The role of genetics in medicine and public health came to broad public consciousness quite dramatically in June 2000, when President Bill Clinton and Prime Minister Tony Blair jointly sponsored the announcement by government, academic, and industrial scientists that a "first draft" of the human genome sequence had been completed. Then, in early 2001, the announcement of the full sequencing and a revised estimate of the number of genes in the human genome was released. No doubt many people were mystified by the term "genome," even if they had some idea about what genes and proteins are. The genome is the complete set of genes of each individual in any species. In humans, there are an estimated 30,000 to 100,000 genes in the forty-six chromosomes of, essentially, all human cells. In early 2001, the same scientific groups reported a nearly complete sequence for the human genome, with an estimated 30,000 to 40,000 genes distributed on twenty-three pairs of chromosomes.
THE MOLECULAR NATURE OF GENES
DNA (deoxyribonucleic acid) molecules carry the code for genetic information and its transmission from one generation to the next. For decades it was thought that DNA was a most unlikely candidate for this role, due to its presumed simplicity (compared with proteins). DNA consists of a string of just four different nucleotide bases (A, T, G, C—for adenine, thymine, guanine, cytosine) held together by a sugar (deoxyribose)—phosphate backbone. In contrast, proteins are polypeptide chains of twenty different amino acids, offering much more variation for coding. In a classic experiment in 1944, scientists at the Rockefeller Institute in New York City working with bacteria that cause pneumococcal pneumonia showed that inherited transformation of the surface characteristics of the bacteria could be accomplished with DNA and not with protein.
In 1953, James Watson and Francis Crick, at Cambridge University in England, published the stunning hypothesis that two intertwined strands of DNA, running in opposite directions, could be joined in a double helix through hydrogen bonds linking the nucleotide bases in the specific combinations of A-T and G-C (see Figure 1). This model was justified by available X-ray pictures of the molecular patterns of DNA. Linear sets of three nucleotide bases generate a "triplet code," with sixty-four combinations, more than enough to code for the twenty amino acids. We now know that the double helix of DNA can separate, through actions of enzymes that facilitate unwinding, so that one strand of the double-stranded DNA can be transcribed into messenger RNA (mRNA) molecules. The mRNA is then translated into polypeptides, which assume highly folded three-dimensional structures to function as enzymes, antibodies, and structural components of cells. Other RNA molecules are involved in supporting the formation of the polypeptides and in delivering the right amino acid to the growing polypeptide chain as directed by the triplet code in the mRNA.
This flow of information from DNA to RNA to protein is a general phenomenon throughout living organisms. There are exceptions, such as viruses (including HIV/AIDS [human immodeficiency virus/acquired immunodeficiency syndrome]) which use RNA as their genetic material. When these viruses infect human (or plant or animal) cells, they must first convert their RNA message into DNA to join the flow of information in the cell from DNA to RNA to protein. Similar "reverse transcription" can occur in cancer cells and during
Figure 1
embryological development. Experimental conversion of mRNA to DNA is utilized very extensively to clone and sequence individual genes, key techniques in the field of biotechnology.
THE BASIS FOR INHERITANCE
Long DNA molecules are carried on structures called chromosomes in the nucleus inside each cell. Human chromosomes occur in pairs, one derived from the mother and one from the father in sexual reproduction. Humans have twenty-three pairs of chromosomes, of which twenty-two are similar in males and females. These are numbered 1 through 22, according to chromosome size (1 is the largest). One chromosome pair is different between females and males: XX in females (one X from each parent) and XY in males (X from the mother and Y from the father).
When ordinary cells divide (during fetal development, normal growth, and the regeneration of skin, other organs, and cells lining the lung, intestine, and uterus), the chromosomes must be duplicated and then be distributed to daughter cells so that every cell gets a full set of twenty-three pairs of chromosomes. When the chromosomes are duplicated, the DNA must be replicated, as well.
Egg-forming cells in the ovary and sperm-forming cells in the testes are unique. They are duplicated in a more complex pattern so that they contain only one each of the twenty-three pairs of chromosomes; when egg and sperm then combine, their aggregate of chromosomes is the expected twenty-three pairs.
Something else important can happen during duplication of chromosomes and replication of DNA. There may be recombination across the pairs of chromosomes between the DNA strands, so that genes (information) from the mother are combined at the molecular level with information from the father, and vice versa. Also, there may be mistakes. Mistakes in an individual gene occurring during replication, or when triggered by X-rays, ultraviolet radiation, or chemical reactions, are called "mutations." The complementary double-stranded structure of DNA is a defense against loss of information when DNA is damaged or broken; the damaged strand is repaired using the complementary strand to direct the repair. Mistakes that occur when chromosomes are duplicated can lead to translocations of part of one chromosome onto another, loss of a chromosome or part of a chromosome, or failure of separation of duplicated chromosomes (gaining a third copy of that chromosome, as in Down syndrome, where there are three copies of chromosome 21). In addition, some genes are actually carried outside the nucleus in the energy factories called mitochondria; these can be passed on only by the mother, in the egg cell, and are associated with certain diseases of muscle and brain.
MAPPING GENES ON CHROMOSOMES
Very effective methods have been developed to identify which genes for specific traits or diseases are located on which chromosomes, and to pinpoint the location on the relevant chromosome. The smallest genes consist of only a few hundred bases of DNA; the largest known human gene, which is mutated in Duchenne muscular dystrophy, is 2 million base pairs in length. A surprising feature of all nonbacterial genes is their organization into "introns" and "exons." Introns are noncoding stretches of DNA within the gene which are transcribed into RNA but then spliced out before the RNA is translated into protein. There are also untranslated noncoding regions at each end of the gene. There are lots of signals built into the sequence of the DNA—for initiation, stopping, splicing, and other functions crucial to defining the genes in the lengthy DNA molecules; for regulating their expression as mRNA and proteins; and for coordinating regulation of related genes. Some genes have only a single exon; others have up to one hundred interruptions with introns. The reasons are still quite obscure.
Gene mapping before 1950 was limited to the X chromosome, deduced by mother-to-son transmission in pedigrees (diagrams of family histories) for X-linked diseases (e.g., hemophilia) or traits (e.g., color blindness). A few genes were located on abnormal chromosomes by careful clinical correlations. Improved chromosomal analyses permitted formation of double-stranded DNA between fluorescent-labeled probe DNA and the DNA in a particular chromosome. Another method used mouse/human hybrid cells with one each of the different human chromosomes—if a human gene could be detected in the presence of the mouse genes, that gene must be coded for on the single human chromosome present. Once one gene is located, another gene which is linked in transmission from generation to generation can be deduced to be on the same chromosome. For example, the gene for cystic fibrosis was placed near linked markers on chromosome 7. This is truly a needle-in-the-haystack approach, since there are some 30,000 to 40,000 genes, distributed on the 23 pairs of chromosomes.
Everything changed with the new methods of recombinant DNA and the polymerase chain reaction—a way to produce millions of copies of a particular DNA molecule isolated or synthesized in tiny amounts. Increasingly, genes are being identified without the benefit of an initial chromosomal localization. A scan of the entire genome (across all chromosomes) is performed in a search for linkage to fairly common variants of genes that serve as well-spaced markers, even without knowing their function. Many steps in this approach are now automated, thousands of samples can be processed, and powerful computer programs sift through hundreds or thousands of markers to find clues for localization of the presumed gene or genes for a disease. Segments of DNA from the suspected chromosomal region can be cloned into specialized vectors. Linking together all such fragments permits scientists to assemble the genome sequence of humans or of many other organisms (e.g., yeast, fruitfly, bacteria, earthworm, mouse). A much more complicated mapping process is helpful in locating multiple genes for complex diseases like diabetes, high blood pressure, or depression.
THE HUMAN GENOME PROJECT
According to Francis Collins, director of the National Human Genome Research Institute, "mapping the human genetic terrain may rank with the great expeditions of Lewis and Clark, Sir Edmund Hillary, and the Apollo Program." In the early 1970s, obtaining sequence information on DNA or RNA was arduous, typically requiring an entire year to deduce about fifteen nucleotides. Advances in laboratory methods triggered hope in the mid-1980s that a massive scale-up could eventually sequence the entire 3 billion base pairs of human DNA. The project was officially launched in October 1990 in the United States and soon became international, with major efforts in Britain, France, Scandinavia, and Japan. Specific goals, by chromosome, were set to achieve the mapping of genes to chromosomes, the physical map of DNA fragments, and then the DNA sequence. By 1994 there were 5,000 highly useful markers for the genetic map, and overlapping cloned fragments of DNA to create physical maps, using various techniques. Many clever schemes have been put to use to assure sufficient overlaps to orient the location and direction of DNA sequence fragments. Powerful sequencing methods accelerated the target completion date from 2005 to 2003, and then to 2001. Work to "clean-up" the sequence is ongoing.
Nevertheless, having the entire sequence has been likened to having the complete works of William Shakespeare as a sequence of the twenty-six English letters with no punctuation of any kind. Figuring out where the genes are and how they are turned on and off, or up and down, during life's events is a huge remaining task. In reality, the work of understanding the genome has only just begun. Computer algorithms, including one called "GRAIL," have been designed to find and use characteristics that may distinguish coding regions from the other 95 percent of the DNA sequence. Working backward from the mRNA molecules by forming double-stranded complementary DNA with the enzyme reverse transcriptase, and then sequencing the cDNA or even partial cDNA as expressed sequence tags, has accelerated this work.
Another powerful approach has utilized the theme of "unity in diversity" that characterizes all living things. There are amazing homologies between genes in humans and genes for similar functions in mice, earthworms, fruitflies, and even yeast cells, all of which have smaller genomes than humans. Computer databases available to scientists throughout the world permit "virtual experiments" using knowledge of a disease-related gene in the mouse, for example, to deduce what gene might account for a similar disease in humans.
Regulation of gene expression is a crucial feature of differentiated cells in complex organisms and of development from the single fertilized egg cell. Except for red blood cells, which have no nucleus, all other cells in any individual have a nucleus, chromosomes, and DNA—the same DNA. So the information content is essentially the same in all cells, yet quite different sets of genes are active in the blood, liver, kidney, brain, heart, and other organs and in cancer cells. Much is being learned about the ways in which genes are regulated in health and disease.
INTERACTION OF GENES AND ENVIRONMENTAL FACTORS
For many decades there were disputes about whether inheritance or environment were more important in determining health status. The debate was framed as genetics versus environment, or nature versus nurture. That kind of thinking is no longer appropriate. It is firmly established that genes act by generating a molecular framework in cells and organisms, including people, that environmental factors act upon. Thus, people are exposed to many kinds of radiation; noise; chemicals and infectious agents in air, water, food, consumer products, cigarettes, alcohol, and drugs; as well as to physical and psychosocial stresses—all of which may interfere with normal cellular functioning. For example, chemicals called polycyclic aromatic hydrocarbons are produced in the combustion of gasoline, oil, cigarettes, and various industrial processes; these chemicals are breathed in through the lungs, enter the circulating blood, are activated in the liver and other organs into very reactive intermediates, and attack the DNA, forming chemical adducts with the DNA. These adducts cause the DNA code letters to be misread, generating mutations in the genetic information of these cells and increasing the risk that these cells will evade normal growth controls and become cancers. Behavioral follow-up studies in Scandinavia of adults who were adopted as infants have provided potent evidence that genetics and biology are crucial to future risks for alcoholism, depression, schizophrenia, and even criminal actions. There is now evidence of relevant inherited variation in dopamine receptors in depression, cigarette-smoking behaviors, and dysfunctional alcoholic intake. Such genetic variation may account for predisposition or resistance to these behavioral disorders.
Many pharmaceutical agents have variable therapeutic effects and variable adverse effects in different patients. In many cases we understand the reason: the drugs are metabolized (changed by enzyme action) into more active, or less active, molecules, depending on the inherited form of the gene coding for that particular metabolizing enzyme. Other chemicals from the external environment may undergo similar variable steps due to the same genes. Interactions of infectious agents with their "hosts," like infected people, may vary with genetic variation in the microbe and genetic variation in the infected person. Responses to high cholesterol foods or to cigarette smoking are subject to marked variation in people with different patterns of relevant genes. The study of these genetic-environmental interactions is called "ecogenetics."
SIGNIFICANCE OF GENETICS IN CLINICAL MEDICINE
There are well-recognized patterns of inheritance involving particular disease genes. If a disorder occurs in a grandparent, parent, and child, such vertical transmission in the pedigree is called dominant (caused by an abnormal gene from just one of the grandparents), and can involve either the X chromosome or any of the twenty-two autosomal chromosomes. Examples are Marfan's syndrome and Huntington's disease. If both parents appear normal, yet carry a recessive mutation, disease may occur when a child receives the mutant gene from each parent; examples include sickle-cell anemia and cystic fibrosis. Finally, the recessive gene may be carried on the X chromosome without manifestation in the female, but with full manifestation in the XY male, who has no normal second X gene; examples are hemophilia and Duchenne muscular dystrophy.
For common diseases like coronary heart disease, diabetes mellitus, breast cancers, depression, cleft lip and palate, and high blood pressure, multiple genes are involved; the heterogeneous causes vary within any group of diagnosed patients. Identical twins are much more likely than nonidentical (fraternal) twins to have the same disease; siblings and other close relatives have higher risks than unrelated individuals. In all cases, environmental factors, maturation factors, and other genes influence the age of onset of disease and the specific manifestations.
It is quite miraculous that such a high proportion of babies appears to be "normal"—within the broad range of normal physical and mental development. Nevertheless, about 3 percent of newborns have major malformations affecting the heart, colon, bones, or other organs. Some 2 to 5 percent have severe or moderate mental retardation or developmental disabilities. Chromosomal abnormalities account for many of the malformations, and various gene mutations contribute to the disabilities. Major chromosomal abnormalities are particularly common in spontaneously aborted fetuses (up to 50%). Variations within the normal range influence height, body habitus, propensity to weight gain, and mental development and temperament.
One of the important concepts in genetic medicine is "inborn errors of metabolism," a phrase introduced by Sir Archibald Garrod in 1908. Specific mutations, usually involving both the maternal and the paternal forms of the gene (autosomal recessive pattern of inheritance, with 25% risk for each offspring), cause deficiency of a key enzyme—as in mental retardation due to a block in the metabolism of the amino acid phenylalanine, which becomes toxic to the developing brain. The effects of this disorder (phenylketonuria, or PKU) can be prevented by diagnosing the condition at birth through newborn screening of a heel-stick blood sample and putting the child on a diet low in phenylalanine for the first five years of life, while the brain is growing rapidly. The special diet can be less stringent (but should, it now seems, be sustained) during childhood and adolescence. For women, it is crucial that they be back on a stringent low-phenylalanine diet during pregnancy; otherwise, the high circulating levels will definitely damage the fetus (100% risk of mental retardation).
Autosomal dominant diseases, like those which affect collagen in bone, cartilage, skin, and teeth, typically distort key proteins that have two or more polypeptides, such that a mutation in one makes the whole protein complex malfunction.
Knowledge from the Human Genome Project should allow identification of susceptibility genes for a broad array of diseases, thereby permitting testing before symptoms become manifest. If a single gene is responsible, testing during pregnancy or at any other appropriate time of life for the particular disease may predict a high risk or eliminate worry about that specific disease. For example, a person found to carry an inherited mutation in one of the colon cancer mutation repair genes could benefit from annual colonoscopy beginning at age thirty, so that any polyps would be detected and removed long before they progress to a potentially invasive cancer. In more complicated inherited conditions, multiple genes will be tested using new microarray and protein expression methods, answers will be couched in terms of increased or decreased risk and the likelihood of favorable responses to treatment. In other situations, the value of testing is limited due to lack of effective treatments, as for Huntington's disease. Of course, it is hoped that research will lead to effective therapies and preventive interventions and patients and families do value having the correct diagnosis, even if therapy is not (yet) available.
THE HUMAN GENOME PROJECT ELSI PROGRAM
One of the distinctive and important features of the Human Genome Project is its Ethical, Legal, and Social Implications (ELSI) program. James Watson committed a part of the annual appropriation from Congress to such matters from the start of the project. Three major categories of issues that have been examined in conferences, workshops, commissioned papers, and surveys are fairness, privacy, and safety.
Fairness. In the use of genetic information, fairness is especially important in preventing discrimination in access to affordable health care and life insurance and in employment. Many Americans fear genetic testing will identify a predisposition that will be (unfairly) considered a "preexisting condition" by insurance companies. As a result, genetic counselors advise that patients and families have good insurance in place before seeking counseling and testing. Even so, many individuals seek to be tested anonymously.
Privacy. Medical records and insurance health exams are not secure. In the state of Michigan, a 1999 report from the Governor's Commission on Genetic Privacy and Progress led to enactment of seven model statutes in 2000. Federal legislation is pending.
Safety and Efficacy of the Tests. Many new tests emerging from research labs need to be converted to high throughput, less expensive methods, with reliable quality-assurance programs. In general, people will be tested only once, and the test results carry implications for relatives. Autonomy of the individual has been the explicit policy for genetic counseling and informed participation in genetic screening for many years; testing must be conducted with similar respect for individual preferences and decisions.
SIGNIFICANCE OF GENETICS IN PUBLIC HEALTH
The sequencing of the human genome and the subsequent demonstration of variation in numerous genes in health and disease will surely stimulate a golden age for the public health sciences. It will be essential to investigate and link data about microbial, chemical, and physical exposures; about nutrition, metabolism, growth, and development; about lifestyle behaviors; and about diagnoses, medications, and health care utilization to information about genetic variation. Such studies must be conducted on a population basis in order to interpret the significance of the genetic variation. Laboratory scientists, clinician-investigators, and health care professionals will rely upon epidemiologists, biostatisticians, environmental health scientists, behavioral scientists, health economists, and health-policy analysts for the collaborative research that will inform evidence-based, cost-effective medical care and public health interventions.
In research, practice, and policy, both genetics and public health focus on populations. Both are interested in clinical preventive services and in prevention of environmental and behavioral risks. Both fields explicitly recognize cultural, societal, ethnic, and racial contexts. Geneticists are particularly sensitive to the legacy of the eugenics movement of several decades ago and to the conundrum of making medical decisions when no treatment or preventive intervention is yet known. So long as the United States lacks universal health insurance, discriminatory use of genetic information by insurers and by employers must be guarded against, as noted above.
More knowledge is needed about the heterogeneity of genetic predispositions, environmental exposures, and disease risks. Unfortunately, most public health research on infectious disease and environmental chemical risks has paid little attention to inherited susceptibility in people, focusing only on the environmental-disease agents. Similarly, heterogeneity of study populations has often been neglected in epidemiologic studies in the effort to generate sufficient numbers to justify the analysis statistically. For quantitative traits, pharmacologists, toxicologists, and psychologists have generally emphasized means and standard errors of the means, and neglected potentially informative people with values outside two standard deviations from the mean. Nevertheless, genetics is now at the core of research on cancers, coronary heart disease, high blood pressure, neurological and psychiatric conditions, and a host of other common conditions.
Complete genome sequences are now available for Mycobacterium tuberculosis, HIV, and hepatitis B virus; sequences will soon be available for cholera, malaria, and other agents. The ability to promptly identify disease-causing strains of these infectious agents has been a boon to epidemiologic surveillance in the community and to clinical management of patients. Genetic variation in both the agents and exposed persons interact. For both HIV and malaria, there are cell-surface variants of blood cells in humans that protect some people from infection. These host-parasite relationships will be a fertile area for new knowledge in public health and for drug development.
Nutrition and genetics interact extensively. Individuals with similar elevated levels of cholesterol have a variety of underlying conditions for which different dietary and pharmacologic approaches are needed. Another important risk factor for coronary heart disease is the amino acid homocysteine, whose level is greatly influenced by folic acids and vitamins B12 and B6, as well as genetic variation in enzymes metabolizing these vitamins. One common disorder, hereditary hemochromatosis, results from an overload of iron from the diet, leading to damage from iron deposition in the heart, liver, pancreas (diabetes), testes (infertility), skin, and joints (arthritis). Simple blood donation can reduce iron burdens in the body and prevent these serious complications. It is easy to test for elevated iron levels and for the gene mutations that predispose to the retention of excess iron. Unfortunately, the American Red Cross refuses to accept blood from these otherwise normal potential donors, and the Centers for Disease Control has been extremely cautious about undertaking screening programs on a population basis.
In the arena of environmental health, variation in susceptibility has been recognized as one of the three key components in assessment of risks, together with the dose-response relationship and the levels of exposure in relevant settings. The U.S. government has mounted an Environmental Genome Initiative to direct emerging knowledge of genes and genetic variation from the Human Genome Project and develop powerful new methods of chip technology for testing lots of genes simultaneously as an aid in identifying and preventing health risks from environmental exposures.
Across all of these fields, genetics will surely contribute to a scientifically sound strategy for improving health, preventing disease, and reducing disparities, the overarching missions of public health.
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