CONSISTENT INTERSTITIAL CHROMOSOMAL DELETIONS IN MYELOID MALIGNANCIES AND THEIR CORRELATION WITH FRAGILE SITES

Chromosomal deletions occurring in myeloid malignancies have sometimes been reported either with no breakpoints or as terminal deletions. It is of importance to deduce whether these deletions are actually terminal or interstitial because this has implications for their biologic consequences and the mechanism of their development. Chromosomal deletions have been observed in 38 patients with myeloid malignancies. Two or more deletions occurred in six cases, and in seven cases this was part of a complex abnormality. In all, 45 deletions were observed. In all cases analyzed, the deletions consistently were interstitial. Of the 38 cases, 16 were myelodysplastic syndromes (MDS) [refractory anemia (RA), three; RA with ringed sideroblasts (RARS) three; RA with excess of blasts (RAEB) eight; RAEB in transformation (RAEB-t) one; and unclassified, one], 11 cases were acute nonlymphocytic leukemia (ANLL), and 11 were other myeloproliferative disorders [polycythemia rubra vera (PRV) seven; essential thrombocytopenia (ET), three; unclassified, one]. In general, no uniformity of breakpoints could be identified other than del(9)(q13q22.2) most of which occurred with t(8;21) and del(20)(q11.2q13.3 or 13.1). The breakpoints corresponded to or were adjacent to fragile sites in 49% (proximal 64%, distal 33%). These data emphasize that chromosomal deletions in myeloid malignancies are interstitial. The uniformity of breakpoints in del 9q and del 20a supports the concept that in some instances the exact breakpoints may be important through juxtaposition of genes rather than loss of critical regions. The data also suggest that there may be different mechanisms for the development of proximal and distal breakpoints.