Serum amyloid A3 (SAA3), not SAA1 appears to be the major acute phase SAA isoform in the pig

The acute-phase serum amyloid A (SAA) protein family comprises two main circulating (systemic) isoforms, SAA1 and SAA2, synthesised in liver and one local isoform, SAA3, produced in extrahepatic tissues. Systemic and local SAA show structural differences, which suggests different functions. In the pig, AA-amyloidosis is extremely uncommon, and the structural protein in swine has characteristics of systemic SAA. The only pig SAA sequences published so far, either derived form hepatic or extrahepatic sites have been designated SAA2, but the translated protein shows the properties of SAA3 proteins. The aim of this study was to characterise all the porcine SAA isoforms by sequencing from cDNA and genomic DNA obtained form multiple porcine tissues. Primer pairs were designed to amplify presumably all isoforms of SAA firstly and then specifically for each isotype. Results show that the only isotype isolated and sequenced both from hepatic and extrahepatic tissues correspond to a SAA3-like amino acid sequence. No SAA1-like sequences were identified, which could be indicative of the gene being very rare and consistent with the observed resistance to AA-amyloidosis. Finally, it is concluded that the pig is unique among other species in that the main circulating hepatic SAA isotype shows the characteristics of local highly alkaline SAA. This likely precludes a function as apolipoprotein.