The genetic basis for adult-onset idiopathic dilated cardiomyopathy in people of African descent

Cardiomyopathies are a heterogeneous group of cardiac muscle disorders that result in dilated, hypertrophic, or restrictive pathophysiological entities. Dilated cardiomyopathy (DCM) is the most common form in sub-Saharan Africa (SSA). However, population-specific research studies reporting the actual burden of DCM in this region are still lacking. Also, little is known about the genetic basis of DCM in this population, and genetic testing is still not readily accessible. This review describes the common pathogenic genes implicated in DCM globally and discusses the evidence-based management of patients with DCM. We also present a summary of studies describing genes implicated or associated with DCM in patients residing in SSA. Supplementary Information The online version contains supplementary material available at 10.1007/s10741-023-10302-9.


Introduction
Dilated cardiomyopathy (DCM) is Africa's second most common cause of heart failure, preceded only by hypertensive heart diseases [1]. Although DCM is prevalent in Africa, little is known about the genetic aetiology or mutations responsible for monogenic forms of this type of cardiomyopathy. In clinical practice, most patients are given a working diagnosis of idiopathic dilated cardiomyopathy (IDCM) without any formal genetic testing. This is partly because of limited access to genetic testing and the lack of cardiomyopathy gene panel tests based on data on genetic mutations found in African patients with DCM. Population-based data on the burden of IDCM in Africa are lacking. In a post-mortem analysis of 90 subjects with underlying cardiovascular disease in South Africa, 17% had idiopathic DCM [2]. In a prospective registry of heart failure cases in a tertiary-level hospital in Johannesburg involving 5328 patients, 9.4% had a primary diagnosis of IDCM [3]. Although DCM is common, patients in low-and middleincome countries still present with advanced heart failure symptoms at a younger age than those in high-income countries [4,5]. In this review, we discuss the mutational profile of DCM genetics and provide clinical guidance to clinicians managing DCM patients.

Definitions
Cardiomyopathies are a heterogeneous group of cardiac muscle disorders associated with cardiac dysfunction. Therefore, the terminology used to define dilated cardiomyopathies varies and may overlap (Table 1).

Clinical manifestation
The onset of heart failure in sub-Saharan African patients generally occurs at a younger age, thus affecting the economically active group [4]. In the early stages, some patients may remain asymptomatic, whilst in others, the nonspecific symptoms of heart failure may easily be misinterpreted as related to exhaustion or pulmonary diseases. Individuals with DCM may present with acute or gradual onset of heart failure symptoms. In some instances, individuals with DCM may also present with conduction abnormalities and sudden cardiac death.

Genetic determinants and mechanisms of genetic dilated cardiomyopathy
Globally, more than 50 genes that play a role in DCM have been identified, with some included in commercially available gene testing panels [9]. Of these genes, twelve have a definitive or strong relationship with DCM, seven had moderate evidence, and 25 had a limited role in DCM. In addition, seven have been disputed or assigned as having no known disease relationship due to the lack of evidence in human studies [9].
Most identified genes responsible for the DCM phenotype encode proteins within the myocytes. These are the sarcomere components, mitochondria, cytoskeleton, and desmosomal proteins. Genetic mutations implicated in DCM are predominantly composed of rare variants, making it challenging to diagnose a truly monogenic form of DCM [10]. A variant is considered pathogenic or likely pathogenic if there is a ≥ 90% likelihood of causing DCM [11]. The variable penetrance implies that although individuals may carry the disease-causing genotype, the DCM phenotype varies in clinical manifestation and severity. The DCM phenotype may only be evident in some candidates if the affected individual is further exposed to additional environmental or toxic insults such as alcohol. An international panel of clinical and scientific experts in DCM genetics has reviewed the available evidence on DCM genetics and identified twelve implicated genes from 8 gene ontologies as having a strong or definitive association with DCM [9]. Figure 1 depicts these "monogenic" genes classified according to their location in cardiac myocytes.

Mechanism of cardiomyopathy based on causative genes
In up to 50% of cases, DCM is inherited in an autosomal dominant pattern [12]. Titin (TTN) has been reported as the primary causative gene [12]. Titin encodes the largest protein in the heart and functions as a spring that ties myosin to the Z-band in sarcomeres [13]. The second common gene implicated in the pathogenesis of DCM is the LMNA gene, which encodes protein lamins A and C. Mutations in this gene account for 5-10% of DCM and are responsible for the disruption of the chromatin organisation in dividing cells and signal transduction in non-dividing cells. Furthermore, LMNA DCM is often associated with atrioventricular blocks, atrial fibrillation, and ventricular arrhythmias. Patients with LMNA gene mutations may require primary prevention therapy, including a cardiac defibrillator device implantation, due to a 46% risk of sudden cardiac death [14,15].

Genetic studies in the African context
We conducted a systematic literature search in PubMed, Scopus, and Web of Science to identify original clinical research studies conducted in Africa that report genes and variants associated with dilated cardiomyopathy. The search string was used "Dilated cardiomyopathy AND genetics AND Africa". We excluded studies reporting on genetic findings in peripartum cardiomyopathy. The Preferred Reporting Items For Systematic Reviews and Meta-Analyses (PRISMA) flow chart showing the selection of studies is available as a supplementary file. The literature search yielded twelve studies (Table 2), with very few studies (n = 2) on familial or monogenic cases of DCM in Africa. Eight studies (67%) were conducted in South Africa, and the rest in Tunisia (n = 2), Morocco (n = 1), and Egypt (n = 1).  A comprehensive approach to diagnosing dilated cardiomyopathy

History taking and clinical examination
The first step in diagnosing DCM entails obtaining a comprehensive clinical history, physical examination, laboratory investigations, and imaging. History taking should include a three-generation family history and a pedigree [10]. Figure 2 demonstrates an example of a four-generation pedigree of a proband with IDCM. Clinical details related to the cohort, including the proband depicted in the pedigree, are described elsewhere [28].
Clinicians should enquire about a family history of sudden cardiac death, unexplained deaths before age 50, heart transplantation, and pacemaker insertion before age 55. Furthermore, a history of death due to unnatural causes or drownings should be elicited. If a family member died due to unnatural causes, post-mortem reports should be reviewed to exclude or confirm primary or secondary causes of death [10]. History taking should be followed by a thorough clinical examination that includes a neurological examination focused on identifying neuromuscular diseases such as muscular dystrophy, which may manifest as muscle wasting, contractures of the elbows, spine, and Achilles tendons [29].

Electrocardiogram
Various electrocardiographic abnormalities may be found in patients with DCM, including a prolonged PR interval, evidence for ventricular hypertrophy, pathological Q waves, or bundle branch block [30][31][32]. In addition, poor prognostic factors on ECG include atrial fibrillation and a left bundle branch block [32].

Echocardiogram
An echocardiogram is mandatory to confirm ventricular dilatation and calculate the LVEF, which may guide the selection of appropriate heart failure therapy. In addition, features of cardiac remodelling, which include increased left atrial size, functional mitral insufficiency, alteration of diastolic function, and involvement of other chambers, should be elicited as these features are also associated with an unfavourable prognosis [32,33]. Speckle-tracking echocardiography may also help to identify asymptomatic patients with left ventricular systolic dysfunction before they manifest with an overt DCM phenotype [34].

Laboratory tests
Laboratory biochemical tests are an integral part in the clinical workup of patients suspected to have IDCM. These may identify DCM's endocrine, infectious, and haematological causes. Detecting micronutrient deficiencies such as selenium or thiamine may suggest malnutrition, which is strongly associated with alcohol misuse (Fig. 3). Abnormally high serial cardiac troponin and creatine kinase serum levels may indicate an acute myocyte injury due to myocarditis. Thyroid stimulating hormone and thyroxine levels should be assessed to exclude hypothyroidism or hyperthyroidism. Low calcium levels may indicate underlying chronic hypocalcaemia, while elevated iron levels may suggest underlying iron overload. When clinically suspected, bacterial and fungal infections should be excluded by performing blood cultures. Furthermore, a significantly elevated brain natriuretic peptide level has been reported to suggest a poor prognosis in patients with DCM [35][36][37].

Coronary angiography
Coronary angiography is currently the gold standard for evaluating coronary epicardial vessels' atherosclerotic disease. Therefore, all patients with DCM should ideally be referred for a diagnostic coronary angiogram to exclude coronary artery disease. Furthermore, this test is mandatory since the therapeutic approach varies in patients with ischaemic and non-ischaemic cardiomyopathy.

Cardiovascular magnetic resonance imaging
Magnetic resonance imaging is crucial for excluding infiltrative conditions such as sarcoidosis and amyloidosis. In addition, the visualisation of gadolinium enhancement on late images may indicate underlying fibrosis. Several research studies have reported an association between the visualisation and burden of late gadolinium enhancement (LGE) and all-cause mortality in DCM [38][39][40][41].

Endomyocardial biopsy
An endomyocardial biopsy provides a definite histological, immunohistochemistry, and molecular evaluation of myocardial tissue [11]. It is indicated in patients suspected to have acute myocarditis or chronic inflammatory cardiomyopathy. The evaluation of endomyocardial biopsy samples of DCM patients may show nonspecific histopathological signs such as hypertrophy and vacuolar changes of myocytes and fibrosis [42]. Although invasive, the risk of complications associated with an endocardial biopsy is low, with 11% of patients experiencing atrioventricular block [42].
Endomyocardial biopsies should be considered if the test will likely alter the therapeutic management of patients, mainly if conditions such as sarcoidosis, giant cell myocarditis, eosinophilic myocarditis, or hemochromatosis are considered differential diagnoses. Strategies that could improve access to myocardial biopsy, particularly in lowand middle-income countries (LMIC), include the implementation of referral pathway protocols that prioritise the performance of biopsies in these patients, ensuring that personnel are adequately trained, as well as increasing the number of available catheterisation laboratories.

Genetic testing
Pre-test genetic counselling should be provided to DCM patients and their families. A detailed family history should be taken, and the possible findings and implications of genetic results should be explored during the counselling session. Defining terminology such as "pathogenic mutations, variants of uncertain significance and benign genetic variants" should be explained [8,13]. Post-test counselling should focus on interpreting results, discussing reproductive risk, and the need for cascade family testing [13,28].
Cardiomyopathy gene testing is still not widely available in most LMICs. However, diagnostic testing should be performed in carefully phenotyped patients with evidence of disease. In contrast, predictive testing is recommended in asymptomatic individuals (usually family members of an individual with a known DCM mutation) to predict the future risk of disease [43]. Currently available genetic tests involve sequencing a single gene or an individual variant, cardiomyopathy gene panel sequencing, whole-exome sequencing, and whole-genome sequencing [44].
Sequencing a single gene or individual variant should be considered a confirmatory test in a family member with a proband carrying a pathogenic/likely pathogenic variant detected through other techniques. Although this approach is cost-effective, it is not appropriate for diagnostic testing in a proband from another family with DCM where the causal mutation has not yet been identified [44,45]. Cardiomyopathy gene panel sequencing involves sequencing the coding regions of several cardiomyopathy genes simultaneously in a single experiment. The diagnostic yield is higher. However, this test may not capture non-coding variants [13].
Whole-exome sequencing involves the analysis of a sequence of the entire coding region of the human genome. Limited non-coding regions may be included, but much of the non-coding DNA is not analysed. Whole-exome sequencing is not limited to genes previously linked with disease, thus enabling the potential to identify novel variants in new genes of interest. However, there is also a higher likelihood of identifying a variant of uncertain significance with whole-exome sequencing, and these usually need further investigation or careful explanation to patients as to their unknown implications [46]. In contrast, wholegenome sequencing captures both coding and non-coding variants, including deep intronic variants. Although more expensive, whole-genome sequencing will capture all variants and can be used to calculate polygenic risk scores for multifactorial causality of DCM and to report pharmacogenetic variants [47]. If a causal genetic mutation for monogenic DCM is identified, cascade screening of family members should be implemented, where a specific mutation rather than a gene panel is evaluated.
Genetic testing within the African context is challenging, partly due to the diverse genetic makeup of populations within the continent, making the possibility of identifying a variant of uncertain significance higher [48]. Furthermore, currently available DCM gene panels could be limiting in the African context as the diagnostic yield may be lower since African-specific DCM-causing genes (should they be present) have not yet been identified. In addition, the current gene panels are based on DCM patients who are primarily not of African descent. Figure 4 summarises an approach to genetic testing in a proband with DCM.

Differential diagnosis
Diagnosing DCM may be challenging in most LMICs and other remote regions without access to diagnostic modalities such as coronary angiography or cardiovascular MRI. In such scenarios, the diagnosis is generally made on clinical grounds. The higher prevalence of infectious diseases in sub-Saharan Africa makes viral myocarditis highly likely. As such, clinicians faced with patients presenting acutely with the clinical syndrome of heart failure should routinely evaluate and monitor serum troponins, creatine kinase-MB levels, and viral antibody titres to exclude viral myocarditis. The viral antibody titre tests may include coronavirus, enterovirus, HIV, cytomegalovirus, Epstein-Barr virus, adenovirus, human herpes virus 6, parvovirus B19, hepatitis, and influenza virus antibodies [49].
A genetic aetiology should be considered in females with idiopathic cardiomyopathy in the peripartum phase [50][51][52][53]. A discussion on the genetic basis of peripartum cardiomyopathy is beyond the scope of this manuscript, and the reader is referred to literature published elsewhere [54].

Management of patients with genetic causes of dilated cardiomyopathy
The pharmacological management of a proband with a genetic cause for DCM is not different from that of any individual with non-genetic causes of DCM. It is guided by the presence of heart failure symptoms, the baseline LVEF, and the presence or risk for potentially lethal arrhythmias. Pharmacological therapy for DCM is mainly based on neurohumoral blockade [55]. Diuretics and inotropes may be considered in the acute phase in patients with congestive heart failure complicated by cardiogenic shock. Agents such as beta-blockers, angiotensinconverting enzyme inhibitors (ACE-I) or angiotensin receptor blockers in ACE-I intolerant individuals, angiotensin receptor neprilysin inhibitors (ARNI), mineralocorticoid antagonists (MRA), and more recently the sodiumglucose cotransporter-2 (SGLT2) inhibitors are currently considered essential foundational therapies in the management of heart failure with reduced ejection fraction [37].
Mutations in the LMNA gene have been linked to atrioventricular block and atrial or ventricular arrhythmias [56,57]. To prevent lethal ventricular arrhythmias, implantable cardioverter defibrillators should be considered, particularly in patients who have survived a malignant ventricular tachyarrhythmia or those presenting with symptomatic ventricular tachycardia [15]. Unfortunately, device therapy is still not widely available in most LMICs due to its high cost.

Targeted therapies
Several strategies that address genetic abnormalities in patients with DCM are available. These methods include targeting specific single mutations, exon skipping, and gene replacement targeting all mutations at once by gene transfer of the full-length complementary DNA [55]. Also, newer molecules such as EMD 57,033 bind in the same region of myosin, increasing the rate of ATP binding, hydrolysis, and actin interactions. Furthermore, CK-1827452, a molecule that accelerates the transition of the actin-myosin complex from weakly to strong bound, is being investigated for potential clinical use [58]. Also, there is an ongoing randomised, double-blind placebocontrolled trial evaluating the efficacy of ARRY-371797, an inhibitor of the p38α MAPK pathway in symptomatic patients with DCM due to a mutation of the gene encoding the lamin A/C protein [59].

Future directions and recommendations
There is an urgent need to establish registries for IDCM/ DCM patients and their families in Africa to facilitate the investigation of a genetic basis for IDCM in patients of African ancestry [60]. This should be coupled with the creation of African genome banks to enable the ease of determining the pathogenicity of VUS against a suitable Females are represented with circles. Crossed-out circles and squares denote demised relatives Fig. 3 Causes of dilated cardiomyopathy that should be considered after excluding coronary artery disease reference population. Establishing African genomic consortia such as the Human Heredity and Health in Africa (H3Africa) Consortium promises to make these goals a reality [61].

Conclusion
IDCM remains a common disease in many African regions and continues to cause significant morbidity and mortality in young patients. Given that Africa has remarkable genetic diversity, a study of the genetic aetiology of this condition in an African setting is likely to yield novel insights and could be of clinical relevance in diagnosis and treatment. This highlights the need for improved access to IDCM genetic testing. Once widely implemented, the term "idiopathic dilated cardiomyopathy" should only be reserved for patients with a comprehensive clinical workup that includes genetic testing that fails to identify a plausible cause for DCM.
Acknowledgements The authors would like to thank Dr Claude Bailly, from the Division of Human Genetics, who assisted with constructing the pedigree included in this manuscript. All figures in this manuscript were created with Biorender software.

Fig. 4
An approach to genetic testing in a proband with dilated cardiomyopathy. Diagram modified from Tayal et al. [10]. DCM dilated cardiomyopathy, ECG electrocardiogram, ICD implantable cardio-verter defibrillator, LP likely pathogenic, LV left ventricular, MRI magnetic resonance imaging, P pathogenic, VUS variant of uncertain significance Author contribution NT was involved in the conceptualisation of the review paper, data collection, writing and review of the first draft of the manuscript, and funding acquisition. DM was involved in the data collection and review and editing of the manuscript. MR, AK, QW, and PM reviewed and edited the manuscript. All authors read and approved the final manuscript.
Funding Open access funding provided by University of the Witwatersrand. The Genetics of idiopathic dilated cardiomyopathy in Johannesburg Study is funded by the Carnegie Corporation of New York Grant No. B8749.RO1, Discovery Foundation Academic Fellowship Award, South African Heart Association Research Scholarship Award, and the South African Medical Association PhD Supplementary Scholarship. None of the funders played a role in the design of the study and collection, analysis, and interpretation of data and in writing the manuscript.
Availability of data and materials Not applicable.

Declarations
Ethical approval Ethical approval waived since the article is a narrative review that does not contain identifiable patient data. However, the data depicted in Fig. 2 was collected as part of "The Genetics of idiopathic dilated cardiomyopathy in Johannesburg Study". The study complied with the Declaration of Helsinki, and informed consent was obtained from the study participants. Approval to conduct the study was granted by the University of the Witwatersrand Human Research Ethics Committee (Ethics Clearance certificate number: M150467).
Competing interests NT is a cardiologist and has received consultation fees from Acino Health Care Group,AstraZeneca, Boston Scientific, Novartis Pharmaceuticals, Novo Nordisk, Pfizer, Sanofi,Phillips, Servier, Takeda, and Merck. He has also received educational and travel grants fromMedtronic, Biotronik, Boston Scientific, and Vertice Health Care Group. None of the otherauthors has any relevant financial or professional disclosures.
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