Megaconial congenital muscular dystrophy due to loss-of-function mutations in choline kinase β

Megaconial congenital muscular dystrophy due to CHKB gene variants, the first report of thirteen Iranian patients

Megaconial congenital muscular dystrophy (OMIM: 602,541) related to CHKB gene mutation is a newly defined rare autosomal recessive disorder, with multisystem involvement presenting from the neonatal period to adolescence. Choline kinase beta, lipid transport enzyme, catalyzes the biosynthesis of phosphatidylcholine and phosphatidylethanolamine, two major components of the mitochondrial membrane, on which respiratory enzyme activities are dependent. CHKB gene variants lead to loss-of-function of choline kinase b and lipid metabolism defects and mitochondrial structural changes. To date, many megaconial congenital muscular dystrophy cases due to CHKB gene variants have been reported worldwide. We describe thirteen Iranian megaconial congenital muscular dystrophy cases related to CHKB gene variants, including clinical presentations, laboratory and muscle biopsy findings, and novel CHKB gene variants. The most common symptoms and signs included intellectual disability, delayed gross-motor developmental milestones, language skills problems, muscle weakness, as well as autistic features, and behavioral problems. Muscle biopsy examination showed the striking finding of peripheral arrangements of large mitochondria in muscle fibers and central sarcoplasmic areas devoid of mitochondria. Eleven different CHKB gene variants including six novel variants were found in our patients. Despite the rarity of this disorder, recognition of the multisystem clinical presentations combined with characteristic findings of muscle histology can properly guide to genetic evaluation of CHKB gene.

Megaconial congenital muscular dystrophy due to novel CHKB variants: a case report and literature review

Background

Choline kinase beta (CHKB) catalyzes the first step in the de novo biosynthesis of phosphatidyl choline and phosphatidylethanolamine via the Kennedy pathway. Derangement of this pathway might also influence the homeostasis of mitochondrial membranes.

Autosomal recessive CHKB mutations cause a rare form of congenital muscular dystrophy known as megaconial congenital muscular dystrophy (MCMD).

Case presentation

We describe a novel proband presenting MCMD due to unpublished CHKB mutations. The patient is a 6-year-old boy who came to our attention for cognitive impairment and slowly progressive muscular weakness. He was the first son of non-consanguineous healthy parents from Sri Lanka. Neurological examination showed proximal weakness at four limbs, weak osteotendinous reflexes, Gowers’ maneuver, and waddling gate. Creatine kinase levels were mildly increased. EMG and brain MRI were normal. Left quadriceps skeletal muscle biopsy showed a myopathic pattern with nuclear centralizations and connective tissue increase. Histological and histochemical staining suggested subsarcolemmal localization and dimensional increase of mitochondria. Ultrastructural analysis confirmed the presence of enlarged (“megaconial”) mitochondria. Direct sequencing of CHKB identified two novel defects: the c.1060G > C (p.Gly354Arg) substitution and the c.448-56_29del intronic deletion, segregating from father and mother, respectively. Subcloning of RT-PCR amplicons from patient’s muscle RNA showed that c.448-56_29del results in the partial retention (14 nucleotides) of intron 3, altering physiological splicing and transcript stability. Biochemical studies showed reduced levels of the mitochondrial fission factor DRP1 and the severe impairment of mitochondrial respiratory chain activity in patient’s muscle compared to controls.

Conclusions

This report expands the molecular findings associated with MCMD and confirms the importance of considering CHKB variants in the differential diagnosis of patients presenting with muscular dystrophy and mental retardation. The clinical outcome of MCMD patients seems to be influenced by CHKB molecular defects. Histological and ultrastructural examination of muscle biopsy directed molecular studies and allowed the identification and characterization of an intronic mutation, usually escaping standard molecular testing.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13395-022-00306-8.

Identification of unprecedented ATP-competitive choline kinase inhibitors

In this article we describe the identification of unprecedented ATP-competitive ChoKα inhibitors starting from initial hit NMS-P830 that binds to ChoKα in an ATP concentration-dependent manner. This result is confirmed by the co-crystal structure of NMS-P830 in complex with Δ75-ChoKα. NMS-P830 is able to inhibit ChoKα in cells resulting in the reduction of intracellular phosphocholine formation. A structure-based medicinal chemistry program resulted in the identification of selective compounds that have good biochemical activity, solubility and metabolic stability and are suitable for further optimization. The ChoKα inhibitors disclosed in this article demonstrate for the first time the possibility to inhibit ChoKα with ATP-competitive compounds.

Reduced mitochondrial fission and impaired energy metabolism in human primary skeletal muscle cells of Megaconial Congenital Muscular Dystrophy

Megaconial Congenital Muscular Dystrophy (CMD) is a rare autosomal recessive disorder characterized by enlarged mitochondria located mainly at the periphery of muscle fibers and caused by mutations in the Choline Kinase Beta (CHKB) gene. Although the pathogenesis of this disease is not well understood, there is accumulating evidence for the presence of mitochondrial dysfunction. In this study, we aimed to investigate whether imbalanced mitochondrial dynamics affects mitochondrial function and bioenergetic efficiency in skeletal muscle cells of Megaconial CMD. Immunofluorescence, confocal and transmission electron microscopy studies revealed impaired mitochondrial network, morphology, and localization in primary skeletal muscle cells of Megaconial CMD. The organelle disruption was specific only to skeletal muscle cells grown in culture. The expression levels of mitochondrial fission proteins (DRP1, MFF, FIS1) were found to be decreased significantly in both primary skeletal muscle cells and tissue sections of Megaconial CMD by Western blotting and/or immunofluorescence analysis. The metabolomic and fluxomic analysis, which were performed in Megaconial CMD for the first time, revealed decreased levels of phosphonucleotides, Krebs cycle intermediates, ATP, and altered energy metabolism pathways. Our results indicate that reduced mitochondrial fission and altered mitochondrial energy metabolism contribute to mitochondrial dysmorphology and dysfunction in the pathogenesis of Megaconial CMD.

ChoK-Full of Potential: Choline Kinase in B Cell and T Cell Malignancies

Aberrant choline metabolism, characterized by an increase in total choline-containing compounds, phosphocholine and phosphatidylcholine (PC), is a metabolic hallmark of carcinogenesis and tumor progression. This aberration arises from alterations in metabolic enzymes that control PC biosynthesis and catabolism. Among these enzymes, choline kinase α (CHKα) exhibits the most frequent alterations and is commonly overexpressed in human cancers. CHKα catalyzes the phosphorylation of choline to generate phosphocholine, the first step in de novo PC biosynthesis. CHKα overexpression is associated with the malignant phenotype, metastatic capability and drug resistance in human cancers, and thus has been recognized as a robust biomarker and therapeutic target of cancer. Of clinical importance, increased choline metabolism and CHKα activity can be detected by non-invasive magnetic resonance spectroscopy (MRS) or positron emission tomography/computed tomography (PET/CT) imaging with radiolabeled choline analogs for diagnosis and treatment monitoring of cancer patients. Both choline-based MRS and PET/CT imaging have also been clinically applied for lymphoid malignancies, including non-Hodgkin lymphoma, multiple myeloma and central nervous system lymphoma. However, information on how choline kinase is dysregulated in lymphoid malignancies is very limited and has just begun to be unraveled. In this review, we provide an overview of the current understanding of choline kinase in B cell and T cell malignancies with the goal of promoting future investigation in this area.

Choline Kinase: An Unexpected Journey for a Precision Medicine Strategy in Human Diseases

Choline kinase (ChoK) is a cytosolic enzyme that catalyzes the phosphorylation of choline to form phosphorylcholine (PCho) in the presence of ATP and magnesium. ChoK is required for the synthesis of key membrane phospholipids and is involved in malignant transformation in a large variety of human tumours. Active compounds against ChoK have been identified and proposed as antitumor agents. The ChoK inhibitory and antiproliferative activities of symmetrical bispyridinium and bisquinolinium compounds have been defined using quantitative structure-activity relationships (QSARs) and structural parameters. The design strategy followed in the development of the most active molecules is presented. The selective anticancer activity of these structures is also described. One promising anticancer compound has even entered clinical trials. Recently, ChoKα inhibitors have also been proposed as a novel therapeutic approach against parasites, rheumatoid arthritis, inflammatory processes, and pathogenic bacteria. The evidence for ChoKα as a novel drug target for approaches in precision medicine is discussed.

Megaconial congenital muscular dystrophy secondary to novel CHKB mutations resemble atypical Rett syndrome

Megaconial congenital muscular dystrophy (CMD)(OMIM #602541), related to CHKB mutation, is a rare autosomal recessive disorder. To date, only 35 confirmed patients are recorded. We present a detailed description of the clinical, histopathological, imaging, and genetic findings of five children from four Indian families. The children had moderate-to-severe autistic behavior, hand stereotypies, and global developmental delay mimicking atypical Rett syndrome. In addition, generalized hypotonia was a common initial finding. The progression of muscle weakness was variable, with two patients having a milder phenotype and three having a severe form. Interestingly, the majority did not attain sphincter control. Only patient 1 had classical ichthyotic skin changes. Muscle biopsy in two patients showed a myopathic pattern with characteristic peripherally placed enlarged mitochondria on modified Gomori trichrome stain and electron microscopy. Genetic analysis in these patients identified three novel null mutations in CHKB [c.1027dupA (p.Ser343LysfsTer86);c.224 + 1G > T (5' splice site); c.1123C > T (p.Gln375Ter)] and one reported missense mutation, c.581G > A (p.Arg194Gln), all in the homozygous state. Megaconial CMD, although rare, forms an important group with a complex phenotypic presentation and accounted for 5.5% of our genetically confirmed CMD patients. Atypical Rett syndrome-like presentation may be a clue towards CHKB-related disorder.

Detection of a DNA Methylation Signature for the Intellectual Developmental Disorder, X-Linked, Syndromic, Armfield Type

A growing number of genetic neurodevelopmental disorders are known to be associated with unique genomic DNA methylation patterns, called episignatures, which are detectable in peripheral blood. The intellectual developmental disorder, X-linked, syndromic, Armfield type (MRXSA) is caused by missense variants in FAM50A. Functional studies revealed the pathogenesis to be a spliceosomopathy that is characterized by atypical mRNA processing during development. In this study, we assessed the peripheral blood specimens in a cohort of individuals with MRXSA and detected a unique and highly specific DNA methylation episignature associated with this disorder. We used this episignature to construct a support vector machine model capable of sensitive and specific identification of individuals with pathogenic variants in FAM50A. This study contributes to the expanding number of genetic neurodevelopmental disorders with defined DNA methylation episignatures, provides an additional understanding of the associated molecular mechanisms, and further enhances our ability to diagnose patients with rare disorders.

Genetic diseases of the Kennedy pathways for membrane synthesis

The two branches of the Kennedy pathways (CDP-choline and CDP-ethanolamine) are the predominant pathways responsible for the synthesis of the most abundant phospholipids, phosphatidylcholine and phosphatidylethanolamine, respectively, in mammalian membranes. Recently, hereditary diseases associated with single gene mutations in the Kennedy pathways have been identified. Interestingly, genetic diseases within the same pathway vary greatly, ranging from muscular dystrophy to spastic paraplegia to a childhood blinding disorder to bone deformations. Indeed, different point mutations in the same gene (PCYT1; CCTα) result in at least three distinct diseases. In this review, we will summarize and review the genetic diseases associated with mutations in genes of the Kennedy pathway for phospholipid synthesis. These single-gene disorders provide insight, indeed direct genotype-phenotype relationships, into the biological functions of specific enzymes of the Kennedy pathway. We discuss potential mechanisms of how mutations within the same pathway can cause disparate disease.

A Rare Cause of Autism Spectrum Disorder: Megaconial Muscular Dystrophy

Megaconial congenital muscular dystrophy (OMIM 602541) is defined by early-onset hypotonia, mildly elevated serum creatine kinase (CK) levels, muscle wasting, cardiomyopathy, psychomotor developmental delay and intellectual disability. The disease is caused by loss-of-function mutations in Choline kinase beta gene (CHKB) and has specific muscle biopsy findings. Here we investigate two patients with weakness of proximal muscles and generalized muscle atrophy, skin changes, agressiveness, social communication and empathy difficulties. Both patients had mildly elevated serum CK levels. Whole exome sequencing (WES) performed for both patients and homozygous c.818+1G>A and homozygous c.1031+1G>A variants were detected in patient 1 and patient 2, respectively. We would like to draw the attention of autism spectrum disorder in early diagnosis of congenital muscular dystrophies.

Megaconial congenital muscular dystrophy: Same novel homozygous mutation in CHKB gene in two unrelated Chinese patients

Megaconial congenital muscular dystrophy (CMD) is a rare form of congenital muscular dystrophy attributed to an autosomal recessive CHKB mutation. We report two unrelated Chinese girls with Megaconial CMD who harbored the same novel homozygous CHKB mutation but exhibited different phenotypes. Patient 1, who is now 8 years old, has autism, intellectual disabilities, mild girdle weakness, and characteristic muscle biopsy with COX-negative fibers. Patient 2, now 12 years old, has limited intelligence and marked weakness, with scoliosis, hip subluxation and early loss of ambulation. Both exhibited mildly elevated creatine kinase levels, have relative sparing of adductor longus and extensor digitorum longus on MRI leg muscles, and a c.598del (p.Gln200Argfs*11) homozygous CHKB loss-of-function mutation. Their parents are heterozygous carriers. This is the first report of Megaconial CMD in Chinese patients demonstrating the pathogenicity of the identified homozygous CHKB variant. A case review of all previously reported patients of different ethnicities is also included.

Myopathology of Adult and Paediatric Mitochondrial Diseases

Mitochondria are dynamic organelles ubiquitously present in nucleated eukaryotic cells, subserving multiple metabolic functions, including cellular ATP generation by oxidative phosphorylation (OXPHOS). The OXPHOS machinery comprises five transmembrane respiratory chain enzyme complexes (RC). Defective OXPHOS gives rise to mitochondrial diseases (mtD). The incredible phenotypic and genetic diversity of mtD can be attributed at least in part to the RC dual genetic control (nuclear DNA (nDNA) and mitochondrial DNA (mtDNA)) and the complex interaction between the two genomes. Despite the increasing use of next-generation-sequencing (NGS) and various omics platforms in unravelling novel mtD genes and pathomechanisms, current clinical practice for investigating mtD essentially involves a multipronged approach including clinical assessment, metabolic screening, imaging, pathological, biochemical and functional testing to guide molecular genetic analysis. This review addresses the broad muscle pathology landscape including genotype–phenotype correlations in adult and paediatric mtD, the role of immunodiagnostics in understanding some of the pathomechanisms underpinning the canonical features of mtD, and recent diagnostic advances in the field.

Molecular structure and differential function of choline kinases CHKα and CHKβ in musculoskeletal system and cancer

Choline, a hydrophilic cation, has versatile physiological roles throughout the body, including cholinergic neurotransmission, memory consolidation and membrane biosynthesis and metabolism. Choline kinases possess enzyme activity that catalyses the conversion of choline to phosphocholine, which is further converted to cytidine diphosphate-coline (CDP-choline) in the biosynthesis of phosphatidylcholine (PC). PC is a major constituent of the phospholipid bilayer which constitutes the eukaryotic cell membrane, and regulates cell signal transduction. Choline Kinase consists of three isoforms, CHKα1, CHKα2 and CHKβ, encoded by two separate genes (CHKA(Human)/Chka(Mouse) and CHKB(Human)/Chkb(Mouse)). Both isoforms have similar structures and enzyme activity, but display some distinct molecular structural domains and differential tissue expression patterns. Whilst Choline Kinase was discovered in early 1950, its pivotal role in the development of muscular dystrophy, bone deformities, and cancer has only recently been identified. CHKα has been proposed as a cancer biomarker and its inhibition as an anti-cancer therapy. In contrast, restoration of CHKβ deficiency through CDP-choline supplements like citicoline may be beneficial for the treatment of muscular dystrophy, bone metabolic diseases, and cognitive conditions. The molecular structure and expression pattern of Choline Kinase, the differential roles of Choline Kinase isoforms and their potential as novel therapeutic targets for muscular dystrophy, bone deformities, cognitive conditions and cancer are discussed.

Importance of Skin Changes in the Differential Diagnosis of Congenital Muscular Dystrophies

Megaconial congenital muscular dystrophy (OMIM 602541) is characterized with early-onset hypotonia, muscle wasting, proximal weakness, cardiomyopathy, mildly elevated serum creatine kinase (CK) levels, and mild-to-moderate intellectual disability. We report two siblings in a consanguineous family admitted for psychomotor delay. Physical examination revealed proximal muscle weakness, contractures in the knee of elder sibling, diffuse mild generalized muscle atrophy, and dry skin with ichthyosis together with multiple nummular eczema in both siblings. Serum CK values were elevated up to 500 U/L. For genetic work-up, we performed whole exome sequencing (WES) after Nimblegen enrichment on the Illumina platform. The WES revealed a novel homozygous missense mutation in the Choline Kinase-Beta (CHKB) gene c.1031G>A (p.R344Q) in exon 9. Ichthyosis-like skin changes with intense pruritus and nummular eczema may lead to clinical diagnosis in cases with megaconial congenital muscular dystrophy.

Proximal myopathy with focal depletion of mitochondria and megaconial congenital muscular dystrophy are allelic conditions caused by mutations in CHKB

We recently evaluated two of the original three patients (siblings) diagnosed with Proximal Myopathy with Focal Depletion of Mitochondria. The condition was named for the distinctive pattern of enlarged mitochondria around the periphery of muscle fibres with a complete absence in the middle. These siblings, aged 37 and 40, are cognitively normal with mild non-progressive muscle weakness and a susceptibility to rhabdomyolysis. Both were shown to be compound heterozygotes for novel mutations (c.263C>T + c.950T>A) in CHKB, the gene currently associated with Megaconial Congenital Muscular Dystrophy. Individuals with this condition have early-onset muscle weakness and profound intellectual disability but share the same unique pattern on muscle biopsy as was noted in Proximal Myopathy with Focal Depletion of Mitochondria; focal depletion of mitochondria was surrounded by abnormally large "megaconial" mitochondria. Thus the phenotypic spectrum of CHKB mutations ranges from a congenital muscular dystrophy with intellectual disability to a later-onset non-progressive muscular weakness with normal cognition.

Congenital neurogenic muscular atrophy in megaconial myopathy due to a mutation in CHKB gene

Choline kinase beta gene (CHKB) mutations have been identified in Megaconial Congenital Muscular Dystrophy (MDCMC) patients, a very rare inborn error of metabolism with 21 cases reported worldwide. We report the case of a Spanish boy of Caucasian origin who presented a generalized congenital muscular hypotonia, more intense at lower limb muscles, mildly elevated creatine kinase (CK), serum aspartate transaminase (AST) and lactate. Electromyography (EMG) showed neurogenic potentials in the proximal muscles. Histological studies of a muscle biopsy showed neurogenic atrophy with enlarged mitochondria in the periphery of the fibers, and complex I deficiency. Finally, genetic analysis showed the presence of a homozygous mutation in the gene for choline kinase beta (CHKB: NM_005198.4:c.810T>A, p.Tyr270(∗)). We describe here the second Spanish patient whit mutation in CHKB gene, who despite having the same mutation, presented an atypical aspect: congenital neurogenic muscular atrophy progressing to a combined neuropathic and myopathic phenotype (mixed pattern).

Clinical characteristics of megaconial congenital muscular dystrophy due to choline kinase beta gene defects in a series of 15 patients

A new form of congenital muscular dystrophy (CMD) with multisystem involvement and characteristic mitochondrial structural changes, due to choline kinase beta (CHKB) gene defects has been characterized by intellectual disability, autistic features, ichthyosis-like skin changes, and dilated cardiomyopathy. We define the clinical characteristics in 15 patients, from 14 unrelated families with so-called 'megaconial CMD', all having mutations in CHKB. Core clinical phenotype included global developmental delay prominent in gross-motor and language domains, severe intellectual disability (ID), and/or muscle weakness in all cases. Muscle biopsies were equivocally 'megaconial' in all. Other peculiarities were: ichthyosis-like skin changes (n = 11), increased serum CK levels (n = 12), microcephaly (n = 6), dysmorphic facial features (n = 7), neonatal hypotonia (n = 3), seizures (n = 3), epileptiform activity without clinically overt seizures (n = 2), dilated cardiomyopathy (n = 2), decreased left ventricular systolic function (n = 2), congenital heart defects (n = 3), sensorineural (n = 1), and conductive hearing loss (n = 1). Ten patients had cranial neuroimaging (MRI-MRS) study, which was notably normal in all, other than one patient having a decreased choline: creatine peak. Intra-familial variability in clinical expression of the disease is noted in four families. Two siblings from the same family, one presenting with global developmental delay and dilated cardiomyopathy, and the other with ichthyosis, ID and proximal weakness without cardiomyopathy died at the ages of 2 years 1 month, and 7 years 4 months respectively. Evolution was progressive (n = 13) and static (n = 2).

Choline kinase β mutant mice exhibit reduced phosphocholine, elevated osteoclast activity, and low bone mass

The maintenance of bone homeostasis requires tight coupling between bone-forming osteoblasts and bone-resorbing osteoclasts. However, the precise molecular mechanism(s) underlying the differentiation and activities of these specialized cells are still largely unknown. Here, we identify choline kinase β (CHKB), a kinase involved in the biosynthesis of phosphatidylcholine, as a novel regulator of bone homeostasis. Choline kinase β mutant mice (flp/flp) exhibit a systemic low bone mass phenotype. Consistently, osteoclast numbers and activity are elevated in flp/flp mice. Interestingly, osteoclasts derived from flp/flp mice exhibit reduced sensitivity to excessive levels of extracellular calcium, which could account for the increased bone resorption. Conversely, supplementation of cytidine 5'-diphosphocholine in vivo and in vitro, a regimen that bypasses CHKB deficiency, restores osteoclast numbers to physiological levels. Finally, we demonstrate that, in addition to modulating osteoclast formation and function, loss of CHKB corresponds with a reduction in bone formation by osteoblasts. Taken together, these data posit CHKB as a new modulator of bone homeostasis.

Novel CHKB mutation expands the megaconial muscular dystrophy phenotype

INTRODUCTION

Mutations in the choline kinase beta (CHKB) gene are associated with a congenital muscular dystrophy with giant mitochondria at the periphery of muscle fibers.

METHODS

We describe a patient of Italian origin in whom whole-exome sequencing revealed a novel homozygous nonsense mutation, c.648C>A, p.(Tyr216*), in exon 5 of CHKB.

RESULTS

The patient presented with limb-girdle weakness and hypotonia from birth with mental retardation, and had sudden and transient deteriorations of muscle strength with acute intercurrent illnesses. Previously undescribed sarcolemmal overexpression of utrophin was noted in the muscle biopsy.

CONCLUSIONS

Pathological features broaden the description of the entity and provide new insight in the pathogenic mechanisms. This case highlights the usefulness of next-generation sequencing in the diagnosis of rare and incompletely understood conditions.

Ultrastructural mitochondrial alterations in equine myopathies of unknown origin

BACKGROUND

Very few mitochondrial myopathies have been described in horses.

OBJECTIVE

To examine the ultrastructure of muscle mitochondria in equine cases of myopathy of unknown origin.

MATERIALS & METHODS

Biopsies of vastus lateralis of the Musculus quadriceps femoris were taken predominantly immediately post mortem and processed for transmission electron microscopy. As a result, electron micrographs of 90 horses in total were available for analysis comprising 4 control horses, 16 horses suffering from myopathy and 70 otherwise diseased horses.

RESULTS

Following a thorough clinical and laboratory work-up, four out of five patients that did not fit into the usual algorithm to detect known causes of myopathy showed ultrastructural mitochondrial alterations. Small mitochondria with zones with complete disruption of cristae associated with lactic acidemia were detected in a 17-year-old pony mare, extremely long and slender mitochondria with longitudinal cristae in a 5-year-old Quarter horse stallion, a mixture of irregular extremely large mitochondria (measuring 2500 by 800 nm) next to smaller ones in an 8-year-old Hanoverian mare and round mitochondria with only few cristae in a 11-year-old pony gelding. It remains uncertain whether the subsarcolemmal mitochondrial accumulations observed in the fifth patient have any pathological significance.

CONCLUSIONS

Ultrastructural alterations in mitochondria were detected in at least four horses. To conclude that these are due to mitochondrial dysfuntions, biochemical tests should be performed.

PRACTICAL APPLICATIONS

The possibility of a mitochondrial myopathy should be included in the differential diagnosis of muscle weakness.

Exome sequencing identifies a CHKB mutation in Spanish patient with megaconial congenital muscular dystrophy and mtDNA depletion

BACKGROUND

Choline kinase beta gene (CHKB) mutations have been identified in Megaconial Congenital Muscular Dystrophy (MDCMC) patients, but never in patients with an additional combined deficiency of complexes I, III and IV and mitochondrial DNA (mtDNA) depletion.

AIMS

To report mutations in carry genes for MDCMC with respiratory chain defects and mtDNA depletion.

METHODS

Whole-exome sequencing (WES) was used to identify the carry genes in a Spanish child with muscle weakness, mild hypotonia at lower limb muscles, mildly elevated creatine kinase (CK), enlarged mitochondria in the periphery of the fibers, combined deficiency of complex I, III and IV and depletion of mtDNA.

RESULTS

With WES data, it was possible to get the whole mtDNA sequencing and discard any pathogenic variant in this genome. The first filter of WES data with the nuclear-encoded mitochondrial genes (MitoCarta) did not get any candidate. However, the analysis of whole exome uncovered a homozygous nonsense pathogenic mutation in CHKB gene (NM_005198.4:c.810T>A, p.Tyr270*).

CONCLUSIONS

Our data confirm the role of CHKB in MDCMC and point to this gene as unique candidate for the combined deficiency of respiratory chain and mtDNA depletion observed in this patient.

Choline kinase beta is required for normal endochondral bone formation

BACKGROUND

Choline kinase has three isoforms encoded by the genes Chka and Chkb. Inactivation of Chka in mice results in embryonic lethality, whereas Chkb(-/-) mice display neonatal forelimb bone deformations.

METHODS

To understand the mechanisms underlying the bone deformations, we compared the biology and biochemistry of bone formation from embryonic to young adult wild-type (WT) and Chkb(-/-) mice.

RESULTS

The deformations are specific to the radius and ulna during the late embryonic stage. The radius and ulna of Chkb(-/-) mice display expanded hypertrophic zones, unorganized proliferative columns in their growth plates, and delayed formation of primary ossification centers. The differentiation of chondrocytes of Chkb(-/-) mice was impaired, as was chondrocyte proliferation and expression of matrix metalloproteinases 9 and 13. In chondrocytes from Chkb(-/-) mice, phosphatidylcholine was slightly lower than in WT mice whereas the amount of phosphocholine was decreased by approximately 75%. In addition, the radius and ulna from Chkb(-/-) mice contained fewer osteoclasts along the cartilage/bone interface.

CONCLUSIONS

Chkb has a critical role in the normal embryogenic formation of the radius and ulna in mice.

GENERAL SIGNIFICANCE

Our data indicate that choline kinase beta plays an important role in endochondral bone formation by modulating growth plate physiology.

Phosphatidylcholine metabolism and choline kinase in human osteoblasts

There is a paucity of information about phosphatidylcholine (PC) biosynthesis in bone formation. Thus, we characterized PC metabolism in both primary human osteoblasts (HOB) and human osteosarcoma MG-63 cells. Our results show that the CDP-choline pathway is the only de novo route for PC biosynthesis in both HOB and MG-63 cells. Both CK activity and CKα expression in MG-63 cells were significantly higher than those in HOB cells. Silencing of CKα in MG-63 cells had no significant effect on PC concentration but decreased the amount of phosphocholine by approximately 80%. The silencing of CKα also reduced cell proliferation. Moreover, pharmacological inhibition of CK activity impaired the mineralization capacity of MG-63 cells. Our data suggest that CK and its product phosphocholine are required for the normal growth and mineralization of MG-63 cells.

Pantethine treatment is effective in recovering the disease phenotype induced by ketogenic diet in a pantothenate kinase-associated neurodegeneration mouse model

Pantothenate kinase-associated neurodegeneration, caused by mutations in the PANK2 gene, is an autosomal recessive disorder characterized by dystonia, dysarthria, rigidity, pigmentary retinal degeneration and brain iron accumulation. PANK2 encodes the mitochondrial enzyme pantothenate kinase type 2, responsible for the phosphorylation of pantothenate or vitamin B5 in the biosynthesis of co-enzyme A. A Pank2 knockout (Pank2^−/−) mouse model did not recapitulate the human disease but showed azoospermia and mitochondrial dysfunctions. We challenged this mouse model with a low glucose and high lipid content diet (ketogenic diet) to stimulate lipid use by mitochondrial beta-oxidation. In the presence of a shortage of co-enzyme A, this diet could evoke a general impairment of bioenergetic metabolism. Only Pank2^−/− mice fed with a ketogenic diet developed a pantothenate kinase-associated neurodegeneration-like syndrome characterized by severe motor dysfunction, neurodegeneration and severely altered mitochondria in the central and peripheral nervous systems. These mice also showed structural alteration of muscle morphology, which was comparable with that observed in a patient with pantothenate kinase-associated neurodegeneration. We here demonstrate that pantethine administration can prevent the onset of the neuromuscular phenotype in mice suggesting the possibility of experimental treatment in patients with pantothenate kinase-associated neurodegeneration.

Mitochondrial dysfunction in neuromuscular disorders

This review deciphers aspects of mitochondrial (mt) dysfunction among nosologically, pathologically, and genetically diverse diseases of the skeletal muscle, lower motor neuron, and peripheral nerve, which fall outside the traditional realm of mt cytopathies. Special emphasis is given to well-characterized mt abnormalities in collagen VI myopathies (Ullrich congenital muscular dystrophy and Bethlem myopathy), megaconial congenital muscular dystrophy, limb-girdle muscular dystrophy type 2 (calpainopathy), centronuclear myopathies, core myopathies, inflammatory myopathies, spinal muscular atrophy, Charcot-Marie-Tooth neuropathy type 2, and drug-induced peripheral neuropathies. Among inflammatory myopathies, mt abnormalities are more prominent in inclusion body myositis and a subset of polymyositis with mt pathology, both of which are refractory to corticosteroid treatment. Awareness is raised about instances of phenotypic mimicry between cases harboring primary mtDNA depletion, in the context of mtDNA depletion syndrome, and established neuromuscular disorders such as spinal muscular atrophy. A substantial body of experimental work, derived from animal models, attests to a major role of mitochondria (mt) in the early process of muscle degeneration. Common mechanisms of mt-related cell injury include dysregulation of the mt permeability transition pore opening and defective autophagy. The therapeutic use of mt permeability transition pore modifiers holds promise in various neuromuscular disorders, including muscular dystrophies.