protein can be identified and analyzed, and the resulting phenotype is clearly defined. Multiple genes, each with multiple variations, contribute in small ways to the overall chronic condition rather than a single variant having a dramatic effect. The genes involved with chronic disease are influenced by environmental factors in addition to the genetic variation. An individual might have gene variants that predispose to a particular chronic disorder, but the disorder may or may not develop.

CDC Genomics

www.cdc.gov/genomics

Center for Nutritional Genomics

www.nutrigenomics.nl

Ethical, Legal, and Social Issues

http://www.ornl.gov/sci/techresources/Human_Genome/elsi/elsi.shtml

Family History Initiative

http://www.hhs.gov/familyhistory

Genetics and Genomics

http://www.genome.gov/Education/

Genetics Core Competencies

http://www.nchpeg.org/core/Core_Comps_English_2007.pdf

Genetic Information Nondiscrimination Act (GINA)

www.gpo.gov/fdsys/pkg/PLAW-110publ233/pdf/PLAW-110publ233.pdf

Genetics Glossary

www.ornl.gov/TechResources/Human_Genome/glossary

Human Genome Project

www.ornl.gov/hgmis/project/info.html

NUGO for Dietitians

http://www.nugo.org/everyone/28182

Nutrigenomics—New Zealand

www.nutrigenomics.org.nz

Nutrigenomics—University of California–Davis

http://nutrigenomics.ucdavis.edu

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