Mosaicism and phenotype in ring chromosome 20 syndrome.

Ring chromosome 20 [r(20)] syndrome is a rare chromosomal disorder characterized by epilepsy, mild to moderate mental impairment, and malformation. Patients generally show mosaicism in 1-100% of lymphocytes with r(20). We report here a patient with r(20) syndrome who exhibited mild phenotype with the small ratio of mosaicism (13%) with r(20). Although previous small-scale studies concluded that the mosaicism ratio was unrelated to clinical phenotype, our reassessment of all 57 reported cases has revealed that the ratio is significantly associated with age at seizure onset, intelligence quotient, and malformation, but not with the response of epilepsy to drug treatment. Our results provide important clinical information and prediction for r(20) syndrome.