Primary familial brain calcification (PFBC) is a genetically heterogeneous disorder characterized by bilateral calcifications in the basal ganglia and other brain regions. The genetic basis of this disorder remains unknown in a significant portion of familial cases. Here, we reported a recessive causal gene, MYORG, for PFBC. Compound heterozygous or homozygous mutations of MYORG co-segregated completely with PFBC in six families, with logarithm of odds (LOD) score of 4.91 at the zero recombination fraction. In mice, Myorg mRNA was expressed specifically in S100β-positive astrocytes, and knockout of Myorg induced the formation of brain calcification at 9 months of age. Our findings provide strong evidence that loss-of-function mutations of MYORG cause brain calcification in humans and mice.

BackgroundHereditary spastic paraplegias (HSP) is a heterogeneous group of rare neurodegenerative disorders affecting the corticospinal tracts. To date, more than 78 HSP loci have been mapped to cause HSP. However, both the clinical and mutational spectrum of Chinese patients with HSP remained unclear. In this study, we aim to perform a comprehensive analysis of clinical phenotypes and genetic distributions in a large cohort of Chinese HSP patients, and to elucidate the primary pathogenesis in this population.MethodsWe firstly performed next-generation sequencing targeting 149 genes correlated with HSP in 99 index cases of our cohort. Multiplex ligation-dependent probe amplification testing was further carried out among those patients without known disease-causing gene mutations. We simultaneously performed a retrospective study on the reported patients exhibiting HSP in other Chinese cohorts. All clinical and molecular characterization from above two groups of Chinese HSP patients were analyzed and summarized. Eventually, we further validated the cellular changes in fibroblasts of two major spastic paraplegia (SPG) patients (SPG4 and SPG11) in vitro.ResultsMost patients of ADHSP (94%) are pure forms, whereas most patients of ARHSP (78%) tend to be complicated forms. In ADHSP, we found that SPG4 (79%) was the most prevalent, followed by SPG3A (11%), SPG6 (4%) and SPG33 (2%). Subtle mutations were the common genetic cause for SPG4 patients and most of them located in AAA cassette domain of spastin protein. In ARHSP, the most common subtype was SPG11 (53%), followed by SPG5 (32%), SPG35 (6%) and SPG46 (3%). Moreover, haplotype analysis showed a unique haplotype was shared in 14 families carrying c.334C > T (p.R112*) mutation in CYP7B1 gene, suggesting the founder effect. Functionally, we observed significantly different patterns of mitochondrial dynamics and network, decreased mitochondrial membrane potential (Δψm), increased reactive oxygen species and reduced ATP content in SPG4 fibroblasts. Moreover, we also found the enlargement of LAMP1-positive organelles and abnormal accumulation of autolysosomes in SPG11 fibroblasts.ConclusionsOur study present a comprehensive clinical spectrum and genetic landscape for HSP in China. We have also provided additional evidences for mitochondrial and autolysosomal-mediated pathways in the pathogenesis of HSP.Electronic supplementary materialThe online version of this article (10.1186/s13024-018-0269-1) contains supplementary material, which is available to authorized users.

Hereditary spastic paraplegias refer to a heterogeneous group of neurodegenerative disorders resulting from degeneration of the corticospinal tract. Clinical characterization of patients with hereditary spastic paraplegias represents progressive spasticity, exaggerated reflexes and muscular weakness. Here, to expand on the increasingly broad pools of previously unknown hereditary spastic paraplegia causative genes and subtypes, we performed whole exome sequencing for six affected and two unaffected individuals from two unrelated Chinese families with an autosomal dominant hereditary spastic paraplegia and lacking mutations in known hereditary spastic paraplegia implicated genes. The exome sequencing revealed two stop-gain mutations, c.247_248insGTGAATTC (p.I83Sfs*11) and c.526G>T (p.E176*), in the ubiquitin-associated protein 1 (UBAP1) gene, which co-segregated with the spastic paraplegia. We also identified two UBAP1 frameshift mutations, c.324_325delCA (p.H108Qfs*10) and c.425_426delAG (p.K143Sfs*15), in two unrelated families from an additional 38 Chinese pedigrees with autosomal dominant hereditary spastic paraplegias and lacking mutations in known causative genes. The primary disease presentation was a pure lower limb predominant spastic paraplegia. In vivo downregulation of Ubap1 in zebrafish causes abnormal organismal morphology, inhibited motor neuron outgrowth, decreased mobility, and shorter lifespan. UBAP1 is incorporated into endosomal sorting complexes required for transport complex I and binds ubiquitin to function in endosome sorting. Patient-derived truncated form(s) of UBAP1 cause aberrant endosome clustering, pronounced endosome enlargement, and cytoplasmic accumulation of ubiquitinated proteins in HeLa cells and wild-type mouse cortical neuron cultures. Biochemical and immunocytochemical experiments in cultured cortical neurons derived from transgenic Ubap1flox mice confirmed that disruption of UBAP1 leads to dysregulation of both early endosome processing and ubiquitinated protein sorting. Strikingly, deletion of Ubap1 promotes neurodegeneration, potentially mediated by apoptosis. Our study provides genetic and biochemical evidence that mutations in UBAP1 can cause pure autosomal dominant spastic paraplegia.

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Spinal muscular atrophy (SMA) is a devastating motor neuron disease caused by mutations of the survival motor neuron 1 (SMN1) gene. SMN2, a paralogous gene to SMN1, can partially compensate for the loss of SMN1. On the basis of age at onset, highest motor function and SMN2 copy numbers, childhood-onset SMA can be divided into three types (SMA I-III). An inverse correlation was observed between SMN2 copies and the differential phenotypes of SMA. Interestingly, this correlation is not always absolute. Using SMA induced pluripotent stem cells (iPSCs), we found that the SMN was significantly decreased in both SMA III and SMA I iPSCs derived postmitotic motor neurons (pMNs) and γ-aminobutyric acid (GABA) neurons. Moreover, the significant differences of SMN expression level between SMA III (3 copies of SMN2) and SMA I (2 copies of SMN2) were observed only in pMNs culture, but not in GABA neurons or iPSCs. From these findings, we further discovered that the neurite outgrowth was suppressed in both SMA III and SMA I derived MNs. Meanwhile, the significant difference of neurite outgrowth between SMA III and SMA I group was also found in long-term cultures. However, significant hyperexcitability was showed only in SMA I derived mature MNs, but not in SMA III group. Above all, we propose that SMN protein is a major factor of phenotypic modifier. Our data may provide a new insight into recognition for differential phenotypes of SMA disease.