Hypertrophic cardiomyopathy (HCM) is the most common cardiovascular hereditary disease, and it is characterized by cardiac hypertrophy that cannot be explained solely by abnormal loading conditions (Ommen et al., 2020). The history of genetic research in HCM goes back to 3 decades, and up to date, over 450 causative mutations in at least 20 genes encoding for sarcomeric and myofilament-related proteins have been described (Geisterfer-Lowrance et al., 1990; Wolf, 2019). Mutations in MYBPC3 and MYH7 are the most frequent genetic cause of HCM and attributed to more than 50% of all HCM cases. Other HCM causal mutations have been identified in TNNT2, TNNI3, and genes encoding for structural proteins (Sabater-Molina et al., 2018; Teekakirikul et al., 2019). However, in 30–40% of the cases, the origin of the disease still remains unclear despite extensive genetic testing using targeted gene panels (Wolf, 2019; Salazar-Mendiguchía et al., 2020). In the case of HCM associated with rare causative genes, the existing risk prediction models may not be accurate enough because they have not been validated for this particular population. Therefore, collecting clinical and genetic data of patient groups with the rare genetic background of HCM may facilitate the development of more accurate and personalized risk stratification algorithms.

TRIM63 (tripartite motif 63) is a gene only recently described in association with HCM (Chen et al., 2012; Salazar-Mendiguchía et al., 2020). TRIM63 encodes muscle-specific RING-finger protein 1 (MuRF1), a member of ubiquitin ligases subfamily, such as MuRF-2 and MuRF-3.

In skeletal myocytes, upregulation of MuRF1 underlies a broad spectrum of muscle atrophy conditions (Peris-Moreno et al., 2020). In cardiac myocytes, overexpression of MuRF1 enhances susceptibility to heart failure in response to pressure overload, and its activation prevents cardiac hypertrophy (Arya et al., 2004; Willis et al., 2009). MuRF1 targets include sarcomere contractile, structural proteins, and signaling molecules (Witt et al., 2005; Polge et al., 2011; Chen et al., 2012; Maejima et al., 2014; Polge et al., 2018; Higashikuse et al., 2019; Bulatov et al., 2018). The involvement of TRIM63 in various pathological processes has been documented in several experimental and functional studies (Chen et al., 2012; Su et al., 2014; Olivé et al., 2015; Salazar-Mendiguchía et al., 2020).

The original study of Chen et al. revealed three TRIM63 variants in five unrelated probands among 302 probands with HCM. Additionally, they identified two loss of function variants (p.Ala48Val and p.Ile130Met) and one protein-truncation variant (p.Gln247*) detected in the heterozygous state. Experimental cellular and animal studies demonstrated the reduced colocalization of MuRF1 with alfa-actinin at the Z-disk level, impaired auto-ubiquitination, and depressed ubiquitination and proteasome degradation of substrates (Chen et al., 2012).

Later, the role of p.Gln247* as a single causative variant leading to HCM was challenged due to its relatively frequent detection in healthy European subjects and observation of its clinical phenotype only in the cases of compound heterozygosity or homozygous variants (Płoski et al., 2014). Su et al. found rare variants in MuRF1 and MuRF2 encoding genes in the healthy population but with a much lower frequency than in HCM. Moreover, carriers of these variants had greater maximum left ventricular wall thickness than non-carriers. Hence, rare variants in MuRF1 and MuRF2 encoding genes were associated with higher penetrance and more severe clinical manifestations of HCM, especially when coincided with other sarcomeric mutations, therefore being considered by the authors as modifiers in HCM. It is important to note that all detected rare variants were heterozygous (Su et al., 2014).

The final clarification of the TRIM63 inheritance pattern became possible after studying 4,867 index cases with HCM (Salazar-Mendiguchía et al., 2020). TRIM63 sequencing and subsequent familial evaluation revealed that only homozygous and compound heterozygous carriers developed full clinical phenotype of the disease (15 patients with HCM and one with restrictive cardiomyopathy), while heterozygous individuals demonstrated mild or almost no phenotype. These data strongly support the hypothesis that TRIM63 causes HCM with autosomal-recessive inheritance (Salazar-Mendiguchía et al., 2020).

Herein, we present two cases of TRIM63-associated HCM due to compound heterozygous mutations along with results of a 5-year follow-up.

The presented cases describe the clinical phenotype of TRIM63-associated HCM and extend our knowledge on rare genetic forms of one of the most common inherited human disorders. While HCM remains the most common genetically predicted cardiovascular disease, the vast majority of the cases are linked to the variants in eight sarcomeric genes (MYBPC3, MYH7, ACTC, TTNI3, TTNT2, TPM1, MYL2, and MYL3) with a clear predominance of MYH7 and MYBPC3 variants. Therefore, most clinical algorithms and guidelines are developed using these cohorts of patients (Ho et al., 2018; Lorenzini et al., 2020). Rare genetic forms of HCM account for only 1–2% of the cases (Lopes and Elliott, 2014). However, despite the small number of patients with rare variants, their total number makes up almost one-third of all patients with sarcomeric mutations (Ingles et al., 2019). Currently, it is not clear to what extent the common guidelines and recommendations are valid for these rare genetic groups of HCM. It is generally accepted that HCM associated with sarcomere gene mutations has a worse prognosis than non-sarcomeric forms of the disease (Ho et al., 2018; Marstrand et al., 2020). However, several exceptions exist, such as HCM, associated with PRKAG2 gene mutations (Ahamed et al., 2020). Thus, such rare clinical variants are waiting for more cases reported along with broader clinical and prospective descriptions.

One of the rare HCM genetic variants is a form of the disease associated with the TRIM63 gene. It has been described recently and represents one of the rare forms of autosomal-recessive or compound heterozygous form of HCM (Salazar-Mendiguchía et al., 2020). Despite the small number of cases reported by now, several characteristic features of the disease could distinguish this form from the “classical” sarcomeric phenotype. These specific characteristics include the relatively rapid increase in LV free wall thickness, the appearance of diastolic dysfunction from II to III grades with mild atria enlargements, and normal or borderline estimated pulmonary artery systolic pressure. For example, Patient 1 had extreme hypertrophy (septum up to 56 mm), progressing mainly due to posterior wall and RV wall thicknesses, shaping LVOT and RV outflow tract dynamic obstruction. To the best of our knowledge, it is the first report of TRIM63-associated HCM with such high magnitude hypertrophy (Chen et al., 2012; Su et al., 2014; Olivé et al., 2015; Salazar-Mendiguchía et al., 2020).

The diastolic dysfunction up to restrictive phenotype was already reported in TRIM63-mutation carriers by other authors (Olivé et al., 2015; Salazar-Mendiguchía et al., 2020). While not rare in some forms of HCM, restrictive phenotype had a malign prognosis in pediatric and adult patients with HCM (Maskatia et al., 2012; Li et al., 2020). In TRIM63-associated HCM, restrictive dysfunction could reflect the altered molecular interaction of MuRF1 with titin (Higashikuse et al., 2019). There is a MuRF1-binding site in titin adjacent to the titin kinase domain. Mutations of this region lead to hypertrophy and diastolic dysfunction in the medaka fish experimental model and Japanese patients with HCM associated with TRIM63 (Higashikuse et al., 2019). In addition, mutations of the titin MuRF1-binding site lead to the expression shift to the stiffer titin isoforms, increased titin binding to MuRF1, and enhanced titin degradation through ubiquitination. Thus, MuRF1–titin interaction contributed to sarcomeric protein turnover and titin isoforms switch, determining muscle compliance and diastolic function (Higashikuse et al., 2019).

The development of systolic dysfunction has been reported as a distinct feature of TRIM63-associated cardiomyopathies (Salazar-Mendiguchía et al., 2020). Thus, Salazar-Mendiguchiá et al. demonstrated that TRIM63-homozygous HCM patients have significant degrees of the LGE phenomenon and progressed to LV systolic dysfunction more often than in the typical HCM, thereby representing a subgroup of increased risk of adverse events (Salazar-Mendiguchía et al., 2020). However, despite the marked degree of fibrosis reflected by LGE on MRI in both cases, no systolic dysfunction was noted in our patients. Possibly, this could be explained by the relatively young patient age, and a thorough follow-up within the next years will shed light on the frequency of systolic dysfunction in patients with TRIM63-associated cardiomyopathies and marked fibrosis.

MuRF1 tissue distribution raises the question of whether TRIM63 mutations can cause skeletal myopathy. Olivé et al. reported a male patient with cardiac and skeletal myosin aggregate myopathy carrying the combination of homozygous TRIM63 null-mutation and heterozygous TRIM54 (encoding MuRF3) mutation (Olivé et al., 2015). Clinically, it presented as proximal muscle weakness and HCM with atrial flutter. Electron microscopy of muscle biopsy revealed, apart from I-bands and Z-discs disorganization, myosin-associated proteins aggregates and abnormal microtubules distribution in skeletal muscle cells. The latter demonstrated the possibility that MuRF1 and MuRF3 regulate not only sarcomere protein degradation but also spatial organization of the microtubules. However, whether the presence of both TRIM63 and TRIM54 variants or the homozygous TRIM63 variant itself contributed to myopathic phenotype remains unclear (Olivé et al., 2015). Later, Jokela et al. described female patient with mild clinical symptoms of skeletal myopathy along with creatine kinase elevation and severe cardiac hypertrophy (in the absence of other diseases capable of producing the observed degree of hypertrophy) in association with TRIM63-homozygous variant alone (Jokela et al., 2019). Given that, the remarkable muscle hypertrophy of upper limbs observed in both patients potentially could reflect pseudohypertrophy due to the myopathic process. Of note, both patients performed sport and had, similar to the case presented by Olive et al., marked muscular hypertrophic phenotype (Olivé et al., 2015). Of note, Patient 1, who did sport with the prevalence of static load, displayed more severe hypertrophy. However, this notion needs further attention and deeper functional studies, including neuromyography and muscle MRI.

Our study has several important limitations. One of them is an inability to perform a genetic test on the parental DNA and identify whether three of four described variants are de novo or inherited. Another important limitation is an inability to verify in both cases if two detected variants belong to the same or different alleles. In light of the recently published data on the polygenic impact of many genetic variants into HCM, the role of these and other concomitant variants in the genes not yet described in connection to the observed phenotype cannot be excluded and may need further elucidation.

In summary, we have described two new patients with HCM due to the compound heterozygous TRIM63 variants. Both patients presented with marked progressed myocardial hypertrophy and diastolic dysfunction from II to III grades and demonstrated clear indications for ICD implantation according to the accepted risk prediction models. Further data collecting regarding rare cases of compound TRIM63 variants associated with inherited cardiac pathology will allow developing a more personalized approach in this rare cardiac disorder.