A case report of recessive restrictive cardiomyopathy caused by a novel mutation in cardiac troponin I (TNNI3)

The proband (patient II2, Fig. 1a) was referred to our hospital at the age of 41 due to progressive dyspnea on exertion over the last 3 months. Her medical history was remarkable for ischemic stroke attributed to paroxysmal atrial fibrillation. She was on oral anticoagulation (acenocumarol). Her electrocardiogram showed atrial fibrillation and her echocardiographic study revealed normal left ventricular (LV) dimensions with borderline contractile function (left ventricular ejection fraction [LVEF] 50%), severe biatrial dilatation and mild mitral regurgitation. Transmitral diastolic flow revealed monophasic flow (atrial fibrillation) with normal average maximal velocity (E) but shortened deceleration time (DT 110 ms), a finding consistent with restrictive LV filling (Table 1). Right heart catheterization demonstrated increased left atrial pressure (estimated through pulmonary capillary wedge pressure), pulmonary hypertension and restrictive diastolic filling pattern with characteristic dip-plateau morphology, confirming echocardiographic findings. Attempts to restore sinus rhythm were unsuccessful. Despite receiving appropriate treatment (metoprolol tartate, acenocoumarol, furosemide, amiloride) and achieving adequate rate control, the patient deteriorated during the following year and a new echocardiographic study revealed worsened LV systolic function (LVEF 35%, LV end-systolic diameter 34 mm), similar left and right atrial volumes, moderate mitral and tricuspid regurgitation and pericardial effusion, while right ventricular function was also impaired. Right heart catheterization confirmed hemodynamic deterioration and the patient was admitted to our center’s heart transplantation list. Due to further clinical aggravation, a pulsatile biventricular assist device was implanted as a bridge to transplant and two months later, successful heart transplantation was performed. The histology of the native heart revealed multiple foci of intestitial and pericellurar fibrosis with features of non-specific myocytic hypertrophy (Fig. 2).

The proband’s twin sister (patient II3), although initially asymptomatic, underwent screening exercise stress test and cardiac ultrasound at the age of 43. The stress test, despite LV strain findings on the resting electrocardiogram, was substantially negative for heart ischemia. The cardiac ultrasound revealed normal LV dimensions (interventricular septum 8 mm, posterior wall 8 mm, LV end-diastolic diameter 44 mm, LV end-systolic diameter 30 mm), systolic function (LVEF 55%) and left atrial enlargement (left atrial volume 94.3 ml/m²). Doppler echocardiography showed restrictive pattern of diastolic transmitral flow (early diastolic transmitral blood flow velocity E higher than late atrial contraction-induced velocity A, E/A > 2, decreased deceleration time of E: DT 120 ms), and increased right ventricular systolic pressure (55 mmHg), as well as mild mitral and tricuspid valve regurgitation. At the age of 45, she started complaining for exertional dyspnea. A new cardiac ultrasound confirmed the previously reported findings with similar E/A ratio and even lower DT (104 ms, Table 1). She underwent right heart catheterization, which confirmed restrictive diastolic filling pattern and pulmonary hypertension. As a result, the diagnosis of a form of restrictive cardiomyopathy was established and the patient was managed medically (low doses of furosemide per os) remaining stable with mild symptoms. Recently, at the age of 51, her symptoms were aggravated due to the occurrence of atrial fibrillation.

Their brother (patient II1) was admitted to hospital at the age of 42 due to recurrent pre-syncope and exertional dyspnea. His electrocardiogram revealed sinus rhythm with incomplete Right Bundle Branch Block and nonspecific repolarization abnormalities. His echocardiographic study was remarkable for asymmetric hypertrophy of the interventricular septum and papillary muscles, left atrial enlargement (milder than his affected sisters’), mild mitral regurgitation and diastolic dysfunction without restrictive pattern (E < A, normal DT), while LV size and function were normal (Table 1). Doppler velocity measurements in the LV outflow tract were normal (absence of outflow tract obstruction). The above findings were consistent with HCM. The patient showed no significant epicardial vessel stenoses, but acetylsalicylic acid and rosuvastatin were initiated. He also underwent 24-h ambulatory heart rhythm recording, which was negative for ventricular arrhythmias. The patient remains stable with minimal symptoms and his cardiac ultrasound has not changed remarkably.

His parents (I1 and I2) along with one of his sisters (II4) remain asymptomatic. The echocardiogram of the proband’s mother (patient I2) presented prominent apical trabeculations, but her electrocardiogram was essentially normal. Similarly all of the children (III1–4) of patients II1 and II3 are asymptomatic. The youngest child (III1) is 5 years old and therefore has not been clinically evaluated yet. The clinical data of children III2 (age 13) and III3 (age 21) are listed in Table 1 and for the child III4 (age 21) although reported asymptomatic no data are available as he is clinically monitored in another medical center.

The molecular basis of the disease was identified for the proband with next generation sequencing technology, using Illumina’s Trusight Cardio sequencing panel, covering 174 genes clinically relevant to cardiac diseases. Alignment, quality filtering, variant calling, and variant annotation were performed using in parallel the standard MiSeq Reporter (Illumina) and the Sophia Genetics pipeline. The variant calling files were filtered using VariantStudio Data Analysis Software (Illumina, San Diego, CA) and the Sophia Genetics DDM platform and the detected variants were characterized according to the recommendations of the American College of Medical Genetics and Genomics (ACMG) [6]. All benign or likely benign variants were filtered out and the retained variants were then evaluated according to the relevance of the gene to the observed phenotype resulting in one plausible candidate variant which was located in the TNNI3 gene: NM_000363.4:c.586G > C, p.(Asp196His). The variant (https://​databases.​lovd.​nl/​shared/​individuals/​00204288) was novel and characterized as likely pathogenic (according to ACMG criteria). It was absent from the population databases of Exome Sequencing Project and Genome Aggregation Database (PM2 criterion) and pathogenicity predictions were all in favor of a damaging effect of this amino acid substitution (PP3 criterion) (SIFT: 0.001, PolyPhen-2 HumVar: 0.948, Mutation Taster: 1.000 and CADD v1.3: 32). The variant affected a highly conserved nucleotide (phyloP: 4.56 [− 14.1;6.4]) and a highly conserved amino acid (Consurf: 8, [1‐9]) and was located in a region where mutations of nearby residues support its functional importance [7], while missense variants in TNNI3 are a common mechanism of RCM (PP2 criterion). Additionally, another amino-acid missense variant at this position, p.Asp196Asn, is classified as likely pathogenic using ACMG criteria (PM5 criterion). Surprisingly, the proband was found homozygous for this variant but it was the second child of parents that are third cousins and originate from the same small mountainous village in Peloponnese, Greece (Fig. 1). The family of the proband underwent TNNI3 genetic testing by targeted Sanger sequencing for the detection of the variant. Her twin sister and her brother were found homozygous for the variant, while both parents and the older sister were found heterozygous (Fig. 1).