Ring chromosome 21 syndrome

Ring chromosome 21 syndrome (also known as ring 21 or r(21) syndrome) is a rare chromosomal disorder in which one copy of chromosome 21 forms a ring structure, typically resulting from deletions at both ends of the chromosome followed by fusion of the broken ends. The clinical presentation is highly variable and depends on the amount of genetic material lost during ring formation. When significant deletions occur, affected individuals may present with intellectual disability of varying severity, growth retardation, microcephaly, and distinctive facial features including a broad nasal bridge, downslanting palpebral fissures, epicanthal folds, low-set ears, and micrognathia. Some patients may also exhibit skeletal anomalies, muscle hypotonia, and susceptibility to infections. Ring chromosome instability during cell division can lead to mosaicism, with some cells losing the ring chromosome entirely (monosomy 21), which contributes to the phenotypic variability observed among affected individuals. The severity of the condition ranges widely. Some individuals with minimal deletions may have a relatively mild phenotype, while those with larger deletions or significant monosomy 21 mosaicism may have more pronounced intellectual disability, congenital anomalies, and thrombocytopenia or other hematologic abnormalities. Notably, loss of the RUNX1 gene region on chromosome 21 has been associated with an increased risk of developing myelodysplastic syndrome or acute myeloid leukemia in some patients. There is no specific cure for ring chromosome 21 syndrome. Management is supportive and multidisciplinary, involving early intervention programs, speech and physical therapy, educational support, and regular monitoring for potential hematologic complications. Genetic counseling is recommended for affected families to discuss recurrence risks and prognosis.

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