Alternative splicing variants and DNA methylation status of BDNF in inbred chicken lines

Brain-derived neurotrophic factor (BDNF) has multiple alternative splicing variants and plays diverse biological functions in mammals, including neuronal survival, cholesterol metabolism, cell differentiation and tumor development. However, genomic structures of chicken BDNF (cBDNF) variants and its potential functions are still undefined. Here, we characterized two novel alternative splicing variants of cBDNF, cBDNF1 and cBDNF2, via combining comparative genomics methods and molecular techniques in inbred chicken line 6(3) and line 7(2), which have been developed to be resistant and susceptible, respectively, to Marek's disease tumor since 1939. Both cBDNFs consist of a bipartite transcript, with different 5' exons, exon I (298 bp) in cBDNF1 and exon II (286 bp) in cBDNF2, each of which is spliced to the common 3' exon IV. Exon I and IV are highly conserved between chicken and mammals, whereas exon II is unique for chicken. The amino acid sequence of cBDNF1 contains 8 additional amino acids in the N terminal compared to cBDNF2. cBDNF1 and cBDNF2 were only expressed in the hypothalamus among eight tissues, and cBDNF2 showed lower expression than that of cBDNF1 in both lines. The expression level of cBDNF1 was significantly higher in line 7(2) than in line 6(3) (P<0.01). Notably, the DNA methylation levels on the cis-regulatory region of cBDNF1 was negatively correlated with its expression level, which suggests that the mRNA expression level of cBDNF1 may be related to the DNA methylation status in the chickens. We also discussed a potential role of variant cBDNF1 in MD tumor resistance and susceptibility.