An update on the genetics, clinical presentation, and pathomechanisms of human riboflavin transporter deficiency

Riboflavin transporter deficiency (RTD) is a rare neurological condition that encompasses the Brown-Vialetto-Van Laere and Fazio-Londe syndromes since the discovery of pathogenic mutations in the SLC52A2 and SLC52A3 genes that encode human riboflavin transporters RFVT2 and RFVT3. Patients present with a deteriorating progression of peripheral and cranial neuropathy that causes muscle weakness, vision loss, deafness, sensory ataxia, and respiratory compromise which when left untreated can be fatal. Considerable progress in the clinical and genetic diagnosis of RTDs has been made in recent years and has permitted the successful lifesaving treatment of many patients with high dose riboflavin supplementation. In this review, we first outline the importance of riboflavin and its efficient transmembrane transport in human physiology. Reports on 109 patients with a genetically confirmed diagnosis of RTD are then summarized in order to highlight commonly presenting clinical features and possible differences between patients with pathogenic SLC52A2 (RTD2) or SLC52A3 (RTD3) mutations. Finally, we focus attention on recent work with different models of RTD that have revealed possible pathomechanisms contributing to neurodegeneration in patients.

A novel ATP1A2 mutation in a patient with hypokalaemic periodic paralysis and CNS symptoms

Hypokalaemic periodic paralysis is a rare genetic neuromuscular disease characterized by episodes of skeletal muscle paralysis associated with low serum potassium. Muscle fibre inexcitability during attacks of paralysis is due to an aberrant depolarizing leak current through mutant voltage sensing domains of either the sarcolemmal voltage-gated calcium or sodium channel. We report a child with hypokalaemic periodic paralysis and CNS involvement, including seizures, but without mutations in the known periodic paralysis genes. We identified a novel heterozygous de novo missense mutation in the ATP1A2 gene encoding the α2 subunit of the Na⁺/K⁺-ATPase that is abundantly expressed in skeletal muscle and in brain astrocytes. Pump activity is crucial for Na⁺ and K⁺ homeostasis following sustained muscle or neuronal activity and its dysfunction is linked to the CNS disorders hemiplegic migraine and alternating hemiplegia of childhood, but muscle dysfunction has not been reported. Electrophysiological measurements of mutant pump activity in Xenopus oocytes revealed lower turnover rates in physiological extracellular K⁺ and an anomalous inward leak current in hypokalaemic conditions, predicted to lead to muscle depolarization. Our data provide important evidence supporting a leak current as the major pathomechanism underlying hypokalaemic periodic paralysis and indicate ATP1A2 as a new hypokalaemic periodic paralysis gene.

Immunological effects of competitive versus recreational sports in cross-country skiing

For a period of two months during the competitive season the effects of endurance training in cross-country skiers were evaluated in order to compare the adaptive and innate immune systems between 10 competitive athletes, 10 moderately trained athletes and 10 untrained healthy controls. The main results were as follows: the peripheral T-lymphocyte count of the competitive athletes was decreased. In contrast the number of peripheral blood NK cells was increased in this group. These data imply a diminution of the adaptive immune system due to repeated bouts of intense exercise and contemporaneous reinforcement of the innate immune response. Moreover the inducible IL-12-expression following monocyte stimulation was significantly decreased in competitive athletes. Compared with the other two groups, the moderately trained athletes showed a significantly increased production of IFN-gamma upon T-cell stimulation. These data suggest that the immune system may profit from moderate endurance training by an increased capacity to generate IFN-gamma while the immune situation following repeated exhausting exercise of competitive athletes tends to deteriorate through downregulation of IFN-gamma and IL-12.

Characterization of a satellite DNA from Antilocapra americana

The nucleotide sequence of a cloned satellite DNA from Antilocapra americana (American pronghorn antelope) is presented. The 1477-bp satellite is composed of degenerate 31-bp sub-repeats which are very similar in sequence to those of the major satellite DNAs from cattle and sheep. The sub-repeat sequence is more degenerate and variable in pronghorn than it is in cattle or sheep. The sequence is organized in the pronghorn genome in multicopy tandem arrays.

Gamekeeper's thumb. Ligament localisation by echography

Twenty-one patients presenting clinically with a lesion of the ulnar collateral ligament complex of the metacarpo-phalangeal joint of the thumb underwent prospective diagnostic echography, in order to recognize the possible dislocation of the ligament over the adductor aponeurosis (Stener's lesion). Seven patients were treated surgically, the others conservatively. The accuracy of the echography was controlled by surgery and by the follow-up. We determined 15 true negatives, 4 true positives, 2 false positives, there was no false negative. Ultrasound imaging is a non-invasive, cost-effective diagnostic aid in differentiating the ruptured non-displaced ligament from the true Stener's lesion, solely requiring surgical repartir.

ANGPTL6 Genetic Variants Are an Underlying Cause of Familial Intracranial Aneurysms

Purpose

To understand the role of the angiopoietin-like 6 gene (ANGPTL6) in intracranial aneurysms (IAs), we investigated its role in a large cohort of familial IAs.

Methods

Individuals with family history of IA were recruited to the Genetic and Observational Subarachnoid Haemorrhage (GOSH) study. The ANGPTL6 gene was sequenced using Sanger sequencing. Identified genetic variants were compared to a control population.

Results

We found 6 rare ANGPTL6 genetic variants in 9/275 individuals with a family history of IA (3.3%) (5 missense mutations and 1 nonsense mutation leading to a premature stop codon), none present in controls. One of these had been previously reported: c.392A>T (p.Glu131Val) on exon 2; another was very close: c.332G>A (p.Arg111His). Two further genetic variants lie within the fibrinogen-like domain of the ANGPTL6 gene, which may influence function or level of the ANGPTL6 protein. The last 2 missense mutations lie within the coiled-coil domain of the ANGPTL6 protein. All genetic variants were well conserved across species.

Conclusion

ANGPTL6 genetic variants are an important cause of IA. Defective or lack of ANGPTL6 protein is therefore an important factor in blood vessel proliferation leading to IA; dysfunction of this protein is likely to cause abnormal proliferation or weakness of vessel walls. With these data, not only do we emphasize the importance of screening familial IA cases for ANGPTL6 and other genes involved in IA, but also highlight the ANGPTL6 pathway as a potential therapeutic target.

Classification of Evidence

This is a Class III study showing some specificity of presence of the ANGPTL6 gene variant as a marker of familial intracranial aneurysms in a small subset of individuals with familial aneurysms.

Brown-Vialetto-Van Laere and Fazio-Londe syndromes: SLC52A3 mutations with puzzling phenotypes and inheritance

BACKGROUND

Brown-Vialetto-Van Laere syndrome (BVVLS) and Fazio-Londe disease (FLD) are rare neurological disorders presenting with pontobulbar palsy, muscle weakness and respiratory insufficiency. Mutations in SLC52A2 (hRFVT-2) or SLC52A3 (hRFVT-3) genes can be responsible for these disorders with an autosomal recessive pattern of inheritance. The aim of this study was to screen for mutations in SLC52A2 and SLC52A3 among Indian families diagnosed with BVVLS and FLD.

METHODS

SLC52A2 and SLC52A3 were screened in one FLD and three BVVLS patients by exon-specific amplification using PCR and sequencing. In silico predictions using bioinformatics tools and confocal imaging using HEK-293 cells were performed to determine the functional impact of identified mutations.

RESULTS

Genetic analysis of a mother and son with BVVLS was identified with a novel homozygous mutation c.710C>T (p.Ala237Val) in SLC52A3. This variant was found to have an autosomal pseudodominant pattern of inheritance, which was neither listed in the Exome Variant Server or in the 1000 Genomes Project database. In silico analysis and confocal imaging of the p.Ala237Val variant showed higher degree of disorderness in hRFVT-3 that could affect riboflavin transport. Furthermore, a common homozygous mutation c.62A>G (p.Asn21Ser) was identified in other BVVLS and FLD patients. Despite having different clinical phenotypes, both BVVLS and FLD can be attributed to this mutation.

CONCLUSION

A rare and peculiar pattern of autosomal pseudodominant inheritance is observed for the first time in two genetically related BVVLS cases with Indian origin and a common mutation c.62A>G (p.Asn21Ser) in SLC52A3 can be responsible for both BVVLS and FLD with variable phenotypes.

PINK1: From Parkinson's disease to mitophagy and back again

The genetics of Parkinson's disease has been key to unravelling the PINK1-dependent mitophagy process. Here, we discuss the implications of a 2010 PLOS Biology paper that shed light on the functional importance of PINK1 in the mitophagy cascade.

Asymptomatic grotesque deformities of the cervical spine. An occupational hazard in railway porters

STUDY DESIGN

A case report.

OBJECTIVE

To illustrate an extremely rare occurrence of chronic, occupational, low-grade trauma leading to asymptomatic grotesque cervical spine deformities in railroad station porters.

SUMMARY OF BACKGROUND DATA

Occupational trauma causing spinal deformities has been described in relation to thoracic and lumbar spines in miners. To the authors' knowledge, this is the first reported case of asymptomatic cervical spinal deformity in railway porters as a result of chronic occupational trauma.

METHODS

A magnetic resonance imaging study was performed on both patients.

RESULTS

The magnetic resonance images showed advanced degenerative changes in the cervical spine causing obvious deformities, along with apparently normal cord signal intensity.

CONCLUSIONS

Chronic, occupational, low-grade trauma of the cervical vertebral region is extremely unusual in industrialized countries. Nevertheless, in view of the increasing mobility of people in general and of the labor force in particular, this complication of an occupational exposure deserves attention as an unusual cause of cervical spinal deformity.

Metabolic remodeling in hiPSC-derived myofibers carrying the m.3243A>G mutation

Mutations in mitochondrial DNA cause severe multisystem disease frequently associated with muscle weakness. The m.3243A>G mutation is the major cause of mitochondrial encephalomyopathy lactic acidosis and stroke-like episodes (MELAS). Experimental models that recapitulate the disease phenotype in vitro for disease modeling or drug screening are very limited. We have therefore generated hiPSC-derived muscle fibers with variable heteroplasmic mtDNA mutation load without significantly affecting muscle differentiation potential. The cells exhibit physiological characteristics of muscle fibers and show a well-organized myofibrillar structure. In cells carrying the m.3243A>G mutation, the mitochondrial membrane potential and oxygen consumption were reduced in relation to the mutant load. We have shown through proteomic, phosphoproteomic, and metabolomic analyses that the m.3243A>G mutation variably affects the cell phenotype in relation to the mutant load. This variation is reflected by an increase in the NADH/NAD+ ratio, which in turn influences key nutrient-sensing pathways in the myofibers. This model enables a detailed study of the impact of the mutation on cellular bioenergetics and on muscle physiology with the potential to provide a platform for drug screening.

[Hemorrhagic shock due to splenic rupture in non-severe acute pancreatitis]

A 54-year old man was hospitalized for a non severe acute pancreatitis of alcoholic aetiology. Four days after his admission, whilst symptoms and laboratory findings were on the mend, he suddenly presented with acute shock. A CT-scan demonstrated a massive haemoperitoneum due to splenic rupture which necessitated an emergency splenectomy. The causes of shock in acute pancreatitis and the factors that may lead to splenic rupture are discussed. In this patient, a possible role of the non-steroidal anti-inflammatory treatment remains undetermined.

KAT8 compound inhibition inhibits the initial steps of PINK1-dependant mitophagy

It has recently been shown that KAT8, a genome-wide association study candidate risk gene for Parkinson's Disease, is involved in PINK1/Parkin-dependant mitophagy. The KAT8 gene encodes a lysine acetyltransferase and represents the catalytically active subunit of the non-specific lethal epigenetic remodelling complex. In the current study, we show that contrary to KAT5 inhibition, dual inhibition of KAT5 and KAT8 via the MG149 compound inhibits the initial steps of the PINK1-dependant mitophagy process. More specifically, our study shows that following mitochondrial depolarisation induced by mitochondrial toxins, MG149 treatment inhibits PINK1-dependant mitophagy initiation by impairing PINK1 activation, and subsequent phosphorylation of Parkin and ubiquitin. While this inhibitory effect of MG149 on PINK1-activation is potent, MG149 treatment in the absence of mitochondrial toxins is sufficient to depolarise the mitochondrial membrane, recruit PINK1 and promote partial downstream recruitment of the autophagy receptor p62, leading to an increase in mitochondrial delivery to the lysosomes. Altogether, our study provides additional support for KAT8 as a regulator of mitophagy and autophagy processes.

Characterization of aminopeptidase N from Torpedo marmorata kidney

A major antigen of the brush border membrane of Torpedo marmorata kidney was identified and purified by immunoprecipitation. The sequence of its 18 N terminal amino acids was determined and found to be very similar to that of mammalian amino-peptidase N (EC 3.4.11.2). Indeed aminopeptidase N activity was efficiently immunoprecipitated by monoclonal antibody 180K1. The purified antigen gives a broad band at 180 kDa after SDS-gel electrophoresis, which, after treatment by endoglycosidase F, is converted to a thinner band at 140 kDa. This antigen is therefore heavily glycosylated. Depending on solubilization conditions, both the antigen and peptidase activity were recovered either as a broad peak with a sedimentation coefficient of 18S (2% CHAPS) or as a single peak of 7.8S (1% CHAPS plus 0.2% C12E9), showing that Torpedo aminopeptidase N behaves as an oligomer stabilized by hydrophobic interactions, easily converted into a 160 kDa monomer. The antigen is highly concentrated in the apical membrane of proximal tubule epithelial cells (600 gold particles/microns2 of brush border membrane) whereas no labeling could be detected in other cell types or in other membranes of the same cells (basolateral membranes, vacuoles or vesicles). Monoclonal antibodies prepared here will be useful tools for further functional and structural studies of Torpedo kidney aminopeptidase N.

Medical graduates of the National University of Ireland in 1978: who and where are they?

To inform debate on medical manpower planning and aspects of medical education, we gathered data on graduates of three Irish medical schools in 1978. Twenty six years later, four of the 236 graduates had died and seven were untraceable. All but one of the remainder were in clinical practice and in a wide range of disciplines. A third were overseas. The implications of these findings are briefly discussed.

Common genetic risk for Parkinson's disease and dysfunction of the endo-lysosomal system

Parkinson's disease is a progressive neurological disorder, characterized by prominent movement dysfunction. The past two decades have seen a rapid expansion of our understanding of the genetic basis of Parkinson's, initially through the identification of monogenic forms and, more recently, through genome-wide association studies identifying common risk variants. Intriguingly, a number of cellular pathways have emerged from these analysis as playing central roles in the aetiopathogenesis of Parkinson's. In this review, the impact of data deriving from genome-wide analyses for Parkinson's upon our functional understanding of the disease will be examined, with a particular focus on examples of endo-lysosomal and mitochondrial dysfunction. The challenges of moving from a genetic to a functional understanding of common risk variants for Parkinson's will be discussed, with a final consideration of the current state of the genetic architecture of the disorder. This article is part of a discussion meeting issue 'Understanding the endo-lysosomal network in neurodegeneration'.