The Human Genome Variation Society (HGVS) has proposed standard nomenclature for variation at both the nucleotide and the protein level (www.​hgvs.​org/​mutnomen/​). In this chapter, all mutations and variants will be discussed referring to protein sequence, or amino acid changes. Previously, the standard nomenclature was to number the first amino acid of the mature, processed protein as amino acid number one. For secreted proteins (such as those involved in familial amyloidoses), this numbering system neglected the signal peptides and propeptides that are cleaved from the amino terminus after translation as the protein is being processed by the cell for secretion. The HGVS standard nomenclature now recommends that proteins be numbered starting with the initiator methionine as amino acid number one. All of the amino acid changes discussed here will use the HGVS standard nomenclature.

For example, the signal peptide of the TTR protein is 20 amino acids in length. One mutation seen in TTR amyloid is Cys30Arg (new nomenclature). Using historical nomenclature, the mutation is termed Cys10Arg (subtract 20 amino acids that account for the signal peptide in the new nomenclature to derive this). Apolipoprotein A1 (ApoA1) is an example of a protein with a signal peptide and a propeptide. The signal peptide is 18 amino acids in length, and the propeptide is 6. Hence, to extrapolate the historical nomenclature from the new nomenclature for a mutation in ApoA1, Gly50Arg would be Gly26Arg. Please refer to Table 27.1 for a listing of all genes and the conversions.

The first protein identified in hereditary amyloidosis with amyloid deposition due to coding sequence missense mutations was transthyretin (TTR). Notably, TTR is by far the most common protein and gene involved in familial amyloidosis, accounting for between 95 and 98 % of reported familial amyloid cases, and often presenting after the age of 50. TTR is a transport protein that has four exons, 127 amino acids, weighs 55 kDa, and is synthesized predominantly in the liver. The function of TTR is to carry thyroxine (T4) and to participate in the thyroxine–retinol binding protein complex. This function is important to bear in mind when testing for mutations in TTR since familial euthyroid hyperthyroxinemia can demonstrate a substitution in position 129 (Ala129Thr, Ala 129 Thr, Ala129Val) of the protein. These patients are clinically euthyroid, with normal free thyroid and triiodothyronine levels, but increased thyroxine due to the thyroxine-binding capacity of TTR [10]. Alternatively, when TTR is mutated, aberrant protein folding often results with deposition as described above and the clinical sequelae including most predominantly peripheral polyneuropathy, and/or cardiomyopathy (with or without eye and brain involvement).

Liver transplantation is the treatment of choice for patients with TTR amyloidosis, due to its predominant hepatic synthesis. Since this treatment is vastly different than the cytotoxic chemotherapeutic regimens and/or bone marrow transplant indicated for AL amyloidosis, the correct diagnosis of these two disorders with supporting laboratory data is paramount. Also, senile amyloid deposition is often composed of wild-type TTR protein. In this case, the conversion of TTR into amyloid fibrils is not driven by pathogenic mutations, and gene sequencing is necessary to distinguish TTR type senile amyloid from a hereditary disorder.

More than 100 mutations have been reported in TTR, almost all of them being single base substitutions in the gene, located on chromosome 18 [11] (Fig. 27.2). Common single base substitutions include V50M, L75P, L78H, T80A, and Y134H [12]. A common three-nucleotide/single codon deletion is ∆Val142. Ethnic propensities exist for a number of TTR mutations [2, 13]. For example, M33I is seen in the German population; A45T, Y89I, and Q112K segregate amongst the Japanese. Variants common in the United States are D38N, A45S, F53C, W61L, T69P, L75Q, A101T, and R123S [2]. Phenotypic clustering is seen in some codon changes (Table 27.2), and Tyr89Ile is the only double nucleotide substitution documented to date. Specifically, Tyr89Ile is seen in the Japanese population, with cardiac and connective tissue involvement, and autonomic neuropathy [6].

V122I is another example of a variant with a strong ethnic predisposition, in this case with African American ancestry. Heterozygosity for this mutation is associated with cardiac amyloidosis, congestive heart failure, and mortality in African Americans, typically after the age of 70 [2, 13]. The frequency of the V122I variant is high in the African American population, approximately 4 %. It is interesting to note that this mutation has the same frequency as the most common cystic fibrosis mutation, deltaF508, in the Caucasian population. However, in contrast to the case of CF, where homozygosity for the common mutation is well appreciated, and indeed, is the most common disease-associated genotype, homozygosity for V122I has only infrequently been reported. A recent publication describes 13 homozygous cases—the largest case series reported to date. These investigators found that homozygosity for the V122I mutation was associated with an earlier onset of disease, by approximately 10 years [14]. Whether the small number of reported cases of V122I homozygosity represents a lack of ascertainment of cardiac disease in the elderly African American population, or whether it represents a penetrance that is substantially less than 100 %, is an important question for future research.

Apolipoprotein A1 (ApoA1), another protein involved with hereditary amyloidosis, contains four exons, 243 amino acids, weighs 28 kDa, and is located on chromosome 11q23–q24. ApoA1 is synthesized in the liver and small intestine, conferring a plasma protein that is the main protein of high-density lipoprotein particles and has a key role in lipoprotein metabolism. ApoA1 is important for the formation of high-density lipoprotein cholesterol esters, promoting efflux of cholesterol from cells [15, 16]. Consequently, mutations in ApoA1 can lead to one of two rare diseases of lipoprotein metabolism: primary hypoalphalipoproteinemia (Tangier’s disease) or ApoA1 amyloidosis, which has no pathophysiology linked directly to lipoprotein metabolism, depending on the mutation. The predominant genetic changes seen in ApoA1 amyloidosis are nucleotide substitutions; however, two deletion mutations and deletion/insertion have been described. Most of the mutations are in-frame, with the exception of Asn122fs and Ala202fs. Hence, the mechanism of amyloid production for all of the ApoA1 mutations involves aberrant folding, and the unstable species produced with the Asn122fs and Ala202fs mutations is a truncated protein rather than a full length one.

The clinical presentation of amyloidosis consistent with ApoA1 involves the liver, kidney, larynx, skin, and myocardium most commonly and rarely the testes and adrenal glands [15]. The most common mutations to date include G50R, L99P, A197P, A199P, and L198S [15–17]. Most of these mutations are present in Northern Europeans. Specifically, G50R is common among British, Scandinavians, and North Americans, L99P in Italians, Germans, and North Americans, A197P in Americans and British, and L198S in Italian and Dutch individuals [16] (Table 27.3).

Apolipoprotein A2 (ApoA2), similar to ApoA1, is an amyloidogenic protein involved in lipid metabolism. Unlike ApoA1, ApoA2 can be found in senile amyloidosis [18]. As is the case with TTR, gene sequencing is required to determine if ApoA2 deposition in a given case is due to deposition of a wild-type protein (senile amyloid) or a mutant one (familial amyloidosis). Structurally, it is a 77-amino acid, 17.4 kDa protein located on chromosome 1p21-1qter [19]. While comprised of four exons, three exons in ApoA2 are coding: exons 2, 3, and the 5′ end of exon 4. The Apo A2 gene is one of the more recently described forms of hereditary amyloid, with a clinical picture of early adult-onset, rapidly progressive renal failure [17]. There is no neuropathy associated with the abrupt renal failure, which occurs in the absence of proteinuria. Mutations in the stop codon are the common genetic change resulting in a 21-amino acid extension at the carboxy terminus of the mature protein [17]. All of these changes occur at codon 101 in exon 4 as follows: Stop101G, Stop101S, and Stop101R (Table 27.4). Geographically, these mutations are seen in North Americans, with the exception of Stop101R, which is also seen in Russians [17].