AMYLOID FIBRIL PROTEIN IN FAMILIAL AMYLOIDOTIC POLYNEUROPATHY, PORTUGUESE TYPE - DEFINITION OF MOLECULAR ABNORMALITY IN TRANSTHYRETIN (PREALBUMIN)

Amyloid fibril protein in patients with familial amyloidotic polyneuropathy is chemically related to transthyretin (TTR), the plasma protein that is usually referred to as prealbumin. A genetically abnormal TTR may be involved in this disease. Studies were conducted on amyloid fibril protein (AFp) isolated from tissues of 2 Portuguese patients who died with familial amyloidosis, and on TTR isolated from sera of patients with this disease. AFp, purified by affinity chromatography on retinol-binding protein linked to Sepharose, resembled plasma TTR in forming a stable tetrameric structure, and in its binding afinities for both thyroxine and retinol-binding protein. The structural studies included: comparative peptide mappings by reverse-phase high performance liquid chromatography (HPLC) after trypsin digestion; cyanogen bromide [CNBr] cleavage studies: and amino acid microsequence analysis of selected tryptic and CNBr peptides. On the basis of the known amino acid sequence of TTR, comparative tryptic peptide maps showed the presence of a single aberrant tryptic peptide (peptide 4, residues 22-34) in AFp as compared with TTR. This aberrant peptide contained a methionine residue, not present in normal tryptic peptide 4. CNBr cleavage of AFp produced 2 extra peptide fragments, which were demonstrated, respectively, by HPLC analysis and by sodium dodecyl sulfate-gel electrophoresis. Sequence analyses indicated the presence of a methionine-for-valine substitution at position 30 in AFp as compared with TTR. Thus, the purified amyloid fibril protein comprised a TTR variant with a methionine-for-valine substitution at position 30. A single nucleotide change in a possible codon for valine 30 could explain the substitution. The variant TTR was also present in the TTR isolated from the pooled sera of amyloidoses patients, together with larger (4- to 6-fold) amounts of the normal TTR. Thus, in these patients, the variant TTR was circulating in plasma, along with larger amounts of normal TTR. The variant TTR probably represents the specific biochemical cause of the disease, and this abnormal form of TTR selectively deposits in tissues as the amyloid characteristic of the disease.