Molecular characterization of major histocompatibility complex class I (B-F) mRNA variants from chickens differing in resistance to Marek's disease

In this study, the relative distributions of two alternatively polyadenylated chicken major histocompatibility complex (MHC) mRNA isoforms of approximately 1.5 and 1.9 kb were analysed in spleen cells from chickens homozygous for the MHC haplotypes B21 and B19v1 as well as in heterozygous B19v1/B21 birds. Both isoforms are likely to encode classical MHC class I (B-F) alpha chains. The B19v1 and B21 MHC haplotypes confer different levels of protection against Marek's disease (MD), which is caused by infection with MD virus (MDV). In spleen cells, MD-resistant B21 birds were shown to have the highest percentage of the 1.5 kb variant relative to the total MHC class I expression, MD-susceptible B19v1 birds the lowest and B19v1/B21 birds an intermediate percentage. Infection of 4-week-old chickens with the GA strain of MDV was shown to cause a significant increase in the relative amount of 1.5 kb transcripts in B21 birds 32 days postinfection (dpi). Alternatively polyadenylated mRNA isoforms may encode identical proteins, but differences in the 3' untranslated region (UTR) can influence polyadenylation, mRNA stability, intracellular localization and translation efficiency. It was shown that the increased 1.5 kb percentage in B21 birds 32 days postinfection may be a result of a change in the choice of poly(A) site rather than a locus-specific upregulated transcription of the BF1 gene that preferentially expresses the 1.5 kb variant. Furthermore, the 3' end of the 1.5 kb mRNA variants deriving from B19v1 and B21 chickens was characterized by Rapid Amplification of cDNA Ends (RACE) and sequencing. No potentially functional elements were identified in the 3' UTR of the RACE products corresponding to this short isoform. However, variation in polyadenylation site was observed between the BF1 and BF2 mRNA transcripts and alternative splicing-out of the sequence (exon 7) encoding the second segment of the cytoplasmic part of the mature BF2*19 molecules. This alternative exon 7 splice variant was also detected in other MD-susceptible haplotypes, but not in the MD-resistant B21 and B21-like haplotypes, suggesting a potential role of exon 7 in MHC-related MD resistance.