Ruth N MacKinnon1,2, Hendrika M Duivenvoorden3, Lynda J Campbell1,2 Meaghan Wall1,2,4
1 Victorian Cancer Cytogenetics Service, PO Box 2900, Fitzroy, Vic 3065
2 Department of Medicine (St Vincent’s), University of Mebourne, Fitzroy, Vic 3065
3 Department of Biochemistry and Genetics, La Trobe Institute for Molecular Science, La Trobe University, Bundoora, Vic 3086
4 St Vincent’s Institute of Medical Research, Princess St, Fitzroy ,Vic 3065
Complex karyotypes in acute myeloid leukaemia (AML) and myelodysplastic syndromes (MDS) are associated with a poor clinical outcome, and often have dicentric chromosomes. These malignancies, particularly with a complex karyotype, can be caused by chemotherapy or radiotherapy for a different cancer.
We describe four examples of the rare recurrent dicentric (20;22) in AML or MDS. These abnormal chromosomes had strikingly similar structural features. Combining cytogenetic and molecular genetic technologies has given us insight into how they arose.
All four retained the 20q subtelomere despite 20q12 deletion, and notably telomere sequence was also detected at the junction of chromosomes 20 and 22 in three of the four examples. As telomeres normally occur at the ends of chromosomes, this suggests that the dicentric chromosome arose by end-to-end fusion of 22p and 20q.
Other characteristics in common were instability, deletion of the common deleted region at 20q12, presence of the nucleolar organiser region at 22p near the breakpoint, and gain of the recurrently amplified region at 20q11.21.
We propose the following steps in the evolution of the dic(20;22). End-to-end joining of chromosomes 20q and 22p produces an unstable dicentric chromosome. This instability allows the chromosome to evolve rapidly, selecting for loss of a tumour suppressor gene at 20q12 and amplification of 20q11.21.
A high incidence of 20q subtelomere retention in dicentrics involving chromosome 20 has been reported. These abnormal chromosomes are usually not recognised as possible telomere fusions because subsequent rearrangements have made the breakpoints appear closer to the centromeres. Our results show that there is enough residual internal telomere sequence for detection by FISH in at least some instances of telomere fusion involving 20q. Testing of other dicentrics may show that telomere
Dr MacKinnon has investigated dicentric chromosome abnormalities in AML and MDS during her time at the Victorian Cancer Cytogenetics Service. Previously she has worked on fragile sites at the Adelaide Children’s Hospital, the Fragile X at the Molecular Institue of Medicine, Oxford, and cattle genomics at CSIRO.