Andreea Manole

Genetic and phenotypic characterization of complex hereditary spastic paraplegia

High-throughput next-generation sequencing can identify disease-causing mutations in extremely heterogeneous disorders. Kara et al . investigate a series of 97 index cases with complex hereditary spastic paraplegia (HSP). They identify SPG11 defects in 30 families, as well as mutations in other HSP genes and genes associated with disorders including Parkinson’s disease.

Recent advances in bulbar syndromes: genetic causes and disease mechanisms.

PURPOSE OF REVIEW With advances in next-generation gene sequencing, progress in deep phenotyping and a greater understanding of the pathogenesis of motor neuron disease, our knowledge of the progressive bulbar syndromes has significantly increased in recent years. This group of heterogeneous conditions, in which the primary disorder is focused around degeneration of the lower cranial nerves, can occur in children or adults and form a spectrum of severity, based around the common feature of bulbar dysfunction. Early genetic diagnosis may allow treatment in some bulbar syndromes. RECENT FINDINGS Brown-Vialetto-Van Laere and Fazio-Londe syndromes are the most recent childhood forms of progressive bulbar palsy to be genetically defined. The clinical phenotype of this group of childhood disorders was first reported over 120 years ago. Recently, it was demonstrated that in a third of these patients Brown-Vialetto-Van Laere is caused by mutations in the SLC52A2 and SLC52A3 genes, both of which encode riboflavin transporters. Importantly, supplementation of riboflavin can lead to significant clinical improvement if started early in the disease process. SUMMARY Here, we outline the clinical features, management and an update on the disease mechanisms and genetic causes of the progressive bulbar syndromes.

Homozygous Mutations in VAMP1 Cause a Presynaptic Congenital Myasthenic Syndrome

We report 2 families with undiagnosed recessive presynaptic congenital myasthenic syndrome (CMS). Whole exome or genome sequencing identified segregating homozygous variants in VAMP1: c.51_64delAGGTGGGGGTCCCC in a Kuwaiti family and c.146G>C in an Israeli family. VAMP1 is crucial for vesicle fusion at presynaptic neuromuscular junction (NMJ). Electrodiagnostic examination showed severely low compound muscle action potentials and presynaptic impairment. We assessed the effect of the nonsense mutation on mRNA levels and evaluated the NMJ transmission in VAMP1lew/lew mice, observing neurophysiological features of presynaptic impairment, similar to the patients. Taken together, our findings highlight VAMP1 homozygous mutations as a cause of presynaptic CMS. Ann Neurol 2017;81:597–603

The phenotypic and molecular spectrum of PEHO syndrome and PEHO-like disorders

1 Department of Molecular Neuroscience, Institute of Neurology, UCL Institute of Neurology, London WC1N 3BG, UK 2 Department of Molecular Medicine and Medical Biotechnologies “DMMBM”, University of Naples “Federico II”, Naples 80131, Italy 3 CEINGE Biotecnologie Avanzate, Naples 80131, Italy 4 European School of Molecular Medicine, SEMM, University of Milan, Italy 5 King Saud bin Abdulaziz University for Health Sciences, Department of Pediatrics, Division of Genetics, Riyadh 14611, Saudi Arabia 6 Genetics and Rare Diseases Research Division, Ospedale Pediatrico “Bambino Gesù”, Rome 00146, Italy

A novel KCNA1 mutation in a family with episodic ataxia and malignant hyperthermia.

Episodic ataxia type 1 (EA1) is an autosomal dominant channelopathy caused by mutations in KCNA1, which encodes the voltage-gated potassium channel, Kv1.1. Eleven members of an EA family were evaluated with molecular and functional studies. A novel c.746T>G (p.Phe249Cys) missense mutation of KCNA1 segregated in the family members with episodic ataxia, myokymia, and malignant hyperthermia susceptibility. No mutations were found in the known malignant hyperthermia genes RYR1 or CACNA1S. The Phe249Cys-Kv1.1 channels did not show any currents upon functional expression, confirming a pathogenic role of the mutation. Malignant hyperthermia may be a presentation of KCNA1 mutations, which has significant implications for the clinical care of these patients and illustrates the phenotypic heterogeneity of KCNA1 mutations.

Riboflavin Responsive Mitochondrial Dysfunction in Neurodegenerative Diseases

Mitochondria are the repository for various metabolites involved in diverse energy-generating processes, like the TCA cycle, oxidative phosphorylation, and metabolism of amino acids, fatty acids, and nucleotides, which rely significantly on flavoenzymes, such as oxidases, reductases, and dehydrogenases. Flavoenzymes are functionally dependent on biologically active flavin adenine dinucleotide (FAD) or flavin mononucleotide (FMN), which are derived from the dietary component riboflavin, a water soluble vitamin. Riboflavin regulates the structure and function of flavoenzymes through its cofactors FMN and FAD and, thus, protects the cells from oxidative stress and apoptosis. Hence, it is not surprising that any disturbance in riboflavin metabolism and absorption of this vitamin may have consequences on cellular FAD and FMN levels, resulting in mitochondrial dysfunction by reduced energy levels, leading to riboflavin associated disorders, like cataracts, neurodegenerative and cardiovascular diseases, etc. Furthermore, mutations in either nuclear or mitochondrial DNA encoding for flavoenzymes and flavin transporters significantly contribute to the development of various neurological disorders. Moreover, recent studies have evidenced that riboflavin supplementation remarkably improved the clinical symptoms, as well as the biochemical abnormalities, in patients with neuronopathies, like Brown-Vialetto-Van-Laere syndrome (BVVLS) and Fazio-Londe disease. This review presents an updated outlook on the cellular and molecular mechanisms of neurodegenerative disorders in which riboflavin deficiency leads to dysfunction in mitochondrial energy metabolism, and also highlights the significance of riboflavin supplementation in aforementioned disease conditions. Thus, the outcome of this critical assessment may exemplify a new avenue to enhance the understanding of possible mechanisms in the progression of neurodegenerative diseases and may provide new rational approaches of disease surveillance and treatment.

De novo KCNA2 mutations cause hereditary spastic paraplegia

grounds, and most of them were recruited through the CanHSP network, which includes 7 centers across Canada. All patients were diagnosed by neurologists with expertise in motor neuron diseases, and all provided informed consent before participating in the study, which was approved by the respective institutional review boards. The average coverage of KCNA2 was 3 217, with a range of 358 to 3 379 across all samples, and in all samples 100% of the coding nucleotides of KCNA2 were covered by>310. No nonsynonymous, splice-site, or stop mutations were identified in any of the HSP patients. Only 3 synonymous variants were found. One is a rare variant c.G372A (p.E124E), with allele frequency (AF) of 0.0002 in ExAC (exac.broadinstitute. org), but it was found in only 1 patient of 6 affected with HSP from the same family. Two more common variants were also identified, c.G1185C (p.A395A, AF 5 0.05, rs78349687) and c.T1026C (p.D342D, AF 5 0.14, rs12407942), which also did not segregate with the HSP phenotype, and the allele frequencies are too common to be considered likely pathogenic. Our data suggest that HSP patients with KCNA2 mutations are rare, at least in our ethnically diverse population. We suggest that the various phenotypes reported in individuals with KCNA2 mutations in the recent studies, including epilepsy, intellectual disability, ataxia, and spasticity, represent a spectrum of the same disorder with the same underlying mechanism, especially because some of the recently reported cases had phenotypes similar to those previously reported. Although novel phenotypes associated with known genes are well recognized, sufficient data that support the proposed genotype–phenotype correlation is necessary. The identification of more families with clinically confirmed HSP and KCNA2 mutations would support this mutation being a separate entity causing HSP.

Severe axonal neuropathy is a late manifestation of SPG11

Complex hereditary spastic paraplegia (HSP) is a clinically heterogeneous group of disorders usually inherited in an autosomal recessive manner. In the past, complex recessive spastic paraplegias have been frequently associated with SPG11 mutations but also with defects in SPG15, SPG7 and a handful of other rare genes. Pleiotropy exists in HSP genes, exemplified in the recent association of SPG11 mutations with CMT2. In this study, we performed whole exome sequence analysis and identified two siblings with novel compound heterozygous frameshift SPG11 mutations. The mutations segregated with disease were not present in control databases and analysis of skin fibroblast derived mRNA indicated that the SPG11 truncated mRNA species were not degraded significantly by non-sense mediated mRNA decay. These siblings had severe early-onset spastic paraplegia but later in their disease developed severe axonal neuropathy, neuropathic pain and blue/black foot discolouration likely caused by a combination of the severe neuropathy with autonomic dysfunction and peripheral oedema. We also identified a similar late-onset axonal neuropathy in a Cypriot SPG11 family. Although neuropathy is occasionally present in SPG11, in our SPG11 patients reported here it was particularly severe, highlighting the association of axonal neuropathy with SPG11 and the late manifestation of axonal peripheral nerve damage.

Next-generation sequencing in neuromuscular diseases.

PURPOSE OF REVIEW Neuromuscular diseases are clinically and genetically heterogeneous and probably contain the greatest proportion of causative Mendelian defects than any other group of conditions. These disorders affect muscle and/or nerves with neonatal, childhood or adulthood onset, with significant disability and early mortality. Along with heterogeneity, unidentified and often very large genes require complementary and comprehensive methods in routine molecular diagnosis. Inevitably, this leads to increased diagnostic delays and challenges in the interpretation of genetic variants. RECENT FINDINGS The application of next-generation sequencing, as a research and diagnostic strategy, has made significant progress into solving many of these problems. The analysis of these data is by no means simple, and the clinical input is essential to interpret results. SUMMARY In this review, we describe using examples the recent advances in the genetic diagnosis of neuromuscular disorders, in research and clinical practice and the latest developments that are underway in next-generation sequencing. We also discuss the latest collaborative initiatives such as the Genomics England (Department of Health, UK) genome sequencing project that combine rare disease clinical phenotyping with genomics, with the aim of defining the vast majority of rare disease genes in patients as well as modifying risks and pharmacogenomics factors.

A homozygous loss-of-function mutation in PDE2A associated to early-onset hereditary chorea: A Homozygous PDE2A Mutation Causing Chorea

Background: We investigated a family that presented with an infantile‐onset chorea‐predominant movement disorder, negative for NKX2‐1, ADCY5, and PDE10A mutations. Methods: Phenotypic characterization and trio whole‐exome sequencing was carried out in the family. Results: We identified a homozygous mutation affecting the GAF‐B domain of the 3’,5’‐cyclic nucleotide phosphodiesterase PDE2A gene (c.1439A>G; p.Asp480Gly) as the candidate novel genetic cause of chorea in the proband. PDE2A hydrolyzes cyclic adenosine/guanosine monophosphate and is highly expressed in striatal medium spiny neurons. We functionally characterized the p.Asp480Gly mutation and found that it severely decreases the enzymatic activity of PDE2A. In addition, we showed equivalent expression in human and mouse striatum of PDE2A and its homolog gene, PDE10A. Conclusions: We identified a loss‐of‐function homozygous mutation in PDE2A associated to early‐onset chorea. Our findings possibly strengthen the role of cyclic adenosine monophosphate and cyclic guanosine monophosphate metabolism in striatal medium spiny neurons as a crucial pathophysiological mechanism in hyperkinetic movement disorders.