Purpose:Implementation of novel genetic diagnostic tests is generally driven by technological advances because they promise shorter turnaround times and/or higher diagnostic yields. Other aspects, including impact on clinical management or cost-effectiveness, are often not assessed in detail prior to implementation.Methods:We studied the clinical utility of whole-exome sequencing (WES) in complex pediatric neurology in terms of diagnostic yield and costs. We analyzed 150 patients (and their parents) presenting with complex neurological disorders of suspected genetic origin. In a parallel study, all patients received both the standard diagnostic workup (e.g., cerebral imaging, muscle biopsies or lumbar punctures, and sequential gene-by-gene–based testing) and WES simultaneously.Results:Our unique study design allowed direct comparison of diagnostic yield of both trajectories and provided insight into the economic implications of implementing WES in this diagnostic trajectory. We showed that WES identified significantly more conclusive diagnoses (29.3%) than the standard care pathway (7.3%) without incurring higher costs. Exploratory analysis of WES as a first-tier diagnostic test indicates that WES may even be cost-saving, depending on the extent of other tests being omitted.Conclusion:Our data support such a use of WES in pediatric neurology for disorders of presumed genetic origin.Genet Med advance online publication 23 March 2017

Autosomal recessive and dominant nephrogenic diabetes insipidus (NDI), a disease in which the kidney is unable to concentrate urine in response to vasopressin, are caused by mutations in the aquaporin-2 (AQP2) gene. Missense AQP2 proteins in recessive NDI have been shown to be retarded in the endoplasmic reticulum, whereas AQP2-E258K, an AQP2 mutant in dominant NDI, was retained in the Golgi complex. In this study, we identified the molecular mechanisms underlying recessive and dominant NDI. Sucrose gradient centrifugation of rat and human kidney proteins and subsequent immunoblotting revealed that AQP2 forms homotetramers. When expressed in oocytes, wild-type AQP2 and AQP2-E258K also formed homotetramers, whereas AQP2-R187C, a mutant in recessive NDI, was expressed as a monomer. Upon co-injection, AQP2-E258K, but not AQP2-R187C, was able to heterotetramerize with wild-type AQP2. Since an AQP monomer is the functional unit and AQP2-E258K is a functional but misrouted water channel, heterotetramerization of AQP2-E258K with wild-type AQP2 and inhibition of further routing of this complex to the plasma membrane is the cause of dominant NDI. This case of NDI is the first example of a dominant disease in which the 'loss-of-function' phenotype is caused by an impaired routing rather than impaired function of the wild-type protein.

Our study demonstrates that compound heterozygous mutations in VPS13D cause movement disorders along the ataxia-spasticity spectrum, making VPS13D the fourth VPS13 paralog involved in neurological disorders. Ann Neurol 2018.

KIF1A is a neuron-specific motor protein that plays important roles in cargo transport along neurites. Recessive mutations in KIF1A were previously described in families with spastic paraparesis or sensory and autonomic neuropathy type-2. Here, we report 11 heterozygous de novo missense mutations (p.S58L, p.T99M, p.G102D, p.V144F, p.R167C, p.A202P, p.S215R, p.R216P, p.L249Q, p.E253K, and p.R316W) in KIF1A in 14 individuals, including two monozygotic twins. Two mutations (p.T99M and p.E253K) were recurrent, each being found in unrelated cases. All these de novo mutations are located in the motor domain (MD) of KIF1A. Structural modeling revealed that they alter conserved residues that are critical for the structure and function of the MD. Transfection studies suggested that at least five of these mutations affect the transport of the MD along axons. Individuals with de novo mutations in KIF1A display a phenotype characterized by cognitive impairment and variable presence of cerebellar atrophy, spastic paraparesis, optic nerve atrophy, peripheral neuropathy, and epilepsy. Our findings thus indicate that de novo missense mutations in the MD of KIF1A cause a phenotype that overlaps with, while being more severe, than that associated with recessive mutations in the same gene.

No abstract

This detailed clinical reference dataset will greatly facilitate diagnostic confirmation and management of patients, and has provided important insights into disease pathogenesis. Ann Neurol 2018;83:1105-1124.

BackgroundMutations of SCN8A encoding the neuronal voltage-gated sodium channel NaV1.6 are associated with early-infantile epileptic encephalopathy type 13 (EIEE13) and intellectual disability. Using clinical exome sequencing, we have detected three novel de novo SCN8A mutations in patients with intellectual disabilities, and variable clinical features including seizures in two patients. To determine the causality of these SCN8A mutations in the disease of those three patients, we aimed to study the (dys)function of the mutant sodium channels.MethodsThe functional consequences of the three SCN8A mutations were assessed using electrophysiological analyses in transfected cells. Genotype–phenotype correlations of these and other cases were related to the functional analyses.ResultsThe first mutant displayed a 10 mV hyperpolarising shift in voltage dependence of activation (gain of function), the second did not form functional channels (loss of function), while the third mutation was functionally indistinguishable from the wildtype channel.ConclusionsComparison of the clinical features of these patients with those in the literature suggests that gain-of-function mutations are associated with severe EIEE, while heterozygous loss-of-function mutations cause intellectual disability with or without seizures. These data demonstrate that functional analysis of missense mutations detected by clinical exome sequencing, both inherited and de novo, is valuable for clinical interpretation in the age of massive parallel sequencing.

Epilepsy is one of the most common neurological diseases and it causes profound morbidity and mortality. We identified the first de novo variant in KCNMA1 (c.2984 A > G (p.(N995S)))-encoding the BK channel-that causes epilepsy, but not paroxysmal dyskinesia, in two independent families. The c.2984 A > G (p.(N995S)) variant markedly increased the macroscopic potassium current by increasing both the channel open probability and channel open dwell time. The c.2984 A > G (p.(N995S)) variant did not affect the calcium sensitivity of the channel. We also identified three other variants of unknown significance (c.1554 G > T (p.(K518N)), c.1967A > C (p.(E656A)), and c.3476 A > G (p.(N1159S))) in three separate patients with divergent epileptic phenotypes. However, these variants did not affect the BK potassium current, and are therefore unlikely to be disease-causing. These results demonstrate that BK channel variants can cause epilepsy without paroxysmal dyskinesia. The underlying molecular mechanism can be increased activation of the BK channel by increased sensitivity to the voltage-dependent activation without affecting the sensitivity to the calcium-dependent activation. Our data suggest that the BK channel may represent a drug target for the treatment of epilepsy. Our data highlight the importance of functional electrophysiological studies of BK channel variants in distinguishing whether a genomic variant of unknown significance is a disease-causing variant or a benign variant.