Hereditary amyloid cardiomyopathy caused by a variant apolipoprotein A1

Abstract

Autosomal dominant hereditary amyloidosis with a unique cutaneous and cardiac presentation and death from heart failure by the sixth or seventh decade was found to be associated with a previously unreported point mutation (thymine to cytosine, nt 1389) in exon 4 of the apolipoprotein A1 (apoA1) gene. The predicted substitution of proline for leucine at amino acid position 90 was confirmed by structural analysis of amyloid protein isolated from cardiac deposits of amyloid. The subunit protein is composed exclusively of NH2-terminal fragments of the variant apoA1 with the longest ending at residue 94 in the wild-type sequence. Amyloid fibrils derived from four previously described apoA1 variants are composed of similar fragments with carboxyl-terminal heterogeneity, but contrary to those variants, which all carry one extra positive charge, the substitution Leu90Pro does not result in any charge modification. It is unlikely, therefore, that amyloid fibril formation is related to change of charge for a specific residue of the precursor protein. This is in agreement with studies on transthyretin amyloidosis in which no unifying factor such as change of charge for amino acid residues has been noted.

Figure 1.

Pedigree of family with cutaneous and cardiac amyloidosis. The propositus (IV:8) is indicated by an arrow. Subjects with histologically proven amyloidosis are indicated by solid symbols. Subjects that died of cardiac disease and are presumed to have been affected are indicated by half-shaded symbols.

Figure 2.

A: Skin biopsy from proband stained with Congo red. Amyloid deposition forms a band in the upper dermis with papillary formation. B: Viewed with polarized light, amyloid deposits give typical green birefringence, whereas collagen gives white birefringence.

Figure 3.

A: Postmortem cardiac muscle stained with Congo red showing extensive amyloid deposition between muscle fibers and in vessel walls. Marked loss of myocardial fibers is noted. B: Same section viewed by polarized light.

Figure 4.

Amino acid sequence of the amyloid subunit protein isolated from heart tissue. The arrows indicate the peptides used to determine the amino acid sequence of the amyloid protein. Peptides labeled with T were isolated from HPLC fractionation of a trypsin digest of the subunit protein. The arrow labeled Intact designates the sequence obtained from the undigested sample for 15 cycles. Dashed arrows indicate peptides recovered at ≤10% the molar amount of the other peptides. Amyloid protein isolated from skin deposits gave the same sequence, but no residues beyond position 88 were found.

Figure 5.

Autoradiograph of DNA sequencing gel of exon 4 of propositus showing heterozygosity at codon 90 with both CTG(Leu) and CCG(Pro).

Figure 6.

RFLP analysis of exon 4 PCR product of propositus (lanes 1 and 2) and a normal subject (lanes 3 and 4). Lanes 1 and 3, digested with BamH1; lanes 2 and 4, before digestion. Only propositus DNA product (889 bp) shows cleavage to give bands of 633 bp and 256 bp (lane 1).