Pathogenic variants in the AFG3L2 proteolytic domain cause SCA28 through haploinsufficiency and proteostatic stress-driven OMA1 activation.

BACKGROUND: Spinocerebellar ataxia type 28 (SCA28) is a dominantly inherited neurodegenerative disease caused by pathogenic variants in AFG3L2. The AFG3L2 protein is a subunit of mitochondrial m-AAA complexes involved in protein quality control. Objective of this study was to determine the molecular mechanisms of SCA28, which has eluded characterisation to date. METHODS: We derived SCA28 patient fibroblasts carrying different pathogenic variants in the AFG3L2 proteolytic domain (missense: the newly identified p.F664S and p.M666T, p.G671R, p.Y689H and a truncating frameshift p.L556fs) and analysed multiple aspects of mitochondrial physiology. As reference of residual m-AAA activity, we included SPAX5 patient fibroblasts with homozygous p.Y616C pathogenic variant, AFG3L2+/- HEK293 T cells by CRISPR/Cas9-genome editing and Afg3l2-/- murine fibroblasts. RESULTS: We found that SCA28 cells carrying missense changes have normal levels of assembled m-AAA complexes, while the cells with a truncating pathogenic variant had only half of this amount. We disclosed inefficient mitochondrial fusion in SCA28 cells caused by increased OPA1 processing operated by hyperactivated OMA1. Notably, we found altered mitochondrial proteostasis to be the trigger of OMA1 activation in SCA28 cells, with pharmacological attenuation of mitochondrial protein synthesis resulting in stabilised levels of OMA1 and OPA1 long forms, which rescued mitochondrial fusion efficiency. Secondary to altered mitochondrial morphology, mitochondrial calcium uptake resulted decreased in SCA28 cells. CONCLUSION: Our data identify the earliest events in SCA28 pathogenesis and open new perspectives for therapy. By identifying similar mitochondrial phenotypes between SCA28 cells and AFG3L2+/- cells, our results support haploinsufficiency as the mechanism for the studied pathogenic variants.

Impact of reimbursement restrictions on the choice of antiepileptic drugs: Belgian Study on Epilepsy Treatment (BESET).

BACKGROUND: In Belgium, new and costly antiepileptic drugs (AEDs) are only reimbursed as second-line treatment, after documented treatment with conventional and cheaper AEDs has failed. The objective of this study was to describe the treatment of epilepsy in Belgium and to analyze the impact of the reimbursement restrictions on the choice of AEDs. METHODS: Between May and June 2003, a sample of 100 neurologists, representative of the entire neurological community in teaching, academic, and regional hospitals in Belgium, were personally interviewed on the basis of a structured questionnaire (modified Rand method). The questionnaire contained questions on treatment choices and strategies in adult epilepsy. RESULTS: Unanimously, initial monotherapy was the preferred treatment strategy in all types of epilepsy. In the opinion of most neurologists, valproate was the first choice for idiopathic generalized and focal epilepsy with/without secondary generalization. Carbamazepine as their first choice for the treatment of focal epilepsy. New AEDs were most often prescribed as second-line therapy. Lamotrigine was the most frequently prescribed new AED and used for both generalized and focal epilepsy. It was followed by levetiracetam, topiramate and oxcarbazepine for focal epilepsy. In the absence of reimbursement restrictions, two new AEDs would be significantly more often prescribed as a first-line therapy: lamotrigine for idiopathic generalized epilepsy and oxcarbazepine for focal epilepsy. CONCLUSIONS: The neurologists reached a high level of consensus on many of the key treatment questions. Monotherapy with valproate and carbamazepine was the standard treatment strategy in Belgium. Lamotrigine and less so levetiracetam, topiramate and oxcarbazepine were commonly prescribed as second-line AEDs. In the absence of reimbursement restrictions, lamotrigine and oxcarbazepine would be more frequently prescribed.

Severe epilepsy resulting from genetic interaction between Scn2a and Kcnq2.

A mutation in the voltage-gated sodium-channel Scn2a results in moderate epilepsy in transgenic Scn2a(Q54) mice maintained on a C57BL/6J strain background. The onset of progressive epilepsy begins in adults with short-duration partial seizures that originate in the hippocampus. The underlying abnormality is an increase in persistent sodium current in hippocampal neurons. The voltage-gated potassium channel Kcnq2 is responsible for generating M current (I(KM)) that is thought to control excitability and limit repetitive firing of hippocampal neurons. To determine whether impaired M current would exacerbate the seizure phenotype of Scn2a(Q54) mice, we carried out genetic crosses with two mutant alleles of Kcnq2. Szt1 mice carry a spontaneous deletion that removes the C-terminal domain of Kcnq2. A novel Kcnq2 missense mutation V182M was identified by screening the offspring of ENU-treated males for reduced threshold to electrically evoked minimal clonic seizures. Double mutant mice carrying the Scn2a(Q54) transgene together with either of the Kcnq2 mutations exhibited severe epilepsy with early onset, generalized tonic-clonic seizures and juvenile lethality by 3 weeks of age. This dramatic exacerbation of the sodium-channel mutant phenotype indicates that M current plays a critical role in preventing seizure initiation and spreading in this animal model. The genetic interaction between Scn2a and Kcnq2 demonstrates that combinations of mild alleles of monogenic epilepsy genes can result in severe disease and provides a model for complex inheritance of human epilepsy. The data suggest that interaction between these genes might contribute to the variable expressivity observed in human families with sodium-channel mutations. In a screen of 23 SMEI patients with missense mutations of SCN1A, no second-site mutations in KCNQ2 were identified.