solving amyloidosis

First described by none other than Rudolph Virchow (pictured, rocking a killer beard) in 1853 in reference to abnormal extracellular material found in the liver on autopsy, amyloidosis refers to the accumulation of misfolded extracellular proteins that are deposited in tissue as bundles of beta-sheet, fibrillar insoluble aggregates and ultimately result in end-organ pathology. It is seen in numerous diseases which can be stratified as follows:

Diseases in which amyloid accumulation is critical to pathogenesis: Alzheimer's disease (APP accumulation in the brain), Creutzfeldt-Jacob Disease (PrP accumulation in the brain), dialysis-associated amyloidosis (beta-2 microglobulin accumulation in the joint spaces), and senile/systemic amyloidosis (transthyretin accumulation in the brain, heart, and kidney).

Diseases in which amyloidosis is a disease-modifying side effect: multiple myeloma (accumulation of immunoglobulin light chain in kidneys and peripheral nerves), and reactive amyloidosis of chronic inflammatory disease (amyloid A accumulation).

How does amyloidosis occur? When proteins are initially translated from mRNA, their three-dimensional structure is determined by specific interactions of the amino acid side chains. With the assistance of chaperone proteins, newly produced polypeptides assemble into the secondary, tertiary, and quatenary structure at the lowest energy configuration. However, certain proteins may have more than one low-energy state, depending on the protein microenvironment: the properly assembled, functional assembly, and a misfolded assembly driven by hydrogen bonding between the amide and carbonyl groups of the main amino acid chain, which is facilitated when the polypeptides are layered in antiparallel linear sheets in which the N and C termini are oriented in opposite directions. Therefore, there are 2 major issues of concern:

1) What type of proteins are susceptible to misfolding? Certain intrinsically susceptible proteins such as transthyretin, proteins that acquire a susceptible point mutation, as seen in hereditary amyloidosis, and proteins that achieve saturating concentrations in serum, such as beta-2 microglobulin in dialysis patients.

2) What conditions support pathologic misfolding of the aforementioned proteins? Conditions that interfere with weak peptide interactions (low pH, accumulation of free radical species, high temperature).

It is important (foreshadowing) to note that amyloid accumulations are not composed simply of these fibrillar beta-sheets. A critical component of amyloid is SAP, a glycoprotein that binds the common conformation of amyloid fibrils and which is protected against proteolysis, making amyloid fibrils resistant to degradation.

The presenting symptoms of amyloidosis depend on the primary tissue in which amyloid is deposited, despite the fact that in patients with amyloidosis, autopsy routinely demonstrates some degree of deposition in every organ system. Perhaps because of this, the most common initial presenting symptoms (although not that which leads to diagnosis) in patients with amyloidosis are fatigue and weight loss. End-organ damage in amyloidosis is primary caused by tissue distortion, although there may a role for an inflammatory response to deposited amyloid as well. The most common organs affected are the heart and kidneys, but other organs can be affected as well:

Renal amyloidosis often presents with the nephrotic syndrome, with renal failure, proteinuria, hyperlipidemia, and hypoalbuminemia.

Cardiac amyloidosis often presents as a restrictive cardiomyopathy with signs of right-sided heart failure (elevated jugular venous pulses, hepatic congestion, and peripheral edema). EKG often shows low voltage, and echocardiography often reveals concentric hypertrophy with elevated filling pressures. Patients may present with an MI despite no coronary artery disease. Amyloid infiltration often has a predilection for the cardiac conduction system resulting in bradyarrhythmias and even asystole.

Neuropathies are usually autonomic (orthostatic hypotension) or sensory (carpal tunnel or symmetric, distal, painful sensory neuropathies).

GI symptoms are often secondary to enteric dysfunction, resulting in constipation and/or diarrhea, but diffuse infiltration can often result in malabsorption

Vascular infiltration results in friable blood vessels, presenting as easy bruising, with the characteristic "raccoon eyes" (spontaneous periorbital bruising following rubbing of the eyes or nose-blowing).

Soft tissue involvement results in macroglossia, submandibular enlargement.

Diagnosis is classically obtained by either involved organ or abdominal fat pad biopsy. Following Congo-Red staining of tissue, apple-green birefringence under polarized light is pathognomonic. Once found, patients should be evaluated for occult plasma cell dyscrasias, transthyretin mutations, or collagen vascular disease.

Treatment of amyloidosis has been notoriously difficult, and has revolved around reducing the rate of synthesis of the amyloidogenic protein. This is based on the idea that slow resorption of amyloid components by tissue-resident macrophages is opposed by constant renewal of amyloid protein. This has proven to be notoriously difficult, other than cases in which the culprit disease (multiple myeloma, collagen vascular disease) can be treated. Vaccination against the amyloidogenic variants of A-beta peptides in Alzheimer's Disease showed some promise in early mouse studies, but has been largely disappointing in human clinical trials. Because of this, overall prognosis of patients with amyloidosis has been overwhelmingly poor.

So why have I wasted this much breath on this topic? Because there's been a breakthrough, that's why!!

We salute you, Mark Pepys' lab at the University College London! Because you did something awesome. You realized that SAP (remember??) is critical for the stability of amyloid fibrils in vivo. You then developed a proline-based compound, CPHPC (the full name is too long), which effectively binds and depletes circulating human SAP (a component of C-reactive protein) and leaves only SAP which is bound to amyloid. You then treated this residual sap with a specific IgG antibody. Right off the bat, you showed that CPHPC followed by anti-SAP treatment significantly reduced tissue amyloid burden without any significant adverse effects in mice with human SAP-induced amyloid A-driven angiopathy (a mouse model for inflammatory amyloidosis). You then carefully demonstrated the mechanism by which this occurs:

- 24 hours after antibody treatment, you show mononuclear, primarily macrophage infiltration of amyloid accumulations, as indicated by F4/80 staining in (b).
- 48 hours after treatment, you show coalescence of macrophages into multinucleated giant cells containing amyloid within endocytic compartments, as seen in (e).
- 96 hours after, you show that diminished, fragmented amyloid fibrils are largely contained within multinucleate giant cells, as seen in (d).
-Finally, you demonstrate that this process is not only macrophage dependent, but also Fc and C3 dependent, providing a step-by-step mechanism for complement mediated clearance.

What else is there to say? With a humanized monoclonal anti-SAP antibody already being tested, this work may ultimately constitute a paradigm shift in the treatment of this disease.

Stay tuned for my next post, about hormones! The lady kinds!