Absence of the Autophagy Adaptor SQSTM1/p62 Causes Childhood-Onset Neurodegeneration with Ataxia, Dystonia, and Gaze Palsy.

SQSTM1 (sequestosome 1; also known as p62) encodes a multidomain scaffolding protein involved in various key cellular processes, including the removal of damaged mitochondria by its function as a selective autophagy receptor. Heterozygous variants in SQSTM1 have been associated with Paget disease of the bone and might contribute to neurodegeneration in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Using exome sequencing, we identified three different biallelic loss-of-function variants in SQSTM1 in nine affected individuals from four families with a childhood- or adolescence-onset neurodegenerative disorder characterized by gait abnormalities, ataxia, dysarthria, dystonia, vertical gaze palsy, and cognitive decline. We confirmed absence of the SQSTM1/p62 protein in affected individuals' fibroblasts and found evidence of a defect in the early response to mitochondrial depolarization and autophagosome formation. Our findings expand the SQSTM1-associated phenotypic spectrum and lend further support to the concept of disturbed selective autophagy pathways in neurodegenerative diseases.

Brain iron quantification by MRI in mitochondrial membrane protein-associated neurodegeneration under iron-chelating therapy.

Therapeutic trials for Neurodegeneration with Brain Iron Accumulation have aimed at a reduction of cerebral iron content. A 13-year-old girl with mitochondrial membrane protein-associated neurodegeneration treated with an iron-chelating agent was monitored by R2 relaxometry, R2* relaxometry, and quantitative susceptibility mapping to estimate the brain iron content. The highly increased brain iron content slowly decreased in the substantia nigra but remained stable for globus pallidus. The estimated iron content was higher by R2* compared to R2 and quantitative susceptibility mapping, a finding not previously observed in the brain of healthy volunteers. A hypothesis explaining this discrepancy is offered.

Mitochondrial membrane protein-associated neurodegeneration (MPAN).

Neurodegeneration with brain iron accumulation (NBIA) is a group of rare and devastating disorders characterized by iron deposition in the brain. Mutations in C19orf12 cause autosomal recessive inherited mitochondrial membrane protein-associated neurodegeneration (MPAN), which may account for up to 30% of NBIA cases. The C19orf12 gene product is an orphan mitochondrial membrane protein, and most mutations are predicted to cause loss of function. From 67 MPAN cases so far reported, we describe here the clinical, radiological, and genetic features. Key clinical features are pyramidal and extrapyramidal signs, cognitive decline, neuropsychiatric abnormalities, optic atrophy, and motor axonal neuropathy. Magnetic resonance imaging shows the eponymous brain iron accumulation in globus pallidus and substantia nigra and in some cases a hyperintense streaking of the medial medullary lamina. The latter sign may discriminate MPAN from other NBIA subtypes. In two postmortem MPAN cases, neuropathology showed axonal spheroids, Lewy bodies, and hyperphosphorylated tau-containing inclusions.

Alterations of red cell membrane properties in neuroacanthocytosis.

Neuroacanthocytosis (NA) refers to a group of heterogenous, rare genetic disorders, namely chorea acanthocytosis (ChAc), McLeod syndrome (MLS), Huntington's disease-like 2 (HDL2) and pantothenate kinase associated neurodegeneration (PKAN), that mainly affect the basal ganglia and are associated with similar neurological symptoms. PKAN is also assigned to a group of rare neurodegenerative diseases, known as NBIA (neurodegeneration with brain iron accumulation), associated with iron accumulation in the basal ganglia and progressive movement disorder. Acanthocytosis, the occurrence of misshaped erythrocytes with thorny protrusions, is frequently observed in ChAc and MLS patients but less prevalent in PKAN (about 10%) and HDL2 patients. The pathological factors that lead to the formation of the acanthocytic red blood cell shape are currently unknown. The aim of this study was to determine whether NA/NBIA acanthocytes differ in their functionality from normal erythrocytes. Several flow-cytometry-based assays were applied to test the physiological responses of the plasma membrane, namely drug-induced endocytosis, phosphatidylserine exposure and calcium uptake upon treatment with lysophosphatidic acid. ChAc red cell samples clearly showed a reduced response in drug-induced endovesiculation, lysophosphatidic acid-induced phosphatidylserine exposure, and calcium uptake. Impaired responses were also observed in acanthocyte-positive NBIA (PKAN) red cells but not in patient cells without shape abnormalities. These data suggest an "acanthocytic state" of the red cell where alterations in functional and interdependent membrane properties arise together with an acanthocytic cell shape. Further elucidation of the aberrant molecular mechanisms that cause this acanthocytic state may possibly help to evaluate the pathological pathways leading to neurodegeneration.

Mitochondrial membrane protein associated neurodegenration: a novel variant of neurodegeneration with brain iron accumulation.

BACKGROUND: Recently, mutations in an open-reading frame on chromosome 19 (C19orf12) were identified as a novel genetic factor in neurodegeneration with brain iron accumulation (NBIA). Because of the mitochondrial localization of the derived protein, this variant is referred to as mitochondrial membrane protein-associated neurodegeneration with brain iron accumulation (MPAN). METHODS/RESULTS: We describe the clinical phenotype and MRI of 3 newly identified individuals with MPAN due to either previously reported or novel homozygous or compound heterozygous genetic alterations in C19orf12. CONCLUSIONS: MPAN is characterized by a juvenile-onset, slowly progressive phenotype with predominant lower limb spasticity, generalized dystonia, and cognitive impairment. Typical additional features include axonal motor neuropathy and atrophy of the optic nerve. MRI showed iron deposition in the globus pallidus and substantia nigra without the eye-of-the-tiger sign, which is typical for PKAN, the most frequent form of NBIA.

Pantothenate kinase-associated neurodegeneration.

Pantothenate kinase-associated neurodegeneration (PKAN) is a hereditary progressive disorder and the most frequent form of neurodegeneration with brain iron accumulation (NBIA). PKAN patients present with a progressive movement disorder, dysarthria, cognitive impairment and retinitis pigmentosa. In magnetic resonance imaging, PKAN patients exhibit the pathognonomic "eye of the tiger" sign in the globus pallidus which corresponds to iron accumulation and gliosis as shown in neuropathological examinations. The discovery of the disease causing mutations in PANK2 has linked the disorder to coenzyme A (CoA) metabolism. PANK2 is the only one out of four PANK genes encoding an isoform which localizes to mitochondria. At least two other NBIA genes (PLA2G6, C19orf12) encode proteins that share with PANK2 a mitochondrial localization and all are suggested to play a role in lipid homeostasis. With no causal therapy available for PKAN until now, only symptomatic treatment is possible. A multi-centre retrospective study with bilateral pallidal deep brain stimulation in patients with NBIA revealed a significant improvement of dystonia. Recently, studies in the PANK Drosophila model "fumble" revealed improvement by the compound pantethine which is hypothesized to feed an alternate CoA biosynthesis pathway. In addition, pilot studies with the iron chelator deferiprone that crosses the blood brain barrier showed a good safety profile and some indication of efficacy. An adequately powered randomized clinical trial will start in 2012. This review summarizes clinical presentation, neuropathology and pathogenesis of PKAN.

Absence of an orphan mitochondrial protein, c19orf12, causes a distinct clinical subtype of neurodegeneration with brain iron accumulation.

The disease classification neurodegeneration with brain iron accumulation (NBIA) comprises a clinically and genetically heterogeneous group of progressive neurodegenerative disorders characterized by brain iron deposits in the basal ganglia. For about half of the cases, the molecular basis is currently unknown. We used homozygosity mapping followed by candidate gene sequencing to identify a homozygous 11 bp deletion in the orphan gene C19orf12. Mutation screening of 23 ideopathic NBIA index cases revealed two mutated alleles in 18 of them, and one loss-of-function mutation is the most prevalent. We also identified compound heterozygous missense mutations in a case initially diagnosed with Parkinson disease at age 49. Psychiatric signs, optic atrophy, and motor axonal neuropathy were common findings. Compared to the most prevalent NBIA subtype, pantothenate kinase associated neurodegeneration (PKAN), individuals with two C19orf12 mutations were older at age of onset and the disease progressed more slowly. A polyclonal antibody against the predicted membrane spanning protein showed a mitochondrial localization. A histopathological examination in a single autopsy case detected Lewy bodies, tangles, spheroids, and tau pathology. The mitochondrial localization together with the immunohistopathological findings suggests a pathomechanistic overlap with common forms of neurodegenerative disorders.

A population-based epidemiological and genetic study of X-linked retinitis pigmentosa.

PURPOSE: To perform a nation-wide elucidation of the prevalence and the mutation spectrum in X-linked retinitis pigmentosa (XLRP), and to make genotype-phenotype comparisons. METHODS: The study comprised 96 affected males and 149 female carriers from 42 families representing all identified XLRP individuals in the Danish population (5.4 million inhabitants). RPGR and RP2 were screened for mutations in 34 families, the medical files of the patients were scrutinized, and phenotype data were extracted. RESULTS: The prevalence of affected males was estimated to be 1:26,200 and 1:18,000 of female carriers. A rough estimate, however, indicates that the real prevalence of affected males was approximately 1:15,000. The cumulated life risk of development of XLRP in carriers was strongly age dependent and included one third of the carriers older than 60 years. Molecular analysis of RP2 and RPGR uncovered 28 different mutations in 33 of 34 index cases analyzed. Twelve patients carried a mutation in RP2, 12 in exons 1 to 14, and 9 in open reading frame (ORF) 15 of RPGR. Males with RP2 mutations tended to have higher degrees of myopia, lower visual acuities, and more preserved visual fields than did males with RPGR mutations at the same age. No significant differences in phenotype were found in age of onset and type of mutation in either RP2 or RPGR. CONCLUSIONS: A very high mutation detection rate in familial cases makes genetic testing a valuable clinical tool for genetic counseling and prenatal testing. The proportion of RP2-mediated XLRP in the Danish population is higher and the proportion of RPGR-ORF15 is lower than reported in other studies. Thus, strategies for diagnostic procedures should take into account the population-specific mutation spectrum.

Genotypic and phenotypic spectrum of PANK2 mutations in patients with neurodegeneration with brain iron accumulation.

OBJECTIVE: Neurodegeneration with brain iron accumulation (NBIA) is a group of disorders characterized by magnetic resonance imaging (MRI) changes in basal ganglia. Both missense and nonsense mutations have been found in such patients in a gene encoding the mitochondrial pantothenate kinase (PANK2). METHODS: We completed a mutation screen in 72 patients with the diagnosis NBIA based on clinical findings and radiological imaging. The entire coding region of the PANK2 gene (20p12.3) was investigated for point mutations and deletions. RESULTS: We uncovered both mutant alleles in 48 patients. Deletions accounted for 4% of mutated alleles. Patients with two loss-of-function alleles (n = 11) displayed symptoms always at an early stage of life. In the presence of missense mutations (n = 37), the age of onset correlated with residual activity of the pantothenate kinase. Progression of disease measured by loss of ambulation was variable in both groups. We did not observe a strict correlation between the eye-of-the-tiger sign and PANK2 mutations. In 24 patients, no PANK2 mutation was identified. INTERPRETATION: Deletion screening of PANK2 should be part of the diagnostic spectrum. Factors other than enzymatic residual activity are determining the course of disease. There are strong arguments in favor of locus heterogeneity.