Molecular characterization of de novo ring chromosome 21 in a child with seizures, growth retardation, and multiple congenital anomalies.

The ring chromosome 21[r(21)] syndrome is a rare disorder, and mainly occurs as a de novo event. However, a wide variation of the phenotype has been reported in r(21) cases depending on breakpoints, loss of genetic material, and mosaicism of cells with r(21) and monosomy 21, causing copy number alterations. A 29-month-old female was referred to the centre for seizures, developmental delay, microcephaly, hypotonia, deafness, and other congenital abnormalities. Physical examination revealed short stature and multiple facial dysmorphism. She was unable to sit, walk or stand by herself. Cytogenetic study with GTG banding revealed a karyotype of mos 46,XX,r(21)(p11.1q22.12)[70]/45,XX,-21[10]/47,XX,r(21),+r(21)[1]/46,XX[10]. Additionally, molecular cytogenetics refined the breakpoints and characterized the deleted region (RP11-410P24/CHR21: 32849565-33019511) in the clone with the r(21) as ~12-14 Mb contiguous region at 21q22.12 to 21qter. The present study has accurately detected copy number alterations caused by ring chromosome formation. The basis of the UCSC Genome Browser on Human (GRCh38/hg38) analysis suggests hemizygous expression of a deleted critical region of chromosome 21 in ring chromosome cell lines. This is likely to be the underlying cause of the present phenotypes in the patient. Overall, the genotype-phenotypic correlation in r(21) cases remains widely diverse, most likely due to tissue-specific mosaicism of the 45, XX,-21 cell line.