Sphingomyelinases generate ceramide from sphingomyelin as a second messenger in intracellular signaling pathways involved in cell proliferation, differentiation, or apoptosis. Children from 12 unrelated families presented with microcephaly, simplified gyral pattern of the cortex, hypomyelination, cerebellar hypoplasia, congenital arthrogryposis, and early fetal/postnatal demise. Genomic analysis revealed bi-allelic loss-of-function variants in SMPD4, coding for the neutral sphingomyelinase-3 (nSMase-3/SMPD4). Overexpression of human Myc-tagged SMPD4 showed localization both to the outer nuclear envelope and the ER and additionally revealed interactions with several nuclear pore complex proteins by proteomics analysis. Fibroblasts from affected individuals showed ER cisternae abnormalities, suspected for increased autophagy, and were more susceptible to apoptosis under stress conditions, while treatment with siSMPD4 caused delayed cell cycle progression. Our data show that SMPD4 links homeostasis of membrane sphingolipids to cell fate by regulating the cross-talk between the ER and the outer nuclear envelope, while its loss reveals a pathogenic mechanism in microcephaly.

Leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation is a disorder caused by recessive mutations in the gene DARS2, which encodes mitochondrial aspartyl-tRNA synthetase. Recent observations indicate that the phenotypic range of the disease is much wider than initially thought. Currently, no treatment is available. The aims of our study were (i) to explore a possible genotype-phenotype correlation; and (ii) to identify potential therapeutic agents that modulate the splice site mutations in intron 2 of DARS2, present in almost all patients. A cross-sectional observational study was performed in 78 patients with two DARS2 mutations in the Amsterdam and Helsinki databases up to December 2012. Clinical information was collected via questionnaires. An inventory was made of the DARS2 mutations in these patients and those previously published. An assay was developed to assess mitochondrial aspartyl-tRNA synthetase enzyme activity in cells. Using a fluorescence reporter system we screened for drugs that modulate DARS2 splicing. Clinical information of 66 patients was obtained. The clinical severity varied from infantile onset, rapidly fatal disease to adult onset, slow and mild disease. The most common phenotype was characterized by childhood onset and slow neurological deterioration. Full wheelchair dependency was rare and usually began in adulthood. In total, 60 different DARS2 mutations were identified, 13 of which have not been reported before. Except for 4 of 42 cases published by others, all patients were compound heterozygous. Ninety-four per cent of the patients had a splice site mutation in intron 2. The groups of patients sharing the same two mutations were too small for formal assessment of genotype-phenotype correlation. However, some combinations of mutations were consistently associated with a mild phenotype. The mitochondrial aspartyl-tRNA synthetase activity was strongly reduced in patient cells. Among the compounds screened, cantharidin was identified as the most potent modulator of DARS2 splicing. In conclusion, the phenotypic spectrum of leukoencephalopathy with brainstem and spinal cord involvement and lactate elevation is wide, but most often the disease has a relatively slow and mild course. The available evidence suggests that the genotype influences the phenotype, but because of the high number of private mutations, larger numbers of patients are necessary to confirm this. The activity of mitochondrial aspartyl-tRNA synthetase is significantly reduced in patient cells. A compound screen established a 'proof of principle' that the splice site mutation can be influenced. This finding is promising for future therapeutic strategies.

RMND1 is an integral inner membrane mitochondrial protein that assembles into a large 240 kDa complex to support translation of the 13 polypeptides encoded on mtDNA, all of which are essential subunits of the oxidative phosphorylation (OXPHOS) complexes. Variants in RMND1 produce global defects in mitochondrial translation and were first reported in patients with severe neurological phenotypes leading to mortality in the first months of life. Using whole-exome sequencing, we identified compound heterozygous RMND1 variants in a 4-year-old patient with congenital lactic acidosis, severe myopathy, hearing loss, renal failure, and dysautonomia. The levels of mitochondrial ribosome proteins were reduced in patient fibroblasts, causing a translation defect, which was rescued by expression of the wild-type cDNA. RMND1 was almost undetectable by immunoblot analysis in patient muscle and fibroblasts. BN-PAGE analysis showed a severe combined OXPHOS assembly defect that was more prominent in patient muscle than in fibroblasts. Immunofluorescence experiments showed that RMND1 localizes to discrete foci in the mitochondrial network, juxtaposed to RNA granules where the primary mitochondrial transcripts are processed. RMND1 foci were not detected in patient fibroblasts. We hypothesize that RMND1 acts to anchor or stabilize the mitochondrial ribosome near the sites where the mRNAs are matured, spatially coupling post-transcriptional handling mRNAs with their translation, and that loss of function variants in RMND1 are associated with a unique constellation of clinical phenotypes that vary with the severity of the mitochondrial translation defect.