Abstract
Aims
Polyglucosan body myopathy 1 (PGBM1) is a type of glycogen storage disease where polyglucosan accumulation leads to cardiomyopathy and skeletal muscle myopathy. Variants of RBCK1 is related with PGBM1. We present a newly discovered pathogenic RBCK1 variant resulting in dilated cardiomyopathy (DCM) and a comprehensive literature review.
Methods and results
Whole‐exome sequencing (WES) was utilized to detect genetic variations in a 7‐year‐old girl considered the proband. Sanger sequencing was performed to validate the variant in the patient and all the available family members, whether affected or unaffected. The variant's pathogenicity was assessed by conducting a cosegregation analysis within the family with in silico predictive software. WES showed that the proband's RBCK1 gene contained a missense likely pathogenic homozygous nucleotide variant, c.598_599insT: p.His200LeufsTer14 (NM_001323956.1), in exon 8. The computational analysis supported the variant's pathogenicity. The variant was identified in a heterozygous form among all the healthy members of the family. Variants with changes in N‐terminal part of the protein were more likely to manifest immunodeficiency and auto‐inflammation than those with C‐terminal protein modifications according to prior variations of RBCK1 reported in the literature.
Conclusions
Our study offers novel findings indicating an RBCK1 variant in individuals of Iranian ancestry presenting with DCM leading to heart transplantation and myopathy without immunodeficiency or auto‐inflammation.
Keywords: RBCK1, DCM, PGBM1, PBD, Genetic, Variant
Introduction
Polyglucosan body disease (PBD) is a type of glycogen storage disease characterized by abnormal polyglucosan accumulation. 1 The pathological accumulation of polyglucosan bodies leads to vacuolar degeneration and PBD. 2 , 3 Polyglucosan body myopathy 1 (PGBM1) is a PBD characterized by skeletal muscle myopathy and cardiac muscle involvement, mostly cardiomyopathy. Cardiomyocytes and skeletal muscles are predominantly affected in PGBM1 because of the high physiological glycogen turnover. 2 , 3 The cardiac involvement of patients with RBCK1 mutations is principally dilated cardiomyopathy (DCM). 4 , 5 A recent study reported cerebral white matter changes and cognitive impairment in a patient with PGBM1. 1 PBD is primarily caused by a mutation in one of the following eight human genes: GYG1, GBE1, RBCK1, PFKM, EPM2A, EPM2B (NHLRC1), PRDM8, and PRKAG2. 1 The inheritance pattern of PGBM1 is autosomal recessive and is caused by homozygous or compound heterozygous mutations. The RBCK1 gene is located on chromosome 20p13 and encodes haem‐oxidized iron‐regulatory protein 2 ubiquitin ligase 1 (HOIL‐1). 1 , 2 , 3 , 6 , 7 HOIL‐1 is part of E3 ubiquitin ligase and plays a crucial role in myogenesis. 2 , 3 , 6 Moreover, RBCK1 interacts with EYA1, involved in myogenesis, and its mutations result in improper myogenesis, usually presenting with progressive muscular weakness and DCM. 8
RBCK1 deficiency has been reported to lead to glycogen accumulation and subsequent formation of polyglucose bodies (PBs) 9 , 10 due to its effect on glycogen architecture pseudobranch‐specific glycogen. 11 Very low branched are atypical structures of proteins and amylopectin‐like polysaccharides. RBCK1 deficiency leads to accumulation of peroxisomes in neural tissues, including the brain and spinal cord. This accumulation subsequently leads to neurodegeneration and neurobehavioural disorders. 12 The absence of RBCK1 is associated with glycogen exhibiting longer chains and hyperphosphorylation, comparable with the symptoms seen in Lafora disease (LD). 13 RBCK1 is part of a system regulating glycogen regulation, including its presence in specific neural regions. Absence of RBCK1 disrupts this regulatory mechanism, leading to glycogen accumulation. 14 Localization of important enzymes involved in glycogen metabolism in perinuclear regions, known as perinuclear bodies, may potentially provide the mechanism of glycogen depletion in muscle fibres and later muscle after loss of performance. 15 The presence of non‐multiple protein quality control mechanisms of those commonly associated with protein aggregation disorders suggests that protein aggregation plays an important role in the pathobiology of polyglucosan storage. 16
Here, we present a review of all reported pathogenic RBCK1 variants and a novel likely pathogenic homozygous RBCK1 variant that gives rise to DCM and muscle weakness and ultimately necessitates heart transplantation.
Methods and materials
Subject
An Iranian girl was referred to Rajaie Cardiovascular Medical and Research Center, Tehran, Iran, for genetic evaluation. Her parents were healthy and first cousins. She also had a healthy sister (Figure 1 ). The patient was diagnosed with DCM based on cardiac magnetic resonance imaging (CMR) and underwent heart transplantation at the age of 7. Two years after heart transplantation, she developed progressive bilateral proximal muscle weakness in the lower limbs. Consequently, she underwent electromyography and the nerve conduction velocity test, whose results were consistent with myopathy.
Figure 1.
The image presents the pedigree of an index family with hereditary dilated cardiomyopathy. The male and female subjects affected are represented with a square and a circle filled in black, respectively. The symbols of squares and circles with diagonal lines are utilized to represent deceased males and females, correspondingly. The proband is indicated in the pedigree with a bold black arrow. The pathogenic nucleotide variation, c.598_599insT, in RBCK1, is shown with a thick pink arrow. The individual designated as the proband (III‐1) exhibited homozygosity for the aforementioned variant. The genotypes of c.598_599insT(p. His200LeufsTer14) were in the heterozygous form (CT) among the parents of the proband (II‐4 and II‐5), while the unaffected member of the family (III‐2) exhibited the wild type.
Cardiac magnetic resonance imaging
CMR (1.5 T MAGNETOM Sola, Siemens Healthcare, Erlangen, Germany) was performed. Breath‐hold steady‐state free‐precession cine imaging was carried out to measure the left ventricular myocardial function and mass in the four‐, two‐, and three‐chamber (long‐axis) and short‐axis stack views. The peak velocity of the right ventricular outflow tract was also acquired. For the assessment of oedema and inflammation, short‐tau inversion recovery sequences (breath‐hold) were taken in the long‐axis (four‐, two‐, and three‐chamber) and short‐axis views. Next, 0.15 mmol/kg of gadoterate meglumine (gadolinium‐DOTA, DOTAREM, Guerbet S.A., Paris, France) was administered to assess myocardial fibrosis. Early and late gadolinium sequences in the magnitude and phase‐sensitive inversion recovery reconstructions were taken in the short‐axis stack and four‐, two‐, and three‐chamber views.
Pathology
For cardiac pathology evaluation, the explanted heart was fixed in formaldehyde. After a gross examination, transverse slides of the ventricles, including the right and left free ventricular walls and septum and sections from the coronary arteries and valves, were prepared. The sections were evaluated by light microscopy and haematoxylin–eosin and trichrome staining.
Genetic analysis
Whole‐exome sequencing
After the patient's parents provided informed consent and on the treating physician's order, a peripheral blood sample was obtained from the patient. Genomic DNA was extracted from blood leucocytes via an in‐house salting‐out method. 17 The quality of the DNA was evaluated by NanoDrop (Thermo Fisher Scientific, USA). Then, the DNA was sent to Macrogen (Amsterdam, The Netherlands) for whole‐exome sequencing (WES) in a wet lab. The raw data were analysed at the Cardiogenetic Research Center, Rajaie Cardiovascular Medical and Research Center, Tehran, Iran. The reads were aligned with the human reference genome (hg19, GRCH37) by using the Burrows–Wheeler Aligner (BWA). 18 The unified GenoTyper module of the Genome Analysis Toolkit (GATK) was used to identify single‐nucleotide polymorphisms, deletions, and insertions. 19 According to their location (splice, intronic, and exonic), the variants were annotated and filtered by utilizing ANNOVAR. 20 Variants with a minor allele frequency of <5% in the Genome Aggregation Database (gnomAD), the Exome Aggregation Consortium (ExAC), and the 1000 Genomes Project were selected. For the in silico analysis, Combined Annotation‐Dependent Depletion (CADD), 21 Sorting Intolerant From Tolerant (SIFT), 22 PROVEAN, 23 and MutationTaster 24 were drawn upon to check the pathogenicity of the variants.
Segregation analysis
The pedigree and blood samples of the patient's family members were segregated and validated. First, DNA was extracted from peripheral blood samples via an in‐house salting‐out method. Next, the genomic candidate region was primer designed with the Primer3 server, Version 0.4.0, generating a pair of forward and reverse primers. Subsequently, polymerase chain reaction (PCR) amplification was carried out using the SimpliAmp Thermal Cycler (Thermo Fisher Scientific) following the protocol outlined in Supporting Information, Table S1 . The PCR products were subjected to Sanger sequencing with the ABI Sequencer 3500XL PE (Applied Biosystems), and the data were analysed and verified with the Codon Code Aligner, Version 7.1.2.
Search strategy
RBCK1 mutation was searched in PubMed and Google Scholar. The location, zygosity, predicted pathogenicity, nucleotide changes, and amino acid changes of the variants were investigated by inserting them into the Single‐Nucleotide Polymorphism Database (dbSNP) (Table 1 ). Concerning pathogenicity, the variants were checked with prediction tools, such as the American College of Medical Genetics and Genomics (ACMG), 25 MutationTaster, ClinVar, and SIFT. In addition, the CADD was calculated.
Table 1.
The pathogenic and likely pathogenic variants in RBCK1 (NM_031229.4)
| No. | Nucleotide change | Protein change | Zygosity | Position | Transcript | Exon | dbSNP | CADD | ClinVar | MutationTaster | ACMG | Ref |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | c.52G>C | p.Ala18Pro | Homozygote | 390554 | Missense | 2 | NA | 26.3 | Uncertain | Disease causing | VUS | 1 |
| 2 | c.121_122delCT | p.Leu41Glufs*7 | Homozygote | 390623 | Frameshift | 2 | rs727503763 | 27.7 | Pathogenic | Disease causing | Pathogenic | 2 |
| 3 | c.456+1G>C | NA | Homozygote | 400113 | Noncoding | 4 | rs1017046170 | 28.6 | Pathogenic | Uncertain | Pathogenic | 3 |
| 4 | c.494delG | p.Gly166AlafsTer110 | Homozygote | 400024 | Frameshift | 5 | NA | 25 | NA | Disease causing | Likely pathogenic | 1 |
| 5 | c.553C>T | p.Gln185Ter | Heterozygote | 400083 | Nonsense | 5 | rs727503762 | 35 | Pathogenic | Disease causing | Pathogenic | 2 |
| 6 | c.664G>T | p.Glu222Ter | NA | 400283 | Nonsense | 6 | NA | 43 | NA | Disease causing | Likely pathogenic | 3 |
| 7 | c.691delC | p.Gln231Serfs*45 | Homozygote | 400310 | Frameshift | 6 | NA | 28.2 | Pathogenic | Disease causing | Pathogenic | 4 |
| 8 | c.722delC | p.Arg241Glyfs*34 | Homozygote | 400341 | Frameshift | 6 | NA | 30 | NA | Disease causing | Likely pathogenic | 2 |
| 9 | c.727G>T | p.Glu243Ter | Heterozygote | 400346 | Nonsense | 6 | rs727503765 | 39 | Pathogenic | Polymorphism | Pathogenic | 1 |
| 10 | c.727_728insGGCG | p.Glu243Glyfs*58 | Heterozygote | 400339 | Frameshift | 6 | rs730880329 | 32 | Pathogenic | Disease causing | Pathogenic | 1 |
| 11 | c.756G>A | p.Gln252Gln | Heterozygote | 400375 | Synonymous | 7 | rs751274254 | 25.3 | NA | Polymorphism | VUS | 5 |
| 13 | c.799C>T | p.Gln267Ter | Heterozygote | 401557 | Nonsense | 7 | NA | 43 | Pathogenic | Disease causing | Pathogenic | 4 |
| 14 | c.817dupC | p.Leu273Profs*27 | Heterozygote | 401576 | Frameshift | 7 | NA | 33 | Pathogenic | Disease causing | Pathogenic | 4 |
| 15 | c.917+3_917+4insG | NA | Homozygote | 401678 | Noncoding | 7 | NA | 29 | NA | Disease causing | VUS | 1 |
| 16 | c.896_899delAGTG | p.Glu299Valfs*46 | Homozygote | 401651 | Frameshift | 7 | rs727503764 | 34 | Pathogenic | Disease causing | Pathogenic | 1 |
| 17 | c.913T>C | p.Cys305Arg | Homozygote | 401671 | Missense | 7 | rs1288748870 | 32 | NA | Disease causing | VUS | 6 |
| 18 | c.969C>A | p.Cys323Ter | Heterozygote | 402822 | Nonsense | 8 | NA | 40 | NA | Polymorphism | Likely pathogenic | 5 |
| 19 | c.1054C>T | p.Arg352Ter | Homozygote | 407981 | Nonsense | 9 | rs780854072 | 40 | Pathogenic | Disease causing | Pathogenic | 1 |
| 20 | c.1160A>G | p.Asn387Ser | Heterozygote | 408087 | Missense | 9 | rs566912235 | 32 | Likely pathogenic | Benign | Likely pathogenic | 1 |
| 21 | c.1411G>A | p.Glu471Lys | Homozygote | 409697 | Missense | 11 | rs1342598444 | 32 | Conflicting | Polymorphism | Likely pathogenic | 7 |
| 22 | c.1465delA | p.Thr489Profs*9 | Heterozygote | 411006 | Frameshift | 12 | NA | 30 | Pathogenic | Disease causing | Likely pathogenic | 4 |
| 23 | c.1522_1526del | p.Asn508ProfsTer4 | Heterozygote | 411062 | Frameshift | 12 | NA | 26 | Pathogenic | Polymorphism | VUS | 4 |
ACMG, American College of Medical Genetics and Genomics; CADD, Combined Annotation‐Dependent Depletion; dbSNP, Single‐Nucleotide Polymorphism Database; VUS, variant of uncertain significance.
Results
Clinical characteristics of the patient
A 7‐year‐old girl presented with dyspnoea and fatigue. CMR revealed severe systolic dysfunction and global hypokinesia in both ventricles, with a left ventricular ejection fraction (LVEF) of 10% and a right ventricular ejection fraction (RVEF) of 15%. Additionally, the left and right ventricles and both atria were severely enlarged. The CMR findings led to a diagnosis of DCM, and she was considered a candidate for heart transplantation, which was successfully performed on the same admission. A microscopic evaluation of the explanted heart demonstrated irregular myocardial atrophy and hypertrophy, focal fibrosis, and myocytolysis in the absence of inflammation, consistent with DCM. All the patient's symptoms were resolved post‐operatively. Serum levels of anti‐ds DNA, antinuclear antibody (ANA), complete overview of anti‐neutrophil cytoplasmic antibody (C‐ANCA), peripheral anti‐neutrophil cytoplasmic antibody (P‐ANCA), immunoglobulin (Ig) M, IgG, and IgA were assessed to test for any subclinical immunodeficiency or auto‐immune disease, and all were found to be normal in this patient. However, after 2 years, she developed proximal muscle weakness. The nerve conduction velocity test was unremarkable, whereas the electromyography was consistent with myopathy. Thus, the patient was referred to the genetics department for WES.
Cardiac magnetic resonance imaging findings
CMR illustrated a severely enlarged left ventricle with an end‐diastolic volume indexed to the body surface area of 217 mL/m2, a severely reduced LVEF (18%), global severe hypokinesia, a severely enlarged right ventricle, an end‐diastolic volume indexed to the body surface area of 155 mL/m2, and a mildly reduced RVEF (15%). The mitral and tricuspid valves showed severe functional regurgitation with a mitral regurgitation fraction of 66% and a tricuspid regurgitation fraction of 42% (Supporting Information, Video S1 ). The short‐tau inversion recovery sequences demonstrated no inflammation or oedema. Late gadolinium enhancement images depicted subendocardial near‐transmural enhancement in the basal to mid‐lateral and inferior walls (Figure 2 A,B ). Given the CMR features of severe biventricular enlargement and dysfunction, the phenotype was compatible with DCM. Nonetheless, considering the presence of subendocardial near‐transmural fibrosis, the possibility of ischaemic myocardial injury was raised, and coronary anatomy evaluation was advised, which was normal. The CMR was repeated 6 months after heart transplantation, which demonstrated normal LVEF and RVEF.
Figure 2.
The image presents the clinical information of the patient. (A, B) The late enhancement images at the mid‐level short‐axis and four‐chamber views, respectively, show subendocardial near‐transmural fibrosis in the basal to mid‐lateral and inferior walls. The microscopic examination (H&E stain) shows (C) some enlarged hyperchromatic nuclei with anisonucleosis and (D) focal myocyte vacuolation.
Pathology findings
A macroscopic examination showed the enlargement of both ventricular cavities and a mild decrease in the free ventricular wall thickness. Histological features revealed irregular enlarged nuclei and myocyte vacuolation because of myofibrils loss and focal interstitial fibrosis. There was no inflammation (Figure 2 C,D ).
Genetic diagnosis
WES identified a likely pathogenic variant in the homozygous state, c.598_599insT: p.His200LeufsTer14, in exon 8 of the RBCK1 gene. PCR and Sanger sequencing were employed to confirm the heterozygous/homozygous pathogenic/normal states for this position in the available members of the pedigree (Figure 1 ). The segregation analysis findings indicated that PGBM1 was transmitted in an autosomal recessive manner in the depicted pedigree. The presence of the variant was ascertained to be heterozygous in the parents of the patient and homozygous in the patient. The CADD Phred quality score for this variant is 26.5. According to the guideline of the ACMG, the RBCK1 gene variant was identified as a likely pathogenic factor linked to PGBM1. The aforementioned variant was identified as pathogenic in the MutationTaster database, while its absence in the ClinVar database was noted. Additionally, this variant was not detected in the gnomAD exomes/genomes.
Literature review findings
The process of the search strategy and data extraction resulted in the acquisition of 26 variants documented concerning the RBCK1 gene. Table 1 presents the genetic characteristics of the aforementioned variants. The majority of these variants (13 out of 26) were situated in the Znf‐RBZ and RING domains (Figure 3 A ). The variants with the highest CADD score were identified as c.799C>T and c.664G>T (the Phred quality score: 43). Additionally, the variant identified in our study was also found within this region (Figure 3 B ). Table 2 presents the mapping of protein domain changes in the retracted variants based on their clinical presentations. Variants with N‐terminal protein changes were more likely to manifest immunodeficiency and auto‐inflammation than those with C‐terminal protein modifications (Figure 3 C ).
Figure 3.
(A) The image provides genetic information regarding the RBCK1 protein, specifically focusing on the location and domain areas of variants reported to cause protein changes. (B) The image presents the Combined Annotation‐Dependent Depletion (CADD) score of the variants. Homozygous variants are presented in bold text, and the red text indicates the only variant with unspecified zygosity. (C) The image shows the presence or absence of each of the three principal clinical manifestations of PGBM1 in different variants and the percentages of variants presenting each manifestation in protein domain areas.
Table 2.
Clinical manifestations of RBCK1 variants
| No. | Nucleotide change | Protein change | Zygosity | Cardiomyopathy | EF % | HT | Initial | Skeletal muscle | Facial weakness | Ptosis | Scoliosis | Contracture | Immunodeficiency | Auto‐inflammation | Cognitive impairment | Ref |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | c.52G>C | p.Ala18Pro | Homozygote | DCM | 15 | — | NR | Walks w/o aid | No | NR | NR | NR | No | No | No | 1 |
| 2 | c.121_122delCT | p.Leu41Glufs*7 | Homozygote | DCM | NR | NR | Infection | NR | NR | NR | NR | NR | Yes | IBD | No | 2 |
| 3 | c.494delG | p.Arg165Argfs*111 | Homozygote | DCM | 28 | 15 | Difficulty running | Walks w/o aid | No | Mild | No | Yes | No | No | No | 1 |
| 4 | c.553C>T | p.Gln185Ter | Heterozygote | DCM | NR | 6 | Fever | Myopathy | NR | NR | NR | NR | Yes | IBD | No | 2 |
| 5 | c.553C>T | p.Gln185Ter | Heterozygote | CM | NR | — | Infection | Myopathy | NR | NR | NR | NR | Yes | IBD | No | 2 |
| 6 | c.664G>T | p.Glu222Ter | NA | Progressive+ | NR | NR | Weakness | Weakness | NR | NR | NR | NR | No | NR | NR | 3 |
| 7 | c.691delC | p.Gln231Serfs*45 | Homozygote | CM | NR | 22 | Difficulty running | Wheelchair | NR | NR | NR | NR | Yes | No | No | 4 |
| 8 | c.691delC | p.Gln231Serfs*45 | Homozygote | CM | NR | 7 | Delayed motor development | Walks with aid | NR | NR | NR | NR | Yes | Arthritis, IBD | No | 4 |
| 9 | c.727G>T | p.Glu243Ter | Heterozygote | DCM | 35–40 | — | Leg weakness | Wheelchair | No | No | No | No | No | No | No | 1 |
| 10 | c.722delC | p.Arg241Glyfs*34 | Homozygote | DCM | 25 | — | Difficulty running | NR | Yes | No | No | No | No | No | No | 1 |
| 11 | c.727_728insGGCG | p.Glu243Glyfs*58 | Heterozygote | DCM | 23 | — | Difficulty running | Wheelchair | No | Mild | Yes | No | No | No | No | 1 |
| 12 | c.756G>A | p.Gln252Gln | Heterozygote | No | NR | — | Weakness | Myopathy | NR | NR | NR | NR | No | No | No | 3 |
| 13 | c.799C>T | p.Gln267Ter | Heterozygote | CM | NR | — | Abdominal pain | Activity limited by fatigue | NR | NR | NR | NR | No | No | No | 4 |
| 14 | c.817dupC | p.Leu273Profs*27 | Heterozygote | CM | NR | 17 | Leg weakness | Wheelchair | NR | NR | NR | NR | No | Generalized non‐specific synovial inflammation | No | 4 |
| 15 | c.917+3_917+4insG | p.Arg298Argfs*40 | Homozygote | No | NR | — | Leg weakness | Minimally restricted | Yes | No | Yes | No | No | No | No | 1 |
| 16 | c.896_899delAGTG | p.Glu299Valfs*18 | Homozygote | DCM | <20 | 14 | Leg weakness | Walks w/o aid | No | Mild | No | No | No | No | No | 1 |
| 17 | c.896_899delAGTG | p.Glu299Valfs*18 | Homozygote | DCM | 18 | 13 | Difficulty running | Walks w/o aid | No | No | No | No | No | No | No | 1 |
| 18 | c.913C>T | p.Cys305Arg | Homozygote | No | NR | — | Abdominal distension | Myopathy | NR | NR | NR | NR | No | No | No | 5 |
| 19 | c.969C>A | p.Cys323Ter | Homozygote | No | NR | — | Weakness | Myopathy | NR | NR | NR | NR | No | No | No | 6 |
| 20 | c.1054C>T | p.Arg352Ter | Homozygote | DCM | 15 | 20 | Leg weakness | NR | No | Mild | Yes | No | No | No | No | 1 |
| 21 | c.1160A>G | p.Asn387Ser | Heterozygote | No | NR | — | Leg weakness | Walks with aid | No | No | No | No | No | No | No | 1 |
| 22 | c.1411G>A | p.Glu471Lys | Homozygote | No | NR | — | Leg weakness |
Walks w/o aid Waddling gait |
No | No | No | No | No | No | Yes | 7 |
| 23 | c.1465delA | p.Thr489Profs*9 | Heterozygote | CM | NR | 17 | Leg weakness | Wheelchair | NR | NR | NR | NR | No | Generalized non‐specific synovial inflammation | No | 4 |
| 24 | c.456+1G>C | NA | Homozygote | Progressive+ | NR | NR | Weakness | Weakness | NR | NR | NR | NR | NR | NR | NR | 3 |
| 25 | C.1522_1526del | p.Asn508Profs*27 | Heterozygote | CM | NR | — | Abdominal pain | Limited by fatigue | NR | NR | NR | NR | No | No | No | 4 |
| 26 | c.896_899del | p.Glu299Valfs*46 | Homozygote | DCM | NR | — | Dyspnoea | Walks w/o aid | No | No | No | No | Yes | ANA mild increased | No | 8 |
| 27 | c.896_899del | p.Glu299Valfs*46 | Homozygote | DCM | NR | 17 | Dyspnoea | Wheelchair | Mild | No | No | No | Yes | Sweet's syndrome | No | 8 |
ANA, antinuclear antibody; CM, cardiomyopathy; DCM, dilated cardiomyopathy; EF, ejection fraction; HT, heart transplantation; IBD, inflammatory bowel disease; NA, not available; NR, not reported; w/o, without.
Discussion
PBD is classified as a glycogen storage disease and is primarily attributed to genetic mutations in eight distinct human genes, namely, GYG1, GBE1, RBCK1, PFKM, EPM2A, EPM2B (NHLRC1), PRDM8, and PRKAG2. 5 A correlation exists between genotype and phenotype, whereby mutations located in the N‐terminal region of RBCK1 may increase the likelihood of immunodeficiency in affected patients, while mutations in the middle or C‐terminal region may lead to the development of myopathic symptoms. 26 Mutations in RBCK1 cause PGBM1, a type of glycogen storage disease. Based on available evidence, the hallmark of RBCK1 mutations is skeletal muscle involvement, present in all reported patients. Proximal lower limb muscles are predominantly involved, with the clinical manifestations varying among patients, including cardiomyopathy, systemic auto‐inflammation, immunodeficiency, hepatomegaly, and cognitive impairment. 1 , 2 , 3 , 6 , 8 , 26 , 27 , 28
In the current study, we presented a new missense variant of RBCK1, c.598_599insT: p.His200LeufsTer14, in a 7‐year‐old girl manifesting with DCM leading to heart transplantation and proximal skeletal muscle weakness. Mutations that impact transcript size, such as deletions, insertions, and nonsense mutations, are more prevalent in this particular gene and exhibit a more pronounced phenotype than missense mutations. A prior investigation identified truncating mutations in six DCM patients who were either homozygous or compound heterozygous; further, the results indicated a significant deletion in 31 799 kb, encompassing the terminal three exons of TRIB3 and the initial four exons of RBCK1, in three individuals exhibiting immune system dysfunction. 6 A previous study reported a comparable deletion in a patient and concluded that the relationship between immune dysfunction and the large deletion in the context of severe cardiomyopathy and a potential, albeit unverified, immune disorder needed clarification. 8 In our proband, although the mutation led to truncation, she manifested myopathy and severe cardiomyopathy but no immunodeficiency or auto‐inflammation. The need for heart transplantation was evident in patients with mutation from N‐terminal to C‐terminal, and no pattern or prediction of the need for heart transplantation was evidenced within reported RBCK1 variants. However, all patients reported to need heart transplantation when younger than 10 years old, including our patient, had a variant located in the middle part. In our opinion, genetic evaluation for detection of potential variants of RBCK1 in patients with PGBM1 not only helps in screening of the family members but also might give the clinician a better insight regarding the potential course of the disease.
The effect of truncated RBCK1 protein in DCM is manifested by polyglucosan formation in skeletal muscle and myocardium, leading to the progressive onset of muscle weakness and myocardial infarction. 29 And also, its deficiency is a rare inflammatory disorder characterized by amylopectin accumulation in skeletal muscle, causing myopathy and cardiomyopathy, and it is a valuable diagnostic indicator and therapeutic target. 8 , 30
The investigation of disease‐causing mutations has involved the examination of protein expression levels and truncation levels. The abstracts presented do not include the specific investigation of protein expression levels or protein truncation levels and their correlation with the phenotype in the context of DCM resulting from RBCK1 mutation. Nevertheless, scholarly articles have been published that examine the impact of missense mutations on the process of protein folding and stability. Additionally, researchers have explored the application of protein truncation tests for the identification of truncation‐type mutations. 31 , 32 These studies emphasize the significance of comprehending the molecular and genetic consequences of mutations in proteins associated with diseases. Additional investigation is warranted to explicitly examine the correlation between RBCK1 mutations, levels of protein expression, levels of protein truncation, and the phenotype observed in DCM.
The medical literature suggests a notable correlation between the location of the RBCK1 variant exon and the emergence of an immunodeficiency or myopathic phenotype. Boisson et al. concluded that individuals with pathogenic variants located in the N‐terminal region of RBCK1 experienced a severe form of premature immunodeficiency, ultimately leading to fatality during infancy. 6 Patients identified by Nilsson et al. and Wang et al. exhibited mutations in the middle or C‐terminal region of RBCK1 and manifested a neuromuscular phenotype. 8 , 28 Subsequently, it was proposed that the clinical manifestation of the disease might be influenced by the nature and distribution of the underlying pathogenic variant, wherein N‐terminal variants were predominantly found to cause immune dysfunction, whereas variants located in the middle or C‐terminal region were postulated to have a different effect. The primary manifestation of this condition is the development of cardiomyopathy and neuromuscular symptoms. 26 Krenn et al. suggested that the exact location of the mutations within the gene correlated with the clinical presentation of the disease and concluded that while N‐terminal mutations mostly presented with immunological dysfunction, middle and C‐terminal part mutations led to cardiomyopathy. 3 The updated mapping of clinical manifestations on protein domains in the present study was also consistent with that in prior studies, and N‐terminal mutations presented immunodeficiency and auto‐inflammation more often.
Conclusions
In the current study, we presented a novel RBCK1 variant with DCM, leading to heart transplantation and myopathy. The variant was located in the middle part of the protein, and similar to most previously reported variants, it presented with cardiomyopathy but not immunodeficiency or auto‐inflammation.
Conflict of interest
None declared.
Funding
None declared.
Supporting information
Table S1: The polymerase chain reaction protocol.
Video S1: The four‐chamber cine image shows severe biventricular enlargement and dysfunction with severe mitral and tricuspid regurgitation.
Acknowledgements
The authors wish to acknowledge the kind contribution of the family described herein.
MozafaryBazargany, M. , Esmaeili, S. , Hesami, M. , Houshmand, G. , Mahdavi, M. , Maleki, M. , and Kalayinia, S. (2024) A novel likely pathogenic homozygous RBCK1 variant in dilated cardiomyopathy with muscle weakness. ESC Heart Failure, 11: 1472–1482. 10.1002/ehf2.14702.
Data availability statement
The data sets generated and/or analysed during the current study are available in ClinVar (https://www.ncbi.nlm.nih.gov/clinvar/variation/2505540/). The accession number of the variant in ClinVar is as follows: RBCK1(NM_001323956.1):c.598_599insT (p.His200LeufsTer14): VCV002505540.1.
References
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Table S1: The polymerase chain reaction protocol.
Video S1: The four‐chamber cine image shows severe biventricular enlargement and dysfunction with severe mitral and tricuspid regurgitation.
Data Availability Statement
The data sets generated and/or analysed during the current study are available in ClinVar (https://www.ncbi.nlm.nih.gov/clinvar/variation/2505540/). The accession number of the variant in ClinVar is as follows: RBCK1(NM_001323956.1):c.598_599insT (p.His200LeufsTer14): VCV002505540.1.