The functional role of Cytoplasmic RNA Binding Element 4 in body fat distribution and Type II Diabetes Risk.

Metabolic syndrome (MetSyn) is a group of metabolic conditions that occur together and promote the development of cardiovascular disease (CVD) and type 2 diabetes. Disease conditions include abdominal obesity, insulin resistance, elevated serum triglyceride levels, depressed serum high-density lipoprotein levels, elevated blood glucose levels, and hypertension. Heritability estimates for individual components of MetSyn vary between 40% to 70%, suggesting a strong contribution of genetic components to disease pathology. A recent genome-wide association study of body fat distribution conducted in approximately quarter million men and women from different ethnic groups identified significant associations of genetic variants in the chromosome 5q35 locus with BMI-adjusted waist-to-hip ratio, a measure of abdominal obesity. Given these results, our study had two aims: (1) to predict the causal gene in the 5q35 locus, and (2) create and characterize a mouse model of the predicted causal gene. Using gene expression data from subcutaneous adipose tissue biopsies collected from 770 participants of the Metabolic Syndrome in Men (METSIM) cohort, we showed that CPEB4 is the likely cis-effector gene for this locus. The A allele of the single nucleotide polymorphism rs875741 at the 5q35 locus was associated with increased abdominal obesity and higher expression of CPEB4 in adipose tissue (P-value = 4.27x10-174; effect size =1.16). In agreement with the human genetic data, CPEB4 expression was significantly positively correlated with waist-to-hip ratio in 770 METSIM participants (r = 0.21, P-value = 2.83x10-9). CPEB4 encodes an RNA-binding protein but its role in obesity is not known. To further study the impact of Cpeb4 on obesity, we generated mice which had three genotypes at the Cpeb4 locus: wild-type (WT), Cpeb4-deficient (KO), or heterozygous (het). First, we isolated total RNA from various fat depots and measured Cpeb4 expression using qRT-PCR. As expected, Het mice had half the amount of Cpeb4 abundance compared to WT mice and KO mice had no Cpeb4 expression. Second, we placed the 8-week old male and female mice on a 12-week high fat diet (45% fat calories) and measured body fat every four weeks using echoMRI. In agreement with the human studies, mice, which were Cpeb4-deficient, gained less body fat compared to WT mice. Collectively, our human results predict Cpeb4 as the causal gene in the 5q35 locus for body fat distribution and our mouse results show that Cpeb4 impacts body fat under high fat diet conditions. Future work will delineate the adipose-specific role of Cpeb4 on obesity in our mouse model, and its function in adipocytes using cell culture models of adipogenesis.