Recruitment

Our main recruitment strategy is to increase awareness of the twin registry's existence, along with the uniqueness of twins in research. This includes mass media advertisements for specific projects as well as publication of results in non-scientific newspapers. Motivation comes from demonstrating value for the general population as well as the individual benefits for the twins who participate. As an incentive for participation, we provide a free health checkup and free zygosity testing to twins. Aside from recruiting new twin pairs, keeping in contact with the current registry members is the second important part of maintaining a vibrant twin registry. While some twins will have been participating in ongoing studies, a larger portion may either not be suitable for the current projects or currently not available. It is crucial to keep ties and maintain involvement even if no immediate participation is possible. Regular newsletters and participation in local twin conventions are currently our main activities; extensions to our Web site for interaction between twins are under development.

Data and Samples

Data storage is accomplished in a MySQL database with Web front end. This includes all personal as well as phenotypic information. A major part of our internal resources are devoted to the continuing database development.

In most projects, complex measurement schemes are applied to balance effort in recruitment, time strain for the twins, and scientific value. As long as there is a balance between research interest and burden to the twins, combining studies is a suitable strategy to maximize the power of phenotyping in twins.

Our twin sample is a self-selected adult sample with a wide age range (38 ± 15 years) (Table 1) without specific diseases; we have directed our phenotyping at intermediate traits and studied the influence of genetic variability on normal physiological variation. Direct measurements of traits like blood pressure were extended by functional assays to determine venous function (e.g., Brinsuk et al., Reference Brinsuk, Tank, Luft, Busjahn and Jordan2004). Molecular phenotyping has been included in one study in which we counted the percentage of neutrophils expressing Proteinase 3 on their cell surface as a potential risk factor for Wegener granulomatosis, finding a heritability of 0.99 with strong indication of epistatic effects (Schreiber et al., Reference Schreiber, Busjahn, Luft and Kettritz2003). Gene expression assays to measure gene activity as an intermediate phenotype are currently under way.

TABLE 1 Number of Twin Pairs by Age, Sex, and Zygosity

While our past studies were based on status variables, we have started intervention studies, looking at changes in response to nutriceuticals. Using the monozygotic (MZ) co-twin control design, a decrease in blood pressure after an 8-week period of supplementation with an olive tree leaf extract was found (Perrinjaquet-Moccetti et al., Reference Perrinjaquet-Moccetti, Busjahn, Schmidlin, Schmidt, Bradl and Aydogan2008).

Major Research Focus and Findings

Our main focus has been on the cardiovascular system. The heritability analysis of blood pressure response to stress (Busjahn et al., Reference Busjahn, Faulhaber, Viken, Rose and Luft1996) was extended to linkage and association with candidate genes (Li et al., Reference Li, Faulhaber, Rosenthal, Schuster, Jordan, Timmermann and Busjahn2001). For resting blood pressure, with its well-established genetic variance (Williams et al., Reference Williams, Hunt, Hasstedt, Hopkins, Wu, Berry and Kuida1990), a number of candidate genes were tested based on findings from animal studies, cell-based assays, or case–control studies in hypertension (Busjahn et al., Reference Busjahn, Li, Faulhaber, Rosenthal, Becker, Jeschke and Luft2000, Reference Busjahn, Aydin, Uhlmann, Krasko, Bahring, Szelestei and Lang2002b; Firouzi et al., Reference Firouzi, Kok, Spiering, Busjahn, Bezzina, Ruijter and de Leeuw2006; Gollasch et al., Reference Gollasch, Tank, Luft, Jordan, Maass, Krasko and Bähring2002; Nagy et al., Reference Nagy, Busjahn, Bahring, Faulhaber, Gohlke, Knoblauch and Luft1999).

Other cardiovascular traits of interest included baroreflex sensitivity (Gollasch et al., Reference Gollasch, Tank, Luft, Jordan, Maass, Krasko and Bähring2002; Tank et al., Reference Tank, Jordan, Diedrich, Stoffels, Franke, Faulhaber and Busjahn2001), the electrocardiographic QT interval (Aydin et al., Reference Aydin, Bahring, Dahm, Guenther, Uhlmann, Busjahn and Luft2005; Bezzina et al., Reference Bezzina, Mannens, van der Lip, Busjahn and Wilde2001, Reference Bezzina, Verkerk, Busjahn, Jeron, Erdmann, Koopmann and Wilde2003; Busjahn et al., Reference Busjahn, Knoblauch, Faulhaber, Boeckel, Rosenthal, Uhlmann and Luft1999b, Reference Busjahn, Seebohm, Maier, Toliat, Nurnberg, Aydin and Lang2004;), lipid metabolism (Al-Kateb et al., Reference Al-Kateb, Bähring, Hoffmann, Strauch, Busjahn, Nürnberg and Luft2002; Knoblauch et al., Reference Knoblauch, Busjahn, Munter, Nagy, Faulhaber, Schuster and Luft1997, Reference Knoblauch, Busjahn, Muller, Faulhaber, Schuster, Uhlmann and Luft1999, Reference Knoblauch, Muller, Busjahn, Ben Avi, Bahring, Baron and Leitersdorf2000), and cardiac function as well as morphology as determined by echocardiography (Busjahn et al., Reference Busjahn, Knoblauch, Knoblauch, Bohlender, Menz, Faulhaber and Luft1997, Reference Busjahn, Li, Faulhaber, Rosenthal, Becker, Jeschke and Luft2000, Reference Busjahn, Schulz-Menger, Abdel-Aty, Rudolph, Jordan, Luft and Busjahn2009).

We have also included obesity-related measurements. New heritability estimates have been obtained for free (h ² = 0.28) and bound leptin (h ² = 0.72) concentrations, together with the leptin receptor (h ² = 0.56; Jordan et al., Reference Jordan, Brabant, Brinsuk, Tank, Horn, Luft and Busjahn2005). Candidate genes for obesity have also been studied (Dieter et al., Reference Dieter, Palmada, Rajamanickam, Aydin, Busjahn, Boehmer and Lang2004; Knoblauch et al., Reference Knoblauch, Busjahn, Muller, Faulhaber, Schuster, Uhlmann and Luft1999). A recent major project focused on the impact of dietary changes on hormones as well as gene expression; following 2 years of data collection, analyses are currently being conducted.

Pharmacogenetic studies have been attempted too, but recruitment proved to be less successful compared to non-interventional studies (Birkenfeld et al., Reference Birkenfeld, Jordan, Hofmann, Busjahn, Franke and Krüger2009). Radiosensitivity, a trait of utmost relevance for cancer in terms of causality as well as therapy, was studied based on cultured blood cells from MZ twins (Borgmann et al., Reference Borgmann, Haeberle, Doerk, Busjahn, Stephan and Dikomey2007, Reference Borgmann, Raabe, Reuther, Szymczak, Schlomm, Isbarn and Dikomey2010).

Psychological traits are not our main focus; however, we have studied ‘stress coping’ because stress has been implicated as a risk factor for various mental and physical diseases (von Känel, Reference von Känel2012). Heritability estimation and gene identification studies for stress coping have been carried out, showing heritability estimates between 0.21 and 0.52 for four major coping factors, as well as a significant association with the beta-2 adrenergic receptor gene (Busjahn et al., Reference Busjahn, Faulhaber, Freier and Luft1999a, Reference Busjahn, Freier, Faulhaber, Li, Rosenthal, Jordan and Luft2002a).

Collaborations

The BTR is open to collaboration with national as well as international networks. Beside common scientific endeavors, this may include the establishment of data management structures for newly emerging twin registries, based on our in-house framework. Hosting of our twin registry by a private company has added great flexibility for collaborations, academic as well as industrial. Traditionally, there have been close ties with the Medical Faculty of the Charité, where most of the clinical phenotyping takes place, and the Max-Delbrück–Center for Molecular Medicine. Other collaborations have included the University Hospital Eppendorf in Hamburg, Tübingen University, the Max-Planck Institute for Psychiatry in Munich, the Dr Margarete Fischer-Bosch Institute for Clinical Pharmacology in Stuttgart, University Hospital Jena, and the Academic Medical Center of the University of Amsterdam.

Collaborative research may include access to clinical data and DNA from earlier studies in the context of candidate gene studies, as well as prospective studies to obtain new data and biological samples. Prospective studies may include intervention studies in the context of nutriceuticals/cosmeceuticals, but not clinical trials in phase I or II.

Depending on the background of the collaboration, development and fine-tuning of methodology can be done by either party or in close collaboration. If necessary, we can involve our local partners in phenotyping and/or genotyping.

Ethical Responsibilities

All our projects have been approved either by the Ethics Committee of the Charité or the partnering research institution. There is no direct access to non-anonymized data or twin addresses. The subjects have the option of withdrawing at any time. Participants receive feedback on clinical measurements — for example, blood pressure or cholesterol — but not on gene variants.