HCM is considered to be primarily a disease of the sarcomere, the contractile unit of myocytes. Disease-causing mutations have been identified in most sarcomere genes, with two genes (MYH7 and MYBPC3) contributing almost 80 % of disease causing mutations [40]. Genes encoding Z-disk proteins, mitochondrial genes, and metabolic genes also have been implicated, though these are less frequently mutated. The majority of mutations are private (only seen in one family), and there is a high prevalence of missense mutations acting in a dominant negative fashion. Mutations leading to loss of function are less frequent but are prevalent in the MYBPC3 gene. The known HCM-causing genes are listed in Table 17.1. Interestingly, very few genotype–phenotype correlations have been identified. Some general associations have emerged through analysis of case studies, though these are broad, usually are defined at the gene level, and may not be applicable to everyone with mutations in those genes. For example, HCM caused by mutations in TNNT2 has been associated with a wide variety of clinical presentations and is usually associated with a reduced disease penetrance, a mild degree of hypertrophy, but a high incidence of SCD and more extensive myocyte disarray, although some mutations are associated with hypertrophy without risk of arrhythmias [41–43]. MYBPC3-related HCM has been associated with a reduced penetrance, relatively mild hypertrophy, a low incidence of SCD, late onset of clinical manifestations, and good prognosis before the age of 40 [44–46]. PRKAG2 related HCM is associated with concentric hypertrophy and Wolff–Parkinson–White syndrome [47, 48]. Finally, MYH7 mutations appear to be associated with severe LVH and an increased risk of heart failure and SCD [10].

Recent studies have shown that approximately 5 % of individuals with HCM have two pathogenic mutations, typically one inherited from each parent [49, 50]. Multiple mutations typically lead to a more severe presentation and an earlier age of onset than those seen with only one mutation [50].

One of the most important uses of genetic testing for HCM is the ability to identify asymptomatic family members who may be at risk to develop HCM. Multiple modes of inheritance have been described in HCM and knowing the genetic cause of an individual’s disease allows for more accurate genetic counseling of family members. Since genotype–phenotype correlations for HCM are minimal, prognostic use of genetic testing is limited. Distinguishing the different genetic causes of heart muscle thickening is extremely important, as the treatment for HCM differs markedly from the treatment of other conditions. Fabry disease can present as isolated LVH in men and women and recent studies have suggested that approximately 6 % of males with late onset HCM had low α-Gal A enzyme activity [51, 52]. Identifying a pathogenic mutation in the GLA gene can provide a definitive diagnosis and enable lifesaving enzyme replacement therapy. Additionally, studies are emerging regarding therapeutic guidance for patients with sarcomeric HCM. Experimental studies and clinical trials of interventions (including diltiazem and angiotensin receptor/aldosterone blockade) show promise in delaying disease onset [53–55].

Clinical molecular testing for the HCM genes listed in Table 17.1 is currently available through several laboratories (www.​genetests.​org). The majority of testing for HCM is done by NGS using a targeted gene panel.

Over 50 % of individuals with a clinical diagnosis of idiopathic HCM and up to 70 % of familial HCM will have a mutation in one of the genes listed in Table 17.1 [3, 20]. Therefore, a negative test result reduces but does not eliminate the likelihood that the individual carries a causative mutation. Copy number variants (CNVs), such as large deletions leading to loss of function, are generally suspected to exist in genes with a high prevalence of other loss-of-function variants (e.g., nonsense and frameshift variants). HCM genes that fit this description include MYBPC3 and LAMP2. Deletion mutations have been described [56] but are thought to be rare. However, Sanger sequencing does not detect large deletions and thus, their prevalence will only emerge as technologies which can detect large deletions are more widely used for clinical testing. Methods of NGS data analysis are being developed to simultaneously detect small mutations as well as CNVs, which will provide a more accurate estimate of the prevalence of CNVs related to HCM and other inherited disorders.

Genetic studies have revealed that approximately 25 % of tested patients have a mutant allele but do not manifest a HCM phenotype (genotype-positive/phenotype-negative HCM) [57]. The variability in phenotypic expression of the mutations could be due to environmental influences (differences in lifestyle, risk factors, and exercise) or genetic modifiers. VUS are common and difficult to interpret and should not be used for predictive testing for at-risk family members. Genetic testing laboratories typically offer targeted testing of family members to establish whether a variant identified in the index family member segregates with the disease. These concordance studies can be helpful in clarifying the clinical significance of VUS. Functional tests, such as the investigation of a possible splice mutation, exist but are currently difficult to incorporate into routine clinical testing so are largely confined to research laboratories.

The majority of molecular genetic testing for inherited cardiac diseases is done by sequencing the exons and splice sites of one or several disease genes because allelic heterogeneity is the norm for all diseases covered in this chapter. Similarly, the assays that are commonly used to detect small deletions or duplications (e.g., multiplex ligation-dependent probe amplification [MLPA] or array-based comparative genomic hybridization) are not unique to any given gene or disease; hence, the laboratory issues common to all genetic CVD testing are reviewed at the end of this chapter.

DCM is, by far, the most heterogeneous of the diseases discussed in this chapter. Mutations in MYH7, LMNA, SCN5A, TNNI3, and TNNT2 make up roughly 20 % of familial DCM cases [3, 23]. Additional genes (Table 17.1) typically contribute a small fraction of the remaining DCM-causing mutations. The recent discovery that loss of function mutations in the TTN gene are responsible for as much as 25 % of genetic DCM is a notable exception and a radical improvement for genetic testing for DCM [12]. Missense mutations in the LMNA and SCN5A genes are responsible for inherited DCM with conduction system disease [66–68]. Autosomal recessive DCM is less frequent and is characterized by a significantly younger age of onset and a worse prognosis compared to the dominant form. This form is typically associated with skeletal myopathy and caused by mutations in the SGCD and TCAP genes. Additionally, autosomal dominant forms of DCM can present with skeletal myopathy and include mutations in the CSRP3, DES, DMD, EMD, LMNA, MYH7, and TTN genes [3, 20] (Table 17.1). The TAZ gene is associated with Barth syndrome but DCM can be the first presenting feature [69–72]. In addition, recent studies have provided evidence that mutations in the LAMP2 gene, which typically cause Danon disease in young boys, can cause DCM in the fourth decade of life in women [37]. Finally, there is emerging evidence of genetic overlap with ARVC as mutations in traditional ARVC genes have been detected in individuals with DCM [25, 26].

Molecular testing for DCM is most useful when there is a confirmed family history and/or conduction system disease in the proband or other affected family members. The profound genetic heterogeneity has long precluded a prominent role for molecular testing in isolated DCM, though this is rapidly changing. Multiple modes of inheritance have been described in DCM, and knowing the cause of an individual’s disease allows for more accurate genetic counseling regarding the risk to other family members. Distinguishing the different genetic causes of heart muscle weakness is extremely important, as the clinical management for primary DCM can differ from DCM associated with other conditions. The prognostic utility of genetic testing is limited to a higher risk for SCD in SCN5A and LMNA mutation carriers, and screening for mitochondrial disease and skeletal myopathy in the genes associated with those features. For those individuals who are at risk for SCD due to arrhythmias caused by SCN5A and LMNA mutations, an implantable cardioverter defibrillator (ICD) can be implanted as a preventative measure.

Molecular testing for the DCM genes listed in Table 17.1 is currently available through clinical laboratories (www.​genetests.​org). Both Sanger sequencing and NGS methods are used.

The detection rate of mutations in individuals with DCM is rapidly evolving. Until 2012, approximately 20 % of individuals with idiopathic DCM and more than 30 % of those with familial DCM were expected to have a mutation in one of the genes listed in Table 17.1 [3, 73]. With the discovery that TTN mutations may account for another 25 %, the detection rate for DCM is now approaching that of HCM. A negative test result reduces but does not eliminate the likelihood that an individual carries a causative mutation. The variability in phenotypic expression of mutations could be due to environmental influences or acquired traits (differences in lifestyle, risk factors, and exercise). Variants of unknown significance are difficult to interpret and should not be used for predictive testing for at-risk family members.

Two modes of inheritance, autosomal dominant and autosomal recessive, are observed with ARVC. This disease is often described as a disease of the desmosome because mutations are predominantly found in genes encoding this multiprotein complex. Desmosomes form cell–cell junctions and are prevalent in tissues that are subjected to mechanical stress (such as the heart and skin). To date, six genes (PKP2, DSP, DSC2, DSG2, JUP, and TMEM43) have been implicated in autosomal dominant ARVC and two of these genes (DSP and JUP) have been associated with rare autosomal recessive forms that have cutaneous involvement (wooly/kinky hair and palmoplantar hyperkeratosis) [79–82]. These variants are referred to as Naxos syndrome (OMIM: #601214) and Carvajal syndrome (OMIM: #605676). Autosomal dominant ARVC has incomplete penetrance and variable phenotypic expression [83]. Catecholaminergic polymorphic ventricular tachycardia (CPVT) is characterized by SCD during physical or emotional stress [84, 85]. Although considered clinically distinct, CPVT shares some features with ARVC and given that ARVC can be difficult to diagnose clinically, genetic testing sometimes includes the RYR2 gene, which is the main gene associated with CPVT. A recent study provided convincing evidence for a role of the TTN gene, which is strongly associated with DCM, in ARVC [30].

The variability of phenotype, disease progression, and underlying genetic cause contribute to the difficulties of diagnosis and risk stratification in ARVC. Given these complexities, genetic testing can be helpful in providing a definitive diagnosis when a pathogenic mutation is found. Although a few patterns have emerged in relation to genotype–phenotype correlations, the evidence is limited and the prognostic and therapeutic implications of genetic testing are still being debated. Because the age of onset, symptoms, and penetrance vary so widely in ARVC, genetic testing may be more accurate at identifying at-risk relatives than clinical screening alone.

Molecular testing for the ARVC genes listed in Table 17.1 is currently available through clinical laboratories (www.​genetests.​org). Both Sanger sequencing and NGS methods are used.

Approximately 50 % of individuals who meet task force criteria for ARVC have a pathogenic mutation in one of the six genes listed in Table 17.1 [86, 87]. PKP2 mutations make up the majority of cases. Therefore, a negative test result reduces but does not eliminate the likelihood that the individual carries a causative mutation. The variability in phenotypic expression of the mutations could be due to environmental influences or acquired traits (differences in lifestyle, risk factors, and exercise). VUS are difficult to interpret and should not be used for predictive testing for at risk family members.

Idiopathic RCM is most commonly sporadic, but familial disease has been reported with autosomal dominant inheritance [89, 90]. Missense mutations in the DES (desmin) gene have been found in several families with desmin-related myopathy with and without RCM [91, 92]. Additionally, mutations in ACTC, MYH7, TNNI3, and TNNT2 have been associated with RCM, confirming that RCM is part of the spectrum of hereditary sarcomeric contractile protein disease (reviewed in refs. 3, 20).

Given the low detection rate for RCM genetic testing, the diagnostic utility of genetic testing is limited to predictive testing for family members. No genotype–phenotype correlations have been established and genetic testing has limited prognostic value.

Tests for most genes which have been associated with RCM are available as single gene tests using Sanger sequencing. Most clinical laboratories do not offer panels of genes specifically tailored to RCM but all genes associated with RCM to date are part of panels offered for HCM testing.

The clinical detection rate of the genes listed in Table 17.1 is estimated to be roughly 10–15 %; however, due to the unclear etiology of the disease and limited numbers, a true detection rate is unknown [3]. Therefore, a negative test result reduces but does not eliminate the likelihood that the individual carries a causative mutation. VUS are difficult to interpret and should not be used for predictive testing for at-risk family members.

Familial recurrence in LVNC is high and found in approximately 40 % of patients [94]. Gene mutations have been detected in sarcomere/Z-disk genes including MYH7, MYBPC3, TNNT2, ACTC1, and LDB3 [97]. In addition, LVNC can be the presenting feature of Barth syndrome caused by mutations in the TAZ gene [98, 99].

The ability to identify asymptomatic family members who may be at risk of developing LVNC is the main utility of genetic testing for this disease. As multiple modes of inheritance (autosomal dominant and X-linked) have been described in LVNC families, knowing the cause of an individual’s disease allows for more accurate genetic counseling regarding the risk to other family members. No genotype–phenotype correlations have been established and as such genetic testing has limited prognostic value.

Molecular testing for the LVNC genes listed in Table 17.1 is currently available through clinical laboratories (www.​genetests.​org). Both Sanger sequencing and NGS methods are used.

Between 17 and 41 % of individuals who meet criteria for LVNC will have a pathogenic mutation in one of the genes listed in Table 17.1 [20, 100]. Therefore, a negative test results reduces but does not eliminate the likelihood that an individual carries a causative mutation. The variability in phenotypic expression of the mutations could be due to environmental influences or acquired traits (differences in lifestyle, risk factors, and exercise). VUS are difficult to interpret and should not be used for predictive testing for at-risk family members.

Congenital LQTS is an inherited cardiac channelopathy characterized by prolongation of the QT interval of the cardiac cycle and increased susceptibility for syncope, seizures, and sudden cardiac death secondary to polymorphic ventricular tachyarrhythmias (torsades de pointes). Congenital LQTS occurs in two main heritable forms: autosomal dominant LQTS, originally described as Romano–Ward syndrome, and autosomal recessive LQTS, originally described as the Jervell and Lange-Nielsen syndrome. LQTS is the first type of arrhythmia to be understood at the molecular level as a primary cardiac channelopathy [101–103].

Over thirteen LQTS genes have been identified. Mutations in KCNQ1 (KVLQT1, LQT1) [104], and KCNH2 (HERG, LQT2) [105] cause the majority (about half) of LQTS. In approximately 25 % of families with LQTS, a genetic defect cannot be identified in the currently known LQTS-causing genes (www.​genereviews.​com). KCNQ1, the gene responsible for LQTS type 1 (LQTS1), encodes the α-subunit of the slowly activating delayed rectifier potassium ion channel (I_Ks). LQTS mutations in this gene cause a loss-of-function; the loss of I_Ks channel function decreases the I_Ks current, resulting in prolongation of the action potential duration and ventricular repolarization. LQTS type 2 (LQTS2) is due to mutations in KCNH2, which codes for the α-subunit of the rapidly activating delayed rectifier potassium ion channel (I_Kr). Mutations in KCNH2 reduce the I_Kr current, resulting in prolongation of the action potential duration and repolarization. LQTS type 3 (LQTS3) results from mutations in SCN5A, which encodes the α-subunit of the cardiac sodium channel [106]. In contrast to the other forms of LQTS, in which a reduction of repolarization causes the prolongation of the action potential, SCN5A mutations that cause LQTS3 cause a “gain-of-function” in the cardiac sodium channel with an increase in late sodium current [107]. Ankyrin-B (ANK2) is a member of a family of versatile membrane adapters [108]. This gene was the first non-cardiac channel gene implicated in LQTS. Since this discovery, other non-cardiac cannel genes (CAV3, AKAP9, and SNT1) have been associated with LQTS [109–111]. The remaining LQTS-causing genes and their associated mutations are shown in Table 17.2.