A novel mutation in SLC39A14 causing hypermanganesemia associated with infantile onset dystonia

Membrane Transporters Involved in Iron Trafficking: Physiological and Pathological Aspects

Iron is an essential transition metal for its involvement in several crucial biological functions, the most notable being oxygen storage and transport. Due to its high reactivity and potential toxicity, intracellular and extracellular iron levels must be tightly regulated. This is achieved through transport systems that mediate cellular uptake and efflux both at the level of the plasma membrane and on the membranes of lysosomes, endosomes and mitochondria. Among these transport systems, the key players are ferroportin, the only known transporter mediating iron efflux from cells; DMT1, ZIP8 and ZIP14, which on the contrary, mediate iron influx into the cytoplasm, acting on the plasma membrane and on the membranes of lysosomes and endosomes; and mitoferrin, involved in iron transport into the mitochondria for heme synthesis and Fe-S cluster assembly. The focus of this review is to provide an updated view of the physiological role of these membrane proteins and of the pathologies that arise from defects of these transport systems.

A simple and highly reproducible method for the detection of erythrocyte membrane ZIP metal transporters by immunoblotting

Manganese is one of the essential trace elements found in erythrocytes. Metal transporters situated on the plasma membrane generally facilitate the movement of manganese into and out of cells. This study aims at determining whether two recently discovered manganese importers, ZIP8 and ZIP14, are located in the erythrocyte membrane. We outline a simple, effective and repeatable method for the isolation of erythrocyte membrane from a minimum of 50 µL mouse blood, followed by the identification of ZIP metal transporters using immunoblotting. Our results revealed that ZIP8 is expressed within the erythrocyte membrane, in contrast to ZIP14 which is not identified using immunoblotting approach. A direct measurement of the ZIP8 protein expression in erythrocyte membranes could provide valuable information for further analyzing its biological function.

Recent progress toward understanding the role of ZIP14 in regulating systemic manganese homeostasis

ZIP14 is a metal transporter essential for the regulation of body manganese homeostasis. The physiological functions of ZIP14 have been uncovered mainly through two lines of in vivo studies that examined the phenotypes of ZIP14 loss, including studies of humans with ZIP14 mutations and animals with ZIP14 deficiency. This mini review aims at presenting an updated view of the important advances made towards understanding the genetic and pathological mechanisms of brain manganese overload caused by ZIP14 deficiency.

Hereditary Disorders of Manganese Metabolism: Pathophysiology of Childhood-Onset Dystonia-Parkinsonism in SLC39A14 Mutation Carriers and Genetic Animal Models

Over the last decade, several clinical reports have outlined cases of childhood-onset manganese (Mn)-induced dystonia-parkinsonism, resulting from loss-of-function mutations in the Mn influx transporter gene SLC39A14. These clinical cases have provided a wealth of knowledge on Mn toxicity and homeostasis. However, our current understanding of the underlying neuropathophysiology is severely lacking. The recent availability of Slc39a14 knockout (KO) murine and zebrafish animal models provide a powerful platform to investigate the neurological effects of elevated blood and brain Mn concentrations in vivo. As such, the objective of this review was to organize and summarize the current clinical literature and studies utilizing Slc39a14-KO animal models and assess the validity of the animal models based on the clinical presentation of the disease in human mutation carriers.

Clinical Profile and Treatment Outcomes of Hypermanganesemia with Dystonia 1 and 2 among 27 Indian Children

Background

Hypermanganesemia with dystonia 1 and 2 (HMNDYT1 and 2) are rare, inherited disorders of manganese transport.

Objectives

We aimed to describe clinical, laboratory features, and outcomes among children with HMNDYT.

Methods

We conducted a retrospective multicenter study involving tertiary centers across India. We enrolled children between 1 month to 18 years of age with genetically confirmed/clinically probable HMNDYT. Clinical, laboratory profile, genetic testing, treatment details, and outcomes scored by treating physicians on a Likert scale were recorded.

Results

We enrolled 27 children (19 girls). Fourteen harbored SLC30A10 mutations; nine had SLC39A14 mutations. The SLC39A14 cohort had lower median age at onset (1.3 [interquartile range (IQR), 0.7-5.5] years) versus SLC30A10 cohort (2.0 [IQR, 1.5-5.1] years). The most frequent neurological features were dystonia (100%; n = 27), gait abnormality (77.7%; n = 21), falls (66.7%; n = 18), and parkinsonism (59.3%; n = 16). Median serum manganese (Mn) levels among SLC39A14 (44.9 [IQR, 27.3-147.7] mcg/L) cohort were higher than SLC30A10 (29.4 [17.1-42.0] mcg/L); median hemoglobin was higher in SLC30A10 (16.3 [IQR, 15.2-17.5] g/dL) versus SLC39A14 cohort (12.5 [8.8-13.2] g/dL). Hepatic involvement and polycythaemia were observed exclusively in SLC30A10 variants. A total of 26/27 children underwent chelation with disodium calcium edetate. Nine demonstrated some improvement, three stabilized, two had marked improvement, and one had normalization. Children with SLC39A14 mutations had poorer response. Two children died and nine were lost to follow-up.

Conclusions

We found female predominance. Children with SLC39A14 mutations presented at younger age and responded less favorably to chelation compared to SLC30A10 mutations. There is emerging need to better define management strategies, especially in low resource settings.

Pathophysiological studies of aging Slc39a14 knockout mice to assess the progression of manganese-induced dystonia-parkinsonism

Over the last decade, several clinical reports have outlined cases of early-onset manganese (Mn)-induced dystonia-parkinsonism, resulting from loss of function mutations of the Mn transporter gene SLC39A14. Previously, we have performed characterization of the behavioral, neurochemical, and neuropathological changes in 60-day old (PN60) Slc39a14-knockout (KO) murine model of the human disease. Here, we extend our studies to aging Slc39a14-KO mice to assess the progression of the disease. Our results indicate that 365-day old (PN365) Slc39a14-KO mice present with markedly elevated blood and brain Mn levels, similar to those found in the PN60 mice and representative of the human cases of the disease. Furthermore, aging Slc39a14-KO mice consistently manifest a hypoactive and dystonic behavioral deficits, similar to the PN60 animals, suggesting that the behavioral changes are established early in life without further age-associated deterioration. Neurochemical, neuropathological, and functional assessment of the dopaminergic system of the basal ganglia revealed absence of neurodegenerative changes of dopamine (DA) neurons in the substantia nigra pars compacta (SNc), with no changes in DA or metabolite concentrations in the striatum of Slc39a14-KO mice relative to wildtype (WT). Similar to the PN60 animals, aging Slc39a14-KO mice expressed a marked inhibition of potassium-stimulated DA release in the striatum. Together our findings indicate that the pathophysiological changes observed in the basal ganglia of aging Slc39a14-KO animals are similar to those at PN60 and aging does not have a significant effect on these parameters.

The Combined Inactivation of Intestinal and Hepatic ZIP14 Exacerbates Manganese Overload in Mice

ZIP14 is a newly identified manganese transporter with high levels of expression in the small intestine and the liver. Loss-of-function mutations in ZIP14 can lead to systemic manganese overload, which primarily affects the central nervous system, causing neurological disorders. To elucidate the roles of intestinal ZIP14 and hepatic ZIP14 in maintaining systemic manganese homeostasis, we generated mice with single-tissue or two-tissue Zip14 knockout, including intestine-specific (Zip14-In-KO), liver-specific (Zip14-L-KO), and double (intestine and liver) Zip14-knockout (Zip14-DKO) mice. Zip14^flox/flox mice were used as the control. Tissue manganese contents in these mice were compared using inductively coupled plasma mass spectrometry (ICP-MS) analysis. We discovered that although the deletion of intestinal ZIP14 only moderately increased systemic manganese loading, the deletion of both intestinal and hepatic ZIP14 greatly exacerbated the body's manganese burden. Our results provide new knowledge to further the understanding of manganese metabolism, and offer important insights into the mechanisms underlying systemic manganese overload caused by the loss of ZIP14.

Loss of slc39a14 causes simultaneous manganese hypersensitivity and deficiency in zebrafish

Manganese neurotoxicity is a hallmark of hypermanganesemia with dystonia 2, an inherited manganese transporter defect caused by mutations in SLC39A14. To identify novel potential targets of manganese neurotoxicity, we performed transcriptome analysis of slc39a14-/- mutant zebrafish that were exposed to MnCl2. Differentially expressed genes mapped to the central nervous system and eye, and pathway analysis suggested that Ca2+ dyshomeostasis and activation of the unfolded protein response are key features of manganese neurotoxicity. Consistent with this interpretation, MnCl2 exposure led to decreased whole-animal Ca2+ levels, locomotor defects and changes in neuronal activity within the telencephalon and optic tectum. In accordance with reduced tectal activity, slc39a14-/- zebrafish showed changes in visual phototransduction gene expression, absence of visual background adaptation and a diminished optokinetic reflex. Finally, numerous differentially expressed genes in mutant larvae normalised upon MnCl2 treatment indicating that, in addition to neurotoxicity, manganese deficiency is present either subcellularly or in specific cells or tissues. Overall, we assembled a comprehensive set of genes that mediate manganese-systemic responses and found a highly correlated and modulated network associated with Ca2+ dyshomeostasis and cellular stress. This article has an associated First Person interview with the first author of the paper.

Nomenclature of Genetic Movement Disorders: Recommendations of the International Parkinson and Movement Disorder Society Task Force - An Update

In 2016, the Movement Disorder Society Task Force for the Nomenclature of Genetic Movement Disorders presented a new system for naming genetically determined movement disorders and provided a criterion-based list of confirmed monogenic movement disorders. Since then, a substantial number of novel disease-causing genes have been described, which warrant classification using this system. In addition, with this update, we further refined the system and propose dissolving the imaging-based categories of Primary Familial Brain Calcification and Neurodegeneration with Brain Iron Accumulation and reclassifying these genetic conditions according to their predominant phenotype. We also introduce the novel category of Mixed Movement Disorders (MxMD), which includes conditions linked to multiple equally prominent movement disorder phenotypes. In this article, we present updated lists of newly confirmed monogenic causes of movement disorders. We found a total of 89 different newly identified genes that warrant a prefix based on our criteria; 6 genes for parkinsonism, 21 for dystonia, 38 for dominant and recessive ataxia, 5 for chorea, 7 for myoclonus, 13 for spastic paraplegia, 3 for paroxysmal movement disorders, and 6 for mixed movement disorder phenotypes; 10 genes were linked to combined phenotypes and have been assigned two new prefixes. The updated lists represent a resource for clinicians and researchers alike and they have also been published on the website of the Task Force for the Nomenclature of Genetic Movement Disorders on the homepage of the International Parkinson and Movement Disorder Society (https://www.movementdisorders.org/MDS/About/Committees--Other-Groups/MDS-Task-Forces/Task-Force-on-Nomenclature-in-Movement-Disorders.htm). © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

Effect of Chelation Therapy on a Korean Patient With Brain Manganese Deposition Resulting From a Compound Heterozygous Mutation in the SLC39A14 Gene

Mutations in the manganese transporter gene SLC39A14 lead to inherited disorders of manganese metabolism. Chelation therapy with edetate calcium disodium (CaNa2EDTA) is known to effectively reduce manganese deposition. We describe the first identified Korean case of SLC39A14-associated manganism and the treatment response to a 5-year chelation therapy. An 18-year-old female presented with childhood-onset dystonia. Magnetic resonance imaging showed T1 hyperintensity throughout the basal ganglia, brainstem, cerebellum, cerebral and cerebellar white matter, and pituitary gland. Blood manganese levels were elevated, and whole-exome sequencing revealed compound heterozygous mutations in SLC39A14. Treatment with intravenous CaNa2EDTA led to a significant reduction in serum manganese levels and T1 hyperintensities. However, her dystonia improved insignificantly. Hence, early diagnosis of this genetic disorder is essential because it is potentially treatable. Even though our treatment did not significantly reverse the establish deficits, chelation therapy could have been more effective if it was started at an earlier stage of the disease.

A novel homozygous SLC39A14 variant in an infant with hypermanganesemia and a review of the literature

BACKGROUND

Manganese (Mn) is an essential trace metal necessary for good health; however, excessive amounts in the body are neurotoxic. To date, three genes (SLC30A10, SLC39A8, and SLC39A14) have been discovered to cause inborn errors in Mn metabolism in humans. As very rare diseases, the clinical features require further clarification.

METHODS

A male Chinese patient who mainly presented with hypermanganesemia and progressive parkinsonism-dystonia was recruited for this study. We collected and analyzed clinical information, performed whole-exome sequencing (WES), and reviewed the relevant literature.

RESULTS

The motor-developmental milestones of the patient were delayed at the age of 4 months, followed by rapidly progressive dystonia. The patient displayed elevated Mn concentrations in blood and urine, and brain magnetic resonance imaging (MRI) showed symmetrical hyperintensity on T1-weighted images and hypointensity on T2-weighted images in multiple regions. A novel homozygous variant of the SLC39A14 gene (c.1058T > G, p.L353R) was identified. The patient was treated with disodium calcium edetate chelation (Na₂CaEDTA). Three months later, mild improvement in clinical manifestation, blood Mn levels, and brain MRI was observed. To date, 15 patients from 10 families have been reported with homozygous mutations of SLC39A14, with a mean age of onset of 14.9 months. The common initial symptom is motor regression or developmental milestone delay, with a disease course for nearly all patients involving development of progressive generalized dystonia and loss of ambulation before treatment. Additionally, hypermanganesemia manifests as Mn values ranging from 4- to 25-fold higher than normal baseline levels, along with brain MRI results similar to those observed in the recruited patient. Nine SLC39A14 variants have been identified. Seven patients have been treated with Na₂CaEDTA, and only one patient achieved obvious clinical improvement.

CONCLUSION

We identified a novel SLC39A14 mutation related to autosomal recessive hypermanganesemia with dystonia-2, which is a very rare disease. Patients present motor regression or delay of developmental milestones and develop progressive generalized dystonia. Chelation therapy with Na₂CaEDTA appears to effectively chelate Mn and increase urinary Mn excretion in some cases; however, clinical response varies. The outcome of the disease was unsatisfactory. This study expands the genetic spectrum of this disease.

The role of manganese dysregulation in neurological disease: emerging evidence

Manganese is an essential trace metal. The dysregulation of manganese seen in a broad spectrum of neurological disorders reflects its importance in brain development and key neurophysiological processes. Historically, the observation of acquired manganism in miners and people who misuse drugs provided early evidence of brain toxicity related to manganese exposure. The identification of inherited manganese transportopathies, which cause neurodevelopmental and neurodegenerative syndromes, further corroborates the neurotoxic potential of this element. Moreover, manganese dyshomoeostasis is also implicated in Parkinson's disease and other neurodegenerative conditions, such as Alzheimer's disease and Huntington's disease. Ongoing and future research will facilitate the development of better targeted therapeutical strategies than are currently available for manganese-associated neurological disorders.

A Multi-center Genome-wide Association Study of Cervical Dystonia

BACKGROUND

Several monogenic causes for isolated dystonia have been identified, but they collectively account for only a small proportion of cases. Two genome-wide association studies have reported a few potential dystonia risk loci; but conclusions have been limited by small sample sizes, partial coverage of genetic variants, or poor reproducibility.

OBJECTIVE

To identify robust genetic variants and loci in a large multicenter cervical dystonia cohort using a genome-wide approach.

METHODS

We performed a genome-wide association study using cervical dystonia samples from the Dystonia Coalition. Logistic and linear regressions, including age, sex, and population structure as covariates, were employed to assess variant- and gene-based genetic associations with disease status and age at onset. We also performed a replication study for an identified genome-wide significant signal.

RESULTS

After quality control, 919 cervical dystonia patients compared with 1491 controls of European ancestry were included in the analyses. We identified one genome-wide significant variant (rs2219975, chromosome 3, upstream of COL8A1, P-value 3.04 × 10^-8 ). The association was not replicated in a newly genotyped sample of 473 cervical dystonia cases and 481 controls. Gene-based analysis identified DENND1A to be significantly associated with cervical dystonia (P-value 1.23 × 10^-6 ). One low-frequency variant was associated with lower age-at-onset (16.4 ± 2.9 years, P-value = 3.07 × 10^-8 , minor allele frequency = 0.01), located within the GABBR2 gene on chromosome 9 (rs147331823).

CONCLUSION

The genetic underpinnings of cervical dystonia are complex and likely consist of multiple distinct variants of small effect sizes. Larger sample sizes may be needed to provide sufficient statistical power to address the presumably multi-genic etiology of cervical dystonia. © 2021 International Parkinson and Movement Disorder Society.

Restriction of Manganese Intake Prevents the Onset of Brain Manganese Overload in Zip14-/- Mice

As a newly identified manganese transport protein, ZIP14 is highly expressed in the small intestine and liver, which are the two principal organs involved in regulating systemic manganese homeostasis. Loss of ZIP14 function leads to manganese overload in both humans and mice. Excess manganese in the body primarily affects the central nervous system, resulting in irreversible neurological disorders. Therefore, to prevent the onset of brain manganese accumulation becomes critical. In this study, we used Zip14^−/− mice as a model for ZIP14 deficiency and discovered that these mice were born without manganese loading in the brain, but started to hyper-accumulate manganese within 3 weeks after birth. We demonstrated that decreasing manganese intake in Zip14^−/− mice was effective in preventing manganese overload that typically occurs in these animals. Our results provide important insight into future studies that are targeted to reduce the onset of manganese accumulation associated with ZIP14 dysfunction in humans.

MINPP1 prevents intracellular accumulation of the chelator inositol hexakisphosphate and is mutated in Pontocerebellar Hypoplasia

Inositol polyphosphates are vital metabolic and secondary messengers, involved in diverse cellular functions. Therefore, tight regulation of inositol polyphosphate metabolism is essential for proper cell physiology. Here, we describe an early-onset neurodegenerative syndrome caused by loss-of-function mutations in the multiple inositol-polyphosphate phosphatase 1 gene (MINPP1). Patients are found to have a distinct type of Pontocerebellar Hypoplasia with typical basal ganglia involvement on neuroimaging. We find that patient-derived and genome edited MINPP1^−/− induced stem cells exhibit an inefficient neuronal differentiation combined with an increased cell death. MINPP1 deficiency results in an intracellular imbalance of the inositol polyphosphate metabolism. This metabolic defect is characterized by an accumulation of highly phosphorylated inositols, mostly inositol hexakisphosphate (IP₆), detected in HEK293 cells, fibroblasts, iPSCs and differentiating neurons lacking MINPP1. In mutant cells, higher IP₆ level is expected to be associated with an increased chelation of intracellular cations, such as iron or calcium, resulting in decreased levels of available ions. These data suggest the involvement of IP₆-mediated chelation on Pontocerebellar Hypoplasia disease pathology and thereby highlight the critical role of MINPP1 in the regulation of human brain development and homeostasis.

Tight regulation of inositol polyphosphate metabolism is essential for proper cell physiology. Here, the authors describe an early-onset neurodegenerative syndrome caused by loss-of-function mutations in the MINPP1 gene, characterised by intracellular imbalance of inositol polyphosphate metabolism.

Brain manganese and the balance between essential roles and neurotoxicity

Manganese (Mn) is an essential micronutrient required for the normal development of many organs, including the brain. Although its roles as a cofactor in several enzymes and in maintaining optimal physiology are well-known, the overall biological functions of Mn are rather poorly understood. Alterations in body Mn status are associated with altered neuronal physiology and cognition in humans, and either overexposure or (more rarely) insufficiency can cause neurological dysfunction. The resultant balancing act can be viewed as a hormetic U-shaped relationship for biological Mn status and optimal brain health, with changes in the brain leading to physiological effects throughout the body and vice versa. This review discusses Mn homeostasis, biomarkers, molecular mechanisms of cellular transport, and neuropathological changes associated with disruptions of Mn homeostasis, especially in its excess, and identifies gaps in our understanding of the molecular and biochemical mechanisms underlying Mn homeostasis and neurotoxicity.

The Functions of ZIP8, ZIP14, and ZnT10 in the Regulation of Systemic Manganese Homeostasis

As an essential nutrient, manganese is required for the regulation of numerous cellular processes, including cell growth, neuronal health, immune cell function, and antioxidant defense. However, excess manganese in the body is toxic and produces symptoms of neurological and behavioral defects, clinically known as manganism. Therefore, manganese balance needs to be tightly controlled. In the past eight years, mutations of genes encoding metal transporters ZIP8 (SLC39A8), ZIP14 (SLC39A14), and ZnT10 (SLC30A10) have been identified to cause dysregulated manganese homeostasis in humans, highlighting the critical roles of these genes in manganese metabolism. This review focuses on the most recent advances in the understanding of physiological functions of these three identified manganese transporters and summarizes the molecular mechanisms underlying how the loss of functions in these genes leads to impaired manganese homeostasis and human diseases.

Intestine-specific deletion of metal transporter Zip14 (Slc39a14) causes brain manganese overload and locomotor defects of manganism

Impaired manganese (Mn) homeostasis can result in excess Mn accumulation in specific brain regions and neuropathology. Maintaining Mn homeostasis and detoxification is dependent on effective Mn elimination. Specific metal transporters control Mn homeostasis. Human carriers of mutations in the metal transporter ZIP14 and whole body Zip14-knockout (WB-KO) mice display similar phenotypes, including spontaneous systemic and brain Mn overload and motor dysfunction. Initially, it was believed that Mn accumulation due to ZIP14 mutations was caused by impaired hepatobiliary Mn elimination. However, liver-specific Zip14-KO mice did not show systemic Mn accumulation or motor deficits. ZIP14 is highly expressed in the small intestine and is localized to the basolateral surface of enterocytes. Thus, we hypothesized that basolaterally localized ZIP14 in enterocytes provides another route for the elimination of Mn. Using wild-type and intestine-specific Zip14-KO (I-KO) mice, we have shown that ablation of intestinal Zip14 is sufficient to cause systemic and brain Mn accumulation. The lack of intestinal ZIP14-mediated Mn excretion was compensated for by the hepatobiliary system; however, it was not sufficient to maintain Mn homeostasis. When supplemented with extra dietary Mn, I-KO mice displayed some motor dysfunctions and brain Mn accumulation based on both MRI imaging and chemical analysis, thus demonstrating the importance of intestinal ZIP14 as a route of Mn excretion. A defect in intestinal Zip14 expresssion likely could contribute to the Parkinson-like Mn accumulation of manganism.NEW & NOTEWORTHY Mn-induced parkinsonism is recognized as rising in frequency because of both environmental factors and genetic vulnerability; yet currently, there is no cure. We provide evidence in an integrative animal model that basolaterally localized ZIP14 regulates Mn excretion and detoxification and that deletion of intestinal ZIP14 leads to systemic and brain Mn accumulation, providing robust evidence for the indispensable role of intestinal ZIP14 in Mn excretion.

ZIP14 is degraded in response to manganese exposure

Manganese (Mn) is an essential element necessary for proper development and brain function. Circulating Mn levels are regulated by hepatobiliary clearance to limit toxic levels and prevent tissue deposition. To characterize mechanisms involved in hepatocyte Mn uptake, polarized human HepaRG cells were used for this study. Western blot analysis and immunofluorescence microscopy showed the Mn transporter ZIP14 was expressed and localized to the basolateral surface of polarized HepaRG cells. HepaRG cells took up 54Mn in a time- and temperature-dependent manner but uptake was reduced after exposure to Mn. This loss in transport activity was associated with decreased ZIP14 protein levels in response to Mn exposure. Mn-induced degradation of ZIP14 was blocked by bafilomycin A1, which increased localization of the transporter in Lamp1-positive vesicles. Mn exposure also down-regulated the Golgi proteins TMEM165 and GPP130 while the ER stress marker BiP was induced. These results indicate that Mn exposure decreases ZIP14 protein levels to limit subsequent uptake of Mn as a cytoprotective response. Thus, high levels of Mn may compromise first-pass-hepatic clearance mechanisms.

Autosomal-recessive iron deficiency anemia, dystonia and hypermanganesemia caused by new variant mutation of the manganese transporter gene SLC39A14

This inborn error of manganese metabolism has only recently been identified. A total of 28 affected individuals from ten families are known worldwide. Mutations in SLC39A14, encoding a Mn uptake transporter, have recently been recognized to cause excessive Mn concentrations in the blood which is believed to be neurotoxic and lead to a parkinsonian-like movement disorder caused by accumulation of Mn in the basal ganglia. We are reporting a new variant of SLC39A14 gene mutation (OMIM 608736 8p21.3) that has never been described in the literature so far. The index case is a 3-year-old female who was born at 30 weeks' gestation by emergency lower segment caesarean section, the second of twins, weighing 1.4 kg. Parents have a consanguineous marriage (first cousins) and have four healthy male children. She presented at 30 months of age with history of unsteady gait of 4 months duration and is progressively worsening. She became stiff and has lost all of her locomotor skills. Apart from low serum iron and iron deficiency anemia, her initial work up was unremarkable. T1-weighted MRI brain showed bilateral hyperintense signal in basal ganglia, mid-brain and pontine tegmentum giving rise to the characteristic eye-of-the-tiger sign. Genetic DNA evaluation (Whole Exome Sequencing WES) identified the homozygous missense variant c.1136.T in exon 7 of SLC39A14 gene which is associated with hypermanganesemia. Whole blood Mn was markedly raised at 150 nmol/L (8 mg/L) (normal 10 nmol/L, 0.7 mg/Bioscientia). This young girl has just started treatment with intravenous disodium calcium edetate and oral iron.

Manganese Uptake by A549 Cells is Mediated by Both ZIP8 and ZIP14

The alveolar epithelia of the lungs require manganese (Mn) as an essential nutrient, but also provide an entry route for airborne Mn that can cause neurotoxicity. Transporters involved in Mn uptake by alveolar epithelial cells are unknown. Recently, two members of the Zrt- and Irt-like protein (ZIP) family of metal transporters, ZIP8 and ZIP14, have been identified as crucial Mn importers in vivo. ZIP8 is by far most abundantly expressed in the lungs, whereas ZIP14 expression in the lungs is low compared to other tissues. We hypothesized that Mn uptake by alveolar epithelial cells is primarily mediated by ZIP8. To test our hypothesis, we used A549 cells, a type II alveolar cell line. Mirroring the in vivo situation, A549 cells expressed higher levels of ZIP8 than cell models for the liver, intestines, and kidney. Quantification of ZIP8 and ZIP14 revealed a strong enrichment of ZIP8 over ZIP14 in A549 cells. Using siRNA technology, we identified ZIP8 and ZIP14 as the major transporters mediating Mn uptake by A549 cells. To our surprise, knockdown of either ZIP8 or ZIP14 impaired Mn accumulation to a similar extent, which we traced back to similar amounts of ZIP8 and ZIP14 at the plasma membrane. Our study highlights the importance of both ZIP8 and ZIP14 in Mn metabolism of alveolar epithelial cells.

The intestinal metal transporter ZIP14 maintains systemic manganese homeostasis

ZIP14 (encoded by the solute carrier 39 family member 14 (SLC39A14) gene) is a manganese transporter that is abundantly expressed in the liver and small intestine. Loss-of-function mutations in SLC39A14 cause severe hypermanganesemia. Because the liver is regarded as the main regulatory organ involved in manganese homeostasis, impaired hepatic manganese uptake for subsequent biliary excretion has been proposed as the underlying disease mechanism. However, liver-specific Zip14 KO mice exhibit decreased manganese only in the liver and do not develop manganese accumulation in other tissues under normal conditions. This suggests that impaired hepatobiliary excretion is not the primary cause for manganese overload observed in individuals lacking functional ZIP14. We therefore hypothesized that increased intestinal manganese absorption could induce manganese hyperaccumulation when ZIP14 is inactivated. To elucidate the role of ZIP14 in manganese absorption, here we used CaCo-2 Transwell cultures as a model system for intestinal epithelia. The generation of a ZIP14-deficient CaCo-2 cell line enabled the identification of ZIP14 as the major transporter mediating basolateral manganese uptake in enterocytes. Lack of ZIP14 severely impaired basolateral-to-apical (secretory) manganese transport and strongly enhanced manganese transport in the apical-to-basolateral (absorptive) direction. Mechanistic studies provided evidence that ZIP14 restricts manganese transport in the absorptive direction via direct basolateral reuptake of freshly absorbed manganese. In support of such function of intestinal ZIP14 in vivo, manganese levels in the livers and brains of intestine-specific Zip14 KO mice were significantly elevated. Our findings highlight the importance of intestinal ZIP14 in regulating systemic manganese homeostasis.

Genetic Disorders of Manganese Metabolism

Purpose of Review

This article provides an overview of the pathogenesis, clinical presentation and treatment of inherited manganese transporter defects.

Recent Findings

Identification of a new group of manganese transportopathies has greatly advanced our understanding of how manganese homeostasis is regulated in vivo. While the manganese efflux transporter SLC30A10 and the uptake transporter SLC39A14 work synergistically to reduce the manganese load, SLC39A8 has an opposing function facilitating manganese uptake into the organism. Bi-allelic mutations in any of these transporter proteins disrupt the manganese equilibrium and lead to neurological disease: Hypermanganesaemia with dystonia 1 (SLC30A10 deficiency) and hypermanganesaemia with dystonia 2 (SLC39A14 deficiency) are characterised by manganese neurotoxicity while SLC39A8 mutations cause a congenital disorder of glycosylation type IIn due to Mn deficiency.

Summary

Inherited manganese transporter defects are an important differential diagnosis of paediatric movement disorders. Manganese blood levels and MRI brain are diagnostic and allow early diagnosis to avoid treatment delay.

Novel founder intronic variant in SLC39A14 in two families causing Manganism and potential treatment strategies

Congenital disorders of manganese metabolism are rare occurrences in children, and medical management of these disorders is complex and challenging. Homozygous exonic mutations in the manganese transporter SLC39A14 have recently been associated with a pediatric-onset neurodegenerative disorder characterized by brain manganese accumulation and clinical signs of manganese neurotoxicity, including parkinsonism-dystonia. We performed whole exome sequencing on DNA samples from two unrelated female children from the United Arab Emirates with progressive movement disorder and brain mineralization, identified a novel homozygous intronic mutation in SLC39A14 in both children, and demonstrated that the mutation leads to aberrant splicing. Both children had consistently elevated serum manganese levels and were diagnosed with SLC39A14-associated manganism. Over a four-year period, we utilized a multidisciplinary management approach for Patient 1 combining decreased manganese dietary intake and chelation with symptomatic management of dystonia. Our treatment strategy appeared to slow disease progression, but did not lead to a cure or reversal of already established deficits. Clinicians should consider testing for noncoding mutations in the diagnosis of congenital disorders of manganese metabolism and utilizing multidisciplinary approaches in the management of these disorders.