PPARG by dietary fat interaction influences bone mass in mice and humans

Abstract

Adult BMD, an important risk factor for fracture, is the result of genetic and environmental interactions. A quantitative trait locus (QTL) for the phenotype of volumetric BMD (vBMD), named Bmd8, was found on mid-distal chromosome (Chr) 6 in mice. This region is homologous to human Chr 3p25. The B6.C3H-6T (6T) congenic mouse was previously created to study this QTL. Using block haplotyping of the 6T congenic region, expression analysis in the mouse, and examination of nonsynonymous SNPs, peroxisome proliferator activated receptor gamma (Pparg) was determined to be the most likely candidate gene for the Bmd8 QTL of the 630 genes located in the congenic region. Furthermore, in the C3H/HeJ (C3H) strain, which is the donor strain for the 6T congenic, several polymorphisms were found in the Pparg gene. On challenge with a high-fat diet, we found that the 6T mouse has a lower areal BMD (aBMD) and volume fraction of trabecular bone (BV/TV%) of the distal femur compared with B6 mice. Interactions between SNPs in the PPARG gene and dietary fat for the phenotype of BMD were examined in the Framingham Offspring Cohort. This analysis showed that there was a similar interaction of the PPARG gene and diet (fat intake) on aBMD in both men and women. These findings suggest that dietary fat has a significant influence on BMD that is dependent on the alleles present for the PPARG gene.

FIG. 1

Systematic identification of the most likely candidate gene for the mouse QTL Bmd8. In generating the 6T congenic strain, 43.6% of the genes on the sixth Chr could be eliminated as candidates for Bmd8. Using block haplotyping, the number of candidates was further reduced to 123 genes. Examination of tissue of expression eliminated another 12 genes, resulting in 111 candidate genes. Of these genes, 16 were found to contain a nonsynonymous SNPs, 12 were found to be differentially expressed in the 6T mouse, and an additional 8 were found to be differentially expressed and contain a nonsynonymous SNP, leaving 36 genes. For 5 of these 36 genes, literature references existed describing a role in bone biology.

FIG. 2

Effects of dietary fat on bone and body composition in the B6 versus 6T strains. Brackets above the graphs denote statistically significant differences (p < 0.05) compared with the group fed the 11% fat diet. Comparisons were done within a strain by ANOVA. *Statistical significance when comparing the two strains fed the same diet, as determined by Student's t-test (p < 0.05). Stipled bars denote B6 and white bar denote 6T. BV/TV% and trabecular number were assessed by μCT. B6 and 6T mice reacted very differently with regards to aBMD (A) and BV/TV% (B) when fed diets containing differing %KJ from fat. Body weight was used as a covariate for aBMD. Neither femoral length nor body weight was found to be an appropriate covariate in the fit model for either BV/TV% or trabecular number. The decrease in BV/TV% associated with the increase in dietary fat seen in 6T was primarily caused by a decrease in trabecular number (C), but no change in trabecular thickness was observed (data not shown). There was an increase in the absolute number of adipocytes in the distal femur of B6 fed the higher %KJ from fat diets, whereas there was no change observed in 6T (D).

FIG. 3

p values for interaction between fat intake and PPARG SNPs. p values for interaction between 13 SNPs across the PPARG gene and dietary fat intake (%KJ in the diet obtained from fat) above vs. below median. The y-axis provides the –log10 of the p values such that values above the line are <0.05. The top two panels present data from the femoral neck, whereas the bottom two panels present data for the lumbar spine.

FIG. 4

Diet by SNP allele interactions in the Framingham Offspring cohort. A significant SNP allele by percent energy derived from dietary fat interaction was observed for SNP 6 (A, rs1151999) and SNP 7 (B, rs709150) in men and for SNP 12 (C, rs1175381) and SNP 13 (D, rs1186464) in women. SNPs 6 and 7 are in strong linkage disequilibrium (LD), with a D′ of 0.996 and an r ² of 0.976, and as a consequence, yielded virtually the same result. In addition, a D′ of 1 and an r ² of 0.338 were noted for SNPs 12 and 13. The p values for the interaction between a given SNP and energy intact (%KJ from total fat) were as follows: SNP 6, p = 0.0004; SNP7, p = 0.0002; SNP12, p = 0.002; SNP13, p = 0.002). This interaction is visually presented in for each SNP in the graph. The results to the right of each graph shows the p values obtained from pairwise comparisons by fat stratum and genotypes.