The HAPI Heart Study was designed to facilitate the mapping of genes that influence response to four different environmental perturbations relevant to CVD. Our design is an efficient way of studying gene by environment interactions since the between person variation across different environments is eliminated. Indeed, the heritability analyses indicate that for nearly all traits, there is statistical evidence that some of this variation can be attributed to the additive effects of genes. Furthermore, since the four interventions were performed in the same individuals, we may discern genes that have pleiotropic effects, e.g., blood pressure response to CPT and blood pressure response to a high salt diet.
The Amish in Lancaster County are predominantly Old Order Amish, the most conservative of the Amish sects in terms of adherence to strict social norms and resistance to using modern technologies. All Amish are rural-living and most earn their living by farming. Marriages between Amish and those of non-Amish descent are rare. Because of their unique ancestral background, there is a high degree of consanguinity. First cousin marriages are uncommon, but on average, any two Amish individuals are more closely related than second cousins, but less related than first cousins once removed, who share 1/64 and 1/32 of their genes in common, respectively.
This Amish population has several unique characteristics that make it particularly well suited for genetic studies. First, the population is a closed founder population that is genetically homogeneous. There are extensive genealogical records spanning 12–14 generations that allow researchers to trace all living Amish to their founding ancestors in the mid-1700s with minimal pedigree errors and/or non-paternity 9;14
. Second, Amish families are characterized by very large sibships averaging 7–9 siblings each who are geographically localized. This, combined with extraordinary cooperation from the Amish community, greatly facilitates high rates of participation, remarkable adherence to complicated intervention protocols, and reliable follow-up. Third, the homogeneity of socioeconomic status and lifestyle (e.g., minimal smoking and alcohol consumption, abundant and consistent physical activity, limited medication use, and similar dietary routines) reduces sources of non-genetic variation. Despite the unique ancestral background of the Amish, it seems reasonable to expect that most common disease-predisposing alleles found in other European Caucasian populations will also be found in the Amish. Indeed, classical population genetic theory and our15
and others’ empirical data show that populations with a modest number of founders like the Amish have allele frequencies (on average) that are similar to those in the ancestral population, particularly for common alleles (e.g., minor allele frequencies > 5%).
The family-based design of the HAPI Heart Study provides additional opportunities for analyses that are not possible in conventional population-based studies. For example, heritability and linkage analysis contrast phenotypic similarities between related and unrelated individuals. Family-based tests of association that test for evidence of association in the presence of linkage, such as the TDT or FBAT, are also possible, although the added value of such tests is debatable in genetically homogenous isolate populations with minimal population stratification, such as the Amish. Lastly, the unique family structure of the HAPI Heart Study cohort facilitates investigations of genetic imprinting, maternal effects, and other non-classical genetic phenomenon that might play a role in complex diseases.
Conventional genetic association tests can also be carried out in families, with the caveat that the nonindependence of family members must be taken into account since failure to do so will inflate the variance of the genotypic effect size 13
. Analytic approaches for doing this include accounting for family structure as a random effect in a mixed effect model or through generalized estimating equations (GEE) modeling or by specifying explicitly the covariance structure as a random effect using variance component modeling. Because of the unique ancestral background of this population with its relatively small number of founders, an additional analytic approach potentially useful in the Amish is to map recessively acting genes with rare alleles through homozygosity by descent mapping 16
In conclusion, through its use of standardized interventions relevant to CVD, the HAPI Heart Study employs a powerful approach for assessing how genes modify the response to particular controlled environments. Identifying these response genes may identify new mechanisms influencing the regulation of cardiovascular health, leading eventually to improved and individualized preventive strategies and treatments and to improved early identification of high-risk individuals.