Monosomy chromosome 21 compensated by 21q22.11q22.3 duplication in a case with small size and minor anomalies.

Background: Partial monosomy 21 is a rare finding with variable sizes and deletion breakpoints, presenting with a broad spectrum of phenotypes.
Case Presentation: We report a 10-month-old boy with short stature, minor anomalies and mild motor delay. The patient had a monosomy 21 and duplication of the 21q22.11q22.3 region on the remaining derivative chromosome 21 which represents a partial 21q uniparental disomy of paternal origin, upd(21q22.11q22.3)pat. The abnormalities were characterized by karyotyping, FISH, chromosomal microarray, and genotyping.
Conclusions: This is the first case showing a monosomy 21 compensated by upd(21q22.11q22.3) as a mechanism of genomic rescue. Because there is no strong evidence showing imprinting on chromosome 21, the uniparental disomy itself is not associated with abnormal phenotype but has reduced phenotype severity of monosomy 21. We reviewed the previously published cases with isolated 21q deletions and identified a common deletion of 5.7 Mb associated with low birth weight, length and head circumference in the 21q21.2 region.
Competing Interests: Our institution does not consider a clinical report as human subjects research and this falls in the realm of routine clinical care. Therefore, an IRB approval is not required for a clinical report. Written informed consent for publication was obtained from the parents of the child. The authors declare that they have no competing interests. Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.