Biochemical characterization of vitreous and cardiac amyloid in Ile84Ser transthyretin amyloidosis

Plasma transthyretin (TTR) is synthesized in the liver and is the source for visceral amyloid deposits in TTR amyloidosis. However, TTR is also synthesized in the retinal pigment epithelium of the eye and choroid plexus of the brain. It has been postulated that vitreous amyloid, which is associated with approximately 20% of the known amyloidogenic TTR mutations, results from local synthesis of TTR in the eye. In order to elucidate if differences in amyloid between organs exists, we have analyzed vitreous and cardiac amyloid fibrils in Ile84Ser TTR patients for comparison. Analysis of guanidine hydrochloride solubilized protein from isolated vitreous and cardiac amyloid fibrils indicated that the amyloid TTR in both organs is highly proteolyzed with minor amounts of intact TTR present. While vitreous protein was amenable to direct Edman sequence analysis, cardiac protein gave low yields indicating it was mostly N-terminally blocked or inaccessible to Edman degradation. While vitreous contained major 11 kDa and minor 9 kDa fragments, cardiac contained at least three major fragments of 7-11 kDa. Vitreous protein was cleaved between Lys48-Thr49, while cardiac protein was cleaved at multiple sites in the residue 46-52 region. While deposits in both tissues were enriched in variant TTR, vitreous fibrils contained more variant protein than cardiac fibrils (80-89% vs. 60-65% Ser84TTR). These differences suggest that the mechanism or pathway of fibril formation may differ in various tissues.