What is amyloidosis2019-09-24T17:14:00+00:00

What is Amyloidosis?

This section of the website is under construction and will be completed by the end of September. It will provide an introduction to amyloidosis, cover definitions and nomenclature, describe  the most common types and basic workup highlighting the pitfalls and misconceptions in the diagnostic algorithm. A summary of its content follows.

Introduction

  • Amyloid is a deposit consisting of normally soluble protein molecules that have formed insoluble fibrils
  • Amyloid can be made from 36 proteins
    • Most of these proteins are derived from endogenous human proteins eg light chain, transthyretin, serum amyloid A, fibrinogen and semenogelin
  • Amyloidosis is the disease caused by deposits of amyloid fibrils that have aggregated in the extracellular tissues resulting in disruption and impairment of organ structure and function of the affected organs
  • Amyloidosis is a heterogenous disorder with amyloid made of different proteins caused by different amyloidogenic mechanisms with different clinical phenotypes, organ tropism, prognosis and treatments
  • It can feel overwhelming when trying to accurately type the wide variety of amyloidoses and it is helpful to focus on the most common forms of amyloidosis
  • The amyloidosis types referred to AAN clinics are;
    • ATTRwt 46%
    • AL 30%
    • Localised Amyloidosis 12%
    • ATTRv 5%
    • AA 3%
    • Non ATTRv hereditary amyloidosis 2%
    • ALECT2 1%
    • Unknown 1%
  • However, from each specialist’s point of view, there is a different distribution of amyloidosis types that will be encountered
  • (The cerebral amyloid angiopathies have a distinctly different workup and management and are not included in this website)
  • The nomenclature system starts with an “A”, signifying amyloid, followed by letters which are an abbreviated form of the constituent protein name e.g.
    • Light chain (L) amyloid is AL and transthyretin (TTR) amyloid is ATTR
      • Sometimes the same precursor protein can form amyloid via both acquired and inherited mechanisms. To distinguish between the two forms of amyloidosis
        • “wt” denotes amyloid derived from wild type protein aka ATTRwt
        • “v” denotes a variant protein created by an amyloidogenic gene aka ATTRv
  • It is important to be familiar with amyloid typing tests and the organ staging approach in order to diagnose and manage amyloidosis
  • A primary role of the AAN clinics is to assist with the typing of amyloidosis. See the Clinics list
    • However, initiating the basic diagnostic workup can expedite the diagnostic process and allow for timely treatment and improved outcomes
  • Amyloid can now be confirmed and typed in two ways
  1. Tissue biopsy and Anatomical Pathology testing
  2. Cardiac amyloid scintigraphy using the validated bone tracers DPD/PYP/HMDP
  • All patients with amyloidosis identified by either of these means still require
  1. Monoclonal gammopathy screening
  2. Organ staging performed by clinical assessment or non-invasive tests
  1. Tissue Biopsy
    • Amyloid is identified on tissue with Congo Red Stain showing salmon pink coloured amorphous deposits which display apple green birefringence under polarised light
    • Anatomical pathology typing starts with immunohistochemical stains for the most common amyloid proteins: TTR, kappa and lambda, AA. See Anatomical Pathology Lab contacts
      • It must be noted that these stains are prone to false negative and false positive results
      • Their positive predictive value is in the order of 60%
        • This is because the stains have been developed against physiologic TTR, AA, kappa and lambda proteins (leading to false positives and increased background staining) and are not specific for variant forms of amyloid proteins such as monoclonal light chain or genetically altered TTR (leading to false negative results)
      • It is important not to accept the immunohistochemical staining result in isolation and to always correlate with organ staging, monoclonal gammopathy screening results and other typing tests (+/- cardiac amyloid scintigraphy, +/-genetic tests)
  1. Cardiac amyloid scintigraphy using bone tracers (DPD/PYP/HMDP)
  • If cardiac uptake is greater than or equal to bone uptake and there is no monoclonal gammopathy then the patient has cardiac ATTR with 100% specificity and 100% positive predictive value
    • Biopsies are not required in these cases
  • These scans can also show uptake in a minority of cardiac AL cases
    • When a monoclonal gammopathy is present, the specificity of uptake on cardiac amyloid bone scintigraphy decreases to approximately 91%
    • In other words, if there is a positive cardiac amyloid scan and coincident monoclonal gammopathy 91% will have ATTR (and an unrelated coincident monoclonal gammopathy) and ≈9% will have cardiac AL
      • For definitive typing, a biopsy and Anatomical Pathology typing of the amyloid deposits is required to differentiate between the two
    • To put this situation into context, ≈20% of ATTR have an unrelated monoclonal gammopathy and so overall ≈ 2% of those with a positive cardiac amyloid scintigraphy scan will have AL

After amyloid has been detected by biopsy or cardiac amyloid scintigraphy then it is mandatory to do

  1. Monoclonal gammopathy screening and
  2. Organ staging
  1. Monoclonal gammopathy screening is required as
    • Almost all plasma or B-cell clones (≈99%) causing amyloidosis can be detected by performing monoclonal gammopathy screening with
      • Serum free light chain assay
      • Serum Protein Electrophoresis + immunofixation (SPEPG + IFE)
      • Urine Protein Electrophoresis + immunofixation (UPEPG + IFE)
    • This is also required as a small percentage (≈1-2%) of those with positive cardiac amyloid scintigraphy will have cardiac AL rather than ATTR
    • Immunohistochemical typing of AL amyloid using kappa and lambda stains has poor diagnostic and predictive value
  1. Organ staging is required to determine if there is localised versus systemic amyloidosis
    • Localised amyloidosis is a disorder characterized by single non-vital organ tissue production and deposition of amyloid
      • Localised amyloid usually involves the mucosal, cutaneous or glandular tissues
    • Systemic amyloidosis is a disorder where an amyloidogenic protein produced at a distant site deposits in vital organs causing organ failure and reduced survival
      • The most common systemic amyloidoses are ATTRwt followed by AL
    • Patterns of organ involvement can also assist with typing as the systemic amyloidoses have differing organ tropism patterns
    • There are simple clinical, pathology and radiologic tests for screening for vital organ involvement by amyloid
      • In summary these assessments are
        • Cardiac: Heart failure assessment/NYHA score, ECG, NTproBNP, troponin (+/- TTE, +/-cardiac MRI)
        • Renal: proteinuria assessment with 24-hour urine protein or random urine protein-creatinine ratio, albumin-creatinine ratio
        • GIT- upper and lower symptoms (and if present +/- confirmatory biopsy)
        • Liver- LFT for an obstructive pattern, assess for hepatomegaly
        • Nerves- clinical assessment for sensory, motor or autonomic neuropathy
      • More difficult to access tests such as cardiac MRI and NCS are not routinely required to screen for organ involvement
  • In certain circumstances light dissection of the amyloid from the paraffin embedded tissue block followed by trypsin digestion and mass spectrometry analysis of the constituent amyloid proteins can identify the precursor amyloid protein. This method has become an invaluable tool in identifying the amyloid precursor protein in difficult cases. See the Laboratory contacts > ‘other tests’ section
    • Mass spectrometry is used when the type remains uncertain after the initial workup and common situations where it is required are:
  • Isolated cardiac amyloidosis with a monoclonal gammopathy where the TTR, kappa, lambda stains are non-diagnostic in order to differentiate between ATTR with an unrelated monoclonal gammopathy vs AL
  • Isolated renal disease without a monoclonal gammopathy or chronic inflammatory disorder as the following amyloidosis types may be present (where there are no readily available immunohistochemical stains)
    • ALect
    • Inherited amyloidosis i.e. AFib, AApoA1
    • Other rarer amyloidosis i.e. AH (heavy chain amyloid)
  • Genetic testing is an important typing tool and indicated in certain circumstances. See the Laboratory contacts > ‘other tests’ section
  • There are over 100 gene mutations that can cause systemic amyloidosis and most lie in 4 proteins’ encoding regions
    • Screening for the known genetic mutations that cause amyloidosis is performed on a blood sample
      • Polymerase chain reaction of the exons of the known gene regions that carry mutations is followed by bi-directional Sanger sequencing
    • Genetic testing is most commonly required in
      • ATTR to differentiate between ATTRwt and ATTRv
      • In non-AL, non-ATTR systemic amyloidosis
        • mass spectrometry analysis is commonly run in parallel in this scenario