A Homozygous Synonymous Variant Likely Cause of Severe Ciliopathy Phenotype

Joubert syndrome (OMIM #213300) is a rare neurodevelopmental disease characterized by abnormal breathing patterns, intellectual impairment, ocular findings, renal cysts, and hepatic fibrosis. It is classified as a ciliopathy disease, where cilia function or structure in various organs are affected. Here, we report a 17-year-old male whose main clinical findings are oculomotor apraxia and truncal ataxia. Magnetic resonance imaging revealed the characteristic molar tooth sign of Joubert syndrome. He also has obsessive–compulsive disorder concomitantly, which is not a known feature of Joubert syndrome. Molecular genetic analysis revealed a homozygous c.2106G>A (p.(Thr702=)) variation in the Abelson helper integration 1 (AHI1) gene and another homozygous c.1739C>T (p.Thr580Ile) variation in the coiled-coil and C2 domain-containing protein 1A (CC2D1A) gene. Even though certain AHI1 variations were previously associated with Joubert syndrome (JS), c.2106G>A (p.(Thr702=)) was only reported in one patient in trans with another known pathogenic JS variant. The CC2D1A c.1739C>T (p.Thr580Ile) variation, on the other hand, has been reported to cause autosomal recessive nonsyndromic mental retardation, but there are conflicting interpretations about its pathogenicity. Overall, to our knowledge, this is the first patient representing a severe ciliopathy phenotype caused by a homozygous synonymous AHI1 variation. Further investigations should be performed to determine any involvement of the CC2D1A gene in ciliopathy phenotypes such as Joubert syndrome.