Two Cases of TMEM151A-Associated Paroxysmal Dyskinesia in a Single-Center Series of PRRT2-Negative Patients.

Paroxysmal dyskinesia (PD) refers to a group of heterogeneous syndromes characterized by recurrent attacks of dystonia and/or chorea, without loss of consciousness.1 PD are classified by triggers as paroxysmal kinesigenic dyskinesia (PKD), paroxysmal nonkinesigenic dyskinesia (PNKD), and paroxysmal exertion-induced dyskinesia (PED). An increasing number of genes have been implicated in their pathogenesis.1 Although defects in the PRRT2 gene are the main cause of PKD (77%–93% familial; 21%–45% sporadic),2 TMEM151A variants have been recently identified in both familial and isolated PRRT2-negative PKD patients with a frequency ranging from 4,8% to 6,9%.2-6 However, the full clinical spectrum of this condition and the exact frequency of new allelic variants in European population have yet to be defined. Sanger sequencing of TMEM151A gene was performed in 26 sporadic and three familial cases from a single pedigree with PD in whom mutations in known-dystonia and paroxysmal movement disorder-associated genes were excluded by targeted resequencing using a customized gene panel including 39 genes (full list of genes available on request [[email protected]]), and phenotypic characteristics were analyzed. Two novel variants of the TMEM151A gene (NM_153266.4, NP_694998.1) were identified in two unrelated patients. Detailed clinical features are reported in Table 1. In silico analysis of the candidate variants was performed using different bioinformatics tools: Polyphen-2, SIFT, Mutation Taster and the VarSome software (https://varsome.com) for gnomAD, phyloP100, and American College of Medical Genetics and Genomics (ACMG) Classification (Table 2). Patient 1 (age 13 years) presented at 11 years of age with daily brief attacks of choreic movements in his right arm. Paroxysmal episodes were either kinesigenic or exertion induced, occurring after 2 or 3 min of running and during physical effort; they were preceded by an aura and were completely suppressed by low-dose carbamazepine. We uncovered a novel heterozygous TMEM151A variant c.890C > T (p.S297L) inherited from his mother suffering from migraine since 20 years of age. This variant shows low frequency in the population database and occurs in highly conserved residue. Multiple lines of in silico algorithms predicted this variant as deleterious. Notably, a different missense mutation in the same codon was described in an affected family (c.889 T > A, p.S297T).2 However according to the ACMG guidelines is classified as variant of “uncertain significance” (VUS) (Table 2). Further future investigations will be needed to verify its causative role. Patient 2 (age 19 years) presented at 9 years of age with kinesigenic dystonic movements and posturing of his right arm, lasting for few seconds; excellent and sustained long-lasting response to low doses of carbamazepine was reported after 6 years of treatment. We identified a heterozygous variant c.445_447delGTC (p.V149del) inherited from his asymptomatic mother. This variant, not previously reported and without frequency in GnomAD, occurs in preserved amino acid. SIFT classified this variant as “damaging” and bioinformatics tools (RaptorX) predict deletion-induced changes in protein folding stability. Therefore, it is classified as “uncertain significance” (PM2, PM4) according to ACMG guidelines (Table 2). In our series, 2 of 29 patients with genetically undefined PD carried pathogenetic mutations in TMEM151A gene attesting a frequency of 6.9%. This data is consistent with literature,2-6 supporting the evidence that TMEM151A is not a frequent cause of PD. TMEM151A molecular function is still largely unknown. It is predicted to be a transmembrane protein localized at endoplasmic reticulum and it is probably involved in neuronal excitability by Ca2+ and Na+ signaling modulation, similarly to PRRT2.2 By comparison with PRRT2-related PKD patients, TMEM151A mutations carriers were reported to present more frequently as pure phenotype with shorter attack duration, a later onset, an incomplete response to carbamazepine.3, 6 In contrast, our patients showed age at onset and a response to carbamazepine comparable to PRRT2 patients, but shorter attack duration (<10 s). Patient 1 showed choreoathetosic attacks that are more typically observed in PRRT2 patients,3 which were kinesigenic or triggered by sustained exercise. PED, also observed in a few PRRT2-related patients, was already reported in one TMEM151A patient.5 Our data supports the hypothesis that TMEM151A variants may have decreased penetrance, as already observed in previous reports.2, 3, 6 In addition, a clinical variability could be suggested with migraine as part of TMEM151A-related phenotypic spectrum. In fact, migraine was previously reported in two TMEM151A patients in association with PKD,3 whereas the mother of patient 2, carrying the same variant, presented migraine as an isolated symptom. To conclude, our observations further expand the genotypic and clinical spectrum of TMEM151A-related paroxysmal manifestations to include also PED and isolated migraine. Additional reports may further contribute to delineate the clinical spectrum and pathogenic relevance of TMEM151A-associated disorders and to uncover the underlying molecular mechanisms. We acknowledge the European Reference Network for Rare Neurological Diseases (ERN-RND). We acknowledge the Pierfranco and Luisa Mariani Foundation and the Paolo Zorzi Association for Neuroscience ONLUS. We thank Dr. Isabel Colangelo for technical support in genetic analysis. Open access funding provided by BIBLIOSAN. (1) Research project: A. Conception, B. Organization, C. Execution; (2) Manuscript: A. Writing of the First Draft, B. Review and Critique. S.S.: 1A, 1B, 1C, 2A F.I.: 1A, 1B, 1C, 2A C.P.: 1C C.R.: 1C F.R.D.: 1B, 2B G.Z.: 1A, 1B, 1C, 2B N.N.: 2B B.G.: 2B. Ethical Compliance Statement: The authors confirm that the approval of an institutional review board was not required for this work. Subjects gave full written consent to genetic analysis for diagnostic and scientific purposes. We confirm that we have read the Journal's position on issues involved in ethical publication and affirm that this work is consistent with those guidelines. Funding Sources and Conflicts of Interest: This work was partially supported by the Italian Ministry of Health (RRC). The authors declare that there are no conflicts of interest relevant to this work. Financial Disclosures for the Previous 12 Months: The authors declare that there are no additional disclosures to report.