A Clinical Diagnosis of Laminopathy without Systolic Dysfunction: When Does Nuclei Malformation Start?

Abstract

Nuclear shape abnormalities in laminopathy are well known to occur in patients with cardiac systolic dysfunction. However, those in patients without systolic dysfunction are still unclear. We herein report a 42-year-old man who presented with advanced atrioventricular block without systolic dysfunction. Genetic testing identified a laminopathic mutation, c.497G>C, and an endocardial biopsy was performed. The hyperperfine structure on electron microscopy showed malformation of the nuclei, euchromatic nucleoplasm, and partial existence of heterochromatin clumps. Intrusion of heterochromatin into the nuclear fibrous lamina was observed. Cardiomyocyte nuclear shape abnormalities were observed before the progression of systolic dysfunction.

Introduction

The lamin A/C (LMNA) gene encodes a nuclear membrane protein, and the mutations cause several pathogenic phenotypes manifested by conduction disorder, tachyarrhythmias, systolic dysfunction, and cardiac death (1). Approximately 30% of mutation carriers are at risk for end-stage heart failure, although half of the carriers present with only atrioventricular block without systolic dysfunction at the first clinical contact (2). Previous studies have focused on patients with systolic dysfunction or autopsy cases with cardiomyocyte nuclear malformation (3,4). However, the relationship between the progression of these morphological abnormalities and the pathological phenotypic manifestations remains unclear.

We herein report a patient with laminopathy showing atrioventricular block without systolic dysfunction. The fine structure of the nucleus in cardiomyocytes was studied in detail by a pathological analysis.

Case Report

Before referral

A 42-year-old man with no notable medical history complained of a recurrent loss of consciousness. He had no paresis or paraesthesia and had a strong family history of conduction disorder and heart failure (Fig. 1). Electrocardiograms recorded 15 years ago had demonstrated first-degree atrioventricular block (Fig. 2A). He was admitted for screening for arrhythmogenic syncope, and paroxysmal complete atrioventricular block with an episode of syncope was found.

Figure 1.

The family pedigree. The patient’s mother presented with atrioventricular block and systolic dysfunction leading to implantable cardioverter defibrillator implantation. Many of his mother’s siblings suffered from cardiac disorders. The arrow indicates the proband; half-shaded circles or squares, pathophysiological positive individuals; full-shaded, both heart failure with systolic dysfunction and cardiac implantable electronic device implantation; plus sign, LMNA p. Arg166Pro carrier. AVB: atrioventricular block, CHF: congestive heart failure, CI: cerebral infarction, CIEDs: cardiac implantable electronic devices, d.: death

Figure 2.

Electrocardiogram findings. (A) Twelve-lead electrocardiograms were recorded 15 years before the visit. (B) Complete atrioventricular block was revealed with the VVI mode of the transvenous pacemaker.

Although he received transvenous pacemaker implantation, the atrial lead dislodged six months later (Fig. 2B). The patient was therefore referred to our institute for investigation of the cause of familial cardiac conduction disorder and atrial lead reimplantation.

Genetic testing

Three individuals (his mother with complete atrioventricular block and systolic dysfunction, his brother without any atrioventricular block, and himself) underwent genetic testing using a gene panel developed at the National Cerebral and Cardiovascular Center (Osaka, Japan), and findings were confirmed by Sanger sequencing. A heterozygous missense variant, LMNA c.497G>C: p.R166P (rs267607570) was identified in the index patient and his mother but not in his brother (Fig. 3). The present patient was diagnosed with laminopathy.

Figure 3.

Genetic testing. DNA sequencing of the proband and his mother (II-6).

In-hospital course

Transthoracic echocardiography demonstrated a preserved left ventricular ejection fraction without valvular disease. An endomyocardial biopsy was performed to exclude other secondary cardiomyopathies leading to conduction block. Although no disease-specific findings were observed, light microscopy revealed partially hypertrophic myocardial fiber and interstitial fibrosis. Of note, the hyperfine structure observed by electron microscopy represented the malformation of the nuclei, the euchromatic nucleoplasm, and the partial existence of heterochromatin clumps (Fig. 4A). The width of the nuclear fibrous lamina was 20 nm, and the nuclear pore size was 100 nm, both within normal limits (Fig. 4B). The high magnification of the nuclear membrane revealed the intrusion of heterochromatin into the nuclear fibrous lamina (Fig. 4C).

Figure 4.

Hyperfine structure evaluated using an electron microscope. (A) Irregularly shaped nuclei. Red arrows indicate heterochromatin clumps. (B) The nuclear pore and the nuclear fibrous lamina. The circle indicates the nuclear pore. (C) High magnification of the nuclear membrane. Yellow arrows indicate the intrusion of heterochromatin into the nuclear fibrous lamina.

After atrial lead reimplantation, he was discharged without any anti-heart failure agents. Despite being placed on calcium blockade because of a diagnosis of vasospastic angina for six months after discharge, he remained free of heart failure symptoms to date.

Discussion

To our knowledge, this is the first report detailing the shapes of cardiomyocyte nuclei in a laminopathic patient without systolic dysfunction. Most previous histologic studies have focused on patients with systolic dysfunction (3,4). In our patient, nuclear malformation was observed despite a preserved ventricular systolic function.

Lamin mutants p.R166P

The present lamin mutant, p.R166P, is established as a pathogenic mutation that is associated with dilated cardiomyopathy accompanied by or lacking conduction abnormalities (5,6). In cell culture experiments, the forced expression of R166P induced mild to moderate abnormal nuclear shapes, and small nuclear envelopes associated with aggregates were observed (6). Our case also demonstrated nuclear malformation and disruption of heterochromatin formation, which are associated with lamin. Thus, R166P likely contributed to the pathogenesis in this case.

Although the index patient and most of his family members had a preserved systolic function, most patients with LMNA mutations will develop left ventricular systolic dysfunction in the future (7). Our patient should therefore be followed up with echocardiography repeatedly from now on.

An LMNA mutation is also a cause of skeletal muscle dystrophy, such as Charcot-Marie-Tooth disease; however, our patient did not show any skeletal muscle or sensory abnormalities (8). A biopsy of the skeletal muscle may provide additional information on the phenotypic differences between myocardium and skeletal muscle.

Pathogenesis of laminopathy

Two pathogenic theories concerning laminopathy have been proposed: “structural” and “gene expression” hypotheses (9,10). According to the structural hypothesis, loss of structural integrity impairs mechanotransduction and increases susceptibility to stress, leading to cardiomyocyte apoptosis, necrosis, and senescence (9). The gene expression hypothesis explains that 20% of the human genome contains lamin-associated domains, and lamin mutants induce abnormal gene expression regulation due to epigenetic modification (10).

Nevertheless, these hypotheses cannot explain why our patient uniquely manifested a conduction disorder rather than systolic dysfunction. His-Purkinje fibers are located in the endocardium and are exposed to a greater pressure load than the mid- or epi-cardiac layers of the left ventricle. Thus, the His-Purkinje fibers responsible for the conduction system may be more susceptible to lamin mutant-induced nucleus malformation than the working myocardium. More advanced laminopathy, rather than the less advanced laminopathy that we encountered, may present systolic dysfunction. An aggressive evaluation and intervention for suspected laminopathy should be conducted, regardless of the presence of systolic dysfunction.

Selection of cardiac implantable electronic devices

Guidelines recommend a cardioverter defibrillator be implanted for laminopathy with systolic dysfunction or nonsustained ventricular tachycardia, even if syncope is not associated with ventricular tachyarrhythmias (11). Our patient had a normal left ventricular systolic function and no ventricular tachyarrhythmias, which led to the selection of a pacemaker for complete atrioventricular block. Careful monitoring for ventricular tachyarrhythmias with home monitoring and a repeated evaluation of the left ventricular systolic function are necessary.

Conclusion

We described the detailed nuclear shape in the cardiomyocytes of a patient with atrioventricular block without systolic dysfunction due to laminopathy. Further micromorphological analyses are expected to help elucidate the pathophysiological mechanisms underlying the clinical presentation in laminopathy.

The authors state that they have no Conflict of Interest (COI).