Infantile cerebellar-retinal degeneration associated with a mutation in mitochondrial aconitase, ACO2

Abstract

Degeneration of the cerebrum, cerebellum, and retina in infancy is part of the clinical spectrum of lysosomal storage disorders, mitochondrial respiratory chain defects, carbohydrate glycosylation defects, and infantile neuroaxonal dystrophy. We studied eight individuals from two unrelated families who presented at 2-6 months of age with truncal hypotonia and athetosis, seizure disorder, and ophthalmologic abnormalities. Their course was characterized by failure to acquire developmental milestones and culminated in profound psychomotor retardation and progressive visual loss, including optic nerve and retinal atrophy. Despite their debilitating state, the disease was compatible with survival of up to 18 years. Laboratory investigations were normal, but the oxidation of glutamate by muscle mitochondria was slightly reduced. Serial brain MRI displayed progressive, prominent cerebellar atrophy accompanied by thinning of the corpus callosum, dysmyelination, and frontal and temporal cortical atrophy. Homozygosity mapping followed by whole-exome sequencing disclosed a Ser112Arg mutation in ACO2, encoding mitochondrial aconitase, a component of the Krebs cycle. Specific aconitase activity in the individuals' lymphoblasts was severely reduced. Under restrictive conditions, the mutant human ACO2 failed to complement a yeast ACO1 deletion strain, whereas the wild-type human ACO2 succeeded, indicating that this mutation is pathogenic. Thus, a defect in mitochondrial aconitase is associated with an infantile neurodegenerative disorder affecting mainly the cerebellum and retina. In the absence of noninvasive biomarkers, determination of the ACO2 sequence or of aconitase activity in lymphoblasts are warranted in similarly affected individuals, based on clinical and neuroradiologic grounds.

Figure 1

Pedigrees of the Two Families (A) Family A and (B) Family B. Affected individuals are represented by filled symbols.

Figure 2

Representative MRI Findings (A) T2-weighted, coronal section image of individual V-2 at 3 months shows a normal cerebellum and mild cortical atrophy. (B) T1-weighted, coronal section scan of the same individual at 4 years displays severe atrophy of the cerebellar hemispheres (arrowheads) and the vermis (arrows). (C) T1-weighted, coronal section image of individual IV-5 at 12 years shows enlarged cerebellar folia and reduced volume indicating generalized (vermian and hemispheral) atrophy. Note also the mild cortical atrophy and visible sulci. (D) Cortical atrophy and white matter abnormal signals (arrows) demonstrated by FLAIR image of coronal section in the same individual. (E) Moderate cortical atrophy, very thin corpus callosum (arrow), and cerebellar atrophy (arrowhead) in a T1-weighted, sagital section of individual V-5 at 2.5 years. (F) FLAIR image, axial section of individual V-5 displaying frontal and temporal cortical atrophy compared with relatively preserved occipital cortex. The abnormal periventricular white matter signal (arrow) is consistent with dysmyelination.

Figure 3

Aconitase-Specific Activity Lymphoblasts from individuals harboring either a wild-type ACO2 (black bars) or a Ser112Arg-ACO2 mutated gene (grey bars) were lysed and cell extracts were assayed for aconitase (A) and fumarase (B) activity at the indicated temperatures. The relative specific enzymatic activity is presented with reference to the wild-type extract at 37°C (100%). Error bars indicate the standard deviation of two independent experiments.

Figure 4

Expression of S112R mutant human mitochondrial aconitase, ACO2, in yeast, only partially complements the depletion of the endogenous aconitase, S. cerevisiae ACO1 Wild-type (WT) yeast and a mutant deleted for the aconitase gene (Δaco1) were transformed with the indicated plasmids. Cultures were serially diluted at 10-fold intervals, and 10 μl of each was spotted onto ethanol plates, which were incubated at 37°C for 5 days.