Amyloid deposition is a pathological process in which misfolded proteins form insoluble fibrous structures that accumulate in tissues and organs. These abnormal protein aggregates, known as amyloid fibrils, progressively disrupt the architecture of healthy tissue and lay the groundwork for organ failure, neurodegeneration, and serious clinical manifestations. The group of diseases associated with amyloid deposition is collectively referred to as amyloidosis.

Structural Properties of Amyloid

Amyloid takes its name not from a specific protein but from a specific structural arrangement. A large number of proteins from different sources can, following a misfolding process, converge on a common fibrous structure. This structure possesses a characteristic architecture known as the beta-sheet conformation; beta-sheets align in parallel to form insoluble fibril bundles. The appearance of apple-green birefringence under polarized light when stained with Congo red dye is the histopathological hallmark of amyloid identification and is considered the gold standard in diagnosis.

The Fundamental Mechanism of Amyloid Formation

Under normal conditions, proteins fold into a specific three-dimensional structure and carry out their functions. When this structure is disrupted — that is, when a protein misfolds — intracellular quality control mechanisms are activated in an attempt to degrade the abnormal protein. When these mechanisms prove insufficient or when the quantity of misfolded protein becomes excessive, the proteins aggregate to form first oligomers, then protofibrils, and ultimately mature amyloid fibrils. These fibrils begin to accumulate within tissues, progressively infiltrating the spaces between cells, vessel walls, and the parenchyma of organs.

The Principal Protein Types That Cause Amyloidosis

More than thirty proteins capable of forming amyloid have been identified to date. Each protein is associated with a specific type of amyloidosis.

AL amyloidosis (immunoglobulin light chain amyloidosis) is the most common type of systemic amyloidosis. Immunoglobulin light chains produced in excess as a result of abnormal proliferation of plasma cells are converted into amyloid fibrils. It may be associated with multiple myeloma and Waldenström macroglobulinemia.

AA amyloidosis (secondary amyloidosis) develops through the accumulation of serum amyloid A (SAA) protein, produced in the liver in the setting of chronic inflammatory disease. Rheumatoid arthritis, inflammatory bowel diseases, chronic infections, and familial Mediterranean fever are among the principal underlying causes.

ATTR amyloidosis (transthyretin-associated amyloidosis) results from misfolding of transthyretin produced in the liver. In hereditary ATTR amyloidosis, mutations in the TTR gene create a predisposition, while wild-type ATTR amyloidosis (formerly known as senile systemic amyloidosis) arises as a process triggered by advancing age itself in the absence of any genetic mutation and presents predominantly with cardiac involvement in elderly men.

Beta-2 microglobulin amyloidosis develops in patients on long-term hemodialysis as a result of beta-2 microglobulin accumulation in joints and surrounding tissues.

Amyloid-beta (Aβ) deposition in Alzheimer’s disease is responsible for the formation of amyloid plaques in the brain parenchyma. Deposition in the walls of cerebral vessels is also observed in a condition known as cerebral amyloid angiopathy.

Which Organs Are Affected?

Amyloid deposition can be systemic or localized. In systemic amyloidosis, multiple organs are involved simultaneously, while in localized amyloidosis the deposition remains confined to a single organ or tissue.

Heart: Cardiac amyloidosis leads to stiffening of the myocardial walls and restrictive cardiomyopathy. It is seen particularly in ATTR and AL amyloidosis; the principal features are dyspnea, easy fatigability, and edema. Characteristic granular sparkling and increased wall thickness are notable findings on echocardiography.

Kidneys: Amyloid deposition concentrates at the level of the glomeruli and leads to nephrotic syndrome. The picture of massive proteinuria, hypoalbuminemia, and edema can progress over time to chronic kidney failure.

Liver: Hepatomegaly and abnormalities in liver function tests may be seen. Portal hypertension can develop in advanced cases.

Nervous system: Peripheral and autonomic neuropathy are particularly prominent in ATTR and AL amyloidosis. Numbness and burning sensations in the hands and feet, postural hypotension, and gastrointestinal dysmotility are typical features.

Gastrointestinal tract: Macroglossia is a near-pathognomonic finding of AL amyloidosis. Malabsorption, weight loss, constipation, or diarrhea may also be encountered.

Skin: Easy bruising and periorbital ecchymosis — bruising around the eyes — are frequently encountered findings in AL amyloidosis.

Brain: Amyloid plaques and neurofibrillary tangles in Alzheimer’s disease are held primarily responsible for progressive cognitive deterioration.

Diagnostic Methods

Amyloidosis is frequently diagnosed late, a consequence of the insidious progression of the disease and the overlap of its features with many other conditions.

Tissue biopsy is the cornerstone of diagnosis. The appearance of apple-green birefringence under polarized light in a biopsy specimen stained with Congo red confirms amyloid deposition. Abdominal fat pad aspiration, rectal biopsy, and salivary gland biopsy may be preferred as minimally invasive sampling methods. Organ biopsy provides a higher diagnostic yield.

Amyloid typing — the identification of the underlying protein after a diagnosis has been established — is mandatory, since different types of amyloidosis require entirely different treatments. Immunohistochemistry, mass spectrometry, and genetic testing are used for this purpose.

Among imaging modalities, scintigraphy has acquired increasing importance in the diagnosis of ATTR cardiac amyloidosis. Echocardiography, cardiac MRI, and PET imaging are also used in the assessment of organ involvement.

Treatment Approaches

The treatment of amyloidosis is determined by the underlying amyloid type and the condition of the affected organs. The general strategy is built around suppressing or eliminating the source of the amyloidogenic protein.

In AL amyloidosis, chemotherapeutic regimens targeting plasma cells are applied. Autologous stem cell transplantation may be possible in eligible patients.

In AA amyloidosis, bringing the underlying inflammatory or infectious process under control is the primary objective. Suppression of inflammation can lower SAA levels and thereby slow or arrest amyloid deposition.

In ATTR amyloidosis, agents that stabilize transthyretin, along with RNA interference and antisense oligonucleotide-based therapies that reduce TTR gene expression, substantially slow the rate of deposition. The advances achieved in this field in recent years have fundamentally transformed the treatment paradigm.

General supportive measures encompass symptomatic management of heart failure, neuropathy, renal disease, and other organ manifestations. Organ transplantation may also be considered in selected patients with advanced organ failure.

Recent Developments in Research

Amyloidosis research has accelerated considerably in recent years. Fibril disruption strategies that directly target amyloid fibrils in an attempt to promote their dissolution, monoclonal antibodies targeting amyloid-beta plaques in Alzheimer’s disease, and next-generation gene silencing technologies are among the most promising advances in this field. Immunotherapeutic approaches aimed at reducing amyloid-beta deposition in Alzheimer’s disease in particular continue to yield increasingly complex clinical findings; the balance between efficacy and side-effect profile remains a contested item on the research agenda.

Conclusion

Amyloid deposition represents not a single disease but a broad spectrum of conditions in which different proteins converge on a common pathological final pathway. Early diagnosis and accurate identification of the amyloid type are decisive for treatment success. The significant advances recorded in both diagnosis and treatment in recent years have begun to fundamentally transform the management of this group of diseases, which has historically been associated with a poor prognosis.