Loss of MECR, an enzyme for mitochondrial fatty acid synthesis, causes iron accumulation, upregulation of ceramides and neurodegeneration
Authors: Debdeep Dutta 1,2; Oguz Kanca 1,2; Seul Kee Byeon 3; Jun Hyoung Park 1; Paul C. Marcogliese 1,2; Zhongyuan Zuo 1,2; Rishi V. Shridharan 1; Guang Lin 1,2; Matthew Wheeler 4; Benny A. Kaipparettu 1; Akhilesh Pandey 3; Hugo J. Bellen 1,2,5; Undiagnosed Diseases Network
Affiliations: 1) Department of Molecular and Human Genetics, Baylor College of Medicine, Houston, TX; 2) Jan and Dan Duncan Neurological Research Institute, Texas Children’s Hospital, Houston, TX; 3) Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, MN, US; 4) Division of Cardiovascular Medicine, Stanford University School of Medicine, Stanford, CA; 5) Department of Neuroscience, Baylor College of Medicine, Houston, TX
Keywords: a. neural degeneration; b. metabolism
Fatty acid synthesis occurs in the cytoplasm as well as in the mitochondria of eukaryotic cells. Autosomal recessive mutations in MECR (Mitochondrial Enoyl CoA Reductase), which encodes an enzyme required for the last step of the mitochondrial fatty acid synthesis, cause a childhood-onset pediatric neurodegenerative disorder called MEPAN syndrome (MIM # 617282). To study the mechanism underlying the neurodegenerative phenotypes observed in MEPAN syndrome, we generated a severe loss-of-function CRIMIC (SA-T2AGAL4-polyA) allele of the fly ortholog of MECR, CG16935/mecr using CRISPR. Homozygous mutants die at larval stages and the lethality is rescued upon expression of the reference human MECR cDNA, showing functional conservation across species. Given the lethality, we performed RNAi-mediated knockdown experiments in neurons and observed progressive locomotor defects and vision loss. Upon successfully modelling the key disease phenotypes in our fly model, we used the fly and fibroblasts from patients to evaluate the mitochondrial function and morphology. A GFP-tagged allele of mecr expressed at the endogenous levels showed that the protein is localized to mitochondria. Transmission Electron Microscopy of Drosophila eye mutant clones and the MEPAN patient fibroblasts revealed reduced mitochondrial numbers and abnormal mitochondrial morphology. Additionally, mitochondrial function (mitochondrial membrane potential, ATP production, electron transport chain activity, and oxygen consumption rates) are all compromised due to loss of mecr/MECR. Lipidomic analyses of fly mutants and MEPAN patient fibroblasts revealed elevated ceramide levels. Hence, our next question was: how does loss of mecr increase ceramide levels? We found that loss of mecr affects the stability of mtACP, a multifunctional protein that helps in the assembly of Fe-S cluster biogenesis. Consistent with decreased mtACP levels, loss of mecr causes Fe-S cluster biogenesis defects and elevated iron levels in the larval brain. Excess iron results in the accumulation of ceramides leading to mitochondrial defects and neurodegeneration in MEPAN syndrome.