In collaboration with the University of Turku and the Turku PET Centre, elderly (born before 1938) MZ and DZ twins are being screened to identify twin pairs discordant with respect to Alzheimer's disease. Screening will be completed in 2006. Out of a large number of twin pairs, we first identified a small sample of MZ pairs discordant for dementia. These pairs were then invited for clinical assessment and underwent extensive neuroimaging using MRI and PET imaging (Järvenpää et al., 2004; Järvenpää et al., 2003a; Järvenpää et al., 2003b). We have shown that binge-drinking in middle age predicts dementia 25 years later (Järvenpää et al., 2005)
The Finnish Twin Cohorts (Kaprio et al., 1978; Kaprio, 1994) form a national resource for genetic epidemiological studies. Twins and their families have been ascertained in three stages from the Central Population Register for studies of the genetic and environmental determinants of common, chronic diseases. In this paper we will focus on recent and in-progress research based on the older twin cohort. A comprehensive list of publications and a review of studies completed by the early 1990s was published in 1994 (Kaprio, 1994).
Cohort of Older Like-sexed Twins (Born Before 1958)
The older part of the Finnish Twin Cohort consists of all Finnish twin pairs of the same gender born before 1958 with both co-twins alive in 1975. These twin pairs were selected from the Central Population Registry of Finland in 1974. Three surveys of the entire cohort have been carried out. The first questionnaire was mailed to all pairs in August-October 1975. Two follow-up questionnaire studies have been carried out in 1981 and 1990. Twin zygosity was determined by a validated questionnaire methods initially in the entire cohort (Sarna et al., 1978). In studies of selected twin pairs, genetic markers have been
used for validation. The total number of MZ and DZ twin pairs was 13,888 in the beginning of prospective follow-up in August 1975.
Expansion of Older Cohort in 1996 to Include Opposite-sex Pairs
In 1996, opposite-sex (OS) twins born 1938--1949 have been identified from the Central Population Registry. So far, questionnaire data and DNA samples from over 1500 pairs have been collected in 1997. These are pairs in which one or both of the twins reported that they have been diagnosed with one of selected chronic diseases. This new material represents a substantial new population of sibpairs, while permitting the expansion of the twin design to assess sex by gene, sex by environment as well as sex by gene by environment interactions. Questionnaire data have been used to study sex-limitation models such as for height
(Silventoinen et al., 2001), but also as phenotypic information to identify affected sibpairs for molecular genetic studies as described in Table 1.
The Components of the Finnish Twin Cohort