AA amyloidosis associated with a mutated serum amyloid A4 protein.

AA amyloidosis invariably has been associated with fibrillar deposits of the acute phase high-density lipoprotein serum amyloid A isotypes SAA1 and SAA2. We now report the first case in a patient with no antecedent history of a chronic inflammatory or neoplastic process whose pathologic renal deposits were comprised of a mutated form of the constitutively expressed serum amyloid A4 (SAA4) protein. Analyses by tandem mass spectrometry of amyloid extracted from kidney biopsies revealed a component identical in sequence to the N-terminal portion of SAA4, except for the substitution of glycine for tryptophan at position 22 (W22G). Sequencing of genomic DNA using SAA4-specific primers showed a TGG to GGG transversion in codon 22 that accounted for the observed modification. Confirmation of the SAA4 nature of the amyloid was obtained immunohistochemically. Notably, only wild-type SAA4 was detected by mass spectrometry in the patient's serum and its concentration was within normal limits. Given the substitution of an amino acid lacking a side chain for a bulky residue, we posit that the W22G alteration would profoundly affect SAA4 stability, rendering it amyloidogenic. Our studies provide the first evidence for a novel type of AA amyloidosis in which the fibrils were formed from a mutated SAA4 protein.