Severe X-Linked Mitochondrial Encephalomyopathy Associated with a Mutation in Apoptosis-Inducing Factor

Division of Molecular Neurogenetics, The Carlo Besta Neurological Institute Foundation, Istituto di Ricovero e Cura a Carattere Scientifico, via Temolo 4, 20126, Milan, Italy.
The American Journal of Human Genetics (Impact Factor: 10.93). 03/2010; 86(4):639-49. DOI: 10.1016/j.ajhg.2010.03.002
Source: PubMed


We investigated two male infant patients who were given a diagnosis of progressive mitochondrial encephalomyopathy on the basis of clinical, biochemical, and morphological features. These patients were born from monozygotic twin sisters and unrelated fathers, suggesting an X-linked trait. Fibroblasts from both showed reduction of respiratory chain (RC) cIII and cIV, but not of cI activities. We found a disease-segregating mutation in the X-linked AIFM1 gene, encoding the Apoptosis-Inducing Factor (AIF) mitochondrion-associated 1 precursor that deletes arginine 201 (R201 del). Under normal conditions, mature AIF is a FAD-dependent NADH oxidase of unknown function and is targeted to the mitochondrial intermembrane space (this form is called AIF(mit)). Upon apoptogenic stimuli, a soluble form (AIF(sol)) is released by proteolytic cleavage and migrates to the nucleus, where it induces "parthanatos," i.e., caspase-independent fragmentation of chromosomal DNA. In vitro, the AIF(R201 del) mutation decreases stability of both AIF(mit) and AIF(sol) and increases the AIF(sol) DNA binding affinity, a prerequisite for nuclear apoptosis. In AIF(R201 del) fibroblasts, staurosporine-induced parthanatos was markedly increased, whereas re-expression of AIF(wt) induced recovery of RC activities. Numerous TUNEL-positive, caspase 3-negative nuclei were visualized in patient #1's muscle, again indicating markedly increased parthanatos in the AIF(R201 del) critical tissues. We conclude that AIF(R201 del) is an unstable mutant variant associated with increased parthanatos-linked cell death. Our data suggest a role for AIF in RC integrity and mtDNA maintenance, at least in some tissues. Interestingly, riboflavin supplementation was associated with prolonged improvement of patient #1's neurological conditions, as well as correction of RC defects in mutant fibroblasts, suggesting that stabilization of the FAD binding in AIF(mit) is beneficial.

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    • "Regions 41 Mb (million base pairs) with very low rates of heterozygosity were selected for further analysis. Candidate genes were identified as mitochondrially targeted using the programs Mitoprot, TargetP, Predotar and Wolf-PSORT and retained if mitochondrial localization was identified by two programs, or if they were present in MitoCarta (Pagliarini et al., 2008; Ghezzi et al., 2010). Candidate genes were sequenced following PCR amplification with M13-tagged primers located at least 20 bp in the intronic regions (primers available upon request), and mutation analysis was performed using CodonCode Aligner (Dedham) and Mutation Surveyor (SoftGenetics). "
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