Two patients with mosaic trisomy ring 20. Doing the right test for the wrong reasons

Abstract

We report two patients with mosaic trisomy ring 20. The first patient was a 16 yo referred for possible Cohen syndrome because of obesity, learning disabilities and speech delay. Additionally, he had a history of Tetralogy of Fallot, myopia, small posterior subcapsular cataracts and dysmophic features inconsistent with the referring diagnosis. Chromosome analysis was performed looking for 22q deletion (given the conotruncal heart lesion). Karyotype was 47,XY,+r[6]/46,XY[24].ish r(20)(P?q?)(D20Z1+),22q11.2(D22S75×2). The second patient was a 10 month old with developmental delay and bypotonia. Additional features included small penis, a broad face and small hands and feet. Chromosome analysis was performed looking for 15q deletion. Karyotype was 47,XY,+mar[35]/46,XY[45].ish 15q11-q13(SNRPN×2; D15S10×2).ish r(20)(p11.2q11.2)(coatasome 20+).

Only three other cases of mosaic trisomy ring 20 have been reported in the medical literature. Only one has been molecularly characterized. A consistent phenotype has not been seen, which likely reflects differences in the genetic material present in the ring, as well as the level of mosaicism present. Comparison with the 3 reported cases as well as with other cases of mosaic trisomy 20p and 20q will be presented. These cases also point out a potential pitfall in that exclusive utilization of molecular cytogenetic techniques for specific syndromic diagnosis would have led to missed diagnoses in both of our patients. Molecular cytogenetics should continue to be use as a complement to standard high resolution cytogenetic analysis.