Analysis of Dystrophin Deletion Mutations Predicts Age of Cardiomyopathy Onset in Becker Muscular DystrophyCLINICAL PERSPECTIVE
Background— Becker muscular dystrophy (BMD) and X-linked dilated cardiomyopathy often result from deletion mutations in the dystrophin gene that may lead to expression of an altered dystrophin protein in cardiac muscle. Cardiac involvement is present in ≈70% of BMD and all X-linked dilated cardiomyopathy cases. To date, the timing of cardiomyopathy development remains unpredictable. We analyzed 78 BMD and X-linked dilated cardiomyopathy patients with common deletion mutations predicted to alter the dystrophin protein and correlated their mutations to cardiomyopathy age of onset. This approach was chosen to connect dystrophin structure with function in the heart.
Methods and Results— Detailed cardiac information was collected for BMD and X-linked dilated cardiomyopathy patients with defined dystrophin gene deletion mutations. Patients were grouped based on the dystrophin protein domain affected by the deletion. Deletions affecting the amino-terminal domain are associated with early-onset dilated cardiomyopathy (DCM; mid-20s), whereas deletions removing part of the rod domain and hinge 3 have a later-onset DCM (mid-40s). Further, we modeled the effects of the most common mutations occurring in the rod domain on the overall structure of the dystrophin protein. By combining genetic and protein information, this analysis revealed a strong correlation between specific protein structural modifications and DCM age of onset.
Conclusions— We identified specific regions of the dystrophin gene that when mutated predispose BMD patients to early-onset DCM. In addition, we propose that some mutations lead to early-onset DCM by specific alterations in protein folding. These findings have potential implications for early intervention in the cardiac care of BMD patients and for therapeutic approaches that target the heart in dystrophinopathies.
Received March 25, 2009; accepted September 21, 2009.
The dystrophin gene, located on the X-chromosome, is the largest known human gene (2.4 Mb, 79 exons), resulting in a high rate of spontaneous disease-causing mutations (30% of cases) with deletions forming the majority (≈60%). Dystrophin plays an essential structural role in both cardiac and skeletal muscle, protecting the sarcolemma from mechanical stresses of muscle contraction. Complete loss of dystrophin leads to Duchenne muscular dystrophy (DMD), the most common severe form of childhood muscular dystrophy, complicated by skeletal muscle degeneration and dilated cardiomyopathy (DCM).
Clinical Perspective on p 544
In contrast to the well-defined clinical course of DMD, mutations that do not disrupt the reading frame can result in expression of an altered dystrophin protein, leading to a more variable clinical presentation. This includes Becker muscular dystrophy (BMD) that presents primarily with progressive skeletal muscle degeneration with variable age of onset and severity, and X-linked DCM (XLDCM) that typically has no detectable skeletal muscle signs accompanying the cardiac involvement. Although BMD is historically diagnosed based on skeletal muscle manifestations, the primary cause of death is heart failure.1 Indeed, ≈70% of BMD patients develop DCM,1–4 and a recent longitudinal study demonstrated an onset of cardiac involvement in the early teens in some BMD patients.5 Similar to DMD and XLDCM, the severity and age of onset of cardiac involvement in BMD show no correlation to skeletal muscle involvement.2,3 Furthermore, DCM is often diagnosed after cardiac symptoms manifest, diminishing the efficacy of cardioprotective drugs. Therefore, the identification of parameters for cardiac risk assessment before symptom manifestation bears undeniable relevance for the clinical care of BMD patients and would assist in patient stratification in clinical trials testing the efficacy of cardiac treatments. It is with this in mind that we extensively examined a large cohort of BMD and XLDCM patients with deletion mutations to explore the hypothesis that loss of specific domains of the dystrophin protein predispose to early-onset cardiomyopathy by potentially affecting protein expression levels, function, and/or structure. This hypothesis was born from previous studies exploring correlation between genotype and the presence of DCM in BMD patients2–4 as well as reports on the association of deletions in specific dystrophin domains with severity of skeletal muscle symptoms.6–8
Materials and Methods
Patient Sources and Data Collection
This is a cross-sectional retrospective study of patient information obtained from 3 major sources: (1) a database from the Muscular Dystrophy Association clinics at Nationwide Children’s Hospital and The Ohio State University Medical Center; (2) the United Dystrophinopathy Project database (PI, Kevin Flanigan, Co-PIs Jerry R Mendell and Alan Pestronk); and (3) published studies. Information was obtained related to age of onset and severity of skeletal and cardiac muscle manifestations, description of cardiac evaluation and skeletal muscle biopsy, the gene mutation, family history, and serum creatine kinase. Echocardiograms at Nationwide Children’s Hospital were interpreted by the same cardiologist (H.D.A.). The Institutional Review Board at Nationwide Children’s Hospital and The Ohio State University approved all protocols.
Inclusion and Exclusion Criteria
Inclusion criteria included (1) a diagnosis of BMD or XLDCM or both, (2) cardiac evaluation, and (3) a confirmed exon deletion mutation spanning up to 11 exons. The 11 exon limit was set to exclude large deletions potentially affecting multiple functional domains of dystrophin, while including most patients who were found to have deletions affecting 1 to 8 exons. Patients with exon 45 to 55 deletions known to be associated with mild, late-onset skeletal muscle involvement9,10 were also included. More stringent criteria potentially compromised statistical power.
Exclusion criteria included (1) reported or suspected cardiac viral infections, (2) cardiac biopsy without dystrophin expression, (3) deletions restricted to noncoding regions of the dystrophin gene potentially containing ill-defined regulatory elements, (4) subjects younger than 12 years without proven family history of BMD, and (5) wheelchair-dependent patients by age 12 years carrying a diagnosis of BMD (preferably considered a severe form of dystrophinopathy11). Deletions selectively affecting myocardial expression of dystrophin have been excluded from this study primarily because of low patient numbers, precluding meaningful statistical analysis. Supplemental Table I lists subjects excluded based on these criteria. Of 320 subjects initially screened for inclusion in this study, 118 satisfied the selection criteria.
Definition of Cardiomyopathy and Disease Onset
Cardiomyopathy was defined as follows: ejection fraction ≤55% or shortening fraction ≤32% or both. The ejection fraction cutoff agrees with previous studies based on the natural history of the disease5,12 and with timing of cardiac drug intervention common in clinical practice for BMD patients. When available, additional parameters were considered to support cardiac dilation: E-point septal separation above 5 mm, left ventricular (LV) end-diastolic diameter above 58 mm or above 2 z scores when indexed to body surface area, or cardiomegaly consistent with cardiomyopathy by chest radiograph. Electrocardiograms were not used to define cardiomyopathy.13 In this cross-sectional study, the time at which abnormal cardiac findings were first reported defines the “onset” of cardiomyopathy. The age of onset of DCM represents the youngest reported age at which cardiac parameters met the definition of cardiomyopathy.
For analyses involving noncardiomyopathic patients, age corresponds to the oldest reported age at which cardiac findings were normal.
For United Dystrophinopathy Project and Muscular Dystrophy Association clinic patients, images from 2D and Doppler ultrasound studies were evaluated by standard techniques. Measurements included LV diameter, shortening fraction (LV diastolic diameter minus LV systolic diameter divided by LV diastolic diameter), and ejection fraction (Simpsons’ formula applied to planimeterized diastolic and systolic LV cavity images derived from the apex view). For published cases, deviations from this methodology can be found in the original articles (supplemental Tables II and III).
Patients were categorized into 3 groups based on the affected functional domain of the dystrophin protein. Group 1: subjects with deletions affecting any portion of the actin-binding amino-terminal domain of dystrophin (exons 2 to 9). Hinge 3 (a specific protein sequence joining 2 segments of dystrophin that allows flexible movement accounting for intrinsic protein folding) has been implicated in skeletal muscle involvement6 and thus served to divide BMD subjects into 2 additional groups. Group 2: subjects with deletions preserving hinge 3 and affecting exons 45 to 49 (spectrin repeats 17 up to 19). Group 3: subjects with deletions affecting exon 50 or 51 or both, removing or disrupting hinge 3.
Dystrophin Protein Modeling
Rod-region spectrin repeats were modeled based on the published structure of repeats 15 and 16 of chicken brain α-spectrin (PDB 1U5P).14 The structure was manually extended by replication and RMS alignment of corresponding terminal residues, using PyMol (http://www.pymol.org/). The structure was briefly minimized using VMD/NAMD (http://www.ks.uiuc.edu/Research/namd/) to remove significant bad contacts. Hinge region and out-of-phase deletion mutation structures were constructed by manual deletion of structurally equivalent residues from the extended spectrin repeat and structural realignment of the resulting fragments. A brief minimization using VMD/NAMD was used to correct any significant misplacement of fragment ends.
Nonparametric Kruskal-Wallis test was performed for cross-group age comparisons (a priori P<0.05), followed by Mann–Whitney U test post hoc comparisons among groups (Bonferroni adjustment was used to achieve overall significance of P<0.05). Mann–Whitney U test was used to compare age of cardiomyopathy onset between in-phase and out-of-phase mutations in group 2 patients. For blood relatives, only 1 patient was randomly selected for inclusion in statistical analyses. Three sibling pairs were identified within cardiomyopathic patients: 224 and 225, 251 and 3, AH11 and MJ13 (supplemental Table II). Concordance in the age of cardiomyopathy onset was observed among siblings. All data were analyzed in SPSS version 15 (SPSS, Chicago).
Patient Selection and Description
A total of 118 BMD and XLDCM patients (supplemental Tables II and III) were enrolled. Table 1 shows the breakdown of patients based on diagnosis, source, and whether they were categorized as cardiomyopathic or noncardiomyopathic. Only subjects with cardiomyopathy (n=78) were required to test our hypothesis that the age of DCM manifestation is associated with deletion of specific dystrophin protein domains. However, we did analyze noncardiomyopathic patients for evidence of a cardioprotective effect of some deletion mutations. We found that the noncardiomyopathic BMD patients were significantly younger than cardiomyopathic BMD patients (P<0.001) and that their deletion mutations overlap with those of cardiomyopathic patients (supplemental Figure 1). This suggests that noncardiomyopathic patients were too young to manifest cardiac involvement and will require follow-up studies to further test the hypothesis under consideration.
Distribution of Mutations Relative to Dystrophin Protein Structure and Diagnosis
To determine whether all cardiomyopathic patients can be combined for maximum statistical power, we first tested whether the source of patient information (published versus United Dystrophinopathy Project) or the diagnosis (BMD versus XLDCM) influences the age of cardiomyopathy onset. No significant effect was found (P>0.9). Therefore, the United Dystrophinopathy Project patient population is comparable with published case reports with respect to age, and XLDCM patients did not differ from cardiomyopathic BMD patients in their median age of cardiac involvement.
Next, we mapped the location of deletion mutations of cardiomyopathic patients to determine whether BMD and XLDCM patients differ in the affected dystrophin protein domains. The deletion mutations found in these patients clustered around 2 dystrophin protein regions: the amino-terminal domain corresponding to exons 2 to 7, and a region in the rod domain centered around hinge 3, corresponding to exons 45 to 55 (Figure 1). This distribution is in agreement with previous reports on mutation hot spots for BMD patients.15,16 Deletions found in XLDCM patients overlapped or in some cases were identical to those reported for BMD patients. Thus, XLDCM and BMD patients do not segregate into separate groups based on deletion mutation site or age of DCM manifestations. Taken together, these results indicate that patients can be combined for statistical analyses regardless of diagnosis or source of information.
Description of Patient Groups
Group 1 (Figure 2A) includes 11 patients with deletions affecting exons 2 to 9 coding for the actin-binding amino-terminal domain of dystrophin. No information on dystrophin expression in the myocardium of these patients is available.
Group 2 represents the majority of patients (67%) and involves deletions affecting exons 45 to 49 (spectrin repeats 17 to 19) that preserve hinge 3 of the dystrophin protein (Figure 2A). This group comprises 56 cardiomyopathic patients and shows the broadest age range of all 3 groups, with most patients falling between 15 and 55 years. A single outlier (161; supplemental Table II) was diagnosed at the age of 70 years with an ejection fraction of 27% suggestive of advanced disease. Cardiac biopsy information was available for 8 patients from published case reports (supplemental Table II). Dystrophin staining could be detected in the myocardium but was often fainter than in control tissues and was discontinuous along the cardiomyocyte membrane.
Group 3 includes 11 patients with deletions between exons 45 and 55 that remove or disrupt hinge 3 (Figure 2A). Of note, none of the patients was younger than 30 years. Cardiac biopsies were available for 4 patients (supplemental Table II) and showed reduced levels of dystrophin expression with a discontinuous pattern along the cardiomyocyte membrane.
Association of Deletion Mutations With DCM
Group 1 patients had the earliest age of DCM manifestations (median: 23 years) followed by group 2 patients (median: 29.5 years; Figure 2B). Group 3 patients developed DCM later in the course of the disease (median: 43 years; Figure 2B). A significant difference was detected among the groups (P<0.001, Kruskal-Wallis test) and for all post hoc pairwise comparisons (P<0.016) except between groups 1 and 2 (P=0.03). Thus, BMD patients with deletions that lie within exons 45 to 55 resulting in a dystrophin protein lacking hinge 3 have a significantly later-onset DCM compared with patients with overlapping deletions that preserve hinge 3 or with mutations affecting the amino-terminal region of dystrophin. By contrast, patients with deletion mutations affecting exons 2 to 9 or exons 45 to 49 are at risk of developing DCM in their second and third decades of life, respectively.
Disruption of Spectrin Repeat Phasing Results in Early DCM
Since group 2 patients showed the widest age range, we further investigated whether a second factor could be responsible for this heterogeneity. Previous studies focusing on this region have suggested a potential association of deletions of exon 48 or 49 or both with a more severe cardiomyopathy.2,4 Subdividing group 2 patients based on the presence or absence of exon 48 or 49 or both did not distinguish 2 subpopulations with significantly different ages of DCM onset (P>0.2).
One mechanism by which genotype can influence the age of DCM manifestations is by causing protein structure rearrangements that are more or less compatible with the cellular functions of dystrophin. Previous evidence in mice has shown that the phasing of the dystrophin spectrin repeats affects function in skeletal muscle.17 Exons 45 to 49 code for spectrin repeats 17 (partial) to 19. Because exon boundaries do not correlate with the physical boundaries of individual spectrin repeats at the protein level, different combinations of exon deletions could affect spectrin repeat phasing. For each group 2 mutation, the amino acid sequence of dystrophin was analyzed to assess whether the deleted sequence would disrupt (out of phase) or preserve (in phase) the known spectrin repeat pattern18 (supplemental Figure II). Subdividing group 2 patients based on phasing pattern revealed that disruption of spectrin repeat phasing is associated with significantly earlier onset of DCM (26 versus 36 years, Figure 3A).
We further compared the age of DCM manifestations between all patient groups taking phasing into account (Table 2). No significant difference was detected in the age of DCM manifestations between groups 1 and 2 out-of-phase mutations. Thus, out-of-phase mutations in the rod domain and deletions in the actin-binding amino-terminal domain are both associated with early age cardiomyopathy. By contrast, group 2 in-phase mutations led to a significantly later age of DCM manifestations compared with group 1 patients but did not differ from group 3 patients (Table 2). These results indicate that the effect of deletion mutations on the phasing pattern of spectrin repeats 17 to 19 is a strong determinant of DCM age of onset. Based on available information from cardiac biopsies, both in-phase and out-of-phase mutations result in expression of a mutant dystrophin protein in cardiomyocytes (supplemental Table II)19–23 suggesting that the observed difference in age of DCM is not due to obvious differences in the level of cardiac dystrophin expression.
Effects of Out-of-Phase and In-Phase Deletions on Dystrophin Structure
To further investigate the mechanism by which phasing affects dystrophin function, we modeled the effects of deletion mutations on the rod domain of dystrophin for group 2 mutations. Each spectrin repeat is composed of a long α-helix 1 connected to a shorter α-helix 2 by a flexible linker sequence (supplemental Figure 2A). Figure 3B illustrates how the α-helices of the spectrin repeats interlock with each other forming a stable yet flexible rod-shaped structure. In-phase mutations remove 1 or more interlocking units and simply result in a shortening of the rod-domain (supplemental Figure 2B). By contrast, out-of-phase mutations join the helices 1 and 2 together, removing the intervening linker sequence (supplemental Figure 2C). This is predicted to result in potentially dramatic changes to the rod domain that would affect the overall dystrophin structure. Based on our modeling, most out-of-phase mutations are predicted to bend the rod domain and reverse the directionality of the entire carboxyl-terminal part of dystrophin (Figure 3C). One exception is deletion of exon 48, which is predicted to introduce a new hinge adjacent to hinge 3 (Figure 3D). Such major alterations to the overall protein structure are likely to affect the function of dystrophin.
An association of genotype with DCM has long been suspected but has not been clearly established.2–4,24 In this study, we demonstrated that genotype is a determinant of the age of DCM manifestations in BMD and XLDCM patients. The large sample size (78 patients) and stringent selection criteria including only patients with small deletions (11 exons) enabled us to capture informative dystrophin protein domains. This allowed patient grouping based on dystrophin mutations affecting a single rather than multiple adjacent functional protein domains, thus increasing the statistical power of the study. We also provided novel evidence of a strong association of cardiomyopathy with specific structural alterations of the dystrophin rod domain. Previous studies reported contradictory findings on the potential link of deletions including exon 48 or 49 or both with severity and occurrence of DCM.2–4,24 Analysis of our 56 patients in group 2 showed that DCM in this region is more sensitive to altered phasing of the spectrin repeats rather than absence or presence of any individual exon. Our analysis of the effects of specific deletions on the 3D structure of dystrophin and their correlation with cardiac phenotype highlights the importance of integrating protein structure information in genotype-phenotype studies.
Our results also indicate that the rod domain of dystrophin may not be as permissive to alterations as previously thought. This study suggests that preservation of phasing delays the onset of DCM by about a decade. This is likely not due to a difference in expression of cardiac dystrophin protein because Arbustini et al20 reported similar amount and distribution of dystrophin in cardiac biopsy samples from BMD patients with either in-phase or out-of-phase mutations. Rather, our modeling suggests that the alterations caused by out-of-phase mutations extend beyond the spectrin repeat unit and may lead to a severely altered configuration of the rod domain, ultimately affecting the entire dystrophin protein. This major structural change is likely a main determinant of early-onset DCM. Interestingly, most group 2 BMD patients have late-onset skeletal muscle symptoms and mild disease progression, irrespective of the effects of their mutation on phasing. This is in agreement with studies in dystrophin-null mdx mice expressing a minidystrophin construct that lacks the exons 45 to 49 region but has an intact hinge 3 domain. In these mice, only a partial restoration of cardiac function was achieved despite a complete rescue of the skeletal muscle pathology.25 Thus, cardiac dystrophin may be particularly sensitive to structural disruptions of the exons 45 to 49 region compared with skeletal muscle dystrophin. The reasons for this disparity are currently unknown but highlight the importance of mapping domains of dystrophin essential for cardiac function to improve on current treatment approaches relying on exon skipping or gene replacement with mini-/microdystrophin constructs.
This study provides expected median ages of onset of DCM associated with 3 distinct regions of the dystrophin protein and with specific rearrangements of its rod domain. The deletion mutations studied here are among the most frequent, rendering our findings relevant to most BMD and XLDCM patients. Of interest, within groups, XLDCM patients did not have an earlier age of cardiomyopathy compared with BMD patients. Instead, the earliest age of DCM is associated with mutations affecting the amino-terminal domain of the protein (early 20s), and out-of-phase mutations in the exons 45 to 49 region of the dystrophin rod domain (mid-20s). Although cardiac expression of dystrophin has been confirmed in several out-of-phase group 2 patients, such information is not available for group 1 patients. The best studied mutations affecting the 5′ region of the dystrophin gene (including the muscle promoter, exon 1, or intronic regions that alter exon splicing26–28) lead to a selective lack of cardiac dystrophin. Although none of our group 1 patients had mutations affecting noncoding regions or exon 1, we cannot exclude the possibility that the early DCM onset in group 1 patients reflects a selective absence of cardiac dystrophin. Of note, the median age of XLDCM patients with lack of cardiac dystrophin (24 years for 6 independent families, supplemental Table I) is very similar to that of group 1 patients (23 years). Further studies are needed to determine the mechanism(s) by which deletion mutations in the amino-terminal region lead to earlier onset of cardiomyopathy compared with mutations affecting other regions. A greater awareness of the value of cardiac tissue sampling at the time of cardiac transplantation and the design of transgenic mdx mice mimicking human mutations could yield important information on cardiac-specific mechanisms regulating this region of dystrophin at a transcriptional and protein level.
Significantly later DCM onset is associated with group 2 in-phase (mid-1930s) and group 3 mutations (mid-1940s). The cardio-protective effect of hinge 3 deletion seen in group 3 patients mirrors findings reported for skeletal muscle in both mice and humans.6,17 However, although loss of hinge 3 delays onset of cardiomyopathy, it correlated with slower disease progression in skeletal muscle but had no effect on age of onset.6 Because of the small number of group 3 patients with identical deletions, we could not determine whether this partially protective effect is associated with a specific structural alteration of the dystrophin protein backbone. Further studies are needed to explain the significant cardio-protective effect conferred by the loss of hinge 3.
The median age of cardiac involvement for each patient group reported here is currently the best approximation available for this patient population. This information is valuable because cardiac involvement in BMD patients is often asymptomatic in its initial stages and can therefore be underestimated. Because genotyping has become a more common practice, the median ages reported here may prove valuable for individualized risk assessment and for timely cardiac evaluation and intervention. An important next step is to conduct a large-scale longitudinal study to further refine the age of DCM onset associated with the dystrophin domains identified here. This information underscores the importance of genotype information in the cardiac care of BMD patients and bears relevance to the design of therapies aimed at the myocardium in BMD, XLDCM, and DMD patients.
We acknowledge the input of the United Dystrophinopathy Project Consortium including the following individuals: Brenda Wong at Cincinnati Children’s Hospital Medical Center, Richard Finkel, Carsten Bonnemann, and Livje Medne at Children’s Hospital of Philadelphia, Julaine Florence and Anne Connolly, Washington University, Katherine Mathews, University of Iowa, Jacinda Sampson, Mark Bromberg, and Kathryn J. Swoboda, University of Utah, and John W. Day, University of Minnesota. We thank Dr Xiomara Rosales for her diagrams of the alignment of dystrophin exons with protein domains and Brent Yetter for assistance in the identification of patients seen at Muscular Dystrophy Association clinics who were suitable for this study. We also thank Drs Carlos Miranda, Jennifer Thomas-Ahner, and Christopher Pierson for editing assistance and for mentorship and support to R.W.K. from Dr Donna McCarthy, Professor of Nursing at The Ohio State University. We are indebted to Dr Christopher Holloman from the College of Mathematical and Physical Sciences at The Ohio State University for assistance with statistical analysis.
Sources of Funding
Supported by a grant from the NIH Roadmap Training Program in Clinical Research (T32-RR023260-03, to R.W.K.). The United Dystrophinopathy Project is supported by grants from the National Institute of Neurological Diseases and Stroke (R01 NS043264) and the National Center for Research Resources (M01-RR00064, to the University of Utah, Dr L. Betz, P.I.).
Melacini P, Fanin M, Danieli GA, Villanova C, Martinello F, Miorin M, Freda MP, Miorelli M, Mostacciuolo ML, Fasoli G, Angelini C, Dalla Volta S. Myocardial involvement is very frequent among patients affected with subclinical Becker’s muscular dystrophy. Circulation. 1996; 94: 3168–3175.
Jefferies JL, Eidem BW, Belmont JW, Craigen WJ, Ware SM, Fernbach SD, Neish SR, Smith EOB, Towbin JA. Genetic predictors and remodeling of dilated cardiomyopathy in muscular dystrophy. Circulation. 2005; 112: 2799–2804.
Comi GP, Prelle A, Bresolin N, Moggio M, Bardoni A, Gallanti A, Vita G, Toscano A, Ferro MT, Bordoni A, Fortunato F, Ciscato P, Felisari G, Tedeschi S, Castelli E, Garghentino R, Turconi A, Fraschini P, Marchi E, Negretto GG, Adobbati L, Meola G, Tonin P, Papadimitriou D, Scarlato G. Clinical variability in Becker muscular dystrophy. Genetic, biochemical and immunohistochemical correlates. Brain. 1994; 117 (pt 1): 1–14.
Nakamura A, Yoshida K, Fukushima K, Ueda H, Urasawa N, Koyama J, Yazaki Y, Yazaki M, Sakai T, Haruta S, Takeda S, Ikeda S. Follow-up of three patients with a large in-frame deletion of exons 45–55 in the Duchenne muscular dystrophy (DMD) gene. J Clin Neurosci. 2008; 15: 757–763.
Worton RG, Molnar MJ, Brais B, Karpati G. The muscular dystrophies. In: Scriver CR, Beaudet A, Sly WS, Valle D, eds. The Metabolic and Molecular Bases of Inherited Disease: New York: McGraw Hill; 2001: 5493–5523.
Politano L, Passamano L, Petretta VR, Nigro V, Papparella S, Nigro G, Santangelo L, Esposito MG, Come LI, Nigro G. Familial dilated cardiomyopathy associated with the typical dystrophin BMD mutation: report on two additional cases. Acta Myol. 1999; 3: 3329–3336.
Muntoni F, Di Lenarda A, Porcu M, Sinagra G, Mateddu A, Marrosu G, Ferlini A, Cau M, Milasin J, Melis MA, Marrosu MG, Cianchetti C, Sanna A, Falaschi A, Camerini F, Giacca M, Mestroni L. Dystrophin gene abnormalities in two patients with idiopathic dilated cardiomyopathy. Heart. 1997; 78: 608–612.
Politano L, Colonna-Romano S, Esposito MG, Nigro V, Comi LI, Passamano L, Nigro G. Genotype-phenotype correlation in patients with deletions of Duchenne/Becker gene. Acta Cardiomyologica. 1991; 3: 239–244.
Milasin J, Muntoni F, Severini GM, Bartoloni L, Vatta M, Krajinovic M, Mateddu A, Angelini C, Camerini F, Falaschi A, Mestroni L, Giacca M. A point mutation in the 5′ splice site of the dystrophin gene first intron responsible for X-linked dilated cardiomyopathy. Hum Mol Genet. 1996; 5: 73–79.
Exon deletions of the dystrophin gene lead to Becker muscular dystrophy and X-linked dilated cardiomyopathy. Both conditions are associated with cardiomyopathy with variable onset between the second and sixth decade of life. Better understanding of the predictive pathogenic factors influencing time of onset and severity of cardiac involvement would enable clinicians to begin early intervention, and potentially prevent premature death. In this study, insight into the evolution of cardiomyopathy was gained from analyzing a large patient population with the most prevalent exon deletions affecting discrete dystrophin protein domains. Four patient groups emerged from our study. Their expected ages of cardiomyopathy onset seem to be associated with the location of the exon deletion mutation and the effects on dystrophin protein structure. The complexity of our findings illustrates that dystrophin exon deletions must be correlated with protein structural alterations to predict outcomes. Prospective testing of these relationships potentially will empower clinicians to use genotype information to intervene more effectively in the treatment of patients with Becker muscular dystrophy or X-linked dilated cardiomyopathy. In addition, the findings pave the way for improvements on current therapeutic approaches targeting the heart in dystrophinopathies and may be valuable for patient stratification in clinical trials.
The online-only Data Supplement is available at http://circgenetics.ahajournals.org/cgi/content/full/CIRCGENETICS.109.867242.