Hereditary Transthyretin Amyloidosis (hATTR)

According to the current nomenclature guidelines, the familial forms of transthyretin-mediated (ATTR) amyloidosis, formerly known as familial amyloid polyneuropathy (FAP) or familial amyloid cardiomyopathy (FAC), should now be referred to as hereditary ATTR (hATTR) amyloidosis, and a characterization of the dominant features and mutations should be included if known.¹ More than 120 TTR gene mutations that contribute to hATTR amyloidosis have been identified. The most common pathogenic variant, Val30Met, is linked mainly to neuropathy.² 

To confirm a diagnosis of amyloidosis, it is essential to demonstrate the presence of amyloid deposits via tissue biopsy.³ Characteristic amyloid deposits can be seen in various tissues throughout the body, including fat pads, skin, nerve, myocardium, kidney, labial salivary gland, and gastrointestinal mucosa.^4,5  In hereditary amyloid syndromes, amyloid deposits can be found in variable locations.⁶ The sensitivity of endoscopic biopsy of the gastrointestinal mucosa is approximately 85%.³ The sensitivity of aspiration of abdominal fat pads is approximately 50%, and salivary gland biopsy has a documented sensitivity of 75% to 90% in skilled hands. In contrast, nerve biopsy is typically regarded as a second-line test, with a sensitivity that has been reported to be as high as 80%. Nonetheless, negative biopsy results in hATTR continue to be a common reason for diagnostic delays.² 

On macroscopic examination, organs with amyloid deposits may appear normal, or nodularity or organomegaly may be seen. The cut tissue appears pale gray or waxy, with a firm consistency.^6,7 On histological examination with routine hematoxylin and eosin staining, amyloid is usually seen in the extracellular space, appearing as pink amorphous material. The deposits are located between cells, frequently near basement membranes. As the amyloid accumulates, it eventually engulfs and kills cells. ^6-8 

A histological diagnosis of amyloidosis is based on the characteristics of amyloid staining. The stain most commonly used is Congo red, which causes amyloid to appear pink or red under ordinary light. The presence of a characteristic apple green birefringence when amyloid stained with Congo red is viewed under polarized light is the gold standard for a generic diagnosis of amyloidosis, irrespective of the protein type. This reaction is due to the cross-β-pleated configuration of amyloid fibrils and is common to all forms of amyloid.^3,6-9 

Other stains that can be used in addition to Congo red include fluorescent stains, such as thioflavin S or T, and metachromatic stains, such as crystal violet. Fluorescent dyes, because of their high sensitivity, may be helpful in screening tissues for amyloid deposits, but positive results should be confirmed by more specific methods.^8,10 

Under the electron microscope, amyloid appears as rigid, nonbranching fibrils 10 to 15 nm in diameter. Each fibril consists of a bundle of 2 to 5 filaments organized in a twisted ribbon-like arrangement. However, electron microscopy alone is not always adequate for a diagnosis of amyloidosis because the ultrastructure of amyloid fibrils is not always recognized correctly.¹⁰ 

Following the demonstration and confirmation of the presence of amyloid, the type of precursor protein should be determined.² Although amyloid has traditionally been typed with immunofluorescence in frozen sections and immunohistochemistry in paraffin sections, mass spectrometry has become the technique of choice, especially for paraffin sections.⁸

It is important to mention that the distribution of amyloid fibrils in tissues is patchy and uneven. A diagnosis of hATTR amyloidosis should therefore not be ruled out in case of a negative biopsy result. In addition to the skill of the pathologist, other factors affecting the sensitivity of biopsy include the type of tissue being tested, the TTR mutation, and the patient’s age; in some circumstances, it may be necessary to obtain more than one specimen for a diagnosis.² 

Cardiac Biopsy Specimens

Any type of systemic amyloidosis can cause cardiac amyloidosis. The heart is usually normal in gross appearance but may be firm and enlarged. Amyloid deposits start to form focally in a subendocardial location and within the myocardium in the spaces between the muscle fibers. Myocardial fiber pressure atrophy ultimately develops as a consequence of the expansion of myocardial amyloid deposits. The cardiac conduction system may be affected when the amyloid deposits are subendocardial, which may explain the electrocardiographic disturbances seen in some patients.⁶ 

Nerve Biopsy Specimens

Presently, sural nerve biopsy is advocated because fat aspiration is thought to be insensitive and nonspecific in the assessment of amyloid neuropathy. Sural nerve biopsy has a sensitivity of about 83% in hATTR amyloidosis, which is the most prevalent form of amyloidosis linked to polyneuropathy in the United States and throughout the world.⁸ The axonal degeneration of short nerve fibers caused by amyloid deposits in the peripheral nerves results in polyneuropathy. Sural nerve biopsy is frequently used to make the diagnosis, although the deposits may be proximal to the sural nerve and thus not detectable in biopsy specimens.⁴ The patchy distribution of amyloid may hinder the usefulness of diagnostic procedures and make it necessary to obtain additional nerve samples or specimens from other organs.⁹ 

A study of sural nerve biopsy specimens from cases of hATTR amyloidosis revealed a marked loss of myelinated fibres in the Val30Met variant and a complete loss of myelinated fibres in the Phe64Leu variant in tissues stained with toluidine blue.² 

References

1. Benson MD, Dasgupta NR, Rao R. Diagnosis and screening of patients with hereditary transthyretin amyloidosis (hATTR): current strategies and guidelines. Ther Clin Risk Manag. 2020;16:749-758. doi:10.2147/TCRM.S185677
2. Luigetti M, Romozzi M, Bisogni G, et al. hATTR pathology: nerve biopsy results from Italian referral centers. Brain Sci. 2020;10(11):780. doi:10.3390/brainsci10110780
3. Ando Y, Coelho T, Berk JL, Cruz MW, et al. Guideline of transthyretin-related hereditary amyloidosis for clinicians. Orphanet J Rare Dis. 2013;8(1):1-8. doi:10.1186/1750-1172-8-31
4. Roberts JR. Transthyretin-related amyloidosis workup. Medscape. Updated June 16, 2022. Accessed on July 19, 2022. 
5. Luigetti M, Romano A, Di Paolantonio A, Bisogni G, Sabatelli M. Diagnosis and treatment of hereditary transthyretin amyloidosis (hATTR) polyneuropathy: current perspectives on improving patient care. Ther Clin Risk Manag. 2020;16:109-123. doi:10.2147/TCRM.S219979
6. Kumar V, Aster JC. Abbas AK. Diseases of the immune system. In: Robbins & Cotran Pathologic Basis of Disease, 10th ed. New York, NY: Elsevier; 2020:chap 6.
7. Kim MJ, Baek D, Truong L, Ro JY. Pathologic findings of amyloidosis: recent advances. In: Kurouski D, ed. Amyloid Diseases. April 3, 2019. doi:10.5772/intechopen.84268  
8. Picken MM. The pathology of amyloidosis in classification: a review. Acta Haematol. 2020;143(4):322-334. doi:10.1159/000506696
9. Carroll A, Dyck PJ, de Carvalho M, et al. Novel approaches to diagnosis and management of hereditary transthyretin amyloidosis. J Neurol Neurosurg Psychiatry. 2022;93(6):668-678. doi:10.1136/jnnp-2021-327909
10. Tan SY, Pepys MB. Amyloidosis. Histopathology. 1994;25(5):403-414. doi:10.1111/j.1365-2559.1994.tb00001.x

Reviewed by Hasan Avcu, MD, on 7/26/2022.

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Harshi Dhingra, MD

Harshi Dhingra is a licensed medical doctor with specialization in Pathology. She is currently employed as faculty in a medical school with a tertiary care hospital and research center in India. Dr. Dhingra has over a decade of experience in diagnostic, clinical, research, and teaching work, and has written several publications and citations in indexed peer reviewed journals. She holds medical degrees for MBBS and an MD in Pathology.