The human pancreatic polypeptide receptor Y4 : Genetic and functional variation

Abstract: Humans are evolutionarily adapted to an environment where food is scarce, but today many live in a world of food abundance. Paired with low physical activity, this may lead to weight gain and obesity. Efficient anti-obesity treatments require understanding of the mechanisms that control hunger, satiety, energy metabolism and body weight. This thesis investigates possible genetic and physiological mechanisms behind these processes.Genetic correlation between body-mass index (BMI) and a highly polymorphic region on chromosome 10 was analysed with regard to single nucleotide polymorphisms (SNPs) and gene copy number variation (CNV). This region contains the gene NPY4R encoding the pancreatic polypeptide (PP) receptor Y4, which has been reported to reduce appetite.The results show that the NPY4R gene was duplicated before the divergence of modern humans from the Neanderthals and the Denisovans (approximately to 400,000–800,000 years ago). The CNV of the NPY4R gene region was investigated by read depth analysis based on genome sequences and droplet digital PCR (ddPCR). The read depth results revealed a CNV range of 3-7 copies per genome, while the ddPCR results demonstrated a range of 2–11. Most humans have a total of 4–5 copies, in contrast to the two copies presumed by previous studies.Investigation of an association between the NPY4R CNV and body mass index (BMI) led to interesting and ambiguous results. A study of 558 Swedish individuals with a wide range of BMI suggested, surprisingly, a positive correlation between NPY4R copy number and BMI for women. On the other hand, a study of 1009 individuals from Northern Sweden found no correlation between BMI and NPY4R copy number. These diverging findings may be due to geographical variation or lack of power in one of these studies.Twelve naturally-occurring amino acid variants of the Y4 receptor were investigated pharmacologically in cell culture. Three of these showed no functional response, which may be explained by altered conformation of the receptors. For two receptor variants PP had a significantly decreased potency. A 3D model of the Y4 receptor was generated based on the crystal structure of the human Y1 receptor. The functional responses of the Y4 variants agree well with the 3D model and with the degree of evolutionary conservation of the positions.In conclusion, these studies reveal unexpectedly large CNV as well as extensive SNP for the NPY4R gene and a possible correlation with BMI that may be due to the differing responses of the naturally occurring receptor variants.