CHARACTERIZATION OF SERUM AMYLOID A PROTEIN MESSENGER-RNA EXPRESSION AND SECONDARY AMYLOIDOSIS IN THE DOMESTIC DUCK

Secondary amyloidosis is a common disease of water fowl and is charact erized by the deposition of extracellular fibrils of amyloid A (AA) pr otein in the liver and certain other organs. Neither the normal role o f serum amyloid A (SAA), a major acute phase response protein, nor the causes of secondary amyloidosis are well understood. To investigate a possible genetic contribution to disease susceptibility, we cloned an d sequenced SAA cDNA derived from livers of domestic ducks. This revea led that the three C-terminal amino acids of SAA are removed during co nversion to insoluble AA fibrils. Analysis of SAA cDNA sequences from several animals identified a distinct genetic dimorphism that may be r elevant to susceptibility to secondary amyloid disease. The duck genom e contained a single copy of the SAA gene that was expressed in liver and lung tissue of ducklings, even in the absence of induction of acut e phase response. Genetic analysis of heterozygotes indicated that onl y one SAA allele is expressed in livers of adult birds. Immunofluoresc ence staining of livers from adult ducks displaying early symptoms of amyloidosis revealed what appear to be amyloid deposits within hepatoc ytes that are expressing unusually high amounts of SAA protein. This o bservation suggests that intracellular deposition of AA may represent an early event during development of secondary amyloidosis in older bi rds.