key: cord-0804453-ip2ny4a7 authors: Bruyndonckx, Luc; Vogelzang, Judith L.; Bugiani, Marianna; Straver, Bart; Kuipers, Irene M.; Onland, Wes; Nannenberg, Eline A.; Clur, Sally‐Ann; & van der Crabben, Saskia N. title: Childhood onset nexilin dilated cardiomyopathy: A heterozygous and a homozygous case date: 2021-05-05 journal: Am J Med Genet A DOI: 10.1002/ajmg.a.62231 sha: c91f3bf41c94f2e272dab62e68035a94d419d106 doc_id: 804453 cord_uid: ip2ny4a7 Pathogenic heterozygous NEXN variants are associated with progressive dilated cardiomyopathy (DCM) usually presenting around 50 years of age. We describe an asymptomatic boy who had transient DCM at 3 months of age, that resolved by 4 months. Presently, at 11 years of age, he has normal cardiac function with signs of mild DCM on cardiac MRI. Genetic diagnostics revealed a paternally derived, heterozygous 1949_1951del class 4 variant in NEXN. His father had mild DCM with mildly reduced systolic function. The second patient presented with fetal hydrops at 33 weeks gestation requiring emergency caesarian delivery. Postnatally she required ventilation and continuous inotropic support for left ventricle systolic dysfunction. She died after 2 weeks when therapy was withdrawn. Homozygous c.1174C > T,p.(R392*) class 4 variants in the NEXN gene were found via WES. Microscopic investigation showed endomyocardial fibroelastosis. Her parents, both heterozygous carriers, had normal cardiac function and the family history was normal. These patients show a new clinical spectrum of pediatric cardiac disease seen in heterozygous and homozygous NEXN variants, ranging from mild, transient DCM to a severe, fatal neonatal DCM. These patients support the inclusion of the NEXN gene in the investigation of pediatric patients with DCM, even in cases with transient DCM. Dilated cardiomyopathy (DCM) is the predominant pediatric cardiomyopathy with an estimated incidence of 0.57 per 10,000 children per year (Towbin et al., 2006) . There is a high risk for mortality and morbidity, such as requirement of circulatory support and heart transplantation (Arola et al., 1998) . There are numerous causes of DCM, but a familial form can be identified in 30%-35% of the patients (Grünig et al., 1998) . In recent years, immense advances have been made to unravel the variants responsible for genetic forms, which mostly encode for structural elements of the myocardium (Haas et al., 2015) . Abbreviations: DCM, dilated cardiomyopathy; EFE, endomyocardial fibroelastosis; LV, left ventricle. Luc Bruyndonckx and Judith L. Vogelzang are equally contributing first authors NEXN was recently discovered as a DCM-associated gene, encoding for nexilin. The first publication demonstrated that nexilin variants lead to Z-disk instability (Hassel et al., 2009) both in a zebrafish model as well as in human tissue from a myocardial biopsy. In addition, recent research in mouse models showed nexilin to be a component of the junctional membrane complexes essential for Calcium influx and T-tubule formation (Liu et al., 2019) . DCM due to NEXN variants is rare and mostly described in the heterozygous form in adults with an average age of onset of 50 years (Hassel et al., 2009) . We present two pediatric cases of nexilin DCM. The first is heterozygous for a NEXN variant. He had a transient DCM at 3 months and is currently asymptomatic at the age of 11 years although his left ventricle is mildly dilated on cardiac MRI. The second patient, homozygous for NEXN variants, presented with fetal hydrops and died 2 weeks postnatally. This boy, the child of healthy, unrelated parents, presented at 5 days of age at the pediatric cardiology outpatient clinic because of a systolic murmur. Echocardiography showed a structurally and functionally normal heart with normal diameters of the left ventricle (LV). However, a mild tricuspid regurgitation prompted a repeat ultrasound. At 3 months of age, he was in good clinical condition and had grown and developed normally. The echocardiogram now unexpectedly revealed a dilated LV with a left ventricular end-diastolic diameter of 34.8 mm (Z score + 5 [Pettersen et al., 2008] ), and a reduced shortening fraction of 19.8% (Figure 1 and video 1: online). Further investigations, including screening for infections, metabolic and neuromuscular diseases, a 12 lead and 24 h Holter electrocardiogram, and a coronary angiography, showed no abnormalities besides evidence for a recent Coronavirus infection. An angiotensin-converting-enzyme inhibitor and diuretics were started and the systolic function and left ventricular dimensions normalized during the following 4 months. All medication could be stopped after 7 months of treatment. The family history was negative for DCM. With the presumptive diagnosis of a recovering myocarditis, genetic testing was not performed at that stage. Further echocardiograms showed normal systolic function and left ventricular dimensions (z score + 1 to +1.5) (Pettersen et al., 2008) . A cardiac MRI performed at the age of 9 years, showed a mildly dilated left (end-diastolic volume 132 ml/m 2 ) and right (120 ml/ m 2 ) ventricles with normal systolic function and no obvious myocardial fibrosis. The MRI was repeated a year later and showed unchanged dilated ventricles with preserved systolic function. Presently, at the age of 11 years, he is asymptomatic and plays soccer at a high level. His most recent electrocardiogram shows a sinus rhythm and normal AV conduction, but abnormal repolarization in V2, V3, and V4. His last 24-h Holter electrocardiograms showed no arrhythmias. (Table 1) (Hassel et al., 2009) and was found 29 times in 279,530 reference alleles from gnomAD (0.01%) (online search in gnomAD Database, 2020) . However, during functional analyses, zebrafish embryos developed DCM after injection of this specific mRNA supporting a dominant negative effect on cardiac function (Hassel et al., 2009) . Cardiac screening in the patient's four siblings was normal and genetic screening has not been performed (yet) but they remain under surveillance. Cardiac examination of his asymptomatic father, carrier of the same variant, revealed a mildly dilated LV. A paternal uncle also has a dilated LV on cardiac evaluation, but he declined genetic testing. His paternal grandmother died of heart failure and his paternal grandfather has no known heart disease. F I G U R E 1 Echocardiograms from patient 1. In panel a the echocardiogram made at 5 days of age is shown. The heart was structurally and functionally normal heart with normal diameters. Panel b shows the severely dilated LV seen at 3 In case 1 the echocardiogram was completely normal at 5 days of age, but unexpectedly a dilated left ventricle was seen at 3 months of age. Pediatric cased described previously (Kean et al., 2019; Klauke et al., 2017 ) also presented at a young age (patients 4, 5, and 7 in Table 1 Table 1 ). In addition, heterozygous mice with knock-out of just one NEXN allele displayed a DCM phenotype with mild dilatation and mildly reduced systolic function up to 6 days postnatally (Aherrahrou et al., 2016) . However, after 3 months, no significant differences could be noted compared to the wild-type mice, similar to our patient. A transient infection in these mouse models is highly inactivating NEXN in zebrafish embryos (Hassel et al., 2009) , the embryos initially show normal morphogenesis of the heart, but 24 h after fertilization their hearts start to dilate and significant F I G U R E 3 Histopathology of the heart of patient 2 showed normally arranged cardiomyocytes without disarray and without cytoplasmic vacuolization (a). There was a pronounced trabecular aspect of the myocardium at the endocardial side (b and c). The blood vessels were normal. A single papillary muscle showed a small focus of dystrophic calcification. The endocardium was broadened. No interstitial or replacement fibrosis was present. No iron or glycogen accumulation was seen. The cardiomyocytes contained abundant glycogen especially subendocardially. systolic dysfunction develops leading to heart failure, similar to our second patient. The functional role of nexilin was further investigated in a NEXN knock-out mice model (Aherrahrou et al., 2016) . These knock-out mice have severely reduced survival, with most dying within 8 days after birth. At birth, the cardiac structure and function appear normal, but after 4 days they have progressive dilation of the LV and EFE develops. EFE was also seen in the microscopic evaluation of the heart of patient 2. EFE is rare in adults, but is seen in 25% of all pediatric DCM patients requiring heart transplantation (Seki et al., 2013) and is associated with a younger median age of transplantation, (10 vs. 142 months in non-EFE heart transplant patients). Without a cardiac transplant, children with EFE do not survive beyond 2 years (Seki et al., 2013) . Electron microscopy of this patient confirmed data of mouse models that nexilin does not affect Z disks, but unfortunately autolysis did not allow evaluation of the T-tubuli. Both the G650del as well as the R392* variant have also been described heterozygously in "asymptomatic" controls in the gnomAD database impeding a class 5 classification. Given the variable age of presentation of heterozygous carriers however, these patients may have still been in the presymptomatic phase of their illness. In summary, the presented patients show the clinical spectrum of pediatric cardiac disease seen in heterozygous and homozygous NEXN variants, ranging from mild, transient DCM to a severe, fatal neonatal DCM. These patients support the inclusion of the NEXN gene in the investigation of pediatric patients with DCM, even in cases with transient DCM. The authors declare no real or perceived conflicts of interest related to this work. Data sharing is not applicable to this article as no new data were created or analyzed in this study. 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