FBXL4: From mtDNA Depletion Syndrome to the Maintenance of Mitochondrial Form & Function Rasha Sabouny1, Rachel Wong1, David Sinasac2, Aneal Khan3, Timothy E Shutt1,2 1Biochemistry & Molecular Biology, University of Calgary, Calgary, AB, Canada; 2Medical Genetics, University of Calgary, Calgary, AB, Canada; 3Pediatrics, Alberta Children's Hospital, Calgary, AB, Canada
Mutations in the mitochondrial intermembrane space (IMS) protein, FBXL4 (F-Box and Leucine Rich Repeat Protein 4) cause an encephalopathic mtDNA depletion phenotype. However, the cellular function of FBXL4 remains unknown. MtDNA depletion syndromes are a class of severe infantile-onset mitochondrial diseases that can arise from defects in mtDNA maintenance genes, reduction in mitochondrial dNTP pools or dysregulation of mitochondrial morphology. Thus, it is likely that FBXL4 is involved in one of these processes. While F-box proteins are typically involved in ubiquitination, little is known about ubiquitination inside the IMS.
To understand the role of FBXL4 in the mitochondria, we have begun to characterize fibroblasts from patients harbouring a previously uncharacterized homozygous FBXL4 mutation (c. 1750T>C; p.C584R). Mitochondrial networks in these patient fibroblasts are fragmented and contain large mtDNA nucleoids, as seen with other FBXL4 mutations. Consistent with these structural abnormalities, impaired mitochondrial respiration is also observed. Our novel findings show that our FBXL4 patient cells display upregulated mitophagy and abnormal lysosomal structures, neither of which have been reported in the context of FBXL4 mutations. We also observe mitochondrial fragments devoid of mtDNA.
Notably, network fragmentation, enlarged nucleoids and altered mtDNA distribution are reminiscent of defects in cells lacking fusion. Thus, we suspect that FBXL4 might be a novel regulator of mitochondrial fusion. To this end, FBXL4 patient cells display altered processing patterns of the inner membrane fusion protein, Opa1. Specifically, we observe an enrichment of pro-fission, short-Opa1 isoforms with mutant FBXL4, suggesting that FBXL4 is a novel regulator of Opa1 processing.
In summary, by seeking to characterize novel mitochondrial disease genes, we can gain valuable mechanistic insights into the regulation of mitochondrial function.
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