Pathogenesis and Classification of Amyloidosis

Slide 1 · Pathogenesis and Classification

of Amyloidosis

Protein misfolding • fibril formation • biochemical and clinical categories

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Trace normal protein quality control to insoluble extracellular fibrils.

Distinguish precursor overproduction from genetically variant proteins.

Classify deposits by biochemical precursor and tissue distribution.

Apply the classification to AL, AA, ATTR and selected localized forms.

Interpretation

Classification begins with the fibril protein—not solely the organ or associated disease.

Slide 3 · Amyloidosis: The unifying lesion

Proteins prone to aggregation assemble into extracellular fibrillar deposits.

Their persistence progressively impairs tissue architecture and function.

Amyloidosis denotes a family of disorders, not a single protein disease.

Interpretation

Common histologic appearance can conceal markedly different precursors.

Slide 4 · Why one morphology has many etiologies

More than 40 proteins can form amyloid, according to the textbook.

Different precursor proteins reach a shared fibrillar conformation.

Disease classification therefore depends on chemical identity and context.

Interpretation

Morphologic similarity does not establish an AL, AA or ATTR subtype.

Slide 5 · From precursor to tissue damage

Soluble precursor

Misfolded protein

Oligomers

Insoluble fibrils

Extracellular deposit

The key transition is formation of aggregation-prone conformations.

Accumulation, persistence and tissue location determine the consequences.

Slide 6 · Amyloid is extracellular

Aggregates coalesce into fibrils deposited between cells and in matrices.

Deposits may accumulate next to basement membranes and blood vessels.

Expanding material compresses adjacent parenchymal structures.

Interpretation

The deposited material lies outside cells even when the precursor originates intracellularly.

Slide 7 · Common fibril architecture

Schematic — fibril arrangement

Nonbranching parallel organization

Mechanistic interpretation

Electron microscopy: Nonbranching fibrils approximately 8–10 nm in diameter.

A cross-β-pleated organization underlies their common structural behavior.

The shared fibrillar geometry helps explain the Congo red reaction.

Slide 8 · Normal protein surveillance

Schematic — two parallel disposal routes

Intracellular misfolded protein

Proteasomal degradation

Extracellular misfolded protein

Macrophage clearance

Mechanistic interpretation

Intracellular misfolded proteins are typically removed by proteasomes.

Extracellular misfolded proteins can be cleared by macrophages.

These are parallel quality-control systems, not sequential steps.

Slide 9 · Two major routes to amyloidogenesis

Route 1: Excess / persistence

A normal protein is intrinsically capable of misfolding.

Overproduction or reduced degradation increases its available pool.

Examples: Light chains, SAA and wild-type TTR.

Route 2: Altered sequence

A variant protein is structurally more prone to abnormal folding.

Aggregation may occur despite production at ordinary levels.

Example: Pathogenic variant transthyretin.

Both routes converge on insoluble fibrils and tissue deposits.

Slide 10 · Amyloidogenesis is a balance problem

Precursor synthesis continuously supplies potentially amyloidogenic protein.

Proteolysis and cellular clearance constrain its effective concentration.

More substrate, less clearance or greater misfolding propensity shifts the balance.

Interpretation

Failure of protein-quality control as a key permissive event.

Slide 11 · Molecular assembly proceeds in stages

Abnormal monomers

Soluble oligomers

β-sheet assemblies

Stable fibrils

Accumulated deposits

Oligomer formation precedes large insoluble fibrillar aggregates.

Stage labels describe a conceptual sequence, not a kinetic timescale.

Slide 12 · Why β-structure matters

Schematic — repeated β-sheet packing

Stacking and side-chain interactions stabilize an ordered fibril.

Mechanistic interpretation

Amyloidogenic monomers adopt a β-sheet-rich structure.

Stacked protofilaments assemble into nonbranching fibrils.

This arrangement—not precursor identity—underlies shared staining properties.

Slide 13 · Deposits contain more than fibril protein

Amyloid associates with proteoglycans and glycosaminoglycans.

Heparan sulfate, dermatan sulfate and serum amyloid P-component occur in deposits.

These associated molecules coexist with the dominant fibrillar precursor.

Interpretation

Fibrillar protein as the great majority of the deposit.

Slide 14 · Excess precursor: General mechanism

Increased production

Higher precursor burden

Misfolding / aggregation

Fibril deposition

Normal sequences can become amyloidogenic when precursor load is excessive.

Reduced breakdown can act as an additional contributor.

Slide 15 · Immunoglobulin light chain: AL pathway

Monoclonal plasma cells

Excess free light chain

Sequence-dependent misfolding

AL fibrils

Systemic deposits

The fibril protein is immunoglobulin light chain or a fragment.

The plasma-cell clone is the source, not the identity of the deposited fibril.

Slide 16 · AL precursor identity

AL denotes amyloid light chain, derived from monoclonal immunoglobulin light chains.

Deposits contain complete light chains, amino-terminal fragments, or both.

The source notes λ chains predominate, although κ chains also form amyloid.

Interpretation

The prefix AL specifies the fibril protein; It does not mean every monoclonal gammopathy causes amyloid.

Slide 17 · Why only some free light chains aggregate

Plasma-cell proliferations can release large quantities of free light chains.

Most patients with circulating light chains do not develop amyloidosis.

Sequence-dependent physicochemical properties influence amyloidogenicity.

Interpretation

Abundance alone is insufficient: The particular light-chain sequence matters.

Slide 18 · AL may occur without overt myeloma

An underlying monoclonal plasma-cell proliferation remains the mechanism.

Classical myeloma features need not be evident when amyloid causes disease.

Monoclonal immunoglobulin and/or free light chains are usually detectable.

Interpretation

“Primary” is a historical clinicopathologic label; AL is the biochemical name.

Slide 19 · Inflammation supplies the AA precursor

Chronic inflammation

Macrophage cytokines

IL-1 and IL-6

Hepatic SAA production

Sustained circulating SAA

AA fibril formation

Serum amyloid A (SAA) is an acute-phase precursor.

Long-standing inflammatory activity sustains its hepatic synthesis.

Slide 20 · SAA is not itself deposited unchanged

Serum amyloid A circulates as a plasma acute-phase protein.

Proteolysis of the SAA precursor yields amyloid A (AA) fibril protein.

Persistent excess SAA increases substrate available for amyloidogenesis.

Interpretation

Precursor SAA and deposited AA are chemically related but not interchangeable labels.

Slide 21 · AA formation involves impaired processing

SAA synthesis

Macrophage-derived

Proteolytic processing

Incomplete degradation

(possible mechanism)

AA aggregation

Systemic deposition

Normal degradation yields soluble products.

The book discusses defective breakdown or resistant SAA structure as possible explanations.

Slide 22 · AA mechanism: Established vs proposed

Established: Inflammation increases SAA production through acute-phase signaling.

Established: Fibrils contain AA, generated from the SAA precursor.

Proposed: Incomplete macrophage degradation and/or structural resistance aid deposition.

Interpretation

Do not present proposed processing abnormalities as proven in every patient.

Slide 23 · AL and AA: Different upstream causes

AL: Monoclonal protein

Plasma-cell clone supplies a particular light-chain sequence.

Free light chain or fragments form the deposited fibril.

Associated with monoclonal plasma-cell disorders.

AA: Inflammatory response

Inflammatory cytokines increase hepatic SAA synthesis.

SAA-derived AA forms the deposited fibril.

Associated with chronic inflammatory diseases.

Both cause extracellular amyloid despite distinct source cells.

Slide 24 · ATTR begins with transthyretin

Transthyretin (TTR) is a circulating transport protein.

Its normal functions include binding and transport related to thyroxine and retinol.

Both wild-type and variant TTR can form amyloid under different conditions.

Interpretation

ATTR specifies transthyretin-derived fibrils, not an exclusively cardiac disease.

Slide 25 · Variant TTR: Sequence-mediated pathway

Inherited TTR variant

Increased propensity

To misfold

Aggregation

ATTRv fibrils

Nerve / heart / other sites

Specific variant polypeptides can favor distinct organ patterns.

The source describes ATTRv as autosomal dominant.

Slide 26 · ATTRv illustrates a normal-quantity problem

The pathogenic property may reside in the amino-acid sequence.

A variant precursor can misfold without exceptional overproduction.

Extracellular fibril assembly produces clinically important deposits.

Interpretation

Variant protein production contrasts mechanistically with sheer precursor excess.

Slide 27 · Wild-type TTR: Aggregation with aging

Wild-type TTR

Age-associated

Misfolding tendency

TTR aggregation

ATTRwt fibrils

Predominantly cardiac

Deposition

No inherited sequence variant is required for ATTRwt.

Older adults, commonly men, and cardiac involvement.

Slide 28 · ATTRwt versus ATTRv

ATTRwt

Protein sequence is wild type.

Typically age-associated; Heart is a major target.

Can also involve connective tissues such as ligaments.

ATTRv

Protein has an inherited amyloidogenic variant.

Autosomal dominant inheritance is described.

Heart, peripheral and autonomic nerves may be affected.

Both are transthyretin amyloidosis; Their etiologic labels are not interchangeable.

Slide 29 · Convergent pathogenesis of AL, AA and ATTR

AL

Light chains

AA

SAA-derived AA

ATTR

TTR protein

Extracellular

Amyloid fibrils

Mechanistic interpretation

AL: Monoclonal light-chain supply and sequence-dependent aggregation.

AA: Cytokine-driven SAA production plus altered processing.

ATTR: Variant or wild-type transthyretin becomes aggregation-prone.

Slide 30 · The deposit and the precursor are not synonyms

The circulating or cellular precursor supplies the substrate for fibrils.

Proteolysis may change the deposited protein, as in AA or AL fragments.

Biochemical typing asks which protein constitutes the amyloid fibril.

Interpretation

For classification, name the deposited fibril protein and identify its precursor.

Slide 31 · Location: At the source or distant

Localized amyloidosis

Amyloid forms at or near the site of precursor production.

Deposits may remain confined to one organ or tissue.

Some nodular forms are produced by local plasma cells.

Systemic amyloidosis

Precursor may circulate from a distant production site.

Deposits accumulate in multiple organs or tissues.

AL, AA and many ATTR forms are central systemic examples.

Distribution is a separate axis from fibril biochemical composition.

Slide 32 · Systemic deposition: Source-to-target separation

Precursor source

Circulating protein

Misfolding / persistence

Distant tissue deposit

Systemic disease is defined by its distribution, not by a unique microscopic morphology.

The exact sites affected vary between patients and subtypes.

Slide 33 · Localized deposition: Source and lesion overlap

Local source cell

Amyloidogenic protein

Local aggregation

Focal tissue deposit

A single organ can contain amyloid without generalized deposition.

Localized nodules can develop in lung, larynx, skin and other sites.

Slide 34 · Classification has independent axes

Protein

AL / AA / ATTR / other

Extent

Localized / systemic

Context

Clonal / inflammatory / inherited / age-related

Mechanistic interpretation

Fibril protein: AL, AA, ATTR and less common types.

Clinical distribution: Systemic or localized.

Etiologic context: Clonal, inflammatory, inherited, age-associated or local.

Slide 35 · Biochemical typing is central

Three common forms are designated by the chemical nature of the deposits.

The associated condition provides context but does not replace fibril typing.

The same organ can be affected by different amyloid proteins.

Interpretation

Avoid a classification scheme based only on “primary” and “secondary.”

Slide 36 · Three major amyloid families

Fibril type

Immediate precursor

Characteristic context

AL

Ig light chains

Monoclonal plasma-cell proliferation

AA

Serum amyloid A

Sustained inflammation

ATTR

Transthyretin

Wild-type aging or inherited variant

Each row has a distinct precursor; All form extracellular amyloid.

Slide 37 · AL is a systemic biochemical category

Usually generalized deposition derived from one amyloidogenic light-chain species.

It is associated with a monoclonal plasma-cell proliferation.

The absence of overt multiple myeloma does not exclude AL amyloidosis.

Interpretation

Etiology: Clone → amyloidogenic light chain → fibrillar deposits.

Slide 38 · AA is the inflammation-linked category

AA is derived from hepatic serum amyloid A rather than an immunoglobulin.

Examples include rheumatoid arthritis and inflammatory bowel disease.

Its deposits are generally systemic classification.

Interpretation

Etiology: Sustained inflammatory signaling → SAA supply → AA deposits.

Slide 39 · ATTR contains two mechanistically distinct forms

ATTRwt uses the ordinary transthyretin sequence and is age-associated.

ATTRv uses variant transthyretin and may be familial.

Organ predilection can differ even though both contain TTR fibrils.

Interpretation

Genetic status subdivides ATTR; It does not create a different fibril-protein family.

Slide 40 · Systemic examples classification

Category

Associated setting

Fibril

AL

Monoclonal plasma-cell disorders

AL

ATTRwt

Aging; heart often affected

ATTRwt

AA

Chronic inflammatory conditions

AA

Table is a teaching abstraction of the textbook Table 6.17.

Slide 41 · An involved organ does not identify a subtype

Heart, kidney and other tissues may be affected in multiple amyloid categories.

The textbook cautions against rigid organ-distribution patterns.

Tissue morphology establishes amyloid; Biochemical analysis establishes type.

Interpretation

A cardiac or renal deposit should not automatically be assumed to be AL or ATTR.

Slide 42 · Hereditary does not mean one fibril protein

Inherited susceptibility may reflect a variant deposited protein.

Alternatively, inherited inflammation can increase a normal precursor.

The category “hereditary” therefore overlaps with multiple biochemical classes.

Interpretation

ATTRv has variant TTR fibrils; Familial Mediterranean fever has AA fibrils.

Slide 43 · Familial Mediterranean fever: Indirect amyloid pathway

Pyrin-related

Autoinflammation

Excess IL-1 signaling

Sustained SAA synthesis

AA amyloid deposition

The familial disorder is associated with an autoinflammatory syndrome.

The deposited amyloid is AA—not mutant pyrin protein.

Slide 44 · Two hereditary mechanisms

Variant fibril protein

ATTRv: Inherited TTR variant is the amyloidogenic substrate.

The molecular change affects precursor fold stability.

Fibril identity follows the variant protein.

Inherited inflammatory driver

Familial Mediterranean fever: Altered inflammatory regulation.

Cytokines raise SAA concentration.

Fibril identity remains AA.

Inherited cause and deposited protein are different classification questions.

Slide 45 · Hemodialysis-associated amyloid

Reduced renal clearance

β₂-microglobulin

Retention

Long-term accumulation

Aβ₂m fibril deposition

Joints / tendons

Soft tissue

β₂-microglobulin is a normal component of MHC class I and plasma protein.

The burden decreased with newer dialysis filters.

Slide 46 · β₂-microglobulin is a different precursor

The amyloid protein derives from β₂-microglobulin rather than AL, AA or TTR.

Long-term dialysis can allow precursor retention and tissue deposition.

Periarticular disease may produce carpal tunnel syndrome.

Interpretation

Dialysis-related amyloidosis illustrates failed systemic clearance.

Slide 47 · Amyloid-β in the brain

Membrane APP precursor

Proteolytic cleavage

Amyloid-β peptide

Cerebral plaques /

Vascular deposits

Amyloid-β is derived from amyloid precursor protein (APP).

The source links these deposits to Alzheimer disease.

Slide 48 · Amyloid-β illustrates organ-restricted biochemistry

Cerebral plaques and cerebral vessels can contain amyloid-β.

Its immediate precursor is a proteolytic product of APP.

It is not equivalent to systemic AL, AA or ATTR deposits.

Interpretation

Identical amyloid staining does not imply an identical source protein.

Slide 49 · Local endocrine products can form amyloid

Some peptide hormones and endocrine-cell products are amyloidogenic.

These deposits are often microscopic and restricted to an organ or neoplasm.

The biochemical identity reflects the local secreted peptide.

Interpretation

Endocrine amyloid is a category of localized deposition, not a single protein.

Slide 50 · Endocrine and organ-localized amyloid

Site / setting

Fibril designation

Precursor

Medullary thyroid carcinoma

ACal

Calcitonin

Type 2 diabetic islets

AIAPP

Islet amyloid polypeptide

Isolated atrial amyloid

AANF

Atrial natriuretic factor

These entries are taken from the textbook classification table.

Slide 51 · Medullary thyroid carcinoma: ACal

Amyloid in the tumor stroma can derive from calcitonin.

The corresponding fibril designation in the source is ACal.

Local deposits provide a useful morphologic clue in this tumor.

Interpretation

Diagnostic significance depends on the tumor context; The deposit is not systemic AL.

Slide 52 · Pancreatic islets: AIAPP

Amyloid in type 2 diabetes can occur in pancreatic islets.

Deposits are derived from islet amyloid polypeptide.

The category demonstrates how a local protein forms amyloid in situ.

Interpretation

AIAPP names the fibril identity; Diabetes is the associated setting.

Slide 53 · Isolated atrial amyloidosis: AANF

Some atrial amyloid deposits derive from atrial natriuretic factor.

The source labels this localized type AANF.

It is biochemically distinct from transthyretin deposits in the heart.

Interpretation

The organ “heart” can contain several molecularly distinct amyloid proteins.

Slide 54 · Localized nodules may still contain AL

Focal amyloid masses can arise in lung, larynx, skin, bladder or tongue.

Some show adjacent lymphocytes and plasma cells.

At least a subset consists of AL made by locally situated plasma cells.

Interpretation

AL does not necessarily mean multi-organ systemic disease.

Slide 55 · Classification: Selected uncommon forms

Fibril

Source / precursor

Representative setting

Aβ

APP-derived peptide

Alzheimer cerebral deposits

Aβ₂m

β₂-microglobulin

Long-term dialysis

ACal

Calcitonin

Medullary thyroid carcinoma

AIAPP

Islet amyloid polypeptide

Type 2 diabetic islets

Examples are not an exhaustive list of human amyloid fibril proteins.

Slide 56 · How deposits produce organ dysfunction

Extracellular fibrils

Expanded interstitium /

Vascular walls

Distorted architecture

Cell pressure atrophy

Functional impairment

The major injury mechanism described is architectural disruption.

Deposits cause tissue compromise without necessarily inducing local inflammation.

Slide 57 · Pressure atrophy links structure to disease

Expanding material compresses cells and disrupts their tissue relationships.

Vascular wall deposits can narrow lumens and compromise perfusion.

Progressive replacement of normal parenchyma reduces functional reserve.

Interpretation

Microscopic “acellular” material can cause profound organ dysfunction.

Slide 58 · Deposits may also perturb cell signaling

Cell

ROS signaling

Ca²⁺ homeostasis

Mechanistic interpretation

Some amyloidogenic proteins can induce reactive oxygen species production.

Altered calcium homeostasis may contribute to cellular dysfunction.

These as additional contributors beyond mass effect.

Slide 59 · Renal versus cardiac consequences

Kidney

Deposits enlarge mesangium and glomerular capillary walls.

Progressive lumen obliteration impairs filtration.

Proteinuria and renal failure may follow.

Heart

Interstitial myocardial deposits separate and compress fibers.

Subendocardial disease may affect conduction structures.

Heart failure or arrhythmia can result.

The same extracellular lesion causes organ-specific dysfunction.

Slide 60 · Native tissue schematic: Vascular encroachment

Residual

Lumen

Schematic vascular cross section

Deposited amyloid

Mechanistic interpretation

Amyloid may deposit in vascular walls and perivascular connective tissue.

Increasing deposits reduce usable lumen and compress nearby cells.

This is an original conceptual diagram, not a histology specimen.

Slide 61 · Morphology raises suspicion—not protein identity

Routine H&E shows amorphous eosinophilic extracellular material.

Collagen and fibrin may produce deceptively similar hyaline appearances.

A specific amyloid stain is required to establish the nature of the deposit.

Interpretation

Fibril typing is a second diagnostic question after deposit recognition.

Slide 62 · Congo red: A structural recognition step

Suspicious H&E deposit

Congo red staining

Polarized light

Birefringent amyloid

The source describes red/pink staining in brightfield and characteristic birefringence.

A common cross-β structure underlies the staining reaction across subtypes.

Slide 63 · Detection and typing are distinct

Demonstrate amyloid

Appropriate tissue sample and histology.

Congo red and polarized microscopy.

Ultrastructure can show nonbranching fibrils.

Identify fibril protein

Type-specific immunohistochemistry may help.

Mass spectrometry can identify protein composition.

The biochemical answer guides clinical classification.

Congo red positivity alone does not identify AL versus AA versus ATTR.

Slide 64 · Biopsy strategy follows organ involvement

Histologic demonstration in tissue is the diagnostic foundation.

Kidney can be biopsied for renal manifestations; Other sites may be sampled.

Abdominal fat aspirate is a less invasive option for some systemic cases.

Interpretation

A negative limited sample cannot automatically exclude systemic amyloid.

Slide 65 · Clinical classification sequence

Demonstrate amyloid

Type deposited protein

Determine systemic

Or localized extent

Identify precursor source

Link to organ injury

Distribution, molecular identity and cause must be integrated.

An organ-based label alone cannot replace biochemical classification.

Slide 66 · Same appearance, different molecular diagnoses

Evidence

Compatible interpretations

Congo red positive kidney biopsy

AL, AA or another systemic protein

Cardiac interstitial amyloid

ATTRwt, ATTRv, AL and others

Localized thyroid tumor amyloid

ACal in appropriate tumor context

Periarticular amyloid during dialysis

Aβ₂m is an important consideration

Examples teach diagnostic discrimination; They are not exclusive mappings.

Slide 67 · Case: Monoclonal precursor

Clinical premise

Patient with systemic tissue amyloid and a monoclonal plasma-cell population.

AL is an important biochemical possibility.

The plasma-cell clone may produce amyloidogenic free light chains.

Confirmation still requires identifying the protein in the deposit.

Slide 68 · Case: Persistent inflammatory stimulus

Clinical premise

Patient with long-standing active inflammatory arthritis and renal amyloid.

AA amyloidosis is mechanistically plausible.

Persistent IL-1/IL-6–linked signaling increases hepatic SAA.

The clinical history alone does not conclusively type the fibrils.

Slide 69 · Case: Older adult with cardiac amyloid

Clinical premise

Older adult with predominantly cardiac amyloid and no specified inherited TTR variant.

ATTRwt belongs in the differential.

Normal transthyretin can form amyloid with aging.

AL and other possibilities must not be dismissed without typing.

Slide 70 · Case: Affected family with neuropathy

Clinical premise

Several relatives develop peripheral or autonomic neuropathy with amyloid deposits.

Consider ATTRv and inherited amyloid disorders.

Specific TTR variants may show neural predilection.

Biochemical and genetic distinctions remain necessary.

Slide 71 · Case: Isolated thyroid tumor deposit

Clinical premise

Medullary thyroid carcinoma includes stromal amyloid without evidence of generalized disease.

Calcitonin-derived ACal is a source-described form.

The amyloid is linked to a local endocrine product.

Do not interpret every tumor-associated amyloid deposit as systemic AL.

Slide 72 · Integrated synthesis: Cause → fibril → distribution

Cause

Clone / inflammation / TTR variant

Fibril identity

AL / AA / ATTR / others

Distribution

Localized / systemic

Mechanistic interpretation

First identify the molecular precursor and amyloidogenic route.

Then establish where deposits occur and how tissue architecture is altered.

Finally connect the subtype to its defining etiologic setting.

Slide 73 · Four distinctions to retain

Misfolded precursor ≠ mature deposited fibril.

Overproduced normal protein ≠ inherently amyloidogenic variant protein.

Systemic versus localized ≠ AL versus AA versus ATTR.

Amyloid detection ≠ amyloid protein typing.

Interpretation

These distinctions prevent the most important category errors in amyloidosis.

Slide 74 · Reference

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026).

Amyloidosis — disease mechanisms, fibril chemistry and clinicopathologic forms.

Primary section: Pathogenesis and Classification of Amyloidosis.

See the adjacent AL, ATTR, AA and Other Types sections for the classification examples.