Homozygous SMN1 loss causes spinal muscular atrophy (SMA), the most common lethal genetic childhood motor neuron disease. SMN1 encodes SMN, a ubiquitous housekeeping protein, which makes the primarily motor neuron-specific phenotype rather unexpected. SMA-affected individuals harbor low SMN expression from one to six SMN2 copies, which is insufficient to functionally compensate for SMN1 loss. However, rarely individuals with homozygous absence of SMN1 and only three to four SMN2 copies are fully asymptomatic, suggesting protection through genetic modifier(s). Previously, we identified plastin 3 (PLS3) overexpression as an SMA protective modifier in humans and showed that SMN deficit impairs endocytosis, which is rescued by elevated PLS3 levels. Here, we identify reduction of the neuronal calcium sensor Neurocalcin delta (NCALD) as a protective SMA modifier in five asymptomatic SMN1-deleted individuals carrying only four SMN2 copies. We demonstrate that NCALD is a Ca-dependent negative regulator of endocytosis, as NCALD knockdown improves endocytosis in SMA models and ameliorates pharmacologically induced endocytosis defects in zebrafish. Importantly, NCALD knockdown effectively ameliorates SMA-associated pathological defects across species, including worm, zebrafish, and mouse. In conclusion, our study identifies a previously unknown protective SMA modifier in humans, demonstrates modifier impact in three different SMA animal models, and suggests a potential combinatorial therapeutic strategy to efficiently treat SMA. Since both protective modifiers restore endocytosis, our results confirm that endocytosis is a major cellular mechanism perturbed in SMA and emphasize the power of protective modifiers for understanding disease mechanism and developing therapies.

Plastin3 is a protective modifier of spinal muscular atrophy (SMA). Janzen et al. report that CHP1 interacts directly with plastin3, and that CHP1 suppression reduces SMA pathology in cellular and animal models by restoring impaired endocytosis. CHP1 suppression holds promise as an SMN-independent component of a combinatorial SMA therapy.

Spinal muscular atrophy (SMA) is a neuromuscular disease causing the most frequent genetic childhood lethality. Recently, nusinersen, an antisense oligonucleotide (ASO) that corrects SMN2 splicing and thereby increases full-length SMN protein, has been approved by the FDA and EMA for SMA therapy. However, the administration of nusinersen in severe and/or post-symptomatic SMA-affected individuals is insufficient to counteract the disease. Therefore, additional SMN-independent therapies are needed to support the function of motoneurons and neuromuscular junctions. We recently identified asymptomatic SMN1-deleted individuals who were protected against SMA by reduced expression of neurocalcin delta (NCALD). NCALD reduction is proven to be a protective modifier of SMA across species, including worm, zebrafish, and mice. Here, we identified Ncald-ASO3-out of 450 developed Ncald ASOs-as the most efficient and non-toxic ASO for the CNS, by applying a stepwise screening strategy in cortical neurons and adult and neonatal mice. In a randomized-blinded preclinical study, a single subcutaneous low-dose SMN-ASO and a single intracerebroventricular Ncald-ASO3 or control-ASO injection were presymptomatically administered in a severe SMA mouse model. NCALD reduction of >70% persisted for about 1 month. While low-dose SMN-ASO rescues multiorgan impairment, additional NCALD reduction significantly ameliorated SMA pathology including electrophysiological and histological properties of neuromuscular junctions and muscle at P21 and motoric deficits at 3 months. The present study shows the additional benefit of a combinatorial SMN-dependent and SMN-independent ASO-based therapy for SMA. This work illustrates how a modifying gene, identified in some asymptomatic individuals, helps to develop a therapy for all SMA-affected individuals.

Spinal muscular atrophy (SMA) is a neurodegenerative disorder characterized by alpha motor neuron loss in the spinal cord due to reduced survival motor neuron (SMN) protein level. While the genetic basis of SMA is well described, the specific molecular pathway underlying SMA is still not fully understood. Areas covered: This review discusses the recent advancements in understanding the molecular pathways in SMA using different omics approaches and genetic modifiers identified in both vertebrate and invertebrate systems. The findings that are summarized in this article were deduced from original articles and reviews with a particular focus on the latest advancements in the field. Expert commentary: The identification of genetic modifiers such as PLS3 and NCALD in humans or of SMA modulators such as Elavl4 (HuD), Copa, Uba1, Mapk10 (Jnk3), Nrxn2 and Tmem41b (Stasimon) in various SMA animal models improved our knowledge of impaired cellular pathways in SMA. Inspiration from modifier genes and their functions in motor neuron and neuromuscular junctions may open a new avenue for future SMA combinatorial therapies.

Neurocalcin delta (NCALD) is a brain-enriched neuronal calcium sensor and its reduction acts protective against spinal muscular atrophy (SMA). However, the physiological function of NCALD and implications of NCALD reduction are still elusive. Here, we analyzed the ubiquitous Ncald knockout in homozygous ( Ncald KO/KO ) and heterozygous ( Ncald KO/WT ) mice to unravel the physiological role of NCALD in the brain and to study whether 50% NCALD reduction is a safe option for SMA therapy. We found that Ncald KO/KO but not Ncald KO/WT mice exhibit significant changes in the hippocampal morphology, likely due to impaired generation and migration of newborn neurons in the dentate gyrus (DG). To understand the mechanism behind, we studied the NCALD interactome and identified mitogen-activated protein kinase kinase kinase 10 (MAP3K10) as a novel NCALD interacting partner. MAP3K10 is an upstream activating kinase of c-Jun N-terminal kinase (JNK), which regulates adult neurogenesis. Strikingly, the JNK activation was significantly upregulated in the Ncald KO/KO brains. Contrary, neither adult neurogenesis nor JNK activation were altered by heterozygous Ncald deletion. Taken together, our study identifies a novel link between NCALD and adult neurogenesis in the hippocampus, possibly via a MAP3K10-JNK pathway and emphasizes the safety of using NCALD reduction as a therapeutic option for SMA.

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Objective: To ascertain the genetic and functional basis of complex autosomal recessive cerebellar ataxia (ARCA) presented by 2 siblings of a consanguineous family characterized by motor neuropathy, cerebellar atrophy, spastic paraparesis, intellectual disability, and slow ocular saccades.Methods: Combined whole-genome linkage analysis, whole-exome sequencing, and focused screening for identification of potential causative genes were performed. Assessment of the functional consequences of the mutation on protein function via subcellular fractionation, sizeexclusion chromatography, and fluorescence microscopy were done. A zebrafish model, using Morpholinos, was generated to study the pathogenic effect of the mutation in vivo.Results: We identified a biallelic 3-bp deletion (p.K19del) in CHP1 that cosegregates with the disease. Neither focused screening for CHP1 variants in 2 cohorts (ARCA: N 5 319 and NeurOmics: N 5 657) nor interrogating GeneMatcher yielded additional variants, thus revealing the scarcity of CHP1 mutations. We show that mutant CHP1 fails to integrate into functional protein complexes and is prone to aggregation, thereby leading to diminished levels of soluble CHP1 and reduced membrane targeting of NHE1, a major Na 1 /H 1 exchanger implicated in syndromic ataxiadeafness. Chp1 deficiency in zebrafish, resembling the affected individuals, led to movement defects, cerebellar hypoplasia, and motor axon abnormalities, which were ameliorated by coinjection with wild-type, but not mutant, human CHP1 messenger RNA. Conclusions: Collectively, our results identified CHP1 as a novel ataxia-causative gene in humans,further expanding the spectrum of ARCA-associated loci, and corroborated the crucial role of NHE1 within the pathogenesis of these disorders. Neurol Genet 2018;4:e209; doi: 10.1212/ NXG.0000000000000209 GLOSSARY ARCA 5 autosomal recessive cerebellar ataxia; CaP-MN 5 caudal primary motor neuron; cDNA 5 complementary DNA; EGFR 5 epidermal growth factor receptor; GFP 5 green fluorescent protein; HMW 5 high molecular weight; HPRT 5 hypoxanthine-guanine phosphoribosyltransferase; HSP90 5 heat shock protein 90; KO 5 knockout; LKS 5 LichtensteinKnorr syndrome; LOD 5 Logarithm of the odds; mRNA 5 messenger RNA; MO 5 Morpholino; OE 5 overexpression; RT-PCR 5 reverse transcription PCR; SEC 5 size-exclusion chromatography; WB 5 Western blot; WES 5 whole-exome sequencing; WT 5 wild type.Autosomal recessive cerebellar ataxias (ARCAs) comprise a heterogeneous group of neurodegenerative disorders associating cerebellar degeneration to a variable combination of central or peripheral neurologic or nonneurologic signs.1 Collectively, ARCAs have an estimated frequency of 1:20,000 and often show overlapping features with spastic paraplegias and peripheral neuropathies.

Nearly two decades after the discovery of the disease causing gene survival motor neuron 1, many therapeutic options for SMA have been developed, some of which made it to clinical trials but could not prove their promising experimental results. Recently, big research efforts from academia, government and the pharmaceutical industry have led to the development of highly promising compounds that are currently in clinical trials, and which could lead to feasible treatment options in the future.