Ring chromosome 20 with loss of telomeric sequences detected by multicolour PRINS

Brandt CA, Kierkegaard O, Hindkjær J, Jensen PKA, Pedersen S. Ring chromosome 20 with loss of telomeric sequences detected by multicolour PRINS. Clin Genet 1993: 44: 26–31. © Munksgaard, 1993 A ring chromosome 20 in a male infant with epileptic seizures, mental and somatic growth retardation, and behavioural disturbances is described. Conventional cytogenetics revealed the karyotype to be 46,XY,r(20)(pter→qter) and no signs of mosaicism were found. Fluorescence in situ hybridisation using the clone p20Z1 identified the ring to be derived from chromosome 20. By counting 111 metaphases, only 7% were found to be missing the ring. The absence of telomeric sequences in the ring chromosome was demonstrated by multicolour PRINS: a three‐step PRimed IN Situ labelling technique, using unlabelled primers. A terminal deletion of both arms thus seems to be the cause of the ring formation in the proband. Bivariate flow‐analysis of chromosomes verified a deletion of the ring chromosome. The clinical and cytogenetic findings are compared with previous cases. A specific ring 20 syndrome seems justified.