In mammalian females, one of the two X chromosomes is inactivated in early embryonic life. Females are therefore mosaics for two cell populations, one with the maternal and one with the paternal X as the active X chromosome. A skewed X inactivation is a marked deviation from a 50:50 ratio. In populations of women past 55–60 years of age, an increased degree of skewing (DS) is found. Here the association between age-related skewing and mortality is analyzed in a 13-year follow-up study of 500 women from three cohorts (73–100 years of age at intake). Women with low DS had significantly higher mortality than the majority of women who had a more skewed DS (hazard ratio: 1.30; 95% CI: 1.04–1.64). The association between X inactivation and mortality was replicated in dizygotic twin pairs for which the co-twin with the lowest DS also had a statistically significant tendency to die first in the twin pairs with the highest intra-pair differences in DS (proportion: 0.71; 95% CI: 0.52–0.86). Both results suggest that lower DS is associated with higher mortality. We therefore propose that age-related skewing may be partly due to a population selection with lower mortality among those with higher DS.
aging; sex; mortality
Rheumatoid arthritis (RA) is an autoimmune disease with a complex origin. Previous studies have reported heritability estimates on RA at about 60%. Only 16% of the genetic background of the disease has been disclosed so far. The purpose of the present investigation was to provide an optimized estimate on the heritability of RA and to study the recurrence risk in a nationwide Caucasian twin population.
Methods and Findings
In a mail survey addressed to 56.707 twin individuals, RA was reported by 479 individuals, mean age 52 (range 16–73). Respondents underwent an interview and clinical examination. Ascertainment probability was 80%. RA was confirmed in 162 twin individuals yielding a prevalence at 0.37% (95% CI 0.31–0.43). The mean discordance time was 19 years (range 0–57). The concordance was 9.1% (95% CI 1.9 to 24.3) in MZ, 6.4% (95% CI 2.1 to 14.3) in DZss. The increased relative risk of attracting RA conditioned on having an affected cotwin compared to the background population risk was 24.6 to 35.4 in MZ twins and 17.3 to 31.6 in DZss twins. The correlation coefficients were 0.60 (0.33 to 0.78) in monozygotic (MZ) and 0.55 (0.33 to 0.72) in dizygotic same sexed (DZss) pairs. Twelve percent (95% CI 0–76%) of the phenotypic variance in the liability to RA was due to additive genetic effects, 50% (95% CI 0–72%) to shared environmental effects and 38% (95% CI 17–61%) to non-shared environmental effects.
This study emphasizes that family factors are important for the development of RA. Although genetic effectors are important, shared and non-shared environmental triggers and/or epigenetic stochastic events seem to be even more significant. However, it should be borne in mind that the genetic and non-genetic components may not be the same across disease subsets.
While a substantial amount of behavioral genetic research has helped to characterize developmental trends in twin similarity in early life, relatively little is known about changes in twin similarity with age in adulthood. We investigated age moderation of twin similarity for a composite measure of cognitive ability, depression symptomatology and hand grip strength in a cross-sectional sample of 2,332 like-sex pairs of Danish twins age 46–96 years. All three outcomes were strongly correlated with age, indicating that the three phenotypes analyzed are not developmentally static. Nonetheless, in moderated regression analysis we found no evidence of declining twin similarity for any of the three outcomes in either zygosity group. Moreover, biometric analysis of the twin data revealed minimal differences in heritability estimates across the age range sampled. While small sample size limits our ability to draw firm conclusions at very advanced ages, these findings call into question the hypothesis that the cumulative impact of life experiences diminishes twin similarity at least through age 80. We hypothesize that twins are able to maintain similarity over extended periods of time because in part they are able to construct similar environments that reinforce that similarity.
Aging; Depression; Cognitive functioning; Grip strength; Age differences in heritability; Twin correlation
To test whether females in families with cleft lip and/or palate (CL/P) have increased breast cancer risk
Using the Danish Facial Cleft Registry, females with CL/P, mothers of children with CL/P, and sisters to CL/P cases were identified for the Danish birth cohorts 1911 to 1975. These females were compared to a 5% random sample of these cohorts regarding the incidence and age of onset for breast cancer registered in the Danish Hospital Discharge Register 1977–2005.
Examining 48,404 person-years for 1,809 female CL/P cases (49 breast cancer cases) and 212,795 person-years for 7935 female relatives (188 breast cancer cases) we found no increased breast cancer risk for either CL/P cases (hazard ratio (HR) = 1.23, 95% confidence interval (CI): 0.92–1.63), mothers of children with CL/P (HR = 0.93, 95%, CI: 0.80–1.08), or sisters of CL/P cases (HR = 0.94, 95% CI: 0.55–1.60). Neither were there any significant differences in age of onset.
Both epidemiological and genetic studies have suggested common etiological factors for breast cancer and cleft lip and/or palate (CL/P). However, this population-based study was not able to confirm a general increase in breast cancer risk among females in families with CL/P.
Cleft lip; cleft palate; breast cancer; recurrence; family study
Despite twinning being common in Africa, few prospective twin studies have been conducted. We studied twinning rate, perinatal mortality and the clinical characteristics of newborn twins in urban Guinea-Bissau.
The study was conducted at the Bandim Health Project (BHP), a health and demographic surveillance site in Bissau, the capital of Guinea-Bissau. The cohort included all newborn twins delivered at the National Hospital Simão Mendes and in the BHP study area during the period September 2009 to August 2011 as well as singleton controls from the BHP study area. Data regarding obstetric history and pregnancy were collected at the hospital. Live children were examined clinically. For a subset of twin pairs zygosity was established by using genetic markers.
Out of the 5262 births from mothers included in the BHP study area, 94 were twin births, i.e. a community twinning rate of 18/1000. The monozygotic rate was 3.4/1000. Perinatal mortality among twins vs. singletons was 218/1000 vs. 80/1000 (RR = 2.71, 95% CI: 1.93-3.80). Among the 13783 hospital births 388 were twin births (28/1000). The hospital perinatal twin mortality was 237/1000.
Birth weight < 2000g (RR = 4.24, CI: 2.39-7.51) and caesarean section (RR = 1.78, CI: 1.06-2.99) were significant risk factors for perinatal twin mortality. Male sex (RR = 1.38, CI: 0.97-1.96), unawareness of twin pregnancy (RR = 1.64, CI: 0.97-2.78) and high blood pressure during pregnancy (RR = 1.77, CI: 0.88-3.57) were borderline non-significant. Sixty-five percent (245/375) of the mothers who delivered at the hospital were unaware of their twin pregnancy.
Twins had a very high perinatal mortality, three-fold higher than singletons. A birth weight < 2000g was the strongest risk factor for perinatal death, and unrecognized twin pregnancy was common. Urgent interventions are needed to lower perinatal twin mortality in Guinea-Bissau.
Genetic variants in the choline acetyltransferase (ChAT) gene have been suggested as risk factors for neurodegenerative Alzheimers Disease (AD). Here we tested the importance of genetic variants in the ChAT gene in normal cognitive function of elderly in a study sample of Danish twins and singletons (N=2070). The ChAT rs3810950 A allele, which has been associated with increased risk for AD, was found to be associated with a decrease cognitive status evaluated by a 5-component cognitive composite score (p=0.03 reg. coef. −0.30 CI 95% −0.57; −0.02), and the rs3810950 and rs8178990 ancestral GC haplotype was also associated with better cross sectional cognitive composite score (p=0.04 reg. coef. 0.59 CI 95% 0.03; 1.16). Growth curve model analyses applied to up to 10 years of follow-up data demonstrated that the rs3810950 A allele was associated with a lower cognitive composite score and MMSE at the lowest age (73 years of age), and was lower in the whole interval 73–82 although the absolute difference became smaller with age. Stratification by presence of the APOE ε4 allele revealed that rs3810950 AG/non-APOE ε4 carriers and rs3810950 AA/APOE ε4 carriers were associated with a lower cognitive composite score in younger elderly 73–83 years of age, similar to previous reports of association with AD.
Dementia; Alzheimer; Aging; Genes; oldest old; MMSE; Cognitive function
Leukocyte telomere length (LTL) shortens with age and is potentially a biomarker of human aging.
We examined the relation of LTL with physical ability and cognitive function in 548 same-sex twins from the Longitudinal Study of Aging Danish Twins. LTL was measured by Southern blots of the terminal restriction fragments (TRF). Physical ability was evaluated using a self reported scale of 11 questions, while cognitive function was scored by MMSE and a cognitive composite score sensitive to age-related decline.
A random intercept model revealed a positive, significant association between LTL and physical ability. For every unit increase in physical ability score, LTL increased by 0.066 kb (p=0.01), equal to approximately three years of age-dependent LTL shortening. A matched case-co-twin design showed that the group consisting of the twins from each pair with the longer LTL also displayed better physical ability (p<0.01). Moreover, the intra-pair difference in LTL was associated with intra-pair difference in physical ability (p<0.01), confirming the association. However, we found no association between cognitive function and LTL.
The LTL-physical ability association in the elderly provides further support to the premise that LTL is an index of somatic fitness in the narrow context of human physical health.
Leukocyte telomere length (LTL) is associated with a number of common age-related diseases and is a heritable trait. Previous genome-wide association studies (GWASs) identified two loci on chromosomes 3q26.2 (TERC) and 10q24.33 (OBFC1) that are associated with the inter-individual LTL variation. We performed a meta-analysis of 9190 individuals from six independent GWAS and validated our findings in 2226 individuals from four additional studies. We confirmed previously reported associations with OBFC1 (rs9419958 P = 9.1 × 10−11) and with the telomerase RNA component TERC (rs1317082, P = 1.1 × 10−8). We also identified two novel genomic regions associated with LTL variation that map near a conserved telomere maintenance complex component 1 (CTC1; rs3027234, P = 3.6 × 10−8) on chromosome17p13.1 and zinc finger protein 676 (ZNF676; rs412658, P = 3.3 × 10−8) on 19p12. The minor allele of rs3027234 was associated with both shorter LTL and lower expression of CTC1. Our findings are consistent with the recent observations that point mutations in CTC1 cause short telomeres in both Arabidopsis and humans affected by a rare Mendelian syndrome. Overall, our results provide novel insights into the genetic architecture of inter-individual LTL variation in the general population.
Non-syndromic cleft palate (CP) is a common birth defect with a complex and heterogeneous etiology involving both genetic and environmental risk factors. We conducted a genome wide association study (GWAS) using 550 case-parent trios, ascertained through a CP case collected in an international consortium. Family based association tests of single nucleotide polymorphisms (SNP) and three common maternal exposures (maternal smoking, alcohol consumption and multivitamin supplementation) were used in a combined 2 df test for gene (G) and gene-environment (G×E) interaction simultaneously, plus a separate 1 df test for G×E interaction alone. Conditional logistic regression models were used to estimate effects on risk to exposed and unexposed children. While no SNP achieved genome wide significance when considered alone, markers in several genes attained or approached genome wide significance when G×E interaction was included. Among these, MLLT3 and SMC2 on chromosome 9 showed multiple SNPs resulting in increased risk if the mother consumed alcohol during the peri-conceptual period (3 months prior to conception through the first trimester). TBK1 on chr. 12 and ZNF236 on chr. 18 showed multiple SNPs associated with higher risk of CP in the presence of maternal smoking. Additional evidence of reduced risk due to G×E interaction in the presence of multivitamin supplementation was observed for SNPs in BAALC on chr. 8. These results emphasize the need to consider G×E interaction when searching for genes influencing risk to complex and heterogeneous disorders, such as non-syndromic CP.
Background. Nasal carriage is a major risk factor for Staphylococcus aureus infection. Approximately, one-quarter of adults carry S. aureus. However, the role of host genetics on S. aureus nasal carriage is unknown.
Methods. Nasal swabs were obtained from a national cohort of middle-aged and elderly Danish twins. Subjects colonized with S. aureus were identified by growth on selective plates and spa typing. A second sample was obtained from twins initially concordant for carriage. Twins found to again be colonized with S. aureus were defined as persistent carriers.
Results. The prevalence of S. aureus carriage among 617 twin pairs (monozygotic/dizygotic pairs: 112/505) was 26.3% (95% confidence interval [CI], 24.0%–28.9%). The concordance rate for carriage did not differ significantly between pairs of monozygotic (37.5%; 95% CI, 22.3%–53.8%) twins and same sex (24.2%; 95% CI, 15.4%–34.5%), and opposite sex (21.4%; 95% CI, 12.0%–33.4%) dizygotic twins. Despite shared childhoods, only 1 of 617 pairs was concordant with respect to lineage. Although heritability increased for S. aureus and lineage persistency, no significant heritability was detected.
Conclusion. In this study, host genetic factors exhibited only a modest influence on the S. aureus carrier state of middle-aged and elderly individuals.
Orofacial clefts are common birth defects of complex etiology, with an excess of males among babies with cleft lip and palate, and an excess of females among those with cleft palate only. Although genes on the X chromosome have been implicated in clefting, there has been no association analysis of X-linked markers.
We added new functionalities in the HAPLIN statistical software to enable association analysis of X-linked markers and an exploration of various causal scenarios relevant to orofacial clefts. Genotypes for 48 SNPs in 18 candidate genes on the X chromosome were analyzed in two population-based samples from Scandinavia (562 Norwegian and 235 Danish case-parent triads). For haplotype analysis, we used a sliding-window approach and assessed isolated cleft lip with or without cleft palate (iCL/P) separately from isolated cleft palate only (iCPO). We tested three statistical models in HAPLIN, allowing for: i) the same relative risk in males and females, ii) sex-specific relative risks, and iii) X-inactivation in females. We found weak but consistent associations with the oral-facial-digital syndrome 1 (OFD1) gene (formerly known as CXORF5) in the Danish iCL/P samples across all models, but not in the Norwegian iCL/P samples. In sex-specific analyses, the association with OFD1 was in male cases only. No analyses showed associations with iCPO in either the Norwegian or the Danish sample.
The association of OFD1 with iCL/P is plausible given the biological relevance of this gene. However, the lack of replication in the Norwegian samples highlights the need to verify these preliminary findings in other large datasets. More generally, the novel analytic methods presented here are widely applicable to investigations of the role of X-linked genes in complex traits.
Mental health is increasingly defined not only by the absence of illness but by the presence of subjective well-being (SWB). Previous cohort studies have consistently shown that indicators of SWB predict favorable life outcomes including better mental and somatic health, including longevity. The favorable effects associated with SWB have prompted new research aimed at raising happiness and well-being through individual interventions and public health initiatives. Standard observational studies of individual-level associations, however, are subject to potential confounding of exposure and outcome by shared genes and environment. The present study explored the association between SWB and increased longevity, using twin pair analyses to determine whether the association is consistent with causality or is due to genetic or environmental confounding. The study sample of 3,966 twins aged 70 or older followed for a median time period of 9 years was drawn from the population-based Longitudinal Study of Aging Danish Twins (LSADT). The association between the exposure of subjective wellbeing, operationalized as affect and life satisfaction, and the outcome of all-cause mortality risk, was examined using between-individual and within-pair survival analyses. As expected, at the individual level (without regard to twin pair membership), SWB predicted increased longevity. Exposure effects were also present in unadjusted and adjusted within-pair analyses of 400 dizygotic (DZ) pairs and 274 monozygotic (MZ) pairs, indicating that SWB is associated with increased longevity independent of familial factors of genes and shared environment.
well-being; life satisfaction; aging; longevity; mortality; cotwin control
Inflammation may play an essential role in the decline of physical performance. In this study we investigated the associations between inflammatory markers, candidate polymorphisms and physical performance in elderly people. Plasma levels of TNF-α, IL-6, CRP, fibrinogen, sICAM-1 and candidate polymorphisms were measured in 600 twin individuals aged 73 years and older participating in the Longitudinal Study of Aging Danish Twins. Physical performance was assessed using a self-reported measure. The inclusion of twins allowed both traditional and within-twin-pair analysis which permitted control for shared environment and genetic factors. Higher levels of inflammatory markers were generally associated with a lower level of physical performance. The TNFα-238G/A polymorphism was significantly associated with physical performance in men, with A allele carriers having significantly better performance than GG homozygotes. However, this gene variation seems to have only a minor role in explaining the associations between the levels of inflammatory markers and physical performance. When using twin pair analysis to test whether genetic factors in general account for this association, results showed that the association between the level of fibrinogen and physical performance could be caused by genetic factors. Also the association between the level of TNF-α and physical performance in males could be caused by genetic factors. However, other gene variations than the candidate gene polymorphisms studied here seem to explain the major part of the genetic proportion of this association.
Inflammation; Functional disability; Aging; Genetic; Twins
The Danish Twin Registry (DTR) has for more than 50 years been based on surveys and clinical investigations and over the two last decades also on register linkage. Currently these two approaches are merged within Statistics Denmark.
Here we report on three major groups of register-based research in the DTR that used the uniqueness of twinning. First, we focus on the “long-termprognosis” of being a twin compared with being a singleton and show that Danish twins have health trajectories in adulthood similar to singletons, which is a result of interest for twins and their families as well as a test of the fetal origins hypothesis that states that fetal growth restriction has long-term health consequences. Secondly, we summarise some of the most important register-based “classical twin studies”, e.g. heritability studies on lifespan and exceptional longevity. Finally, we illustrate how the co-twin control method in a register setting can be used to control for the effect of rearing environment and genetic factors in studies of the association between exposures and health.
The spectrum of register-based twin studies is very wide and have changed in accordance with methodological and data resource developments.
Aging; fetal origin; mortality; register; twin
The Danish Twin Registry is a unique source for studies of genetic, familial and environmental factors on life events, health conditions and diseases.
More than 85,000 twin pairs born 1870–2008 in Denmark.
Validity and coverage
Four main ascertainment methods have been employed. Completeness of ascertainment varies according to birth cohorts. For birth cohorts 1870–1930 both twins should survive to age 6 years. From 1931–1968 72% of all twin pairs has been ascertained, with complete ascertainment of all live born twins since 1968.
Because twins have been identified independent of traits and on a population basis, the Danish Twin Registry is well suited for studies to understand the influence of genetic and environmental factors for a wide variety of diseases and traits.
Twin register; ascertainment; multiple births; Denmark
Small studies have indicated that twinning increases the risk of oral cleft.
We used data from a Danish national population-based cohort study to investigate whether twinning was associated with isolated oral cleft, and to estimate the twin probandwise concordance rate and heritability. Twins (207 affected/130,710) and singletons (7766 affected/4,798,526) born from 1936 through 2004 in Denmark were ascertained by linkage among the Danish Facial Cleft Database, the Danish Twin Registry and the Civil Registration System. We computed oral cleft prevalence and prevalence proportion ratio for twins versus singletons, stratified for three sub-phenotypes. Probandwise concordance rates and heritability for twins were estimated for two phenotypes—cleft lip with or without cleft palate (CL/P) and cleft palate (CP).
The prevalence of oral cleft was 15.8 per 10,000 twins and 16.6 per 10,000 singletons (prevalence proportion ratio = 0.95; 95% confidence interval = 0.83 – 1.1). This prevalence was similar for monozygotic and dizygotic twins. The probandwise concordance rate was higher for CL/P for monozygotic twins than for dizygotic twins (50 % vs. 8%, respectively). A similar contrast was present for CP. Recurrence risk for both types of clefts was greater in dizygotic twins than in non-twin siblings. Heritability estimates were above 90% for both CL/P and CP.
No excess risk of oral cleft could be demonstrated for twins compared with singletons. The concordance rates and heritability estimates for both types of clefts show a strong genetic component.
We used an approach of cumulative deficits to evaluate the rate of aging in 4954 participants of the Long-Life Family Study (LLFS) recruited in the U.S. (Boston, New York, and Pittsburg) and Denmark. We used an array of 85 health-related deficits covering major health dimensions including depression, cognition, morbidity, physical performance, and disability to construct several deficit indices (DIs) with overlapping and complementary sets of deficits to test robustness of the estimates. Our study shows that the DIs robustly characterize accelerated rates of aging irrespective of specific of deficits. When a wider spectrum of health dimensions is considered these rates are better approximated by quadratic law. Exponential rates are more characteristic for more severe health dimensions. The aging rates are the same for males and females. Individuals who contracted major diseases and those who were free of them exhibited the same aging rates as characterized by the DI constructed using mild deficits. Unlike health, disability can qualitatively alter the aging patterns of the LLFS participants. We report on systemic differences in health among the LLFS centenarians residing in New York and Boston. This study highlights importance of aggregated approaches to better understand systemic mechanisms of health deterioration in long-living individuals.
aging rates; longevity; cumulative indices; Long-Life Family Study
Oral clefts are one of the most common birth defects worldwide. They require multiple healthcare interventions and add significant burden on the health and quality of life of affected individuals. However, not much is known about the long term effects of oral clefts on health and healthcare use of affected individuals. In this study, we evaluate the effects of oral clefts on hospital use throughout the lifespan.
We estimate two-part regression models for hospital admission and length of stay for several age groups up to 68 years of age. The study employs unique secondary population-based data from several administrative inpatient, civil registration, demographic and labor market databases for 7,670 individuals born with oral clefts between 1936 and 2002 in Denmark, and 220,113 individuals without oral clefts from a 5% random sample of the total birth population from 1936 to 2002.
Oral clefts significantly increase hospital use for most ages below 60 years by up to 233% for children ages 0-10 years and 16% for middle age adults. The more severe cleft forms (cleft lip with palate) have significantly larger effects on hospitalizations than less severe forms.
The results suggest that individuals with oral clefts have higher hospitalization risks than the general population throughout most of the lifespan.
Genetic variation in FOXO3A has previously been associated with human longevity. Studies published so far have been case–control studies and hence vulnerable to bias introduced by cohort effects. In this study we extended the previous findings in the cohorts of oldest old Danes (the Danish 1905 cohort, N = 1089) and middle-aged Danes (N = 736), applying a longitudinal study design as well as the case–control study design. Fifteen SNPs were chosen in order to cover the known common variation in FOXO3A. Comparing SNP frequencies in the oldest old with middle-aged individuals, we found association (after correction for multiple testing) of eight SNPs; 4 (rs13217795, rs2764264, rs479744, and rs9400239) previously reported to be associated with longevity and four novel SNPs (rs12206094, rs13220810, rs7762395, and rs9486902 (corrected P-values 0.001–0.044). Moreover, we found association of the haplotypes TAC and CAC of rs9486902, rs10499051, and rs12206094 (corrected P-values: 0.01–0.03) with longevity. Finally, we here present data applying a longitudinal study design; when using follow-up survival data on the oldest old in a longitudinal analysis, we found no SNPs to remain significant after the correction for multiple testing (Bonferroni correction). Hence, our results support and extent the proposed role of FOXO3A as a candidate longevity gene for survival from younger ages to old age, yet not during old age.
human longevity; Forkhead box O3A (FOXO3A); association study; case–control and longitudinal data
Identification of gene variants that contribute to exceptional survival may provide critical biologic information that informs optimal health across the life span.
As part of phenotype development efforts for the Long Life Family Study, endophenotypes that represent exceptional survival were identified and heritability estimates were calculated. Principal components (PCs) analysis was carried out using 28 physiologic measurements from five trait domains (cardiovascular, cognition, physical function, pulmonary, and metabolic).
The five most dominant PCs accounted for 50% of underlying trait variance. The first PC (PC1), which consisted primarily of poor pulmonary and physical function, represented 14.3% of the total variance and had an estimated heritability of 39%. PC2 consisted of measures of good metabolic and cardiovascular function with an estimated heritability of 27%. PC3 was made up of cognitive measures (h2 = 36%). PC4 and PC5 contained measures of blood pressure and cholesterol, respectively (h2 = 25% and 16%).
These PCs analysis–derived endophenotypes may be used in genetic association studies to help identify underlying genetic mechanisms that drive exceptional survival in this and other populations.
Heritability; Longevity; Endophenotypes
People aged 80 or older are the fastest growing population in high-income countries. One of the most common causes of death among the elderly is the cardiovascular disease (CVD). Lipid-lowering treatment is common, e.g. one-third of 75–84-year-old Swedes are treated with statins [
3]. The assumption that hypercholesterolaemia is a risk factor at the highest ages seems to be based on extrapolation from younger adults. A review of observational studies shows a trend where all-cause mortality was highest when total cholesterol (TC) was lowest (‘a reverse J-shaped’ association between TC and all-cause mortality). Low TC (<5.5 mmol/l) is associated with the highest mortality rate in 80+-year olds. No clear optimal level of TC was identified. A review of the few randomised controlled trials including 80+-year olds did not provide evidence of an effect of lipid-lowering treatment on total mortality in 80+-year-old people. There is not sufficient data to recommend anything regarding initiation or continuation of lipid-lowering treatment for the population aged 80+, with known CVD, and it is even possible that statins may increase all-cause mortality in this group of elderly individuals without CVD.
cholesterol; aged; 80+-year olds; lipid-lowering treatment and all-cause mortality; elderly
Our objective in this Danish population-based cohort study was to estimate the recurrence risk of isolated oral cleft (OC) for offspring of the unaffected co-twins of OC discordant twin pairs and to compare this risk to the recurrence risk in the offspring of the affected co-twin as well as to the risk in the background population.
During 1936–2004, 207 twin pairs were ascertained, among whom at least one twin had an OC. The index persons were twins discordant for OC who had children (N=117), and their offspring (N=239). The participants were ascertained by linkage between The Danish Facial Cleft Database, The Danish Twin Registry and The Danish Civil Registration System. In the study OC recurrence risk for offspring of the affected and unaffected twin and relative risk were compared to the background prevalence. We found that among 110 children of the 54 OC affected twins, two (1.8%) children had OC corresponding to a significantly increased relative risk (RR = 10; 95% CI 1.2 to 35) when compared to the frequency in the background population. Among the 129 children of the 63 unaffected twins, three (2.3%) children were affected, corresponding to a significantly increased relative risk (RR = 13; 95% CI 2.6 to 36) when compared the background prevalence. We concluded that in OC discordant twin pairs similar increased recurrence risks were found among offspring of both OC affected and OC unaffected twins. This provides further evidence for a genetic component in cleft etiology and is useful information for genetic counseling of twin pairs discordant for clefting.
recurrence risk; oral cleft; cleft lip and palate; twins; genetics
Identifying the various gene expression response patterns is a challenging issue in expression microarray time-course experiments. Due to heterogeneity in the regulatory reaction among thousands of genes tested, it is impossible to manually characterize a parametric form for each of the time-course pattern in a gene by gene manner. We introduce a growth curve model with fractional polynomials to automatically capture the various time-dependent expression patterns and meanwhile efficiently handle missing values due to incomplete observations. For each gene, our procedure compares the performances among fractional polynomial models with power terms from a set of fixed values that offer a wide range of curve shapes and suggests a best fitting model. After a limited simulation study, the model has been applied to our human in vivo irritated epidermis data with missing observations to investigate time-dependent transcriptional responses to a chemical irritant. Our method was able to identify the various nonlinear time-course expression trajectories. The integration of growth curves with fractional polynomials provides a flexible way to model different time-course patterns together with model selection and significant gene identification strategies that can be applied in microarray-based time-course gene expression experiments with missing observations.
Background. The study aimed to examine sex differences in healthcare use before and after widowhood to investigate whether reduced healthcare use among widowers compared with widows may partially explain excess mortality and more adverse health outcomes among men than women after spousal loss.
Methods. All individuals alive and aged at least 60 years in 1996 and who became widowed in the period from 1996 to 2003 were selected from the 5% sample of the total Danish population and all Danish twins. The healthcare use was assessed as the average daily all-cause and major system-specific medication use and the average annual number of visits to general physicians (GPs).
Results. The average daily use of all-cause and major system-specific medications, as well as the number of GP visits increased over the period from 1 year before and up to 5 years after a spouse's death, but there were no sex-specific patterns in the trajectories of medication use and number of GP visits after conjugal loss. Conclusion. We found little support for the hypothesis that reduced healthcare use contributes to the explanation of more adverse health outcomes after conjugal loss in men compared with women in Denmark.
Valid causal inference is central to progress in theoretical and applied psychology. Although the randomized experiment is widely considered the gold standard for determining whether a given exposure increases the likelihood of some specified outcome, experiments are not always feasible and in some cases can result in biased estimates of causal effects. Alternatively, standard observational approaches are limited by the possibility of confounding, reverse causation, and the nonrandom distribution of exposure (i.e., selection). We describe the counterfactual model of causation and apply it to the challenges of causal inference in observational research, with a particular focus on aging. We argue that the study of twin pairs discordant on exposure, and in particular discordant monozygotic twins, provides a useful analog to the idealized counterfactual design. A review of discordant-twin studies in aging reveals that they are consistent with, but do not unambiguously establish, a causal effect of lifestyle factors on important late-life outcomes. Nonetheless, the existing studies are few in number and have clear limitations that have not always been considered in interpreting their results. It is concluded that twin researchers could make greater use of the discordant-twin design as one approach to strengthen causal inferences in observational research.
discordant-twin design; causal inference; twin research; lifestyle influences in aging