Isolation of natural-resistance-associated macrophage protein (Nramp) gene and identification of a potential molecular indicator of enteric septicemia of catfish (ESC) resistance in channel catfish

Enteric septicemia of catfish (ESC), caused by the gram-negative bacterium Edwardsiella ictaluri, is one of the most serious diseases that affects commercial catfish culture. The development of natural ESC resistance in channel catfish has the potential to control and prevent ESC and several channel catfish families that are resistant to ESC have been identified. However, the genetic basis for ESC resistance/susceptibility is unknown.; In this study, the channel catfish Nramp gene was isolated and its expression under immunostimulant (LPS) was evaluated. In mammals, Nramp1 controls the innate resistance/susceptibility to mycobacteria and intracellular parasites. Channel catfish Nramp structure is more close related to mammalian Nramp2 than Nramp1, but demonstrates positive to LPS induction like Nramp1. The availability of the channel catfish Nramp gene allows for the comparison of sequence and gene expression of this gene between ESC resistant/susceptible families.; In Differential Display Reverse Transcription-Polymerase Chain Reaction, 4 partial cDNA fragments representing differentially expressed mRNA by ESC resistant family B2(1) and susceptible family B2(5) were identified. In one partial cDNA fragment, ct5-3, a d(TTA)n trinucleotide repeat was identified as a highly polymorphic locus in channel catfish genome. This d(TTA) n is located in the 3'-UTR of a novel gene with unidentified function. Because of the relatively short length, the trinucleotide repeat is unlikely to cause gene function disruption as human myotonic dystrophy disease which is caused by the d(CTG) expansion in 3'-UTR. Screening the entire length of d(TTA)n among 13 channel catfish (survival rate after E. ictaluri challenge ranging from 15.0% to 99.3%) showed that d(TTA)15 repeat only exists in the two most susceptible families while d(TTA)13 repeat is associated mainly with the families having survival rates equal to or greater than 90.0%, suggesting that the d(TTA)n locus might be able to serve as a genetic indicator for ESC resistance/susceptibility. However, families tested in this study are mainly heterozygous at the d(TTA)n locus and breeding experiments are needed to verify this observation.