**5. Concluding remarks**

Over two decades ago the first HCM-causing mutation in a sarcomeric gene was identified in β-myosin heavy chain. Since then both HCM and DCM have come to be known as diseases of the sarcomere. In fact, sarcomeric dysfunction is the underlying cause of many genetically mediated HCM and DCM disorders and accounts for ~60% of HCM and ~10-20% of DCM reported cases [42]-[45]. To date, more than 1500 distinct mutations of sarcomeric proteins have been linked to cardiomyopathies [46], [47]. Given the many roles that have been described for titin, nebulette, and obscurin in cardiac muscle, and the effects of the identified mutations in their localization, activity, and regulation, it is not surprising that many human diseases of heart muscle have been linked to these proteins. Notably, a striking 50 missense mutations within *TTN, NEBL,* and *OBSCN* (Table 1) with an additional 16 splice site donor/acceptor mutations (Table 2) and 47 deletion or insertion mutations within *TTN* (Table 3) have been associated with the development of different forms of cardiomyopathy. The severity of these diseases can vary from moderate to severe, depending on the nature of the mutation. The characterization of these mutations and their effects on cardiac pathophysiology is just beginning to be elucidated, however it is clear that this is just the tip of the iceberg. Under‐ standing how these mutations alter sarcomeric structure and contractile activity could aid in improving clinical diagnosis and developing individualized therapies for cardiomyopathic patients.

response and also during hypertrophic growth. Concurrent with this, targeted loss of the obscurin RhoGEF domain resulted in myocytes lacking intercalated discs and in more severe cases in failure of the contractile filaments to organize into mature sarcomeres [41]. It is likely that upregulation of obscurins, is associated with the increase in contractile structures observed during hypertrophy, however, the mechanism by which this occurs remains unresolved.

More direct evidence for the involvement of *OBSCN* in the development of heart disease was demonstrated in a single patient with HCM [3]. Linkage analysis revealed a sequence variation in the *OBSCN* gene in the region encoding the site of interaction for the Z-disc region of titin (Ig58/59), specifically an R4344Q variant in the Ig58 domain of obscurin. *In vitro* studies showed that this variant resulted in decreased binding of obscurin to titin as well as mis-localization of obscurin to the Z-disc. Despite this single case, it suggests that, like titin and nebulette,

**Figure 6.** Schematic representation of the obscurin isoforms, illustrating their motifs and cardiomyopathy associated mutation. The missense mutation in the *OBSCN* gene is shown relative to the domain in which it is localized with a

Over two decades ago the first HCM-causing mutation in a sarcomeric gene was identified in β-myosin heavy chain. Since then both HCM and DCM have come to be known as diseases of the sarcomere. In fact, sarcomeric dysfunction is the underlying cause of many genetically mediated HCM and DCM disorders and accounts for ~60% of HCM and ~10-20% of DCM reported cases [42]-[45]. To date, more than 1500 distinct mutations of sarcomeric proteins have been linked to cardiomyopathies [46], [47]. Given the many roles that have been described for

magenta background.

72 Cardiomyopathies

**5. Concluding remarks**

mutations in the *OBSCN* gene lead to the development cardiomyopathies.



**Splice Site Donor/Acceptor Mutations of Titin Disease Mutation Region on Protein Sarcomeric Region Effect Reference** Mutations of TTN DCM IVS118-g>a^ PEVK I-band Unknown [6] HCM IVS155+g>t^ PEVK I-band Unknown [6] DCM IVS172-g>c^ PEVK I-band Unknown [6] DCM IVS172+g>a^ PEVK I-band Unknown [6] DCM IVS185-2a>g^ Ig85 I-band Unknown [6] DCM IVS230+g>t^ FNIII19 A-band Unknown [6] DCM IVS237+3a>g^ FNIII22 A-band Unknown [6] DCM IVS253-5t>a^ Ig106 A-band Unknown [6] DCM IVS254-g>a^ FNIII41 A-band Unknown [6] DCM IVS255+g>a^ Ig109 A-band Unknown [6] DCM IVS271+5g>a^ FNIII53 A-band Unknown [6] DCM IVS274-2a>g^ FNIII55 A-band Unknown [6] DCM IVS276+5g>c^ FNIII99 A-band Unknown [6] DCM IVS277+g>a^ FNIII100 A-band Unknown [6] DCM IVS279+2t>a^ FNIII102 A-band Unknown [6] DCM IVS302+g>c^ FNIII128 A-band Unknown [6]

**Table 2.** Listing of splice site donor/acceptor mutations found in *TTN* that have been causally linked to the development of cardiomyopathies. Sequence correspond to accession number NM\_001256850.1^ for titin. DCM:

**Deletion and Insertion Mutations of Titin**

**Deletion Mutations** DCM 6247 del g^ R2083fs Ig11 I-band Unknown [6] DCM 19183 del g^ S6395fs Ig45 I-band Unknown [6] HCM 23798-23810 del gtcaagatatctg^ G7933fs Ig61 I-band Unknown [6] DCM 44336 del a^ E14779fs FNIII6 A-band Unknown [6] DCM 44725 del t^ D14909fs FNIII8 A-band Unknown [6] DCM 45322 del t^ F15108fs FNIII10 A-band Unknown [6] DCM 53935 del c^ E17978fs FNIII20 A-band Unknown [6] HCM 60147 del c^ P20049fs FNIII45 A-band Unknown [6] DCM 64925 del t^ K21640fs FNIII57 A-band Unknown [6] DCM 65867 del a^ E21956fs FNIII59 A-band Unknown [6] DCM 67745 del t^ P22582fs FNIII64 A-band Unknown [6] DCM 81536-81537 del ct^ S27179fs Ig129 A-band Unknown [6] DCM 84977-84980 del atta^ Y28326fs FNIII106 A-band Unknown [6] DCM 82381 del g\* A27460 FNIII107 A-band Truncation of titin [24] DCM 89180-89184 del ttaaa^ T29725fs FNIII116 A-band Unknown [6]

**Protein**

**Sarcomeric**

**Region Effect Reference**

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dilated cardiomyopathy, HCM: hypertrophic cardiomyopathy, IVS: intron

**Disease Mutation Amino Acid Region on**

**Table 1.** Listing of missense mutations found in *TTN, NEBL,* and *OBSCN* that have been causally linked to the development of cardiomyopathies. Sequences correspond to the following accession numbers: NM\_133378.4\*, NM\_003319.4#, and NM\_001256850.1^ for titin; NM\_006393.2 for nebulette; and NM\_052843.2 for obscurin. DCM: dilated cardiomyopathy, HCM: hypertrophic cardiomyopathy, ARVC: arrhythmogenic right ventricular cardiomyopathy, IDC: idiopathic dilated cardiomyopathy


**Missense Mutations of Titin, Nebulette, and Obscurin**

DCM R18858X^ Ig107 A-band Truncation of titin [6] DCM R18985X^ FNIII37 A-band Truncation of titin [6] ARVC A18579T\* FNIII41 A-band Unknown [9] DCM R19560X^ Ig109 A-band Truncation of titin [6] ARVC A19309S\* Ig111 A-band Unknown [9] DCM R20858X^ FNIII51 A-band Truncation of titin [6] DCM Q25689X^ Ig125 A-band Truncation of titin [6] DCM W26632X^ FNIII94 A-band Truncation of titin [6] DCM R26949X^ FNIII96 A-band Truncation of titin [6] DCM K27016X^ FNIII97 A-band Truncation of titin [6] DCM W27147X^ Ig129 A-band Truncation of titin [6] DCM Y27567X^ FNIII100 A-band Truncation of titin [6] DCM W29318X^ Ig134 A-band Truncation of titin [6] DCM R29415X^ FNIII114 A-band Truncation of titin [6] DCM E29510X^ FNIII115 A-band Truncation of titin [6] DCM Q30081X^ FNIII119 A-band Truncation of titin [6] DCM R31195X^ FNIII127 A-band Truncation of titin [6] DCM K31371X^ FNIII129 A-band Truncation of titin [6] ARVC P30847L\* FNIII131 A-band Unknown [9] DCM S31841X^ FNIII131 A-band Truncation of titin [6] DCM R32069Q\* Ig146 A-band Unknown [21] ARVC M33291T\* Ig152 A-band Unknown [9] Mutations of NEBL DCM K60N Repeat 1 Sarcomeric structural abnormalities [8] DCM Q128R Repeat 3 Sarcomeric structural abnormalities [8] DCM G202R Repeat 5 Disruption of I-band and Z-disc proteins [8] DCM A592E Repeat 16 Disruption of I-band and Z-disc proteins [8] IDC N654K Repeat 18 Unknown [4] Mutations of OBSCN HCM R4344Q Ig58 Loss of titin Binding [3]

**Region Effect Reference**

**Sarcomeric**

**Table 1.** Listing of missense mutations found in *TTN, NEBL,* and *OBSCN* that have been causally linked to the development of cardiomyopathies. Sequences correspond to the following accession numbers: NM\_133378.4\*, NM\_003319.4#, and NM\_001256850.1^ for titin; NM\_006393.2 for nebulette; and NM\_052843.2 for obscurin. DCM:

dilated cardiomyopathy, HCM: hypertrophic cardiomyopathy, ARVC: arrhythmogenic right ventricular

cardiomyopathy, IDC: idiopathic dilated cardiomyopathy

**Disease Mutation Region on**

74 Cardiomyopathies

**Protein**

**Table 2.** Listing of splice site donor/acceptor mutations found in *TTN* that have been causally linked to the development of cardiomyopathies. Sequence correspond to accession number NM\_001256850.1^ for titin. DCM: dilated cardiomyopathy, HCM: hypertrophic cardiomyopathy, IVS: intron



**Acknowledgements**

**Author details**

**References**

Medicine, Baltimore, MD, USA

2008;77:659-66.

Genet 2012;5:391-9.

National Institutes of Health (F32 AR058079).

Maegen A. Ackermann and Aikaterini Kontrogianni-Konstantopoulos

Rev Genomics Hum Genet 2005;6:185-216.

opathy. N Engl J Med 2012;366:619-28.

dilated cardiomyopathy. Hum Genet 2000;107:440-51.

dilated cardiomyopathy. Am J Cardiol 2012;109:1644-50.

Our research has been supported by grants to A.K.K. through the National Institutes of Health (R21 HL106197) and the American Heart Association (GRNT 3780035) and to M.A.A from the

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Department of Biochemistry and Molecular Biology, University of Maryland, School of

[1] Morimoto S. Sarcomeric proteins and inherited cardiomyopathies. Cardiovasc Res

[2] Ahmad F, Seidman JG, Seidman CE. The genetic basis for cardiac remodeling. Annu

[3] Arimura T, Matsumoto Y, Okazaki O, et al. Structural analysis of obscurin gene in hypertrophic cardiomyopathy. Biochem Biophys Res Commun 2007;362:281-7. [4] Arimura T, Nakamura T, Hiroi S, et al. Characterization of the human nebulette gene: a polymorphism in an actin-binding motif is associated with nonfamilial idiopathic

[5] Golbus JR, Puckelwartz MJ, Fahrenbach JP, Dellefave-Castillo LM, Wolfgeher D, McNally EM. Population-based variation in cardiomyopathy genes. Circ Cardiovasc

[6] Herman DS, Lam L, Taylor MR, et al. Truncations of titin causing dilated cardiomy‐

[7] Kontrogianni-Konstantopoulos A, Ackermann MA, Bowman AL, Yap SV, Bloch RJ. Muscle giants: molecular scaffolds in sarcomerogenesis. Physiol Rev 2009;89:1217-67.

[8] Purevjav E, Varela J, Morgado M, et al. Nebulette mutations are associated with dilated cardiomyopathy and endocardial fibroelastosis. J Am Coll Cardiol 2010;56:1493-502. [9] Taylor M, Graw S, Sinagra G, et al. Genetic variation in titin in arrhythmogenic right ventricular cardiomyopathy-overlap syndromes. Circulation 2011;124:876-85. [10] Yoskovitz G, Peled Y, Gramlich M, et al. A novel titin mutation in adult-onset familial

**Table 3.** Listing of deletion and insertion mutations found in TTN that have been causally linked to the development of cardiomyopathies. Sequences correspond to the following accession numbers: NM\_133378.4\* and NM\_001256850.1^ for titin DCM: dilated cardiomyopathy, HCM: hypertrophic cardiomyopathy, del: deletion, ins: insertion, fs: frameshift
