It is now known that some amyloid fibrils may serve biological function(s). These protease-resistant β-pleated sheet assemblies are widely used in nature and comprise the so-called functional amyloids. In fact, amyloid formation seems to be an intrinsic propensity of polypeptides in general and the amyloid β-pleated sheet is a highly conserved structure through evolution. Functional amyloids have been found in a wide range of organisms, from bacteria to mammals, with functions as diverse as biofilm formation, development of aerial structures, scaffolding, regulation of melanin synthesis, epigenetic control of polyamines, and information transfer [11]. Presently, one of the methods under development for the treatment of amyloid disease involves directly inhibiting the formation of pathological amyloid . Given that pathological and functional amyloid share a common structure, some amyloidogenesis inhibitor drugs intended to prevent disease could disrupt functional amyloid formation. This could lead to undesirable side effects because functional amyloid seems to have a role in vital physiological processes in humans, including hemostasis and melanin synthesis [7]. Thus, amyloidogenesis inhibitor drugs must be designed with sufficient specificity to avoid interfering with these functions.

The kidney is most frequently affected in AL , AA, and several of the hereditary amyloidoses. Kidney biopsy is often required to identify the underlying disease. Amyloid deposits can be seen throughout the kidney but predominate in the glomerulus [12]. By light microscopy, amyloid appears as an amorphous, eosinophilic material in the mesangium and capillary loops. When amyloid is suspected, the tissue for Congo red staining should be cut at 10-micron thickness, rather than the 2-micron sections that are normally prepared from renal biopsies. Amyloid deposition in the tubulointerstitium can also lead to tubular atrophy and interstitial fibrosis even in the absence of significant glomerular deposition. Regardless of the location of the amyloid deposits, birefringent Congo red staining is seen. Because the amyloid fibrils are composed primarily of the amyloid protein and not extracellular matrix polysaccharides such as collagen, periodic acid-Schiff (PAS) staining is only weakly positive. Glomerular deposition often leads to significant proteinuria in the nephrotic range with rates up to 20 g/day. Since the protein is primary albumin, the associated edema can be severe and refractory to diuretics, as well as difficult to manage, especially in the setting of cardiac dysfunction and autonomic neuropathy seen in some amyloid subtypes. Conversely, when glomerular involvement is minimal and amyloid deposition is primarily in the tubulointerstitium, the resulting proteinuria is minimal, glomerular filtration is reduced, and creatinine is increased. Immunofluorescence (IF) and immunohistochemistry (IHC) are negative for intact immunoglobulin, complement and fibrin, but in AL amyloid are frequently positive for immunoglobulin light chain .

Although amyloid fibril deposition is often evident on kidney biopsy by electron microscopy, it can be overlooked if the index of suspicion is not sufficiently high. The fibrils are nonbranching, randomly arrayed, and have a diameter of 8–10 nm. The diagnosis of amyloid may be missed if there is secondary effacement of epithelial foot processes affected by amyloid deposition in the setting of only mild amyloid deposition in the mesangium. Such pathologic findings can give the false impression of minimal change glomerulonephritis [13]. Misdiagnosis may also occur due to confusion with the morphologic appearance of diabetic nephropathy. In a study of 26 cases of both AA and AL renal amyloidosis, there were many morphologic changes seen that mimic diabetic nephropathy; such as diffuse and nodular patterns, capsular drop-like deposits along Bowman’s capsule, deposition in the afferent and efferent arterioles, early stage glomerular microaneurysm, and accumulation of amyloid along the tubular basement membrane [14] .

Serum amyloid P component is a normal plasma protein that constitutes approximately 5 % of all amyloid deposits. It is a member of the pentraxin family of proteins that are involved in the acute inflammatory process, another example of such is C-reactive protein. Of note, radiolabeled serum amyloid P component scintigraphy is a noninvasive and quantitative method for imaging amyloid deposits, which produces diagnostic images in most patients with systemic amyloidosis, and can be used repeatedly to monitor the course of the disease. However, it has not been produced commercially and has very limited availability [15] .

Renal disease is common in many forms of amyloid and a major source of morbidity. Without treatment, end stage kidney disease (ESKD) will usually occur at variable rates over time. However, some forms of amyloid such as senile systemic (TTR protein) and dialysis-related amyloid (β2-microglobulin) do not typically involve the kidney.

The most common form of systemic amyloidosis is AL amyloidosis, with a reported incidence of 8.9 per million person years [10]. AL amyloidosis is of interest to the hematologist because it is caused by a neoplastic plasma cell or B-cell clone which synthesizes abnormal amounts of a specific immunoglobulin (Ig) which results in the dysfunction of one or more involved organs. Systemic amyloid may also be seen in 5–15 % of individuals with multiple myeloma . AL amyloidosis should be suspected in any patient with nondiabetic nephrotic syndrome, nonischemic cardiomyopathy with an echocardiogram showing concentric hypertrophy, increase of NTproBNP in the absence of primary heart disease, presence of hepatomegaly or increase of alkaline phosphatase without an imaging abnormality, peripheral and/or autonomic neuropathy, unexplained facial or neck purpura or macroglossia. Any patient who presents with any one of these signs should undergo a biopsy to detect amyloid deposits and blood screening for monoclonal immunoglobulin light chains . If a monoclonal protein is present, a bone marrow examination should be performed to evaluate for the presence of multiple myeloma . In a retrospective review of 100 known AL amyloid patients, bone marrow core biopsy revealed a plasma cell dyscrasia in 83 % (λ, 65; κ, 18) of cases [16]. Amyloid deposits were observed in 60 % of the bone marrow core biopsy specimens and, when present, were detected most often in blood vessel walls only (39 out of 60). Congo red staining of subcutaneous fat obtained by aspiration is a reliable and noninvasive test that positively identified amyloid deposits in 78 % of patients. If negative, a biopsy of the labial salivary glands may detect amyloid deposits in 50 % of patients. If this is also negative, then an involved organ should be biopsied when the clinical index of suspicion is high (see Fig. 14.4). Both IF and IHC staining are negative for intact immunoglobulin (Ig), but often positive for Ig light chain, which should be restricted to either of the two light chains (κ or λ). A limitation of IHC is reduced specificity due to background staining by normal light chain deposition. Another is the failure of commercial agents to detect the amyloid light chain because of conformational changes in the amyloid fibril that masks the relevant epitope. In one study, 12 of 34 patients (35.3 %) with known AL amyloidosis had negative IF staining for both κ and λ chains [17]. In contrast, AA amyloid is usually more accurately detected with standard antibodies against the AA protein .

The differential diagnosis of AL amyloid includes light chain deposition disease (LCDD). The amyloid deposits of LCDD are distributed in a uniform, granular pattern throughout the glomerulus and the tubular basement membranes. The PAS staining is much more intense than AL amyloid due to the inflammatory response stimulated by the light chain deposition. In AL amyloid the λ light chain isotype predominates, whereas in LCDD the κ isotype is more common. Given the lack of β-pleated fibril formation, LCDD does not emit apple-green birefringence.

Other considerations in the pathologic differential diagnosis of amyloidosis include fibrillary glomerulonephritis and immunotactoid glomerulopathy. In fibrillary glomerulonephritis, the morphologic appearance is indistinguishable from amyloid in that there is glomerular accumulation of nonbranching, randomly arranged fibrils. Like amyloid, there is often a lack of inflammatory cell infiltrate in the glomerulus. It differs from amyloid in that the fibrils are larger (usually 18–20 nm) and lack reactivity with Congo red and light chain IHC stains [18]. Immunotactoid glomerulopathy may be considered a subtype of fibrillary with similar morphologic and histochemical characteristics. However, this is a process where much larger fibrils (ranging from 34 to 49 nm) are deposited in an ordered and parallel orientation. An association with lymphoproliferative disorders has been identified .

It is now clear that an individual patient may have both a monoclonal gammopathy and a hereditary variant which creates the confounding scenario of two possible sources of the amyloid forming protein. AL amyloidosis often responds to chemotherapy that suppresses the underlying clonal plasma-cell disorder, but chemotherapy has no role in the treatment of hereditary amyloidosis and may be harmful. This was initially described in theUK where 350 patients with systemic amyloidosis in whom a diagnosis of sporadic AL amyloid was suggested by clinical findings (negative family history and the presence of a monoclonal gammopathy). However, DNA genotyping of whole blood for the most common causes of hereditary amyloidosis yielded mutations in 10 % of patients, most often in the genes encoding fibrinogen Aα and TTR [19]. Thus, in these cases the monoclonal gammopathy was considered incidental and these patients were not treated with chemotherapy. Since hereditary amyloid has variable penetrance, the family history proved to be an ineffective screening test. This series also revealed that in those patients with true AL amyloidosis, the Ig light chain fibrils were identified by IHC staining in only 38 % of the cases . This low value reflects the failure of anti-light chain antibodies to bind to light chain fragments once they have formed into an amyloid fibril. Thus, although IHC staining for amyloid forming proteins can be useful, protein- or DNA-based screening is recommended due to the limitations of IHC. The Memorial Sloan Kettering group took a more targeted approach to hereditary screening for all patients referred for an evaluation of systemic amyloidosis. In their study of 178 patients, screening took place for those who met the following criteria: (1) asymptomatic African Americans were screened for the presence of a mutant TTR (the Val122Ile variant of TTR occurs in 4 % of African Americans); (2) patients with dominant peripheral nervous system involvement were screened for variants of TTR, apolipoprotein AI and AII, fibrinogen Aα, and lysozyme (peripheral neuropathy is a common presentation of AL amyloidosis and also several of the hereditary variants); (3) and patients with isolated renal amyloidosis and no amyloid in the bone marrow were screened for the fibrinogen Aα variant [20]. Of those who were screened, 6 % had both an incidental monoclonal gammopathy and a true hereditary amyloid protein identified in the same patient .

Because of sample size limitations of the tissue biopsy and the aforementioned limitations of IHC, a newer technique of LCM-MS-based proteomic analysis has been developed at Mayo Clinic to identify amyloid protein in both a specific and sensitive fashion. Laser capture microdissection (LCM) is used to specifically study those areas in the biopsy sample that are positive for Congo red staining, thus increasing the yield of the result. In brief, the process involves the microdissection of amyloid from the tissue biopsy. This is then digested into tryptic peptides and analyzed by liquid chromatography electrospray in tandem mass spectrometry (MS). The MS raw data files are queried by different computer algorithms to search protein databases for the compatible protein. A training set of 50 patients with cardiac amyloid was compared with the current gold standard approach (includes an extensive clinical investigation for plasma cell disorders, serum and genetic testing for amyloidogenic TTR variants, and IHC for TTR, SAA, Ig κ, Ig λ, and serum amyloid P component). This was later validated in 41 additional cases yielding a specificity of 100 % and sensitivity of 98 %, whereas IHC was comparatively informative in only 42 % [21, 22]. In a later study on amyloid diseases associated with neuropathy (includes AL, ATTR, AGel, and AApoAI), the specific amyloid subtype was identified in 21 different nerve biopsies by LCM-MS without assistance from clinical information [22]. In future, LCM-MS will likely become the new gold standard for identifying the protein forming the amyloid deposits when it becomes more generally available. However, there are limitations in LCM-MS, including that the mutations must be both known and available in protein databases and that the amino acid changes must lead to alterations significant enough to be detected by mass spectroscopy .