CHARACTERIZATION OF CRYPTIC REARRANGEMENTS AND VARIANT TRANSLOCATIONSIN ACUTE PROMYELOCYTIC LEUKEMIA

Acute promyelocytic leukemia (APL) is typified by the reciprocal trans location. t(15; 17)(q22; q21), leading to the formation of PML-RAR alp ha and RAR alpha-PML fusion genes. We have characterized 7 cases of mo rphologic APL found to lack the t(15;17) on conventional cytogenetic a ssessment. In 6 of 7 cases, cryptic PML-RAR alpha rearrangements were identified by reverse transcriptase-polymerase chain reaction and fluo rescent in situ hybridization (FISH); whereas, in the remaining patien t, APL was associated with the variant translocation, t(11; 17)(q23; q 12-21), leading to the formation of PLZF-RAR alpha and RAR alpha-PLZF fusion genes. In each of the cases with cryptic PML-RAR alpha rearrang ements, PML-RAR alpha transcripts were detected in the absence of RAR alpha-PML, consistent with the concept that PML-RAR alpha is the criti cal oncogenic fusion protein. In 4 of these cases with evaluable metap hase spreads, the occurrence of a nonreciprocal translocation was conf irmed by FISH with sole formation of the PML-RAR alpha fusion gene; in 3 cases with morphologically normal chromosomes 15 and 17, RAR alpha was inserted into PML on 15q, whereas in the remaining patient the PML -RAR alpha fusion arose due to insertion of 15q-derived material inclu ding PML into RAR alpha on 17q. Immunofluorescence studies were perfor med using antibodies raised against PML and PIC 1, a ubiquitin-homolog y domain protein previously identified as an interaction partner of PM L. In acute myeloid leukemia (AML) of subtypes other than Ma, PIC 1 wa s localized to the nuclear membrane and colocalized with PML within di screte nuclear bodies. In APL cases with cryptic PML-RAR alpha rearran gements, the characteristic microparticulate pattern of PML staining w as detected with partial colocalization with PIC 1, indicative of disr uption of the nuclear bodies; whereas in t(11; 17)-associated APL, PML and PIC 1 remained colocalized within discrete nuclear bodies, as obs erved in non-APL cases. Although deregulation of the putative growth s uppressor PML and delocalization of other nuclear body constituents ha ve been advocated to play a key role in the development of t(15; 17)-a ssociated APL. the present study shows that disruption of PML nuclear bodies per se is not a prerequisite for the pathogenesis of APL. (C) 1 997 by The American Society of Hematology.