Pathology of Intercalated Discs in Friedreich Cardiomyopathy

Friedreich ataxia (FA) is best known for its neurological phenotype, but the most common cause of death is heart disease (1). The pathogenesis of FA cardiomyopathy includes failure to clear iron from myocytes, chronic inflammation, fiber necrosis, and scarring (2). On cross section, heart fibers are significantly enlarged and excessively lobulated (2). In the longitudinal dimension, the pathogenesis also involves modifications of intercalated discs (ICDs), the plasma membrane specializations that connect heart fibers end-to-end. Many proteins participate in the assembly of fascia adherens junctions, desmosomes, and gap junctions (GJs) within or near ICDs (3).

Paraffin-embedded heart sections of 15 FA patients with confirmed homozygous guanine-adenine-adenine trinucleotide repeat expansions (13 autopsies, 1 biopsy specimen, 1 explant) and 12 controls (all autopsies) were stained with antibodies to N-cadherin (Figures 1A and 1B), α-actinin, vinculin, and desmoplakin to visualize fascia adherens junctions and desmosomes and ZO-1 and connexin 43 to reveal GJs. N-cadherin reaction product was used to measure distances between ICDs (Figure 1B, inset) in sections of the left ventricular wall, right ventricular wall, and ventricular septum (VS). Strips of fixed VS were processed for ultrastructural visualization of ICDs (Figures 1C and 1D).

FIGURE 1. Abnormal ICDs in FA.

(A, C) FA. (B, D) Normal controls. (A, B) Immunohistochemistry of N-cadherin left ventricular wall. (C, D) Electron microscopy of ultrathin sections (ventricular septum; insets: toluidine blue–stained semithin sections). ICDs in FA (A, C) are larger and less compact than normal (B, D). ICDs also display duplication or fragmentation (A, arrow). The double-headed arrow in B (inset) shows how inter-ICD distances were measured in end-to-end connected fibers. At the ultrastructural level, ICDs are broader (~2.3 μm) (C) than normal (~0.6 μm) (D) and display discontinuities (C). The semithin section (C, inset) shows exaggerated convolutions over the entire extent of the ICDs (arrow). In the normal control (D), ICDs are smaller, more compact, and less convoluted (D, inset, arrow). Bars: A and B, 50 μm; C and D, 1 μm (insets, 10 μm). FA = Friedrich ataxia; ICD = intercalated disc.

In FA, all ICDs revealed by immunohistochemistry (Figure 1A), toluidine blue staining (Figure 1C, inset), or electron microscopy (Figure 1C) were disorganized, discontinuous, fragmented, and hyperconvoluted. N-cadherin reaction product showed an overall paucity of ICDs (Figure 1A). Connexin 43 reaction product revealed disorganization of ICDs as well as lateralization to plasma membranes (not illustrated). Inter-ICD distances were significantly and uniformly greater across all heart sections in FA (76 ± 11 μm) than in controls (54 ± 10 μm; p <0.001, main effect of FA in analysis of variance), but did not correlate with age of onset or death, disease duration, or guanine-adenine-adenine trinucleotide repeat expansion. Distances between Z discs remained normal (Figures 1C and 1D).

The underlying mutation in FA causes frataxin deficiency, which may adversely affect ICDs and GJs before the onset of heart disease and perhaps prenatally. The critical step in the faulty assembly and maintenance of ICDs may target the most important ICD protein, N-cadherin (4), and also cause reduced expression of other adhesion proteins. Incorrectly constructed ICDs may make FA hearts vulnerable to progressive secondary changes that ultimately lead to fatal cardiomyopathy. Although frataxin replacement may prevent cardiomyocyte necrosis, it is unlikely to reverse incorrect maturation of ICD that occurs long before clinical manifestations of FA cardiomyopathy.