Reporting a rare form of myopathy, myopathy with extrapyramidal signs, in an Iranian family using next generation sequencing: a case report

MICU1's calcium sensing beyond mitochondrial calcium uptake

The discovery of MICU1 as gatekeeper of mitochondrial calcium (mCa2+) entry has transformed our understanding of mCa2+ flux. Recent studies revealed an additional role of MICU1 as a Ca2+ sensor at MICOS (mitochondrial contact site and cristae organizing system). MICU1's presence at MICOS suggests its involvement in coordinating Ca2+ signaling and mitochondrial ultrastructure. Besides its role in Ca2+ regulation, MICU1 influences cellular signaling pathways including transcription, epigenetic regulation, metabolism, and cell death, thereby affecting human health. Here, we summarize recent findings on MICU1's canonical and noncanonical functions, and its relevance to human health and diseases.

Dolichocephaly, Arachnodactyly, Diplopia, and Distal Myopathy - Novel Phenotype of MICU1 Variant c.553C>T

Pathogenic variants in mitochondrial calcium uptake 1 (MICU1) manifest phenotypically heterogeneously but most frequently in the brain and skeletal muscle. Dolichocephaly, arachnodactyly, diplopia, and distal myopathy have not been reported in carriers of a pathogenic MICU1 variant. The patient is a 23-year-old female with consanguineous parents (first cousins) who was a carrier of the homozygous MICU1 variant c.553C>T, phenotypically presenting with developmental delay, intellectual disability, ataxia, dysmorphia (dolichocephaly, arachnodactyly, clinodactyly, hypertelorism, wide nasal bridge), myopathy (ptosis, double vision, strabismus, distal limb weakness, diffuse wasting, hypotonia), hyperextensible joints and hyperkyphosis. Features not previously described were dolichocephaly, arachnodactyly, broad nasal bridge, double vision, and distal myopathy. She was treated with physical therapy, speech therapy, and occupational therapy and received escitalopram and mirtazapine for concomitant depression, anxiety disorder, and insomnia. The presented case shows that the phenotypic heterogeneity of pathogenic MICU1 variants is even greater than previously assumed. Treatment of MICU1-related phenotypes is symptomatic, but these patients benefit from physical therapy, behavioral therapy, speech therapy, and antidepressant treatment.

Preserving and enhancing mitochondrial function after stroke to protect and repair the neurovascular unit: novel opportunities for nanoparticle-based drug delivery

The neurovascular unit (NVU) is composed of vascular cells, glia, and neurons that form the basic component of the blood brain barrier. This intricate structure rapidly adjusts cerebral blood flow to match the metabolic needs of brain activity. However, the NVU is exquisitely sensitive to damage and displays limited repair after a stroke. To effectively treat stroke, it is therefore considered crucial to both protect and repair the NVU. Mitochondrial calcium (Ca²⁺) uptake supports NVU function by buffering Ca²⁺ and stimulating energy production. However, excessive mitochondrial Ca²⁺ uptake causes toxic mitochondrial Ca²⁺ overloading that triggers numerous cell death pathways which destroy the NVU. Mitochondrial damage is one of the earliest pathological events in stroke. Drugs that preserve mitochondrial integrity and function should therefore confer profound NVU protection by blocking the initiation of numerous injury events. We have shown that mitochondrial Ca²⁺ uptake and efflux in the brain are mediated by the mitochondrial Ca²⁺ uniporter complex (MCU_cx) and sodium/Ca²⁺/lithium exchanger (NCLX), respectively. Moreover, our recent pharmacological studies have demonstrated that MCU_cx inhibition and NCLX activation suppress ischemic and excitotoxic neuronal cell death by blocking mitochondrial Ca²⁺ overloading. These findings suggest that combining MCU_cx inhibition with NCLX activation should markedly protect the NVU. In terms of promoting NVU repair, nuclear hormone receptor activation is a promising approach. Retinoid X receptor (RXR) and thyroid hormone receptor (TR) agonists activate complementary transcriptional programs that stimulate mitochondrial biogenesis, suppress inflammation, and enhance the production of new vascular cells, glia, and neurons. RXR and TR agonism should thus further improve the clinical benefits of MCU_cx inhibition and NCLX activation by increasing NVU repair. However, drugs that either inhibit the MCU_cx, or stimulate the NCLX, or activate the RXR or TR, suffer from adverse effects caused by undesired actions on healthy tissues. To overcome this problem, we describe the use of nanoparticle drug formulations that preferentially target metabolically compromised and damaged NVUs after an ischemic or hemorrhagic stroke. These nanoparticle-based approaches have the potential to improve clinical safety and efficacy by maximizing drug delivery to diseased NVUs and minimizing drug exposure in healthy brain and peripheral tissues.

MICU1 regulates mitochondrial cristae structure and function independently of the mitochondrial Ca2+ uniporter channel

MICU1 is a calcium (Ca2+)-binding protein that regulates the mitochondrial Ca2+ uniporter channel complex (mtCU) and mitochondrial Ca2+ uptake. MICU1 knockout mice display disorganized mitochondrial architecture, a phenotype that is distinct from that of mice with deficiencies in other mtCU subunits and, thus, is likely not explained by changes in mitochondrial matrix Ca2+ content. Using proteomic and cellular imaging techniques, we found that MICU1 localized to the mitochondrial contact site and cristae organizing system (MICOS) and directly interacted with the MICOS components MIC60 and CHCHD2 independently of the mtCU. We demonstrated that MICU1 was essential for MICOS complex formation and that MICU1 ablation resulted in altered cristae organization, mitochondrial ultrastructure, mitochondrial membrane dynamics, and cell death signaling. Together, our results suggest that MICU1 is an intermembrane space Ca2+ sensor that modulates mitochondrial membrane dynamics independently of matrix Ca2+ uptake. This system enables distinct Ca2+ signaling in the mitochondrial matrix and at the intermembrane space to modulate cellular energetics and cell death in a concerted manner.

Mitochondrial Calcium Uptake 1 (MICU1) Gene-Related Myopathy with Extrapyramidal Signs: A Clinico-Radiological Case Report from India

Myopathy with extrapyramidal signs (MPXPS) is a rarely reported entity worldwide, manifesting as a muscular dystrophy with movement disorders. It results from mutations in the mitochondrial calcium uptake 1 (MICU1) gene. We hereby describe a 17-year-old boy who presented with proximal myopathy, calf muscle hypertrophy, and skeletal deformities along with choreiform movements of his upper extremities. Muscle MRI revealed a distinctively early involvement of adductors with sparing of antero-lateral compartment of thigh. This report expands the clinico-radiological presentation and to the best of our knowledge, is the first report of MICU-related MPXPS from India.

Genetic diagnosis of Bartter syndrome in Iranian patients and detection of a novel homozygous CLCNKB mutation

An Iranian girl with clinical symptoms of Bartter syndrome like hypokalemia, polyuria, polydipsia, hyponatremia, and hypochloremic alkalosis was referred to us in whom the CLCNKB gene was genetically evaluated using Sanger sequencing. A homozygous pathogenic variant of c.1332_1335delCTCT was detected in this patient.

Case report: Unusual episodic myopathy in a patient with novel homozygous deletion of first coding exon of MICU1 gene

We present a patient with unusual episodes of muscular weakness due to homozygous deletion of exon 2 in the MICU1 gene. Forty-three patients from 33 families were previously described with homozygous and compound heterozygous, predominantly loss of function (LoF) variants in the MICU1 gene that lead to autosomal recessive myopathy with extrapyramidal signs. Most described patients developed muscle weakness and elevated CK levels, and half of the patients had progressive extrapyramidal signs and learning disabilities. Our patient had a few severe acute episodes of muscle weakness with minimal myopathy features between episodes and a strongly elevated Creatinine Kinase (CK). Whole exome sequencing (WES) was performed and the homozygous deletion of exon 2 was suspected. To validate the diagnosis, we performed an RNA analysis of all family members. To investigate the possible impact of this deletion on the phenotype, we predicted a new Kozak sequence in exon 4 that could lead to the formation of a truncated MICU1 protein that could partly interact with MCU protein in a mitochondrial Ca2+ complex. We suspect that this unusual phenotype of the proband with MICU1-related myopathy could be explained by the presence of the truncated but partly functional protein. This work helps to define the clinical polymorphism of MICU1 deficiency better.

Shedding light on the phenotypic–genotypic correlation of rare treatable and potentially treatable pediatric movement disorders

Background

Advances in genetic science have led to the identification of many rare treatable pediatric movements disorders (MDs). We explored the phenotypic–genotypic spectrum of pediatric patients presenting with MDs. By this, we aimed at raising awareness about such rare disorders, especially in our region. Over the past 3 years, we reviewed the demographic data, clinical profile, molecular genetics and other diagnostic workups of pediatric patients presenting with MDs.

Results

Twelve patients were identified; however, only six patients were genetically confirmed. The phenomenology of MDs ranged from paroxysmal kinesigenic choreoathetosis (1 patient), exercise-induced dyskinesia (2 patients), ataxia (2 patients) and dystonia (2 patients). Whole-exome sequencing in addition to the functional studies for some patients revealed a specific genetic diagnosis being responsible for their MDs. The genetic diagnosis of our patients included infantile convulsions and paroxysmal choreoathetosis syndrome and episodic ataxia due to “pathogenic homozygous mutation of PRRT2 gene,” glucose transporter type 1 deficiency-exercise induced dyskinesia due to “De Novo pathogenic heterozygous missense mutation of exon 4 of SLC2A1 gene,” aromatic L amino acid decarboxylase deficiency due to “pathogenic homozygous mutation of the DDC gene,” myopathy with extrapyramidal signs due to “likely pathogenic homozygous mutations of the MICU1 gene,” mitochondrial trifunctional protein deficiency due to “homozygous variant of uncertain significance (VUS) of HADHB gene” and glutaric aciduria II with serine deficiency due to “homozygous VUS for both ETFDH and PHGDH genes.” After receiving the treatment as per recognized treatment protocols, two patients showed complete resolution of symptoms and the rest showed variable responses.

Conclusion

Identifying the genetic etiology of our patients guided us to provide either disease-specific treatment or redirected our management plan. Hence, highlighting the value of molecular genetic analysis to avoid the diagnostic odyssey and identify treatable MDs.

Uncontrolled mitochondrial calcium uptake underlies the pathogenesis of neurodegeneration in MICU1-deficient mice and patients

Dysregulation of mitochondrial Ca²⁺ homeostasis has been linked to neurodegenerative diseases. Mitochondrial Ca²⁺ uptake is mediated via the calcium uniporter complex that is primarily regulated by MICU1, a Ca²⁺-sensing gatekeeper. Recently, human patients with MICU1 loss-of-function mutations were diagnosed with neuromuscular and cognitive impairments. While studies in patient-derived cells revealed altered mitochondrial calcium signaling, the neuronal pathogenesis was difficult to study. To fill this void, we created a neuron-specific MICU1-KO mouse model. These animals show progressive, abnormal motor and cognitive phenotypes likely caused by the degeneration of motor neurons in the spinal cord and the cortex. We found increased susceptibility to mitochondrial Ca²⁺ overload-induced excitotoxic insults and cell death in MICU1-KO neurons and MICU1-deficient patient-derived cells, which can be blunted by inhibiting the mitochondrial permeability transition pore. Thus, our study identifies altered neuronal mitochondrial Ca²⁺ homeostasis as causative in the clinical symptoms of MICU1-deficient patients and highlights potential therapeutic targets.

Mitochondrial calcium exchange in physiology and disease

The uptake of calcium into and extrusion of calcium from the mitochondrial matrix is a fundamental biological process that has critical effects on cellular metabolism, signaling, and survival. Disruption of mitochondrial calcium (_mCa²⁺) cycling is implicated in numerous acquired diseases such as heart failure, stroke, neurodegeneration, diabetes, and cancer, and is genetically linked to several inherited neuromuscular disorders. Understanding the mechanisms responsible for _mCa²⁺ exchange therefore holds great promise for the treatment of these diseases. The past decade has seen the genetic identification of many of the key proteins that mediate mitochondrial calcium uptake and efflux. Here, we present an overview of the phenomenon of _mCa²⁺ transport, and a comprehensive examination of the molecular machinery that mediates calcium flux across the inner mitochondrial membrane: the mitochondrial uniporter complex (consisting of MCU, EMRE, MICU1, MICU2, MICU3, MCUB, and MCUR1), NCLX, LETM1, the mitochondrial ryanodine receptor, and the mitochondrial permeability transition pore. We then consider the physiological implications of _mCa²⁺ flux and evaluate how alterations in _mCa²⁺ homeostasis contribute to human disease. This review concludes by highlighting opportunities and challenges for therapeutic intervention in pathologies characterized by aberrant _mCa²⁺ handling and by summarizing critical unanswered questions regarding the biology of _mCa²⁺ flux.

Identification of a novel MICU1 nonsense variant causes myopathy with extrapyramidal signs in an Iranian consanguineous family

BACKGROUND

Ca²⁺ as a universal second messenger regulates basic biological functions including cell cycle, cell proliferation, cell differentiation, and cell death. Lack of the protein mitochondrial calcium uptake1 (MICU1), which has been regarded as a gatekeeper of Ca ions, leads to the abnormal mitochondrial Ca²⁺ handling, excessive production of reactive oxygen species (ROS), and increased cell death. Mutations in MICU1 gene causes a very rare neuromuscular disease, myopathy with extrapyramidal signs (MPXPS), due to primary alterations in mitochondrial calcium signaling which demonstrates the key role of mitochondrial Ca²⁺ uptake. To date, 13 variants have been reported in MICU1 gene in 44 patients presented with the vast spectrum of symptoms.

CASE PRESENTATION

Here, we report a 44-year-old Iranian patient presented with learning disability, muscle weakness, easy fatigability, reduced tendon reflexes, ataxia, gait disturbance, elevated hepatic transaminases, elevated serum creatine kinase (CK), and elevated lactate dehydrogenase (LDH). We identified a novel nonsense variant c.385C>T; p.(R129*) in MICU1 gene by whole exome sequencing (WES) and segregation analysis.

CONCLUSIONS

Our finding along with previous studies provides more evidence on the clinical presentation of the disease caused by pathogenic mutations in MICU1. Finding more variants and expanding the spectrum of the disease increases the diagnostic rate of molecular testing in screening of this kind of diseases and in turn improves the quality of counseling for at risk couples and helps them to minimize the risks of having affected children.