Variants in Miro1 cause alterations of ER-mitochondria contact sites in fibroblasts from Parkinson's disease patients

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.