Manju A. Kurian

Exome sequencing reveals de novo WDR45 mutations causing a phenotypically distinct, X-linked dominant form of NBIA.

Neurodegeneration with brain iron accumulation (NBIA) is a group of genetic disorders characterized by abnormal iron deposition in the basal ganglia. We report that de novo mutations in WDR45, a gene located at Xp11.23 and encoding a beta-propeller scaffold protein with a putative role in autophagy, cause a distinctive NBIA phenotype. The clinical features include early-onset global developmental delay and further neurological deterioration (parkinsonism, dystonia, and dementia developing by early adulthood). Brain MRI revealed evidence of iron deposition in the substantia nigra and globus pallidus. Males and females are phenotypically similar, an observation that might be explained by somatic mosaicism in surviving males and germline or somatic mutations in females, as well as skewing of X chromosome inactivation. This clinically recognizable disorder is among the more common forms of NBIA, and we suggest that it be named accordingly as beta-propeller protein-associated neurodegeneration.

Phenotypic spectrum of neurodegeneration associated with mutations in the PLA2G6 gene (PLAN)

Background: Neurodegeneration associated with brain iron accumulation (NBIA) comprises a heterogeneous group of disorders in which disruption of cellular mechanisms leads to accumulation of iron in the basal ganglia. This group includes patients with recently discovered mutations in the PLA2G6 gene encoding a calcium-independent phospholipase A2 enzyme that catalyzes the hydrolysis of glycerophospholipids. Previously, children with PLA2G6 mutations have been diagnosed with several different disorders and we wished to better define the phenotype of PLA2G6- associated neurodegeneration. Methods: Detailed review of the clinical and genetic features of 14 and radiologic features of 13 of these patients with PLA2G6 mutations was undertaken. Results: Median age of symptom presentation was 14 months. One third of the cohort presented following an intercurrent illness. The children had progressive cognitive and motor skill regression, with evidence of axial hypotonia, four limb spasticity, bulbar dysfunction, and strabismus. All patients developed cerebellar ataxia and dystonia. Most patients had optic atrophy. Brain imaging demonstrated cerebellar cortical atrophy and gliosis in all patients. Changes consistent with increased iron deposition were identified in the globus pallidus and substantia nigra. Novel corpus callosum changes are also reported. Conclusion: We describe a cohort of patients with PLA2G6-associated neurodegeneration (PLAN). Although patients with PLAN have previously been diagnosed with infantile neuroaxonal dystrophy, neurodegeneration associated with brain iron accumulation, and Karak syndrome, they display a characteristic clinical and radiologic phenotype. PLA2G6 mutational analysis will negate the need for more invasive diagnostic procedures such as tissue biopsy.

The monoamine neurotransmitter disorders: an expanding range of neurological syndromes

The monoamine neurotransmitter disorders consist of a rapidly expanding heterogeneous group of neurological syndromes characterised by primary and secondary defects in the biosynthesis degradation, or transport of dopamine, norepinephrine, epinephrine, and serotonin. Disease onset can occur any time from infancy onwards. Clinical presentation depends on the pattern and severity of neurotransmitter abnormalities, and is predominated by neurological features (encephalopathy, epilepsy, and pyramidal and extrapyramidal motor disorders) that are primarily attributed to deficiency of cerebral dopamine, serotonin, or both. Many neurotransmitter disorders mimic the phenotype of other neurological disorders (eg, cerebral palsy, hypoxic ischaemic encephalopathy, paroxysmal disorders, inherited metabolic diseases, and genetic dystonic or parkinsonian syndromes) and are, therefore, frequently misdiagnosed. Early clinical suspicion and appropriate investigations, including analysis of neurotransmitters in CSF, are essential for accurate clinical diagnosis. Treatment strategies focus on the correction of monoamine deficiency by replacement of monoamine precursors, the use of monoamine analogues, inhibition of monoamine degradation, and addition of enzyme cofactors to promote monoamine production.

Homozygous loss-of-function mutations in the gene encoding the dopamine transporter are associated with infantile parkinsonism-dystonia

Genetic variants of the SLC6A3 gene that encodes the human dopamine transporter (DAT) have been linked to a variety of neuropsychiatric disorders, particularly attention deficit hyperactivity disorder. In addition, the homozygous Slc6a3 knockout mouse displays a hyperactivity phenotype. Here, we analyzed 2 unrelated consanguineous families with infantile parkinsonism-dystonia (IPD) syndrome and identified homozygous missense SLC6A3 mutations (p.L368Q and p.P395L) in both families. Functional studies demonstrated that both mutations were loss-of-function mutations that severely reduced levels of mature (85-kDa) DAT while having a differential effect on the apparent binding affinity of dopamine. Thus, in humans, loss-of-function SLC6A3 mutations that impair DAT-mediated dopamine transport activity are associated with an early-onset complex movement disorder. Identification of the molecular basis of IPD suggests SLC6A3 as a candidate susceptibility gene for other movement disorders associated with parkinsonism and/or dystonic features.

Clinical whole-genome sequencing in severe early-onset epilepsy reveals new genes and improves molecular diagnosis

In severe early-onset epilepsy, precise clinical and molecular genetic diagnosis is complex, as many metabolic and electro-physiological processes have been implicated in disease causation. The clinical phenotypes share many features such as complex seizure types and developmental delay. Molecular diagnosis has historically been confined to sequential testing of candidate genes known to be associated with specific sub-phenotypes, but the diagnostic yield of this approach can be low. We conducted whole-genome sequencing (WGS) on six patients with severe early-onset epilepsy who had previously been refractory to molecular diagnosis, and their parents. Four of these patients had a clinical diagnosis of Ohtahara Syndrome (OS) and two patients had severe non-syndromic early-onset epilepsy (NSEOE). In two OS cases, we found de novo non-synonymous mutations in the genes KCNQ2 and SCN2A. In a third OS case, WGS revealed paternal isodisomy for chromosome 9, leading to identification of the causal homozygous missense variant in KCNT1, which produced a substantial increase in potassium channel current. The fourth OS patient had a recessive mutation in PIGQ that led to exon skipping and defective glycophosphatidyl inositol biosynthesis. The two patients with NSEOE had likely pathogenic de novo mutations in CBL and CSNK1G1, respectively. Mutations in these genes were not found among 500 additional individuals with epilepsy. This work reveals two novel genes for OS, KCNT1 and PIGQ. It also uncovers unexpected genetic mechanisms and emphasizes the power of WGS as a clinical tool for making molecular diagnoses, particularly for highly heterogeneous disorders.

PRRT2 gene mutations From paroxysmal dyskinesia to episodic ataxia and hemiplegic migraine

ABSTRACT Objective: The proline-rich transmembrane protein (PRRT2) gene was recently identified using exome sequencing as the cause of autosomal dominant paroxysmal kinesigenic dyskinesia (PKD) with or without infantile convulsions (IC) (PKD/IC syndrome). Episodic neurologic disorders, such as epilepsy, migraine, and paroxysmal movement disorders, often coexist and are thought to have a shared channel-related etiology. To investigate further the frequency, spectrum, and phenotype of PRRT2 mutations, we analyzed this gene in 3 large series of episodic neurologic disorders with PKD/IC, episodic ataxia (EA), and hemiplegic migraine (HM). Methods: The PRRT2 gene was sequenced in 58 family probands/sporadic individuals with PKD/IC, 182 with EA, 128 with HM, and 475 UK and 96 Asian controls. Results: PRRT2 genetic mutations were identified in 28 out of 58 individuals with PKD/IC (48%), 1/182 individuals with EA, and 1/128 individuals with HM. A number of loss-of-function and coding missense mutations were identified; the most common mutation found was the p.R217Pfs*8 insertion. Males were more frequently affected than females (ratio 52:32). There was a high proportion of PRRT2 mutations found in families and sporadic cases with PKD associated with migraine or HM (10 out of 28). One family had EA with HM and another large family had typical HM alone. Conclusions: This work expands the phenotype of mutations in the PRRT2 gene to include the frequent occurrence of migraine and HM with PKD/IC, and the association of mutations with EA and HM and with familial HM alone. We have also extended the PRRT2 mutation type and frequency in PKD and other episodic neurologic disorders.

Clinical and molecular characterisation of hereditary dopamine transporter deficiency syndrome: an observational cohort and experimental study

Summary Background Dopamine transporter deficiency syndrome is the first identified parkinsonian disorder caused by genetic alterations of the dopamine transporter. We describe a cohort of children with mutations in the gene encoding the dopamine transporter (SLC6A3) with the aim to improve clinical and molecular characterisation, reduce diagnostic delay and misdiagnosis, and provide insights into the pathophysiological mechanisms. Methods 11 children with a biochemical profile suggestive of dopamine transporter deficiency syndrome were enrolled from seven paediatric neurology centres in the UK, Germany, and the USA from February, 2009, and studied until June, 2010. The syndrome was characterised by detailed clinical phenotyping, biochemical and neuroradiological studies, and SLC6A3 mutation analysis. Mutant constructs of human dopamine transporter were used for in-vitro functional analysis of dopamine uptake and cocaine-analogue binding. Findings Children presented in infancy (median age 2·5 months, range 0·5–7) with either hyperkinesia (n=5), parkinsonism (n=4), or a mixed hyperkinetic and hypokinetic movement disorder (n=2). Seven children had been initially misdiagnosed with cerebral palsy. During childhood, patients developed severe parkinsonism-dystonia associated with an eye movement disorder and pyramidal tract features. All children had raised ratios of homovanillic acid to 5-hydroxyindoleacetic acid in cerebrospinal fluid, of range 5·0–13·2 (normal range 1·3–4·0). Homozygous or compound heterozygous SLC6A3 mutations were detected in all cases. Loss of function in all missense variants was recorded from in-vitro functional studies, and was supported by the findings of single photon emission CT DaTSCAN imaging in one patient, which showed complete loss of dopamine transporter activity in the basal nuclei. Interpretation Dopamine transporter deficiency syndrome is a newly recognised, autosomal recessive disorder related to impaired dopamine transporter function. Careful characterisation of patients with this disorder should provide novel insights into the complex role of dopamine homoeostasis in human disease, and understanding of the pathophysiology could help to drive drug development. Funding Birmingham Childrens Hospital Research Foundation, Birth Defects Foundation Newlife, Action Medical Research, US National Institutes of Health, Wellchild, and the Wellcome Trust.

The genetic landscape of the epileptic encephalopathies of infancy and childhood.

Epileptic encephalopathies of infancy and childhood comprise a large, heterogeneous group of severe epilepsies characterised by several seizure types, frequent epileptiform activity on EEG, and developmental slowing or regression. The encephalopathies include many age-related electroclinical syndromes with specific seizure types and EEG features. With the molecular revolution, the number of known monogenic determinants underlying the epileptic encephalopathies has grown rapidly. De-novo dominant mutations are frequently identified; somatic mosaicism and recessive disorders are also seen. Several genes can cause one electroclinical syndrome, and, conversely, one gene might be associated with phenotypic pleiotropy. Diverse genetic causes and molecular pathways have been implicated, involving ion channels, and proteins needed for synaptic, regulatory, and developmental functions. Gene discovery provides the basis for neurobiological insights, often showing convergence of mechanistic pathways. These findings underpin the development of targeted therapies, which are essential to improve the outcome of these devastating disorders.

Loss-of-Function Mutations in RAB18 Cause Warburg Micro Syndrome

Warburg Micro syndrome and Martsolf syndrome are heterogenous autosomal-recessive developmental disorders characterized by brain, eye, and endocrine abnormalities. Previously, identification of mutations in RAB3GAP1 and RAB3GAP2 in both these syndromes implicated dysregulation of the RAB3 cycle (which controls calcium-mediated exocytosis of neurotransmitters and hormones) in disease pathogenesis. RAB3GAP1 and RAB3GAP2 encode the catalytic and noncatalytic subunits of the hetrodimeric enzyme RAB3GAP (RAB3GTPase-activating protein), a key regulator of the RAB3 cycle. We performed autozygosity mapping in five consanguineous families without RAB3GAP1/2 mutations and identified loss-of-function mutations in RAB18. A c.71T > A (p.Leu24Gln) founder mutation was identified in four Pakistani families, and a homozygous exon 2 deletion (predicted to result in a frameshift) was found in the fifth family. A single family whose members were compound heterozygotes for an anti-termination mutation of the stop codon c.619T > C (p.X207QextX20) and an inframe arginine deletion c.277_279 del (p.Arg93 del) were identified after direct gene sequencing and multiplex ligation-dependent probe amplification (MLPA) of a further 58 families. Nucleotide binding assays for RAB18(Leu24Gln) and RAB18(Arg93del) showed that these mutant proteins were functionally null in that they were unable to bind guanine. The clinical features of Warburg Micro syndrome patients with RAB3GAP1 or RAB3GAP2 mutations and RAB18 mutations are indistinguishable, although the role of RAB18 in trafficking is still emerging, and it has not been linked previously to the RAB3 pathway. Knockdown of rab18 in zebrafish suggests that it might have a conserved developmental role. Our findings imply that RAB18 has a critical role in human brain and eye development and neurodegeneration.

Exome sequence reveals mutations in CoA synthase as a cause of neurodegeneration with brain iron accumulation

Neurodegeneration with brain iron accumulation (NBIA) comprises a clinically and genetically heterogeneous group of disorders with progressive extrapyramidal signs and neurological deterioration, characterized by iron accumulation in the basal ganglia. Exome sequencing revealed the presence of recessive missense mutations in COASY, encoding coenzyme A (CoA) synthase in one NBIA-affected subject. A second unrelated individual carrying mutations in COASY was identified by Sanger sequence analysis. CoA synthase is a bifunctional enzyme catalyzing the final steps of CoA biosynthesis by coupling phosphopantetheine with ATP to form dephospho-CoA and its subsequent phosphorylation to generate CoA. We demonstrate alterations in RNA and protein expression levels of CoA synthase, as well as CoA amount, in fibroblasts derived from the two clinical cases and in yeast. This is the second inborn error of coenzyme A biosynthesis to be implicated in NBIA.