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

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We describe a novel proband presenting MCMD due to unpublished CHKB mutations.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.

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.

Acupuncture alleviates chronic pain and comorbid conditions in a mouse model of neuropathic pain: the involvement of DNA methylation in the prefrontal cortex

ABSTRACT

Chronic pain reduces life quality and is an important clinical problem associated with emotional and cognitive dysfunction. Epigenetic regulation of DNA methylation is involved in the induction of abnormal behaviors and pathological gene expression. We examined whether acupuncture can restore epigenetic changes caused by chronic pain, and identified the underlying mechanisms in neuropathic pain mice. Acupuncture treatment for 6 months (3 days/week) improved mechanical/cold allodynia and the emotional/cognitive dysfunction caused by left partial sciatic nerve ligation (PSNL)-induced neuropathic pain. The effects of acupuncture were associated with global DNA methylation recovery in the prefrontal cortex (PFC). Analysis of DNA methylation patterns in PFC indicated that 1364 overlapping genes among 4442 and 4416 methylated genes in the PSNL vs sham and PSNL vs acupuncture points groups, respectively, were highly associated with the DNA methylation process. Acupuncture restored the reduced expression of 5-methylcytosine, methyl-cytosine-phospho-guanine binding protein 2, and DNA methyltransferase family enzymes induced by PSNL in PFC. Methylation levels of Nr4a1 and Chkb associated with mitochondrial dysfunction were decreased in PFC of the PSNL mice, and increased by acupuncture. By contrast, high expression of Nr4a1 and Chkb mRNA in PSNL mice decreased after acupuncture. We also found that acupuncture inhibited the expression of Ras pathway-related genes such as Rasgrp1 and Rassf1. Finally, the expression of Nr4a1, Rasgrp1, Rassf1, and Chkb mRNA increased in the neuronal cells treated with Mecp2 small interfering RNA. These results suggest that acupuncture can relieve chronic pain-induced comorbid conditions by altering DNA methylation of Nr4a1, Rasgrp1, Rassf1, and Chkb in the PFC.

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.

Functional rescue in a mouse model of congenital muscular dystrophy with megaconial myopathy

Congenital muscular dystrophy with megaconial myopathy (MDCMC) is an autosomal recessive disorder characterized by progressive muscle weakness and wasting. The observation of megamitochondria in skeletal muscle biopsies is exclusive to this type of MD. The disease is caused by loss of function mutations in the choline kinase beta (CHKB) gene which results in dysfunction of the Kennedy pathway for the synthesis of phosphatidylcholine. We have previously reported a rostrocaudal MD (rmd) mouse with a deletion in the Chkb gene resulting in an MDCMC-like phenotype, and we used this mouse to test gene therapy strategies for the rescue and alleviation of the dystrophic phenotype. Introduction of a muscle-specific Chkb transgene completely rescues motor and behavioral function in the rmd mouse model, confirming the cell-autonomous nature of the disease. Intramuscular gene therapy post-disease onset using an adeno-associated viral 6 (AAV6) vector carrying a functional copy of Chkb is also capable of rescuing the dystrophy phenotype. In addition, we examined the ability of choline kinase alpha (Chka), a gene paralog of Chkb, to improve dystrophic phenotypes when upregulated in skeletal muscles of rmd mutant mice using a similar AAV6 vector. The sum of our results in a preclinical model of disease suggest that replacement of the Chkb gene or upregulation of endogenous Chka could serve as potential lines of therapy for MDCMC patients.

Late-onset megaconial myopathy in mice lacking group I Paks

BACKGROUND

Group I Paks are serine/threonine kinases that function as major effectors of the small GTPases Rac1 and Cdc42, and they regulate cytoskeletal dynamics, cell polarity, and transcription. We previously demonstrated that Pak1 and Pak2 function redundantly to promote skeletal myoblast differentiation during postnatal development and regeneration in mice. However, the roles of Pak1 and Pak2 in adult muscle homeostasis are unknown. Choline kinase β (Chk β) is important for adult muscle homeostasis, as autosomal recessive mutations in CHKβ are associated with two human muscle diseases, megaconial congenital muscular dystrophy and proximal myopathy with focal depletion of mitochondria.

METHODS

We analyzed mice conditionally lacking Pak1 and Pak2 in the skeletal muscle lineage (double knockout (dKO) mice) over 1 year of age. Muscle integrity in dKO mice was assessed with histological stains, immunofluorescence, electron microscopy, and western blotting. Assays for mitochondrial respiratory complex function were performed, as was mass spectrometric quantification of products of choline kinase. Mice and cultured myoblasts deficient for choline kinase β (Chk β) were analyzed for Pak1/2 phosphorylation.

RESULTS

dKO mice developed an age-related myopathy. By 10 months of age, dKO mouse muscles displayed centrally-nucleated myofibers, fibrosis, and signs of degeneration. Disease severity occurred in a rostrocaudal gradient, hindlimbs more strongly affected than forelimbs. A distinctive feature of this myopathy was elongated and branched intermyofibrillar (megaconial) mitochondria, accompanied by focal mitochondrial depletion in the central region of the fiber. dKO muscles showed reduced mitochondrial respiratory complex I and II activity. These phenotypes resemble those of rmd mice, which lack Chkβ and are a model for human diseases associated with CHKβ deficiency. Pak1/2 and Chkβ activities were not interdependent in mouse skeletal muscle, suggesting a more complex relationship in regulation of mitochondria and muscle homeostasis.

CONCLUSIONS

Conditional loss of Pak1 and Pak2 in mice resulted in an age-dependent myopathy with similarity to mice and humans with CHKβ deficiency. Protein kinases are major regulators of most biological processes but few have been implicated in muscle maintenance or disease. Pak1/Pak2 dKO mice offer new insights into these processes.