Worming along in amyloid cardiotoxicity

In this issue of Blood, Diomede and colleagues establish a novel Caenorhabditis elegans model of primary amyloid cardiotoxicity. Using pharyngeal pumping as a surrogate for cardiac function, they find that human amyloidogenic light-chain proteins directly impair pharyngeal activity. Direct toxicity was associated with the release of reactive oxygen species, and pharyngeal activity was restored with use of antioxidant agents.¹ 

Primary amyloidosis is the most common systemic amyloidosis in the developed world, characterized by the release of amyloidogenic mutant light-chain proteins from plasma cells and systemic deposition in peripheral tissues as proteinaceous β-sheet fibrils.²  Involvement of the heart is associated with the worst prognosis and results in the development of an amyloid cardiomyopathy, often refractory to commonly used heart-failure agents and accompanied by progressive cardiac deterioration and early cardiovascular death.³  Although the origins of systemic amyloidosis dates back to the mid-19th century and the observations of the famed pathologist, Rudolph Virchow, our understanding of this disease process, and in particular amyloid cardiomyopathy, has evolved little since.⁴  Clinical work has detailed a curious observation with a lack of correlation between the amount of amyloid fibril deposition within the heart and the degree of cardiac dysfunction.⁵  Furthermore, recent work has identified a direct cardiotoxic response specific for amyloidogenic light-chain proteins isolated from patients with amyloid cardiomyopathy, but not for control, nonamyloidogenic light-chain proteins⁶  with activation of specific stress-responsive signaling cascades.⁷  Accelerated understanding of this cardiotoxic response, however, has been limited by the lack of a suitable model organism of disease for study. Although a mouse model has been generated with genetic overexpression of a mutant amyloidogenic light-chain protein, the model fails to replicate the cardiotoxicity or cardiomyopathy that occurs in humans.⁸  Chronic infusion of human amyloidogenic light-chain proteins has also been attempted in mice, with a subtle cardiotoxic response, but requires use of significant qualities of isolated human light-chain proteins and thereby precludes its application in routine study.⁷ 

Diomede and colleagues have now established a unique nematode model of amyloid cardiotoxicity.¹  Although nematodes do not have a traditional heart organ, they do have a pharyngeal muscle that resembles the vertebrate heart. The pharynx of C elegans and the vertebrate heart are both tube-like structures, composed of binucleate muscle cells, with continuous, autonomous rhythmic pumping under neurohormonal regulation. Although human cardiomyopathy has previously been modeled in C elegans, this typically has involved study of body-wall muscle cells.⁹  The authors cleverly reasoned that the pharynx in C elegans, given its close association to vertebrate heart muscle, may similarly exhibit a toxic response to amyloidogenic light-chain proteins. Indeed, the authors found that amyloidogenic light-chain proteins isolated from patients with cardiac involvement resulted in impaired pharyngeal pumping, whereas pharyngeal activity was not altered by amyloidogenic light-chain proteins isolated from patients with predominately renal involvement or with nonamyloidogenic light-chain proteins (see figure). Moreover, this impairment of muscle function was dose dependent over a range of light-chain protein concentrations typically observed in patients with systemic amyloidosis and was unrelated to light-chain oligomerization, secondary structure, stability, or surface hydrophobicity, providing further support for a direct cardiotoxic effect of light-chain proteins. Similar to what has been observed in primary cardiomyocytes exposed to human amyloidogenic light-chain proteins,¹⁰  increased oxidant stress was noted in C elegans in this study, with increased reactive radicals, and localized to the mitochondria using redox-sensitive dyes. Of translational significance, antioxidant agents known to quell reactive oxygen species, and currently under phase 2 investigation in patients with primary amyloidosis, protected against pharyngeal dysfunction.

The scientific and translational implications of this paper cannot be overstated. For one, the work presented by Diomede and colleagues¹  confirms prior observations made both in vitro in isolated cardiac muscle cells and in vivo in zebrafish, supporting a proposed direct cardiotoxicity specifically with human amyloidogenic light-chain proteins isolated from patients with underlying cardiac involvement.^6,7,10  Moreover, the genetic tractability of C elegans and amenability to high-throughput screening using either RNA interference or pharmacologic agents will now allow for systematic investigation of the underlying molecular mechanisms dictating amyloid light-chain–induced cardiotoxicity and potential small-molecule therapies. In addition, given the exceedingly poor prognosis associated with primary amyloidosis once it involves the heart, the C elegans model system described here¹  may potentially be used as a bioassay to screen light-chain proteins isolated from patients early in the disease state and prior to overt cardiac dysfunction, allowing for early intervention. Finally, screening of light-chain proteins from large cohorts of patients using the C elegans model system may finally provide some critical insight into the intrinsic light-chain properties that dictate organ specificity, a key unanswered question. This novel C elegans disease model of amyloid cardiotoxicity will enable whole new areas of study and represents a major step forward in the understanding and treatment of systemic amyloidosis.