Mitochondrial cerebellar ataxia, renal failure, neuropathy, and encephalopathy (MCARNE)

Focal segmental glomerulosclerosis with a mutation in the mitochondrially encoded NADH dehydrogenase 5 gene: A case report

NADH dehydrogenase 5 (ND5) is one of 44 subunits composed of Complex I in mitochondrial respiratory chain. Therefore, a mitochondrially encoded ND5 (MT-ND5) gene mutation causes mitochondrial oxidative phosphorylation (OXPHOS) disorder, resulting in the development of mitochondrial diseases. Focal segmental glomerulosclerosis (FSGS) which had podocytes filled with abnormal mitochondria is induced by mitochondrial diseases. An MT-ND5 mutation also causes FSGS. We herein report a Japanese woman who was found to have proteinuria and renal dysfunction in an annual health check-up at 29 years old. Because her proteinuria and renal dysfunction were persistent, she had a kidney biopsy at 33 years of age. The renal histology showed FSGS with podocytes filled with abnormal mitochondria. The podocytes also had foot process effacement and cytoplasmic vacuolization. In addition, the renal pathological findings showed granular swollen epithelial cells (GSECs) in tubular cells, age-inappropriately disarranged and irregularly sized vascular smooth muscle cells (AiDIVs), and red-coloured podocytes (ReCPos) by acidic dye. A genetic analysis using peripheral mononuclear blood cells and urine sediment cells detected the m.13513 G > A variant in the MT-ND5 gene. Therefore, this patient was diagnosed with FSGS due to an MT-ND5 gene mutation. Although this is not the first case report to show that an MT-ND5 gene mutation causes FSGS, this is the first to demonstrate podocyte injuries accompanied with accumulation of abnormal mitochondria in the cytoplasm.

Impact of the m.13513G>A Variant on the Functions of the OXPHOS System and Cell Retrograde Signaling

Mitochondria are involved in many vital functions in living cells, including the synthesis of ATP by oxidative phosphorylation (OXPHOS) and regulation of nuclear gene expression through retrograde signaling. Leigh syndrome is a heterogeneous neurological disorder resulting from an isolated complex I deficiency that causes damage to mitochondrial energy production. The pathogenic mitochondrial DNA (mtDNA) variant m.13513G>A has been associated with Leigh syndrome. The present study investigated the effects of this mtDNA variant on the OXPHOS system and cell retrograde signaling. Transmitochondrial cytoplasmic hybrid (cybrid) cell lines harboring 50% and 70% of the m.13513G>A variant were generated and tested along with wild-type (WT) cells. The functionality of the OXPHOS system was evaluated by spectrophotometric assessment of enzyme activity and high-resolution respirometry. Nuclear gene expression was investigated by RNA sequencing and droplet digital PCR. Increasing levels of heteroplasmy were associated with reduced OXPHOS system complex I, IV, and I + III activities, and high-resolution respirometry also showed a complex I defect. Profound changes in transcription levels of nuclear genes were observed in the cell lines harboring the pathogenic mtDNA variant, indicating the physiological processes associated with defective mitochondria.

Case Report: Optic Atrophy and Nephropathy With m.13513G>A/MT-ND5 mtDNA Pathogenic Variant

Isolated complex I deficiency represents the most common mitochondrial respiratory chain defect involved in mitochondrial disorders. Among these, the mitochondrial DNA (mtDNA) m.13513G>A pathogenic variant in the NADH dehydrogenase 5 subunit gene (MT-ND5) has been associated with heterogenous manifestations, including phenotypic overlaps of mitochondrial encephalomyopathy with lactic acidosis and stroke-like episodes, Leigh syndrome, and Leber’s hereditary optic neuropathy (LHON). Interestingly, this specific mutation has been recently described in patients with adult-onset nephropathy. We, here, report the unique combination of LHON, nephropathy, sensorineural deafness, and subcortical and cerebellar atrophy in association with the m.13513G>A variant.

Charcot-Marie-Tooth Disease and Other Hereditary Neuropathies

PURPOSE OF REVIEW

This article provides an overview of Charcot-Marie-Tooth disease (CMT) and other inherited neuropathies. These disorders encompass a broad spectrum with variable motor, sensory, autonomic, and other organ system involvement. Considerable overlap exists, both phenotypically and genetically, among these separate categories, all eventually exhibiting axonal injury and neurologic impairment. Depending on the specific neural and non-neural localizations, patients experience varying morbidity and mortality. Neurologic evaluations, including neurophysiologic testing, can help diagnose and predict patient disabilities. Diagnosis is often complex, especially when genetic and acquired components overlap.

RECENT FINDINGS

Next-generation sequencing has greatly improved genetic diagnosis, with many third-party reimbursement parties now embracing phenotype-based panel evaluations. Through the advent of comprehensive gene panels, symptoms previously labeled as idiopathic or atypical now have a better chance to receive a specific diagnosis. A definitive molecular diagnosis affords patients improved care and counsel. The new classification scheme for inherited neuropathies emphasizes the causal gene names. A specific genetic diagnosis is important as considerable advances are being made in gene-specific therapeutics. Emerging therapeutic approaches include small molecule chaperones, antisense oligonucleotides, RNA interference, and viral gene delivery therapies. New therapies for hereditary transthyretin amyloidosis and Fabry disease are discussed.

SUMMARY

Comprehensive genetic testing through a next-generation sequencing approach is simplifying diagnostic algorithms and affords significantly improved decision-making processes in neuropathy care. Genetic diagnosis is essential for pathogenic understanding and for gene therapy development. Gene-targeted therapies have begun entering the clinic. Currently, for most inherited neuropathy categories, specific symptomatic management and family counseling remain the mainstays of therapy.

Movement Disorders and Renal Diseases

Movement disorders often emerge from the interplay of complex pathophysiological processes involving the kidneys and the nervous system. Tremor, myoclonus, ataxia, chorea, and parkinsonism can occur in the context of renal dysfunction (azotemia and electrolyte abnormalities) or they can be part of complications of its management (dialysis and renal transplantation). On the other hand, myoglobinuria from rhabdomyolysis in status dystonicus and certain drugs used in the management of movement disorders can cause nephrotoxicity. Distinct from these well-recognized associations, it is important to appreciate that there are several inherited and acquired disorders in which movement abnormalities do not occur as a consequence of renal dysfunction or vice versa but are manifestations of common pathophysiological processes affecting the nervous system and the kidneys. These disorders are the emphasis of this review. Increasing awareness of these conditions among neurologists may help them to identify renal involvement earlier, take timely intervention by anticipating complications and focus on therapies targeting common mechanisms in addition to symptomatic management of movement disorders. Recognition of renal impairment in a patient with complex neurological presentation may narrow down the differentials and aid in reaching a definite diagnosis.