New Hyperekplexia Mutations Provide Insight into Glycine Receptor Assembly, Trafficking, and Activation Mechanisms

• Published in: The Journal of biological chemistry 2013-11, Vol.288 (47), p.33745-33759
• Main Authors: Bode, Anna, Wood, Sian-Elin, Mullins, Jonathan G.L., Keramidas, Angelo, Cushion, Thomas D., Thomas, Rhys H., Pickrell, William O., Drew, Cheney J.G., Masri, Amira, Jones, Elizabeth A., Vassallo, Grace, Born, Alfred P., Alehan, Fusun, Aharoni, Sharon, Bannasch, Gerald, Bartsch, Marius, Kara, Bulent, Krause, Amanda, Karam, Elie G., Matta, Stephanie, Jain, Vivek, Mandel, Hanna, Freilinger, Michael, Graham, Gail E., Hobson, Emma, Chatfield, Sue, Vincent-Delorme, Catherine, Rahme, Jubran E., Afawi, Zaid, Berkovic, Samuel F., Howell, Owain W., Vanbellinghen, Jean-François, Rees, Mark I., Chung, Seo-Kyung, Lynch, Joseph W.
• Format: Article
• Language: English
• Subjects: Amino Acid Substitution; Chloride Channels; Cys-Loop Receptors; Female; Gene Expression Regulation; Glycine Receptors; Humans; Male; Molecular Bases of Disease; Muscle Rigidity - genetics; Muscle Rigidity - metabolism; Mutation, Missense; Neurological Diseases; Patch Clamp Electrophysiology; Protein Structure, Secondary; Protein Structure, Tertiary; Receptors, Glycine - genetics; Receptors, Glycine - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M113.509240

Hyperekplexia is a syndrome of readily provoked startle responses, alongside episodic and generalized hypertonia, that presents within the first month of life. Inhibitory glycine receptors are pentameric ligand-gated ion channels with a definitive and clinically well stratified linkage to hyperekplexia. Most hyperekplexia cases are caused by mutations in the α1 subunit of the human glycine receptor (hGlyR) gene (GLRA1). Here we analyzed 68 new unrelated hyperekplexia probands for GLRA1 mutations and identified 19 mutations, of which 9 were novel. Electrophysiological analysis demonstrated that the dominant mutations p.Q226E, p.V280M, and p.R414H induced spontaneous channel activity, indicating that this is a recurring mechanism in hGlyR pathophysiology. p.Q226E, at the top of TM1, most likely induced tonic activation via an enhanced electrostatic attraction to p.R271 at the top of TM2, suggesting a structural mechanism for channel activation. Receptors incorporating p.P230S (which is heterozygous with p.R65W) desensitized much faster than wild type receptors and represent a new TM1 site capable of modulating desensitization. The recessive mutations p.R72C, p.R218W, p.L291P, p.D388A, and p.E375X precluded cell surface expression unless co-expressed with α1 wild type subunits. The recessive p.E375X mutation resulted in subunit truncation upstream of the TM4 domain. Surprisingly, on the basis of three independent assays, we were able to infer that p.E375X truncated subunits are incorporated into functional hGlyRs together with unmutated α1 or α1 plus β subunits. These aberrant receptors exhibit significantly reduced glycine sensitivity. To our knowledge, this is the first suggestion that subunits lacking TM4 domains might be incorporated into functional pentameric ligand-gated ion channel receptors. Background: Hyperekplexia mutations have provided much information about glycine receptor structure and function. Results: We identified and characterized nine new mutations. Dominant mutations resulted in spontaneous activation, whereas recessive mutations precluded surface expression. Conclusion: These data provide insight into glycine receptor activation mechanisms and surface expression determinants. Significance: The results enhance our understanding of hyperekplexia pathology and glycine receptor structure-function.