Dilated cardiomyopathy in Friedreich's ataxia: 2D echo and tissue-Doppler analysis of left ventricular and atrial function

The Cardiac Calcium Handling Machinery is Remodeled in Friedreich's Ataxia

Background

Friedreich's ataxia (FA) is an inherited neurodegenerative disorder that causes progressive nervous system damage resulting in impaired muscle coordination. FA is the most common autosomal recessive form of ataxia and is caused by an expansion of the DNA triplet guanine-adenine-adenine (GAA) in the first intron of the Frataxin gene (FXN), located on chromosome 9q13. In the unaffected population, the number of GAA repeats ranges from 6 to 27 repetitions. In FA patients, GAA repeat expansions range from 44 to 1,700 repeats which decreases frataxin protein expression. Frataxin is a mitochondrial protein essential for various cellular functions, including iron metabolism. Reduced frataxin expression is thought to negatively affect mitochondrial iron metabolism, leading to increased oxidative damage. Although FA is considered a neurodegenerative disorder, FA patients display heart disease that includes hypertrophy, heart failure, arrhythmias, conduction abnormalities, and cardiac fibrosis.

Objective

In this work, we investigated whether abnormal Ca 2+ handling machinery is the molecular mechanism that perpetuates cardiac dysfunction in FA.

Methods

We used the frataxin knock-out (FXN-KO) mouse model of FA as well as human heart samples from donors with FA and from unaffected donors. ECG and echocardiography were used to assess cardiac function in the mice. Expression of calcium handling machinery proteins was assessed with proteomics and western blot. In left ventricular myocytes from FXN-KO and FXN-WT mice, the IonOptix system was used for calcium imaging, the seahorse assay was utilized to measure oxygen consumption rate (OCR), and confocal imaging was used to quantify the mitochondrial membrane potential (Δψm) and reactive oxygen species (ROS).

Results

We found that major contractile proteins, including SERCA2a and Ryr2, were downregulated in human left ventricular samples from deceased donors with FA compared to unaffected donors, similar to the downregulation of these proteins in the left ventricular tissue from FXN-KO compared to FXN-WT. On the ECG, the RR, PR, QRS, and QTc were significantly longer in the FXN-KO mice compared to FXN-WT. The ejection fraction and fractional shortening were significantly decreased and left ventricular wall thickness and diameter were significantly increased in the FXN-KO mice versus FXN-WT. The mitochondrial membrane potential Δψm was depolarized, ROS levels were elevated, and OCR was decreased in ventricular myocytes from FXN-KO versus FXN-WT.

Conclusion

The development of left ventricular contractile dysfunction in FA is associated with reduced expression of calcium handling proteins and mitochondrial dysfunction.

Familial dilated cardiomyopathy. Clinical and genetic characteristics

Familial dilated cardiomyopathy (F-DCM) describes a clinically and genetically heterogeneous group of diseases, mostly inherited as autosomal dominant traits, having idiopathic left ventricular dilatation and dysfunction as a common phenotype. The age of onset, rate of progression, disease complications, as well as overall prognosis and outcome vary both amongst and within families. Clinical traits, both cardiac and extracardiac, may recur in association with the DCM phenotype. The former include conduction defects, structural abnormalities such as left ventricular noncompaction, of right ventricular involvement, and recurrence of atrial or ventricular arrhythmias; the latter commonly affect the musculoskeletal (myopathies/dystrophies, both clinically overt and subclinical), ocular, auditory, nervous, and integument systems. These traits may help guide genetic testing. In parallel to the clinical heterogeneity, F-DCM also shows genetic heterogeneity: more than 40 genes have been causally linked to F-DCM, with mutations recurring more commonly in a few known genes, and less frequently in rare, less commonly known genes. Based on the known prevalence of mutations in disease genes, more than 50% of F-DCM cases can be regarded as still genetically orphan, implying that further disease genes have to be discovered. Family screening and genetic testing are now established as the gold standard for diagnosis, care, and prevention in F-DCM. Diagnostic tests are performed using Sanger-based sequencing. Furthermore, new biotechnology tools, based on next-generation sequencing, are now being implemented in the research setting and will dramatically modify the future of the nosology of F-DCM.

[The various forms of left ventricular hypertrophy: diagnostic value of echocardiography]

Left ventricular hypertrophy is a non-specific physiological or maladaptive cardiac response to a large array of stimuli mediated by exercise and numerous cardiac and systemic diseases. The precise characterization and quantification of left ventricular hypertrophy may allow a more timely diagnosis of the underlying condition. The clinical reference standard to assess left ventricular hypertrophy is echocardiography, but a comprehensive description of how to approach this frequent finding in clinical practice is lacking. The current review systematically describes the typical echocardiographic patterns of important types of cardiac hypertrophy using both established and advanced imaging modalities. In hypertrophic obstructive cardiomyopathy a markedly reduced regional systolic function is found in the prominent thickened septum, whereas in essential arterial hypertension a typical concentric left ventricular hypertrophy with a less prominent basal septal bulge is present. The echocardiographic characteristics of cardiac amyloidosis are ventricular hypertrophy with sparkling granular myocardial texture and a small epicardial effusion. In addition, the strain rate curve for longitudinal function shows a typically reduced function which reaches maximum already in early systole. The typical feature of Friedreich cardiomyopathy is concentric left ventricular hypertrophy and sparkling granular texture with preserved regional systolic function. In Fabry cardiomyopathy a prominent papillary muscle is presented and a typical strain rate curve can be extracted from the basal lateral wall, indicating replacement fibrosis. Prominent hypertrabecularisation (ratio of non-compacted to compacted myocardium >2) in the apical and mid left ventricular segments is typical for non-compaction cardiomyopathy. Knowledge of these typical echocardiographic features enables the cardiologist to distinguish between the different hypertrophic entities, thus paving the way to early diagnosis.

Use of tissue Doppler imaging to identify and manage systemic diseases

In systemic diseases such as amyloidosis, sarcoidosis, Friedreich's ataxia, Fabry's disease and muscular dystrophy the clinician has to judge the presence and the amount of cardiac involvement. In most of these patients conventional echocardiographic parameters are not sensitive enough to detect sub-clinical dysfunction. Tissue Doppler imaging and in addition strain rate imaging has proven to be very sensitive for the assessment of myocardial dysfunction. This review explores the impact of these new techniques to identify and to manage cardiac aspects of the different systemic diseases.