CLASSIFICATION OF DELETIONS AND IDENTIFICATION OF CRYPTIC TRANSLOCATIONS INVOLVING 7Q BY FLUORESENCE IN-SITU HYBRIDIZATION (FISH)

Monosomy 7 (-7) and deletion of the long arm of chromosome 7, del(7q), are frequent non-random findings in the myelodysplastic syndromes (MD S) and acute myeloid leukaemia (AML), particularly associated with the rapy-related disease (t-MDS and t-AML). The cytogenetic breakpoints of 7q deletions are variable, with both terminal and interstitial deleti ons reported. It is now believed that most deletions are interstitial, and that the variability in reported breakpoints may be due to the di fficulty in determining whether the terminal, pale staining G band is present. it has also been suggested that some reported deletions of 7q may be cryptic translocations. To address these questions, we carried out fluorescence in situ hybridization (FISH) studies on leukaemic ce lls from a large series of patients using a chromosome 7-specific pain t and a 7q telomere-specific probe. Of the 26 cases studied, seven wer e 'pure' deletions (ie without the involvement of other chromosomes); four were interstitial and two terminal. One further patient had two c lones each with a different deletion: one with a terminal del(7)(q22) and the second with an interstitial del(7)(q32-qter), A further nine c ases had unbalanced translocations with deletion of 7q terminal sequen ces. The remaining 10 cases were translocations and complex rearrangem ents, some involving interstitial deletions of 7q. In two cases in whi ch del(7q) was reported as the sole cytogenetic abnormality by G-bandi ng, FISH revealed cryptic translocations involving 7q.