Dysregulated mitophagy and mitochondrial organization in optic atrophy due to OPA1 mutations.

OBJECTIVE: To investigate mitophagy in 5 patients with severe dominantly inherited optic atrophy (DOA), caused by depletion of OPA1 (a protein that is essential for mitochondrial fusion), compared with healthy controls. METHODS: Patients with severe DOA (DOA plus) had peripheral neuropathy, cognitive regression, and epilepsy in addition to loss of vision. We quantified mitophagy in dermal fibroblasts, using 2 high throughput imaging systems, by visualizing colocalization of mitochondrial fragments with engulfing autophagosomes. RESULTS: Fibroblasts from 3 biallelic OPA1(-/-) patients with severe DOA had increased mitochondrial fragmentation and mitochondrial DNA (mtDNA)-depleted cells due to decreased levels of OPA1 protein. Similarly, in siRNA-treated control fibroblasts, profound OPA1 knockdown caused mitochondrial fragmentation, loss of mtDNA, impaired mitochondrial function, and mitochondrial mislocalization. Compared to controls, basal mitophagy (abundance of autophagosomes colocalizing with mitochondria) was increased in (1) biallelic patients, (2) monoallelic patients with DOA plus, and (3) OPA1 siRNA-treated control cultures. Mitophagic flux was also increased. Genetic knockdown of the mitophagy protein ATG7 confirmed this by eliminating differences between patient and control fibroblasts. CONCLUSIONS: We demonstrated increased mitophagy and excessive mitochondrial fragmentation in primary human cultures associated with DOA plus due to biallelic OPA1 mutations. We previously found that increased mitophagy (mitochondrial recycling) was associated with visual loss in another mitochondrial optic neuropathy, Leber hereditary optic neuropathy (LHON). Combined with our LHON findings, this implicates excessive mitochondrial fragmentation, dysregulated mitophagy, and impaired response to energetic stress in the pathogenesis of mitochondrial optic neuropathies, potentially linked with mitochondrial mislocalization and mtDNA depletion.

Two sisters with Rett syndrome and non-identical paternally-derived microdeletions in the MECP2 gene.

The unique case of two sisters with symptoms of RTT and two quite distinct, novel, and apparently de novo microdeletions of the MECP2 gene is described. One sister possessed an 18 base-pair (bp) deletion (c.1155_1172del18) within the deletion hotspot region of exon 4, whereas the other sister exhibited a 43 bp deletion at a different location in the same exon (c.1448_1461del14+29). Although these lesions occurred on the same paternally-derived X chromosome, this is probably due to chance co-occurrence owing to the relatively high mutation rate of the MECP2 gene rather than to a constitutional mutator phenotype.

Severe hypohidrotic ectodermal dysplasia in a girl caused by a de novo 9;X insertion that includes XIST and disrupts the EDA gene.

X-linked hypohidrotic ectodermal dysplasia (XLHED) is caused by mutations in the EDA gene. A girl with severe hypohidrotic ectodermal dysplasia and normal mental development had completely skewed X chromosome inactivation with only the paternal X active in peripheral blood cells. Routine chromosome analysis and sequencing of the EDA gene were normal. However, whole chromosome painting revealed a 9;X insertion. FISH analyses with BAC probes towards the EDA gene and the more distal region containing the XIST locus showed that an X chromosome fragment of at least 4 Mb containing XIST was inserted into 9p13 in conjunction with a de novo pericentric inversion of chromosome 9. The proximal breakpoint was within the EDA gene and the distal breakpoint was distal to the XIST locus. Both parents had normal chromosomes, and the mother had random X inactivation in peripheral blood cells. Because XIST was lacking on the X chromosome with the disrupted EDA gene, the normal X chromosome was inactivated resulting in severe XLHED.