Two Siblings with Cerebellar Ataxia, Mental Retardation, and Disequilibrium Syndrome 4 and a Novel Variant of <i>ATP8A2</i>

Cerebellar ataxia, mental retardation, and disequilibrium syndrome 4 (CAMRQ4) is early onset neuromotor disorder and intellectual disabilities caused by variants of ATP8A2. We report sibling cases and systematically analyze previous literature to increase our understanding of CAMRQ4. Japanese siblings presented with athetotic movements at 1 and 2 months of age. They also had ptosis, ophthalmoplegia, feeding difficulty, hypotonia, and severely delayed development. One patient had retinal degeneration and optic atrophy. Flattening of the auditory brainstem responses and areflexia developed. At the last follow-up, neither patient could sit or achieve head control, although some nonverbal communication was preserved. Whole exome sequencing revealed compound heterozygous variants of ATP8A2: NM_016529.6:c.[1741C>T];[2158C>T] p.[(Arg581*)];[(Arg720*)]. The p.(Arg581*) variant has been reported, while the variant p.(Arg720*) was novel. The symptoms did not progress in the early period of development, which makes it difficult to distinguish from dyskinetic cerebral palsy, particularly in solitary cases. However, visual and hearing impairments associated with involuntary movements and severe developmental delay may be a clue to suspect CAMRQ4.