RING CHROMOSOME-20 WITH LOSS OF TELOMERIC SEQUENCES DETECTED BY MULTICOLOR PRINS

A ring chromosome 20 in a male infant with epileptic seizures, mental and somatic growth retardation, and behavioural disturbances is descri bed. Conventional cytogenetics revealed the karyotype to be 46,XY,r(20 )(pter-->qter) and no signs of mosaicism were found. Fluorescence in s itu hybridisation using the clone p20Z1 identified the ring to be deri ved from chromosome 20. By counting 111 metaphases, only 7% were found to be missing the ring. The absence of telomeric sequences in the rin g chromosome was demonstrated by multicolour PRINS: a three-step PRime d IN Situ labelling technique, using unlabelled primers. A terminal de letion of both arms thus seems to be the cause of the ring formation i n the proband. Bivariate flow-analysis of chromosomes verified a delet ion of the ring chromosome. The clinical and cytogenetic findings are compared with previous cases. A specific ring 20 syndrome seems justif ied.