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Functional analysis of a novel mutation in the TIMM8A gene that causes Deafness-Dystonia-Optic Neuronopathy Syndrome

Addison Neighbors, Tonya Moss, Lynda Holloway, Seok-Ho Yu, Fran Annese,  Steve Skinner, Russell Saneto, Richard Steet

Background: The rare, X-linked neurodegenerative disorder, Mohr-Tranebjaerg syndrome (also called Deafness-Dystonia-Optic Neuronopathy (DDON) syndrome), is caused by mutations in theTIMM8A gene. DDON syndrome is characterized by dystonia, early-onset deafness, and various other neurological manifestations. The TIMM8A gene product localizes to the 
intermembrane space in mitochondria where it functions in the import of nuclear-encoded proteins into the mitochondrial inner membrane. Frameshifts or premature stops represent the majority of mutations in TIMM8A that cause DDON syndrome. However, missense mutations have also been reported that result in loss of the TIMM8A gene product.

Methods: We report a novel TIMM8A variant in a patient with DDON syndrome that alters the initiation codon and employed functional analyses to determine the significance of the variant and its impact on mitochondrial morphology.

Results: The novel base change in the TIMM8A gene (c. 1A>T, p.Met1Leu)  results in no detectable protein and a reduction in TIMM8A transcript abundance. We observed a commensurate decrease in the steady-state level of the Tim13 protein (the binding partner of Tim8a) but no decrease in TIMM13 
transcripts. Patient fibroblasts exhibited elongation and/or increased fusion of mitochondria, consistent with prior reports. 

Conclusion: This case expands the spectrum of mutations that cause DDON syndrome and demonstrates effects on mitochondrial morphology that are consistent with prior reports.

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