Ring Chromosome 17

Ring chromosome 17 (RC17) that replaces a normal chromosome 17 homologue is extremely rare, and only about 20 RC17 cases were reported in literature. RC17 is usually associated with deletion of the terminal regions of chromosome 17. When the deletion included the PAFAH1B1 gene at 17p13.3, the carriers showed phenotypes resembling Miller–Dieker lissencephaly syndrome (MDLS), with lissencephaly, intellectual disability, seizures, and severe postnatal growth deficiency. If PAFAH1B1 was not deleted, the individuals with RC17 may show clinical features, including growth failure, mild intellectual disability, seizures, flecked retina, and café au lait spots. These features were even milder in RC17 cases with the presence of normal cells. Some RC17 cases were mosaic for RC17 and monosomy 17. Loss of a copy of tumor suppressor genes on chromosome 17, such as TP53, NF1, and BRCA1, potentially increased the risk of cancer. Since each RC17 can be unique for its ring structure, type and level of mosaicism, and genomic content, detailed cytogenomic characterization and comprehensive clinical evaluation are essential for accurate diagnostic interpretation, genetic counseling, and clinical management for patients of RC17.