Dynamic tissue-specific mosaicism in constitutional non-supernumerary ring chromosome 21 with a Down syndrome-like phenotype: a case report and genotype-phenotype analysis.

An 8-month-old boy with constitutional non-supernumerary ring chromosome 21 shows tissue-specific mosaicism and a 18.77-Mb 21q duplication plus a 3.09-Mb terminal 21q deletion linked to a Down syndrome-like phenotype.

• Why it matters: Ring chromosome 21 can produce complex imbalances and dynamic mosaicism, but how instability and tissue-specific mosaicism shape the clinical presentation remains unclear.
• What they did: The team used cytogenomic analyses to define a 18.77-Mb duplication and 3.09-Mb terminal deletion, then applied FISH to map ring structure and identify four instability-derived clones across lymphocytes, oral mucosa epithelial cells, and urinary sediment epithelial cells.
• The result: Across four previously reported constitutional non-supernumerary r(21) cases, they found a recurrent DSCR duplication with terminal 21q deletion despite substantial phenotypic variability, suggesting the DS-like presentation is not explained solely by classical DSCR dosage.

AI summary of the abstract · may contain errors · not medical advice · read the paper.