Novel compound heterozygous ACO2 mutations in an infant with progressive encephalopathy: A newly identified neurometabolic syndrome

Background: The ACO2 gene encodes mitochondrial aconitase, the enzyme involved in the second step of the tricarboxylic acid cycle, catalyzing the interconversion of citrate into isocitrate. To date, fewer than 20 families harboring ACO2 mutations have been identified since the first report of a neurodegenerative disorder such as infantile cerebellar retinal degeneration in 2012. Subsequently, various phenotypes, from isolated optic atrophy to spastic paraplegia, have been recognized. Here, we report a case of a newly identified neurometabolic syndrome resulting from novel ACO2 mutations, which expands the genetic spectrum and increases clinical awareness in real-world clinical practice.

Case report: A 2-month-old boy presented with hypotonia, cyanosis, and abnormal eye movements. He had severe psychomotor retardation and intractable seizures manifesting with cyanotic episodes. Diffuse cerebral atrophy and bilateral optic atrophy were noted without cerebellar atrophy. With unremarkable results on comprehensive diagnostic work-up and targeted genetic tests, whole exome sequencing revealed novel compound heterozygous variants in ACO2 (p.Met393Ile and p.Cys448Ser), which were confirmed by Sanger sequencing. Although no definitive signs suggestive of metabolic disturbances or mitochondrial dysfunction have been noted in patients with ACO2 mutations to date, elevated plasma glutamate levels were noted in our case.

Conclusion: A high index of clinical suspicion and awareness of this disease may aid in the diagnosis of cases with unknown neurodegenerative diseases, facilitated by deep sequencing.