Age Differences in the Etiology of the Relationship between Life Satisfaction and Self-Rated Health

This study explores age differences in the genetic and environmental influences that may mediate the relationship between life satisfaction and self-rated health. The participants are from the Swedish Adoption/Twin Study of Aging (SATSA), which includes 99 pairs of identical twins reared apart, 166 pairs of identical twins reared together, 238 pairs of fraternal twins reared apart, and 221 pairs of fraternal twins reared together. A mailed questionnaire included four items inquiring about perceived health and the 13-item shortened version of the Life Satisfaction Index, which yielded a general principal component for life satisfaction. Important age differences were found in the etiology of individual differences for each scale and in the factors that mediate the relationship between the measures. In the group younger than 65 years, the correlation between life satisfaction and perceived health can be explained entirely by environmental influences. For the group aged 65 and older, both genetic and environmental effects contribute substantially to the relationship. These age differences may reflect age-related changes, cohort effects, or both.

Swedish Early Separated Twins: Identification and Characterization

A sample of twins separated early in life has been identified in the Swedish Twin Registry. When the registry was compiled in 1961 (old cohort) and 1973 (young cohort), one or both members of 961 pairs indicated that they were separated by the age of 10. In May 1979, both members of 698 pairs were alive and were sent a questionnaire concerning the circumstances of separation. Items included reasons and timing of separation, biological relatedness of rearing parents, degree of contact after separation (including whether they lived in the same area, attended the same school, or lived together again), rough measures of selective placement, and current frequency of contact. An attempt was then made to categorize the pairs based on degree of separation. A total of 257 pairs met the criteria: rearing parents of one twin biologically unrelated to rearing parents of the cotwin, twins not living together again after separation, and contact after separation a few times a year or less. As much as 50% were separated by their first birthday, and 80% by the age of five. Various data from the twin registry are presented describing the entire sample of early separated twins as compared to a matched sample of twins reared together.

Genetic architecture for hole-board behaviors across substantial time intervals in young, middle-aged and old mice

A quantitative trait locus (QTL) analysis of behaviors across the life span was conducted in F(2) mice from a C57BL/6J x DBA/2J cross and 22 BXD recombinant inbred (RI) strains. Mice of three age groups were tested in a hole-board apparatus for 3 min on three occasions approximately 1 month apart (average age at test 150, 450 and 750 days, approximately 400 mice per group, divided equally by sex). Quantitative trait loci with small effect size were found on 11 chromosomes for hole-board activity (Hbact) and hole-board rearing (Hbrear). Analysis of 22 RI strains tested at 150 and 450 days of age found only suggestive linkage, with four QTL for Hbact overlapping with those from the F(2) analysis. There was a significant phenotypic correlation between Hbact and Hbrear (approximately 0.55-0.69) and substantial commonality among QTL for the two behaviors. QTL analyses of head-pokes (HP) and fecal boli (FB) only identified QTL at the suggestive level of significance. Age accounted for approximately 15% of the phenotypic variance (sex approximately 3%), and there were genotype by age interactions at approximately 25% of the Hbact and Hbrear QTL. Quantitative trait loci for Hbrear were relatively stable across the three measurement occasions (those for Hbact somewhat less so), although mean levels of each index declined markedly comparing the first to subsequent trials. Considered as a whole, the polygenic system influencing exploratory behaviors accounts for approximately the same amount of phenotypic variance as age (within the range studied), is stable across substantial periods of time, and acts, for the most part, independently of age and sex.

What matters for life satisfaction in the oldest-old?

The purpose of this study was to examine factors associated with life satisfaction in the oldest-old within a spectrum of psychosocial and health related variables. Scores on the life satisfaction index (LSI-Z) were related to scales and questions regarding, demographics, depression, locus of control, cognitive function, functional capacity (instrumental and personal activities of daily living), self-rated overall health and medically based health, and social network. The sample consisted of 315 participants, aged 80-98 years; (M = 83 years, 66% women). Regression analyses indicated that social network quality, self-rated overall health, sense of being in control of one's life, and depressive symptoms were significantly associated with life satisfaction. There was no gender difference in overall life satisfaction. However, there were different patterns of variables associated with life satisfaction in men and women. Self-rated overall health and depressive symptoms were related to life satisfaction in women, whereas widowhood was significantly associated with lower life satisfaction among men. The results emphasize the need to analyse associates of life satisfaction within a broader context of psychosocial variables and separately for men and women.

Genetic determinants of weight of fast- and slow-twitch skeletal muscle in 500-day-old mice of the C57BL/6J and DBA/2J lineage

C57BL/6J (B6) and DBA/2J (D2) strains and two derivative populations, BXD recombinant inbred strains (BXD RIs) and B6D2F2, were used to explore genetic basis for variation in muscle weight at 500 days of age. In parallel with findings in 200-day-old mice (Lionikas A, Blizard DA, Vandenbergh DJ, Glover MG, Stout JT, Vogler GP, McClearn GE, and Larsson L. Physiol Genomics 16: 141-152, 2003), weight of slow-twitch soleus, mixed gastrocnemius, and fast-twitch tibialis anterior (TA) and extensor digitorum longus (EDL) muscles was 13-22% greater (P < 0.001) in B6 than in D2. Distribution of BXD RI strain means indicated that genetic influence on muscle weight (strain effect P < 0.001, all muscles) was of polygenic origin, and effect of genetic factors differed between males and females (strain-by-sex interaction: P < 0.01 for soleus, EDL; P < 0.05 for TA, gastrocnemius). Linkage analyses in B6D2F2 population identified QTL affecting muscle weight on Chr 1, 2, 6, and 9. Pleiotropic influences were observed for QTL on Chr 1 (soleus, TA), 2 (TA, EDL, gastrocnemius), and 9 (soleus, TA, EDL) and were not related to muscle type (fast/slow-twitch) or function (flexor/extensor). Effect of QTL on Chr 9 on soleus muscle was male specific. QTL on Chr 2 and 6 were previously observed at 200 days of age, whereas QTL on Chr 1 and 9 are novel muscle weight QTL. In summary, muscle weight in B6/D2 lineage is affected by a polygenic system that has variable influences at different ages, between males and females, and across muscles in a manner independent of muscle type.

Gender differences in heritability of depressive symptoms in the elderly

BACKGROUND

The present study aimed to investigate the relative importance of genetic and environmental influences on depressive symptoms in the elderly.

METHOD

Depressive symptoms were assessed through the Center for Epidemiological Studies-Depression (CES-D) scale. The CES-D scale was administered to 959 twin pairs (123 female MZs, 90 male MZs, 207 same-sex female DZs, 109 same-sex male DZs and 430 opposite-sex DZs) aged 50 years or older (mean age 72 years). A dichotomous depressed state variable was constructed based on CES-D cut-offs and self-reported use of antidepressant medication. Structural equation models were fitted to the data to dissect genetic and environmental variance components.

RESULTS

The sex-specific heritability estimates for depressive symptoms were 14% for males and 29% for females and 23% when constrained to be equal for men and women. The prevalence of clinically significant depressive symptoms was 16% for men and 24% for women. Heritability estimates for the dichotomous depressed state measure were 7% for males and 49% for females in the full model and 33% when constrained to be equal.

CONCLUSION

Our results suggest that depressive symptoms in the elderly are moderately heritable, with a higher heritability for women than men, although differences in heritability estimates were not statistically significant.

Genetic architecture of fast- and slow-twitch skeletal muscle weight in 200-day-old mice of the C57BL/6J and DBA/2J lineage

The aim of the study was to explore the genetic architecture influencing weight of fast- and slow-twitch skeletal muscles. The weights of the slow-twitch soleus, the mixed gastrocnemius, the fast-twitch tibialis anterior (TA), and extensor digitorum longus (EDL) muscles were 11-34% greater (P < 0.001) in 200-day-old C57BL/6J (B6) than in DBA/2J (D2) mice. Male muscles were 13-28% larger than female (P < 1 x 10(-5), no strain by sex interaction). The sex-related difference in muscle weight, however, varied significantly among the 23 derivative BXD recombinant inbred (RI) strains (strain by sex interaction for soleus, P < 0.01; TA, P < 1 x 10(-4); EDL, not significant; and gastrocnemius, P < 0.001). Quantitative trait loci (QTL) affecting muscle weight were mapped in an F2 intercross of B6 and D2 mice (B6D2F2) and BXD RIs. A total of 10 autosomal, muscle-specific, but not muscle-type-specific, QTL, explaining a total of 5.4, 7.7, 22.9, and 8.6% of phenotypic variance for soleus, TA, EDL, and gastrocnemius muscles, respectively, were found across chromosomes 1 (Chr 1), 2, 3 (female-specific), 5 (two), 6, 7, 8, and 9 in B6D2F2 mice. The QTL on Chr 8 for EDL and the female-specific QTL on Chr 3 for gastrocnemius muscles were statistically significant, but the remaining QTL were at the suggestive level of statistical significance. Ten QTL on Chr 1, 2, 4, 5, 7, 8, 14, 17 (two), and 19 were identified in BXD RIs. Half of the QTL in BXD RIs had pleiotropic effects and were at the suggestive level of significance (except for the significant QTL for gastrocnemius muscle on Chr 17). The B6D2F2 nominated QTL on Chr 8 for EDL weight was validated in BXD RIs (P < 0.03). Support intervals for the QTL on Chr 1 and 5 overlapped between B6D2F2 and BXD RIs. An epistatic interaction between markers on Chr 1 and 17 affected gastrocnemius weight in BXD RIs. The interaction was not, however, validated in the B6D2F2 population. Our results indicate that the differences in muscle weight in the B6 and D2 segregating populations were the outcome of a polygenic system, with each factor contributing a small amount to the phenotypic variance and the genetic architecture affecting muscle weight was muscle specific, but not muscle-type specific, and in some instances sex specific.

Association between depressed mood in the elderly and a 5-HTR2A gene variant

The aim of this study was to investigate any possible association between depressed mood in the elderly and two candidate SNPs in the serotonin system: one in the 5-HTR2A gene promotor (-1438 G/A) and one in the 5-HT transporter gene (-925 C/A). DNA from a population-based Swedish twin sample (N = 1,592; mean age = 73) was genotyped using Pryosequencing trade mark. An association was found between the 5-HTR2A gene promotor polymorphism and depressed mood (OR: 1.5, CI: 1.1-2.1) for the A/A genotype in the total sample. When the sample was analyzed by gender, a significant association (OR: 2.4, CI: 1.4-4.4) was found for males and the A/A genotype, but not for females. The 5-HT transporter gene was not associated with depressed mood in this elderly population. These results suggest that there might be different genetic mechanisms for males and females contributing to the development of depressed mood in the elderly.

Sex distinctiveness in effective genotype

The difference between sexes in incidence and prevalence of alcohol-related problems is a central feature of alcohol research. It is inevitable that these differences will receive escalating attention as it becomes increasingly apparent that the interests of both equity and good science are served by the study of sex differences in health-related processes. For several reasons, genetic methods promise to offer powerful tools for the elucidation of sex differences. In the first place, the determination of sex depends on a genetic mechanism. Furthermore, there is an abundant literature showing the relevance of heredity to a broad variety of alcohol-related processes. Moreover, there is evidence of major differences in genetic influences in males and females in respect to alcoholism specifically. It is important to appreciate that genetic influence on sex distinctiveness may operate through several different mechanisms, with quite different implications. The purpose of this chapter is to provide an elementary description of these different genetic routes to sex differences.

Health locus of control in late life: a study of genetic and environmental influences in twins aged 80 years and older

The factor structure of health locus of control (Form A; K. A. Wallston, B. S. Wallston, & R. DeVellis, 1978) was examined in 420 octogenarians (M age = 83.2 years), and the contributions of genetic and environmental factors to health-control beliefs in 141 octogenarian twin pairs (71 identical, 70 same-sex fraternal) were estimated. Factor analyses reproduced previously proposed factors (Internal, Chance, and Powerful Others). Associations between health-control beliefs and life satisfaction, depression, and other health-related measures (e.g., self-rated health, outpatient contacts, and hospitalization), were modest. Quantitative genetic analyses revealed significant shared environmental influence on the Chance subscale, and significant familiality (attributable to a combination of genetic and shared environmental influences) on the Powerful Others subscale; there was no evidence of familiality on the Internal subscale.

Health locus of control in late life: a study of genetic and environmental influences in twins aged 80 years and older

The factor structure of health locus of control (Form A; K. A. Wallston, B. S. Wallston, & R. DeVellis, 1978) was examined in 420 octogenarians (M age = 83.2 years), and the contributions of genetic and environmental factors to health-control beliefs in 141 octogenarian twin pairs (71 identical, 70 same-sex fraternal) were estimated. Factor analyses reproduced previously proposed factors (Internal, Chance, and Powerful Others). Associations between health-control beliefs and life satisfaction, depression, and other health-related measures (e.g., self-rated health, outpatient contacts, and hospitalization), were modest. Quantitative genetic analyses revealed significant shared environmental influence on the Chance subscale, and significant familiality (attributable to a combination of genetic and shared environmental influences) on the Powerful Others subscale; there was no evidence of familiality on the Internal subscale.

Epistatic quantitative trait loci for alcohol preference in mice

Alcohol consumption is a complex trait, responding to the influence of various genes and environmental influences acting in a quantitative fashion. Various studies in alcohol consumption processes have identified quantitative trait locus (QTL) regions across the mouse genome that appear to contribute to this phenotype. The purpose of this study was to examine the influence of interactions between alleles at different loci, a phenomenon known as epistasis, on previously identified QTLs for alcohol consumption in mice. A multiple regression model was developed and applied to test for the significance of the interaction between two QTLs and to quantify this interaction. Our results indicate the presence of epistasis between loci on mouse chromosomes 2 and 3 accounting for 7-8% of the variation in alcohol preference, respectively.

Association between ethanol and sucrose intake in the laboratory mouse: exploration via congenic strains and conditioned taste aversion

BACKGROUND

A substantial body of literature indicates that intakes of "sweet" solutions and ethanol are positively correlated across inbred strains of rats and mice but there has been speculation that the correlation is fortuitous and there is no agreement on the underlying mechanism.

METHODS AND RESULTS

We assessed the correlation between intake of sucrose and ethanol in congenic mice created by backcrossing alleles favoring sucrose intake from the BXD RI-5 strain into DBA/2J. In addition, to probe more specifically the interrelationship between intake of the two solutions, we examined aversion generalization from sucrose to ethanol in C57BL/6J mice. Among the congenic mice, a statistically significant product-moment correlation of r = 0.36 (p < 0.02) was found between 6-hr intake of sucrose corrected for differences in baseline water intake and preference for 10% ethanol presented in a 96-hr 2-bottle test. Furthermore, C57BL/6J male mice conditioned to avoid a 0.2 M sucrose solution generalized their aversion to a 10% ethanol solution presented in the same 2-bottle test, drinking 42.1 +/- 9.38% (mean +/- SE) of their total fluid intake from the ethanol tube, compared with the control group mean of 69.86 +/- 8.84%.

CONCLUSIONS

The positive association between intake of sucrose and ethanol in congenic mice provides strong evidence that the previously demonstrated genetic correlation between intake of these solutions is not the result of fortuitous fixation of unrelated alleles and provides suggestive evidence that, at least in the B6/D2 lineage, the genetic association between intakes of the two solutions reflects close linkage or the pleiotropic effects of the same genes. The demonstration that a conditioned taste aversion to sucrose generalized to ethanol in the C57BL/6J inbred mouse strain is an extension of similar observations in outbred rats and specifically demonstrates that intake of the two solutions is controlled by some of the same physiologic or neurological processes and thus is consistent with the pleiotropic interpretation of the genetic correlation.

Sex-exclusive quantitative trait loci influences in alcohol-related phenotypes

During the past half century, researchers have identified and examined sex differences in alcohol-related phenotypes, focusing more recently on understanding of the mechanisms underlying these differences. In general, the genetic contributions influencing these differences are not consistent with an interpretation of sex linkage and must, therefore, reflect some form of sex limitation in which allelic differences at particular autosomal loci have different consequences in males and females. Significant sex differences in measures of alcohol consumption in mice have been demonstrated in previous work in our laboratory. To investigate these differences further, we explore the limiting case of sex-exclusive effects using data from (BXD) recombinant inbred (RI) strains of mice and from an intercross derived from the same progenitors, C57BL/6J (B) and DBA/2J (D). By the use of two statistical approaches (examination of residual scores as a sex-exclusive phenotypic value for the RI strains and multivariate regression on sex and genotype in the F(2)) we have identified and confirmed female-exclusive markers for alcohol acceptance on chromosomes 9 and 12 and one marker for alcohol preference on chromosome 2. Am. J. Med. Genet. (Neuropsychiatr. Genet.) 88:647-652, 1999.

The importance of genetic and environmental effects for self-reported health symptoms: a 30-year follow-up considering survival and selection effects

OBJECTIVES

This study delineated the nature of individual differences in self-reported health status across a 30-year period. Potential survival and selection effects on mean levels, variances, and genetic and environmental sources of variance were evaluated.

METHODS

Self-reported health status in 1963 was evaluated in 5,229 pairs of twins from the Swedish Twin Registry and in a subsample of 351 pairs surviving to the age of 80 years in 1993. Structural equation modeling evaluated genetic and environmental contributions to total variance and change in variance.

RESULTS

For men but not women, the genetic and environmental influences on health symptoms differed between survivors and nonsurvivors. Total variance increased, reflecting an increase in environmental variance, across the 30 years for both genders. Genetic variance decreased longitudinally for men.

DISCUSSION

The increase in variation from the mid-50s to the mid-80s appears to be due to an accumulation in environmental variation. There are gender differences that deserve further exploration.

Quantitative-trait loci analysis of cocaine-related behaviours and neurochemistry

We recently conducted a dose-response study of the effects of cocaine on several activity measures in the panel of BxD/Ty recombinant inbred mice. Animals were tested in an automated activity chamber over 2 days with i.p. saline on day 1 and i.p. cocaine on day 2, at one of four doses, 5, 15, 30 or 45 mg kg(-1). The monitor recorded total distance traveled, nosepokes in a holeboard, repeated movements and time spent by an individual in proximity to the centre of the apparatus. Dose-response curves for locomotor activation, i.e. the difference between cocaine and saline scores, showed that for all strains tested, scores increased 5-30 mg kg(-1). With few exceptions, locomotor activity at 45 mg kg(-1) was not significantly higher than that at 30 mg kg(-1). Repeated movement scores showed patterns similar to locomotor activity and nosepokes tended to be progressively inhibited by increasing doses of cocaine. Recombinant inbred strain mean distributions for all behaviours and at all doses exhibited continuous, rather than discrete variation, thus providing evidence of multiple-gene effects on cocaine-related behaviours. Quantitative trait loci (QTL) analysis pointed to several chromosomal locations associated with variations in cocaine-related behaviours and some are either identical or close to QTL reported by others. In separate groups of animals, densities of dopamine D1, and D2 receptors and dopamine uptake transporters were measured in the medial prefrontal cortex, caudate-putamen, nucleus accumbens and ventral midbrain. In all areas, all measures showed distributions consistent with polygenic influence and were associated with QTL. Of particular interest was our finding of a large segment on chromosome 15, which is related to dopamine receptor densities and cocaine-related behaviours.

Differences in locomotor activity across the lifespan of Drosophila melanogaster

The identification of differential patterns of change across the lifespan in quantitative traits is of abiding interest in the biological and gerontological research communities. These differential phenotypic patterns in complex systems illuminate developmental processes and form the foundation for the identification of putative biomarkers of aging. The goal of the present study was to explore changes in locomotor activity through the lifespan of Drosophila melanogaster. A replicated serial cross-sectional sampling design was used to test activity in five genetically independent inbred strains at 7, 14, 21, 28, 35, and 42 days of age. Differences were observed in activity level across ages and strains, suggesting that patterns of activity throughout the lifespan of D. melanogaster are influenced by genetic factors. Observed sex differences in change in activity level indicate that the aging processes may proceed differently in males and females.

Origins of individual differences in episodic memory in the oldest-old: a population-based study of identical and same-sex fraternal twins aged 80 and older

The relative importance of genetic and environmental influences on episodic memory in very late life was studied using a quantitative genetic approach. Identical (n = 125) and same-sex fraternal (n = 157) twin pairs, aged 80 and older (mean age = 83.3; SD = 3.1) and without a diagnosis of dementia were tested with seven memory measures: (1-2) Digit Span Forward and Backwards, (3) Prose Recall, (4) Thurstone's picture memory test, and the Memory in Reality (MIR) test, including the subtasks of (5) free recall, (6) recognition, and (7) relocation. Heritabilities, estimated by structural equation modeling, ranged from .04 to .49. The digit span backward test showed the highest heritability (h2 = .49), while heritabilities were typically lower for the long-term memory measures. The results demonstrate genetic influences on memory in the oldest-old, but suggest that the magnitude of these effects differs across memory measures.

Longitudinal and cross-sectional twin data on cognitive abilities in adulthood: the Swedish Adoption/Twin Study of Aging

Cross-sequential methods of analysis, designed to separate age and cohort effects, were applied to data from the Swedish Adoption/Twin Study of Aging. Thirteen cognitive variables were collected at 3 times of measurement separated by 3-year intervals. Data were available from 85 individuals from monozygotic (MZ) pairs reared apart, 132 from MZ pairs reared together, 207 from dizygotic (DZ) pairs reared apart, and 178 from DZ pairs reared together (age range at first assessment: 41-84 years). Time x Cohort interactions were found for mean performance on 8 of the measures, revealing stable mean performance in the younger cohorts and longitudinal decreases in mean performance in the older cohorts. Cohort and time effects for total variance were mixed; little evidence was found for increases in variance with age. Age changes and cohort differences in genetic and environmental components of variance were test-specific; several Cohort x Time interactions attained significance. Heritability of the general cognitive ability factor showed significant longitudinal decreases over time in the older cohorts.

Genes as gerontological variables: uniform genotypes

Genetic conceptualizations and procedures have become integral to the conduct of research across the spectrum of life sciences, including gerontology, even when genetics is not the focus of inquiry. Among the research tools thus provided, one of the most basic is that of inbred strains. A close approximation to genetic uniformity is achieved by a sufficient number of successive generations of matings of relatives, and, once this near-uniformity is attained, the members of an inbred strain constitute a reference group relatively stable over time and available to diverse investigators. Different inbred strains possess different genotypes, so that numerous distinctive reference groups are available. The stability of these groups enhances prospects of replication-testing, and makes possible the focused accumulation of pertinent data. Phenotypic differences among strains identify particular groups that can be most appropriate for particular subsequent research objectives (and also provide ipso facto evidence of genetic influence on the phenotype). The very substantial advantages of the uniform genotypes provided by inbred strains (and by their F1 offspring) are purchased at the cost of limited generalizability of results and constraints on assessment of co-variation among variables. Uniform genotypes are, thus, not a tool for all purposes but must be seen as a powerful basic tool within an abundant genetic tool-kit. Particular research purposes will require use of more than one tool from the kit.

Exotic mice as models for aging research: polemic and prospectus by R. Miller et al

Miller and colleagues present a polemic against uniform genotypes in gerontological research and a paean for genetically heterogeneous populations. Both on simple sampling considerations, and because of selective loss of alleles through inbreeding depression, inbred strains are idiosyncratic, and the lack of genetic variance within strains limits their utility in examining relationships among phenotypes. Heterogeneous stocks, by contrast, are not impaired by inbreeding depression, and the presence of genetic and environmental variance provides for effective assessment of correlations among variables. Those assembled by intermating of inbred strains are replicable in terms of gene pool parameters, even if no individual can ever be replicated, and those derived from wild-trapping may include alleles absent for whatever reason from current laboratory strains. This author concurs that there are limitations on research with uniform genotypes and advantages of heterogeneous populations. However, for specific purposes, the uniformity and stability of inbred strains are extremely valuable attributes, and heterogeneous stocks have their own limitations. Researchers should select the animal model most appropriate for their specific purpose.

Genes as gerontological variables: genetically heterogeneous stocks and complex systems

In gerontological research utilizing animal models, a major general strategy has been the use of uniform genotypes of inbred strains or their F1 hybrids. These animal models provide standard reference groups that are of major importance in establishing a reliable data base on aging phenomena. There are limitations to their usage, however, particularly in respect to descriptions or evaluations of variances or of covariance relationships. For these purposes, genetically heterogeneous stocks have the advantage that phenotypic variance (and covariance) has a genetic as well as an environmental component. The advantages of genetic heterogeneity are best realized when the stock has been systematically derived (usually by intercrossing of inbred strains) and maintained by a mating scheme of sufficient size to minimize inbreeding. Genetically heterogeneous stocks are of particularly high potential value in the study of complex systems. Some examples of their use in a gerontological context are provided.

Developmental loss of effect of a Chromosome 15 QTL on alcohol acceptance

Human alcohol abuse and alcoholism have clear developmental features, suggesting the possibility of changes over time in heritability and in quantitative genetic architecture, and raising prospects of identifying individual genes or quantitative trait loci (QTLs) that display different influence on alcohol-related phenotypes at different ages. The identification of specific loci showing such age-related changes will open up opportunities of focused association studies and of genotype manipulation by various mating procedures. Most animal model research in alcohol assesses the phenotypes of the animals at an early age; developmental studies are rare. Here we report on a QTL on Chromosome (Chr) 15 of the mouse that has been shown in several populations, including BXD recombinant inbred strains, an F2, and genotypically selected lines, to affect a measure of alcohol consumption. In the present study, we measured alcohol acceptance in the genotypically selected animals and in an F4 sample at about 100 days and again at about 300 days of age. In both groups, and in both sexes, significant differences were observed at 100 days between animals that were homozygous for the "increasing" haplotype defining the QTL region and those homozygous for the "decreasing" haplotype. At 300 days of age, the effect is absent in females and has diminished or disappeared in males. The results provide a further confirmation of the Chr 15 QTL in young mice, offer a new perspective on the development of alcohol-related phenotypes, and have strong implications for research design.

High genetic susceptibility to ethanol withdrawal predicts low ethanol consumption

C57BL/6J (B6) inbred mice are well known to drink large amounts of alcohol (ethanol) voluntarily and to have only modest ethanol-induced withdrawal under fixed dose conditions. In contrast, DBA/2J (D2) mice are "teetotallers" and exhibit severe ethanol withdrawal. Speculation that an inverse genetic relationship existed between these two traits was substantiated by meta-analysis of existing data collected in multiple genetic models, including large panels of standard and recombinant inbred strains, their crosses, and selectively bred mouse lines. Despite methodological differences among laboratories in measurement of both preference drinking and withdrawal, a nearly universal finding was that genotypes consuming large amounts of 10% ethanol (calculated as g/kg/day) during two-bottle choice preference drinking were genetically predisposed to low withdrawal scores in independent studies after either acute or chronic ethanol treatment. Conversely, low-drinking genotypes had higher withdrawal severity scores. The genetic relationship appears to be strongest in populations derived from B6 and D2, where data from more genotypes (BXD RIs, B6D2F2s, BXD RI F1s, and B6D2F2-derived selectively bred lines) were available for analysis. Gene mapping studies in these populations identified four chromosome regions [on Chromosomes (Chrs) 1, 2, 4, and 15] where genes might potentially influence both traits. Among genotypes with greater genetic diversity (for example, a panel of standard inbred strains or selectively bred lines), the relationship was less pronounced. Thus, reduced susceptibility to the development of high alcohol use may be supported by increased genetic susceptibility to ethanol withdrawal symptoms.

The effect of genetic factors for longevity: a comparison of identical and fraternal twins in the Swedish Twin Registry

BACKGROUND

The relative importance of genetic influences on longevity was studied on data from the population-based Swedish Twin Registry.

METHODS

A sample of 3,656 identical and 6,849 like-sexed fraternal twin pairs was studied regarding mortality rates and within-pair similarity for age at death. Genetic and environmental contributions to variation in longevity, expressed by integrated mortality rates, were estimated from a subsample of 1,734 twin pairs reared together and 130 twin pairs reared apart from the cohorts born 1886 to 1900.

RESULTS

The intraclass correlation coefficients suggested that the genetic effect was small, and, for males, perhaps absent. Among pairs in which both twins died relatively young and among pairs in which both twins lived until very old age, the variance in age at death seemed to have no genetic component. Model fitting procedures based on twins reared apart and twins reared together indicated that most of the variance in longevity was explained by environmental factors.

CONCLUSIONS

Over the total age range examined, a maximum of around one third of the variance in longevity is attributable to genetic factors, and almost all of the remaining variance is due to nonshared, individual specific environmental factors. The evidence that genetic factors play a minor role depending upon age at death merits further examination.

Longitudinal and cross-sectional twin data on cognitive abilities in adulthood: the Swedish Adoption/Twin Study of Aging

Cross-sequential methods of analysis, designed to separate age and cohort effects, were applied to data from the Swedish Adoption/Twin Study of Aging. Thirteen cognitive variables were collected at 3 times of measurement separated by 3-year intervals. Data were available from 85 individuals from monozygotic (MZ) pairs reared apart, 132 from MZ pairs reared together, 207 from dizygotic (DZ) pairs reared apart, and 178 from DZ pairs reared together (age range at first assessment: 41-84 years). Time x Cohort interactions were found for mean performance on 8 of the measures, revealing stable mean performance in the younger cohorts and longitudinal decreases in mean performance in the older cohorts. Cohort and time effects for total variance were mixed; little evidence was found for increases in variance with age. Age changes and cohort differences in genetic and environmental components of variance were test-specific; several Cohort x Time interactions attained significance. Heritability of the general cognitive ability factor showed significant longitudinal decreases over time in the older cohorts.

Longitudinal and genetic effects in the relationship between pulmonary function and cognitive performance

Previous studies have found cognitive deficits in patients with impaired pulmonary function, and recent data from healthy older adults suggest an association of pulmonary function with cognitive function. This 6-year longitudinal study evaluated genetic and environmental sources of covariation in the association of pulmonary function and cognitive performance. The sample included 222 Swedish twin pairs (60% women) with a mean age of 62.3 (+/- 7.7) years (age range: 40-84). Hierarchical multiple regression analyses, controlling for the effects of age, gender, and height, were employed to predict performance on cognitive tests of fluid intelligence (Digit Symbol, Block Design, Digit Span-Backward) and crystallized intelligence (Information) from forced expiratory volume in one second (FEV1). Bivariate cross-twin correlations were used to evaluate the contribution of genetic and environmental factors in the association of pulmonary function and cognitive performance. Results indicated that FEV1 predicted performance on tests of fluid intelligence but not crystallized intelligence at the initial assessment and at the 6-year follow-up. Cross-twin correlational analyses indicated that genetic effects accounted for a greater share of the association of pulmonary function and cognitive performance than environmental effects, but environment also accounted for a substantial share of the covariance.

Confirmation of quantitative trait loci for alcohol preference in mice

An F2 intercross derived from C57BL/6 and DBA/2 progenitor inbred strains was used to test for replication of quantitative trait loci (QTLs) for alcohol preference nominated by a previous study using BXD recombinant inbred (RI) strains (Rodriguez et al., Alcohol. Clin. Exp. Res. 19:367-379, 1995). Fourteen provisional QTLs were nominated in the original RI study with a p < 0.05 criterion. In the present study, a genome scan (101 microsatellite markers) was conducted on an F2 population (n = 218). Three significant QTLs were detected on chromosomes 1, 4, and 9, and three suggestive QTLs were detected on chromosomes 2, 3, and 10. Of these six QTLs, four were consistent with the previous RI nominations. The replication rate of 28.6% (4 of 14) is in agreement with the results of simulation studies performed by Belknap et al. (Behav. Genet. 26:149-160, 1996) and supports the methodological argument for a multistage research design for nominating and replicating QTLs.

A genetic basis for neuroendocrine-immune interactions

Psychoneuroimmunology is an exciting, complex field that elucidates interactions among the nervous, endocrine, and immune systems. The contribution of psychosocial factors and behavioral processes to these interactions has been the focus of numerous studies designed to investigate the intricate pathways that are involved in the "mind-body connection." In addition, the effects of this connection on the development and progression of various disease conditions are of considerable interest. Although efforts have been made to identify the cellular and molecular mechanisms underlying these relationships, the impact of genetic makeup on the communication among these systems has yet to be fully realized. The development of sophisticated genetic analytical methods and gene mapping techniques now provide the "tools" to determine the influence of genetics on behavior-neuroendocrine-immune interactions--an area of study that may represent the next frontier in psychoneuroimmunology.

Mortality and biomarkers of aging in heterogeneous stock (HS) mice

A longitudinal study was undertaken to evaluate the relationships among a battery of aging biomarkers and subsequent survival time in 319 genetically heterogenous stock (HS) mice. The biomarker variables chosen were selected from the broad domains of behavior, homeostatic physiology, oxidative defense, and immune function; biomarkers were measured at 45, 90, 360, 630, and 900 days of age. Sex differences were found in the survivor and mortality functions, with a mortality rate crossover occurring at about 525 days and a survival curve crossover at about 750 days of age. Females experienced lower initial mortality but had more sharply increasing mortality with age than did males. Survival analysis using Gompertz parametric models with biomarkers as time-varying covariates yielded significant biomarkers from each domain. Following backward elimination procedures, the final set of independent mortality predictors included headpokes in the File activity apparatus, maximum cord drop time, weight, hematocrit, urine concentration, natural killer cell activity, and concanavalin A response.

Selection bias in samples of older twins? A comparison between octogenarian twins and singletons in Sweden

Twin studies are a powerful approach for estimating genetic and environmental influences in later life, but the usual requirement that both twins are alive may introduce a selection bias in gerontological studies relative to representative samples of nontwins. In the present study, samples of older twins and nontwins in Sweden were compared across the domains of vitality, well-being, physical and cognitive functioning, and health utilization to evaluate possible selection bias. One member of each twin dyad in the OCTO-Twin Study of intact twin pairs older than age 80 was randomly selected (N = 128) and compared with a population-based sample of nontwins (N = 324) from the OCTO Study. Multiple regressions adjusting for differences in demographic variables showed significant effects for twin status in only 3 of 20 comparisons. The results suggest that twin pairs surviving into very late life are similar to a representative sample of nontwins of the same age in health status and biobehavioral functioning. These findings support the generalizability of twin studies for understanding genetic and environmental influences on aging, health, and behavior.

Genotypic selection provides experimental confirmation for an alcohol consumption quantitative trait locus in mouse

Quantitative genetic research has produced a wealth of basic information concerning genetic influence on alcohol-related processes. Recent developments in quantitative trait locus (QTL) methodology were promptly applied to the task of individuating polygenes affecting alcohol-related attributes in animal models and a body of reliable data is gradually coming into focus as a result of replication and convergence of evidence from a variety of methods. A key issue in QTL research is the need to distinguish true positive results from the false positive results that are inherent in analytical procedures requiring large numbers of significance tests. One school of thought holds that stringent significance levels should be imposed; another suggests more modest criteria for QTL nomination, with subsequent confirmation trials with independent samples. Recombinant inbred strains and various types of intercrosses have been used in correlational designs, both for nomination and confirmation studies. Alternative experimental procedures include knockout preparations and short-term phenotypic selective breeding. We present here results from a third experimental method-that of marker-based genotypic selection--in evaluation of two nominated QTLs for alcohol acceptance in mice.

Substantial genetic influence on cognitive abilities in twins 80 or more years old

General and specific cognitive abilities were studied in intact Swedish same-sex twin pairs 80 or more years old for whom neither twin had major cognitive, sensory, or motor impairment. Resemblance for 110 identical twin pairs significantly exceeded resemblance for 130 fraternal same-sex twin pairs for all abilities. Maximum-likelihood model-fitting estimates of heritability were 62 percent for general cognitive ability, 55 percent for verbal ability, 32 percent for spatial ability, 62 percent for speed of processing, and 52 percent for memory. There was also evidence for the significant influence of idiosyncratic experience as the environmental component that most determines individual differences in cognitive abilities late in life.

Can personality explain genetic influences on life events?

Previous research in the Swedish Adoption/Twin Study of Aging (SATSA) has found genetic influences on life events (R. Plomin, P. Lichtenstein, N.L. Pedersen, G.E. McClearn, & J.R. Nesselroade, 1990). The present study extends this finding by examining sex differences in genetic and environmental contributions to life events and by examining personality as a mediator of genetic influences on life events in SATSA. Analyses were based on 320 twin pairs, including identical and fraternal twins reared together and apart (mean age = 58.6 years). Controllable, desirable, and undesirable life events were revealed significant genetic variance for women. There was no significant genetic variance for either sex for uncontrollable events. Multivariate analyses of personality (as indexed by Neuroticism, Extraversion, and Openness to Experience) and life events suggest that all of the genetic variance on controllable, desirable, and undesirable life events for women is common to personality. Thus, in this sample of older adult women, genetic influences on life events appear to be entirely mediated by personality.

Biogerontologic theories

Theories of age and aging engage a wide variety of phenomena and levels of explanation. There is a general consensus that aging is a complex process or set of processes, involving many "causal" inputs and manifold consequences. Two general theoretical orientations that explicitly address complexity are sketched here-the differential theory of quantitative genetics and systems theory. Among the suggestions derived are that it may be advantageous to consider aging to be hierarchically organized, with the corollary that subsystems of an organism can have different functional or biological ages, and that several or many indices will be required to provide an adequate characterization of a single individual. Aging probably proceeds by saltation rather than continuously. Uncertainty associated with bifurcations in complex systems, together with individual differences in timing and magnitude of step changes, may constitute fundamental limitations to the predictability across the full life trajectory. Genetic and environmental influences will differ from hierarchical subsystem to subsystem, and may differ within a subsystem across chronological age.

Biomarkers of age and aging

The escalating interest in research on interventions that may affect aging processes has necessarily focussed attention on the outcome measures. The desirable characteristics of these "biomarker variables" have been widely discussed. This article offers some reflections on validity, reliability, and generalizability of biomarkers. It is argued that our comprehension of aging will evolve iteratively from application of a diversity of biomarker variables. Each of these will have strengths and shortcomings from methodological and measurement points of view. The siren song that a "gold standard" index of aging can be found should be ignored.

No association between general cognitive ability and the A1 allele of the D2 dopamine receptor gene

Berman and Noble (1995) reported significantly reduced visuospatial performance in children with the TAQI A1 allele of the D2 dopamine receptor (DRD2) gene. Given that visuospatial performance loads highly on an unrotated principal component indexing general cognitive ability, we tested the association between DRD2 and WISC-R IQ comparing 51 high-IQ, 51 average-IQ, and 35 low-IQ children in the IQ Quantitative Trait Loci (QTL) Project. No statistically significant association between the TAQI A DRD2 alleles and IQ was found. Given that a statistically significant portion of genetic variance for specific cognitive abilities is independent of general cognitive ability, it is possible that the TAQI DRD2 association is specific to visuospatial performance and independent of general cognitive ability.

Can personality explain genetic influences on life events?

Previous research in the Swedish Adoption/Twin Study of Aging (SATSA) has found genetic influences on life events (R. Plomin, P. Lichtenstein, N.L. Pedersen, G.E. McClearn, & J.R. Nesselroade, 1990). The present study extends this finding by examining sex differences in genetic and environmental contributions to life events and by examining personality as a mediator of genetic influences on life events in SATSA. Analyses were based on 320 twin pairs, including identical and fraternal twins reared together and apart (mean age = 58.6 years). Controllable, desirable, and undesirable life events were revealed significant genetic variance for women. There was no significant genetic variance for either sex for uncontrollable events. Multivariate analyses of personality (as indexed by Neuroticism, Extraversion, and Openness to Experience) and life events suggest that all of the genetic variance on controllable, desirable, and undesirable life events for women is common to personality. Thus, in this sample of older adult women, genetic influences on life events appear to be entirely mediated by personality.

Prospects for quantitative trait locus methodology in gerontology

The remarkable advances in mapping of genomes have made it feasible to search among the hitherto anonymous polygenes of complex systems for loci of intermediate effect size (quantitative trait loci-QTL). Pursuing the strategy of identifying these QTL in complex systems will open the possibility of exploring the genetic architecture and the anatomical, physiological, biochemical, and molecular mechanisms underlying the phenotypes under investigation. One study (Gelman et al., 1988) has pioneered the exploration of QTL in the study of aging. Their engaging results, dealing with age at death as the endpoint measure, portend the power of the search for quantitative trait loci in aging processes.

Genetics and behavioral medicine: risk factors for cardiovascular disease

This is the second in a series of three articles addressing the intersection of interests in behavioral genetics and behavioral medicine. In this article, we use risk factors for cardiovascular disease as a prototypical trait for which behavioral genetic approaches provide powerful tools for understanding how risk factors, behavior, and health outcomes are related. The approach synthesizes a number of methods and areas of interest in an attempt to arrive at a comprehensive, whole-organism understanding of health-related risk factors and their response to behavioral interventions.

Genetics and behavioral medicine

Genetics has substantial relevance to behavioral medicine. A rapidly growing body of evidence indicates the influence of genetics on health and disease and on the behavioral factors related to them. The model of quantitative genetics provides a general interpretational scheme for this burgeoning field. The model focuses on variability, and a major research objective is the decomposition of observed individual differences into portions attributable to various types of genetic and environmental sources of variability. This approach emphasizes the coaction of genes and environments and stands in sharp contrast to the archaic view that places nature and nurture in opposition. Some relevant examples are given in this first article to illustrate the general analytic process. A detailed application to cardiovascular health and disease is provided in the second article, and some policy implications are briefly considered in the third article.

A co-twin--control study of response to widowhood

The effects of long-term and recent conjugal bereavement were investigated in a sample of 2,104 Swedish twins followed between 1984 and 1993. In co-twin-control analyses, the bereaved twin experienced significantly more depressive symptoms, more loneliness, and less life satisfaction than the married co-twin. This association existed for recently widowed (< 3 years) of both sexes. Long-term widowed (> 5 years) reported more loneliness than married individuals, and for women there was also a difference in life satisfaction. There were no effects of bereavement on perceived physical health. Individual analyses, which included all respondents regardless of the co-twin's bereavement status, showed the same pattern of results. There was also evidence for an anticipation effect of widowhood indicated by elevated depressive symptoms prior to the spouse's death. Finally, longitudinal analyses showed that it is more stressful to be bereaved when young-old than old-old, but revealed no age differences in adaptation.

Potential environmental effects on adult lipoprotein(a) levels: results from Swedish twins

Two hundred and ninety four pairs of Swedish twins reared apart and twins reared together were used to evaluate the importance of genetic and environmental influences on lipoprotein(a) (Lp(a)) levels. Lp(a) levels ranged from <10 mg/l to 926 mg/l with 7.9% of the sample having undetectable Lp(a) levels (i.e. <10 mg/l). A substantial genetic component in Lp(a) variation was indicated by a heritability estimate of approximately 90%. No difference in heritability was found across age groups. Quantitative genetic analyses also suggest correlated environmental effects most likely composed of maternal, neonatal and postnatal environmental influences. However, these effects did not reach statistical significance, partly due to a lack of power. Results from analyses of co-twin differences in Lp(a) levels for monozygotic twins indicate that sex hormone use may be of importance for Lp(a) variation in women. There was no evidence of potential influences of alcohol consumption, beta-blocker and diuretic administration on Lp(a) levels in either men or women.

Heritability of cognitive abilities in adult twins: comparison of Minnesota and Swedish data

Cross-sectional reports suggest heritability of cognitive ability increases throughout adulthood. To investigate this hypothesis, quantitative genetic analyses were conducted on four measures of cognitive ability (verbal, spatial, perceptual speed, memory). Data from Minnesota and Swedish twin studies of aging were compared. Heritability estimates and the factor structure of cognitive abilities could be equated across younger twins (age, 27-50) and middle-aged twins (age, 50-65) from both studies, suggesting stability of heritability during adulthood. The heritability of 81% for a general cognitive factor confirmed earlier findings of high heritability in younger and middle-aged samples. Older Swedish twins (age, 65-85) demonstrated significantly lower heritability estimates for cognitive abilities (54%) and a significantly different factor structure of cognitive ability.

Alcohol acceptance, preference, and sensitivity in mice. II. Quantitative trait loci mapping analysis using BXD recombinant inbred strains

Quantitative trait loci (QTL) mapping of complex phenotypes has emerged as an important feature of the recombinant inbred (RI) strain methodology. In this second study of our series on alcohol-related behaviors in mice, we examine alcohol acceptance, preference, and hypnotic dose sensitivity (HDS) to a standard dose of alcohol measured in BXD RI strains to identify candidate QTL regions responsible for their heritability. We detected highly significant marker associations for acceptance on chromosome 12 (Eif4e), for preference on chromosome 1 (D1Rti2) and chromosome 7 (D7Mit7), and for HDS on chromosome 7 (Mpmv1). These are the strongest QTL associations that we detected, but several other candidate QTL regions are reported. Given the limited number of BXD RI strains available, the large number of markers used herein, and the consequent chance of identifying false marker associations, these RI QTL mapping results must be seen as tentative, but an important first step toward identifying QTL for alcohol-related behaviors.