Protein Variation Associated With Facial Eczema Resistance or Susceptibility in Romney Sheep

Abstract

The detection of plasma and liver protein markers for facial eczema resistance or susceptibility in Romney sheep was undertaken. A pooling protocol was used to allow rapid comparison of variation between populations. A 2-D PAGE technigue was developed for protein separation. In general, proteins separated by 2-D PAGE were examined on Coomassie blue or silver stained gels. Greater sensitivity was achieved by labelling proteins with radioactive isotopes. Reductive methylation of the free amino groups of proteins with radioactively labelled formaldehyde and sodium cyanoborohydride was used for isotopic labelling of proteins. A double-labelling technique involving 14C and 3H was used to label plasma or liver proteins from facial eczema resistant and susceptible sheep. The labelled proteins were subsequently separated by 2-D PAGE and detected by autoradiography and fluorography. Any detected variation was further analysed for individuals on one-dimensional polyacrylamide gels which allowed more rapid analysis of multiple samples. No significant difference was detected among the liver proteins of resistant and susceptible sheep. However, among the approximately twenty major plasma protein families visualised on 2-D PAGE gels, significant variation between sheep selected for facial eczema resistance or susceptibility occurred at the transferrin locus. Sheep selected for resistance showed a predominance of acidic transferrins while sheep selected for susceptibility contained a basic transferrin in greater abundance. These results were confirmed and their significance was assessed by transferrin phenotyping on one-dimensional polyacrylamide gels. The transferrin A allele was more abundant in sheep selected for resistance while the transferrin D allele showed a greater association with facial eczema susceptibility. The A allele frequency was 0.57 in resistants and 0.05 in susceptibles while the D allele frequency was 0.18 in resistants and 0.68 in susceptibles. The results suggest some separation of transferrin A and D alleles between the animals selected for resistance and susceptibility. The basis of this variation is unknown. It may reflect either a physiological association of transferrin alleles with a character of importance in facial eczema resistance, or it may be a phenomenon unrelated to facial eczema resistance produced as a result of the way in which the facial eczema resistant and susceptible flocks were generated. It is expected that subsequent genetic studies will show whether transferrin phenotype can be used as a marker to select for facial eczema resistance as a means of controlling the disease.