Genetics of obesity in Hong Kong Chinese : a candidate gene approach focusing on the melanocortin-4 receptor and adiponectin

Abstract

of thesis entitled Genetics of Obesity in Hong Kong Chinese: A Candidate Gene Approach Focusing on the Melanocortin-4 Receptor and Adiponectin Submitted by RONG RONG for the degree of Doctor of Philosophy at the University of Hong Kong in June 2005 Various genes have been implicated in the pathogenesis of obesity. Melanocortin-4 receptor (MC4R) has been shown to play a key role in energy homeostasis. MC4R deficiency has been shown to account for 1-6% of early-onset human obesity. Adiponectin also plays an important role in energy, glucose and lipid homeostasis. Single nucleotide polymorphisms (SNPs) and haplotypes of adiponectin gene have been reported to be associated with BMI, insulin, glucose, and adiponectin levels in several ethnic populations. However, no systematic study has been conducted for these two candidate genes in Chinese. The aim of the present study was to explore the contribution of the MC4R and adiponectin genes to obesity in Chinese. For the MC4R gene, 227 obese subjects with a mean BMI of 35.29 kg/m and 100 normal controls with a mean BMI of 21.57 kg/m were screened for mutations in the putative promoter region and coding region by PCR-direct sequencing. Apart from two known SNPs, V103I and -176A>C, 3 novel missense mutations: Y35C, C40R, and M218T were also identified. The prevalence of carriers of V103I in normal controls was significantly higher than that in obese subjects, in keeping with findings in other ethnic groups. C40R and M218T were only detected in obese but not in normal subjects. All three novel MC4R missense mutations were functionally normal in terms of cAMP accumulation and cell surface expression, suggesting that they probably did not contribute significantly to obesity in the mutation carriers. For the adiponectin gene, firstly, a 4.6 kb fragment of the adiponectin gene was sequenced for SNPs in 30 obese and 30 normal subjects. 15 SNPs including -12140G>A, -11426A>G, -11377C>G, -11156insCA, -11043C>T, -10677C>T, -4041A>C, -3971A>G, +45T>G, +276G>T, +349A>G, +639T>C, +712A>G, +967G>A, +2019del/insA with 4 novel ones: SNPs -12140G>A, -10677C>T, +639T>C and +967G>A were identified. Secondly, a case-control study including 226 obese and 207 normal subjects was conducted to examine the contribution of SNPs -11377, +45, +276, +349, +639, +712, and +967 to obesity. However, the allelic frequency of the aforementioned SNPs was similar in 226 obese and 207 normal subjects. In subjects with normal weight, homozygous +45 GG and 45-276 GG/GG had high insulin levels and insulin resistance index compared to those with other genotypes or haplotypes, suggesting the contribution of SNPs and haplotype of adiponectin gene to obesity-related traits. However, we could not detect a correlation of adiponectin levels with SNPs or haplotypes, implying the limited contribution of adiponectin gene SNPs and haplotypes to adiponectin concentrations in Chinese. In summary, 2 SNPs and 3 novel missense mutations of MC4R were identified. The frequency of MC4R mutations (C40R and M218T) was 0.8% in our population. The functional properties of the 3 novel missense mutations were characterized. Fifteen SNPs of the adiponectin gene, 4 being novel, were identified. The association of homozygous +45 GG and homozygous 45-276 GG/GG haplotype with two obesity-related traits, hyperinsulinemia and insulin resistance index, is in keeping with a role of adiponectin in the regulation of energy homeostasis. Genetics of Obesity in Hong Kong Chinese: A Candidate Gene Approach Focusing on the Melanocortin-4 Receptor and Adiponectin