**3. Etiology and pathophysiology of pediatric CM**

Phenotypes as well as the etiology of pediatric CMs are very heterogeneous. Thus, in inherited CMs, genetic defects may occur in genes encoding proteins of the sarcomere, including the Z-disc proteins, structural proteins (e.g., costameric, desmosomal, cytoskeletal, nucleoskeletal proteins), mitochondrial or Ca<sup>2</sup>þ-handling proteins, signaling proteins such as Ras, or proteins of the Notch signaling pathway, and even mutations in non-coding regions of the genome have been described (see also **Figure 2**). Other causes of pediatric CMs might be inflammation due to viral or bacterial infections and toxins, including chemotherapeutics or neurohormonal or metabolic disorders [48–62]. Inborn errors of metabolism associated with

#### **Figure 2.**

*Cellular compartments affected by genetic mutations causing cardiomyopathy in children. Arrows indicate interactions between different compartments altered in the presence of mutations (e.g., sarcomeric mutations leading to energy depletion and altered transcription). IF: Intermediate filaments.*

cardiomyopathies are nicely summarized by [63]. One of these diseases, the Barth syndrome, is associated with CM in early childhood. Characteristically, the Barth syndrome may induce DCM or LNVC, skeletal muscle myopathy, mitochondrial dysfunction, neutropenia, and growth retardation [64]. According to Neuwald [65], a defect in the *TAZ* gene encoding several tafazzins due to alternative splicing might be causative. The taffazin protein family includes acyltransferases involved in phospholipid formation, indicating that the mitochondrial membranes might be defective, leading to an inefficient oxidative phosphorylation and thus to an inefficient energy production for cardiac performance.

LNVC can be caused not only by systemic diseases such as the Barth syndrome but also by gene defects also described in congenital or arrhythmogenic heart disease and restrictive, hypertrophic, or dilated CM. The affected genes identified include those encoding sarcomeric or cytoskeletal proteins as for example *MYH7* (myosin heavy chain 7), *MYL2,3* (myosin light chain 2,3), *MYBPC3* (cardiac myosin binding protein C),*TTN* (titin), *ACTC1* (cardiac alpha actin),*TPM1* (cardiac tropomyosin),*TNNT2* (cardiac troponin T),*TNNI3* (cardiac troponin I), and *ZASP* (LIM binding domain 3), a Z-disc protein [4, 66–71]. Non-sarcomeric gene defects have also been identified, for example, in *HCN4* (hyperpolarisation activated cyclic nucleotide gated potassium channel 4) coding for an ion channel located in pacemaker cells or in genes encoding intermediate filament proteins such as *LMNA* (Lamin A/C), which is involved in heart development, or *DES*, encoding desmin [72, 73]. A list of affected genes in LNVC and their functions can be found in the National Library of Medicine, Medline PLUS. LNVC is mainly due to developmental failure, and it is associated with increased trabeculation, leading to a sponge-like appearance of the heart chambers. Such a remodeling may result in contractile dysfunction, arrhythmia, and sudden cardiac death. The latter is also characteristic of other CMs such as ACM, HCM, or DCM.

DCM, a systolic disorder, is characterized by dilation of the left ventricle or of both ventricles associated with suppressed contractile function. DCM based on genetic defects seems to form the main cause of pediatric DCM, whereby the vast majority of gene mutations is found in sarcomeric proteins, especially in *MYH7*,*TTN*,*TNNT2*, *TPM1*, *MYBPC3*, *MYL2*,*TNNC1* (Troponin C), and *ACTN2*. Others are found in *LMN, DES, VCL* (Vinculin),*TTR* (transthyretin), *BAG3* (associated with apoptosis), *MT-TS2* (encoding a mitochondrial small RNA), or transcription factor encoding PRDM16 [74–78]. In adult and pediatric DCM, the same genes seem to be affected, though unfortunately, most genetic testings have been performed in adults [79]. Besides the genetic causes, inflammation may lead to DCM, the main non-genetic cause in children. Myocarditis is mainly caused by viral infections and is rather challenging for diagnosis and treatment [80]. In most cases, Coxsackieviruses B and adenovirus infections are the causes of myocarditis in children [81]. Recently, SARS-CoV-2 virus has also been described as a possible cause [82]. Besides viral infections, toxins and chemotherapeutics are also emerging as DCM-inducing agents. Especially anthracyclines, which often are used to treat tumors in children as well as in adults, have been linked to the development of heart failure, DCM, or RCM [83]. The onset of cardiomyopathy in cancer patients varies largely; it may develop within a week or less than one year after starting the treatment, depending on various risk factors (e.g., sex, age, dosage, and subtype of the agent) [84].

Most pediatric HCM cases are due to genetic defects mainly in genes encoding sarcomeric proteins, though diagnosis in respect of etiology remains challenging. The HCM mutations often may also cause other CMs such as DCM, RCM, ARVC, or LNVC, even within the same family, indicating additional factors such as further gene

#### *Cardiomyopathies in Children: Genetics, Pathomechanisms and Therapeutic Strategies DOI: http://dx.doi.org/10.5772/intechopen.109896*

mutations, polymorphisms, or comorbidities. Mutations have been identified in genes encoding thick- and thin-filament proteins, whereby most mutations have been identified in *MYBPC3* and *MYH7*. Frequently, mutations have also been described in *TNNT2,TNNI3,* and *MYL2* or more seldom in *TPM1* and *ACTN*. HCM in children is even more heterogeneous than in adults. There are also several non-sarcomeric causes recently summarized in [85], which may occur alone or in combination with sarcomeric mutations and then modify the severity and prognosis of the disease [86]. These non-sarcomeric causes of HCM in children include rasopathies, a set of diseases with genetic defects encoding proteins of the Ras signaling cascade, for example, the Noonan syndrome. Others are glycogen storage diseases, including Pompe or Danon disease, characterized by glycogen-filled vacuoles within the cardiomyocytes [87]. In lysosomal storage diseases, for example, mucopolysaccharidoses, partially or undigested macromolecules are accumulated due to dysfunctional lysosomal enzymes [88]. In addition, mitochondrial disorders may occur, which mostly affect oxidative phosphorylation and thus energy production. Thus, HCM is often characterized by an energy deficiency probably due to over-contractility, as might also be the case in RCM.

In contrast to HCM, RCM is characterized by restrictive ventricular filling mostly without an increase in wall thickness. The genetic/familial causes of RCM are mainly due to mutations in genes encoding sarcomeric proteins (listed in [4]). Here, mutations in *TNNI3* occur frequently, but mutations have also been identified in genes of cytoskeletal or nuclear envelope proteins, leading to storage diseases (e.g., Danon disease, Friedrich ataxia, etc.) or infiltrative diseases (cardiac amyloidosis, cardiac sarcoidosis). The latter may also develop due to non-genetic causes.

All CM types are associated with an increased risk of developing severe arrhythmias, but they are most prominent in ACM. ACM is a genetically based disease, whereby most studies have concentrated on adult patients, underlining the need for more studies with children under 18 years of age. Histologically, ACM is characterized by the loss of cardiomyocytes being replaced by fibrofatty tissue [42, 89]. As a consequence of the cardiac remodeling, life-threatening arrhythmia and sudden cardiac death can occur. Causes for this familial disease are mutations in genes encoding desmosomal proteins with a deletion mutation in the junctional plakoglobin gene (*JUP*) being the first identified in ACM [90]. It seems that truncation mutations and splice variants in the *PKP2*, encoding plakophilin 2, are the most frequent causes of ACM, together with mutations in *DSP* (desmoplakin), *DSC2* (desmocollin 2), *DSG2* (desmoglein 2), and *JUP* [91]. Less frequently, mutations in *TMEM43* (Transmembrane Protein 43) and *PLN* (phospholamban) have been described.

### **4. Molecular mechanisms**

Molecular mechanisms of CM development—as far as known to date—often are not specific for a distinct CM type. Generally, contractile function is affected via various mechanisms, and involvement of several cell compartments and cardiac remodeling is common (**Figure 2**).

#### **4.1 Sarcomeric dysfunction**

The smallest contractile unit of cross-striated muscles is the sarcomere. It is bordered by the Z-discs from which thin and elastic filaments reach out to the center of the sarcomere (M-Line). The thick filaments emanate bidirectionally from the M-line. Cardiac thin filaments are mainly composed of filamentous cardiac actin, cardiac tropomyosin (cTpm), and the cardiac heterotrimeric troponin complex composed of the tropomyosin-binding subunit (cTnT), the inhibitory subunit (cTnI), and the calcium-binding subunit (cTnC). Cardiac thick filaments are composed of cardiac myosin containing the essential and regulatory myosin light chains (MyLC) and heavy chains (MyHC) and cardiac myosin-binding protein C (cMyBPC), linking thick, thin, and elastic filaments. This implies that MyBPC coordinates the action of the filaments in contraction and relaxation processes. The elastic filament is formed by the giant protein titin. Ca2þ-binding to cTnC triggers contraction by enabling actin-myosin interaction and the power stroke (for review, see [92]). In addition, the contraction can be fine-tuned by reversible phosphorylation of many sarcomeric proteins, for example, titin, myosin-binding protein C, myosin light chains, cTnT, cTnI, cTpm, and also Z-disc proteins [93, 94].

CM-inducing mutations may occur in genes encoding filament proteins as well as proteins of the Z-disc. They may lead to single amino acid replacements or loss of a single or several amino acids. As a result, the mutations may alter the very sensitive interplay between the components of the sarcomere and/or protein dynamics and thereby induce contractile dysfunction (for review, see [4]). Furthermore, interactions of sarcomeric proteins with associated proteins might be altered, affecting signaling and thereby protein transcription, metabolism, inflammation, oxidation, cell death, protein degradation, fibrosis, cell–cell communication, Ca2<sup>þ</sup> homeostasis, and so on [95–97].

The intracellular Ca<sup>2</sup>þ‐ concentration is pivotal for the muscles' contractile function. At submicromolar concentrations, the cardiac muscle is at rest. It contracts upon an up to 100-fold increase in intracellular Ca2<sup>þ</sup>‐ concentration following a nervous impulse and opening of the voltage-gated L-type Ca-channels, which are located in the T-tubules of cardiomyocytes. The resultant Ca<sup>2</sup><sup>þ</sup> inward current triggers the opening of the ryanodine receptors, the Ca2<sup>þ</sup>‐ channels within the membrane of the sarcoplasmatic reticulum (SR). Thereby, Ca2<sup>þ</sup> is released from the SR Ca2<sup>þ</sup>‐ stores, which leads to a massive increase in intracellular Ca2þ- concentration and to Ca2<sup>þ</sup>‐ binding to cTnC. This finally enables the power-generating interaction of myosin and actin, leading to contraction. On the other hand, relaxation occurs when Ca2<sup>þ</sup> is dissociated from cTnC. The released Ca2<sup>þ</sup> is then pumped back into the SR via the SR Calcium ATPase (SERCA), into the extracellular space via NCX (Na<sup>þ</sup>‐Ca2<sup>þ</sup>‐ exchanger) and plasma membrane Ca<sup>2</sup><sup>þ</sup> ATPases, and into mitochondria [98]. These complex procedure makes clear that alterations in the regulation of intracellular Ca<sup>2</sup>þconcentrations lead to contractile dysfunction and thus to myocardial diseases. Besides mutations in genes of proteins involved in calcium fluxes (for review, see [99]), mutations in sarcomeric protein genes affect the calcium response of sarcomeric proteins and/or the calcium-binding affinity of the sarcomeres' Ca2<sup>þ</sup>‐ sensor, cTnC. On a simplified level, it is thought that DCM mutations decrease the calcium sensitivity of the actin-myosin interaction, thereby reducing the contractile capacity, which makes DCM a systolic disorder. Though, in pediatric end-stage DCM, cardiomyocytes exhibit an increased calcium sensitivity [100]. The authors assumed that this is caused by a substantial decrease in cTnI phosphorylation, which however cannot be the main reason, since in adult DCM, cTnI phosphorylation is also reduced (see below). In HCM and RCM, often the Ca<sup>2</sup><sup>þ</sup>‐ sensitivity of the actin-myosin

#### *Cardiomyopathies in Children: Genetics, Pathomechanisms and Therapeutic Strategies DOI: http://dx.doi.org/10.5772/intechopen.109896*

interaction is increased (at least in adults), leading to faster relaxation and recontraction. Since intracellular Ca2<sup>þ</sup> is increased also due to an increased phosphorylation of Ca2<sup>þ</sup> channels, relaxation is impaired. Increased intracellular Ca2þ‐ concentration leads to hypercontractility, causing energy deficiency and increased production of reactive oxygen species (ROS) due to increased oxidative phosphorylation [101]. Posttranslational modifications and increased ROS might contribute to the development of arrhythmias. ROS induce oxidation of proteins, for example, of the calcium calmodulin kinase (CaMKII), which upon oxidation of methionine residues within the autophosphorylation domain is constantly active, because it cannot be inactivated via dephosphorylation and Ca2<sup>þ</sup>‐ CaM (calmodulin) dissociation [102, 103]. CaMKII regulates several proteins including those involved in calcium fluxes. Its activation increases intracellular Ca2<sup>þ</sup> levels due to increased open probabilities of Ca<sup>2</sup><sup>þ</sup> channels as the L-type Ca<sup>2</sup><sup>þ</sup> channel or the Ryanodine receptor. This might contribute to the remodeling of T-tubules further affecting EC coupling and Ca2<sup>þ</sup> homeostasis (for review see [92]).

However, calcium regulation might not only be impaired directly by mutations but also disturb the intermolecular interaction between the components of the sarcomere [104, 105]. Additionally, calcium responses might be affected by cross bridge kinetics or phosphorylation of myosin-binding protein C and cardiac TnI [106]. In pediatric and in adult DCM, cAMP-dependent protein kinase (PKA)-dependent hypophosphorylation of cTnI and MyBPC has been described, leading to a reduced stress response. Furthermore, maximal force and passive force were reduced. Hereby, reduced myofiber densities as proposed by [107] might contribute to the impaired force production. The reduced myofiber density seems not to be caused by an impaired protein quality control system [107]. The underlying mechanism is still unknown.

Most mutations, preferentially truncations leading to adult DCM, have been identified in titin. Controversial study results have been obtained with pediatric DCM. *TTN* and *MYH7* mutations were identified as predominant in a cohort of 106 pediatric patients [74]. In another study analyzing 36 patients, only one *TTN* mutation, a truncation (p.Arg33703\*), was identified in a 16-year-old male DCM patient [108]. Mutations occur most frequently in A-band N2Ba/N2B-titin and thus may induce structural and contractile dysfunctions [109]. In A-band titin, interaction sites for myosin and myosin-binding protein C are distributed in a regular pattern [110]. This implies that in the case of A-band titin sequence alterations or truncations, its interaction with myosin and/or myosin-binding protein C might be impaired.

The location of mutations within *MYH7* - another frequent target for pathogenic mutations - seems to determine the DCM type according to Khan et al. [74]. They found that mostly single amino acid replacements in ß-Myosin heavy chain (MYH7) in the range of amino acids 1–600 lead to mixed DCM, a DCM–LNVC phenotype, whereas mutations in the C-terminal part from amino acid 600 lead to pure DCM [74]. The C-terminal rod of myosin heavy chain interacts with other myosin rods, which is essential to build up the thick filament. Myosin consists of 2 heavy chains, with the rod region forming a supercoil. The N-terminal part of each myosin heavy chain consists of a lever arm, a converter region with binding regions for the myosin light chains, and the globular motor domain containing the ATPase domain and actin binding domain. Mutations in the motor domain may affect ATPase activity via altered ATP-binding affinities and/or ATP hydrolysis rate and/or dissociation of the hydrolysis products ADP and Pi. Furthermore, it may impair the interaction of the

motor domain with actin. Thus, most DCM mutations seem to weaken the affinity for actin. Furthermore, in contrast to MYH7 HCM mutants, in case of DCM, less force is produced due to a lower occupancy of the force generating state and a reduced ATPase velocity [111]. They predict that less ATP is used to hold a specific force.

Besides MYH7, the gene of cardiac MyBPC is the main target for pediatric and adult cardiomyopathies, whereby mutations in the cardiac MyBPC predominantly lead to HCM and, typically for HCM, are associated with hypercontractility. According to Toepfer et al. [112], mutations in *MYBPC3*, which result in either truncations or single amino acid replacements, affect the dynamics of myosin conformations and the super-relaxed state of myosin. cMyBPC interacts with myosin at several sites (rod, lever arm) and with titin, actin, and troponin and is thought to regulate the number of force-producing myosin motor domains. When phosphorylated, at submaximal Ca2<sup>þ</sup>‐ levels, it promotes the actin-myosin interaction [113]. Mutations in *MYBPC3* or in genes of interacting proteins might impair the interplay between these proteins and thereby the regulation of contraction. Hereby, an impaired interaction of different proteins with cardiac troponin might also play a role, though the role of the MyBPC-cardiac troponin interaction as well as of its interaction with titin remains to be elucidated [104, 114]. Two fascinating studies revealed that MyBPC regulates sarcomeric contractile oscillations, which might be based on its interplay with all sarcomeric partners [115, 116].

Mutations in the cardiac actin gene are relatively rare and mostly are associated with the development of HCM and DCM. They seem to affect the interaction with myosin heads and/or tropomyosin or troponin [117]. Interactions with MyBPC or actin-associated proteins regulating formation and length of the actin filament have not been considered yet. A nice overview of these proteins and their role is given in the review by Ehler [118]. The first hint that filament formation/structure could be impaired by pathogenic mutations in the cardiac actin gene comes from [119]. They showed that different mutations incorporate differently into the actin filament and destabilize the filaments. A destabilization of actin filaments has also been described by Hassoun et al. [114], and pediatric RCM mutations in *TNNI3* have been demonstrated to largely affect the integrity of reconstituted thin filament structure [104].

Other targets for pathogenic mutations are the genes of the three subunits of the troponin complex: cTnC, cTnI, and cTnT. They are associated mostly with HCM and DCM. However,*TNNI3* mutations frequently result in RCM [4]. They affect Ca<sup>2</sup><sup>þ</sup>‐ sensitivity via impaired intra- and intermolecular interactions as well as via impaired posttranslational modifications. Hypercontractility-induced increase in ROS leads to oxidation of not only lipids, nucleic acids, or protein kinases but also sarcomeric proteins. It has been shown by Budde et al. [120] that oxidation of cardiac troponin I and cardiac MyBPC reduced phosphorylation by PKA and PKC and thereby contributed to the impairment of force production. The effects could be reversed fully by using antioxidants and partly by supplementing PKA. Also, specific oxidations of actin or titin have been associated with development of heart diseases [121, 122].

#### **4.2 Cardiac remodeling**

Enhanced ROS production leading to oxidative stress contributes to cardiac remodeling as fibrosis, another hallmark of cardiomyopathies, which leads to further contractile dysfunction via increased stiffness of the ventricular walls and also contributes to arrhythmia. Fibrosis occurs due to the activation of fibroblasts via stimulation of pro-fibrotic factors as TGF-ß, PDGF (platelet-derived growth factor), or

#### *Cardiomyopathies in Children: Genetics, Pathomechanisms and Therapeutic Strategies DOI: http://dx.doi.org/10.5772/intechopen.109896*

cytokines and increased production and deposition of collagen I and III as well as cross-linking of the extracellular matrix (ECM). According to Li et al. [123], TGF-ß increases the expression of SerpinE2/nexin-1, leading to increased collagen deposition. Fibrosis as well as hypertrophic growth have been described to be linked also to ERK1/ 2, JNK, and p38 pathways. Furthermore, there seems to be an association between fibrosis, oxidative stress, and inflammation (for review see [124]). Even in young children, fibrosis could be observed, though the pathways in children have not been investigated. But there might be differences in signaling between children and adults [125]. Thus, in the case of DCM, a study revealed that children show much less interstitial and perivascular fibrosis than adult patients [126]. Furthermore, it seems that genes leading to an inflammatory response in DCM are expressed in adult but not pediatric patients. Also, differences between young children and adults in receptor physiology have been described. These aberrances will have consequences for the therapy of pediatric heart diseases [127].

Fibrous and/or fibrofatty infiltration leading to life-threatening arrhythmia is a hallmark of ACM, which is caused by genetic defects [128]. Genes modified include those encoding mostly desmosomal proteins (e.g., placophilin 2) and rarely junctional proteins as catenins, cytoskeletal proteins as TMEM43, and so on [129]. The molecular mechanisms of ACM development especially in children are not quite understood. ACM is strongly correlated to cardiomyocyte loss, which might be due to a stimulated apoptosis, since defects in desmosomal or junctional proteins lead to reduced cell adhesion and impaired sarcolemmal structure [130]. Apoptosis might be triggered by stimulated hippo pathways and fibrosis via WNT inhibition and TGF-ß pathways [129]. Fibrosis (together with deposits of protein aggregates, amyloidosis, glycogen storage defects), probably due to an impaired protein control system, might also be the causative for the stiffness observed especially with RCM [4]. Increased myocardial stiffness, however, is also observed in other heart diseases than RCM, as in heart failure with preserved rejection fraction, LNVC, and HCM. Increased myocardial stiffness leads to a reduced filling of the heart chambers with blood. Besides fibrosis and protein aggregates, the microtubular network, that is, microtubule density and its cross-linking with intermediate filaments, also contributes to the myocardial elasticity [131]. In addition, sarcomeric titin is a major player in myocardial stiffness. The ratio of the isoforms N2Ba and N2B is decisive [132]. But posttranslational modifications such as changes in titin phosphorylation also contribute to the alterations in stiffness [132–136].

For HCM and RCM but not ACM, another hallmark is the myocyte disarray, which according to Garcia-Canadilla et al. [137] in case of HCM mutations occurs very early, even before birth, indicating a developmental impairment at least in mice. In addition, cardiomyocyte disarray might contribute to arrhythmia associated with HCM/RCM. Molecular mechanisms leading to myocyte disarray are not known. In [138], a cell-tocell imbalance in the expression of mutant proteins was described, which might lead to arrhythmias, myocyte disarray, and fibrosis.

Epigenetic modulations play a role in all cardiomyopathies. Thus, in HCM, hypertrophic growth is mediated by stimulating the expression of sarcomeric proteins. Hereby, a reprogramming occurs; fetal instead of adult proteins are expressed in adult HCM patients. In newborn CM patients, there might be a different mechanism, since some cardiac-specific genes such as *TNNI3* are expressed within the first year of life and gradually replace the fetal skeletal muscle isoform. In hypertrophic growth and reprogramming, calicneurin and the transcription factors NFAT, GATA4, NFkappaB, and MEF2 play a central role, nicely summarized by Dirkx et al. [139]. NFAT also

regulates the expression of micro RNAs (miRNAs), which regulate mRNAs. In hypertrophy, miR23 is induced by targeting MURF1 and FOXO3a, both involved in cardiac remodeling [140, 141]. Epigenetic studies are largely missing in infant cardiomyopathy patients, and investigations are urgently needed. One of the few studies is [142], investigating miRNA profiles in children with heart failure. They found 17 miRNAs that were either not regulated (miR-130b, miR-204, miR-331-3p, miR-188-5p, miR-1281, miR-572, miR-765, miR-223, miR-125a-3p, and miR-1268) or antithetically regulated (miR-638, miR-7, miR-132, and miR-146a) in adult heart failure. Several of these miRs regulate genes, such as SMAD4, which are involved in the transition of hypertrophy to heart failure. In DCM, altered DNA methylation patterns and histone modifications and altered miR and lncRNA (long noncoding RNA) regulation have been identified in adult CM patients, but again, investigations in children are missing [143].

## **5. Diagnosis of cardiomyopathy in children**

Considering the substantial risk of developmental disorders, disabilities, and mortality of children with cardiomyopathies, early detection, accurate classification, and treatment of cardiomyopathies in children are imperative [144–146]. Nevertheless, there is a gap in knowledge and a lack of consensus in the diagnostic approach, definition, and classification of cardiomyopathies in children [2]. Thus, the American Heart Association, in their latest publication, provided some suggestions in the form of a scientific statement instead of a clinical practice guideline. In this statement, a classification system for cardiomyopathy based on a hierarchy incorporating the required elements of the MOGE(S) classification was suggested. Manifestations of cardiomyopathy in children can range from a sole histopathological variation in cardiac tissue to congestive heart failure or sudden death [147–149]. The clinical presentation of patients with cardiomyopathy can resemble those with heart failure with reduced ejection fraction, including dyspnea on exertion, fluid retention and edema, lethargy, orthopnea, presyncope, syncope, and paroxysmal nocturnal dyspnea [26, 32]. Nevertheless, clinicians should be attentive to the rare types of cardiomyopathies such as LVNC or arrhythmogenic right ventricular (RV) dysplasia [37, 150]. Considering the variety of manifestations of cardiomyopathy, taking a thorough history of the patient and other family members as well as general physical examination concerning not only cardiac disorders but also possible extracardiac abnormalities, that is, Noonan syndrome, are essential for choosing the most relevant diagnostic modalities [2].

#### **5.1 Laboratory parameters and biomarkers**

A few biomarkers are used in the routine clinical approach to children with cardiomyopathies, which to some extent is due to the lack of reliable evidence. A high cardiac troponin concentration in serum can support a diagnosis of myocarditis considering that other causes of ischemia (e.g., acute coronary syndrome and acute myocardial strain such as that induced by pulmonary embolism or recreational drug use) are uncommon in children. Given that natriuretic peptides have been reported in a study on adult patients to be markedly higher in patients with RCM, N-terminal pro-B-type natriuretic peptide may offer supportive evidence for RCM versus constrictive pericarditis [151]. However, studies are warranted to verify their applicability in children. Thus, for example, fibrotic pathways seem to vary age–dependently.

#### *Cardiomyopathies in Children: Genetics, Pathomechanisms and Therapeutic Strategies DOI: http://dx.doi.org/10.5772/intechopen.109896*

According to Woulfe et al. [152], fibrotic pathways in children (<18 years) were less active than in adults, though here again many more studies are needed [126, 153, 154]. A study of Miyamoto et al. [155] showed that the ß1: ß2 adrenoreceptor ratio differed significantly in pediatric HF patients from the one in adult patients. cAMP levels were commonly decreased in both adult and pediatric HF but were significantly higher in HF children than in HF adults. In addition, gene expressions of BNP, Cx43, and PP1ß and PP2A were regulated antithetically, indicating that signaling is differentially regulated in children and adults. However, it is already difficult to determine a normal BNP level since it alters with age of the children probably due to the maturation process of the heart from fetal to adult including alterations in gene expression and metabolism [156]. This development largely takes place within the first year after birth.

Several biological roles of miRNAs have been identified so far in cardiac development and diseases [157]. Accordingly, emerging studies have revealed the potential utility of miRNAs as biomarkers for the detection of DCM, myocarditis, and the evaluation of heart failure in children [158–160]. The study of Jiao et al. [159] reported an area under the curve (AUC) of up to 0.992, suggesting that these circulating miRNAs may be useful for DCM detection and diagnosis in children. Nevertheless, the studies evaluating the diagnostic role of miRNA in cardiomyopathy in children are sparse. Furthermore, the study of Miyamoto et al. [158] emphasized the inapplicability of employing an adult miRNA profile as a circulating biomarker for pediatric patients, highlighting the significance of developing a signature of circulating miRNAs in this population. Moreover, there are not many miRNAs that are consistent among studies. Therefore, further studies are warranted to find common miRNAs to be accepted as validated biomolecules in the diagnostics of cardiomyopathies.

#### **5.2 Genetic testing**

Considering that genetic causes play a major role in pediatric cardiomyopathies, genetic tests are indicated in most cases, not only for better classification but also for determining the cause and screening other family members. Autosomal-dominant inheritance is the most common mode of inheritance in familial isolated cardiomyopathy diagnosed in childhood, but X-linked inheritance and autosomal-recessive inheritance are also reported less frequently [18, 20]. Children with HCM and those with an affected first-degree relative have the highest likelihood of inheritance of a diseasecausing mutation among those with isolated cardiomyopathy. Furthermore, HCM that develops in childhood is more likely to be caused by multiple disease-causing mutations compared with HCM that emerges in adulthood [161]. Therefore, it is advised to take into account a broad panel rather than targeted genetic testing when HCM manifests throughout childhood [161]. Thus, whole exome or even whole genome screening should be the gold standard to detect pathogenic or likely pathogenic mutations [162, 163]. Moreover, because the signs of syndromes may be missed at the initial presentation, particularly in infants or critically ill children, a comprehensive genetic examination is helpful. Nevertheless, comprehensive genetic counseling and a thorough family pedigree are essential for understanding the scope and implications of genetic testing. When a child with cardiomyopathy is confirmed to have a pathogenic mutation known to be related to cardiomyopathy, cascade genetic testing of family members is typically advised [2]. Notably, there are various limitations of genetic testing in children. A negative or nondiagnostic test result does not rule out the diagnosis of cardiomyopathy or the possibility that the cardiomyopathy may have a hereditary etiology. Moreover, adult data is the only source of information for commercial panels that may not be applicable to children [2].

#### **5.3 Functional and structural assessment**

Assessment of myocardial structural, valvular or coronary artery abnormalities, and cardiac functions by means of proper imaging techniques is considered the cornerstone for the diagnosis and classification of cardiomyopathies.

Electrocardiography and electrophysiology are essential in the diagnosis of some types of pediatric cardiomyopathies, such as ACM. Slow intraventricular conduction in electrophysiology examinations is typically detected in ACM, and most often, right bundle-branch block with right precordial repolarization variations can be detected in electrocardiography. Scar or delayed conduction can be evaluated in 3D using an emerging approach for the diagnosis of ACM called electroanatomic mapping [164–166].

Echocardiography, which is often the initial imaging modality in cardiomyopathy evaluation, provides an overview of structural parameters including chamber dimensions, volumes, wall dimensions, assessment of cardiac functional features such as Doppler traces of ventricular contractility (dP/dt), systolic-to-diastolic ratio, as well as tissue Doppler imaging and extents of myocardial deformation (strain and strain rate) [167]. Notably, no absolute values are attainable as the cutoff point for morphological parameters obtained from echocardiography in children, such as LV end-systolic dimension (LVESD), LV end-diastolic volume, and LV end-diastolic dimension (LVEDD), and they all should be interpreted regarding the z scores adjusted for patient size [168–170]. These parameters are essential to the classification of morphological types. A high LVEDD or LV end-diastolic volume, besides low LV functional parameters, is marker of a dilated, hypokinetic type. Increased wall thickness proposes HCM. Measuring the thickness-to-dimension ratio can aid in distinguishing between idiopathic DCM and myocarditis [2]. However, it is not easy to determine deviations from normal LV morphology, as somatic growth has to be taken into account while monitoring the progression, for example, of HCM or DCM [171]. Thus, performing an accurate assessment by echocardiography in children can be difficult. Specifically, evaluation of diastolic function in children has low interobserver consistency, and the results are not properly associated with invasive haemodynamic investigations; thus, it may not be able to accurately discriminate between cardiomyopathy phenotypes [172, 173].

Cardiac magnetic resonance imaging (cMRI) offers great advantages over echocardiography for the diagnosis and assessment of cardiomyopathies and transcends some limitations of echocardiography in children. Determination of structural features including chamber dimensions, wall thicknesses, and ventricular mass as well as functional parameters including flow rates, shunts, and regional wall motion abnormalities using cMRI can aid in the diagnosis and accurate classification of the cardiomyopathy [174, 175]. Furthermore, assessment of the presence and pattern of fibrosis in tissue with late gadolinium enhancement and also the determination of edema and hyperemia in cMRI are some exceptional properties of cMRI that aid in the noninvasive investigation of patients with cardiomyopathy (e.g., to distinguish the different types of HCM or to discriminate DCM versus myocarditis) [176]. The information acquired from cMRI can also help determine the possible causes of cardiomyopathies (e.g., cardiomyopathies secondary to iron overload). Strain parameters and RV morphology and physiology, which are important in diagnosing and classifying

*Cardiomyopathies in Children: Genetics, Pathomechanisms and Therapeutic Strategies DOI: http://dx.doi.org/10.5772/intechopen.109896*

cardiomyopathies, can be evaluated more accurately with cMRI than with echocardiography.

Cardiac computed tomography (CT) and cardiac catheterization are rarely considered for the diagnosis and evaluation of pediatric cardiomyopathies. Cardiac catheterization can be helpful for haemodynamic assessment, performing endomyocardial biopsy, and surgical interventions in patients with amenable lesions [177]. In general, morphological evaluation of the heart in children and infants by imaging techniques as well as cardiac catheterization can be challenging due to the requirement of specialized training and equipment. The availability of both specialized medical professionals as well as equipment still is a major obstacle worldwide to improve the quality of care for patients with pediatric cardiomyopathies.

#### **6. Treatment of pediatric cardiomyopathy**

Pediatric cardiomyopathy is treated according to the distinct symptoms that each patient presents [15]. Various factors, including the specific type of cardiomyopathy, the disease's progression at the time of diagnosis, the patient's age, any coexisting medical conditions, the patient's tolerance for particular medications, and other factors, may affect the specific therapeutic procedures and interventions [15]. The therapeutic approach for pediatric cardiomyopathy options may include staged therapy for heart failure, lifestyle modifications, nutrition and strenuous physical activity restrictions, patient and parent education, implanted cardioverter-defibrillator installation, and, in refractory situations, consideration of heart transplantation [178, 179]. A multidisciplinary team of healthcare professionals, including pediatricians, pediatric cardiologists, hematologists, surgeons who specialize in pediatric cardiothoracic surgery, physical therapists, occupational therapists, and/or other healthcare professionals, may be needed to provide this type of treatment [179]. It is crucial to consider that medications and surgical methods for treating cardiomyopathy have mostly been tried and evaluated in adults. Thus, as also stated in a review by Loss et al. [180], pediatric management in general follows the guidelines for adult HF treatment. This is due to the lack of clinical trials with children, which are especially challenging due to difficulties in recruiting an adequate number of probands, high costs, and so on.

#### **6.1 Noninvasive treatment**

The usefulness of drugs developed mainly for adults in treating pediatric cardiomyopathy is only dimly documented. There are some studies showing that utilization of medicals as well as signaling differ in children and in adult heart failure patients. In the study of Pan et al. [156], diastolic diseases in children were treated with ACEinhibitors (angiotensin-converting enzyme), ß-blockers, digitalis, calcium channel blockers, dopamine, and diuretics. In children with diastolic diseases but not CM, the medical treatment was largely successful and improved cardiac function. However, children with CM did not improve. Though, in this study, only few children with CM were included. However, the results underline the observation that medical treatments prescribed for adult patients with CM may not be proper for children with CM. For review on possible medical treatments in pediatric RCM, see [29]. Another nice review on HF drug therapies highlighting the gaps for pediatric HF management is by Das et al. [181].

Thus, to ascertain the long-term safety and efficacy of such medicines in the pediatric population, more studies are required. Some ongoing studies evaluate emerging methods for treatment of pediatric cardiomyopathies such as reducing mitochondrial oxidative stress by MitoQ (mitoquinol mesylate) for DCM, oral Ifetroban for Duchenne muscular dystrophy cardiomyopathy and DCM, or gene therapy for male patients with Danon disease [182–184]. Nonetheless, more investigations are required to determine the optimal therapy approaches for cardiomyopathies in children.

#### **6.2 Minimally invasive and invasive treatments**

Altarabsheh et al. evaluated the early and late results of children (<21 years) who underwent transaortic septal myectomy for obstructive HCM [185]. Although the results of this study supported the safety and efficiency of this approach, technical challenges are reported to be augmented in children due to limited exposure during the procedure and subsequently increased risk of suboptimal removal of obstructive muscle, iatrogenic injuries to aortic/mitral valves or papillary muscles, and aneurysm formation in ventricular apex due to excessive muscular resection. Some alternative approaches are also introduced in adult patients such as radiofrequency catheter ablation (RFCA) and alcohol septal ablation, but limited or no data are available for children with HCM [186–188]. There are also ongoing studies evaluating novel methods like intracoronary transplantation of stem cells in pediatric CM [189, 190]. Additionally, surgery in adults is also considered in cases with complex LV morphologic abnormalities, such as papillary muscle anomalies, aberrant intraventricular muscle bundles, intrinsic mitral valve disease (requiring repair or replacement), and associated CAD that necessitates bypass grafting [191].

Cardiac resynchronization therapy (with or without implantable cardioverterdefibrillator) is also available as another option for pediatric and adult patients with DCM with ventricular conduction delay. The goal of this treatment is to enhance heart function by reducing the delay in activation of the left ventricular free wall, which is frequently observed in patients with left ventricular systolic dysfunction. The treatment has been demonstrated to enhance survival in this group, restore coordination and relaxation of the heart chambers, and cause favorable cardiac remodeling. However, using the current suggested criteria, up to a third of patients do not see any therapeutic improvement [192]. Furthermore, recommendations about electrical device therapies for children with DCM are mainly adopted from those for adults but based on considerably less evidence [193].

Despite these promising approaches, often heart transplantation remains as the only treatment option for pediatric CM. The main problem is that waiting for a suitable heart often takes a long time, which the children do not have to survive. The mortality of children while on transplant wait-list might be as high as 17–30% and even higher for infants, due to the general shortage of donor hearts and the high refusal rate due to poor quality (80%) [194, 195]. Also here, a lack of consensus between different centers and listing programs leads to prominent differences in the potential outcomes. Thus, the waiting time has to be bridged by a mechanical circulatory support or total artificial heart, which is suboptimal especially for newborns and infants, due to the limited availability of suitable devices and the adverse effects on the following heart transplantation due to, for example, adhesions. Alternatively, an allograft transplantation can be tried, although here the same problems as the donor heart availability and utilization apply [196]. After transplantation, the problem of

*Cardiomyopathies in Children: Genetics, Pathomechanisms and Therapeutic Strategies DOI: http://dx.doi.org/10.5772/intechopen.109896*

immunosuppression arises, bearing the risk of infections or developing cancer. Thus, at Duke University hospital, recently a newborn who in addition to heart failure had a T-cell deficiency has been successfully transplanted a heart together with thymus for the first time in spring 2022. In general, the posttransplant survival rates in children are highly variable and largely depend on comorbidities, general clinical status, the use of mechanical support devices, and, as a major factor, disease progression due to wait-list time.

#### **7. Conclusion and future perspectives**

Although pediatric cardiomyopathies share many aspects with CMs in adults, there is rising evidence of unique features that have implications for diagnosis and treatment. First of all, a standard guideline for the classification of CM by the scientific society is desperately needed, as a mutual approach is required for developing protocols and reporting the findings. Age-based scales for diagnostic parameters for distinguishing between normal and abnormal conditions have to be established and adjusted for children.

Furthermore, we need to increase awareness of the medical community to avoid using the same CM therapeutic protocols of adults for children when their experimental evidence did not include children or excluded them. In the future, besides genetic testing (whole exome or whole genome), family analysis and analysis of specific biomarkers and investigation of molecular mechanisms in children will further support diagnosis and treatment design. In addition, large well-conceived trials are needed to increase the efficacy of medical treatments in children. Not only physical/mental effects but also developmental effects of therapeutic practices have to be investigated in long-term cohort studies.

Specifically optimized, mechanical circulatory supports for small children have to be developed that will reduce the deaths while awaiting heart transplantation. The assessment of donor heart suitability needs to be improved and standardized to reduce transplantation wait-list time. Also, increased allograft utilization may contribute to improved survival rates until transplantation, together with modified postoperative care and monitoring optimized for pediatric patients.

An interesting, though controversial, topic in the future might be xenotransplantation, considering the increasing demands for pediatric heart transplantation and advances in tissue engineering and genetic modification of, for example, animal donors. Though ethically highly debated, a survey of congenital heart surgeons revealed a generally high acceptance (80–88%) of xenotransplantation [197].

#### **Acknowledgements**

Part of this work was kindly supported by the Boehringer Ingelheim Fonds (S.-R. Sadat-Ebrahimi).

#### **Conflict of interest**

The authors declare no conflict of interest.

*New Insights on Cardiomyopathy*
