Abstract
We report a case of a neonate who was shown with routine chromosome analysis on peripheral blood lymphocytes to have full monosomy 21. Further investigation on fibroblast cells using conventional chromosome and FISH analysis revealed two additional mosaic cell lines; one is containing a ring chromosome 21 and the other a double ring chromosome 21. In addition, chromosome microarray analysis (CMA) on fibroblasts showed a mosaic duplication of chromosome region 21q11.2q22.13 with approximately 45% of cells showing three copies of the proximal long arm segment, consistent with the presence of a mosaic ring chromosome 21 with ring instability. The CMA also showed complete monosomy for an 8.8 Mb terminal segment (21q22.13q22.3). Whilst this patient had a provisional clinical diagnosis of trisomy 21, the patient also had phenotypic features consistent with monosomy 21, such as prominent epicanthic folds, broad nasal bridge, anteverted nares, simple ears, and bilateral overlapping fifth fingers, features which can also be present in individuals with Down syndrome. The patient died at 4.5 months of age. This case highlights the need for additional studies using multiple tissue types and molecular testing methodologies in patients provisionally diagnosed with monosomy 21, in particular if detected in the neonatal period.
Highlights
Apparent full monosomy 21 has been reported in ten cases in the pre- and postnatal settings, with most cases being lethal in utero [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
The patient was shown to have full monosomy 21 on a conventional blood karyotype (60 cells analysed) and interphase FISH analysis (100 cells analysed, using the chromosome 21 specific probe provided in the AneuVysion kit (AneuVysion, Abbott Molecular, Illinois, USA))
This is due to many factors including a lack of molecular cytogenetic protocols available at the time of testing and the unavailability of multiple tissue types for the exclusion of tissue limited mosaicism
Summary
Apparent full monosomy 21 has been reported in ten cases in the pre- and postnatal settings (excluding early pregnancy loss), with most cases being lethal in utero [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. The presence of a ring chromosome 21, due to their mitotic instability and propensity for tissue limited mosaicism, provides a plausible explanation for some of the previously reported cases of full monosomy 21.
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