Characterization of a UQCRC1 variant in a patient with progressive weakness, pain and sleep issues reveals a functional mitochondrial defect restored by mitochondrial transplantation

Primary mitochondrial defects underlie the heterogeneity of many rare inherited disorders. Pathogenic variants that disrupt the function of the multi-subunit protein complexes of the mitochondrial respiratory chain contribute to a range of neurological phenotypes and other clinical manifestations. These variants are also thought to contribute to the onset and progression of numerous more common neurodegenerative conditions such as Parkinson's and Alzheimer's disease. Here we describe an individual affected with progressive muscle weakness and pain harboring a paternally inherited missense variant in <i>UQCRC1</i>, encoding a subunit of Complex III. Biochemical characterization of cells from the proband and his father demonstrated normal steady-state levels of UQCRC1 and UQCRC2 protein. Functional assessment of mitochondrial respiration in lymphoblasts and fibroblasts, however, showed a clear deficit in respiratory parameters in the proband, with a more attenuated response in the father. Lastly, we demonstrate that healthy mitochondria isolated from HEK293 cells can be transferred to the patient lymphoblasts, restoring basal mitochondrial respiration and ATP production. Perspectives on the contribution of this variant to the patient phenotypes, and the potential of mitochondrial transplantation and different compounds as treatment modalities for patients with primary mitochondrial deficits, is discussed.