C19orf12 gene mutations in patients with neurodegeneration with brain iron accumulation

Genetic Targets and Applications of Iron Chelators for Neurodegeneration with Brain Iron Accumulation

Neurodegeneration with brain iron accumulation (NBIA) is a group of neurodegenerative diseases that are typically caused by a monogenetic mutation, leading to development of disordered movement symptoms such as dystonia, hyperreflexia, etc. Brain iron accumulation can be diagnosed through MRI imaging and is hypothesized to be the cause of oxidative stress, leading to the degeneration of brain tissue. There are four main types of NBIA: pantothenate kinase-associated neurodegeneration (PKAN), PLA2G6-associated neurodegeneration (PLAN), mitochondrial membrane protein-associated neurodegeneration (MKAN), and beta-propeller protein-associated neurodegeneration (BPAN). There are no causative therapies for these diseases, but iron chelators have been shown to have potential toward treating NBIA. Three chelators are investigated in this Review: deferoxamine (DFO), desferasirox (DFS), and deferiprone (DFP). DFO has been investigated to treat neurodegenerative diseases such as Alzheimer's disease (AD) and Parkinson's disease (PD); however, dose-related toxicity in these studies, as well as in PKAN studies, have shown that the drug still requires more development before it can be applied toward NBIA cases. Iron chelation therapies other than the ones currently in clinical use have not yet reached clinical studies, but they may possess characteristics that would allow them to access the brain in ways that current chelators cannot. Intranasal formulations are an attractive dosage form to study for chelation therapy, as this method of delivery can bypass the blood-brain barrier and access the CNS. Gene therapy differs from iron chelation therapy as it is a causal treatment of the disease, whereas iron chelators only target the disease progression of NBIA. Because the pathophysiology of NBIA diseases is still unclear, future courses of action should be focused on causative treatment; however, iron chelation therapy is the current best course of action.

Autosomal Dominant MPAN: Mosaicism Expands the Clinical Spectrum to Atypical Late-Onset Phenotypes

BACKGROUND

Mitochondrial membrane protein-associated neurodegeneration (MPAN) is caused by mutations in the C19orf12 gene. MPAN typically appears in the first two decades of life and presents with progressive dystonia-parkinsonism, lower motor neuron signs, optic atrophy, and abnormal iron deposits predominantly in the basal ganglia. MPAN, initially considered as a strictly autosomal recessive disease (AR), turned out to be also dominantly inherited (AD).

OBJECTIVES

Our aim was to better characterize the clinical, molecular, and functional spectra associated with such dominant pathogenic heterozygous C19orf12 variants.

METHODS

We collected clinical, imaging, and molecular information of eight individuals from four AD-MPAN families and obtained brain neuropathology results for one. Functional studies, focused on energy and iron metabolism, were conducted on fibroblasts from AD-MPAN patients, AR-MPAN patients, and controls.

RESULTS

We identified four heterozygous C19orf12 variants in eight AD-MPAN patients. Two of them carrying the familial variant in mosaic displayed an atypical late-onset phenotype. Fibroblasts from AD-MPAN showed more severe alterations of iron storage metabolism and autophagy compared to AR-MPAN cells.

CONCLUSION

Our data add strong evidence of the realness of AD-MPAN with identification of novel monoallelic C19orf12 variants, including at the mosaic state. This has implications in diagnosis procedures. We also expand the phenotypic spectrum of MPAN to late onset atypical presentations. Finally, we demonstrate for the first time more drastic abnormalities of iron metabolism and autophagy in AD-MPAN than in AR-MPAN. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

A novel C19ORF12 mutation in two MPAN sisters treated with deferiprone

BACKGROUND

Mitochondrial membrane protein-associated neurodegeneration (MPAN) is a rare and devastating disease caused by pathogenic mutations in C19orf12 gene. MPAN is characterized by pathological iron accumulation in the brain and fewer than 100 cases of MPAN have been described. Although the diagnosis of MPAN has achieved a great breakthrough with the application of the whole exome gene sequencing technology, the therapeutic effect of iron chelation therapy in MPAN remains controversial.

CASE PRESENTATION

We reported that two sisters from the same family diagnosed with MPAN had dramatically different responses to deferiprone (DFP) treatment. The diagnosis of MPAN were established based on typical clinical manifestations, physical examination, brain magnetic resonance imaging (MRI), cerebrospinal fluid analysis (CSF) and gene sequencing results. The clinical presentations of the two sisters with MPAN due to novel gene locus mutations were similar to those previously reported. There is no other difference in basic information except that the proband had a later onset age and fertility history. Both the proband and his second sister were treated with deferiprone (DFP), but they had dramatically different responses to the treatment. The proband's condition deteriorated sharply after treatment with DFP including psychiatric symptoms and movement disorders. However, the second sister of the proband became relatively stable after receiving the DFP treatment. After four years of follow-up, the patient still denies any new symptoms of neurological deficits.

CONCLUSION

The findings of this study enriched the MPAN gene database and indicated that DFP might ameliorate symptom progression in patients without severe autonomic neuropsychiatric impairment at the early stage of the disease.

A human adipose tissue cell-type transcriptome atlas

The importance of defining cell-type-specific genes is well acknowledged. Technological advances facilitate high-resolution sequencing of single cells, but practical challenges remain. Adipose tissue is composed primarily of adipocytes, large buoyant cells requiring extensive, artefact-generating processing for separation and analysis. Thus, adipocyte data are frequently absent from single-cell RNA sequencing (scRNA-seq) datasets, despite being the primary functional cell type. Here, we decipher cell-type-enriched transcriptomes from unfractionated human adipose tissue RNA-seq data. We profile all major constituent cell types, using 527 visceral adipose tissue (VAT) or 646 subcutaneous adipose tissue (SAT) samples, identifying over 2,300 cell-type-enriched transcripts. Sex-subset analysis uncovers a panel of male-only cell-type-enriched genes. By resolving expression profiles of genes differentially expressed between SAT and VAT, we identify mesothelial cells as the primary driver of this variation. This study provides an accessible method to profile cell-type-enriched transcriptomes using bulk RNA-seq, generating a roadmap for adipose tissue biology.

Case Report: Identification of a De novo C19orf12 Variant in a Patient With Mitochondrial Membrane Protein-Associated Neurodegeneration

Background: Mitochondrial membrane protein–associated neurodegeneration (MPAN) mostly arises as an autosomal recessive disease and is caused by variants in the chromosome 19 open reading frame 12 (C19orf12) gene. However, a few C19orf12 monoallelic truncating de novo variants have been reported and segregated as autosomal dominant traits in some cases.

Methods: We performed whole-exome sequencing and analyzed genes related to neurodegeneration associated with brain iron accumulation for pathogenic variants. The identified variants were confirmed by Sanger sequencing and tested using in silico tools.

Results: The patient had an onset of depression at the age of 22 years, which rapidly progressed to severe dystonia, dementia, and bladder and bowel incontinence. Neuroimaging showed hypointensity in the substantia nigra and the globus pallidum, with additional frontotemporal atrophy. Genetic analysis revealed a single complex de novo variant [c.336_338delinsCACA (p.Trp112CysfsTer40)] in the C19orf12 gene.

Conclusion: This study enriches the genetic spectrum and clinical features of C19orf12 variants and provides additional evidence of the variable inheritance pattern of MPAN.

SPG43 and ALS-like syndrome in the same family due to compound heterozygous mutations of the C19orf12 gene: a case description and brief review

C19orf12 gene biallelic mutations lead mainly to neurodegeneration with brain iron accumulation-4. A 15-year-old male and his 17-year-old sister complained of cramps and exercise intolerance. Clinical examination of the boy mainly showed distal amyotrophy and mild weakness, while the sister predominantly had a tetrapyramidal syndrome. Widespread chronic neurogenic signs and hypointense signals on the striatum were present in both patients. Clinical exome sequencing identified, on both patients, the compound heterozygous pathogenic mutations c.204_214del p.(Gly69ArgfsTer10) and c.32C>T p.(Thr11Met). The description of these rare SPG43 and ALS-like phenotypes in the same family contributes to improve genotype-phenotype correlation in C19orf12-related diseases.

Dominant mitochondrial membrane protein-associated neurodegeneration (MPAN) variants cluster within a specific C19orf12 isoform

Mitochondria membrane protein-associated neurodegeneration (MPAN) neurodegenerative disorder is typically associated with biallelic C19orf12 variants. Here we describe a new and review candidate previous monoallelic de novo C19orf12 variants to define loss of function mutations located in the putative non-membrane spanning C19orf12 isoform as the potential basis of monoallelic MPAN.

A Novel Mutation in Neurodegeneration with Brain Iron Accumulation - A Case Report

Neurodegeneration with brain iron accumulation (NBIA), previously called Hallervorden Spatz disease, is a group of disorders which share the hallmark of iron deposition in the brain. They are collectively characterized by extrapyramidal movement disorders, particularly those of parkinsonism, dystonia, cognitive regression, neuropsychiatric abnormalities, pyramidal features, optic atrophy, and retinal abnormalities. There is aberrant brain iron metabolism, with large amounts of iron deposited in the globus pallidus and the substantia nigra pars reticulata. NBIA displays a marked genetic heterogeneity, and 10 genes have been associated with different NBIA subtypes at present. We present a 12-year-old boy with a one and a half-year history of a slow, progressive gait disturbance. An MRI of his brain revealed T2, FLAIR bilateral symmetrical hypointensities in globus pallidus and substantia nigra s/o NBIA. His genetic analysis revealed a novel homozygous missense variation in exon 2 of the C19orf12 gene (chr19:30199203; A>C) that results in the amino acid substitution of valine for phenylalanine at codon 51 (p.F51V; ENST00000392278). This is consistent with the MPAN (mitochondrial membrane protein-associated neurodegeneration) subtype.

Neurodegeneration with brain iron accumulation: Insights into the mitochondria dysregulation

NBIA (Neurodegeneration with brain iron accumulation) is a group of inherited neurologic disorders characterized by marked genetic heterogeneity, in which iron atypical accumulates in basal ganglia resulting in brain magnetic resonance imaging changes, histopathological abnormalities, and neuropsychiatric clinical symptoms. With the rapid development of high-throughput sequencing technologies, ten candidate genes have been identified, including PANK2, PLA2G6, C19orf12, WDR45, FA2H, ATP13A2, FTL, CP, C2orf37, and COASY. They are involved in seemingly unrelated cellular pathways, such as iron homeostasis (FTL, CP), lipid metabolism (PLA2G6, C19orf12, FA2H), Coenzyme A synthesis (PANK2, COASY), and autophagy (WDR45, ATP13A2). In particular, PANK2, COASY, PLA2G6, and C19orf12 are located on mitochondria, which associate with certain subtypes of NBIA showing mitochondria dysregulation. However, the relationships among those four genes are still unclear. Therefore, this review is specifically focused on dysregulation of mitochondria in NBIA and afore-mentioned four genes, with summaries of both pathological and clinical findings.

Autosomal dominant mitochondrial membrane protein-associated neurodegeneration (MPAN)

BACKGROUND

Mitochondrial membrane protein-associated neurodegeneration (MPAN) is caused by pathogenic sequence variants in C19orf12. Autosomal recessive inheritance has been demonstrated. We present evidence of autosomal dominant MPAN and propose a mechanism to explain these cases.

METHODS

Two large families with apparently dominant MPAN were investigated; additional singleton cases of MPAN were identified. Gene sequencing and multiplex ligation-dependent probe amplification were used to characterize the causative sequence variants in C19orf12. Post-mortem brain from affected subjects was examined.

RESULTS

In two multi-generation non-consanguineous families, we identified different nonsense sequence variations in C19orf12 that segregate with the MPAN phenotype. Brain pathology was similar to that of autosomal recessive MPAN. We additionally identified a preponderance of cases with single heterozygous pathogenic sequence variants, including two with de novo changes.

CONCLUSIONS

We present three lines of clinical evidence to demonstrate that MPAN can manifest as a result of only one pathogenic C19orf12 sequence variant. We propose that truncated C19orf12 proteins, resulting from nonsense variants in the final exon in our autosomal dominant cohort, impair function of the normal protein produced from the non-mutated allele via a dominant negative mechanism and cause loss of function. These findings impact the clinical diagnostic evaluation and counseling.

Mitochondrial membrane protein-associated neurodegeneration: a case report and literature review

Mitochondrial membrane protein-associated neurodegeneration (MPAN) is an autosomal recessive disorder caused by mutation in the C19orf12 gene. We report a compound heterozygous c.[32C>T];[205G>A;424A>G] (p.[Thr11Met];[Gly69Arg;Lys142Glu]) Czech patient who manifested with right foot dystonia, impaired handwriting, attention deficit, and signs of iron accumulation on brain MRI. Gradually, he developed dysarthria, spastic-dystonic gait, pedes cavi, and atrophy of leg muscles. Additionally, we report demographic parameters, clinical signs, and allelic frequencies of C19orf12 mutations of all published MPAN cases. We compared the most frequent mutations, p.Thr11Met and p.Gly69ArgfsX10; the latter was associated with younger age at onset and more frequent optic atrophy in homozygotes.

The p.Thr11Met mutation in c19orf12 is frequent among adult Turkish patients with MPAN

INTRODUCTION

Mutations in the C19orf12 gene cause mitochondrial membrane protein associated neurodegeneration (MPAN), an autosomal recessive form of neurodegeneration with brain iron accumulation (NBIA). A limited number of patients with C19orf12 mutations, particularly those with adult onset of symptoms, have been reported.

METHODS

We sequenced the entire coding region of C19orf12 in 15 Turkish adult probands with idiopathic NBIA. We also performed haplotype analysis in families with a recurrent C19orf12 mutation. Clinical features were collected using a standardized form.

RESULTS

Nine of our 15 probands (60%) carried the homozygous c.32C > T mutation in C19orf12 (predicted protein effect: p.Thr11Met). This homozygous mutation co-segregated with the disease in all affected relatives available for testing (16 homozygous subjects). Haplotypes across the C19orf12 locus were identical for a very small region, closest to the mutation, suggesting an old founder, or, two independent founders. The clinical phenotype was characterized by adult onset in most cases (mean 24.5 years, range 10-36), and broad spectrum, including prominent parkinsonism, pyramidal signs, psychiatric disturbances, cognitive decline, and motor axonal neuropathy, in various combinations. On T2- or susceptibility weighted-MRI images, all patients displayed bilateral hypointensities in globus pallidus and substantia nigra, without an eye-of-the-tiger sign; however, hyperintense streaking of the medial medullary lamina between the external and internal parts of globus pallidus was observed frequently.

CONCLUSION

The C19orf12 p.Thr11Met mutation is frequent among adult Turkish patients with MPAN. These findings contribute to the characterization of this important NBIA form, and have direct implications for genetic testing of patients of Turkish origin.

Mitochondrial Membrane Protein-Associated Neurodegeneration Mimicking Juvenile Amyotrophic Lateral Sclerosis

BACKGROUND

Mitochondrial membrane protein associated neurodegeneration (MPAN) is the third most common subtype of neurodegeneration with brain iron accumulation (NBIA) and caused by mutations of the orphan gene C19ORF12 encoding a transmembrane mitochondrial protein. Like other NBIA disorders, the hallmark of neuropathology is iron deposition in the basal ganglia, but the clinical presentation is highly variable.

METHODS

We present the relevant clinical history, neurological examination, electrophysiological and neuroimaging tests of a currently ten-year-old girl. The genetic analysis was carried out by exome sequencing focused on known NBIA and juvenile amyotrophic lateral sclerosis (ALS) genes.

RESULTS

The patient presented at four years of age with progressive lower extremity weakness and generalized hypotonia. She was initially diagnosed with juvenile ALS based on clinical signs, negative brain magnetic resonance imaging (MRI) and electromyography findings. As the disease progressed, a repeat brain MRI showed iron deposition in the basal ganglia at nine years of age. Exome sequencing of genes known to be associated with NBIA revealed a compound heterozygous mutation of C19ORF12 gene.

CONCLUSIONS

A C19orf12 gene mutation should be considered in young children with clinical signs of progressive upper and lower motor neuron disease. Finding iron accumulation in the basal ganglia helps to focus the genetic testing, but it may not be apparent for several years.

Clinical and imaging characteristics of late onset mitochondrial membrane protein-associated neurodegeneration (MPAN)

Young onset dementias present significant diagnostic challenges. We present the case of a 35-year-old Kuwaiti man with social withdrawal, drowsiness, irritability, anxiety, aphasia, memory loss, hypereflexia, and Parkinsonism. Brain MRI showed bilateral symmetric gradient echo hypointensities in the globi pallidi and substantiae nigrae. Left cortical hypometabolism was seen on brain fluorodeoxyglucose positron emission tomography. A cortical brain biopsy revealed a high Lewy body burden. Genetic testing revealed a homozygous p.T11M mutation in the C19orf12 gene consistent with mitochondrial membrane protein-associated neurodegeneration. This is the oldest onset age of MPAN reported.

Mitochondrial dysfunction and defects in lipid homeostasis as therapeutic targets in neurodegeneration with brain iron accumulation

The PLA2G6 gene encodes a group VIA calcium independent phospholipase A2 (iPLA2β), which hydrolyses glycerophospholipids to release fatty acids and lysophospholipids. Mutations in PLA2G6 are associated with a number of neurodegenerative disorders including neurodegeneration with brain iron accumulation (NBIA), infantile neuroaxonal dystrophy (INAD), and dystonia parkinsonism, collectively known as PLA2G6-associated neurodegeneration (PLAN). Recently Kinghorn et al. demonstrated in Drosophila and PLA2G6 mutant fibroblasts that loss of normal PLA2G6 activity is associated with mitochondrial dysfunction and mitochondrial lipid peroxidation. Furthermore, they were able to show the beneficial effects of deuterated polyunsaturated fatty acids (D-PUFAs), which reduce lipid peroxidation. D-PUFAs were able to rescue the locomotor deficits of flies lacking the fly ortholog of PLA2G6 (iPLA2-VIA), as well as the mitochondrial abnormalities in PLA2G6 mutant fibroblasts. This work demonstrated that the iPLA2-VIA knockout fly is a useful organism to dissect the mechanisms of pathogenesis of PLAN, and that further investigation is required to determine the therapeutic potential of D-PUFAs in patients with PLA2G6 mutations. The fruit fly has also been used to study some of the other genetic causes of NBIA, and here we also describe what is known about the mechanisms of pathogenesis of these NBIA variants. Mitochondrial dysfunction, defects in lipid metabolism, as well as defective Coenzyme A (CoA) biosynthesis, have all been implicated in some genetic forms of NBIA, including PANK2, CoASY, C12orf19 and FA2H.