Amyloidosis is a heterogeneous disease that results from the deposition of toxic insoluble beta-sheet fibrillar protein aggregates in different tissues. Amyloidosis can be acquired or hereditary. The disease can be localized or systemic. Amyloid can accumulate in liver, spleen, kidney, heart, nerves, and blood vessels causing different clinical syndromes including cardiomyopathy, hepatomegaly, proteinuria, macroglossia, autonomic dysfunction, ecchymoses, neuropathy, renal failure, hypertension, and corneal and vitreous abnormalities.
The most common causes of amyloidosis are the immunoglobulin-light-chain relate amyloidosis (AL), ATTR amyloidosis, and reactive amyloidosis (AA) due to chronic inflammatory diseases like chronic infections and rheumatoid arthritis. AL amyloidosis is acquired and is caused by a small plasma cell clone that produces misfolded amyloidogenic light chains that deposit in different organs and tissues.
AL amyloidosis has an incidence of 1 case per 100,000 person-years in Western countries. In the United States, there are approximately 1275 to 3200 new cases per year. The annual proportion of new cases with AL is 78%. Familial transthyretin-associated amyloidosis (ATTR) is a less common systemic type of amyloidosis with unknown incidence, but approximately 10% to 20% of diagnosed cases in tertiary centers are secondary to ATTR amyloidosis. Of these cases, seven percent are hereditary and result from mutated transthyretin (ATTRm) and approximately six percent represent the acquired age-related, wild-type ATTRwt amyloidosis. ATTRwt is seen more commonly in males and was formerly known as senile systemic amyloidosis. Secondary or AA amyloidosis represents 6% of all the amyloidosis cases diagnosed each year. AA amyloidosis is an acquired process and reactive due to chronic inflammation.
Twenty-one different proteins have been identified as amyloidogenic agents. Polypeptides can adopt alternative misfolded states making them prone to aggregate. There are multiple processes by which misfolding of protein precursors occur. The protein may be intrinsically likely to acquire a pathologic conformation with aging as seen in patients with wild-type transthyretin senile systemic amyloidosis. This can also happen when there is high serum concentration of protein precursors as seen in long-term hemodialysis patients with increased levels of beta-2-microglobulin. In hereditary amyloidosis, replacement of single amino acids can lead to amyloidogenic misfolded proteins losing the biologic function of the native proteins and aggregate. Proteolytic remodeling of beta-amyloid precursor proteins has been identified in Alzheimer disease. In patients with AA amyloidosis, serum amyloid A, an acute phase protein deposits in different tissues. The amyloidogenic variants of the various precursor proteins are thermodynamically less stable, resulting in tetramers dissociation into monomers in the case of transthyretin and destabilization of the tertiary structure leading to partially folded conformers in the case of lysozyme. Transthyretin monomers and lysozyme partially folded conformers have the propensity to aggregate and assemble into fibrils. Charged residues also play a role in modulating the aggregation using repulsive forces resulting in partial unfolding, rendering the protein susceptible to proteases attacks and release of amyloidogenic polypeptides. This is the case of gelsolin, an amyloidogenic protein which causes systemic amyloidosis. Low pH, increased temperatures, limited proteolysis, osmolytes, and metal ions can alter the tridimensional structure of proteins shifting the equilibrium towards the amyloidogenic state. The mechanism of tissue damage in amyloidosis involves alteration of tissue architecture, interaction with cell surface receptors, inflammation elicited by the amyloid protein deposition, oxidative stress, and apoptosis activation.
The clinical features of amyloidosis vary depending on which type of amyloid fibrils are responsible. Systemic amyloidosis can lead to heart failure with left ventricular hypertrophy on echocardiogram with standard or low voltage electrocardiogram. Hepatomegaly, nephrotic syndrome, macroglossia, orthostatic hypotension, ecchymosis and autonomic and peripheral neuropathy can be present. Carpal tunnel syndrome, jaw claudication, and articular deposits of amyloid can also be a manifestation of systemic amyloidosis. In secondary amyloidosis (AA), hepatosplenomegaly, proteinuria, renal failure, and orthostasis can be seen. ATTR amyloidosis onset is during midlife and presents with peripheral and autonomic neuropathy, cardiomyopathy, and vitreous opacities. Amyloid beta-amyloidosis is localized to the central nervous system and presents as sporadic Alzheimer disease and aging.
Other physical exam findings can lead to suspicion of amyloidosis diagnoses like shoulder pads from amyloid deposition, amyloid purpura, and raccoon eyes secondary to factor-X deficiency in the case of AL amyloidosis and prolonged PT, PTT, that correct with mixing studies pointing towards the involvement of factor deficiencies in the common pathway of coagulation.
Clinical suspicion, family history, and tissue biopsy establish the diagnosis.
Plasma cell clone identification
Serum and urine electrophoresis with immunofixation and free light chains (FLC) should be requested to rule out plasma cell dyscrasia. When monoclonal light chains are not present, bone marrow biopsy can help establish the diagnosis by immunohistochemical staining. Immunofluorescence in situ hybridization should be ordered as well and skeletal survey.
When plasma cell dyscrasia is ruled out, other types of amyloidosis should be considered. Tissue typing can be done by mass spectrometry, immune electron microscopy or immunohistochemistry. Transthyretin can be detected by isoelectric focusing on the serum which separates wild-type (or formerly known senile cardiac amyloidosis) and variant transthyretin.
Should be done when hereditary amyloidosis has to be ruled out based on clinical grounds.
When transthyretin has not been identified, and patients have macroglossia and other typical organ involvement, AL should still be suspected despite plasma cell dyscrasia absence. ATTRwt is diagnosed by cardiac biopsy showing positivity to antibodies against normal transthyretin. Cardiac Scintigraphy with bone tracers can help differentiate AL amyloidosis which shows mild or no uptake from transthyretin amyloidosis which has strong uptake. This could potentially spare cardiac biopsy.
AA amyloidosis is considered when transthyretin and AL have been ruled out, and there are kidney involvement and neuropathy. Immunohistochemistry aids in the diagnosis of AA.
Organ involvement and staging of the disease should be established to design the treatment plan. For cardiac function, evaluation should include an echocardiogram with an assessment of strain, NT-proBNP, troponins, ECG, Holter ECG, and cardiac MRI should be solicited. For kidney function, evaluation 24-hour urinary protein and eGFR are needed. Liver function test and imaging (ultrasound (US), MRI, or CT scan) can help with hepatic function assessment.
AL amyloidosis: This disease is treated most likely in the framework of clinical trials and depending on the risk stratification according to the Standard Mayo Clinic staging system.
ATTR amyloidosis: Transthyretin is a protein predominantly synthesized in the liver. Upon liver transplant, mutant transthyretin disappeared from blood and neuropathy improvement was observed.
Supportive therapy: The treatment of systemic amyloidosis involves supportive therapy aiming to maintain the quality of life and prevent organ dysfunction. If patients are on a heart transplant waiting list, they should receive low-dose chemotherapy due to increased survival.
Splenectomy: An option with proper prior vaccination when there is severe factor-X deficiency leading to bleeding diathesis. Factor X deficiency is seen in 2.5% of patients with AL amyloidosis. Splenectomy is effective in patients who have splenomegaly, but not usually in patients with a normal size spleen.