Complex III Mitochondrial Leukoencephalopathy Masquerading Acute Demyelinating Syndrome Due to a Novel Variant in CYC1

Background Complex III (CIII) is the third out of five mitochondrial respiratory chain complexes residing at the mitochondrial inner membrane. The assembly of 10 subunits encoded by nuclear DNA and one by mitochondrial DNA result in the functional CIII which transfers electrons from ubiquinol to cytochrome c. Deficiencies of CIII are among the least investigated mitochondrial disorders and thus clinical spectrum of patients with mutations in CIII is not well defined. Resultswe report on a 10-year-old girl born to consanguineous Iranian parents presenting with acute neurological deficits reminiscent of acquired demyelination, mainly acute bilateral vision loss, who was ultimately confirmed to have a novel homozygous missense variant, c.949C>T; p.(Arg317Trp) in complex III of the mitochondrial chain. Sanger sequencing confirmed the segregation of this variant with disease in the family. ConclusionWe present a patient with a mitochondrial leukoencephalopathy due to complex III deficiency that manifested with features suggestive of an acquired demyelinating syndrome. The effect of this variant on the protein structure was shown in-silico. Our findings, not only expand the clinical spectrum due to defects in CYC1 gene but also highlight that mitochondrial disease should be considered in children with acute CNS demyelination.

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