Deletion of <b><i>TOP3B</i></b> Is Associated with Cognitive Impairment and Facial Dysmorphism

Abstract

Deletions of different regions of chromosome 22q11 have been extensively characterized in the literature, with a recent review outlining common deletions with a standardized system proposed for classification and nomenclature. The genotype-phenotype relationships have not been sufficiently elucidated for these deletions, and it remains unclear which specific genes play the dominant roles in producing associated clinical features. Several deletions involve entirely distinct regions of chromosome 22q11 but do not overlap, suggesting that a number of different genes contribute to the clinical features. Studies of patients with small deletions involving only 1 or 2 genes may provide more convincing evidence for the impact of individual genes on the observed phenotype. In this case report, we present a 12-year-old female with autism, cognitive impairment, dysmorphic features, and behavioral concerns and a 268-kb deletion of chromosome 22q11.22 including TOP3B, the only recognized disease-causing gene in the deletion. The mechanism of pathogenesis contributing significantly to our patient's clinical findings may relate to interaction between TOP3B and fragile X mental retardation protein (FMRP), an mRNA-binding protein that regulates translation and is altered in fragile X syndrome, a condition involving developmental delay, learning disability, and autism. All these features are recognized in our patient.