In Project HeartBeat!, the proportions of males and females were nearly identical (50.2% and 49.8%, respectively), while in the Bogalusa cohort, there were proportionally fewer males than females (42.4% and 57.6%, respectively); the difference between cohorts was significant (p = 0.0009). The proportion of African Americans was higher in the Bogalusa cohort (30.7%) than in the Project HeartBeat! cohort (20.6%; p < 0.0001), but these proportions reflected those in the respective communities from which the cohorts were drawn. Distributions of subjects by cohort and race/sex group are shown in . Due to differences in the study designs, subjects from Project HeartBeat! were examined more frequently than those from Bogalusa; the mean ∀ SD number of examinations per subject was 8.28 ∀ 2.86 in Project HeartBeat! (ranging from 1 - 12 examinations) and 2.58 ∀ 0.88 in the Bogalusa Heart Study (ranging from 1 - 5). The mean age at the time of examination was slightly higher in the Bogalusa cohort (12.9 ∀ 2.7 vs. 12.1 ∀ 2.6 years; p < 0.0001). Numbers of examinations in each cohort and age group are shown in .
Numbers of subjects in each cohort, by race/sex group.
Distribution of observations within two-year age groups, by cohort.
SNP genotype frequencies in each race/cohort group met Hardy-Weinberg expectations (data not shown). Genotype counts and minor allele frequencies for the four FTO intron 1 polymorphisms typed are shown in by cohort and race group. Within race/ethnic groups, there were no statistically significant differences between the Bogalusa and Project HeartBeat! cohorts in genotype or allele frequencies for any of the polymorphisms, even though minor allele frequencies appeared slightly higher for three SNPs (rs17817449, rs8050136, and rs9939609) in African Americans from Bogalusa, and slightly lower for all four SNPs in non-Hispanic whites from Bogalusa. In the Bogalusa cohort, genotype frequencies differed significantly between African Americans and non-Hispanic whites for all SNPs except rs17817449; in the smaller Project HeartBeat! cohort, genotype frequencies differed significantly between African Americans and non-Hispanic whites only for rs1421085.
FTO genotypes and minor allele frequencies by race group and cohort.
There was strong pairwise linkage disequilibrium among all markers in both African Americans and non-Hispanic whites in both cohorts. R2
values exceeded 0.9 for each SNP pair in non-Hispanic whites, and while the R2
values in African Americans ranged only from 0.114 to 0.858 (Supplementary Table 1
), D’ values, which are normalized for allele frequencies, exceeded 0.9 for every marker pair in both groups (data not shown). We analyzed associations between BMI and both the individual FTO
polymorphisms and four-locus FTO
haplotypes, but with the strong linkage disequilibrium among all four polymorphisms, analyzing haplotypes provided no additional information (data not shown). To facilitate comparisons with other studies, we here report results for rs9939609; results for the other three SNPs were essentially identical.
In longitudinal mixed models that included all three genotypes, rs9939609 was associated with variation in BMI profiles in non-Hispanic whites in both cohorts, but there was no evidence of an association in African Americans in either cohort (data not shown). In non-Hispanic whites in both cohorts, the model coefficients for the A/T genotype were non-significant and closer to zero (the coefficient for the reference genotype, T/T) than they were to the coefficients for the A/A genotype, while the A/A genotype was significantly associated with an increase in BMI of approximately one kg/m2
. Thus, we combined the T/T and A/T genotypes for the analyses reported here. (Supplementary Figure 1
shows the results of models in each cohort with all three genotypes included.)
When the Bogalusa and Project HeartBeat! cohorts were analyzed separately, the A/A genotype was significantly associated with higher BMI in both cohorts, but an age-by-FTO group interaction was statistically significant only in the Bogalusa cohort (). The significant age-by-FTO group interaction in the Bogalusa cohort indicates that the difference in BMI between those with the A/A genotype and those with either T/T or A/T genotypes increases with age, with the A/A genotype being associated with a mean BMI approximately 0.7 kg/m2 higher at age 8, and 1.6 kg/m2 higher at age 17. The BMI profiles by genotype group predicted by the models shown in for males and females in both cohorts are shown in .
Table 4 Coefficients, standard errors, and p-values from the best models in each cohort for FTO rs9939609 predicting BMI profiles in non-Hispanic whites. There were no statistically significant interaction terms in the best model for the Project HeartBeat! cohort. (more ...)
Figure 1 Predicted longitudinal BMI profiles by FTO rs9939609 genotype in non-Hispanic whites in the Bogalusa Heart Study and Project HeartBeat! cohorts. Mean (∀ one SE) profiles are for males and females combined, to smooth out fluctuations in age-, sex-, (more ...)
It is possible that the age-by-genotype interaction observed in the Bogalusa cohort, with the A/A genotype having a greater effect on BMI in late adolescence, might be due to a propensity of those with higher BMI associated with the A/A genotype to gain even more weight over time. If higher BMI fosters subsequent weight gain, however, then higher BMI in early adolescence should be associated with greater BMI increases in late adolescence, regardless of FTO genotype. In Bogalusa Heart Study subjects measured at 12, 13, or 14 years of age, the mean increase in BMI after three years (the most frequent interval between examinations in the Bogalusa cohort) was actually smaller in those from the highest BMI z-score tertile at the earlier measurement (1.5 ∀ 2.6 kg/m2) than in those from the lowest tertile (1.9 ∀ 1.2 kg/m2), though the difference among z-score tertiles was not statistically significant (p = 0.09). In multiple regression models, however, the effect on subsequent BMI increases of having higher BMI when younger differed by FTO genotype. Among groups defined by FTO genotype and BMI z-score tertile at 12 – 14 years of age, the largest mean increase in BMI after three years (3.3 ∀ 2.9 kg/m2) occurred in those from the highest BMI z-score tertile who had the A/A genotype, while the smallest mean increase (1.3 ∀ 2.5 kg/m2) occurred in those from the highest BMI z-score tertile who had T/T or T/A genotypes; the interaction between BMI z-score tertile and FTO genotype was statistically significant (p = 0.01). Comparable results were found when BMI z-score at 12 – 14 years of age was included in models as a continuous variable. By itself, FTO genotype, but not earlier BMI z-score, significantly predicted change in BMI three years later; with both factors in the model, FTO genotype modified the effect of earlier BMI z-score on subsequent change in BMI (data not shown). These findings suggest that the greater effect of the A/A genotype on BMI in older adolescents from the Bogalusa cohort is not due to a tendency for higher BMI at earlier ages to accelerate subsequent weight gain.
These findings could not be replicated in the Project HeartBeat! cohort, in which no associations between BMI z-scores at 12 – 14 years of age or FTO genotype and change in BMI at 15 – 17 years of age were significant (data not shown). Because subjects in Project HeartBeat! were enrolled at 8, 11, or 14 years of age, those examined both in late adolescence and three years earlier were all from the oldest cohort, and only seven pairs of observations three years apart were available for the A/A genotype in this group. This may help account for the failure to find an age-by-FTO interaction in the Project HeartBeat! cohort comparable to that observed in the Bogalusa cohort, in which the interaction only became apparent in late adolescence.
superimposes mean BMI profiles by cohort, sex, and genotype on BMI growth curves predicted by the models in . The smaller numbers of individuals and observations with the A/A genotype produce greater variability in the mean profiles for that group, as expected, though the overall tendency for higher BMI with this genotype is apparent. The convergence or even crossover of the mean profiles by genotype around 12 - 13 years of age, which is particularly noticeable in males, may in part reflect the greater variation in maturity levels among individuals at these ages. Tanner maturity scores of 2, 3, and 4 occurred in approximately equal proportions in this age range (29%, 27%, and 25%, respectively), while at each age below 12 or above 13, one or, at most, two Tanner stages accounted for more than 80% of the scores for that age. However, adjusting for Tanner scores did not appreciably change the mean or predicted profiles shown in (data not shown).
Figure 2 Longitudinal profiles by sex and FTO rs9939609 genotype in non-Hispanic whites from the Bogalusa Heart Study (BHS) and Project HeartBeat! (PHB) cohorts, constructed from mean BMI values (∀ one SE) at each age, superimposed on predicted longitudinal (more ...)