Clinical-genetic features and peculiar muscle histopathology in infantile DNM1L-related mitochondrial epileptic encephalopathy

Serine 39 in the GTP-binding domain of Drp1 is involved in shaping mitochondrial morphology

Continuous fusion and fission are critical for mitochondrial health. In this study, we further characterize the role played by dynamin-related protein 1 (Drp1) in mitochondrial fission. We show that a single amino acid change in Drp1 at position 39 from serine to alanine (S39A) within the GTP-binding (GTPase) domain results in a fused mitochondrial network in human SH-SY5Y neuroblastoma cells. Interestingly, the phosphorylation of Ser-616 and Ser-637 of Drp1 remains unaffected by the S39A mutation, and mitochondrial bioenergetic profile and cell viability in the S39A mutant were comparable to those observed in the control. This leads us to propose that the serine 39 residue of Drp1 plays a crucial role in mitochondrial distribution through its involvement in the GTPase activity. Furthermore, this amino acid mutation leads to structural anomalies in the mitochondrial network. Taken together, our results contribute to a better understanding of the function of the Drp1 protein.

Systemic phospho-defective and phospho-mimetic Drp1 mice exhibit normal growth and development with altered anxiety-like behavior

Mitochondrial division controls the size, distribution, and turnover of this essential organelle. A dynamin-related GTPase, Drp1, drives membrane division as a force-generating mechano-chemical enzyme. Drp1 is regulated by multiple mechanisms, including phosphorylation at two primary sites: serine 579 and serine 600. While previous studies in cell culture systems have shown that Drp1 S579 phosphorylation promotes mitochondrial division, its physiological functions remained unclear. Here, we generated phospho-mimetic Drp1 S579D and phospho-defective Drp1 S579R mice using the CRISPR-Cas system. Both mouse models exhibited normal growth, development, and breeding. We found that Drp1 is highly phosphorylated at S579 in brain neurons. Notably, the Drp1 S579D mice showed decreased anxiety-like behaviors, whereas the Drp1 S579R mice displayed increased anxiety-like behaviors. These findings suggest a critical role for Drp1 S579 phosphorylation in brain function. The Drp1 S579D and S579R mice thus offer valuable in vivo models for specific analysis of Drp1 S579 phosphorylation.

A novel variant of DNM1L expanding the clinical phenotypic spectrum: a case report and literature review

BACKGROUND

Mitochondrial diseases are heterogeneous in terms of clinical manifestations and genetic characteristics. The dynamin 1-like gene (DNM1L) encodes dynamin-related protein 1 (DRP1), a member of the GTPases dynamin superfamily responsible for mitochondrial and peroxisomal fission. DNM1L variants can lead to mitochondrial fission dysfunction.

CASE PRESENTATION

Herein, we report a distinctive clinical phenotype associated with a novel variant of DNM1L and review the relevant literature. A 5-year-old girl presented with paroxysmal hemiplegia, astigmatism, and strabismus. Levocarnitine and coenzyme Q10 supplement showed good efficacy. Based on the patient's clinical data, trio whole-exome sequencing (trio-WES) and mtDNA sequencing were performed to identify the potential causative genes, and Sanger sequencing was used to validate the specific variation in the proband and her family members. The results showed a novel de novo heterozygous nonsense variant in exon 20 of the DNM1L gene, c.2161C>T, p.Gln721Ter, which is predicted to be a pathogenic variant according to the ACMG guidelines. The proband has a previously undescribed clinical manifestation, namely hemiparesis, which may be an additional feature of the growing phenotypic spectrum of DNM1L-related diseases.

CONCLUSION

Our findings elucidate a novel variant in DNM1L-related disease and reveal an expanding phenotypic spectrum associated with DNM1L variants. This report highlights the necessity of next generation sequencing for early diagnosis of patients, and that further clinical phenotypic and genotypic analysis may help to improve the understanding of DNM1L-related diseases.

Application of super-resolution microscopy in mitochondria-dynamic diseases

Limited by spatial and temporal resolution, traditional optical microscopy cannot image the delicate ultra-structure organelles and sub-organelles. The emergence of super-resolution microscopy makes it possible. In this review, we focus on mitochondria. We summarize the process of mitochondrial dynamics, the primary proteins that regulate mitochondrial morphology, the diseases related to mitochondrial dynamics. The purpose is to apply super-resolution microscopy developed during recent years to the mitochondrial research. By providing the right research tools, we will help to promote the application of this technique to the in-depth elucidation of the pathogenesis of diseases related to mitochondrial dynamics, assistdiagnosis and develop the therapeutic treatment.

Disease-associated mutations in Drp1 have fundamentally different effects on the mitochondrial fission machinery

Patient mutations have been identified throughout dynamin-related protein 1 (Drp1), the key protein mediator of mitochondrial fission. These changes generally impact young children and often result in severe neurological defects and, in some instances, death. Until now, the underlying functional defect leading to patient phenotypes has been largely speculative. We therefore analyzed six disease-associated mutations throughout the GTPase and middle domains (MD) of Drp1. The MD plays a role in Drp1 oligomerization, and three mutations in this region were predictably impaired in self-assembly. However, another mutant in this region (F370C) retained oligomerization capability on pre-curved membranes despite being assembly-limited in solution. Instead, this mutation impaired membrane remodeling of liposomes, which highlights the importance of Drp1 in generating local membrane curvature before fission. Two GTPase domain mutations were also observed in different patients. The G32A mutation was impaired in GTP hydrolysis both in solution and in the presence of lipid but remains capable of self-assembly on these lipid templates. The G223V mutation also exhibited decreased GTPase activity and was able to assemble on pre-curved lipid templates; however, this change impaired membrane remodeling of unilamellar liposomes similar to F370C. This demonstrates that the Drp1 GTPase domain also contributes to self-assembly interactions that drive membrane curvature. Overall, the functional defects caused by mutations in Drp1 are highly variable even for mutations that reside within the same functional domain. This study provides a framework for characterizing additional Drp1 mutations to provide a comprehensive understanding of functional sites within this essential protein.

Genetic defects in peroxisome morphogenesis (Pex11β, dynamin-like protein 1, and nucleoside diphosphate kinase 3) affect docosahexaenoic acid-phospholipid metabolism

Peroxisomes are essential organelles involved in lipid metabolisms including plasmalogen biosynthesis and β-oxidation of very long-chain fatty acids. Peroxisomes proliferate by the growth and division of pre-existing peroxisomes. The peroxisomal membrane is elongated by Pex11β and then divided by the dynamin-like GTPase, DLP1 (also known as DRP1 encoded by DNM1L gene), which also functions as a fission factor for mitochondria. Nucleoside diphosphate kinase 3 (NME3) localized in both peroxisomes and mitochondria generates GTP for DLP1 activity. Deficiencies of either of these factors induce abnormal morphology of peroxisomes and/or mitochondria, and are associated with central nervous system dysfunction. To investigate whether the impaired division of peroxisomes affects lipid metabolisms, we assessed the phospholipid composition of cells lacking each of the different division factors. In fibroblasts from the patients deficient in DLP1, NME3, or Pex11β, docosahexaenoic acid (DHA, C22:6)-containing phospholipids were found to be decreased. Conversely, the levels of several fatty acids such as arachidonic acid (AA, C20:4) and oleic acid (C18:1) were elevated. Mouse embryonic fibroblasts from Drp1- and Pex11β-knockout mice also showed a decrease in the levels of phospholipids containing DHA and AA. Collectively, these results suggest that the dynamics of organelle morphology exert marked effects on the fatty acid composition of phospholipids.

DRP1 mutations associated with EMPF1 encephalopathy alter mitochondrial membrane potential and metabolic programs

Mitochondria and peroxisomes are dynamic signaling organelles that constantly undergo fission, driven by the large GTPase dynamin-related protein 1 (DRP1; encoded by DNM1L). Patients with de novo heterozygous missense mutations in DNM1L present with encephalopathy due to defective mitochondrial and peroxisomal fission (EMPF1) - a devastating neurodevelopmental disease with no effective treatment. To interrogate the mechanisms by which DRP1 mutations cause cellular dysfunction, we used human-derived fibroblasts from patients who present with EMPF1. In addition to elongated mitochondrial morphology and lack of fission, patient cells display lower coupling efficiency, increased proton leak and upregulation of glycolysis. Mitochondrial hyperfusion also results in aberrant cristae structure and hyperpolarized mitochondrial membrane potential. Peroxisomes show a severely elongated morphology in patient cells, which is associated with reduced respiration when cells are reliant on fatty acid oxidation. Metabolomic analyses revealed impaired methionine cycle and synthesis of pyrimidine nucleotides. Our study provides insight into the role of mitochondrial dynamics in cristae maintenance and the metabolic capacity of the cell, as well as the disease mechanism underlying EMPF1.

Mitochondrial quality control in the brain: The physiological and pathological roles

The human brain has high energetic expenses and consumes over 20% of total oxygen metabolism. Abnormal brain energy homeostasis leads to various brain diseases. Among multiple factors that contribute to these diseases, mitochondrial dysfunction is one of the most common causes. Maintenance of mitochondrial integrity and functionality is of pivotal importance to brain energy generation. Mitochondrial quality control (MQC), employing the coordination of multiple mechanisms, is evolved to overcome many mitochondrial defects. Thus, not surprisingly, aberrant mitochondrial quality control results in a wide range of brain disorders. Targeting MQC to preserve and restore mitochondrial function has emerged as a promising therapeutic strategy for the prevention and treatment of brain diseases. Here, we set out to summarize the current understanding of mitochondrial quality control in brain homeostasis. We also evaluate potential pharmaceutically and clinically relevant targets in MQC-associated brain disorders.

Novel DNM1L variants impair mitochondrial dynamics through divergent mechanisms

Novel DNM1L variants underlie a spectrum of clinical phenotypes and impair mitochondrial and peroxisomal dynamics via divergent mechanisms, with effects on DRP1 protein stability, GTPase activity, and oligomerisation in vitro.

Imbalances in mitochondrial and peroxisomal dynamics are associated with a spectrum of human neurological disorders. Mitochondrial and peroxisomal fission both involve dynamin-related protein 1 (DRP1) oligomerisation and membrane constriction, although the precise biophysical mechanisms by which distinct DRP1 variants affect the assembly and activity of different DRP1 domains remains largely unexplored. We analysed four unreported de novo heterozygous variants in the dynamin-1-like gene DNM1L, affecting different highly conserved DRP1 domains, leading to developmental delay, seizures, hypotonia, and/or rare cardiac complications in infancy. Single-nucleotide DRP1 stalk domain variants were found to correlate with more severe clinical phenotypes, with in vitro recombinant human DRP1 mutants demonstrating greater impairments in protein oligomerisation, DRP1-peroxisomal recruitment, and both mitochondrial and peroxisomal hyperfusion compared to GTPase or GTPase-effector domain variants. Importantly, we identified a novel mechanism of pathogenesis, where a p.Arg710Gly variant uncouples DRP1 assembly from assembly-stimulated GTP hydrolysis, providing mechanistic insight into how assembly-state information is transmitted to the GTPase domain. Together, these data reveal that discrete, pathological DNM1L variants impair mitochondrial network maintenance by divergent mechanisms.

Case report: Thirty-year progression of an EMPF1 encephalopathy due to defective mitochondrial and peroxisomal fission caused by a novel de novo heterozygous DNM1L variant

Mutations in DNM1L (DRP1), which encode a key player of mitochondrial and peroxisomal fission, have been reported in patients with the variable phenotypic spectrum, ranging from non-syndromic optic atrophy to lethal infantile encephalopathy. Here, we report a case of an adult female patient presenting with a complex neurological phenotype that associates axonal sensory neuropathy, spasticity, optic atrophy, dysarthria, dysphasia, dystonia, and ataxia, worsening with aging. Whole-exome sequencing revealed a heterozygous de novo variant in the GTPase domain of DNM1L [NM_001278464.1: c.176C>A p.(Thr59Asn)] making her the oldest patient suffering from encephalopathy due to defective mitochondrial and peroxisomal fission-1. In silico analysis suggested a protein destabilization effect of the variant Thr59Asn. Unexpectedly, Western blotting disclosed profound decrease of DNM1L expression, probably related to the degradation of DNM1L complexes. A detailed description of mitochondrial and peroxisomal anomalies in transmission electron and 3D fluorescence microscopy studies confirmed the exceptional phenotype of this patient.

Fission Impossible (?)-New Insights into Disorders of Peroxisome Dynamics

Peroxisomes are highly dynamic and responsive organelles, which can adjust their morphology, number, intracellular position, and metabolic functions according to cellular needs. Peroxisome multiplication in mammalian cells involves the concerted action of the membrane-shaping protein PEX11β and division proteins, such as the membrane adaptors FIS1 and MFF, which recruit the fission GTPase DRP1 to the peroxisomal membrane. The latter proteins are also involved in mitochondrial division. Patients with loss of DRP1, MFF or PEX11β function have been identified, showing abnormalities in peroxisomal (and, for the shared proteins, mitochondrial) dynamics as well as developmental and neurological defects, whereas the metabolic functions of the organelles are often unaffected. Here, we provide a timely update on peroxisomal membrane dynamics with a particular focus on peroxisome formation by membrane growth and division. We address the function of PEX11β in these processes, as well as the role of peroxisome–ER contacts in lipid transfer for peroxisomal membrane expansion. Furthermore, we summarize the clinical phenotypes and pathophysiology of patients with defects in the key division proteins DRP1, MFF, and PEX11β as well as in the peroxisome–ER tether ACBD5. Potential therapeutic strategies for these rare disorders with limited treatment options are discussed.

Predicting Mitochondrial Dynamic Behavior in Genetically Defined Neurodegenerative Diseases

Mitochondrial dynamics encompass mitochondrial fusion, fission, and movement. Mitochondrial fission and fusion are seemingly ubiquitous, whereas mitochondrial movement is especially important for organelle transport through neuronal axons. Here, we review the roles of different mitochondrial dynamic processes in mitochondrial quantity and quality control, emphasizing their impact on the neurological system in Charcot–Marie–Tooth disease type 2A, amyotrophic lateral sclerosis, Friedrich’s ataxia, dominant optic atrophy, and Alzheimer’s, Huntington’s, and Parkinson’s diseases. In addition to mechanisms and concepts, we explore in detail different technical approaches for measuring mitochondrial dynamic dysfunction in vitro, describe how results from tissue culture studies may be applied to a better understanding of mitochondrial dysdynamism in human neurodegenerative diseases, and suggest how this experimental platform can be used to evaluate candidate therapeutics in different diseases or in individual patients sharing the same clinical diagnosis.

Mitochondrial dynamics and its impact on human health and diseases: inside the DRP1 blackbox

Mitochondria are essential organelles that play a significant role in various cellular processes apart from providing energy in eukaryotic cells. An intricate link between mitochondrial structure and function is now unequivocally accepted. Several molecular players have been identified, which are important in maintaining the structure of the organelle. Dynamin-related protein 1 (DRP1) is one such conserved protein that is a vital regulator of mitochondrial dynamics. Multidisciplinary studies have helped elucidate the structure of the protein and its mechanism of action in great detail. Mutations in various domains of the protein have been identified that are associated with debilitating conditions in patients. The involvement of the protein in disease conditions such as neurodegeneration, cancer, and cardiovascular disorders is also gaining attention. The purpose of this review is to highlight recent findings on the role of DRP1 in human disease conditions and address its importance as a therapeutic target.

Pathogenic DNM1L Variant (1085G>A) Linked to Infantile Progressive Neurological Disorder: Evidence of Maternal Transmission by Germline Mosaicism and Influence of a Contemporary in cis Variant (1535T>C)

Mitochondria are dynamic organelles undergoing continuous fusion and fission with Drp1, encoded by the DNM1L gene, required for mitochondrial fragmentation. DNM1L dominant pathogenic variants lead to progressive neurological disorders with early exitus. Herein we report on the case of a boy affected by epileptic encephalopathy carrying two heterozygous variants (in cis) of the DNM1L gene: a pathogenic variant (PV) c.1085G>A (p.Gly362Asp) accompanied with a variant of unknown significance (VUS) c.1535T>C (p.Ile512Thr). Amplicon sequencing of the mother’s DNA revealed the presence of the PV and VUS in 5% of cells, with the remaining cells presenting only VUS. Functional investigations performed on the patient and his mother’s cells unveiled altered mitochondrial respiratory chain activities, network architecture and Ca²⁺ homeostasis as compared with healthy unrelated subjects’ samples. Modelling Drp1 harbouring the two variants, separately or in combination, resulted in structural changes as compared with Wt protein. Considering the clinical history of the mother, PV transmission by a maternal germline mosaicism mechanism is proposed. Altered Drp1 function leads to changes in the mitochondrial structure and bioenergetics as well as in Ca²⁺ homeostasis. The novel VUS might be a modifier that synergistically worsens the phenotype when associated with the PV.

Mitochondrial Dysfunction: A Common Denominator in Neurodevelopmental Disorders?

Mitochondria, the organelles classically seen as the powerhouse of the cell, are increasingly associated with a wide variety of neurodevelopmental disorders. Although individually rare, a myriad of pediatric neurogenetic disorders have been identified in the last few years, thanks to advances in clinical genetic sequencing and data analysis. As this exponential growth continues, mitochondrial dysfunction is increasingly implicated in childhood neurodevelopmental disorders, with clinical presentations ranging from syndromic autism, intellectual disability, and epileptic encephalopathies to childhood onset neurodegeneration. Here we review recent evidence demonstrating mitochondrial involvement in neurodevelopmental disorders, identify emerging mechanistic trends, and reconsider the long-standing question of the role of mitochondria in light of new evidence: causation versus mere association.

Dominant optic atrophy: Culprit mitochondria in the optic nerve

Dominant optic atrophy (DOA) is an inherited mitochondrial disease leading to specific degeneration of retinal ganglion cells (RGCs), thus compromising transmission of visual information from the retina to the brain. Usually, DOA starts during childhood and evolves to poor vision or legal blindness, affecting the central vision, whilst sparing the peripheral visual field. In 20% of cases, DOA presents as syndromic disorder, with secondary symptoms affecting neuronal and muscular functions. Twenty years ago, we demonstrated that heterozygous mutations in OPA1 are the most frequent molecular cause of DOA. Since then, variants in additional genes, whose functions in many instances converge with those of OPA1, have been identified by next generation sequencing. OPA1 encodes a dynamin-related GTPase imported into mitochondria and located to the inner membrane and intermembrane space. The many OPA1 isoforms, resulting from alternative splicing of three exons, form complex homopolymers that structure mitochondrial cristae, and contribute to fusion of the outer membrane, thus shaping the whole mitochondrial network. Moreover, OPA1 is required for oxidative phosphorylation, maintenance of mitochondrial genome, calcium homeostasis and regulation of apoptosis, thus making OPA1 the Swiss army-knife of mitochondria. Understanding DOA pathophysiology requires the understanding of RGC peculiarities with respect to OPA1 functions. Besides the tremendous energy requirements of RGCs to relay visual information from the eye to the brain, these neurons present unique features related to their differential environments in the retina, and to the anatomical transition occurring at the lamina cribrosa, which parallel major adaptations of mitochondrial physiology and shape, in the pre- and post-laminar segments of the optic nerve. Three DOA mouse models, with different Opa1 mutations, have been generated to study intrinsic mechanisms responsible for RGC degeneration, and these have further revealed secondary symptoms related to mitochondrial dysfunctions, mirroring the more severe syndromic phenotypes seen in a subgroup of patients. Metabolomics analyses of cells, mouse organs and patient plasma mutated for OPA1 revealed new unexpected pathophysiological mechanisms related to mitochondrial dysfunction, and biomarkers correlated quantitatively to the severity of the disease. Here, we review and synthesize these data, and propose different approaches for embracing possible therapies to fulfil the unmet clinical needs of this disease, and provide hope to affected DOA patients.

Mimicking human Drp1 disease-causing mutations in yeast Dnm1 reveals altered mitochondrial dynamics

The dynamin-related protein 1 (Drp1) and its homologs in various eukaryotes are essential to maintain mitochondrial morphology and regulate mitochondrial division. Several mutations in different domains of Drp1 have been reported, which result in debilitating conditions. Four such disease-causing mutations of the middle domain of Drp1 were mimicked in the yeast dynamin-related GTPase (Dnm1) and were characterized in this study. Mitochondrial morphology and protein function were observed to be altered to a variable extent in cells expressing the mutated variants of Dnm1. Several aspects related to the protein such as punctate formation, localization to mitochondria, dynamic behavior and structure were analyzed by microscopy, biochemical studies and molecular dynamics simulations. Significant effects on the protein structure and function were observed in cells expressing A430D and G397D mutations. Overall, our data provide insight into the molecular and cellular alterations resulting from middle domain mutations in Dnm1.

Focal status and acute encephalopathy in a 13-year-old boy with de novo DNM1L mutation: Video-polygraphic pattern and clues for differential diagnosis

BACKGROUND

Pathogenic variants in the dynamin 1 like gene are related to abnormal mitochondrial dynamics and distributions and are associated to variable clinical phenotypes. A few patients harboring the p.Arg403Cys missense variant appears to be different from the classical, more severe phenotypes, showing sudden onset of drug resistant seizures after a previously normal or slightly delayed development.

CASE REPORT

We report on a boy with abrupt onset of focal status and coma at the age of 13, initially treated as autoimmune encephalitis, with final diagnosis of de novo missense p.Arg403Cys variant in the DNM1L gene.

DISCUSSION

We compare his clinical, electrophysiological, biochemical, neuroradiological and histopathological picture to the rare cases reported to date and provide diagnostic clues that can help clinicians in differentiate p.Arg403Cys-related phenotype from that of immune-mediated encephalopathies.

CONCLUSION

The clinical picture related to p.Arg403Cys mutations should be considered alongside acquired pathologies in the differential diagnosis of young patients with focal refractory epilepsy and encephalopathy, also occurring during late childhood or adolescence. Prompt genetic testing allows to avoid unnecessary treatments and procedures and to better define the prognosis and management strategies.

Genetic Neuropathy Due to Impairments in Mitochondrial Dynamics

Mitochondria are dynamic organelles capable of fusing, dividing, and moving about the cell. These properties are especially important in neurons, which in addition to high energy demand, have unique morphological properties with long axons. Notably, mitochondrial dysfunction causes a variety of neurological disorders including peripheral neuropathy, which is linked to impaired mitochondrial dynamics. Nonetheless, exactly why peripheral neurons are especially sensitive to impaired mitochondrial dynamics remains somewhat enigmatic. Although the prevailing view is that longer peripheral nerves are more sensitive to the loss of mitochondrial motility, this explanation is insufficient. Here, we review pathogenic variants in proteins mediating mitochondrial fusion, fission and transport that cause peripheral neuropathy. In addition to highlighting other dynamic processes that are impacted in peripheral neuropathies, we focus on impaired mitochondrial quality control as a potential unifying theme for why mitochondrial dysfunction and impairments in mitochondrial dynamics in particular cause peripheral neuropathy.

Molecular Mechanisms behind Inherited Neurodegeneration of the Optic Nerve

Inherited neurodegeneration of the optic nerve is a paradigm in neurology, as many forms of isolated or syndromic optic atrophy are encountered in clinical practice. The retinal ganglion cells originate the axons that form the optic nerve. They are particularly vulnerable to mitochondrial dysfunction, as they present a peculiar cellular architecture, with axons that are not myelinated for a long intra-retinal segment, thus, very energy dependent. The genetic landscape of causative mutations and genes greatly enlarged in the last decade, pointing to common pathways. These mostly imply mitochondrial dysfunction, which leads to a similar outcome in terms of neurodegeneration. We here critically review these pathways, which include (1) complex I-related oxidative phosphorylation (OXPHOS) dysfunction, (2) mitochondrial dynamics, and (3) endoplasmic reticulum-mitochondrial inter-organellar crosstalk. These major pathogenic mechanisms are in turn interconnected and represent the target for therapeutic strategies. Thus, their deep understanding is the basis to set and test new effective therapies, an urgent unmet need for these patients. New tools are now available to capture all interlinked mechanistic intricacies for the pathogenesis of optic nerve neurodegeneration, casting hope for innovative therapies to be rapidly transferred into the clinic and effectively cure inherited optic neuropathies.

The Power of Yeast in Modelling Human Nuclear Mutations Associated with Mitochondrial Diseases

The increasing application of next generation sequencing approaches to the analysis of human exome and whole genome data has enabled the identification of novel variants and new genes involved in mitochondrial diseases. The ability of surviving in the absence of oxidative phosphorylation (OXPHOS) and mitochondrial genome makes the yeast Saccharomyces cerevisiae an excellent model system for investigating the role of these new variants in mitochondrial-related conditions and dissecting the molecular mechanisms associated with these diseases. The aim of this review was to highlight the main advantages offered by this model for the study of mitochondrial diseases, from the validation and characterisation of novel mutations to the dissection of the role played by genes in mitochondrial functionality and the discovery of potential therapeutic molecules. The review also provides a summary of the main contributions to the understanding of mitochondrial diseases emerged from the study of this simple eukaryotic organism.

Dynamic properties of mitochondria during human corticogenesis

Mitochondria are signaling hubs responsible for the generation of energy through oxidative phosphorylation, the production of key metabolites that serve the bioenergetic and biosynthetic needs of the cell, calcium (Ca2+) buffering and the initiation/execution of apoptosis. The ability of mitochondria to coordinate this myriad of functions is achieved through the exquisite regulation of fundamental dynamic properties, including remodeling of the mitochondrial network via fission and fusion, motility and mitophagy. In this Review, we summarize the current understanding of the mechanisms by which these dynamic properties of the mitochondria support mitochondrial function, review their impact on human cortical development and highlight areas in need of further research.

The relevance of mitochondrial morphology for human disease

Mitochondria are highly dynamic organelles, which undergo frequent structural and metabolic changes to fulfil cellular demands. To facilitate these processes several proteins are required to regulate mitochondrial shape and interorganellar communication. These proteins include the classical mitochondrial fusion (MFN1, MFN2, and OPA1) and fission proteins (DRP1, MFF, FIS1, etc.) as well as several other proteins that are directly or indirectly involved in these processes (e.g. YME1L, OMA1, INF2, GDAP1, MIC13, etc.). During the last two decades, inherited genetic defects in mitochondrial fusion and fission proteins have emerged as an important class of neurodegenerative human diseases with variable onset ranging from infancy to adulthood. So far, no causal treatment strategies are available for these disorders. In this review, we provide an overview about the current knowledge on mitochondrial dynamics under physiological conditions. Moreover, we describe human diseases, which are associated with genetic defects in these pathways.

Mitochondrial Dynamics: Molecular Mechanisms, Related Primary Mitochondrial Disorders and Therapeutic Approaches

Mitochondria do not exist as individual entities in the cell—conversely, they constitute an interconnected community governed by the constant and opposite process of fission and fusion. The mitochondrial fission leads to the formation of smaller mitochondria, promoting the biogenesis of new organelles. On the other hand, following the fusion process, mitochondria appear as longer and interconnected tubules, which enhance the communication with other organelles. Both fission and fusion are carried out by a small number of highly conserved guanosine triphosphatase proteins and their interactors. Disruption of this equilibrium has been associated with several pathological conditions, ranging from cancer to neurodegeneration, and mutations in genes involved in mitochondrial fission and fusion have been reported to be the cause of a subset of neurogenetic disorders.

Function and regulation of the divisome for mitochondrial fission

Mitochondria form dynamic networks in the cell that are balanced by the flux of iterative fusion and fission events of the organelles. It is now appreciated that mitochondrial fission also represents an end-point event in a signalling axis that allows cells to sense and respond to external cues. The fission process is orchestrated by membrane-associated adaptors, influenced by organellar and cytoskeletal interactions and ultimately executed by the dynamin-like GTPase DRP1. Here we invoke the framework of the 'mitochondrial divisome', which is conceptually and operationally similar to the bacterial cell-division machinery. We review the functional and regulatory aspects of the mitochondrial divisome and, within this framework, parse the core from the accessory machinery. In so doing, we transition from a phenomenological to a mechanistic understanding of the fission process.

DNM1L-Related Mitochondrial Fission Defects Presenting as Encephalopathy: A Case Report and Literature Review

Background: Mitochondrial dynamics, including mitochondrial fission and fusion, transport and distribution, biogenesis and degradation, are critical to neuronal function. The dynamin-1 like (DNM1L) gene encodes dynamin-related protein 1 (DRP1/DLP1), which is an evolutionarily conserved member of the dynamin family and is responsible for mitochondrial division. DNM1L variants can lead to mitochondrial fission dysfunction and neurological disorders. Methods: We report a case of DNM1L-related mitochondrial disease admitted to Tianjin Children's Hospital. We searched for similar reported cases in the PubMed database using the terms "DNM1L" and "mitochondrial," reviewed recent literature to summarize the clinical and genetic characteristics, and analyzed genotype-phenotype correlations. Results: The patient presented with psychomotor retardation, motor disturbance (muscle weakness with paroxysmal hypermyotonia), and a de novo variant (c.116G>A, g.22229G>A, p.S39N) in the GTPase domain of DNM1L (reference sequence NM_012062), which has not previously been reported in the literature. This case was combined with an additional 35 cases identified in 20 relevant references in order to analyze a total of 36 patients. The male-to-female ratio was 1:1.06, and the median age of onset was 6 months (range, neonatal period to 9 years). The cardinal symptoms included psychomotor retardation in 77.8% (28/36), limb paralysis in 66.7% (18/27), dystonia in 82.8% (24/29), and epilepsy in 59.4% (19/32). The clinical manifestations of variants in the GTPase domain of DRP1 were milder than those identified in the middle domain. Conclusion: This case report describes a new variant of the DNM1L gene, and summarizes previously reported cases. Furthermore, the clinical phenotype and the genotype of DNM1L gene-associated mitochondrial disease was analyzed to improve the understanding of this disease.

De novo DNM1L variant presenting with severe muscular atrophy, dystonia and sensory neuropathy

DNM1L encodes dynamin-related protein 1 (DRP1), a multi-domain GTPase essential for mitochondrial and peroxisomal division. Autosomal dominant and recessive variants in DNM1L cause encephalopathy due to defective mitochondrial and peroxisomal fission 1 (EMPF1), which presents as a complex and clinically heterogeneous neurological disorder of variable severity, often accompanied by seizures. Clinical features are diverse, and no clear phenotype-genotype correlations were drawn to date. DNM1L-related sensory neuropathy has recently been reported as a predominant feature in one case with a de novo variant in the GTPase domain. Herein we present a second case with DNM1L-related sensory neuropathy as the predominant underlying feature without motor neuron involvement, which resulted in severe muscular atrophy and generalized dystonia.

Mitochondrial division, fusion and degradation

The mitochondrion is an essential organelle for a wide range of cellular processes, including energy production, metabolism, signal transduction and cell death. To execute these functions, mitochondria regulate their size, number, morphology and distribution in cells via mitochondrial division and fusion. In addition, mitochondrial division and fusion control the autophagic degradation of dysfunctional mitochondria to maintain a healthy population. Defects in these dynamic membrane processes are linked to many human diseases that include metabolic syndrome, myopathy and neurodegenerative disorders. In the last several years, our fundamental understanding of mitochondrial fusion, division and degradation has been significantly advanced by high resolution structural analyses, protein-lipid biochemistry, super resolution microscopy and in vivo analyses using animal models. Here, we summarize and discuss this exciting recent progress in the mechanism and function of mitochondrial division and fusion.

Impaired turnover of hyperfused mitochondria in severe axonal neuropathy due to a novel DRP1 mutation

Mitochondria undergo continuous cycles of fusion and fission in response to physiopathological stimuli. The key player in mitochondrial fission is dynamin-related protein 1 (DRP1), a cytosolic protein encoded by dynamin 1-like (DNM1L) gene, which relocalizes to the outer mitochondrial membrane, where it assembles, oligomerizes and drives mitochondrial division upon guanosine-5'-triphosphate (GTP) hydrolysis. Few DRP1 mutations have been described so far, with patients showing complex and variable phenotype ranging from early death to encephalopathy and/or optic atrophy. The disease is the consequence of defective mitochondrial fission due to faulty DRP1 function. However, the underlying molecular mechanisms and the functional consequences at mitochondrial and cellular level remain elusive. Here we report on a 5-year-old girl presenting psychomotor developmental delay, global hypotonia and severe ataxia due to axonal sensory neuropathy harboring a novel de novo heterozygous missense mutation in the GTPase domain of DRP1 (NM_012062.3:c.436G>A, NP_036192.2: p.D146N variant in DNM1L). Patient's fibroblasts show hyperfused/balloon-like giant mitochondria, highlighting the importance of D146 residue for DRP1 function. This dramatic mitochondrial rearrangement phenocopies what observed overexpressing DRP1-K38A, a well-known experimental dominant negative version of DRP1. In addition, we demonstrated that p.D146N mutation has great impact on peroxisomal shape and function. The p.D146N mutation compromises the GTPase activity without perturbing DRP1 recruitment or assembly, causing decreased mitochondrial and peroxisomal turnover. In conclusion, our findings highlight the importance of sensory neuropathy in the clinical spectrum of DRP1 variants and, for the first time, the impact of DRP1 mutations on mitochondrial turnover and peroxisomal functionality.

Staying in Healthy Contact: How Peroxisomes Interact with Other Cell Organelles

Peroxisomes share extensive metabolic connections with other cell organelles. Membrane contact sites (MCSs) establish and maintain such interactions, and they are vital for organelle positioning and motility. In the past few years peroxisome interactions and MCSs with other cellular organelles have been explored extensively, resulting in the identification of new MCSs, the tethering molecules involved, and their functional characterization. Defective tethering and compartmental communication can lead to pathological conditions that can be termed 'organelle interaction diseases'. We review peroxisome-organelle interactions in mammals and summarize the most recent knowledge of mammalian peroxisomal organelle contacts in health and disease.

Genome sequencing and implications for rare disorders

The practice of genomic medicine stands to revolutionize our approach to medical care, and to realize this goal will require discovery of the relationship between rare variation at each of the ~ 20,000 protein-coding genes and their consequent impact on individual health and expression of Mendelian disease. The step-wise evolution of broad-based, genome-wide cytogenetic and molecular genomic testing approaches (karyotyping, chromosomal microarray [CMA], exome sequencing [ES]) has driven much of the rare disease discovery to this point, with genome sequencing representing the newest member of this team. Each step has brought increased sensitivity to interrogate individual genomic variation in an unbiased method that does not require clinical prediction of the locus or loci involved. Notably, each step has also brought unique limitations in variant detection, for example, the low sensitivity of ES for detection of triploidy, and of CMA for detection of copy neutral structural variants. The utility of genome sequencing (GS) as a clinical molecular diagnostic test, and the increased sensitivity afforded by addition of long-read sequencing or other -omics technologies such as RNAseq or metabolomics, are not yet fully explored, though recent work supports improved sensitivity of variant detection, at least in a subset of cases. The utility of GS will also rely upon further elucidation of the complexities of genetic and allelic heterogeneity, multilocus rare variation, and the impact of rare and common variation at a locus, as well as advances in functional annotation of identified variants. Much discovery remains to be done before the potential utility of GS is fully appreciated.