Ferrous Iron Up-regulation in Fibroblasts of Patients with Beta Propeller Protein-Associated Neurodegeneration (BPAN)

WDR45-dependent impairment of cell cycle in fibroblasts of patients with beta propeller protein-associated neurodegeneration (BPAN)

De novo mutations in the WDR45 gene have been found in patients affected by Neurodegeneration with Brain Iron Accumulation type 5 (NBIA5 or BPAN), with Non-Transferrin Bound Iron (NTBI) accumulation in the basal ganglia and WDR45-dependent impairment of autophagy. Here we show the downregulation of TFEB and cell cycle impairment in BPAN primary fibroblasts. Noteworthy, TFEB overexpression rescued this impairment, depicting a novel WDR45-dependent cell cycle phenotype.

Targeting selective autophagy in CNS disorders by small-molecule compounds

Autophagy functions as the primary cellular mechanism for clearing unwanted intracellular contents. Emerging evidence suggests that the selective elimination of intracellular organelles through autophagy, compared to the increased bulk autophagic flux, is crucial for the pathological progression of central nervous system (CNS) disorders. Notably, autophagic removal of mitochondria, known as mitophagy, is well-understood in an unhealthy brain. Accumulated data indicate that selective autophagy of other substrates, including protein aggregates, liposomes, and endoplasmic reticulum, plays distinctive roles in various pathological stages. Despite variations in substrates, the molecular mechanisms governing selective autophagy can be broadly categorized into two types: ubiquitin-dependent and -independent pathways, both of which can be subjected to regulation by small-molecule compounds. Notably, natural products provide the remarkable possibility for future structural optimization to regulate the highly selective autophagic clearance of diverse substrates. In this context, we emphasize the selectivity of autophagy in regulating CNS disorders and provide an overview of chemical compounds capable of modulating selective autophagy in these disorders, along with the underlying mechanisms. Further exploration of the functions of these compounds will in turn advance our understanding of autophagic contributions to brain disorders and illuminate precise therapeutic strategies for these diseases.

Metabolic impairments in neurodegeneration with brain iron accumulation

Neurodegeneration with brain iron accumulation (NBIA) is a broad, heterogeneous group of rare inherited diseases (1-3 patients/1,000,000 people) characterized by progressive symptoms associated with excessive abnormal iron deposition in the brain. Approximately 15,000-20,000 individuals worldwide are estimated to be affected by NBIA. NBIA is usually associated with slowly progressive pyramidal and extrapyramidal symptoms, axonal motor neuropathy, optic nerve atrophy, cognitive impairment and neuropsychiatric disorders. To date, eleven subtypes of NBIA have been described and the most common ones include pantothenate kinase-associated neurodegeneration (PKAN), PLA2G6-associated neurodegeneration (PLAN), mitochondrial membrane protein-associated neurodegeneration (MPAN) and beta-propeller protein-associated neurodegeneration (BPAN). We present a comprehensive overview of the evidence for disturbed cellular homeostasis and metabolic alterations in NBIA variants, with a careful focus on mitochondrial bioenergetics and lipid metabolism which drives a new perspective in understanding the course of this infrequent malady.

Ischemic Neuroprotection by Insulin with Down-Regulation of Divalent Metal Transporter 1 (DMT1) Expression and Ferrous Iron-Dependent Cell Death

Background: The regulation of divalent metal transporter-1 (DMT1) by insulin has been previously described in Langerhans cells and significant neuroprotection was found by insulin and insulin-like growth factor 1 treatment during experimental cerebral ischemia in acute ischemic stroke patients and in a rat 6-OHDA model of Parkinson's disease, where DMT1 involvement is described. According to the regulation of DMT1, previously described as a target gene of NF-kB in the early phase of post-ischemic neurodegeneration, both in vitro and in vivo, and because insulin controls the NFkB signaling with protection from ischemic cell death in rat cardiomyocytes, we evaluated the role of insulin in relation to DMT1 expression and function during ischemic neurodegeneration. Methods: Insulin neuroprotection is evaluated in differentiated human neuroblastoma cells, SK-N-SH, and in primary mouse cortical neurons exposed to oxygen glucose deprivation (OGD) for 8 h or 3 h, respectively, with or without 300 nM insulin. The insulin neuroprotection during OGD was evaluated in both cellular models in terms of cell death, and in SK-N-SH for DMT1 protein expression and acute ferrous iron treatment, performed in acidic conditions, known to promote the maximum DMT1 uptake as a proton co-transporter; and the transactivation of 1B/DMT1 mouse promoter, already known to be responsive to NF-kB, was analyzed in primary mouse cortical neurons. Results: Insulin neuroprotection during OGD was concomitant to the down-regulation of both DMT1 protein expression and 1B/DMT1 mouse promoter transactivation. We also showed the insulin-dependent protection from cell death after acute ferrous iron treatment. In conclusion, although preliminary, this evaluation highlights the peculiar role of DMT1 as a possible pharmacological target, involved in neuroprotection by insulin during in vitro neuronal ischemia and acute ferrous iron uptake.

Human WIPI β-propeller function in autophagy and neurodegeneration

The four human WIPI β-propellers, WIPI1 through WIPI4, belong to the ancient PROPPIN family and fulfill scaffold functions in the control of autophagy. In this context, WIPI β-propellers function as PI3P effectors during autophagosome formation and loss of WIPI function negatively impacts autophagy and contributes to neurodegeneration. Of particular interest are mutations in WDR45, the human gene that encodes WIPI4. Sporadic WDR45 mutations are the cause of a rare human neurodegenerative disease called BPAN, hallmarked by high brain iron accumulation. Here, we discuss the current understanding of the functions of human WIPI β-propellers and address unanswered questions with a particular focus on the role of WIPI4 in autophagy and BPAN.

Antioxidants Prevent Iron Accumulation and Lipid Peroxidation, but Do Not Correct Autophagy Dysfunction or Mitochondrial Bioenergetics in Cellular Models of BPAN

Neurodegeneration with brain iron accumulation (NBIA) is a group of rare neurogenetic disorders frequently associated with iron accumulation in the basal nuclei of the brain. Among NBIA subtypes, β-propeller protein-associated neurodegeneration (BPAN) is associated with mutations in the autophagy gene WDR45. The aim of this study was to demonstrate the autophagic defects and secondary pathological consequences in cellular models derived from two patients harboring WDR45 mutations. Both protein and mRNA expression levels of WDR45 were decreased in patient-derived fibroblasts. In addition, the increase of LC3B upon treatments with autophagy inducers or inhibitors was lower in mutant cells compared to control cells, suggesting decreased autophagosome formation and impaired autophagic flux. A transmission electron microscopy (TEM) analysis showed mitochondrial vacuolization associated with the accumulation of lipofuscin-like aggregates containing undegraded material. Autophagy dysregulation was also associated with iron accumulation and lipid peroxidation. In addition, mutant fibroblasts showed altered mitochondrial bioenergetics. Antioxidants such as pantothenate, vitamin E and α-lipoic prevented lipid peroxidation and iron accumulation. However, antioxidants were not able to correct the expression levels of WDR45, neither the autophagy defect nor cell bioenergetics. Our study demonstrated that WDR45 mutations in BPAN cellular models impaired autophagy, iron metabolism and cell bioenergetics. Antioxidants partially improved cell physiopathology; however, autophagy and cell bioenergetics remained affected.

WDR45 mutation dysregulates iron homeostasis by promoting the chaperone-mediated autophagic degradation of ferritin heavy chain in an ER stress/p38 dependent mechanism

Ferritin is the main iron storage protein that plays a pivotal role in the regulation of iron homeostasis. Mutations in the autophagy protein WD repeat domain 45 (WDR45) that lead to iron overload is associated with the human β-propeller protein-associated neurodegeneration (BPAN). Previous studies have demonstrated that ferritin was decreased in WDR45 deficient cells, but the mechanism remains unclear. In this study, we have demonstrated that the ferritin heavy chain (FTH) could be degraded via chaperone-mediated autophagy (CMA) in ER stress/p38-dependent pathway. In HeLa cells, inducing the ER stress activated CMA, therefore facilitated the degradation of FTH, and increased the content of Fe²⁺. However, the increased CMA activity and Fe²⁺ as well as the decreased FTH by ER stress inducer were restored by pre-treatment with p38 inhibitor. Overexpression of a mutant WDR45 activated CMA thus promoted the degradation of FTH. Furthermore, inhibition of ER stress/p38 pathway resulted in reduced activity of CMA, which consequently elevated the protein level of FTH but reduced the Fe²⁺ level. Our results revealed that WDR45 mutation dysregulates iron homeostasis by activating CMA, and promotes FTH degradation through ER stress/p38 signaling pathway.

Cell-Specific Dysregulation of Iron and Oxygen Homeostasis as a Novel Pathophysiology in PSP

OBJECTIVE

Progressive supranuclear palsy (PSP) is a 4R-tauopathy showing heterogeneous tau cytopathology commencing in the globus pallidus (GP) and the substantia nigra (SN), regions also associated with age-related iron accumulation. Abnormal iron levels have been extensively associated with tau pathology in neurodegenerative brains, however, its role in PSP pathogenesis remains yet unknown. We perform the first cell type-specific evaluation of PSP iron homeostasis and the closely related oxygen homeostasis, in relation to tau pathology in human postmortem PSP brains.

METHODS

In brain regions vulnerable to PSP pathology (GP, SN, and putamen), we visualized iron deposition in tau-affected and unaffected neurons, astroglia, oligodendrocytes, and microglia, using a combination of iron staining with immunolabelling. To further explore molecular pathways underlying our observations, we examined the expression of key iron and oxygen homeostasis mRNA transcripts and proteins.

RESULTS

We found astrocytes as the major cell type accumulating iron in the early affected regions of PSP, highly associated with cellular tau pathology. The same regions are affected by dysregulated expression of alpha and beta hemoglobin and neuroglobin showing contrasting patterns. We discovered changes in iron and oxygen homeostasis-related gene expression associated with aging of the brain, and identified dysregulated expression of rare neurodegeneration with brain iron accumulation (NBIA) genes associated with tau pathology to distinguish PSP from the healthy aging brain.

INTERPRETATION

We present novel aspects of PSP pathophysiology highlighting an overlap with NBIA pathways. Our findings reveal potential novel targets for therapy development and have implications beyond PSP for other iron-associated neurodegenerative diseases. ANN NEUROL 2023;93:431-445.

WDR45 variants cause ferrous iron loss due to impaired ferritinophagy associated with nuclear receptor coactivator 4 and WD repeat domain phosphoinositide interacting protein 4 reduction

Static encephalopathy of childhood with neurodegeneration in adulthood/β-propeller protein-associated neurodegeneration is a neurodegenerative disorder with brain iron accumulation caused by the variants of WDR45, a core autophagy-related gene that encodes WD repeat domain phosphoinositide interacting protein 4. However, the pathophysiology of the disease, particularly the function of WDR45/WD repeat domain phosphoinositide interacting protein 4 in iron metabolism, is largely unknown. As no other variants of core autophagy-related genes show abnormalities in iron metabolism, the relation between autophagy and iron metabolism remains to be elucidated. Since iron deposition in the brain is the hallmark of static encephalopathy of childhood with neurodegeneration in adulthood/β-propeller protein-associated neurodegeneration, iron chelation therapy has been attempted, but it was found to worsen the symptoms; thus, the establishment of a curative treatment is essential. Here, we evaluated autophagy and iron metabolism in patient-derived cells. The expression of ferritin and ferric iron increased and that of ferrous iron decreased in the patient cells with WDR45 variants. In addition, the expression of nuclear receptor coactivator 4 was markedly reduced in patient-derived cells. Furthermore, divalent metal transporter 1, which takes in ferrous iron, was upregulated, while ferroportin, which exports ferrous iron, was downregulated in patient-derived cells. The transfer of WDR45 via an adeno-associated virus vector restored WD repeat domain phosphoinositide interacting protein 4 and nuclear receptor coactivator 4 expression, reduced ferritin levels, and improved other phenotypes observed in patient-derived cells. As nuclear receptor coactivator 4 mediates the ferritin-specific autophagy, i.e. ferritinophagy, its deficiency impaired ferritinophagy, leading to the accumulation of ferric iron-containing ferritin and insufficiency of ferrous iron. Because ferrous iron is required for various essential biochemical reactions, the changes in divalent metal transporter 1 and ferroportin levels may indicate a compensatory response for maintaining the intracellular levels of ferrous iron. Our study revealed that the pathophysiology of static encephalopathy of childhood with neurodegeneration in adulthood/β-propeller protein-associated neurodegeneration involves ferrous iron insufficiency via impaired ferritinophagy through nuclear receptor coactivator 4 expression reduction. Our findings could aid in developing a treatment strategy involving WDR45 manipulation, which may have clinical applications.

WIPI proteins: Biological functions and related syndromes

WIPI (WD-repeat protein Interacting with PhosphoInositides) are important effectors in autophagy. These proteins bind phosphoinositides and recruit autophagy proteins. In mammals, there are four WIPI proteins: WIPI1, WIPI2, WIPI3 (WDR45B), and WIPI4 (WDR45). These proteins consist of a seven-bladed β-propeller structure. Recently, pathogenic variants in genes encoding these proteins have been recognized to cause human diseases with a predominant neurological phenotype. Defects in WIPI2 cause a disease characterized mainly by intellectual disability and variable other features while pathogenic variants in WDR45B and WDR45 have been recently reported to cause El-Hattab-Alkuraya syndrome and beta-propeller protein-associated neurodegeneration (BPAN), respectively. Whereas, there is no disease linked to WIPI1 yet, one study linked it neural tube defects (NTD). In this review, the role of WIPI proteins in autophagy is discussed first, then syndromes related to these proteins are summarized.

Cerebral Iron Deposition in Neurodegeneration

Disruption of cerebral iron regulation appears to have a role in aging and in the pathogenesis of various neurodegenerative disorders. Possible unfavorable impacts of iron accumulation include reactive oxygen species generation, induction of ferroptosis, and acceleration of inflammatory changes. Whole-brain iron-sensitive magnetic resonance imaging (MRI) techniques allow the examination of macroscopic patterns of brain iron deposits in vivo, while modern analytical methods ex vivo enable the determination of metal-specific content inside individual cell-types, sometimes also within specific cellular compartments. The present review summarizes the whole brain, cellular, and subcellular patterns of iron accumulation in neurodegenerative diseases of genetic and sporadic origin. We also provide an update on mechanisms, biomarkers, and effects of brain iron accumulation in these disorders, focusing on recent publications. In Parkinson’s disease, Friedreich’s disease, and several disorders within the neurodegeneration with brain iron accumulation group, there is a focal siderosis, typically in regions with the most pronounced neuropathological changes. The second group of disorders including multiple sclerosis, Alzheimer’s disease, and amyotrophic lateral sclerosis shows iron accumulation in the globus pallidus, caudate, and putamen, and in specific cortical regions. Yet, other disorders such as aceruloplasminemia, neuroferritinopathy, or Wilson disease manifest with diffuse iron accumulation in the deep gray matter in a pattern comparable to or even more extensive than that observed during normal aging. On the microscopic level, brain iron deposits are present mostly in dystrophic microglia variably accompanied by iron-laden macrophages and in astrocytes, implicating a role of inflammatory changes and blood–brain barrier disturbance in iron accumulation. Options and potential benefits of iron reducing strategies in neurodegeneration are discussed. Future research investigating whether genetic predispositions play a role in brain Fe accumulation is necessary. If confirmed, the prevention of further brain Fe uptake in individuals at risk may be key for preventing neurodegenerative disorders.

Vitamin E prevents lipid peroxidation and iron accumulation in PLA2G6-Associated Neurodegeneration

BACKGROUND

PLA2G6-Associated Neurodegeneration (PLAN) is a rare neurodegenerative disease with autosomal recessive inheritance, which belongs to the NBIA (Neurodegeneration with Brain Iron Accumulation) group. Although the pathogenesis of the disease remains largely unclear, lipid peroxidation seems to play a central role in the pathogenesis. Currently, there is no cure for the disease.

OBJECTIVE

In this work, we examined the presence of lipid peroxidation, iron accumulation and mitochondrial dysfunction in two cellular models of PLAN, patients-derived fibroblasts and induced neurons, and assessed the effects of α-tocopherol (vitamin E) in correcting the pathophysiological alterations in PLAN cell cultures.

METHODS

Pathophysiological alterations were examined in fibroblasts and induced neurons generated by direct reprograming. Iron and lipofuscin accumulation were assessed using light and electron microscopy, as well as biochemical analysis techniques. Reactive Oxygen species production, lipid peroxidation and mitochondrial dysfunction were measured using specific fluorescent probes analysed by fluorescence microscopy and flow cytometry.

RESULTS

PLAN fibroblasts and induced neurons clearly showed increased lipid peroxidation, iron accumulation and altered mitochondrial membrane potential. All these pathological features were reverted with vitamin E treatment.

CONCLUSIONS

PLAN fibroblasts and induced neurons reproduce the main pathological alterations of the disease and provide useful tools for disease modelling. The main pathological alterations were corrected by Vitamin E supplementation in both models, suggesting that blocking lipid peroxidation progression is a critical therapeutic target.

Consensus clinical management guideline for beta-propeller protein-associated neurodegeneration

This review provides recommendations for the evaluation and management of individuals with beta-propeller protein-associated neurodegeneration (BPAN). BPAN is one of several neurodegenerative disorders with brain iron accumulation along with pantothenate kinase-associated neurodegeneration, PLA2G6-associated neurodegeneration, mitochondrial membrane protein-associated neurodegeneration, fatty acid hydroxylase-associated neurodegeneration, and COASY protein-associated neurodegeneration. BPAN typically presents with global developmental delay and epilepsy in childhood, which is followed by the onset of dystonia and parkinsonism in mid-adolescence or adulthood. BPAN is an X-linked dominant disorder caused by pathogenic variants in WDR45, resulting in a broad clinical phenotype and imaging spectrum. This review, informed by an evaluation of the literature and expert opinion, discusses the clinical phenotype and progression of the disease, imaging findings, epilepsy features, and genetics, and proposes an approach to the initial evaluation and management of disease manifestations across the life span in individuals with BPAN. What this paper adds The complex epilepsy profile of beta-propeller protein-associated neurodegeneration (BPAN) often resolves in adolescence. The treatment for an individual with BPAN is supportive, with attention to sleep disorders, complex epilepsy, and behavioral problems. Individuals with BPAN have shifting needs throughout their life span requiring multidisciplinary care.

A neurodegeneration gene, WDR45, links impaired ferritinophagy to iron accumulation

Neurodegeneration with brain iron accumulation (NBIA) is a clinically and genetically heterogeneous group of neurodegenerative diseases characterized by the abnormal accumulation of brain iron and the progressive degeneration of the nervous system. One of the recently identified subtypes of NBIA is β‐propeller protein‐associated neurodegeneration (BPAN). BPAN is caused by de novo mutations in the WDR45/WIPI4 (WD repeat domain 45) gene. WDR45 is one of the four mammalian homologs of yeast Atg18, a regulator of autophagy. WDR45 deficiency in BPAN patients and animal models may result in defects in autophagic flux. However, how WDR45 deficiency leads to brain iron overload remains unclear. To elucidate the role of WDR45, we generated a WDR45‐knockout (KO) SH‐SY5Y neuroblastoma cell line using CRISPR‐Cas9‐mediated genome editing. Using these cells, we demonstrated that the non‐TF (transferrin)‐bound iron pathway dominantly mediated the accumulation of iron. Moreover, the loss of WDR45 led to defects in ferritinophagy, a form of autophagy that degrades the iron storage protein ferritin. We showed that impaired ferritinophagy contributes to iron accumulation in WDR45‐KO cells. Iron accumulation was also detected in the mitochondria, which was accompanied by impaired mitochondrial respiration, elevated reactive oxygen species, and increased cell death. Thus, our study links WDR45 to specific iron acquisition pathways and ferritinophagy.

WDR45, one gene associated with multiple neurodevelopmental disorders

The WDR45 gene is localized on the X-chromosome and variants in this gene are linked to six different neurodegenerative disorders, i.e., ß-propeller protein associated neurodegeneration, Rett-like syndrome, intellectual disability, and epileptic encephalopathies including developmental and epileptic encephalopathy, early-onset epileptic encephalopathy and West syndrome and potentially also specific malignancies. WDR45/WIPI4 is a WD-repeat β-propeller protein that belongs to the WIPI (WD repeat domain, phosphoinositide interacting) family. The precise cellular function of WDR45 is still largely unknown, but deletions or conventional variants in WDR45 can lead to macroautophagy/autophagy defects, malfunctioning mitochondria, endoplasmic reticulum stress and unbalanced iron homeostasis, suggesting that this protein functions in one or more pathways regulating directly or indirectly those processes. As a result, the underlying cause of the WDR45-associated disorders remains unknown. In this review, we summarize the current knowledge about the cellular and physiological functions of WDR45 and highlight how genetic variants in its encoding gene may contribute to the pathophysiology of the associated diseases. In particular, we connect clinical manifestations of the disorders with their potential cellular origin of malfunctioning and critically discuss whether it is possible that one of the most prominent shared features, i.e., brain iron accumulation, is the primary cause for those disorders.

Abbreviations: ATG/Atg: autophagy related; BPAN: ß-propeller protein associated neurodegeneration; CNS: central nervous system; DEE: developmental and epileptic encephalopathy; EEG: electroencephalograph; ENO2/neuron-specific enolase, enolase 2; EOEE: early-onset epileptic encephalopathy; ER: endoplasmic reticulum; ID: intellectual disability; IDR: intrinsically disordered region; MRI: magnetic resonance imaging; NBIA: neurodegeneration with brain iron accumulation; NCOA4: nuclear receptor coactivator 4; PtdIns3P: phosphatidylinositol-3-phosphate; RLS: Rett-like syndrome; WDR45: WD repeat domain 45; WIPI: WD repeat domain, phosphoinositide interacting

Mitochondrial Dysfunction, Oxidative Stress and Neuroinflammation in Neurodegeneration with Brain Iron Accumulation (NBIA)

The syndromes of neurodegeneration with brain iron accumulation (NBIA) encompass a group of invalidating and progressive rare diseases that share the abnormal accumulation of iron in the basal ganglia. The onset of NBIA disorders ranges from infancy to adulthood. Main clinical signs are related to extrapyramidal features (dystonia, parkinsonism and choreoathetosis), and neuropsychiatric abnormalities. Ten NBIA forms are widely accepted to be caused by mutations in the genes PANK2, PLA2G6, WDR45, C19ORF12, FA2H, ATP13A2, COASY, FTL1, CP, and DCAF17. Nonetheless, many patients remain without a conclusive genetic diagnosis, which shows that there must be additional as yet undiscovered NBIA genes. In line with this, isolated cases of known monogenic disorders, and also, new genetic diseases, which present with abnormal brain iron phenotypes compatible with NBIA, have been described. Several pathways are involved in NBIA syndromes: iron and lipid metabolism, mitochondrial dynamics, and autophagy. However, many neurodegenerative conditions share features such as mitochondrial dysfunction and oxidative stress, given the bioenergetics requirements of neurons. This review aims to describe the existing link between the classical ten NBIA forms by examining their connection with mitochondrial impairment as well as oxidative stress and neuroinflammation.

DMT1 Expression and Iron Levels at the Crossroads Between Aging and Neurodegeneration

Iron homeostasis is an essential prerequisite for metabolic and neurological functions throughout the healthy human life, with a dynamic interplay between intracellular and systemic iron metabolism. The development of different neurodegenerative diseases is associated with alterations of the intracellular transport of iron and heavy metals, principally mediated by Divalent Metal Transporter 1 (DMT1), responsible for Non-Transferrin Bound Iron transport (NTBI). In addition, DMT1 regulation and its compartmentalization in specific brain regions play important roles during aging. This review highlights the contribution of DMT1 to the physiological exchange and distribution of body iron and heavy metals during aging and neurodegenerative diseases. DMT1 also mediates the crosstalk between central nervous system and peripheral tissues, by systemic diffusion through the Blood Brain Barrier (BBB), with the involvement of peripheral iron homeostasis in association with inflammation. In conclusion, a survey about the role of DMT1 and iron will illustrate the complex panel of interrelationship with aging, neurodegeneration and neuroinflammation.

Neurodegeneration with Brain Iron Accumulation Disorders: Valuable Models Aimed at Understanding the Pathogenesis of Iron Deposition

Neurodegeneration with brain iron accumulation (NBIA) is a set of neurodegenerative disorders, which includes very rare monogenetic diseases. They are heterogeneous in regard to the onset and the clinical symptoms, while the have in common a specific brain iron deposition in the region of the basal ganglia that can be visualized by radiological and histopathological examinations. Nowadays, 15 genes have been identified as causative for NBIA, of which only two code for iron-proteins, while all the other causative genes codify for proteins not involved in iron management. Thus, how iron participates to the pathogenetic mechanism of most NBIA remains unclear, essentially for the lack of experimental models that fully recapitulate the human phenotype. In this review we reported the recent data on new models of these disorders aimed at highlight the still scarce knowledge of the pathogenesis of iron deposition.

Iron overload is accompanied by mitochondrial and lysosomal dysfunction in WDR45 mutant cells

Beta-propeller protein-associated neurodegeneration is a subtype of monogenic neurodegeneration with brain iron accumulation caused by de novo mutations in WDR45. The WDR45 protein functions as a beta-propeller scaffold and plays a putative role in autophagy through its interaction with phospholipids and autophagy-related proteins. Loss of WDR45 function due to disease-causing mutations has been linked to defects in autophagic flux in patient and animal cells. However, the role of WDR45 in iron homeostasis remains elusive. Here we studied patient-specific WDR45 mutant fibroblasts and induced pluripotent stem cell-derived midbrain neurons. Our data demonstrated that loss of WDR45 increased cellular iron levels and oxidative stress, accompanied by mitochondrial abnormalities, autophagic defects, and diminished lysosomal function. Restoring WDR45 levels partially rescued oxidative stress and the susceptibility to iron treatment, and activation of autophagy reduced the observed iron overload in WDR45 mutant cells. Our data suggest that iron-containing macromolecules and organelles cannot effectively be degraded through the lysosomal pathway due to loss of WDR45 function.

Pantothenate Rescues Iron Accumulation in Pantothenate Kinase-Associated Neurodegeneration Depending on the Type of Mutation

Neurodegeneration with brain iron accumulation (NBIA) is a group of inherited neurologic disorders in which iron accumulates in the basal ganglia resulting in progressive dystonia, spasticity, parkinsonism, neuropsychiatric abnormalities, and optic atrophy or retinal degeneration. The most prevalent form of NBIA is pantothenate kinase-associated neurodegeneration (PKAN) associated with mutations in the gene of pantothenate kinase 2 (PANK2), which is essential for coenzyme A (CoA) synthesis. There is no cure for NBIA nor is there a standard course of treatment. In the current work, we describe that fibroblasts derived from patients harbouring PANK2 mutations can reproduce many of the cellular pathological alterations found in the disease, such as intracellular iron and lipofuscin accumulation, increased oxidative stress, and mitochondrial dysfunction. Furthermore, mutant fibroblasts showed a characteristic senescent morphology. Treatment with pantothenate, the PANK2 enzyme substrate, was able to correct all pathological alterations in responder mutant fibroblasts with residual PANK2 enzyme expression. However, pantothenate had no effect on mutant fibroblasts with truncated/incomplete protein expression. The positive effect of pantothenate in particular mutations was also confirmed in induced neurons obtained by direct reprograming of mutant fibroblasts. Our results suggest that pantothenate treatment can stabilize the expression levels of PANK2 in selected mutations. These results encourage us to propose our screening model as a quick and easy way to detect pantothenate-responder patients with PANK2 mutations. The existence of residual enzyme expression in some affected individuals raises the possibility of treatment using high dose of pantothenate.

Classification and molecular pathogenesis of NBIA syndromes

Brain iron accumulation is the hallmark of a group of seriously invalidating and progressive rare diseases collectively denominated Neurodegeneration with Brain Iron Accumulation (NBIA), characterized by movement disorder, painful dystonia, parkinsonism, mental disability and early death. Currently there is no established therapy available to slow down or reverse the progression of these conditions. Several genes have been identified as responsible for NBIA but only two encode for proteins playing a direct role in iron metabolism. The other genes encode for proteins either with various functions in lipid metabolism, lysosomal activity and autophagic processes or with still unknown roles. The different NBIA subtypes have been classified and denominated on the basis of the mutated genes and, despite genetic heterogeneity, some of them code for proteins, which share or converge on common metabolic pathways. In the last ten years, the implementation of genetic screening based on Whole Exome Sequencing has greatly accelerated gene discovery, nevertheless our knowledge of the pathogenic mechanisms underlying the NBIA syndromes is still largely incomplete.