Autosomal dominant retinal vasculopathy with cerebral leukodystrophy is a microvascular endotheliopathy with middle-age onset. In nine families, we identified heterozygous C-terminal frameshift mutations in TREX1, which encodes a 3'-5' exonuclease. These truncated proteins retain exonuclease activity but lose normal perinuclear localization. These data have implications for the maintenance of vascular integrity in the degenerative cerebral microangiopathies leading to stroke and dementias.

RNA exosomes are multi-subunit complexes conserved throughout evolution1 and emerging as the major cellular machinery for processing, surveillance, and turnover of a diverse spectrum of coding and non-coding RNA substrates essential for viability2. By exome sequencing, we discovered recessive mutations in exosome component 3 (EXOSC3) in four siblings with infantile spinal motor neuron disease, cerebellar atrophy, progressive microcephaly, and profound global developmental delay, consistent with pontocerebellar hypoplasia type 1 [PCH1; OMIM 607596]3–6. We identified mutations in EXOSC3 in an additional 8 of 12 families with PCH1. Morpholino knockdown of exosc3 in zebrafish embryos caused embryonic maldevelopment with small brain and poor motility, reminiscent of human clinical features and largely rescued by coinjected wildtype but not mutant exosc3 mRNA. These findings represent the first example of an RNA exosome gene responsible for a human disease and further implicate dysregulation of RNA processing in cerebellar and spinal motor neuron maldevelopment and degeneration.

Disturbed mitochondrial fusion and fission have been linked to various neurodegenerative disorders. In siblings from two unrelated families who died soon after birth with a profound neurodevelopmental disorder characterized by pontocerebellar hypoplasia and apnoea, we discovered a missense mutation and an exonic deletion in the SLC25A46 gene encoding a mitochondrial protein recently implicated in optic atrophy spectrum disorder. We performed functional studies that confirmed the mitochondrial localization and pro-fission properties of SLC25A46. Knockdown of slc24a46 expression in zebrafish embryos caused brain malformation, spinal motor neuron loss, and poor motility. At the cellular level, we observed abnormally elongated mitochondria, which was rescued by co-injection of the wild-type but not the mutant slc25a46 mRNA. Conversely, overexpression of the wild-type protein led to mitochondrial fragmentation and disruption of the mitochondrial network. In contrast to mutations causing non-lethal optic atrophy, missense mutations causing lethal congenital pontocerebellar hypoplasia markedly destabilize the protein. Indeed, the clinical severity appears inversely correlated with the relative stability of the mutant protein. This genotype-phenotype correlation underscores the importance of SLC25A46 and fine tuning of mitochondrial fission and fusion in pontocerebellar hypoplasia and central neurodevelopment in addition to optic and peripheral neuropathy across the life span.

Background: Episodic ataxia (EA) is variably associated with additional neurologic symptoms. At least 4 genes have been implicated. Recently, a mutation in the SLC1A3 gene encoding the glutamate transporter EAAT1 was identified in a patient with severe episodic and progressive ataxia, seizures, alternating hemiplegia, and migraine headache. The mutant EAAT1 showed severely reduced uptake of glutamate. The syndrome was designated EA6 and shares overlapping clinical features with EA2, which is caused by mutations in CACNA1A. Objective: To test the role of the SLC1A3 gene in EA. Design: Genetic and functional studies. We analyzed the coding region of the SLC1A3 gene by direct sequencing. Setting: Academic research. Patients: DNA samples from 20 patients with EA (with or without interictal nystagmus) negative for CACNA1A mutations were analyzed. Main Outcome Measures: We identified 1 novel EAAT1 mutation in a family with EA and studied the functional consequences of this mutation using glutamate uptake assay. Results: We identified a missense C186S mutation that segregated with EA in 3 family members. The mutant EAAT1 showed a modest but significant reduction of glutamate uptake. Conclusions: We broadened the clinical spectrum associated with SLC1A3 mutations to include milder manifestations of EA without seizures or alternating hemiplegia. The severity of EA6 symptoms appears to be correlated with the extent of glutamate transporter dysfunction.

The Na(+)/K(+)-ATPases are ion pumps of fundamental importance in maintaining the electrochemical gradient essential for neuronal survival and function. Mutations in ATP1A3 encoding the alpha3 isoform cause rapid-onset dystonia-parkinsonism (RDP). We report a de novo ATP1A3 mutation in a patient with typical RDP, consisting of an in-frame insertion of a tyrosine residue at the very C terminus of the Na(+)/K(+)-ATPase alpha3-subunit-the first reported RDP mutation in the C terminus of the protein. Expression studies revealed that there is no defect in the biogenesis or plasma membrane targeting, although cells expressing the mutant protein showed decreased survival in response to ouabain challenge. Functional analysis demonstrated a drastic reduction in Na(+) affinity in the mutant, which can be understood by structural modelling of the E1 and E2 conformations of the wild-type and mutant enzymes on the basis of the strategic location of the C terminus in relation to the third Na(+) binding site. The dramatic clinical presentation, together with the biochemical findings, provides both in vivo and in vitro evidence for a crucial role of the C terminus of the alpha-subunit in the function of the Na(+)/K(+)-ATPase and a key impact of Na(+) affinity in the pathophysiology of RDP.