The Missense Variation Landscape of FTO, MC4R, and TMEM18 in Obese Children of African Ancestry

Objective: Common variation at the loci harboring fat mass and obesity (FTO), melanocortin receptor 4 (MC4R), and transmembrane protein 18 (TMEM18) is consistently reported as being statistically most strongly associated with obesity. Investigations if these loci also harbor rarer missense variants that confer substantially higher risk of common childhood obesity in African American (AA) children were conducted. Design and Methods: The exons of FTO, MC4R, and TMEM18 in an initial subset of our cohort were sequenced, that is, 200 obese (BMI >= 95th percentile) and 200 lean AA children (BMI >= 5th percentile). Any missense exonic variants that were uncovered went on to be further genotyped in a further 768 obese and 768 lean (BMI <= 50th percentile) children of the same ethnicity. Results: A number of exonic variants were observed from our sequencing effort: seven in FTO, of which four were non-synonymous (A163T, G182A, M400V, and A405V), thirteen in MC4R, of which six were non-synonymous (V103I, N123S, S136A, F202L, N240S, and I251L), and four in TMEM18, of which two were non-synonymous (P2S and V113L). Follow-up genotyping of these missense variants revealed only one significant difference in allele frequency between cases and controls, namely with N240S in MC4R (Fisher's exact P = 0.0001). Conclusion: In summary, moderately rare missense variants within the FTO, MC4R, and TMEM18 genes observed in our study did not confer risk of common childhood obesity in African Americans except for a degree of evidence for one known loss-of-function variant in MC4R.