Homozygous loss of SMN1 causes spinal muscular atrophy (SMA), the most common and devastating childhood genetic motor-neuron disease. The copy gene SMN2 produces only ∼10% functional SMN protein, insufficient to counteract development of SMA. In contrast, the human genetic modifier plastin 3 (PLS3), an actin-binding and -bundling protein, fully protects against SMA in SMN1-deleted individuals carrying 3-4 SMN2 copies. Here, we demonstrate that the combinatorial effect of suboptimal SMN antisense oligonucleotide treatment and PLS3 overexpression-a situation resembling the human condition in asymptomatic SMN1-deleted individuals-rescues survival (from 14 to >250 days) and motoric abilities in a severe SMA mouse model. Because PLS3 knockout in yeast impairs endocytosis, we hypothesized that disturbed endocytosis might be a key cellular mechanism underlying impaired neurotransmission and neuromuscular junction maintenance in SMA. Indeed, SMN deficit dramatically reduced endocytosis, which was restored to normal levels by PLS3 overexpression. Upon low-frequency electro-stimulation, endocytotic FM1-43 (SynaptoGreen) uptake in the presynaptic terminal of neuromuscular junctions was restored to control levels in SMA-PLS3 mice. Moreover, proteomics and biochemical analysis revealed CORO1C, another F-actin binding protein, whose direct binding to PLS3 is dependent on calcium. Similar to PLS3 overexpression, CORO1C overexpression restored fluid-phase endocytosis in SMN-knockdown cells by elevating F-actin amounts and rescued the axonal truncation and branching phenotype in Smn-depleted zebrafish. Our findings emphasize the power of genetic modifiers to unravel the cellular pathomechanisms underlying SMA and the power of combinatorial therapy based on splice correction of SMN2 and endocytosis improvement to efficiently treat SMA.

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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.

Autosomal recessive primary microcephaly (MCPH) is caused by mutations in at least eight different genes involved either in cell division or DNA repair. Most mutations are identified in consanguine families from Pakistan, Iran and India. To further assess their genetic heterogeneity and mutational spectra, we have analyzed 57 consanguine Pakistani MCPH families. In 34 MCPH families, we detected linkage to five out of the eight well-characterized disease loci and identified mutations in 27 families, leaving seven families without mutations in the coding exons of the presumably underlying MCPH genes. In the MCPH cohort 23 families could not be linked to any of the known loci, pointing to remarkable locus heterogeneity. The majority of mutations were found in ASPM followed by WDR62, CENPJ, CEP152 and MCPH1. One ASPM mutation (p.Trp1326*) was found in as many as eight families suggesting a Pakistani founder mutation. One third of the families were linked to ASPM followed by WDR62 confirming previous data. We identified three novel ASPM mutations, four novel WDR62 mutations, one novel MCPH1 mutation and two novel CEP152 mutations. CEP152 mutations have not been described before in the Pakistani population.

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.