Familial Factors in Mortality with Control of Epidemiological Covariables. Swedish Twins Born 1886-1925

Genealogical data and the biodemography of human longevity

Biodemography of human longevity is an emerging interdisciplinary field of sociobiological research with deep historical roots. Two research questions are examined in this article: (1) What evidence is there for the familial transmission of human longevity?, and (2) what are the effects of parental age at reproduction on offspring longevity, and in particular, are there long-term adverse health consequences associated with the trend toward delayed reproduction? The ability of scientists to conduct biodemographic studies depends not only on merging theoretical and methodological elements from the biological and demographic/actuarial sciences, but unique sources of data and statistical methods must also be developed. In this article we describe how gencalogical data have been used for over a century to explore basic questions about human longevity, and how similar kinds of data now being developed are driving the formation of new testable research hypotheses in the field of biodemography.

Communication between identical twins: health behavior and social factors are associated with longevity that is greater among identical than fraternal U.S. World War II veteran twins

BACKGROUND

Longevity is greater for identical twins than for fraternal twins from the same population. Factors that are explanatory for this difference are not known.

METHODS

Multivariate survival analysis is applied to current mortality data for 26,974 male twins with known zygosities of the National Academy of Science-National Research Council World War II Veteran Twins Registry, and this analysis is applied to their health and social behavior and personal histories, as collected from two survey questionnaires distributed in 1967 and 1983 (with 14,300 and 9475 responses received, respectively). To explain this difference in longevity, social, health, and personal history factors are evaluated for associations with longevity.

RESULTS

Survival functions of identical and fraternal twins differed significantly (p<.0001). Median lifetimes were 82 years for identical and 80.5 years for fraternal twins. The correlation between lifetimes of identical twin partners was greater than that of fraternal twins. For identical but not for fraternal twins, the risk of mortality was significantly lower for twin partners who communicated 1 or more times per month, compared with those who communicated less frequently (relative risk.80, 95% confidence interval 0.68-0.94, p=.008, with control for other factors associated with longevity: smoking, exercise, a current marriage, living with both parents until age 15 or older, and having a live co-twin). Distributions of communication, exercise level, and smoking prevalence were more beneficial with regard to longevity for identical than for fraternal twins as a group.

CONCLUSIONS

Frequent communication between identical but not fraternal twin partners, and both level of exercise and prevalence of smoking, distributed more beneficially in terms of longevity for identical compared with fraternal twins, are explanatory for the greater longevity of identical than fraternal twins.

Mortality from birth to adult life: a longitudinal study of twins

We have examined mortality from birth through adult life in a cohort of 2562 twins born in Birmingham, UK, between 1950 and 1954. Their birthweights and obstetric details had been recorded as part of a longitudinal study of births in Birmingham. There were a total of 151 perinatal deaths (perinatal mortality rate = 116 per 1000 births) and 227 infant deaths (infant mortality rate = 94 per 1000 live births). 70 deaths occurred after the age of one year. In comparison with national mortality rates in the UK, overall mortality in the twins was high (standard mortality rate, SMR = 259, 95% CI 221-300). Mortality was highest in the first year of life and, although it then declined progressively, it remained significantly higher that that of the general population until age 5 years. The excess mortality was largely due to conditions originating in the perinatal period but there were excess rates of congenital abnormalities, diseases of the respiratory system, digestive system and nervous and sensory organs. A Cox proportional Hazards analysis showed that the risk of death was related to low birthweight, prematurity and male sex. Death of the co-twin was highly predictive of mortality throughout the period of follow up. These studies not only underline the excess mortality associated with twin birth but show for the first time that this excess mortality extends into childhood.

How frailty models can be used for evaluating longevity limits: Taking advantage of an interdisciplinary approach

In this paper we discuss an approach to the analysis of mortality and longevity limits when survival data on related individuals with and without observed covariates are available. The approach combines the ideas of demography and survival analysis with methods of quantitative genetics and genetic epidemiology. It allows us to analyze the genetic structure of frailty in the Cox-type hazard model with random effects. We demonstrate the implementation of this strategy to survival data on Danish twins. We then evaluate the resulting lower bounds for biological limits of human longevity. Finally, we discuss the limitations of this approach and directions of further research.

A study of twins and stroke

BACKGROUND AND PURPOSE

Although there are strong genetic contributions to coronary artery disease, only a few studies have considered heritable influences on stroke.

METHODS

We investigated the role of genetic factors in stroke using the Twin Registry maintained by the National Academy of Sciences-National Research Council. The registry includes 15,948 male twin pairs born between 1917 and 1927. In 1985, 9,475 twins responded to a mailed questionnaire, which covered vascular risk factors, cardiac events, and stroke.

RESULTS

Analysis of twin pairs in which both responded to the questionnaire, and a question on stroke, indicated proband concordance rates of 17.7% for monozygotic pairs and 3.6% for dizygotic pairs (relative risk = 4.3; chi 2 = 4.94, df = 1; p less than 0.05).

CONCLUSIONS

This nearly fivefold increase in the prevalence of stroke among the monozygotic compared with the dizygotic twin pairs suggests that genetic factors are involved in the etiology of stroke. The twin study paradigm holds considerable promise for identifying both genetic and environmental influences on stroke.

Bivariate survival models for analysis of genetic and environmental effects in twins

Classic methods in genetics for the analysis of binary attributes, based on an assumption of a "threshold" on a normally distributed latent variable called "liability," estimate the strength of genetic and environmental effects from differences in correlations between relatives of differing genetic relatedness. Two problems that are not easily addressed by these methods are the need to take the age of onset into account (particularly in chronic diseases in which incidence rates vary considerably with age and the lengths of time at risk can vary between individuals) and the desirability of incorporating measured covariates (genetic or environmental). The standard methods of cohort analysis used in epidemiology allow for both of these features, but until recently have been restricted to independent individuals. Recent developments in survival analysis have extended the widely used "proportional hazards" model of Cox by the addition of latent variable, epsilon, reflecting the shared susceptibility of related subjects because of their shared genes or shared environment. We show how this approach can be combined with more traditional models of gene-environment interaction to allow the main effects of measured genetic markers and environmental variables to be estimated, as well as the residual variance of genetic and environment and their interactions. The approaches are applied to a cohort of female twin births in Sweden from 1886 to 1958, linked with the Swedish cancer registry from 1961 to 1982.

Inherited frailty and longevity

The lifespansof parents and children appearonly weakly related,even though parents affect their children’s longevity through both genetic and environmental influences. These influences can be summarizedas a correlation betweenparents’and children’s frailty. It isshownthat even ifchildren perfectly inherit their frailty from their parents, parents’ life spans explain little of the variance in children’s life spans, because the variance in life expectancies among people with different frailties is small compared with the variancein life spansamongpeopleat the same leveloffrailty. Byinterpreting frailty as a relative risk in a proportional-hazard model, longevity as a duration or waiting time, and inheritance as an invariance in relative risk over time, one can extend this result to repeatable events involving fertility, migration, marriage, unemployment, and so forth.

Genetic and environmental influences on premature death in adult adoptees

To assess genetic and environmental influences on adult mortality, we followed 960 families that included children born during the period 1924 through 1926 who were placed early in life with adoptive parents unrelated to them. We evaluated the risks of dying from all causes or from specific groups of causes between the ages of 16 and 58 years for adoptees with a biologic or adoptive parent who died of the same cause before the age of either 50 or 70. We compared these risks with the adoptees' risk of dying from the same causes between the ages of 16 and 58 when either the biologic or adoptive parents were still alive at the ages of 50 and 70. The death of a biologic parent before the age of 50 resulted in relative risks of death in the adoptees of 1.71 (95 percent confidence interval, 1.14 to 2.57) for all causes, 1.98 (1.25 to 3.12) for natural causes, 5.81 (2.47 to 13.7) for infections, 4.52 (1.32 to 15.4) for cardiovascular and cerebrovascular causes, and 1.19 (0.16 to 8.99) for cancers. The death of an adoptive parent resulted in relative risks of death in the adoptees that were close to unity for all causes, natural causes, and infections, 3.02 (0.72 to 12.8) for vascular causes, and 5.16 (1.20 to 22.2) for cancers. A similar but weaker pattern was observed when either a biologic or adoptive parent died before the age of 70. We conclude that premature death in adults has a strong genetic background--especially death due to infections and vascular causes.(ABSTRACT TRUNCATED AT 250 WORDS)

Toward a theory of human multiple birthing: sociobiology and r/K reproductive strategies

Using symbols from population biology, a continuum of reproductive strategies can be distinguished ranging from r, the production of large numbers of offspring provided with minimal care, to K, the production of few offspring nurtured intensively. While all humans are at the K end of the continuum, some are proposed to be more so than others. If multiple egg production is part of an r-reproductive strategy, certain facts may be ordered. Compared to mothers of singletons, for example, mothers of DZ twins have a lower age of menarche, a shorter menstrual cycle, a higher number of marriages, a higher rate of coitus, more illegitimate children, a closer spacing of births, a greater fecundity, more wasted pregnancies, a larger family, an earlier menopause, and an earlier mortality. Further, all twins have a shorter gestation period, a lower birth weight, and a greater incidence of infant mortality, with DZ twins having a greater frequency of health disorders, a higher mortality rate, and a lower rate of enrollment in volunteer registries. Multiple birthing also occurs more frequently in families of lower than of higher social status, and in those of African than of European and especially than of Oriental descent.

Genetic aspects of arteriosclerosis

This review discusses the genetic factors in the development of arteriosclerosis and coronary heart disease (CHD). In several studies, multivariate analysis of prospective mortality/morbidity data and angiographic findings have indicated that a family history of CHD contributed to CHD risk independently of the established risk factors. In addition, ethnic groups that differ in the prevalence and incidence of CHD also markedly differ in blood groups and protein-enzymatic markers. These or other genetic differences may affect CHD rates. Data from fraternal and identical twins, the source of some early genetic CHD findings, are reviewed. Genetic disorders of lipoprotein metabolism and transport, such as familial hypercholesterolemia, as well as other monogenic disorders are discussed. The role of apoprotein E polymorphism i other monogenic disorders are discussed. The role of apoprotein E polymorphism in determining plasma LDL variability among individuals is considered. Recombinant DNA technology, molecular cloning, and the identification of restriction fragment length polymorphisms are new tools for investigators who assess DNA polymorphism. Recent advances in that domain include: DNA polymorphisms affecting blood levels of apo A-I and A-II, association of a DNA insertion on chromosome 19 with severe premature atherosclerosis, and information concerning linkage of the genes for various apolipoproteins. In addition, the evidence for a major genetic component in diabetes mellitus and research into the genetic aspects of hypertension are reviewed. The male/female ratio in pathologically and epidemiologically assessed atherosclerosis may provide clues to the role of genetics. Early structural changes in the coronary artery intima are compatible with the ethnic and gender predilection. A key question in understanding underlying mechanisms in atherosclerosis is why coronary arteries are occluded in individuals whose other arterial systems are largely unaffected. The review concludes with a discussion of the directions and implications of future genetic research in arteriosclerosis with an emphasis on uncovering genetically determined differences in arterial wall response to blood flow. Subpopulations with different genetic risks may be identified, in which case universal preventive strategies might be replaced with specific ones.