Second Case of Gonadal Mosaicism and a Novel Nonsense <scp>NR2F1</scp> Gene Variant as the Cause of Bosch–Boonstra–Schaaf Optic Atrophy Syndrome

Bosch-Boonstra-Schaaf optic atrophy syndrome (BBSOAS) is an autosomal dominant disease characterized by developmental delay, intellectual disability, and optic atrophy with a variable expression of other clinical features (dysmorphic features, autistic behaviour, corpus callosum hypoplasia and seizures). To date, approximately a hundred cases of the syndrome have been described, with an estimated prevalence of 1 in 100 000-250 000. BBSOAS is caused by the loss of function of the NR2F1 gene (nuclear receptor subfamily 2 group F member 1), which encodes the COUP-TFI (Chicken ovalbumin upstream promotor-transcription factor 1). COUP-TFI functions as a homodimer and is one of the major transcriptional regulators directing cortical arealization, cell differentiation and maturation. Most cases of BBSOAS occur de novo, and one case was previously described in which the disease resulted from gonadal mosaicism. In the present case, we report two sisters with BBSOAS, a novel nonsense mutation in the NR2F1 gene and potential gonadal mosaicism as the cause of this rare disease, making it the second such case described in the literature.