Salvage pathway of vitamin B absorption in chickens with mutant tumor virus a receptor.

The tumor virus A receptor (TVA), a member of the low-density lipoprotein receptor (LDLR) family, serves as an entry receptor for Avian Leukosis Virus (ALV) subgroups A and K, as well as a receptor for vitamin B bound to transcobalamin. Naturally occurring genetic variants in the TVA gene determine susceptibility or resistance to ALV-A and -K, but the effects of these mutated TVA on vitamin B uptake have not been investigated systemically. We found four TVA variants comprising the wild type (TVA), a single nucleotide polymorphism variant (TVA), and two partial deletions in the splicing branch point region (TVA). This study investigates the relationship between the various genotypes of TVA alleles and uptake of vitamin B in chickens. A protein interaction model suggested that mutant TVAs (i.e., TVA, TVA) may have reduced ability to take up vitamin B due to a disrupted LDL-A domain, a pivotal region involved in vitamin B uptake; however, we found no significant difference in absorption of vitamin B in TVA and TVA chickens, or levels of its metabolite in serum. Notably, TVA chickens had significantly higher levels of vitamin B than TVA chickens, a finding contrary to the predicted lower uptake. Expression of vitamin B carrier related genes (i.e., AMN, GIF, and TCN2) in chickens showed a stepwise increase: TVA > TVA > TVA. These results suggest a mechanism by which mutant TVA chickens with a disrupted TVA protein acquire natural resistance to ALV-A -K, with no impairment of vitamin B metabolism.