**1. Introduction**

Amyloids are an aggregation of misfolded protein that creates fibrillary structures for various causes including hereditary or de novo mutations of the proteins and errors in the normal folding processes. This abnormally misfolded protein can aggregate into insoluble polymers or develop resistance against proteolysis. As a result, amyloid is deposited extracellularly or intracellularly within the tissue, causing tissue damage. This condition is called amyloidosis.

More than 90% of amyloid deposition is composed of protein fibrils, and the remainder of the deposition is proteoglycans, glycoproteins, or serum amyloid P components. Because more than 30 previously described species of amyloid protein share the same fibril structure, they may look similar when they are deposited within an organ. The structure of amyloid is observed by electron microscopy showing fibrils with a diameter of approximately 7.5–10 nm. Furthermore, β-pleated sheet structures within the fibrils were shown by X-ray crystallography and infrared spectroscopy.

#### *Amyloid Diseases*

Depending on the chemical nature and the origin of the amyloid, each amyloid type has a tendency to be deposited in certain tissue or organ. However, the clinical signs and symptoms of different types can be remarkably similar when those types of amyloid are deposited within the same organ. Moreover, the deposited amyloid will show the same bright pink homogeneous amorphous materials when the affected organ is examined microscopically. This can be confirmed by apple-green birefringence on Congo red stain with polarization and fluorescence microscopy. Even if they have a similar morphologic appearance and similar clinical pictures, the treatment and prognosis vary significantly according to the type of amyloid. Thus, distinguishing the amyloid type is essential.

## **2. Molecular pathogenesis**

The mechanisms of amyloidogenesis of each protein are quite variable and involve different overlapping mechanisms and environmental factors. The four recurring themes include intrinsic amyloidogenic tendency, increased concentration, altered proteolytic cleavage, and genetic mutation.

Several indigenous proteins have an innate tendency to fold into amyloid structure. Such proteins include transthyretin (TTR) and atrial natriuretic peptide (ANF). The former can deposit in the heart, joint, and other organs in the elderly, even without genetic mutations, and cause **wild-type TTR amyloidosis (wtATTR)** (formerly, **senile systemic amyloidosis**). In contrary to transthyretin, which is deposited in both the atria and ventricle, ANF deposits selectively in the atria and causes **isolated atrial amyloidosis (IAA)**. This condition is also more common among the elderly and associated with atrial fibrillation. Other intrinsically prone proteins include apolipoproteins A-I, A-IV, and E and serum amyloid protein (SAP), which are incorporated in other forms of amyloid plaques nonspecifically.

Interestingly, exogenous proteins that have an amyloidogenic property can also cause amyloidosis. Two peptide drugs, insulin and enfuvirtide, have recently been described to cause localized amyloidosis [1]. Both drugs are injected subcutaneously, and the drug polypeptides may aggregate into amyloid fibril forming a localized amyloidoma. The amyloid fibrils are composed of the drug peptides themselves, which was confirmed by mass spectrometry. This **pharmaceutical amyloidosis** is an important differential diagnosis in a patient who presents with abdominal nodules and has been on insulin or enfuvirtide therapy.

Another contributing factor is persistently high concentrations of amyloidogenic proteins. Such elevated concentrations make it easier for the proteins to deposit and form a nidus for fibril extension and stabilization. These high levels can be achieved by either overproduction or undersecretion. For instance, SAP expression is greatly increased under inflammatory conditions, where the protein can aggregate to cause **AA amyloidosis**. Another example is **dialysis-associated amyloidosis (Aβ2M)**, where β-2 microglobulin (β2M) level is increased in patients with end-stage renal disease due to ineffective elimination.

Amyloidosis caused by altered proteolytic cleavage is classically exemplified by **Alzheimer's disease**, in which amyloid-β precursor protein (AβPP) forms neuritic plaques. When AβPP is cleaved by β- and γ-secretases rather than normal α- and γ-secretases, a highly amyloidogenic and neurotoxic oligomer Aβ is produced. The Aβ is believed to cause cellular dysfunction and neurodegeneration in **Alzheimer's disease**.

Lastly, genetic mutations of proteins can form amyloid fibrils by one or more mechanisms previously mentioned. These alterations may involve either gene overexpression or structural changes. The former promotes amyloidogenesis by increased concentrations and the latter by either conferring amyloidogenic

**61**

*Pathologic Findings of Amyloidosis: Recent Advances DOI: http://dx.doi.org/10.5772/intechopen.84268*

**familial amyloidosis of Finnish type**.

**3. Major types of amyloidosis**

between the two is important.

instability to the protein or making the protein subject to amyloidogenic proteolytic cleavage. One important example is duplication or triplication of the *SNCA* gene, which results in increased production of the gene product α-synuclein. It aggregates into toxic oligomer and amyloid plaques (Lewy body) and causes familial **Parkinson's disease**. In **hereditary transthyretin amyloidosis (hereditary** 

**ATTR)**, more than 130 mutations in *TTR* gene induce further instability to intrinsically unstable transthyretin and result in amyloid deposition in the heart, kidney, and peripheral nerves. Another interesting kind of mutation that can potentially produce amyloid by altered proteolytic cleavage involves gelsolin protein. Several mutations of gelsolin make the protein vulnerable to cleavage by furin, a ubiquitous protein convertase, producing amyloidogenic fragment C68 and causing **hereditary** 

Although different mechanisms are shown to be involved in amyloidogenesis, how some amyloid fibrils are deposited selectively in certain organs is not well known. In localized amyloidosis, the location of amyloid deposition may be related to the tissue where the amyloid protein is originated. For instance, islet amyloid polypeptide (IAPP or amylin) is an amyloidogenic peptide with physiologic roles in glucose regulation and secreted along with insulin by Langerhans islet cells. In **type 2 diabetes** and **insulinoma**, IAPP is deposited only in the islet of Langerhans, not exocrine pancreas. Other factors that may explain organotropism of amyloid fibrils include physiochemical environment and extracellular matrix. In **dialysis-associated amyloidosis**, β2m deposition in joint and bone tissue may be explained by the affinity of β2m for collagen, enhanced fibril extension by glycosaminoglycans such as heparan sulfate and bone resorption and nidus formation by proinflammatory cytokines and acidosis. Another unsettled issue is how amyloid formation can damage the target tissue. Extracellular deposition of amyloid fibrils itself may disrupt the organ integrity as in **cerebral amyloid angiopathy (CAA)**, where Aβ is deposited in the walls of meningeal or cortical vessels, weakens the vessel, and leads to lobar hemorrhage. However, available evidence indicates that the primary mechanism of tissue damage in the majority of amyloidosis involves toxic oligomers rather than mature amyloid fibrils themselves. Despite various cellular defensive mechanisms to prevent proteins from misfolding like molecular chaperones and cochaperones, ubiquitinprotease pathway, and autophagy, such "proteostasis" machineries can be overwhelmed by mechanisms mentioned above. The resultant misfolded oligomers may exhibit hydrophobic residues that are normally buried inside the normal quaternary structure of the protein, and they seem to induce aberrant interactions with other proteins, triggering unfolded protein response, cell death, inflammation, and other pathways of cell injury. Moreover, these toxic oligomers seem to "spread" to the surrounding tissue in a prion-like manner, further propagating cell injury.

The most recent classification of amyloidosis has been published by the Nomenclature Committee of the International Society of Amyloidosis (ISA) in 2016. The classification listed 36 different extracellular fibril proteins seen in humans and animals, whose sequence is identified unequivocally (**Table 1**). According to this scheme, amyloid proteins can be broadly divided into systemic or localized in relation to the extent of organ involved by the condition. Systemic forms of amyloidosis are common and may result in serious clinical consequences, while localized forms tend to be less common and clinically indolent unless they involve critical organs such as CNS. Therefore, the distinction

#### *Pathologic Findings of Amyloidosis: Recent Advances DOI: http://dx.doi.org/10.5772/intechopen.84268*

*Amyloid Diseases*

Depending on the chemical nature and the origin of the amyloid, each amyloid type has a tendency to be deposited in certain tissue or organ. However, the clinical signs and symptoms of different types can be remarkably similar when those types of amyloid are deposited within the same organ. Moreover, the deposited amyloid will show the same bright pink homogeneous amorphous materials when the affected organ is examined microscopically. This can be confirmed by apple-green birefringence on Congo red stain with polarization and fluorescence microscopy. Even if they have a similar morphologic appearance and similar clinical pictures, the treatment and prognosis vary significantly according to the type of amyloid.

The mechanisms of amyloidogenesis of each protein are quite variable and involve different overlapping mechanisms and environmental factors. The four recurring themes include intrinsic amyloidogenic tendency, increased concentra-

Several indigenous proteins have an innate tendency to fold into amyloid structure. Such proteins include transthyretin (TTR) and atrial natriuretic peptide (ANF). The former can deposit in the heart, joint, and other organs in the elderly, even without genetic mutations, and cause **wild-type TTR amyloidosis (wtATTR)** (formerly, **senile systemic amyloidosis**). In contrary to transthyretin, which is deposited in both the atria and ventricle, ANF deposits selectively in the atria and causes **isolated atrial amyloidosis (IAA)**. This condition is also more common among the elderly and associated with atrial fibrillation. Other intrinsically prone proteins include apolipoproteins A-I, A-IV, and E and serum amyloid protein (SAP),

which are incorporated in other forms of amyloid plaques nonspecifically.

abdominal nodules and has been on insulin or enfuvirtide therapy.

disease due to ineffective elimination.

Interestingly, exogenous proteins that have an amyloidogenic property can also cause amyloidosis. Two peptide drugs, insulin and enfuvirtide, have recently been described to cause localized amyloidosis [1]. Both drugs are injected subcutaneously, and the drug polypeptides may aggregate into amyloid fibril forming a localized amyloidoma. The amyloid fibrils are composed of the drug peptides themselves, which was confirmed by mass spectrometry. This **pharmaceutical amyloidosis** is an important differential diagnosis in a patient who presents with

Another contributing factor is persistently high concentrations of amyloidogenic proteins. Such elevated concentrations make it easier for the proteins to deposit and form a nidus for fibril extension and stabilization. These high levels can be achieved by either overproduction or undersecretion. For instance, SAP expression is greatly increased under inflammatory conditions, where the protein can aggregate to cause **AA amyloidosis**. Another example is **dialysis-associated amyloidosis (Aβ2M)**, where β-2 microglobulin (β2M) level is increased in patients with end-stage renal

Amyloidosis caused by altered proteolytic cleavage is classically exemplified by **Alzheimer's disease**, in which amyloid-β precursor protein (AβPP) forms neuritic plaques. When AβPP is cleaved by β- and γ-secretases rather than normal α- and γ-secretases, a highly amyloidogenic and neurotoxic oligomer Aβ is produced. The Aβ is believed to cause cellular dysfunction and neurodegeneration in **Alzheimer's disease**. Lastly, genetic mutations of proteins can form amyloid fibrils by one or more mechanisms previously mentioned. These alterations may involve either gene overexpression or structural changes. The former promotes amyloidogenesis by increased concentrations and the latter by either conferring amyloidogenic

Thus, distinguishing the amyloid type is essential.

tion, altered proteolytic cleavage, and genetic mutation.

**2. Molecular pathogenesis**

**60**

instability to the protein or making the protein subject to amyloidogenic proteolytic cleavage. One important example is duplication or triplication of the *SNCA* gene, which results in increased production of the gene product α-synuclein. It aggregates into toxic oligomer and amyloid plaques (Lewy body) and causes familial **Parkinson's disease**. In **hereditary transthyretin amyloidosis (hereditary ATTR)**, more than 130 mutations in *TTR* gene induce further instability to intrinsically unstable transthyretin and result in amyloid deposition in the heart, kidney, and peripheral nerves. Another interesting kind of mutation that can potentially produce amyloid by altered proteolytic cleavage involves gelsolin protein. Several mutations of gelsolin make the protein vulnerable to cleavage by furin, a ubiquitous protein convertase, producing amyloidogenic fragment C68 and causing **hereditary familial amyloidosis of Finnish type**.

Although different mechanisms are shown to be involved in amyloidogenesis, how some amyloid fibrils are deposited selectively in certain organs is not well known. In localized amyloidosis, the location of amyloid deposition may be related to the tissue where the amyloid protein is originated. For instance, islet amyloid polypeptide (IAPP or amylin) is an amyloidogenic peptide with physiologic roles in glucose regulation and secreted along with insulin by Langerhans islet cells. In **type 2 diabetes** and **insulinoma**, IAPP is deposited only in the islet of Langerhans, not exocrine pancreas. Other factors that may explain organotropism of amyloid fibrils include physiochemical environment and extracellular matrix. In **dialysis-associated amyloidosis**, β2m deposition in joint and bone tissue may be explained by the affinity of β2m for collagen, enhanced fibril extension by glycosaminoglycans such as heparan sulfate and bone resorption and nidus formation by proinflammatory cytokines and acidosis.

Another unsettled issue is how amyloid formation can damage the target tissue. Extracellular deposition of amyloid fibrils itself may disrupt the organ integrity as in **cerebral amyloid angiopathy (CAA)**, where Aβ is deposited in the walls of meningeal or cortical vessels, weakens the vessel, and leads to lobar hemorrhage. However, available evidence indicates that the primary mechanism of tissue damage in the majority of amyloidosis involves toxic oligomers rather than mature amyloid fibrils themselves. Despite various cellular defensive mechanisms to prevent proteins from misfolding like molecular chaperones and cochaperones, ubiquitinprotease pathway, and autophagy, such "proteostasis" machineries can be overwhelmed by mechanisms mentioned above. The resultant misfolded oligomers may exhibit hydrophobic residues that are normally buried inside the normal quaternary structure of the protein, and they seem to induce aberrant interactions with other proteins, triggering unfolded protein response, cell death, inflammation, and other pathways of cell injury. Moreover, these toxic oligomers seem to "spread" to the surrounding tissue in a prion-like manner, further propagating cell injury.
