Study of POMC (Pro-Opiomelanocortin) Gene Polymorphism and its Association with Growth Traits of Broiler Chicks

The central melanocortin system appears to be an important mediator of the actions of both leptin and insulin, which are key elements in the control of energy balance. Proopiomelanocortin (POMC) is a complex precursor protein that is proteolytically cleaved to a variety of biologically active and important neuroendocrine peptides. The POMC gene is expressed mainly in the anterior and intermediate lobes of the pituitary and in the arcuate nucleus of the hypothalamus, and at a lower level also in a wide variety of peripheral tissues and of brain regions in mammals. It produces many biologically active peptides via a series of enzymatic steps in tissue-specific manners, which have important roles in the regulation of appetite, sexual behavior, the movement of melanin produced from melanocytes in skin and the production of endogenous opioid peptides with widespread actions in the brain. In chicken, the POMC gene consisted of three exons and two introns and its protein has 251 amino acid residues with nine proteolytic cleavage sites, suggesting that it could be processed to give rise to all members of the melanocortin family, including adrenocorticotropic hormone and alpha-, beta- and gamma-melanocyte-stimulating hormones, as well as the other POMC-derived peptides.
Considerable evidence has been collected indicating that POMC mutations are associated with obesity.

Blood samples were collected in EDTA vials from one hundred Ross and sixty Cobb broiler chicks, stored at -20 and their DNA was extracted using the modified salting-out chloroform method. Polymerase chain reaction (PCR) was carried out by specific primer pairs to amplify a 444bp fragment from a part of exon two of the Pro-Opiomelanocortin gene. The pattern of all samples was determined through single stranded conformation polymorphism (SSCP) analyses by Acrylamide gel using silver nitrate staining.
The associations between polymorphisms (patterns) and the growth traits (live and carcass weight, and the weight of breast, thigh, back and neck, wings, liver, heart, bursa of Fabricius, pancreas, paraventricular, gizzard and spleen) were evaluated using the GLM procedure of the SAS software.

The extraction or genomic DNA and amplification of 444bp fragment of Pro-Opiomelanocortin gene were successfully done and it was polymorph in both strains. Two different patterns were found in Ross strain, E and F patterns with the frequencies of 0.56 and 0.44, respectively. Four different patterns were found in Cobb strain, A, B, C and D patterns with the frequencies of 0.63, 0.09, 0.14 and 0.14, respectively. There was no significant association between the patterns and the growth traits. In Ross strain, the effect of genotype (pattern) tend o be significant for carcass weight (p value = 0.054) and the chickens with F pattern have more carcass weight than those with pattern E. In Cobb strain, chickens with B pattern tend to have better slaughter yields compared to other patterns. Our results revealed that Cobb strain has more diversity in the studied fragment of POMC gene than Ross strain.

Energy homeostasis and body weight (BW) are regulated by coordinated actions of multiple genes. For significant economically traits, improvements in BW can be achieved through mass selection whereas feed conversion is relatively more difficult to improve. Gene polymorphisms can be used for improvement of the production traits by genetic selection, if the allelic association with the traits be determined. The variable associations of the identified polymorphisms may be a result of the differences in the population characteristics,
sex, or both, indicating that the selection criteria may influence the production trait associations. This should be taken into consideration while selecting for the desired production traits. Additional studies are required to expand the genetic and physiological aspects involved in feed intake, digestion, and metabolism. The genomic diversity also has important implications in the evolutionary dynamics of species. Investigations of polymorphisms are useful for better understanding of the gene function, and those associated with commercially significant production traits have a potential for usage as molecular markers for selection programs. In summary, the identified polymorphisms and their associations with the traits of economic importance in the present study provides greater insight into the role POMC gene involved in energy balance in poultry and points toward the
potential application of the findings for the enhancement of production traits by marker assisted selection.