Background: Although most cases of Parkinson´s disease (PD) are idiopathic with unknown cause, an increasing number of genes and genetic risk factors have been discovered that play a role in PD pathogenesis. Many of the PD-associated proteins are involved in mitochondrial quality control, e.g., PINK1, Parkin, and LRRK2, which were recently identified as regulators of mitochondrial-endoplasmic reticulum (ER) contact sites (MERCs) linking mitochondrial homeostasis to intracellular calcium handling. In this context, Miro1 is increasingly recognized to play a role in PD pathology. Recently, we identified the first PD patients carrying mutations in RHOT1, the gene coding for Miro1. Here, we describe two novel RHOT1 mutations identified in two PD patients and the characterization of the cellular phenotypes. Methods: Using whole exome sequencing we identified two PD patients carrying heterozygous mutations leading to the amino acid exchanges T351A and T610A in Miro1. We analyzed calcium homeostasis and MERCs in detail by live cell imaging and immunocytochemistry in patient-derived fibroblasts. Results: We show that fibroblasts expressing mutant T351A or T610A Miro1 display impaired calcium homeostasis and a reduced amount of MERCs. All fibroblast lines from patients with pathogenic variants in Miro1, revealed alterations of the structure of MERCs. Conclusion: Our data suggest that Miro1 is important for the regulation of the structure and function of MERCs. Moreover, our study supports the role of MERCs in the pathogenesis of PD and further establishes variants in RHOT1 as rare genetic risk factors for neurodegeneration.

Background: Niemann-Pick type C1 disease (NPC1) is a rare progressive neurodegenerative disorder caused by mutations in the NPC1 gene. In this lysosomal storage disorder the intracellular transport and sequestration of several lipids like cholesterol is severely impaired, resulting in an accumulation of lipids in late endosomes and lysosomes. The neurological manifestation of the disease is caused by dysfunction and cell death in the central nervous system. Several animal models were used to analyze the impaired pathways. However, the underlying pathogenic mechanisms are still not completely understood and the genetic variability in humans cannot be reflected in these models. Therefore, a human model using patient-specific induced pluripotent stem cells provides a promising approach. Methods: We reprogrammed human fibroblasts from a NPC1 patient and a healthy control by retroviral transduction with Oct4, Klf4, Sox2 and c-Myc. The obtained human induced pluripotent stem cells (hiPSCs) were characterized by immunocytochemical analyses. Neural progenitor cells were generated and patch clamp recordings were performed for a functional analysis of derived neuronal cells. Filipin stainings and the Amplex Red assay were used to demonstrate and quantify cholesterol accumulation. Results: The hiPSCs expressed different stem cell markers, e.g. Nanog, Tra-1-81 and SSEA4. Using the embryoid body assay, the cells were differentiated in cells of all three germ layers and induced teratoma in immunodeficient mice, demonstrating their pluripotency. In addition, neural progenitor cells were derived and differentiated into functional neuronal cells. Patch clamp recordings revealed voltage dependent channels, spontaneous action potentials and postsynaptic currents. The accumulation of cholesterol in different tissues is the main hallmark of NPC1. In this study we found an accumulation of cholesterol in fibroblasts of a NPC1 patient, derived hiPSCs, and neural progenitor cells, but not in cells derived from fibroblasts of a healthy individual. These findings were quantified by the Amplex Red assay, demonstrating a significantly elevated cholesterol level in cells derived from fibroblasts of a NPC1 patient. Conclusions: We generated a neuronal model based on induced pluripotent stem cells derived from patient fibroblasts, providing a human in vitro model to study the pathogenic mechanisms of NPC1 disease.

Background: It is generally believed that the pathogenesis of PINK1/parkin‐related Parkinson's disease (PD) is due to a disturbance in mitochondrial quality control. However, recent studies have found that PINK1 and Parkin play a significant role in mitochondrial calcium homeostasis and are involved in the regulation of mitochondria–endoplasmic reticulum contact sites (MERCSs).Objective: The aim of our study was to perform an in‐depth analysis of the role of MERCSs and impaired calcium homeostasis in PINK1/Parkin‐linked PD.Methods: In our study, we used induced pluripotent stem cell–derived dopaminergic neurons from patients with PD with loss‐of‐function mutations in PINK1 or PRKN. We employed a split‐GFP‐based contact site sensor in combination with the calcium‐sensitive dye Rhod‐2 AM and applied Airyscan live‐cell super‐resolution microscopy to determine how MERCSs are involved in the regulation of mitochondrial calcium homeostasis.Results: Our results showed that thapsigargin‐induced calcium stress leads to an increase of the abundance of narrow MERCSs in wild‐type neurons. Intriguingly, calcium levels at the MERCSs remained stable, whereas the increased net calcium influx resulted in elevated mitochondrial calcium levels. However, PINK1‐PD or PRKN‐PD neurons showed an increased abundance of MERCSs at baseline, accompanied by an inability to further increase MERCSs upon thapsigargin‐induced calcium stress. Consequently, calcium distribution at MERCSs and within mitochondria was disrupted.Conclusions: Our results demonstrated how the endoplasmic reticulum and mitochondria work together to cope with calcium stress in wild‐type neurons. In addition, our results suggests that PRKN deficiency affects the dynamics and composition of MERCSs differently from PINK1 deficiency, resulting in differentially affected calcium homeostasis. © 2023 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

ImportanceWe sought to unravel the genetic cause in a consanguineous Pakistani family with a complex neurological phenotype.ObservationsNeurological and ophthalmological examination, including videotaping and fundoscopy, and genetic investigations, including homozygosity mapping and exome sequencing, were performed at the University of the Punjab and the University of Lübeck. Participants included 2 severely affected cousins from consanguineous parents, 10 of their reportedly unaffected relatives, and 342 Pakistani controls. Motor symptoms in the 2 patients started at the age of 3 to 4 years and included chorea, cerebellar ataxia, dystonia, and pyramidal tract signs. Genome-wide genotyping delineated 2 regions of homozygosity on chromosomes 13q12.11 to 13q12.13 and 19q12 to 19q13.41. Exome sequencing revealed 2 rare, homozygous variants (c.32 T>A [p.L11Q] in OPA3 and c.941 C>G [p.A314G] in TSHZ3) that segregated with the disease. Only the OPA3 variant was absent in the control subjects and predicted to be damaging. Subsequent ophthalmological assessment revealed bilateral optic atrophy in both patients.Conclusions and RelevanceMutations in OPA3 have been reported in Costeff optic atrophy syndrome. We identify a novel missense mutation in OPA3 as the cause of a complex neurological disorder, expanding the OPA3 -linked phenotype by early-onset pyramidal tract signs and marked lower limb dystonia. Investigation of optic atrophy was initiated only after genetic analysis, a phenomenon referred to as reverse phenotyping.

Myoclonus-dystonia (DYT-SGCE, formerly DYT11) is characterized by alcohol-sensitive, myoclonic-like appearance of fast dystonic movements. It is caused by mutations in the SGCE gene encoding ε-sarcoglycan leading to a dysfunction of this transmembrane protein, alterations in the cerebello-thalamic pathway and impaired striatal plasticity. To elucidate underlying pathogenic mechanisms, we investigated induced pluripotent stem cell (iPSC)-derived striatal medium spiny neurons (MSNs) from two myoclonus-dystonia patients carrying a heterozygous mutation in the SGCE gene (c.298T>G and c.304C>T with protein changes W100G and R102X) in comparison to two matched healthy control lines. Calcium imaging showed significantly elevated basal intracellular Ca²⁺ content and lower frequency of spontaneous Ca²⁺ signals in SGCE MSNs. Blocking of voltage-gated Ca²⁺ channels by verapamil was less efficient in suppressing KCl-induced Ca²⁺ peaks of SGCE MSNs. Ca²⁺ amplitudes upon glycine and acetylcholine applications were increased in SGCE MSNs, but not after GABA or glutamate applications. Expression of voltage-gated Ca²⁺ channels and most ionotropic receptor subunits was not altered. SGCE MSNs showed significantly reduced GABAergic synaptic density. Whole-cell patch-clamp recordings displayed elevated amplitudes of miniature postsynaptic currents and action potentials in SGCE MSNs. Our data contribute to a better understanding of the pathophysiology and the development of novel therapeutic strategies for myoclonus-dystonia.

X-linked dystonia–parkinsonism (XDP) is a severe neurodegenerative disorder that manifests as adult-onset dystonia combined with parkinsonism. A SINE-VNTR-Alu (SVA) retrotransposon inserted in an intron of the TAF1 gene reduces its expression and alters splicing in XDP patient-derived cells. As a consequence, increased levels of the TAF1 intron retention transcript TAF1-32i can be found in XDP cells as compared to healthy controls. Here, we investigate the sequence of the deep intronic region included in this transcript and show that it is also present in cells from healthy individuals, albeit in lower amounts than in XDP cells, and that it undergoes degradation by nonsense-mediated mRNA decay. Furthermore, we investigate epigenetic marks (e.g., DNA methylation and histone modifications) present in this intronic region and the spanning sequence. Finally, we show that the SVA evinces regulatory potential, as demonstrated by its ability to repress the TAF1 promoter in vitro. Our results enable a better understanding of the disease mechanisms underlying XDP and transcriptional alterations caused by SVA retrotransposons.