Mitochondrial DNA instability disorders are responsible for a large clinical spectrum, among which amyotrophic lateral sclerosis-like symptoms and frontotemporal dementia are extremely rare. We report a large family with a late-onset phenotype including motor neuron disease, cognitive decline resembling frontotemporal dementia, cerebellar ataxia and myopathy. In all patients, muscle biopsy showed ragged-red and cytochrome c oxidase-negative fibres with combined respiratory chain deficiency and abnormal assembly of complex V. The multiple mitochondrial DNA deletions found in skeletal muscle revealed a mitochondrial DNA instability disorder. Patient fibroblasts present with respiratory chain deficiency, mitochondrial ultrastructural alterations and fragmentation of the mitochondrial network. Interestingly, expression of matrix-targeted photoactivatable GFP showed that mitochondrial fusion was not inhibited in patient fibroblasts. Using whole-exome sequencing we identified a missense mutation (c.176C>T; p.Ser59Leu) in the CHCHD10 gene that encodes a coiled-coil helix coiled-coil helix protein, whose function is unknown. We show that CHCHD10 is a mitochondrial protein located in the intermembrane space and enriched at cristae junctions. Overexpression of a CHCHD10 mutant allele in HeLa cells led to fragmentation of the mitochondrial network and ultrastructural major abnormalities including loss, disorganization and dilatation of cristae. The observation of a frontotemporal dementia-amyotrophic lateral sclerosis phenotype in a mitochondrial disease led us to analyse CHCHD10 in a cohort of 21 families with pathologically proven frontotemporal dementia-amyotrophic lateral sclerosis. We identified the same missense p.Ser59Leu mutation in one of these families. This work opens a novel field to explore the pathogenesis of the frontotemporal dementia-amyotrophic lateral sclerosis clinical spectrum by showing that mitochondrial disease may be at the origin of some of these phenotypes.

IMPORTANCE Mutations in the SQSTM1 gene, coding for p62, are a cause of Paget disease of bone and amyotrophic lateral sclerosis (ALS). Recently, SQSTM1 mutations were confirmed in ALS, and mutations were also identified in 3 patients with frontotemporal dementia (FTD), suggesting a role for SQSTM1 in FTD. OBJECTIVE To evaluate the exact contribution of SQSTM1 to FTD and FTD with ALS (FTD-ALS) in an independent cohort of patients. DESIGN A SQSTM1 mutation was first identified in a multiplex family with FTD by use of whole-exome sequencing. To evaluate the frequency of SQSTM1 mutations, we sequenced this gene in a cohort of patients with FTD or FTD-ALS, with no mutations in known FTD and ALS genes. SETTING Primary care or referral center. PARTICIPANTS An overall cohort of 188 French patients, including 132 probands with FTD and 56 probands with FTD-ALS. MAIN OUTCOMES AND MEASURES Frequency of SQSTM1 mutations in patients with FTD or FTD-ALS; description of associated phenotypes. RESULTS We identified 4 heterozygous missense mutations in 4 unrelated families with FTD; only 1 family had clinical symptoms of Paget disease of bone, and only 1 family had clinical symptoms of FTD-ALS, possibly owing to the low penetrance of some of the clinical manifestations. CONCLUSIONS AND RELEVANCE Although the frequency of the mutations is low in our series (4 of 188 patients [2%]), our results, similar to those already reported, support a direct pathogenic role of p62 in different types of FTD.

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