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Cells and Surveys: Should Biological Measures be Included in Social Science Research? 6 Opportunities for Population-Based Research on Aging Human Subjects: Pathology and Genetics George M.Martin and Qubai Hu Pathologists and geneticists have recently been empowered with new or greatly improved sets of instruments and methodologies that provide greatly enhanced sensitivity, specificity, and scope for the quantitation of biological parameters. These innovations should interest demographers and social scientists concerned with varying patterns of health and disease within human populations over the life course. The term “human populations” is used in various contexts in this essay. Most social scientists and demographers will have particular interest in population-based samples. Geneticists and pathologists can make important contributions to such investigations. But highly selected populations, such as large kindreds, and large populations used for genetic association studies, such as comparisons between patients with a late-life disease and age-matched controls, have often been used to enhance our understanding of the basis for varying susceptibilities to late-life disorders. A cogent example of a new tool that is waiting to be exploited for studies of populations of human subjects is the analysis of gene expression using massive microarrays. This powerful methodology can provide a simultaneous description of how tens of thousands of human genes change their expression during development and aging (Brown and Botstein, 1999; Ferea and Brown, 1999; Friend, 1999; Thieffry, 1999). With the anticipated sequencing of virtually the entire human genome, essentially all human genes will eventually be subject to such evaluation. These and other exciting innovations will be described below in the section on
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? Genetics. The communication of these new research opportunities to the community of population scientists is the first goal of this chapter. The second goal is to increase awareness by the greater community of scientists of neglected resources under the control of pathologists. Two major points will be made. The first is the astonishing neglect of the autopsy, which has never been employed for a population-based statistical sampling of any segment of the American population. Death certificates are notoriously unreliable instruments for documenting causes of death. Moreover, they can never provide the comprehensive analysis of the burden of preclinical and clinical disease that can be documented via a thorough autopsy. The second major point will be to describe the vast store of cells, tissues, and body fluids that are potentially available for research. Our third and final goal will be to underscore the value of middle age as a stage of the life course that should receive greater attention by geneticists, physiologists, physicians, and population scientists concerned with the elucidation of varying patterns of aging in our species. GENETICS New Methods for the Comprehensive Assessment of Changes in Gene Expression During the Life Course The expression of a large number of genes changes during the life course. These changes are thought to underlie the mechanism of tissue differentiation during development, the modulation of physiology during pubescence, the maintenance of physiological homeostasis in the adult, the reaction of tissues to disease states, and, perhaps, some fundamental processes of aging. Proteins, rather than the ribonucleic acid messages (messenger RNA or mRNA) that relay the information in the DNA genetic code to the protein synthesizing machinery of cells, are the proximal agents of these modulations. It is difficult, however, to comprehensively assess changes in proteins over the life course. Only a small proportion of the tens of thousands of proteins in a cell can be specifically identified by available antibodies. Many critically important proteins, such as transcription factors, which regulate the expression of many other genes, are present in only trace amounts. Methods such as two-dimensional electrophoresis lack sufficient sensitivity for their identification. Highly specific sensitive-quantitative methods have been developed, however, for the identification of the levels of a wide range of mRNA in cells and tissues. These offer the best hope for the development of a battery of molecular biomarkers of aging, although the protein approach, proteomics, is being vigorously pursued by a number of laboratories.
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? The physical vehicles for the large-scale screening of rnRNAs are sometimes referred to as massive microarrays or gene chips because the thousands of stretches of the DNA sequences that represent specific genes that allow one to identify levels of expression of mRNAs are systematically arrayed as minute samples in rows and columns on various physical substrates, such as glass slides, membranes, or composite materials. The DNA sequences are of two types. The approach pioneered by the Affymetrix company (Table 6–1) uses short strings of the nucleotide coding units of a single strand of DNA (oligonucleotides) from several exons of interest. Exons are the multiple, physically separated segments of the DNA sequence of a gene that are spliced together to provide the mRNA; the intervening sequences, which may have regulatory information, are called introns. These oligonucleotides hybridize with mRNA that is purified from various cells or tissues (e.g., from cells or tissues from a young individual) and labeled with radioactive or fluorescent tags. For the latter, one can label the material from one specimen with a fluorescent chemical that, when optically excited by a laser, emits a signal in the green region of the spectrum of light. A different set of mRNAs (for example, from the homologous cell type or tissue from an older subject) can be labeled with another fluorescent compound, such as one that emits a red signal. These two sets of labeled mRNAs can be mixed and allowed to jointly hybridize to the oligonucleotide array of human cognate genes to determine the intensity of hybridization. If the expression of a given gene does not change with age, one would observe a yellow signal (an equal mixture of the red and green signals). A dominance of a red or green signal would indicate either an increase or decrease in the expression of that gene in the cells or tissues of the older individual. A second approach uses cDNAs instead of DNA oligonucleotides to make the microarrays. The symbol “cDNA” refers to a DNA copy made from the cognate mRNA, using an enzyme called reverse transcriptase. These cDNAs are denatured so that single strands are available to hybridize with single-stranded molecules of mRNA, forming duplex molecules. The cDNAs are typically arrayed on glass slides or membranes. Directions for making such arrays can be found in the web site of the laboratory of Dr. Pat Brown of Stanford University (http://cmgm.stanford.edu/pbrown/). Commercially available sources of such microarrays can now be obtained, and can include access to software for the analysis of the results. Table 6–1 summarizes some current major sources of such materials. The reader will note, however, that these are very expensive reagents. The associated informatics software (e.g., Rosetta Inpharmatics, Inc., http://www.rii.com/about/overview.htm) required for the analysis of the vast amounts of data that emerge from such studies, including algorithms for cluster analysis, can also be very expensive. Moreover, there is
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? TABLE 6–1 Some Commercial Resources for Assay of Human Gene Expression Using cDNA Microarrays Company Name Products Substratesa mRNA-Labeling Methods Number of Genesb Cost Affymetrix (www.affymetrix.com) GeneChip® Chipsc Biotin-labeled 12,000 full-length cDNA & 50,000 ESTd clusters in 5 subsets List: $10,000/subset Contract: $5,000/subset Custom arrays Available Clontech (www.clontech.com) Atlas™ Array Glass slides Radio-labeled or Fluorescence-labeled 1,081 cDNAs $750 Nylon Membranes Radio-labeled 3,600 cDNAs in 3 subsets $1,395/subset Cell function-specific (e.g., apoptosis, cell cycle, etc.) arrays Available Invitrogen (Research Genetics) (www.invitrogen.com) GeneFilters® Nylon Membranes Radio-labeled 30,000 cDNAs on 6 membrans $960/membrane Tissue-specific (prostate, ovary, breast, colon, DermArray skin) cDNA arrays $1,440/membrane Stratagene (www.stratagene.com) GeneConnection™ Discovery-3’ Microarrays Glass slides Fluorecence-labeled 4,000 cDNAs $995 aThe materials on which oligonucleotides (short single strand DNA fragments) or cDNA fragments are immobilized. bEach gene is represented by 16–20 pairs of specific 25-mer oligonucleotides (e.g., Affymetrix) or by a denatured double strand cDNA fragment, typically 300–1000 base pairs, from the 3’ end of each of these cDNAs. cThe Affymetrix’s substrate is proprietary. They contain oligonucleotides in high-density arrays immobilized on glass wafers by combinations of solid-phase chemical synthesis with photolithographic fabrication techniques employed in the semiconductor industry. dEST: expressed sequence tags. These are short (~200 base pairs) sequences from the 3’ end of a cDNA clone (i.e., an expressed gene).
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? still a lack of rigorous statistical analysis of many of the results. The need for replicate independent experiments for such statistical analysis will put such an approach out of the range of most laboratories and certainly could not reasonably be applied for the study of large populations. This situation is likely to change, however, for several reasons. First, improvements in the technology are likely to decrease the variance. Second, the enormous interest and demand for such approaches will certainly lead to substantial decreases in costs. Third, more modest arrays designed for the investigations of genes of special interest to specific studies are likely to be developed by individual laboratories or by specialized centers within major institutions at reduced costs. Fourth, and most importantly, once a few particularly robust biomarkers of specific processes of aging have been discovered and replicated using microarrays, one can employ other, more rigorously quantitative and more rapid methods based upon the polymerase chain reaction (PCR). PCR greatly amplifies small amounts of DNA, either directly from genomic DNA or from mRNA that has been converted to DNA using reverse transcriptase enzymes. The authors have successfully employed one such method in studies of groups of subjects with autopsy-documented dementias of the Alzheimer type (DAT) and compared the results with materials from other neurological degenerative disorders and neurologically normal age-matched controls. We found changes in the expression of a specific gene in relatively unaffected portions of the brain of DAT patients that we believe could represent a relatively early stage in the evolution of the disease process (Hu et al., 2000). The method is called quantitative reverse transcription-competitive PCR (RT-cPCR). The assay is extremely sensitive and very specific. For example, we could measure, in the same reaction, two alternatively spliced forms of the same gene (called isoforms) that differed in only six nucleotide base pairs. This should be considered in the context of the fact that each of the two sets of human genes in an individual cell has about 3.1 billion base pairs. The isoform with the extra base pairs, which results in a protein with an additional two amino acids, turned out to be an exclusive marker of neuronal cells (Hu et al., 1999). It would be feasible to perform approximately 1,000 such assays, which would score for both isoforms, in no more than 50 days. We estimate the cost per assay of both isoforms to be no more than $7, including the salary of a technician and the costs of all reagents. The application of robotic methods should greatly decrease the cost per specimen for very large-scale studies. Once suitable conditions have been worked out, the methodology could be applied, in principle, to any species of mRNA from any source of mRNA, including cells from peripheral blood samples; from small biopsies of subcutaneous fat, skin, or skeletal muscle; or from cultured cells derived
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? from such biopsies. There are already interesting clues as to what general classes of mRNA are deserving of further analysis in larger populations of aging animals and aging human subjects. A microarray analysis of the skeletal muscles of aging cohorts of mice revealed alterations of gene regulation involving loci included in several functionally related disease clusters (Lee et al., 1999). These included genes involved in energy metabolism, protein metabolism, DNA repair, and in the responses of cells to a variety of stress factors, such as oxidative and thermal stress. A large proportion of these changes could be reversed by partial caloric restriction, a well-established method in rodents for the enhancement of life span and the postponement of several late-life diseases. An interesting example of a specific gene whose expression was markedly diminished in aged muscles was one coding for a mitochondrial protease that is involved in the biogenesis of mitochondria, an organelle that is in abundance in skeletal muscles and that is the powerhouse of our cells. The use of expression arrays for the study of aging in human subjects has so far been confined to cultured skin fibroblast-like cells from only a few subjects (Shelton et al., 1999; Ly et al., 2000), but there is much more research in progress. Other Uses of Microarray Technologies These microarray technologies could also be adapted to determine the prevalence of a wide variety of inborn alterations in gene dosage, either deletions or DNA amplifications (Pinkel et al., 1998; Trent et al., 1997). To do this, one hybridizes fragments of genomic DNA, rather than messenger RNA, against an array of cognate DNA sequences. This approach has only just begun to be applied to human tissues and has so far been used only to demonstrate that one can get linear signals of gene dosage for genes on the X chromosome over the range of one to five copies of X chromosomes. Given a number of technical improvements, we may soon be in the position of being able to determine the contribution of inborn changes in gene dosage to late-life disorders. Malignant neoplasms are cogent examples. We know that individuals born with only one copy of a tumor suppressor gene are more likely to develop cancer or to progress towards cancer as a result of a lesion developing in the remaining good copy of that gene sometime during the individual’s life span. There are likely to be hundreds of such genes. It would be of great interest to compare the relative prevalence of deletions in such genes in various populations and in different epochs. There is epidemiological evidence of secular trends towards the postponement of reproduction in many developed countries (Armitage and Babb, 1996; Guyer et al., 1998; Speroff, 1994; Ford and Nault, 1996; Kaufmann et al., 1998; Hoshi et al.,
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? 1999). It is known that advanced paternal age is associated with increased risk of various mutations (Risch et al., 1987; Vogel and Ralhenberg, 1975). James Crow, a leading scholar in this field, has concluded that “the age of the father is the main factor determining the human spontaneous mutation rate, and probably the total mutation rate” (1993). An example of the importance of these issues to late-life diseases is the recent evidence that advanced paternal age is associated with increased risk of prostate cancer in offspring (Zhang et al., 1999). Could this be due, in part, to an increasing burden of deletions in tumor suppressor genes? The use of massive oligonucleotide arrays can also be used for the detection of a wide range of polymorphisms (Halushka et al., 1999; Sapolsky et al., 1999; Chee et al., 1996). In contrast to mutations, which are rare events (~10–6), polymorphisms represent common genetic variants (alleles). By definition, their allelic frequencies are at least 1–2 percent of all alleles at the particular genetic locus. Polymorphisms are being used for a very exciting emerging technology involving the use of ordered arrays of human genes cloned in what are called bacterial artificial chromosomes (BAC clones). These can be used for mapping disease genes via hybridization of genomic DNA using the principles of genomic mismatch repair and linkage disequilibrium (Cheung et al., 1998). (For further details, see the internet site of the laboratory of Vivian G.Cheung, http://genomics.med.upenn.edu/vcheung/.) The method takes advantage of the fact that we humans have single nucleotide polymorphisms (SNPs) at an average of at least one for each one thousand of our base pairs. Chromosomal domains indicative of identity by descent for such SNPs permit one to map and clone disease genes of interest in the context of a vast amount of information on other regions of the genome. The term “identity by descent” refers to the sharing of an allelic form of a gene in two individuals because they have both been inherited from the same common ancestor. By contrast, the term “identity by state” refers to the situation in which such sharing is coincidental. This technology may well be the method of choice for the genomic characterization of large populations. The polymerase chain reaction (PCR) has also been adapted to the analysis of tissue sections so that levels of gene expression (Barlati et al., 1999; Lee et al., 1996), the detection of genomic deletions (Chiang et al., 1999), and the detection of infectious agents (Fredericks and Relman, 1996; Montone and Litzky, 1995) can be related to specific cell types. The development of methods for the quantitative analysis of gene expression in single cells is of exceptional importance to gerontology because as people age, there is a shift in the population heterogeneity of complex tissues. Methods such as preparative cell sorting can provide one solution to this problem (Darzynkiewicz et al., 1994). It is often used, for example, to analyze specific subsets of peripheral blood lymphocytes.
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? Genetic Linkage Studies: The Identification of Pedigrees and Their Use for the Discovery of Major Genes that Modulate Life Span and Senescent Phenotypes Although not systematic, large-scale screenings of populations with specific phenotypes of interest to gerontologists have led to the identification of unusual pedigrees with patterns of segregation consistent with the hypothesis that mutation at a single major genetic locus is responsible for the phenotype. These studies take advantage of the availability of a large number of genetic markers and the principle that a marker that is in close physical proximity to a disease gene is rarely separated from that disease gene during meiosis, thus “traveling” together from generation to generation. This is the basis of genetic linkage analysis, which permits the mapping of a disease gene to a specific segment of a specific chromosome. Given such mapping, it is possible to eventually clone the responsible mutant gene. The discoveries of autosomal dominant mutations at the beta amyloid precursor protein locus and at the two presenilin loci are good examples (Blacker and Tanzi, 1998). These mutations cause earlyonset DAT. Mutation at the presenilin 1 locus is particularly virulent, leading to dementia as early as age 26 in a pedigree with the diagnosis confirmed by autopsy (Martin et al., 1991). These mutations affect virtually all carriers if they live long enough. The dementias are usually interpreted as resulting from dominant gain-of-function mutations, meaning that the mutation produced a protein having a new, deleterious function. The evidence is far from definitive on this point, however. It is quite interesting that there is as yet no convincing evidence for autosomal recessive loci resulting in unusual susceptibility to DAT. Such mutations represent a loss of function, typically of an enzyme. One certainly has reason to suspect that such mutations might be found if one were to examine very large numbers of pedigrees in different parts of the world. One would especially want to search within populations having high degrees of consanguineous marriages, as two rare recessive mutations are more likely to show up in the offspring of such matings. Examples would include populations of Hindus in the Southern part of India, where first cousin and uncle-niece marriages are still prevalent. Muslims throughout India and other parts of the world, such as the Middle East, also sanction first cousin marriages. The advantage of a focus on India, of course, is that its population continues to grow at a rapid rate and is now approaching one billion. The rationale for suspecting that an enzyme deficiency can cause a form of DAT comes from current views concerning its pathogenesis. These views center upon the processing of the beta amyloid precursor protein (Selkoe, 2000). This processing is carried out by enzymes that catalyze the
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? cutting of the protein at various positions in the sequence of its amino acids. A “bad” way to process the protein is with enzymes known as beta and gamma secretases. The products of such processing are small peptides, called beta amyloids; these form insoluble aggregates that deposit in the brain, including its blood vessels. Beta amyloid is thought by many to be a major cause of the loss of synaptic connections between neurons and of the neurons themselves. There is evidence that a product of the presenilin 1 gene is in fact a gamma secretase or a protein closely associated with the relevant gamma secretase (Li et al., 2000). The “good” way to process the beta amyloid precursor protein is with an enzyme known as alpha secretase. It cuts roughly in the middle of the piece of protein destined to make the beta amyloid peptide, thus preventing the accumulation of this substance. It is therefore reasonable to believe that a deficiency of alpha secretase would accelerate the development of DAT. Genetic diseases caused by single Mendelian genes, whether autosomal dominant, autosomal recessive, or X-linked recessive, although quite rare, are relatively easy to diagnose. Nowadays, once diagnosed, it is relatively straightforward to map and clone the responsible gene, especially if multiple pedigrees with the same phenotype or large affected kindreds can be identified. This is the reason why screening very large populations or focusing on screening a particular geographic area or ethnic group is important. An example of the importance of the latter strategy is the success in collecting a group of related pedigrees that permitted the mapping and cloning of the presenilin 2 gene mutation. It turned out that the original set of affected pedigrees were all ethnic Volga Germans (Bird et al., 1988). Their ancestors migrated from the Hesse area of Germany to one of two small villages on the west bank of the Volga River in about 1760. They did this at the behest of Catherine the Great, the Czarina of Russia, who was a fellow German. This “genetic founder effect” gave assurance that one was dealing with a common cause of familial DAT. One could therefore map the responsible mutations even with a relatively small number of pedigrees, since they were basically part of one large kindred. All three autosomal dominant DAT mutations, those at the presenilin 1 and 2 loci and those at the beta amyloid precursor protein locus, result in relatively early-onset dementias. There is considerable variation in the age of onset for the case of the presenilin 2 mutations, however, including rare apparent “escapees” who may live into the ninth decade without becoming affected. Pedigrees are now being collected by T.T.Perls and others for the purpose of using linkage analysis to identify genes that contribute to unusual longevities. One rationale for such a study was a finding consistent with the conclusion that siblings of centenarians tended to have rela-
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? tively long life spans (Perls et al., 1998). It is too early to evaluate the success or failure of these studies, but there are reasons for pessimism. First of all, life span is subject to stochastic events (Finch and Kirkwood, 1999). Cancer, for example, can result in chance “hits” at particularly vulnerable parts of the genome (e.g., tumor suppressor genes). Two individuals might have the same basic rate of somatic mutations, but one may be lucky and never sustain “hits” in a tumor suppressor gene, thus escaping premature death from a cancer. Second, there is also concern that, at least for the case of human subjects, there are likely to be a very large number of genes impacting upon late-life disorders and, hence, upon life span (Martin, 1978). A proportion of such genes may be of exceptional importance, however. One such example may be the gene coding for a gene that helps to unwind double stranded DNA; when one inherits two doses of a severe mutation in that gene, it causes the Werner syndrome (Yu et al., 1996). People with that disorder display such features as premature arteriosclerosis, a variety of cancers, cataracts, osteoporosis, type 2 diabetes, gonadal atrophy, atrophy of skin and subcutaneous tissues, and premature graying and thinning of hair (Epstein et al., 1966). Twin studies indicate that perhaps a quarter of the variance of life span in humans has a genetic basis (Herskind et al., 1996). A search for these genes by large scale genomic screening of pedigrees with members exhibiting unusual longevities may therefore reveal several important genetic loci, perhaps including the Werner locus. The ascertainment of such families will require large-scale screening of populations and documentation of birth dates. Genetic Association Studies and the Difficulties Inherent in Case-Control Designs Most cases of DAT are not associated with single autosomal dominant genes and occur well after the age of 65 years, typically after the age of 85 years, when perhaps 50 percent of the population can be affected (Katzman and Kawas, 1998). Such a high “background” of affected individuals makes the search for genetic susceptibility factors very difficult. There is also the difficulty of matching cases and controls in genetic association studies. Typically, one sees the statement, in publications, that only Caucasian subjects were studied. But what is a Caucasian? That word obscures the enormous genetic heterogeneity in the U.S. population. Thus, differences in the prevalence of a particular polymorphism may be biased by an enrichment, in either controls or in diseased subjects, of a particular ethnic subtype having a different distribution frequency of the polymorphism of interest. This leads to both false positives and false negatives. These uncertainties have led to the development of various
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? pedigree-based genetic association studies, including the use of various sib-pair strategies. The failure to replicate an apparently robust conclusion, however, may simply be due to the fact that the polymorphism of interest may not be relevant in a particular population in a particular environment. Despite these difficulties, however, there is evidence that polymorphic variations at a number of different genetic loci each contributes to differential susceptibility to DAT. Most of these are likely to make only small contributions to susceptibility. The best example of a major modulator of susceptibility to the common late-onset forms of DAT is the apolipoprotein E (APOE) polymorphism (Corder et al., 1993). There are likely to be other major modulators, however; one of these loci may even have a much greater impact than APOE, at least in some populations (Daw et al., 2000). There have been several confirmations (Castro et al., 1999) of the report (Schachter et al., 1994) that long-lived individuals such as centenarians are statistically more likely to carry the APOE e2 allele and less likely to carry the e4 allele, but with interesting regional variations (Panza et al., 1999). This is consistent with a deleterious role of APOE e4 on cardiovascular disease (Lehtinen et al., 1995) and on DAT (Corder et al., 1993). The robustness of the Alzheimer effect, however, varies among different ethnic groups, the impact being less for American black and Hispanic populations (Tang et al., 1996). These association studies are also consistent with the reported protective effect of the e2 allele against DAT (Corder et al., 1993). Preliminary results have indicated enrichments of other polymorphic alleles in certain populations (e.g., De Benedictis et al., 1999). The Paucity of Genetic Investigations of Differential Rates of Change of Specific Physiological Functions in Middle Age and the Many Advantages of Such an Approach From the point of view of gerontology, it can be argued that medical scientists have been preoccupied with deleterious phenotypes. One would very much like to know the genetic contributions to unusually robust retention of specific physiological functions during aging. I have argued that one should begin such studies with middle-aged cohorts. First of all, population geneticists have determined that the force of natural selection has essentially disappeared for phenotypes that do not emerge until after around age 40 or 45 (reviewed by Martin et al., 1996). Thus, we are in a position to detect differential rates of change of relevant phenotypes beginning in middle age. Secondly, assays for specific physiological functions are less likely to be compromised by various co-morbidities that would be the case were one to investigate differential rates of decline of specific functions in much older individuals. Thirdly, a genetic analysis
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? Population scientists should participate in such research, as the prevalence of colon cancer varies substantially in different regions of the country (Devesa et al., 1999; some of these data are available as county-specific maps on the National Cancer Institute website: www.nci.nih.gov/atlas/). Is this related to regional differences in diet or environmental mutagens such as the JC virus? Are there secular changes in the prevalence of the molecular alterations described above? Is the epigenetic component a robust marker of the aging colonic mucosa in all populations? Cytology Another vast storehouse of preserved cellular materials from subjects of various ages, particularly of females, can be found in the files of Divisions of Cytology in virtually all Departments of Pathology in the United States. Millions of females have been periodically examined with the famous “Pap” (Papanicolaou) smear for many decades. There has already been at least one important research application using such materials from various populations. Molecular techniques were used for the detection of specific strains of the human papillomavirus in the cells of archived Pap smears (Pillai et al., 1996; Wagner et al., 1985). Specific strains were found to be associated with cervical cancers (Evans and Mueller, 1990). Preserved cytological materials are also available from other sources, such as pleural and peritoneal fluids, amniotic fluids, and bronchoscopic lavages. Autopsy Pathology Numerous studies have documented the inadequacy of the death certificate as a source of information on causes of morbidity and mortality (Nielsen et al., 1991; Kircher et al., 1985; Engel et al., 1980; Lee, 1994). Nothing can take the place of a thorough autopsy for the full description of the burden of disease in an individual and in populations of individuals. Unfortunately, once hospital accreditation no longer required autopsies for a significant proportion of hospital deaths, the autopsy rates, especially in private hospitals, dropped precipitously. A recent survey by the American College of Pathology indicated a median rate of 8.5 percent (Nakhleh et al., 1999). These figures are often inflated by autopsies on stillborns and neonates. The rates for geriatric subjects are typically much below average (Campion et al., 1986; Nemetz et al., 1997). Moreover, there is great variation in the degree of thoroughness with which autopsies are performed. There are a number of reasons for this striking decline in the quality and quantity of the medical necropsy. First, there is the notion that laboratory tests such as X-rays and clinical chemistry will almost always
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? suffice for the clarification of disease status. This has provided a false sense of security. Second, and most importantly, there is the fact that insurance companies, hospitals, medical schools, families, governmental agencies, and pathologists are reluctant to cover the substantial costs for a modern academic autopsy, or even simple focused autopsies to determine the major cause of death. At the University of Washington, a routine full autopsy currently costs about $3,000. This includes histological examination of all organs, a routine neuropathological examination, laboratory assays for HIV and hepatitis B, and the typical costs of transportation to and from funeral homes. Special studies, such as immunohistochemistry, electron microscopy, biochemical assays, postmortem imaging and photomicrography, cell culture, and molecular biological assays can double those costs. Third, there is the fear of lawsuits against attending physicians should the diagnosis prove erroneous or should additional treatable pathologies be discovered. Fourth, pathologists and residents avoid doing autopsies, or do them in a very cursory fashion, in part because of the pressures of time and because there is typically little or no financial incentive (the pathologist is not reimbursed for his autopsy services by Medicare). Surgical pathology is considered to be a more challenging occupation. Thus, we no longer have many highly experienced autopsy pathologists, as trainees have been getting minimal exposure to the autopsy as a tool in medical diagnosis and research. We urgently need solutions to these impediments to the maintenance and enhancement of a vital component of our system for the monitoring of the public health. There are two major sources of autopsy materials in the United States, those resulting from the autopsy services of hospitals and those coming from medical examiner offices. By law, the latter are required to determine causes of accidental, suicidal, and homicidal deaths, and deaths that occur in hospitals and elsewhere under unusual circumstances. For purposes of population-based research in gerontology, by far the most valuable material is that which can be obtained from modern medical examiner facilities, given suitable informed consent. With a few exceptions, the research community has sadly neglected this material. Perhaps this is because medical examiners have only recently evolved in most urban areas of the country from the politically controlled system of county coroners. They are thus typically independent of or only loosely connected with academic medical centers. Their missions are clearly circumscribed by the need to determine a probable cause of death. Budgetary components for research are scarce and usually involve issues related to forensic pathology. Basic research could certainly be addressed, however, via the usual vehicle of research grants. Finland provides a very good model for how forensic pathologists can effectively interact with researchers (Kortelainen and Sarkioja, 1999; Viitanen et al., 1998; Lahti et al., 1998).
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? Fatal accidents, the most numerous being motor vehicle accidents, affect all age groups, all ethnic groups, and all regions of the country. They have been occurring over many decades and, unfortunately, are likely to continue to occur for the foreseeable future. Autopsies of such individuals can, in principle, provide samples of well-preserved normal tissues from essentially all organs of the body, as well as body fluids such as cerebrospinal fluid and fluid from the vitreous of the eye. They could thus be utilized, for example, to give information on the rates of deposition of soluble and insoluble moieties of the family of beta amyloid peptides (Lue et al., 1999) in the cerebral cortex and hippocampus from large numbers of individuals. The results could be related to age, ethnicity, and genotype (e.g., APOE polymorphisms). Secular changes in these patterns might suggest important environmental influences. This is just one of an almost unlimited range of research applications. Correlations with cognitive function could even be carried out retrospectively via standardized telephone interviews of informants (Gallo and Breitner, 1995). A second example would be an extension to human subjects of the recent exciting application of the microarray expression technology demonstrating altered gene expression, as previously discussed. Hospital autopsies, of course, are never population-based samples of the spectrum of disease, as there are different kinds of selection biases, particularly in the academic centers, with respect to both the kinds of patients referred to those centers and the types of patients for whom autopsies are sought. To the best of our knowledge, there has never been a valid systematic population-based autopsy study of the geriatric population in the United States. One such study was reported for an Eastern German city (population 78,484) conducted in 1987 (Modelmog et al., 1992). Of the 1,060 subjects who died that year, 1,023 (96.5 percent) were autopsied. Despite the fact that thorough neuropathological evaluations appear not to have been pursued, the overall discrepancy between death certificate diagnoses and autopsy diagnoses was 47 percent, with higher proportions for the nursing home subset. It would be of great interest to see the results of follow-up studies over the next several decades, but this is unlikely to occur, given the dismantling of the system of socialized medicine that existed in East Germany at the time of the initial study. Clinical Pathology (Laboratory Medicine) Practicing pathologists are responsible for two major domains of activities. The first, anatomic pathology, has been discussed above. The second is known as clinical pathology, or, in some regions of the country, as laboratory medicine. This includes responsibility for the direction of a group of clinical hematology activities (blood counts, blood smears, bone
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? marrow smears or sections, coagulation assays, transfusion services, etc.), clinical chemistry labs (charged with the assays of a very large number of biochemical determinations from body fluids), clinical immunology (assays for autoantibodies, cryoglobulins, antibodies to phospholipids, etc.), clinical microbiology (the isolation and characterization of infectious bacterial agents and fungi), clinical virology (the isolation and characterization of viral agents), clinical toxicology (assays for inorganic and organic substances, including drugs), and clinical genetics (tests for various heritable disorders). There is also increasing recognition of the importance of supporting associated divisions of informatics in order to expedite retrieval, analysis, and archiving of clinical laboratory data. As one can imagine, an enormous repository of test results has emerged from these activities. Virtually all members of our population eventually undergo many of the assays listed above, particularly the geriatric subset. Many jurisdictions have implemented the screening of all newborn infants for certain genetic disorders, notably for phenylketonuria and congenital hypothyroidism. These programs have often provided for long-term storage of excess samples of capillary blood dried on filter papers. Imaginative uses of this material have been employed for population-based research. One example was its use for the determination of the prevalence of neonatal infection with Toxoplasma gondii by the detection of specific IgM antibodies (Petersen and Eaton, 1999; Paul et al., 2000). This protozoan is among the most prevalent causes of latent infection of the central nervous system throughout the world. Infection occurs primarily via the oral route, typically from eating undercooked meat or via contact with household cats. In a European study, cerebral toxoplasmosis was found in 34 percent of middle-aged subjects with AIDS (Martinez et al., 1995). It is probably the case that many patients thought to have dementia on the basis of their HIV infections are demented because of secondary infection with Toxoplasma gondii. No population-based study of the contribution of T. gondii to neuropathology in geriatric subjects has been carried out to the best of our knowledge. Another example of an imaginative use of archival clinical pathology material for population-based research has been the application of the powerful method of tandem mass spectrometry for the screening of newborns, using the spare dried-filter paper samples of DNA mentioned above, for a great variety of inborn errors of metabolism about 700,000 newborns, 163 inborn errors of metabolism were detected, including 86 with amino acid metabolic errors, 32 with errors in the metabolism of organic acids, and 45 with errors in fatty acid oxidation (Naylor and Chace, 1999). These are largely recessive disorders, severe mutations having been inherited from both parents. It would be of great interest to discover the prevalence of heterozygous carriers for such mutations in
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Cells and Surveys: Should Biological Measures be Included in Social Science Research? middle-aged and older subjects. Such carriers are of course far more prevalent than are the homozygous deficient individuals. While there are often no obvious detectable phenotypic abnormalities in obligate heterozygous carriers (e.g., the parents of the homozygous deficient children), we have little or no information on the extent to which haploinsufficiency for at least a proportion of this substantial mutational load may increase the vulnerability of carriers to basic processes of aging and to late-life disease. Given the availability of population-based sampling of peripheral blood from such older subjects, one is in a position to apply a variety of methodologies to this question. Some of these methodologies have been reviewed under the section on Genetics. SUMMARY There is a vast repertoire of data being collected by anatomical and clinical pathologists. Their value has been greatly enhanced by the development of powerful new methodologies for chemical and genetic analysis. Despite these new opportunities, these materials are rarely used for population-based studies of interest to gerontologists. It is particularly disturbing that autopsy rates have declined precipitously in recent years, especially for the geriatric population. Remarkably, there has never been a valid, systematic population-based autopsy study of geriatric diseases in the United States. The result has been the over-reliance upon highly inaccurate death certificate data. The screening of large populations for unusual pedigrees has already led to the identification of rare mutations of importance to our understanding of the pathogenesis of late-life disease. More attention, however, should be given to the discovery of genetic polymorphisms that modulate how we age. Of special interest would be programs of research designed to identify alleles that confer unusual resistance to age-related declines in specific functions. Large-scale screenings of populations of middle-aged subjects have the potential to identify such alleles. REFERENCES Arendt, G., H.Hefter, C.Figge, E.Neuen-Jakob, H.W.Nelles, C.Elsing, and H.J.Freund 1991 Two cases of cerebral toxoplasmosis in AIDS patients mimicking HIV-related dementia. Journal of Neurology 238:439–442. Armitage, B., and P.Babb 1996 Population review: (4). Trends in fertility. Population Trends 7–13. Barlati, S., N.Zoppi, A.Copeta, D.Tavian, G.De Petro, and M.Colombi 1999 Quantitative in situ hybridization for the evaluation of gene expression in asynchronous and synchronized cell cultures and in tissue sections. Histology and Histopathology 14:1231–1240.
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Representative terms from entire chapter: