A novel biallelic frameshift variant in C2orf69 causing developmental regression, seizures, microcephaly, autistic features, and hypertonia.

Combined oxidative phosphorylation deficiency type 53 (COXPD53) is an autosomal recessive neurodevelopmental disorder (NDD) caused by homozygous variants in the gene C2orf69. Here, we report a novel frameshift variant c.187_191dupGCCGA, p.D64Efs*56 identified in an individual with clinical presentation of COXPD53 with developmental regression and autistic features. The variant c.187_191dupGCCGA, p.D64Efs*56 represents the most N-terminal part of C2orf69. Notable clinical features of COXPD53of the proband include developmental delay, developmental regression, seizures, microcephaly, and hypertonia. Structural brain defects of cerebral atrophy, cerebellar atrophy, hypomyelination, and thin corpus callosum were also observed. While we observe strong phenotypic overlap among affected individuals with C2orf69 variants, developmental regression and autistic features have not been previously described in individuals with COXPD53. Together, this case expands the genetic and clinical phenotypic spectrum of C2orf69-associated COXPD53.