Osteoporosis is a systemic skeletal disease characterized by low bone mass and micro-architectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture (1). This disorder is most common in women after menopause due to a rapid drop in estrogen levels.
In adults, bone mineral density (BMD) is the single best predictor of fragility fractures and is used as an important diagnostic criterion for osteoporosis (2). It is now widely recognized from twin studies (3) that BMD is highly heritable and recent discoveries resulting from genome wide association studies (GWAS) of this trait further confirm a genetic component. Since the first such report in 2008, outlining OPG (TNFRSF11B) and LRP5 (4), additional loci have been reported at RANKL, ESR1, 1p36 (ZBTB40), MHC SOST, MARK3, SP7 (Osterix), and TNFRSF11A (RANK) (5,6). In early 2009, a meta-analysis of five GWAS of femoral neck and lumbar spine BMD in a total of 19,195 subjects of northern European decent added another 13 new loci, namely GPR177, SPTBN1, CTNNB1, MEPE, MEF2C, STARD3NL, FLJ42280, LRP4, DCDC5, SOX6, FOXL1, HDAC5, CRHR1 (7). Altogether, variants at 23 loci have been established to date with GWAS of adult BMD or related traits in populations of European ancestry.
In contrast to the studies of adult BMD, there have been few reports of genetic studies of BMD determination in childhood. Familial resemblance of BMD is expressed in prepuberty and is greater in adolescence and early adulthood than in later life (8). Bone accrual during childhood and adolescence is a determinant of peak bone size and mass, which are traits that are crucial for bone structural strength later in life and ultimately the degree of risk of presenting with osteoporosis (9). To date, the only GWAS of BMD in children revealed a single locus near the Osterix (SP7) gene (10), which was also observed in the adult studies.
During childhood, BMI is positively associated with increased bone mass and bone dimensions (11,12,13). A recent report from a pediatric cohort in the Avon Longitudinal Study of Parents and Children (ALSPAC) showed that total fat mass was strongly correlated with total body, spinal, and upper and lower limb bone mineral content and suggested that fat mass influences bone mass in children (14).
In this study, we have leveraged our existing genome-wide genotyped pediatric cohort with height and weight measures to examine the known BMD GWAS loci in the context of childhood obesity. In our analyses, single-nucleotide polymorphisms (SNPs) corresponding to the 23 BMD loci discovered in adult studies (4,5,6,7,10) were investigated in our childhood obesity cohort (Table 1). Evidence of association was only observed with rs2016266 at the Osterix locus (P = 2.85 × 10−3), the only GWAS-discovered BMD locus to date that has also been reported to impact BMD specifically in children (10). The minor G allele of rs2016266 was associated with childhood obesity; this is the same allele that has been previously associated with increased BMD. We did not observe evidence of association at any of the other loci investigated which may be due to limited power, but probably indicates that they do not play a major role in the pathogenesis of childhood obesity. Additionally, our positive control of FTO lends credibility to our results. The corresponding results when treating BMI as a quantitative trait are summarized in Supplementary Table S1 online.
Our findings lend an intriguing perspective on the association between BMI and BMD (and other bone outcomes) in children, since both are associated with the same genetic variant. These findings suggest that the increased BMD-associated with obesity may be due to more than simply mechanical loading of bone through excess weight, since the minor G allele of rs2016266 confers a higher BMD in both children and adults and we observe that it confers risk for childhood obesity.
Timpson et al. suggested that Osterix may influence bone development via longitudinal and periosteal bone growth (10). Adipocytes and osteoblasts are both derived from mesenchymal stem cells, so it is possible that Osterix influences both BMD and BMI in children by its effect on this cell lineage.
At least one study previously suggested that fat mass influences cortical bone mass more in female children than in males (15). As such, we examined rs2016266 in each gender separately (males: 629 cases and 3,130 controls; females: 477 cases and 2,867 controls) with respect to our phenotypes of interest. We observed strong association between rs2016266 and obesity in female children (P = 3.56 × 10−4) (Table 2), which survived adjustment for all tests applied. Breaking this observation down further with respect to age, the association is strongly driven by females aged 10 years or older (see Supplementary Table S2 online). Conversely, we did not observe association between this locus and obesity in male children (P = 0.42). In addition, we found no consistent gender-specific association at any of the other known BMD loci (see Supplementary Tables S3 and S4 online).
As such, our study finds that the Osterix association with childhood obesity is very gender-specific i.e., the association was primarily observed in females, despite more males being present in this cohort. Analysis with respect to gender was not reported in the original GWAS report of this locus with respect to BMD in either adults or children (6,10). Despite not knowing that outcome, this observation is in keeping with the concept that females are more susceptible to osteoporosis in later life; however, it is not known how this locus confers its influence in a gender-specific manner. The fact that the association is observed primarily in females over the age of 10 years old suggests that pubertal hormones may be involved, influencing the action of this zinc-containing transcription factor which is already known to be essential for osteoblast differentiation and bone formation (16). Alternatively, since changes in BMD are very slow, it is possible that the age association is simply caused by the time that it takes to create a measurable change in BMD. It will be important in later studies to explore and refine the molecular significance of the present correlation between increased BMD and obesity in female children.
In conclusion, variation at the only GWAS locus implicated in the determination of pediatric BMD to date, i.e., Osterix, is also associated with childhood obesity. This association was primarily driven by an influence on female children; however, it remains to be determined how Osterix confers this gender-specific effect and future experiments investigating the repertoire of genes that are transcriptionally regulated by Osterix are underway that could shed light on its dual role of impacting bone density and adiposity in children.