KCNB1 Genotypes and Phenotypes

Research JAMA Neurology | Original Investigation Neurodevelopmental Disorders Caused by De Novo Variants in KCNB1 Genotypes and Phenotypes Carolien G. F. de Kovel, PhD; Steffen Syrbe, MD, PhD; Eva H. Brilstra, MD, PhD; Nienke Verbeek, MD, PhD; Bronwyn Kerr, PhD; Holly Dubbs, MD; Allan Bayat, MD, PhD; Sonal Desai, MGC, CGC; Sakkubai Naidu, MD; Siddharth Srivastava, MD; Hande Cagaylan, MD; Uluc Yis, MD, PhD; Carol Saunders, PhD; Martin Rook, PhD; Susanna Plugge, MSc; Hiltrud Muhle, MD; Zaid Afawi, MD, PhD; Karl-Martin Klein, MD, PhD; Vijayakumar Jayaraman, MSc; Ramakrishnan Rajagopalan, PhD; Ethan Goldberg, MD, PhD; Eric Marsh, MD, PhD; Sudha Kessler, MD, MSCE; Christina Bergqvist, MD; Laura K. Conlin, PhD; Bryan L. Krok, PhD; Isabelle Thiffault, PhD; Manuela Pendziwiat, MSc; Ingo Helbig, MD; Tilman Polster, MD, PhD; Ingo Borggraefe, MD, PhD; Johannes R. Lemke, MD, PhD; Marie-José van den Boogaardt, MD, PhD; Rikke S. Møller, MSc, PhD; Bobby P. C. Koeleman, PhD IMPORTANCE Knowing the range of symptoms seen in patients with a missense or loss-of-function variant in KCNB1 and how these symptoms correlate with the type of variant will help clinicians with diagnosis and prognosis when treating new patients. OBJECTIVES To investigate the clinical spectrum associated with KCNB1 variants and the genotype-phenotype correlations. DESIGN, SETTING, AND PARTICIPANTS This study summarized the clinical and genetic information of patients with a presumed pathogenic variant in KCNB1. Patients were identified in research projects or during clinical testing. Information on patients from previously published articles was collected and authors contacted if feasible. All patients were seen at a clinic at one of the participating institutes because of presumed genetic disorder. They were tested in a clinical setting or included in a research project. MAIN OUTCOMES AND MEASURES The genetic variant and its inheritance and information on the patient's symptoms and characteristics in a predefined format. All variants were identified with massive parallel sequencing and confirmed with Sanger sequencing in the patient. Absence of the variant in the parents could be confirmed with Sanger sequencing in all families except one. RESULTS Of 26 patients (10 female, 15 male, 1 unknown; mean age at inclusion, 9.8 years; age range, 2-32 years) with developmental delay, 20 (77%) carried a missense variant in the ion channel domain of KCNB1, with a concentration of variants in region S5 to S6. Three variants that led to premature stops were located in the C-terminal and 3 in the ion channel domain. Twenty-one of 25 patients (84%) had seizures, with 9 patients (36%) starting with epileptic spasms between 3 and 18 months of age. All patients had developmental delay, with 17 (65%) experiencing severe developmental delay; 14 (82%) with severe delay had behavioral problems. The developmental delay was milder in 4 of 6 patients with stop variants and in a patient with a variant in the S2 transmembrane element rather than the S4 to S6 region. CONCLUSIONS AND RELEVANCE De novo KCNB1 missense variants in the ion channel domain and loss-of-function variants in this domain and the C-terminal likely cause neurodevelopmental disorders with or without seizures. Patients with presumed pathogenic variants in KCNB1 have a variable phenotype. However, the type and position of the variants in the protein are (imperfectly) correlated with the severity of the disorder. Author Affiliations: Author affiliations are listed at the end of this article. Corresponding Author: Bobby P. C. Koeleman, PhD, Department of Genetics, University Medical Center Utrecht, PO Box 85500, 3508 GA JAMA Neurol. 2017;74(10):1228-1236. doi:10.1001/jamaneurol.2017.1714 Utrecht, the Netherlands Published online August 14, 2017. ([email protected]). 1228 (Reprinted) jamaneurology.com © 2017 American Medical Association. All rights reserved. Downloaded From: by a Radboud University Nijmegen User on 10/27/2017 Neurodevelopmental Disorders Caused by KCNB1 De Novo Variants Original Investigation Research arly infantile epileptic encephalopathies (EEs) form a group of disorders that are characterized by epileptic sei- Key Points zures starting during the first year of life. Patients have E Question How does the phenotype of patients with a KCNB1 developmental delay (DD) or even regression to which the epi- mutation correlate with the type of mutation? leptic discharges are presumed to contribute. The seizures are Findings This study found that patients with a de novo mutation often unresponsive to treatment. Many of those epilepsies have in KCNB1 present a variable phenotype of neurodevelopmental a genetic disposition.1 disorder. In the past few years, several new genes have been identified that can cause EE when mutated. One of the more re- Meaning De novo mutations in KCNB1 are associated with a variable neurodevelopmental disorder. cently discovered genes is KCNB1 (potassium voltage-gated channel subfamily B member 1 or delayed rectifier potassium channel 1 [DRK1]) (OMIM 616056), which acts in a dominant KCNQ3 (OMIM 602232).14 The protein forms homotetramers manner. Variants are described by Torkamani et al,2 who iden- and heterotetramers with various other voltage-gated potas- tified a de novo KCNB1 variant when exome sequencing a fam- sium channel proteins, such as KCNB2 (OMIM 607738), and ily with a child with EE. They subsequently found 2 other in- proteins from the KCNG (Kv6), KCNE, and KCNS (Kv9) dependent patients with EE and KCNB1 variants. One of those families.15-17 patients had earlier been described in the Epi4K effort3 as hav- ing Lennox-Gastaut syndrome. Subsequently, more patients 4-10 with variants in KCNB1 were described. Methods The KCNB1 gene encodes the Kv2.1 pore-forming, voltage- sensing α-subunit of a delayed rectifier potassium channel. It is In a recent screen of 358 children with EEs, we identified 3 novel expressed in various neuronal cells in the brain.11 In Ensembl KCNB1 variants (approximately 1%), all of which were found (GRCh38.p3), only 1 transcript is known (ENST00000371741). to be de novo.18 We subsequently collected data from 13 ad- It consists of only 2 exons and contains 4 known domains ditional patients from research and diagnostic sources who (Figure): the T1 domain, which is involved in multimeriza- were reported to carry a de novo novel KCNB1 variant not pres- tion; the ion_trans domain, which contains the transmem- ent in public databases. In this article, we describe the phe- brane elements; and 2 intracellular Kv2 channel–specific do- notypic and genetic characteristics of these patients. In addi- mains (Pfam; http://smart.embl-heidelberg.de/).12,13 The tion, we collected more information about 10 patients who were protein has 6 transmembrane elements in the ion_trans do- described previously. For 3 of the 10 patients in previously pub- main; the first 4 form the voltage-sensor domain, with S4 being lished articles, the authors have provided updated informa- the actual voltage sensor, whereas S5 and S6 form the pore tion for the current article. We present the available informa- domain of the protein. The extracellular loop between S5 tion on 26 patients for a broad overview of the phenotypic and and S6 contains the selectivity filter (TVGYG amino acids; genetic variability and similarities. Statistical tests were per- UNIPROT.org). This structure is similar to other voltage- formed using χ2 2 × 2 contingency tables; the significance of gated potassium channels, such as KCNQ2 (OMIM 602235) and the enrichment of de novo mutation was calculated using a Figure. Distribution of Pathogenic Variants in KCNB1 Selectivity filter 220 Pathogenic missense variant Pathogenic LoF variant BTB Kv2-specific domain Extracellular i ii iii iv v vi Cytoplasmic Milder 430 Developmental delay 160 850 No seizures Schematic view of the Kv2.1 (KCNB1) protein in the membrane, showing pathogenic missense and loss-of-function (LoF) variants. BTB indicates broad-complex, tramtrack, and bric-brac domain. jamaneurology.com (Reprinted) JAMA Neurology October 2017 Volume 74, Number 10 1229 © 2017 American Medical Association. All rights reserved. Downloaded From: by a Radboud University Nijmegen User on 10/27/2017 Research Original Investigation Neurodevelopmental Disorders Caused by KCNB1 De Novo Variants χ2 goodness-of-fit test. A 2-sided χ2 test with P < .05 determined carried the prominent variant G381R within the selectivity fil- statistical significance. Collection of data procedures agreed ter of the P-loop.7 In most patients with epilepsy, the semio- with the local ethics guidelines at the different institutes. logic features developed over time into multiple seizure types, including tonic, focal-clonic, myoclonic, and atypical ab- sences. Three patients did not have epileptic seizures (pa- Results tients 1, 3, and 24), 1 reported only infantile spasms (patient 2), and no data were available for 1 (patient 8). The patients with Description of the Variants LoF variants in the C-terminal region had no seizures or had We identified 16 patients with KCNB1 variants who were pre- infantile spasms only; in addition, 1 patient with a variant in viously not described and collected information about 10 pre- the S1 to S2 linker reported no seizures. However, one patient viously described patients (10 female, 15 male, 1 unknown; (patient 3) with LoF variant in the C-terminus reported se- mean age at inclusion, 9.8 years; age range, 2-32 years).2-9 In vere DD. Of the patients with LoF variants, 2 were reported to these 26 patients, we identified 23 different variants; 3 vari- have severe DD and 4 had moderate or moderate-severe DD ants were seen twice each. Twenty patients (77%) carried mis- (1 unspecified). In contrast, the only patient with a missense sense variants, and 6 (23%) carried nonsense or frameshift vari- variant for whom moderate DD was reported was patient 23, ants leading to a premature termination codon (Table 1).

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