Identifying genetic variants regulating MGMT gene expression - A study in monozygotic Danish twins

Identifying genetic factors affecting the regulation of the O-6-Methylguanine-DNA Methyltransferase (MGMT) gene and estimating the genetic contribution of the MGMT gene through within-pair correlation in monozygotic twin pairs is of particular importance in various types of cancer such as glioblastoma. We used gene expression data in whole blood from 448 monozygotic twins from the Middle Age Danish Twins (MADT) study to investigate genetic regulation of the MGMT gene by performing a genome-wide association study (GWAS) of the variation in MGMT expression. Additionally, we estimated within-pair dependence measures of the expression values looking for the genetic influence of significant identified genes. We identified 243 single nucleotide polymorphisms (SNPs) significantly (p < 5e-8) associated with expression of MGMT, all located on chromosome 10 near the MGMT gene. Of the 243 SNPs, 7 are novel cis-eQTLs. By further looking into the suggestively significant SNPs (increasing cutoff to p = 1e-6), we identified 11 suggestive trans-eQTLs located on chromosome 17. These variants were in or proximal to a total of seven genes, which may regulate MGMT expression. The within-pair correlation of the expression of MGMT, TRIM37, and SEPT4 provided the upper bound genetic influence of these genes. Overall, identifying cis- or trans-acting genetic variations regulating the MGMT gene can pave the way for a better understanding of the MGMT gene function and ultimately in understanding the patient's sensitivity to therapeutic alkylating agents.

Small hard drusen and associated factors in early seniority

PURPOSE

The purpose of this study was to examine the ocular and systemic risk profile of the fundus phenotype ≥ 20 small hard (macular) drusen (< 63 μm in diameter).

METHODS

This single-center, cross-sectional study of 176 same-sex twin pairs aged 30 to 80 (median 60) years was a component of a framework study of the transition from not having age-related macular degeneration to having early AMD. Drusen categories assessed using fundus photography and optical coherence tomography included small hard drusen (diameter < 63 μm), intermediate soft drusen (63-125 μm), and large soft drusen (> 125 μm), of which the soft drusen are compatible with a diagnosis of AMD.

RESULTS

Having ≥ 20 small hard drusen within or outside the macula was associated with increasing age, lower body mass index, shorter axial length, hyperopia, female sex, increasing high-density lipoprotein (HDL), high alcohol consumption, and with the presence of soft drusen.

CONCLUSIONS

Having ≥ 20 small hard drusen was associated with some AMD-related risk factors, but not with smoking, increasing body mass index, and higher blood pressure. Having ≥ 20 small hard drusen was also associated with soft drusen, in agreement with previous studies. These findings suggest that small hard drusen are not an early manifestation of AMD but the product of a distinct process of tissue alteration that promotes the development of AMD or some subtype thereof.

Heritability of retinal drusen in the Copenhagen Twin Cohort Eye Study

PURPOSE

To study age- and sex-adjusted heritability of small hard drusen and early age-related macular degeneration (AMD) in a population-based twin cohort.

METHODS

This was a single-centre, cross-sectional, classical twin study with ophthalmic examination including refraction, biometry, best-corrected visual acuity assessment, colour and autofluorescence fundus photography, and fundus optical coherence tomography. Grading and categorization of drusen was by diameter and location.

RESULTS

The study enrolled 176 same-sex pairs of twins of mean (SD) age 58.6 (9.9) years. The prevalence of the four phenotypes ≥20 small hard macular drusen (largest diameter < 63 μm), ≥20 small hard extramacular drusen, intermediate drusen (63-125 μm) anywhere, and large drusen (>125 μm) anywhere was 12.4%, 36.4%, 5.8%, and 8.4%, respectively, and the respective heritabilities, adjusted for age and sex, were 78.2% [73.5-82.9], 69.1% [62.3-75.9], 30.1% [4.1-56.1], and 65.6% [26.4-100]. Age trajectory analysis supported a gradual transition to larger numbers of small hard drusen with increasing age. The heritability of ≥20 small hard drusen was markedly lower than the 99% found in the 40% overlapping twin cohort that was seen 20 years earlier.

CONCLUSION

Numerous (≥20) small hard drusen and larger drusen that fit the definition of dry AMD were highly heritable. Small hard drusen counts increased with age. Decreasing heritability with increasing age suggests that the impact of behavioural and environmental factors on the development of small hard drusen increases with age.

Heritability and risk factors of incident small and large drusen in the Copenhagen Twin Cohort Eye Study: a 20-year follow-up

INTRODUCTION

The transition from a normal fundus to one with early drusen (≥ 20 small hard drusen) to age-related macular degeneration (AMD) in the form of drusen ≥ 63 µm in diameter is of interest, because small hard drusen may be precursors of large drusen. Study of AMD precursors lesions may provide valuable insight into factors that initiate AMD. Here, the progression of drusen was studied over an interval of 20 years in a population-based twin cohort.

METHODS

Single-center, 20-year follow-up of 138 twins, including biometry, fundus optical coherence tomography, and fundus photography. Macular characteristics were hierarchically classified as (per eye) 1) < 20 small hard drusen, 2) ≥ 20 small hard drusen, 3) drusen ≥ 63 µm, or 4) ≥ 20 small hard drusen combined with drusen ≥ 63 µm. Additive and dominant genetic effects as well as shared and non-shared environmental effects were analyzed in a bivariate biprobit model with a classic liability-threshold approach and polygenic modeling with random effects.

RESULTS

Median participant age was 59 (range 41 - 66) years. Of 25 (18%) cases of incident macular drusen, 7 had ≥ 20 small hard drusen, and 18 had drusen ≥ 63 µm at follow-up, whereas no participant had developed both traits simultaneously. Smoking was associated with incident ≥ 20 small hard drusen (p = 0.04) and incident drusen ≥ 63 µm (p = 0.003). Having ≥ 20 small hard drusen at baseline was associated with incident drusen ≥ 63 µm at follow-up (p = 0.02). Development of drusen ≥ 63 µm was attributable to 49% genetic effects and 51% environmental effects.

CONCLUSION

The risk of progressing from 0-19 small hard macular drusen per eye to having ≥ 20 small hard drusen or drusen ≥ 63 µm at follow-up was associated with smoking and genetic predisposition. Having ≥ 20 small hard drusen in the absence of drusen ≥ 63 µm at baseline was associated with incident drusen ≥ 63 µm when examined 20 years later. The study confirms that small hard macular drusen is a forewarning of AMD and that progression to AMD may be hindered by avoidance of smoking.

Cancer-related reductions in survival: extent and duration evaluated using a large cohort study of twins, 1943-2011

BACKGROUND

The time during which there is an increased risk of death for cancer survivors was evaluated in a large twin study, which allows for matching on shared components such as age, genes, and socioeconomic factors in childhood.

METHODS

By use of data from Danish registers, time to death from initial cancer was studied prospectively in twins in two different settings. The twins were diagnosed with at least one cancer in the period 1943-2011. Setting I included 5,680 same-sex twin pairs aged six and over, while Setting II included 3,218 twin individuals from age 70 and over. The study provides comparisons within twin pairs and across birth cohorts, age at diagnoses, and time at diagnosis.

RESULTS

In 2001-2011, the five-year mortality risk for a twin surviving cancer after the age of 70 was twofold that of the co-twin, regardless of sex and zygosity, and it was 1.5-fold if the twin survived the initial nine months. After five to six years, the mortality risk corresponded to that of the co-twin. In previous decades, the excess hazard risk was considerably higher for both older and younger cohorts. There were no indications of change in relative survival across old birth cohorts.

CONCLUSIONS

This large twin study suggested that for a cancer-treatment survivor diagnosed at age 70 or later, the additional mortality risk was largely absent five years later, by which time the survival relative to the co-twin was 60%.

IMPACT

Elevated mortality risk after cancer is offset after five-six years.

Familial risk and heritability of depression by age at first diagnosis in Danish twins

OBJECTIVE

Familial and genetic factors seem to contribute to the development of depression but whether this varies with age at diagnosis remains unclear. We examined the influence of familial factors on the risk of depression by age at first diagnosis.

METHODS

We included 23 498 monozygotic and 39 540 same-sex dizygotic twins from the population-based Danish Twin Registry, followed from 1977 through 2011 in nationwide registers. We used time-to-event analyses accounting for censoring and competing risk of death to estimate cumulative incidence, casewise concordance, relative recurrence risk, and heritability of first depression by age using monozygotic and same-sex dizygotic twin pairs.

RESULTS

During follow-up, a total of 1545 twins were diagnosed with depression. For twins at age 35 or younger at first depression, heritability was estimated to be 24.8% (95% confidence interval [CI], 4.6-43.1%), whereas at age 90 it was 14.7% (95% CI, 3.1-26.3%). The relative recurrence risk was higher at younger ages: At age 35, the risk was 27.7-fold (95% CI, 20.0-35.5) and 6.9-fold (95% CI, 3.9-9.8) higher than the population risk for monozygotic and same-sex dizygotic twins, respectively, while the corresponding numbers were 3.0 (95% CI, 2.3-3.6) and 1.8 (95% CI, 1.3-2.2) at age 90. Heritability seemed similar for male and female twins.

CONCLUSION

Familial risk of depression, caused either by genes or shared environment, seemed to slightly decrease with age at diagnosis and an elevated concordance risk for monozygotic over same-sex dizygotic pairs suggested a genetic contribution to the development of depression.

Posttreatment Effect of MGMT Methylation Level on Glioblastoma Survival

The DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) removes temozolomide-induced alkylation, thereby preventing DNA damage and cytotoxicity. We investigated the prognostic effect of different MGMT methylation levels on overall and progression-free survival in 327 patients with primary glioblastoma undergoing standard treatment. We obtained MGMT methylation level in 4 CpG sites using pyrosequencing. The association between MGMT methylation level and survival was investigated using Cox proportional hazards model and an extension to detect time-varying effects. We found an association between MGMT methylation level and overall survival (OS) from around 9 months after the diagnosis, with no association between MGMT methylation level and OS before that. For patients surviving at least 9 months even small increases in MGMT methylation level are significantly beneficial (HR = 0.97, 95% CI [0.96, 0.98]). The predictive ability of MGMT methylation level on OS from 9 months after diagnosis has a Harrel's C of 66%. We conclude that the MGMT methylation level is strongly associated with survival only for patients surviving beyond 9 months with considerable effects for levels much lower than previously reported. Prognostic evaluation of cut-points of MGMT methylation levels and of CpG island site selection should take the time-varying effect on overall survival into account.

Single-cell high-content imaging parameters predict functional phenotype of cultured human bone marrow stromal stem cells

Cultured human bone marrow stromal (mesenchymal) stem cells (hBM-MSCs) are heterogenous cell populations exhibiting variable biological properties. Quantitative high-content imaging technology allows identification of morphological markers at a single cell resolution that are determinant for cellular functions. We determined the morphological characteristics of cultured primary hBM-MSCs and examined their predictive value for hBM-MSC functionality. BM-MSCs were isolated from 56 donors and characterized for their proliferative and differentiation potential. We correlated these data with cellular and nuclear morphological features determined by Operetta; a high-content imaging system. Cell area, cell geometry, and nucleus geometry of cultured hBM-MSCs exhibited significant correlation with expression of hBM-MSC membrane markers: ALP, CD146, and CD271. Proliferation capacity correlated negatively with cell and nucleus area and positively with cytoskeleton texture features. In addition, in vitro differentiation to osteoblasts as well as in vivo heterotopic bone formation was associated with decreased ratio of nucleus width to length. Multivariable analysis applying a stability selection procedure identified nuclear geometry and texture as predictors for hBM-MSCs differentiation potential to osteoblasts or adipocytes. Our data demonstrate that by employing a limited number of cell morphological characteristics, it is possible to predict the functional phenotype of cultured hBM-MSCs and thus can be used as a screening test for "quality" of hBM-MSCs prior their use in clinical protocols.

Resting Heart Rate Is Not Associated with Cognitive Function

AIMS

In order to examine the hypothesis that elevated resting heart rate (RHR) is associated with impaired cognitive score, we investigated the relationship between RHR and cognitive score in middle-aged, elderly and old Danish subjects from the general population.

METHODS

Composite cognitive scores derived from the result of 5 age-sensitive cognitive tests for a total of 7,002 individuals (Middle-aged Danish twin: n = 4,132, elderly Danish twins: n = 2,104 and Danish nonagenarian: n = 766) divided according to RHR and compared using linear regression models adjusted for sex, age, previous heart conditions and hypertension. RHR was assessed by palpating radial pulse. Genetic and shared environmental confounding was addressed in intrapair analyses of 2,049 twin pairs.

RESULTS

In unadjusted multivariate models and in multivariable models adjusting for age, sex, heart conditions and hypertension, RHR was not associated with cognitive function. Furthermore, the intrapair analyses showed that RHR was not associated with cognitive score testing within twin pairs, as measured by the proportion of twin pairs in which the twin with higher RHR also was the twin with the lowest composite cognitive score (1,049 pairs of 2,049 pairs [51% (95% CI 49-53), p < 0.289]).

CONCLUSION

While elevated RHR has been shown to be associated with adverse health events and poor fitness level, RHR has no relation to cognitive function in the general population.

DNA Methylation and All-Cause Mortality in Middle-Aged and Elderly Danish Twins

Several studies have linked DNA methylation at individual CpG sites to aging and various diseases. Recent studies have also identified single CpGs whose methylation levels are associated with all-cause mortality. In this study, we perform an epigenome-wide study of the association between CpG methylation and mortality in a population of 435 monozygotic twin pairs from three Danish twin studies. The participants were aged 55-90 at the time of blood sampling and were followed for up to 20 years. We validated our results by comparison with results from a British and a Swedish cohort, as well as results from the literature. We identified 2806 CpG sites associated with mortality (false discovery rate ( FDR ) < 0.05 ), of which 24 had an association p-value below 10 - 7 . This was confirmed by intra-pair comparison controlling for confounding effects. Eight of the 24 top sites could be validated in independent datasets or confirmed by previous studies. For all these eight sites, hypomethylation was associated with poor survival prognosis, and seven showed monozygotic correlations above 35%, indicating a potential moderate to strong heritability, but leaving room for substantial shared or unique environmental effects. We also set up a predictor for mortality using least absolute shrinkage and selection operator (LASSO) regression. The predictor showed good performance on the Danish data under cross-validation, but did not perform very well in independent samples.

Familial Risk and Heritability of Colorectal Cancer in the Nordic Twin Study of Cancer

BACKGROUND & AIMS

We analyzed data from twins to determine how much the familial risk of colorectal cancer can be attributed to genetic factors vs environment. We also examined whether heritability is distinct for colon vs rectal cancer, given evidence of distinct etiologies.

METHODS

Our data set included 39,990 monozygotic and 61,443 same-sex dizygotic twins from the Nordic Twin Study of Cancer. We compared each cancer's risk in twins of affected co-twins relative to the cohort risk (familial risk ratio [FRR]). We then estimated the proportion of variation in risk that could be attributed to genetic factors (heritability).

RESULTS

From earliest registration in 1943 through 2010, there were 1861 individuals diagnosed with colon cancer and 1268 diagnosed with rectal cancer. Monozygotic twins of affected co-twins had an FRR for colorectal cancer of 3.1 (95% confidence interval [CI], 2.4-3.8) relative to the cohort risk. Dizygotic twins of affected co-twins had an FRR for colorectal cancer of 2.2 (95% CI, 1.7-2.7). We estimated that 40% (95% CI, 33%-48%) of the variation in colorectal cancer risk could be attributed to genetic factors; unique environment only accounted for the remaining liability. For colon cancer, the FRR was 3.3 (95% CI, 2.1-4.5) for monozygotic twins and 2.6 (95% CI, 1.7-3.5) for dizygotic twins. For rectal cancer, comparable estimates were 3.3 (95% CI, 1.5-5.1) for monozygotic twins and 2.6 (95% CI, 1.2-4.0) for dizygotic twins. Heritability estimates for colon and rectal cancer were 16% (95% CI, 0-46%) and 15% (95% CI, 0-50%), common environment estimates were 15% (95% CI, 0-38%) and 11% (95% CI, 0-38%), and unique environment estimates were 68% (95% CI, 57%-79%) and 75% (95% CI, 61%-88%), respectively.

CONCLUSIONS

Interindividual genetic differences could account for 40% of the variation in susceptibility to colorectal cancer; risk for colon and rectal cancers might have less of a genetic component than risk for colorectal cancer. Siblings, and particularly monozygotic co-twins, of individuals with colon or rectal cancer should consider personalized screening.

Differences in genetic and environmental variation in adult BMI by sex, age, time period, and region: an individual-based pooled analysis of 40 twin cohorts

Background: Genes and the environment contribute to variation in adult body mass index [BMI (in kg/m²)], but factors modifying these variance components are poorly understood.Objective: We analyzed genetic and environmental variation in BMI between men and women from young adulthood to old age from the 1940s to the 2000s and between cultural-geographic regions representing high (North America and Australia), moderate (Europe), and low (East Asia) prevalence of obesity.Design: We used genetic structural equation modeling to analyze BMI in twins ≥20 y of age from 40 cohorts representing 20 countries (140,379 complete twin pairs).Results: The heritability of BMI decreased from 0.77 (95% CI: 0.77, 0.78) and 0.75 (95% CI: 0.74, 0.75) in men and women 20-29 y of age to 0.57 (95% CI: 0.54, 0.60) and 0.59 (95% CI: 0.53, 0.65) in men 70-79 y of age and women 80 y of age, respectively. The relative influence of unique environmental factors correspondingly increased. Differences in the sets of genes affecting BMI in men and women increased from 20-29 to 60-69 y of age. Mean BMI and variances in BMI increased from the 1940s to the 2000s and were greatest in North America and Australia, followed by Europe and East Asia. However, heritability estimates were largely similar over measurement years and between regions. There was no evidence of environmental factors shared by co-twins affecting BMI.Conclusions: The heritability of BMI decreased and differences in the sets of genes affecting BMI in men and women increased from young adulthood to old age. The heritability of BMI was largely similar between cultural-geographic regions and measurement years, despite large differences in mean BMI and variances in BMI. Our results show a strong influence of genetic factors on BMI, especially in early adulthood, regardless of the obesity level in the population.

Heritability of resting heart rate and association with mortality in middle-aged and elderly twins

OBJECTIVE

Resting heart rate (RHR) possibly has a hereditary component and is associated with longevity. We used the classical biometric twin study design to investigate the heritability of RHR in a population of middle-aged and elderly twins and, furthermore, studied the association between RHR and mortality.

METHODS

In total, 4282 twins without cardiovascular disease were included from the Danish Twin Registry, hereof 1233 twin pairs and 1816 'single twins' (twins with a non-participating co-twin); mean age 61.7 (SD 11.1) years; 1334 (31.2%) twins died during median 16.3 (IQR 13.8-16.5) years of follow-up assessed through Danish national registers. RHR was assessed by palpating radial pulse.

RESULTS

Within pair correlations for RHR adjusted for sex and age were 0.23 (95% CI 0.14 to 0.32) and 0.10 (0.03 to 0.17) for RHR in monozygotic (MZ) and dizygotic (DZ) twin pairs, respectively. Overall, heritability estimates were 0.23 (95% CI 0.15 to 0.30); 0.27 (0.15 to 0.38) for males and 0.17 (0.06 to 0.28) for females. In multivariable models adjusting for age, gender, body mass index, diabetes, hypertension, pulmonary function, smoking, physical activity and zygosity, RHR was significantly associated with mortality (eg, RHR >90 vs 61-70 beats per min: all-cause HR 1.56 (95% CI 1.21 to 2.03); cardiovascular 2.19 (1.30 to 3.67). Intrapair twin comparison revealed that the twin with the higher RHR was significantly more likely to die first and the probability increased with increase in intrapair difference in RHR.

CONCLUSIONS

RHR is a trait with a genetic influence in middle-aged and elderly twins free of cardiovascular disease. RHR is independently associated with longevity even when familial factors are controlled for in a twin design.

Familial Risk and Heritability of Cancer Among Twins in Nordic Countries

IMPORTANCE

Estimates of familial cancer risk from population-based studies are essential components of cancer risk prediction.

OBJECTIVE

To estimate familial risk and heritability of cancer types in a large twin cohort.

DESIGN, SETTING, AND PARTICIPANTS

Prospective study of 80,309 monozygotic and 123,382 same-sex dizygotic twin individuals (N = 203,691) within the population-based registers of Denmark, Finland, Norway, and Sweden. Twins were followed up a median of 32 years between 1943 and 2010. There were 50,990 individuals who died of any cause, and 3804 who emigrated and were lost to follow-up.

EXPOSURES

Shared environmental and heritable risk factors among pairs of twins.

MAIN OUTCOMES AND MEASURES

The main outcome was incident cancer. Time-to-event analyses were used to estimate familial risk (risk of cancer in an individual given a twin's development of cancer) and heritability (proportion of variance in cancer risk due to interindividual genetic differences) with follow-up via cancer registries. Statistical models adjusted for age and follow-up time, and accounted for censoring and competing risk of death.

RESULTS

A total of 27,156 incident cancers were diagnosed in 23,980 individuals, translating to a cumulative incidence of 32%. Cancer was diagnosed in both twins among 1383 monozygotic (2766 individuals) and 1933 dizygotic (2866 individuals) pairs. Of these, 38% of monozygotic and 26% of dizygotic pairs were diagnosed with the same cancer type. There was an excess cancer risk in twins whose co-twin was diagnosed with cancer, with estimated cumulative risks that were an absolute 5% (95% CI, 4%-6%) higher in dizygotic (37%; 95% CI, 36%-38%) and an absolute 14% (95% CI, 12%-16%) higher in monozygotic twins (46%; 95% CI, 44%-48%) whose twin also developed cancer compared with the cumulative risk in the overall cohort (32%). For most cancer types, there were significant familial risks and the cumulative risks were higher in monozygotic than dizygotic twins. Heritability of cancer overall was 33% (95% CI, 30%-37%). Significant heritability was observed for the cancer types of skin melanoma (58%; 95% CI, 43%-73%), prostate (57%; 95% CI, 51%-63%), nonmelanoma skin (43%; 95% CI, 26%-59%), ovary (39%; 95% CI, 23%-55%), kidney (38%; 95% CI, 21%-55%), breast (31%; 95% CI, 11%-51%), and corpus uteri (27%; 95% CI, 11%-43%).

CONCLUSIONS AND RELEVANCE

In this long-term follow-up study among Nordic twins, there was significant excess familial risk for cancer overall and for specific types of cancer, including prostate, melanoma, breast, ovary, and uterus. This information about hereditary risks of cancers may be helpful in patient education and cancer risk counseling.

The Heritability of Breast Cancer among Women in the Nordic Twin Study of Cancer

BACKGROUND

Family history is an established risk factor for breast cancer. Although some important genetic factors have been identified, the extent to which familial risk can be attributed to genetic factors versus common environment remains unclear.

METHODS

We estimated the familial concordance and heritability of breast cancer among 21,054 monozygotic and 30,939 dizygotic female twin pairs from the Nordic Twin Study of Cancer, the largest twin study of cancer in the world. We accounted for left-censoring, right-censoring, as well as the competing risk of death.

RESULTS

From 1943 through 2010, 3,933 twins were diagnosed with breast cancer. The cumulative lifetime incidence of breast cancer taking competing risk of death into account was 8.1% for both zygosities, although the cumulative risk for twins whose co-twins had breast cancer was 28% among monozygotic and 20% among dizygotic twins. The heritability of liability to breast cancer was 31% [95% confidence interval (CI), 10%-51%] and the common environmental component was 16% (95% CI, 10%-32%). For premenopausal breast cancer these estimates were 27% and 12%, respectively, and for postmenopausal breast cancer 22% and 16%, respectively. The relative contributions of genetic and environmental factors were constant between ages 50 and 96. Our results are compatible with the Peto-Mack hypothesis.

CONCLUSION

Our findings indicate that familial factors explain almost half of the variation in liability to develop breast cancer, and results were similar for pre- and postmenopausal breast cancer

IMPACT

We estimate heritability of breast cancer, taking until now ignored sources of bias into account.

Paternal age and telomere length in twins: the germ stem cell selection paradigm

Telomere length, a highly heritable trait, is longer in offspring of older fathers. This perplexing feature has been attributed to the longer telomeres in sperm of older men and it might be an ‘epigenetic’ mechanism through which paternal age plays a role in telomere length regulation in humans. Based on two independent (discovery and replication) twin studies, comprising 889 twin pairs, we show an increase in the resemblance of leukocyte telomere length between dizygotic twins of older fathers, which is not seen in monozygotic twins. This phenomenon might result from a paternal age-dependent germ stem cell selection process, whereby the selected stem cells have longer telomeres, are more homogenous with respect to telomere length, and share resistance to aging.

Estimating Twin Pair Concordance for Age of Onset

Twin and family data provide a key source for evaluating inheritance of specific diseases. A standard analysis of such data typically involves the computation of prevalences and different concordance measures such as the casewise concordance, that is the probability that one twin has the disease given that the co-twin has the disease. Most diseases have a varying age-of-onset that will lead to age-specific prevalence. Typically, this aspect is not considered, and this may lead to severe bias as well as make it very unclear exactly what population quantities that we are estimating. In addition, one will typically need to deal with censoring in the data, that is the fact that we for some subjects only know that they are alive at a specific age without having the disease. These subjects needs to be considered age specifically, and clearly if they are young there is still a risk that they will develop the disease. The aim of this contribution is to show that the standard casewise concordance and standard prevalence estimators do not work in general for age-of-onset data. We show how one can in fact do something easy and simple even with censored data. The key is to take age into account when analysing such data.

Measuring early or late dependence for bivariate lifetimes of twins

We consider data from the Danish twin registry and aim to study in detail how lifetimes for twin-pairs are correlated. We consider models where we specify the marginals using a regression structure, here Cox's regression model or the additive hazards model. The best known such model is the Clayton-Oakes model. This model can be extended in several directions. One extension is to allow the dependence parameter to depend on covariates. Another extension is to model dependence via piecewise constant cross-hazard ratio models. We show how both these models can be implemented for large sample data, and suggest a computational solution for obtaining standard errors for such models for large registry data. In addition we consider alternative models that have some computational advantages and with different dependence parameters based on odds ratios of the survival function using the Plackett distribution. We also suggest a way of assessing how and if the dependence is changing over time, by considering either truncated or right-censored versions of the data to measure late or early dependence. This can be used for formally testing if the dependence is constant, or decreasing/increasing. The proposed procedures are applied to Danish twin data to describe dependence in the lifetimes of the twins. Here we show that the early deaths are more correlated than the later deaths, and by comparing MZ and DZ associations we suggest that early deaths might be more driven by genetic factors. This conclusion requires models that are able to look at more local dependence measures. We further show that the dependence differs for MZ and DZ twins and appears to be the same for males and females, and that there are indications that the dependence increases over calendar time.

The heritability of leucocyte telomere length dynamics

Background

Leucocyte telomere length (LTL) is a complex trait associated with ageing and longevity. LTL dynamics are defined by LTL and its age-dependent attrition. Strong, but indirect evidence suggests that LTL at birth and its attrition during childhood largely explains interindividual LTL variation among adults. A number of studies have estimated the heritability of LTL, but none has assessed the heritability of age-dependent LTL attrition.

Methods

We examined the heritability of LTL dynamics based on a longitudinal evaluation (an average follow-up of 12 years) in 355 monozygotic and 297 dizygotic same-sex twins (aged 19–64 years at baseline).

Results

Heritability of LTL at baseline was estimated at 64% (95% CI 39% to 83%) with 22% (95% CI 6% to 49%) of shared environmental effects. Heritability of age-dependent LTL attrition rate was estimated at 28% (95% CI 16% to 44%). Individually unique environmental factors, estimated at 72% (95% CI 56% to 84%) affected LTL attrition rate with no indication of shared environmental effects.

Conclusions

This is the first study that estimated heritability of LTL and also its age-dependent attrition. As LTL attrition is much slower in adults than in children and given that having a long or a short LTL is largely determined before adulthood, our findings suggest that heritability and early life environment are the main determinants of LTL throughout the human life course. Thus, insights into factors that influence LTL at birth and its dynamics during childhood are crucial for understanding the role of telomere genetics in human ageing and longevity.

The heritability of prostate cancer in the Nordic Twin Study of Cancer

BACKGROUND

Prostate cancer is thought to be the most heritable cancer, although little is known about how this genetic contribution varies across age.

METHODS

To address this question, we undertook the world's largest prospective study in the Nordic Twin Study of Cancer cohort, including 18,680 monozygotic (MZ) and 30,054 dizygotic (DZ) same-sex male twin pairs. We incorporated time-to-event analyses to estimate the risk concordance and heritability while accounting for censoring and competing risks of death, essential sources of biases that have not been accounted for in previous twin studies modeling cancer risk and liability.

RESULTS

The cumulative risk of prostate cancer was similar to that of the background population. The cumulative risk for twins whose co-twin was diagnosed with prostate cancer was greater for MZ than for DZ twins across all ages. Among concordantly affected pairs, the time between diagnoses was significantly shorter for MZ than DZ pairs (median, 3.8 versus 6.5 years, respectively). Genetic differences contributed substantially to variation in both the risk and the liability [heritability = 58% (95% confidence interval, 52%-63%)] of developing prostate cancer. The relative contribution of genetic factors was constant across age through late life with substantial genetic heterogeneity even when diagnosis and screening procedures vary.

CONCLUSIONS

Results from the population-based twin cohort indicate a greater genetic contribution to the risk of developing prostate cancer when addressing sources of bias. The role of genetic factors is consistently high across age.

IMPACT

Findings affect the search for genetic and epigenetic markers and frame prevention efforts.

Estimating heritability for cause specific mortality based on twin studies

There has been considerable interest in studying the magnitude and type of inheritance of specific diseases. This is typically derived from family or twin studies, where the basic idea is to compare the correlation for different pairs that share different amount of genes. We here consider data from the Danish twin registry and discuss how to define heritability for cancer occurrence. The key point is that this should be done taking censoring as well as competing risks due to e.g.  death into account. We describe the dependence between twins on the probability scale and show that various models can be used to achieve sensible estimates of the dependence within monozygotic and dizygotic twin pairs that may vary over time. These dependence measures can subsequently be decomposed into a genetic and environmental component using random effects models. We here present several novel models that in essence describe the association in terms of the concordance probability, i.e., the probability that both twins experience the event, in the competing risks setting. We also discuss how to deal with the left truncation present in the Nordic twin registries, due to sampling only of twin pairs where both twins are alive at the initiation of the registries.

Abstract A51: The heritability of colorectal cancer in the Nordic Twin Study of Cancer (NorTwinCan)

Background: Family history is a strong risk factor for colorectal cancer, but the extent to which familial risk can be attributed to heritability versus shared environment remains obscure. Whether familial risk is distinct for colon versus rectal cancer also remains unclear and is important to examine given accumulating evidence that their etiologies are distinct.

Methods: We estimated the concordance and heritability of colorectal, colon and rectal cancer using information from 40,634 monozygotic and 111,897 dizygotic twin pairs from the Nordic Twin Study of Cancer. Time-to-event analyses accounted for left-censoring due to variable initiation of cancer registration and right-censoring resulting from end of follow-up or competing risk of death.

Results: From earliest cancer registration in 1943 through 2009, 2,286 twins were diagnosed with colon cancer and 1,493 were diagnosed with rectal cancer. The cumulative risk of colorectal cancer overall was 4.5%, while the cumulative risk for twins whose co-twins had colorectal cancer was 15.7% for monozygotic and 11.0% for dizygotic twins. The heritability of colon cancer was 16.2% (95% CI: 2.3%-61.8%), with shared environment accounting for 14.2% (95% CI: 2.7%-49.1%) of the variation in disease liability. For rectal cancer, a model including only environmental effects best fit the data; heritability was not statistically important. Shared environment accounted for 16.6% (95% CI: 8.8%-28.8%) of the variation in liability.

Conclusions: These results suggest a greater genetic contribution to the risk of colon cancer than the risk of rectal cancer. The contribution of shared environment to both cancers may be larger than previously thought.

Citation Format: Rebecca E. Graff, Axel Skytthe, Hans-Olov Adami, Kamila Czene, Jennifer R. Harris, Jaakko Kaprio, Edward Giovannucci, Lorelei A. Mucci, Jacob B. Hjelmborg. The heritability of colorectal cancer in the Nordic Twin Study of Cancer (NorTwinCan). [abstract]. In: Proceedings of the Twelfth Annual AACR International Conference on Frontiers in Cancer Prevention Research; 2013 Oct 27-30; National Harbor, MD. Philadelphia (PA): AACR; Can Prev Res 2013;6(11 Suppl): Abstract nr A51.

Estimating twin concordance for bivariate competing risks twin data

For twin time-to-event data, we consider different concordance probabilities, such as the casewise concordance that are routinely computed as a measure of the lifetime dependence/correlation for specific diseases. The concordance probability here is the probability that both twins have experienced the event of interest. Under the assumption that both twins are censored at the same time, we show how to estimate this probability in the presence of right censoring, and as a consequence, we can then estimate the casewise twin concordance. In addition, we can model the magnitude of within pair dependence over time, and covariates may be further influential on the marginal risk and dependence structure. We establish the estimators large sample properties and suggest various tests, for example, for inferring familial influence. The method is demonstrated and motivated by specific twin data on cancer events with the competing risk death. We thus aim to quantify the degree of dependence through the casewise concordance function and show a significant genetic component.

Offspring's leukocyte telomere length, paternal age, and telomere elongation in sperm

Leukocyte telomere length (LTL) is a complex genetic trait. It shortens with age and is associated with a host of aging-related disorders. Recent studies have observed that offspring of older fathers have longer LTLs. We explored the relation between paternal age and offspring's LTLs in 4 different cohorts. Moreover, we examined the potential cause of the paternal age on offspring's LTL by delineating telomere parameters in sperm donors. We measured LTL by Southern blots in Caucasian men and women (n=3365), aged 18-94 years, from the Offspring of the Framingham Heart Study (Framingham Offspring), the NHLBI Family Heart Study (NHLBI-Heart), the Longitudinal Study of Aging Danish Twins (Danish Twins), and the UK Adult Twin Registry (UK Twins). Using Southern blots, Q-FISH, and flow-FISH, we also measured telomere parameters in sperm from 46 young (<30 years) and older (>50 years) donors. Paternal age had an independent effect, expressed by a longer LTL in males of the Framingham Offspring and Danish Twins, males and females of the NHLBI-Heart, and females of UK Twins. For every additional year of paternal age, LTL in offspring increased at a magnitude ranging from half to more than twice of the annual attrition in LTL with age. Moreover, sperm telomere length analyses were compatible with the emergence in older men of a subset of sperm with elongated telomeres. Paternal age exerts a considerable effect on the offspring's LTL, a phenomenon which might relate to telomere elongation in sperm from older men. The implications of this effect deserve detailed study.