We investigated two male infant patients born from monozygotic
twin sisters and unrelated fathers, suggesting an X-linked trait.
Several months after birth, both patients developed severe psycho-
motor regression, hypotonia and seizures associated with bilateral
lesions in the striatum and brainstem. Patient #1 died at 16 months,
patient #2 is now 5 years, alert but unable to communicate,
tetraplegic and wheelchair-bound under permanent artificial ventila-
tion. A muscle biopsy of Pt#1 showed ragged red, complex-IV
negative fibers and fibrosis, with severe reduction of mitochondrial
respiratory complexes cI, cIII and cIV. The muscle biopsy of Pt #2
showed massive connective and fat tissue replacement with hardly
any residual muscle fiber. Fibroblasts from both patients showed
reduction of cIII and cIV but not of cI. We SNP-screened the X
chromosome, and found a disease-segregating haploidentical region
containing a mutation in the Apoptosis Inducing Factor 1 (AIF1) gene
that eliminates aminoacid R201. AIF1 is bound to the inner
mitochondrial membrane (AIFmit) as an NADH oxidase flavoprotein
of unknown function; under apoptogenic stimuli, a soluble form
(AIFsol) is released by proteolytic cleavage of AIFmit, and migrates to
the nucleus, where induces parthanatos, i.e. caspase-independent
fragmentation of chromosomal DNA. We found that in vitro the
R201del mutation increases the DNA binding affinity of AIF1sol and
decreases the stability of both forms. Staurosporine-induced partha-
natos, and galactose-induced mitochondrial fragmentation were
markedly increased, whereas re-expression of AIF1wt induced
recovery of cIII and cIV activities, in AIFR201del fibroblast cells. This
is the first report of an AIF1 mutation in a human mitochondrial
encephalomyopathy. Unlike the Harlequin mouse, a spontaneous
AIF1 knockdown model, which displays partial cI deficiency, the
human disease showed multiple respiratory enzyme defects asso-
ciated with reduced mtDNA amount in skeletal muscle. Interestingly,
riboflavin supplementation improved the neurological conditions of
patient 2 for several months, suggesting that stabilization of the FAD-
associated NADH oxidase activity of AIF1mit is beneficial
.
