Abstract
Background: Harel-Yoon syndrome (HAYOS) is a recently described, rare neurodevelopmental disorder characterized by developmental delay, hypotonia, appendicular hypertonia, axonal neuropathy, and other variable features, such as spasticity and optic atrophy. With only a few reports in the literature, both heterozygous and homozygous mutations have been reported in ATPase Family AAA Domain Containing 3A (ATAD3A).
Case Presentation: Herein, we present the first case of HAYOS in Saudi Arabia. A 3-month-old girl presented with global developmental delay, hypotonia, bilateral severe sensorineural hearing loss, and vision impairment. Brain magnetic resonance imaging showed mild brain atrophy and delayed myelination. Laboratory tests showed high serum lactate and increased urinary excretion of 3-hydroxy methyl glutaconic acid. Whole exome
sequencing revealed a pathogenic heterozygous variant in ATAD3A gene (c.1726C>T; p. R576W: NM_018188.4 or c.1582C>T; p. R528W: NM_001170535.1) which is the same recurrent variant reported in patients with the dominant form of HAYOS.
Conclusion: Our report provides further evidence of the clinical relevance of ATAD3A gene variant (c. 1726C>T; p. R576W) in the pathogenesis of HAYOS. The therapeutic options for HAYOS are limited to supportive measures as in other mitochondrial diseases.

Key words: Mitochondrial disorder, HAYOS, ATAD3A, whole exome sequencing