The prevalence of obesity is rising globally, constituting a major health burden. The interplay of environmental and genetic factors influences the body weight variation between individuals. Genetic mechanisms underlying the infrequent monogenic forms of obesity revealed major pathways for weight regulation. Through genome-wide association studies (GWAS), a large number of variants relevant for the prevalent polygenic forms of obesity have been identified. Yet, their functional implications remain mainly unknown. To improve our understanding of the body weight regulation, further studies considering both forms of obesity are warranted. The here pursued objectives pertain analyses for (1) monogenic obesity, (2) candidate gene analyses and (3) functional analyses of GWAS hits.

1. Monogenic obesity is predominantly characterised by mutations in genes of the leptin-melanocortin system. So far, few patients with obesity homozygous for functionally relevant leptin (LEP) variants have been reported worldwide. Therefore, prevalence estimates of potentially pathogenic LEP variants based on the Genome Aggregation Database (gnomAD, v2.1.1), representing an approximation to the general population, were determined. After the pathogenicity predictions by in silico tools, prevalence rates were estimated. Across all populations, prevalence rates for hetero- and homozygosity (including compound heterozygous variants) were approximately 1:2,100 and 1:17,830,000, respectively. These estimates varied considerably between the studied subpopulations. The obtained findings highlight the infrequency of pathogenic LEP variants within the general population and emphasize the relevance of genetic screening in individuals with obesity.
2. Mouse models and findings of GWAS demonstrated the relevance of the thermogenic creatine cycle for weight regulation. The here performed analyses in three genes of the creatine biosynthesis (GATM, CKB, and CKMT1B) initially identified a sexually dimorphic single nucleotide polymorphism (SNP) in CKB (rs1136165) based on data of a sex stratified body mass index (BMI, kg/m²) GWAS. A mutation screen in children and adolescents with severe obesity, female patients with anorexia nervosa, and lean controls revealed five variants in GATM and CKB as well as nine variants in CKMT1B. Subsequent genotyping of a large number of family-based samples consisting of the offspring with severe obesity and both biological parents, provided evidence for an obesity-protective effect of the rare allele at rs149544188. Subsequent transcriptomic analyses of 1,479 samples from the Leipzig Obesity Biobankdetected specific correlations of the three genes and higher gene expressions of these in visceral than subcutaneous adipose tissue in individuals with obesity. Future in vitro studies need to elucidate the relevance of these variants in relation to polygenic obesity.
3. Deciphering the functional implications of GWAS-identified variants remains challenging, given their predominant localisation in non-coding genomic regions. Previously, evidence emerged that these variants can affect the expression of circular RNAs (circRNAs). Accordingly, interactions between BMI-associated variants and circRNAs were investigated. Applying a custom R script, an enrichment of BMI-associated variants (P < 5*10^- 8) on circRNAs in relation to non-significant variants (P >= 5*10^-8) was detected. This enrichment was also determined for variants associated with chronic kidney disease, body height, autism spectrum disorder and anorexia nervosa. An in vitroanalysis of a BMI-associated SNP (rs4752856) located on the circRNA hsa_circ_0022025 revealed a differential allelic expression of the BMI-increasing allele on the circRNA. Consequently, initial indications of putative correlations between BMI-associated variants and circRNAs were hereby provided.