DIABETES CARE, VOLUME 23, NUMBER 5, MAY 2000 595 D iabetes is one of the most common chronic diseases in the U.S., having a prevalence of 7.5% among people aged �20 years (1). Diabetes prevalence rates vary widely in different regions of the world. In some traditional communities, such as those of the Mapuche Indians and Melanesians, diabetes is rare or absent, whereas high prevalence rates are observed in some Arab, Asian Indian, Chinese, and Hispanic American populations (2). In the U.S., diabetes is more common in African- Americans, Mexican Americans, Japanese Americans, and Native Americans than in non-Hispanic Caucasians (1). Type 2 diabetes accounts for 95% of all diabetes. Although both insulin resistance and �-cell dysfunction are well docu- mented, the molecular basis of type 2 dia- betes is poorly understood (3,4). It has long been considered a disorder resulting from both genetic and nongenetic influ- ences (5). The current understanding of the genetic basis of diabetes is largely restricted to a few distinct monogenic forms of the disease with clear Mendelian modes of inheritance (3,4); there has been limited progress in identifying specific genetic defects responsible for the most common form(s) of type 2 diabetes, which is likely to be heterogeneous and polygenic. The Amish Family Diabetes Study was initiated in 1995 with the goal of identify- ing the genetic determinants of type 2 dia- betes and related traits through positional cloning approaches. The Amish, named after their original leader Jacob Ammann, immigrated from western Europe (mainly Switzerland) to the U.S. to escape religious persecution over a 50-year period begin- ning in 1727 (6). The earliest immigrants settled in Pennsylvania. Later groups settled in Ohio, Indiana, and Illinois. There are no longer any Amish living in Europe. Approximately 200 of these families settled in Lancaster County, Pennsylvania, and can be considered the founders of today’s Lan- caster Amish community (7). Today’s Amish population in the Lancaster area exceeds 30,000 (8). All Amish are ruralites, and most earn their living by farming. They are resistant to From the Southwest Foundation for Biomedical Research (W.-C.H., B.D.M.), San Antonio, Texas; Axys Phar- maceuticals, La Jolla, California (R.A., H.S., C.J.B.); the University of Maryland School of Medicine (T.P., A.R.S.), Baltimore, Maryland; Glaxo Wellcome (M.G.E., M.J.W., P.L.S.J., D.K.B.), Research Triangle Park, North Carolina; the Hagedorn Research Institute, Gentofte, Denmark (B.K.M.); and the National Institute of Diabetes and Digestive and Kidney Diseases (W.C.K.), the National Institutes of Health, Phoenix, Arizona. Address correspondence and reprint requests to Alan R. Shuldiner, MD, Division of Endocrinology, Dia- betes and Nutrition, University of Maryland School of Medicine, 725 W. Lombard St., Room S-422, Balti- more, MD 21201. E-mail: ashuldin@medicine.umaryland.edu. Received for publication 28 September 1999 and accepted in revised form 22 December 1999. D.K.B., M.G., P.L.S.J., and M.J.W. are employed by Glaxo Wellcome; W.-C.H. and A.R.S. have received funding and salary support from Glaxo Wellcome; and T.P. is employed by the Division of Endocrinology, Diabetes and Nutrition at the University of Maryland, which has received research funds and salary support from Glaxo Wellcome. H.S. is employed by Parke-Davis Pharmaceuticals, and B.M. is employed by and holds stock in Novo Nordisk A/S. C.J.B. is employed by and R.A. holds stock in Axys Pharmaceuticals. Abbreviations: ANOVA, analysis of variance; AUC, area under the curve; dBP, diastolic blood pressure; IFG, impaired fasting glucose; IGT, impaired glucose tolerance; LADA, latent autoimmune diabetes of adults; NHANES III, Third National Health and Nutrition Examination Survey; OGTT, oral glucose tolerance test; OOA, Old Order Amish; sBP, systolic blood pressure; STR, subscapular-to-triceps ratio; WHR, waist-to-hip ratio. A table elsewhere in this issue shows conventional and Système International (SI) units and conversion factors for many substances. Diabetes in the Old Order Amish Characterization and heritability analysis of the Amish Family Diabetes Study O R I G I N A L A R T I C L E OBJECTIVE — The Old Order Amish (OOA) are a genetically well-defined closed Caucasian founder population. The Amish Family Diabetes Study was initiated to identify susceptibility genes for type 2 diabetes. This article describes the genetic epidemiology of type 2 diabetes and related traits in this unique population. RESEARCH DESIGN AND METHODS — The study cohort comprised Amish probands with diabetes who were diagnosed between 35 and 65 years of age and their extended adult family members. We recruited 953 adults who represented 45 multigenerational families. Phenotypic characterization included anthropometry, blood pressure, diabetes status, lipid profile, and leptin levels. RESULTS — The mean age of study participants was 46 years, and the mean BMI was 26.9 kg/m2. Subjects with type 2 diabetes were older, more obese, and had higher insulin levels. The prevalence of diabetes in the OOA was approximately half that of the Caucasian individuals who participated in the Third National Health and Nutrition Examination Survey (95% CI 0.23–0.84). The prevalence of diabetes in the siblings of the diabetic probands was 26.5% compared with a prevalence of 7.0% in spouses (λs = 3.28, 95% CI 1.58–6.80). The heritability of diabetes-related quantitative traits was substantial (13–70% for obesity-related traits, 10–42% for glucose levels, and 11–24% for insulin levels during the oral glucose tolerance test; P = 0.01 to �0.0001). CONCLUSIONS — Type 2 diabetes in the Amish has similar phenotypic features to that of the overall Caucasian population, although the prevalence in the Amish community is lower than that of the Caucasian population. There is significant familial clustering of type 2 diabetes and related traits. This unique family collection will be an excellent resource for investigating the genetic underpinnings of type 2 diabetes. Diabetes Care 23:595–601, 2000 WEN-CHI HSUEH, PHD BRAXTON D. MITCHELL, PHD RAMI ABUROMIA, BA TONI POLLIN, MS HAKAN SAKUL, PHD MARGARET GELDER EHM, PHD BIRGITTE K. MICHELSEN, MSC MICHAEL J. WAGNER, PHD PAMELA L. ST. JEAN, PHD WILLIAM C. KNOWLER, MD, DR PH DANIEL K. BURNS, PHD CALLUM J. BELL, PHD ALAN R. SHULDINER, MD E p i d e m i o l o g y / H e a l t h S e r v i c e s / P s y c h o s o c i a l R e s e a r c h 596 DIABETES CARE, VOLUME 23, NUMBER 5, MAY 2000 Diabetes in the Amish assimilation into the surrounding dominant culture and are widely known for their old- fashioned social and technological practices and characteristic dress. They do not pros- elytize and do not allow outsiders to marry into the sect. They represent both a religious and a genetic isolate (6). There is a high degree of consanguinity in the Old Order Amish (OOA). Although first-cousin mar- riages are not permitted, on average, OOA married couples are more closely related than second cousins once removed but less related than second cousins (9). Other fea- tures of this population include low reloca- tion rates, a relatively high standard of living, large family sizes (average sibship size 6–7), and essentially complete genealo- gies dating back to the early 1700s (12–14 generations). All of these characteristics facilitate the collection of large families and extended pedigrees for genetic studies. In this initial report, we describe the design of the Amish Family Diabetes Study, the genetic epidemiology of diabetes, and the heritability of diabetes-related traits in this unique founder population. RESEARCH DESIGN AND METHODS Study recruitment Subject recruitment for the Amish Family Diabetes Study began in February 1995. The protocol was approved by the Institu- tional Review Board of the University of Maryland. Informed consent, including permission to contact relatives, was obtained before participation. Individuals with adult-onset diabetes were identified by door-to-door interviews and by word- of-mouth. Probands were defined as indi- viduals with previously diagnosed diabetes with age at diagnosis between 35 and 65 years. The diabetic probands’ first- and sec- ond-degree family members aged �18 years were recruited. If another diabetic individual was identified in the family (e.g., an aunt or uncle), then the family was expanded further to include that person’s first- and second-degree relatives aged �18 years. The efficiency of this type of sequen- tial sampling strategy for genetic linkage studies has been previously described (10). Phenotypic characterization Study subjects were examined either at the Amish Diabetes Research Clinic in Stras- burg, Pennsylvania, or at their homes. Height and weight were measured using a stadiometer and calibrated scale with shoes removed and in light clothing. Waist cir- cumference was measured at the level of the umbilicus, and hip circumference was measured at the widest protuberance across the pelvis. Skinfold thickness was mea- sured with calipers in triplicate at 2 sites, the subscapular and triceps. The ratios of waist-to-hip circumference (WHR) and subscapular-to-triceps skinfold thickness (STR) were calculated as indexes of abdom- inal and central adipose distributions. Sys- tolic (first phase) and diastolic (fifth phase) blood pressure were obtained in duplicate by use of a standard sphygmomanometer with the subject sitting for at least 5 min and was recorded to the nearest 1 mmHg. After an overnight fast, an indwelling angiocatheter was placed in an antecubital vein. After acquisition of a fasting blood sample, a 75-g oral glucose tolerance test (OGTT) was administered. Blood samples were then drawn for determination of glu- cose and insulin values at 30-min intervals for 3 h during the OGTT. Glucose concen- trations were assayed with a Beckman glu- cose analyzer (Beckman Coulter, Fullerton, CA) using the glucose oxidase method (interassay coefficient of variation = 1.52%) (11). Insulin and leptin levels were deter- mined by radioimmunoassay (Linco, St. Louis, MO) (interassay coefficients of varia- tion = 4.42 and 4.25%, respectively). The total glucose and insulin areas under the curve (AUCs) during the 3-h OGTT were determined with the trapezoid method. HbA1c levels were measured by high- pressure liquid chromatography (interassay coefficient of variation = 4.3% for low stan- dard and 2.5% for high standard), and the fasting lipid profile (total cholesterol, HDL cholesterol, and triglyceride levels) was assayed by Quest Diagnostics (Baltimore, MD) (interassay coefficient of variation = 1.6% for total cholesterol, 5.0% for HDL cholesterol, and 1.6% for triglycerides). Levels of antibodies to GAD, a marker of immune destruction of pancreatic �-cells, were measured by radioligand-binding assay (12) in a subset of 455 subjects (48 subjects with diabetes). Previously known diabetes was deter- mined by a self-report of diabetes and any of the following: 1) a single fasting venous plasma glucose level �7 mmol/l or a 2-h OGTT venous plasma glucose level �11.1 mmol/l; 2) current treatment with insulin or oral hypoglycemic agents; or 3) con- firmed diagnosis by a physician. Newly defined diabetes was determined by the lack of diabetes history by self-report and positive OGTT results (either a fasting venous plasma glucose level �7 mmol/l or a 2-h OGTT venous plasma glucose level �11.1 mmol/l). Impaired glucose toler- ance (IGT) was defined as a 2-h OGTT venous plasma glucose level �7.8 mmol/l but �11.1 mmol/l, and impaired fasting glucose (IFG) was defined as a fasting plasma glucose level �6.1 mmol/l but �7 mmol/l. The definitions of IGT and IFG excluded subjects with diabetes. Statistical methods To obtain descriptive details of the collec- tion, clinical characteristics of study sub- jects were computed as unadjusted means ± SD. Means were compared using analysis of variance (ANOVA) (SYSTAT 8.0; SPSS, Chicago); P values were adjusted for age and sex, and, where appropriate, BMI. To obtain unbiased estimates of the prevalence of diabetes and IGT in the overall OOA population, we compared age-specific prevalence rates in spouses of OOA dia- betic probands and their family members. We compared diabetes and IGT prevalence in the OOA with corresponding rates from the general U.S. Caucasian population, as estimated from the Third National Health and Nutrition Examination Sur vey (NHANES III) (1). Using the indirect method of age adjustment (13), we applied prevalence rates obtained from the NHANES III to determine the expected number of diabetes cases in the OOA. Direct comparisons of diabetes prevalence between OOA and the NHANES III were then obtained by comparing the number of cases observed in the OOA with the num- ber expected if the OOA had the same age- specific prevalence rates as the NHANES III. The confidence interval for the ratio of observed-to-expected was also calculated (13). The λs was estimated by comparing the prevalence rates of type 2 diabetes between siblings of diabetic probands and spouses of OOA diabetic probands and their family members, adjusting for age with the Mantel-Haenszel procedure (14). The heritabilities of diabetes- and obe- sity-related traits were computed using standard quantitative genetic methods (15). Heritability was defined in the narrow sense as the proportion of total phenotypic vari- ance that could be attributed to the additive effects of genes. It was estimated as a func- tion of the covariance among all possible relationship pairs in the pedigree. We simul- taneously adjusted for the effects of age, age2, and sex. Parameter estimates were obtained using maximal likelihood proce- dures, as implemented in the SOLAR soft- ware package (Southwest Foundation for Biomedical Research, San Antonio, TX) (16). Although it is possible to combine all of the study participants into a single 11- generation pedigree (17), it was not com- putationally feasible to perform heritability analysis with such a complex pedigree structure. Therefore, we combined families with overlapping individuals so that the original 45 families were reduced to 8 pedi- grees ranging in size from 3 to 844 individ- uals. Subjects with unknown diabetes status due to missing data (n = 25) were excluded from all analyses, and subjects with diabetes (n = 109) were excluded from heritability analysis of insulin. Similarly, subjects cur- rently taking antihypertensive medications were excluded from heritability analysis of blood pressure. Data on HbA1c, insulin, lep- tin, and triglycerides were transformed by their natural logarithms to normalize the data distributions. RESULTS — The Amish Family Dia- betes Study was well received by the OOA community, and the participation rate was excellent (�80%). Of those who did not participate, the most common reasons given were 1) lived over 60 miles from the clinic, 2) personal reasons, and 3) debili- tating illnesses that precluded their partic- ipation. By the end of March 1998, 953 adults aged �18 years from 45 multigen- erational families were studied. The mean sibship size was 4.5 (range 1–16). The 8 extended pedigrees formed by combining these 45 families provided a very large number of relationship pairs for our genetic analyses (Table 1). Overall, 53% of the study subjects were women. The overall frequency of diabetes in this study population ascertained through a diabetic proband was 11.7%. Diabetes was significantly more common in women (13.5%) than men (9.7%) (P = 0.02), although after removing probands from the sample, the prevalence in men and women was approximately similar (10.5% in women, 7.3% in men, P = 0.1). Clinical characteristics of OOA subjects with dia- betes and IGT and/or IFG were similar to those observed in other Caucasian popula- tions (Table 2) (18,19). Compared with euglycemic subjects, those with diabetes or IGT and/or IFG were older, more obese, and had higher blood pressure and triglyc- eride levels. Amish individuals with diabetes and impaired glucose tolerance also had higher insulin levels than euglycemic indi- viduals, suggesting insulin resistance. Table 3 compares the age-specific prevalence rates of diabetes and IGT b e t w e e n s p o u s e s o f O O A d i a b e t i c probands and their family members and Caucasians from NHANES III (1). For the sake of comparison, diabetes status was defined by World Health Organization cri- teria (20) for both the OOA and NHANES III. As in NHANES III Caucasians, the prevalence of diabetes increased with age in the OOA. Diabetes prevalence was lower in the OOA across all age groups than in the general U.S. Caucasian population. The overall prevalence of diabetes in the OOA was 5.0% (14 of 280 subjects). A total of 22.4 cases would be expected among sub- jects aged 40–74 years if the OOA experi- enced the same prevalence rates as the NHANES III. Thus, the diabetes prevalence in the OOA was only 0.54 times that of the national rate for U.S. Caucasians (95% CI 0.23–0.84). In contrast, the prevalence of IGT in the OOA was 1.38 times that of N H A N E S I I I C a u c a s i a n s ( 9 5 % C I 0.93–1.83). The mean BMI in all of the age groups was comparable with those of NHANES III (data not shown) (21). Diabetes was diagnosed in a total of 109 OOA individuals. Characteristics of these diabetic subjects are summarized in Table 4. Of those individuals, 57% were newly diagnosed at our research clinic. Approximately 16% of the diabetic subjects DIABETES CARE, VOLUME 23, NUMBER 5, MAY 2000 597 Hsueh and Associates Table 1—Number of pairwise relationships among examined subjects in the Amish Family Diabetes Study Pairwise relationship n Parent-offspring 658 Sib-sib 1,568 Grandparent-grandchild 139 Avuncular 2,346 Half-sib 8 Grand-avuncular 490 Great-grand–avuncular 22 First cousins 4,750 First cousins, once removed 5,560 First cousins, twice removed 1,124 First cousins, three times removed 25 Second cousins 4,169 Second cousins, once removed 3,231 Third cousins 2,064 Third cousins, once removed 518 Total 26,672 Data are n from 8 pedigrees consisting of 953 subjects. Table 2—Clinical characteristics of study subjects by diabetes status Variable Euglycemic IGT and/or IFG* Diabetes n 659 160 109 Age 41.7 ± 14.4 51.1 ± 15.4† 60.8 ± 14.8† Sex (M/F) 335/324 56/104 42/67 BMI (kg/m2) 26.4 ± 4.4 28.3 ± 5.1‡ 28.0 ± 5.7‡ Leptin (ng/ml) 9.0 ± 9.5 13.7 ± 12.6‡ 13.5 ± 9.7† Waist (cm) 90.1 ± 11.2 94.1 ± 11.4‡ 95.9 ± 12.3† WHR 0.86 ± 0.06 0.86 ± 0.06 0.88 ± 0.06‡ STR 0.38 ± 0.22 0.41 ± 0.20 0.46 ± 0.23‡ sBP (mmHg) 118.6 ± 13.8 127.3 ± 18.0† 136.1 ± 24.1† dBP (mmHg) 77.2 ± 9.0 80.6 ± 9.7‡ 81.0 ± 11.9‡ Total cholesterol (mmol/l) 5.38 ± 1.13 5.77 ± 1.33 5.83 ± 1.33‡ HDL cholesterol (mmol/l) 1.32 ± 0.35 1.31 ± 0.32 1.28 ± 0.35 Triglyceride (mmol/l) 0.85 ± 0.48 1.09 ± 0.63‡ 1.15 ± 0.66† Glucose (mmol/l) Fasting 5.0 ± 0.4 5.3 ± 0.6† 8.1 ± 3.8† OGTT at 2 h 5.6 ± 1.2 8.7 ± 1.2† 14.6 ± 4.4† Insulin (pmol/l) Fasting 65 ± 39 74 ± 39 86 ± 41† OGTT at 2 h 206 ± 161 426 ± 296† 400 ± 287† Data are n unadjusted means ± SD. P values for BMI, leptin, waist, WHR, and STR were adjusted for age and sex; P values for all other traits were adjusted for age, sex, and BMI. Of the study subjects, 25 with unknown diabetes status were excluded. *Of 160 subjects, 142 had IGT, 12 had IFG, and 6 had both IGT and IFG. †P � 0.001 between the euglycemic group and the IGT/IFG or diabetic groups; ‡P � 0.05 between the eugly- cemic group and the IGT/IFG or diabetic groups. dBP, Diastolic blood pressure; sBP, systolic blood pressure. were diagnosed before the age of 35 years. More than half of all previously known cases were on insulin (53.2%), whereas only 6.4% of previously known cases did not take any medications for diabetes. The prevalence of GAD antibody pos- itivity in diabetic subjects whose age at diagnosis was �35 years was 10.0%. This rate did not differ significantly from that in subjects with either normal glucose toler- ance (6.6%) or IGT (5.6%). In contrast, the prevalence of GAD antibody positivity was 50% in diabetic subjects with an age at diagnosis �35 years, which is significantly higher than that in nondiabetic subjects (P � 0.001). These results are in accor- dance with the hypothesis that a large pro- portion of diabetes cases with an age at diagnosis �35 years may have autoim- mune type 1 diabetes or latent autoim- mune diabetes of adults (LADA) (22), and, therefore, we excluded subjects with age of diagnosis �35 years (n = 17) from further analyses of type 2 diabetes–related traits. To determine the magnitude of familial aggregation of type 2 diabetes in the OOA, we compared prevalence rates of diabetes (age at diagnosis �35 years) between sib- lings of diabetic probands and the spouses of probands and their family members. The prevalence of type 2 diabetes was signifi- cantly higher in the probands’ siblings aged �35 years (9 of 34 [26.5%]) than in the spouse group (13 of 187 [7.0%]). The sib- ling relative risk (λs) adjusted for age by the Mantel-Haenszel procedure was 3.28 (95% CI 1.58–6.80). In contrast, the prevalence of IGT was similar between the 2 groups (23.5% among diabetic probands’ siblings vs. 20.9% in the spouse group). Heritability estimates for obesity, blood pressure, lipids, HbA1c, glucose, and insulin in the Amish Family Diabetes Study are shown in Table 5. Heritability of both BMI and leptin was 42%. Heritability of the body composition measures were 37% for waist circumference, 13% for WHR, and 70% for STR (P � 0.001 for all). Concen- trations of lipids were also highly heritable, with familiality accounting for 35, 50, and 54% of variation in triglycerides, HDL-cho- lesterol, and total cholesterol, respectively (P � 0.0001 for all). Familiality accounted for 18 and 24% of the variation in systolic and diastolic blood pressure (P � 0.0001), 31% of the variation in HbA1c values (P = 0.002), 10–42% of the variation in glucose levels (P � 0.0001), and 11–24% of the variation in insulin levels during the OGTT (P = 0.014 for fasting insulin and P � 0.0001 for insulin at 120 min). Among adult subjects, the majority of men made their living as farmers (40%) and laborers (33%), and the majority of women identified their occupations as housewives (64%), farmers’ wives (10%), or shopkeepers, teachers, or craft makers (e.g., quilting) (21%). Of our respondents, 87% said they had no leisure physical activ- ity, and for those who did, such activities were mostly walking and playing ball (baseball, volleyball, or table tennis). Approximately 42% of men reported they had ever smoked cigarettes, compared with only 1.4% of women. Among ever-smok- ing men, most started smoking cigarettes between ages 16 and 20 years, but then stopped smoking in their early 20s. Less than 3% of all OOA subjects reported that they currently smoked. CONCLUSIONS — The Amish Fam- ily Diabetes Study was designed to elucidate the genetic epidemiology of type 2 diabetes in a genetically well-defined founder popu- lation. The study population included �5% of the total adult OOA population in the Lancaster area. Participation rates for the study were high, resulting in the enroll- ment of large families. We were further able to reconstruct the complex pedigree struc- 598 DIABETES CARE, VOLUME 23, NUMBER 5, MAY 2000 Diabetes in the Amish Table 3—Age-specific prevalence rates of diabetes and IGT in the spouses of OOA diabetic probands, their family members, and the NHANES III Caucasian cohort, based on WHO criteria Diabetes IGT NHANES III Observed Expected NHANES III Observed Expected Age-group (years) Caucasians* in OOA in OOA† Caucasians* in OOA in OOA‡ 20–39 NA† 1.1 (1/93) NA NA 6.5 (6/93) NA 40–49 5.2 2.8 (2/72) (3.7/72) 11.1 12.5 (9/72) (8.0/72) 50–59 13.0 7.4 (4/54) (7.0/54) 13.3 20.4 (11/54) (7.2/54) 60–74 22.3 11.5 (6/52) (11.6/52) 20.9 30.8 (16/52) (10.9/52) �75 NA 11.1 (1/9) NA NA 33.3 (3/9) NA Overall§ 14.3 6.7 (12/178) 12.6 (22.4/178) 15.3 20.2 (36/178) 14.6 (26.1/178) Data are % (n). *Data are from reference 1; †ratio of observed-to-expected among those subjects aged 40–74 years was 0.54 (95% CI 0.23–0.84); ‡ratio of observed-to- expected among those subjects aged 40–74 years was 1.38 (0.93–1.83); §prevalence rate for subjects aged 40–74 years. NA, not available. Table 4—Characteristics of 109 subjects with diabetes Diabetic characteristic n (%) Newly diagnosed diabetes 62/109 (56.9) Age at diagnosis �35 years 17/109 (15.6) Mean HbA1c (%) 7.04 ± 2.26 Previously diagnosed diabetes 47/109 (43.1) On insulin 25/47 (53.2) On oral agents 19/47 (40.4) On no medication 3/47 (6.4) Diabetic cases with positive GAD antibody* — Age at diabetes diagnosis �35 years 4/8 (50.0) Age at diabetesdiagnosis �35 years 4/40 (10.0) Data are n (%) or means ± SD. *GAD antibody was measured in a subset of 455 subjects (48 with diabetes). The prevalence rates of GAD antibody positivity in euglycemic subjects and subjects with IGT were 6.6 and 5.6%, respectively. ture of the study population through exten- sive interviews with participating subjects and by record linkage with the extensive genealogical database previously compiled for the Lancaster County OOA population (17). The mean kinship coefficient in this population is �0.037, indicating that the average degree of relatedness between any two random individuals was less than that of first-cousins but greater than that of sec- ond-cousins (23). The phenotypic characteristics of adult- onset diabetes in the OOA are similar to typ- ical type 2 diabetes and, thus, atypical and/or monogenic forms of diabetes (e.g., maturity onset diabetes of the young, mater- nally-inherited diabetes and deafness, and type A syndrome of extreme insulin resis- tance) are uncommon in the Amish. Based on the association between age of diabetes diagnosis and GAD antibody prevalence, it is likely that both types 1 and 2 diabetes exist in this population. To minimize the possi- bility that probands for our study had type 1 diabetes, we required that age of diabetes onset in the proband be at least 35 years. An unexpected result obtained from this study was the lower prevalence of diabetes observed among the OOA compared with the general U.S. Caucasian population. The observed prevalence of diabetes among spouses of probands and their family mem- bers was only 0.54 as high as that expected if these individuals had experienced the same diabetes risk as in the general U.S. Caucasian population. Interestingly, the prevalence of IGT in the OOA was similar to or slightly higher in spouse control subjects, as compared with the general U.S. Cau- casian population. This raises the interesting hypothesis that the OOA may not be pro- tected against glucose intolerance, but fewer individuals with impaired glucose tolerance eventually develop overt diabetes. By virtue of their predominantly agrarian lifestyle, the OOA may be more physically active than the general U.S. Caucasian population. One speculation is that this relatively active lifestyle provides partial protection against conversion from IGT to type 2 diabetes. Several studies have reported beneficial effects of physical activity on insulin sensi- tivity and glucose tolerance (24,25). Diabetes aggregates in families in the OOA as it does in other populations (26,27). In this study, the prevalence of dia- betes was 3.28 times as high in the siblings of diabetic probands than in the spouse control group, a finding similar to that observed in other Caucasian populations (28–30). By contrast, there was no familial aggregation of IGT in the OOA. These find- ings suggest that genes may be important determinants of progression of IGT to dia- betes in the Amish. To define type 2 diabetes genes, there may be value in dissecting the type 2 dia- betes phenotype into genetically less com- plex traits that may be more proximal to the underlying pathophysiology. We observed moderate heritabilities for many of these quantitatively distributed traits, including glucose, insulin, obesity, blood pressure, and lipid levels. The heritability estimates we obtained from the Amish are in the range of those reported for other popula- tions (Table 5). The OOA (41,42) and other founder populations (43,44) have been used suc- cessfully to map genes for simple Mendelian diseases, especially those characterized by recessive transmission. More recently, founder populations have been used to map genes for complex diseases (45,46), includ- ing diabetes and related quantitative traits (47,48). To understand further the genetic contribution to type 2 diabetes, we have ini- tiated additional studies of the OOA, including a genome scan. Because of their unique ancestral history, there may be advantages to trying to identify diabetes susceptibility genes in this population. First, because of the relatively small number of founders, it is possible that a complex genetic disease like type 2 diabetes will have equally strong genetic determinants in this population, as compared with the general Caucasian population, but will be attributed to a smaller number of genes, which should facilitate their identification and characteri- zation. Second, diabetes susceptibility genes present in the Amish are likely to be a sub- set of those that are relevant to the general Caucasian population. Third, the large fam- ily structure characteristic of the OOA and the large number of individuals for which quantitative trait information has been obtained lead to greater power because of the very large number of relative pairs that are present. Fourth, uniformity of lifestyle may minimize potential confounding effects of variable phenotypic expression of sus- ceptibility genes, which would further enhance our ability to identify these genes. However, the use of founder populations to map genes for complex disease poses spe- cial challenges. For example, the complex pedigree structures can greatly complicate the estimation of allele-sharing among pedi- gree members. In addition, once linkage is observed, fine mapping may be difficult if linkage disequilibrium extends over large regions of the chromosome. Furthermore, the genetic defects identified in these pop- ulations will need to be verified in other populations to evaluate their implication in public health. In summary, the OOA are a genetically and socioculturally well-defined founder population. Type 2 diabetes in the OOA is DIABETES CARE, VOLUME 23, NUMBER 5, MAY 2000 599 Hsueh and Associates Table 5—Heritability estimates of diabetes-related traits in the OOA h2 Reported Phenotype n h2 P by others References BMI 889 0.42 ± 0.07 �0.0001 0.21–0.79 31–33,35–37,39,40 Ln (leptin) 868 0.42 ± 0.07 �0.0001 0.39–0.73 35–37 Waist 887 0.37 ± 0.07 �0.0001 0.81 38 WHR 887 0.13 ± 0.05 0.0005 0.06–0.39 34,36,37 STR 887 0.70 ± 0.05 �0.0001 0.24–0.32 33,39 sBP (mmHg) 855 0.18 ± 0.06 �0.0001 0.15–0.42 32,33,40 dBP (mmHg) 855 0.24 ± 0.07 �0.0001 0.25–0.30 32,33,40 Total cholesterol 846 0.54 ± 0.08 �0.0001 0.37–0.51 31–33,40 HDL cholesterol 847 0.50 ± 0.07 �0.0001 0.45–0.66 31–33,40 Ln (triglycerides) 843 0.35 ± 0.07 �0.0001 0.13–0.53 31–33,40 Ln (HbA1c) 847 0.31 ± 0.06 0.0022 NA NA Fasting glucose 886 0.10 ± 0.04 �0.0001 0.18–0.64 33,35,40 Glucose 120 807 0.30 ± 0.06 �0.0001 0.16–0.51 33,35,40 Glucose AUC 761 0.42 ± 0.06 �0.0001 NA NA Ln (fasting insulin) 790 0.11 ± 0.06 0.0143 0.19–0.65 33,35,38,40 Ln (insulin 120) 742 0.24 ± 0.08 �0.0001 0.13–0.48 33,40 Insulin AUC 691 0.15 ± 0.08 0.0092 NA NA Data are n from 8 pedigrees consisting of 953 subjects. dBP, Diastolic blood pressure; h2, heritability estimates; Ln, natural log transformed; NA, not available; sBP, systolic blood pressure. phenotypically similar to that of the general Caucasian population, although its preva- lence is lower. 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