Cryptic mosaicism for monosomy 20 identified in renal tract cells

We report a post-natal case of mosaic aneuploidy for chromosome 20 in a 4 months old male baby with an abnormal phenotype including dysmorphic features (asymmetric facial growth), ventricular septal defect, hypotonia and bilateral vesicoureteric reflux. Conventional cytogenetics on peripheral blood showed 1 cell of 200 with 47,XY,+20. Further investigations using fluorescent in situ hybridization (FISH) on a urine sample, with a centromere probe for chromosome 20, revealed 39 of 50 cells giving one signal indicative of monosomy 20. FISH analysis of a buccal smear was consistent with disomy 20 as was conventional cytogenetics on skin fibroblasts. This is the fourth reported case of mosaic monosomy 20, the second case where monosomy 20 is present with a trisomy 20 cell and the first case with each aneuploidy found in two separate tissues. The identification of mosaicism is a difficult task since the abnormal cells can be present only in certain tissues and may disappear with selection as the fetus develops, thus leading to single-cell abnormalities that may get dismissed (pseudomosaicism). The use of FISH in this case was crucial in identifying the cryptic mosaic monosomy 20 cell line. The likely mechanism of origin is post-zygotic nondisjunction giving rise to monosomy, disomy and trisomy cell(s) in the same or different tissues. Although no other trisomy 20 cells were found, the abnormal phenotype plus the finding of a monosomy 20 cell line make this mechanism the most plausible explanation. Had we dismissed the single-cell abnormality, the cryptic mosaicism of monosomy 20 would not have been identified. A detailed analysis of all tissues accessible in conjunction with careful consideration of all clinical information available is the best course of action in suspected mosaicism.