Cardiovascular research in twin samples has suggested a clear-cut genetic contribution to hypertension (Kupper et al, 2005b; McCaffery et al, 2008), diabetes (Poulsen et al, 1999), stroke (Bak et al, 2002), and coronary heart disease (Zdravkovic et al, 2002). A landmark paper was published by twin researchers of the Karolinska Institute in Sweden (Marenberg et al, 1994). They searched the National Death Registry for death certificates on ~21,000 twins born in Sweden between 1886 and 1925, where both twins within a pair still lived within the country in 1961. The risk to have died from coronary heart disease when one's co-twin died before the age of 55 years was 8.1 among male monozy-gotic (MZ) twins as compared to 3.8 among male dizygotic (DZ) twins. Re-analysis using a correlated c-frailty model, which translates discrete yes/no traits into a continuously distributed latent liability, yielded a heritability to die from coronary heart disease of 57% in males and 38% in females (Zdravkovic et al, 2004).
The genetic contribution to cardiovascular disease end points most likely results from the joint effects of risk genes on the large number of biological and behavioral risk factors that impact on the atherosclerotic process. Table 28.8 summarizes the heritabil-ity estimates for many of these risk factors based on the study of twins and their family members registered in the Netherlands Twin Registry (NTR). The NTR was established in 1987 as a resource for prospective studies in behavior genetics and genetic epidemiology (www.tweelingenregister.org). The adult branch of register now includes over 23,000 participants that can be MZ or DZ twins, parents, siblings, children, or spouses of twins. They have been longitudinally assessed for a large number of behavioral and medical traits using biennial surveys and multiple visits to the laboratory or the university hospital. It is clear from Table 28.8 that all cardiovascular risk factors assessed were significantly heritable and often more than half of the variance in the traits was explained by genetic factors.
Again, I stress that the heritability estimates in Table 28.8 are based on a single population in a single country. Many twin studies in different countries and ethnicities, however, have confirmed the importance of genetic factors not just in the established risk factors like smoking (Li et al, 2003), physical inactivity (Stubbe et al, 2006; Beunen and Thomis, 1999), blood pressure (Evans et al, 2003), cholesterol (Beekman et al, 2002; Iliadou et al, 2005), and BMI (Schousboe et al, 2003; Silventoinen et al, 2003) but also for insulin resistance (Poulsen et al, 2001), inflammation (Su et al, 2008; Worns et al, 2006), hemostasis (Peetz et al, 2004), cardiac autonomic control (Wang et al, 2009), personality (Wray et al, 2007), and depression (Sullivan et al, 2000), with very comparable heritability estimates to those in the Dutch population.
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