INTRAGENIC REORGANIZATION OF RB1 IN A COMPLEX (4 13) REARRANGEMENT DEMONSTRATED BY FISH

Reciprocal chromosome translocations with no apparent loss of material are the most common de novo structural rearrangements in man. The lar ge majority of these cases have been characterized cytogenetically but very few have been investigated at the molecular level. Using fluores cence in situ hybridization (FISH) we have studied the organization of the tumor supressor gene RB1 in a patient with retinoblastoma and a r earrangement between chromosomes 4 and 13. In addition to the hybridiz ation signal on the normal chromosome 13, three distinct sites of hybr idization of RB1 probes on the translocated chromosomes were detected. These findings show that a complex rearrangement occurred involving a t least three breaks on chromosome 13, two of them in the RB1 gene. Th is also demonstrates that FISH, which offers resolution between that o f fine molecular methods and classical cytogenetics, is a valuable too l for investigating organization of sequences at breakpoints of chromo somal rearrangements.