[Mitochondrial trifunctional protein deficiency: an adult patient with similar progress to Charcot-Marie-Tooth disease]

Case report: Mitochondrial trifunctional protein deficiency caused by HADHB gene mutation (c.1175C>T) characterized by higher brain dysfunction followed by neuropathy, presented gadolinium enhancement on brain imaging in an adult patient

Mitochondrial trifunctional protein (MTP) deficiency is an autosomal recessive disorder caused by impaired metabolism of long-chain fatty acids (LCFAs). Childhood and late-onset MTP deficiency is characterized by myopathy/rhabdomyolysis and peripheral neuropathy; however, the features are unclear. A 44-year-old woman was clinically diagnosed with Charcot-Marie-Tooth disease at 3 years of age due to gait disturbance. Her activity and voluntary speech gradually decreased in her 40s. Cognitive function was evaluated and brain imaging tests were performed. The Mini-Mental State Examination and frontal assessment battery scores were 25/30 and 10/18, respectively, suggesting higher brain dysfunction. Peripheral nerve conduction studies revealed axonal impairments. Brain computed tomography showed significant calcification. Magnetic resonance imaging revealed an increased gadolinium contrast-enhanced signal in the white matter, suggesting demyelination of the central nervous system (CNS) due to LCFAs. The diagnosis of MTP deficiency was confirmed through genetic examination. Administration of L-carnitine and a medium-chain fatty triglyceride diet was initiated, and the progression of higher brain dysfunction was retarded within 1 year. This patient's presentation was suggestive of CNS demyelination. The presence of brain calcification, higher brain dysfunction, or gadolinium enhancement in the white matter in patients with peripheral neuropathy may be suggestive of MTP deficiency.

Complex hereditary peripheral neuropathies caused by novel variants in mitochondrial-related nuclear genes

Mitochondrial disorders are a group of clinically and genetically heterogeneous multisystem disorders and peripheral neuropathy is frequently described in the context of mutations in mitochondrial-related nuclear genes. This study aimed to identify the causative mutations in mitochondrial-related nuclear genes in suspected hereditary peripheral neuropathy patients. We enrolled a large Japanese cohort of clinically suspected hereditary peripheral neuropathy patients who were mutation negative in the prescreening of the known Charcot–Marie–Tooth disease-causing genes. We performed whole-exome sequencing on 247 patients with autosomal recessive or sporadic inheritance for further analysis of 167 mitochondrial-related nuclear genes. We detected novel bi-allelic likely pathogenic/pathogenic variants in four patients, from four mitochondrial-related nuclear genes: pyruvate dehydrogenase beta-polypeptide (PDHB), mitochondrial poly(A) polymerase (MTPAP), hydroxyacyl-CoA dehydrogenase/3-ketoacyl-CoA thiolase/enoyl-CoA hydratase, beta subunit (HADHB), and succinate-CoA ligase ADP-forming beta subunit (SUCLA2). All these patients showed sensory and motor axonal polyneuropathy, combined with central nervous system or multisystem involvements. The pathological analysis of skeletal muscles revealed mild neurogenic changes without significant mitochondrial abnormalities. Targeted screening of mitochondria-related nuclear genes should be considered for patients with complex hereditary axonal polyneuropathy, accompanied by central nervous system dysfunctions, or with unexplainable multisystem disorders.

Charcot-Marie-Tooth Disease With Episodic Rhabdomyolysis Due to Two Novel Mutations in the β Subunit of Mitochondrial Trifunctional Protein and Effective Response to Modified Diet Therapy

A 29-year-old female experienced chronic progressive peripheral neuropathy since childhood and was diagnosed with Charcot–Marie–Tooth disease (CMT) at age 15. She developed recurrent, fever-induced rhabdomyolysis (RM) at age 24. EMG studies showed decreased amplitude of compound muscle action potential, declined motor conductive velocity, and absence of sensor nerve action potential. Acylcarnitine analysis revealed elevated C16-OH, C18-OH, and C18:1-OH. Muscle biopsy showed scattered foci of necrotic myofibers invaded by macrophages, occasional regenerating fibers, and remarkable muscle fiber type grouping. Whole-exome sequencing identified two novel heterozygous mutations: c.490G>A (p.G164S) and c.686G>A (p.R229Q) in HADHB gene encoding the β-subunit of mitochondrial trifunctional protein (MTP). Reduction of long-chain fatty acid via dietary restrictions alleviated symptoms effectively. Our study indicates that the defect of the MTP β-subunit accounts for both CMT and RM in the same patient and expands the clinical spectrum of disorders caused by the HADHB mutations. Our systematic review of all MTPD patients with dietary treatment indicates that the effect of dietary treatment is related to the age of onset and the severity of symptoms.

MTP deficiency caused by HADHB mutations: Pathophysiology and clinical manifestations

Mutations in the HADHB gene lead to Mitochondrial Trifunctional Protein (MTP) deficiency. MTP deficiency is a rare autosomal recessive disorder affecting long-chain fatty acid oxidation. Patients affected by MTP deficiency are unable to metabolize long-chain fatty-acids and suffer a variety of symptoms exacerbated during fasting. The three phenotypes associated with complete MTP deficiency are an early-onset cardiomyopathy and early death, an intermediate form with recurrent hypoketotic hypoglycemia and a sensorimotor neuropathy with episodic rhabdomyolysis with small amount of residual enzyme activities. This review aims to discuss the pathophysiological mechanisms and clinical manifestations of each phenotype, which appears different and linked to HADHB expression levels. Notably, the pathophysiology of the sensorimotor neuropathy is relatively unknown and we provide a hypothesis on the qualitative aspect of the role of acylcarnitine buildup in Schwann cells in MTP deficiency patients. We propose that acylcarnitine may exit the Schwann cell and alter membrane properties of nearby axons leading to axonal degeneration based on recent findings in different metabolic disorders.

An update on diagnosis and therapy of metabolic myopathies

INTRODUCTION

Metabolic myopathies are a heterogeneous group of disorders characterized by inherited defects of enzymatic pathways involved in muscle fiber energetics. Diagnosing metabolic myopathies requires a thoroughly taken individual and family history, a meticulous neurologic exam, exercise tests, blood and urine tests, needle-electromyography, nerve-conduction studies, muscle biopsy, targeted genetic tests, or next-generation sequencing. There is limited evidence from the literature to guide treatment of metabolic myopathies. Treatment is largely limited to non-invasive/invasive symptomatic measures. However, promising results have been achieved with enzyme replacement therapy in Pompe disease (GSD-II). Primary coenzyme-Q deficiency responds favorably to coenzyme-Q supplementation. MNGIE responds to allogeneic hematopoietic stem cell transplantation, orthotopic liver transplantation, and carrier erythrocyte entrapped thymidine phosphorylase enzyme therapy. MADD may respond to riboflavin. Areas covered: This review aims to summarize and discuss recent findings and new insights concerning diagnosis and treatment of metabolic myopathies. Expert commentary: Except for GSD-II, coenzyme-Q deficiency, and MNGIE, treatment of metabolic myopathies is usually palliative and supportive (non-invasive or invasive). Non-invasive symptomatic treatment includes physiotherapy, diet, administration of drugs, conservative orthopedic measures, and respiratory non-invasive support. Important is the avoidance of triggers for episodic forms of fatty acid oxidation disorders. Invasive measures include orthopedic surgery and invasive mechanical ventilation.