Unbalanced (major route) additional
cytogenetic aberrations (ACA) at diagnosis of
chronic myeloid leukemia (CML) indicate an increased risk of progression and shorter survival. Moreover, newly arising ACA under
imatinib treatment and clonal evolution are considered features of acceleration and define failure of
therapy according to the European LeukemiaNet (ELN) recommendations. On the basis of 1151
Philadelphia chromosome positive chronic phase patients of the randomized CML-study IV, we examined the incidence of newly arising ACA under
imatinib treatment with regard to the p210BCR-ABL breakpoint variants b2a2 and b3a2. We found a preferential acquisition of unbalanced ACA in patients with b3a2 vs. b2a2 fusion type (ratio: 6.3 vs. 1.6, p = 0.0246) concurring with a faster progress to
blast crisis for b3a2 patients (p = 0.0124). ESPL1/
Separase, a
cysteine endopeptidase, is a key player in chromosomal segregation during mitosis.
Separase overexpression and/or hyperactivity has been reported from a wide range of
cancers and cause defective mitotic spindles, chromosome missegregation and
aneuploidy. We investigated the influence of p210BCR-ABL breakpoint variants and
imatinib treatment on expression and proteolytic activity of
Separase as measured with a specific fluorogenic assay on CML cell lines (b2a2: KCL-22, BV-173; b3a2: K562, LAMA-84). Despite a drop in
Separase protein levels an up to 5.4-fold increase of
Separase activity under
imatinib treatment was observed exclusively in b3a2 but not in b2a2 cell lines. Mimicking the influence of
imatinib on BV-173 and LAMA-84 cells by ESPL1 silencing stimulated
Separase proteolytic activity in both b3a2 and b2a2 cell lines. Our data suggest the existence of a fusion type-related feedback mechanism that posttranslationally stimulates
Separase proteolytic activity after
therapy-induced decreases in
Separase protein levels. This could render b3a2 CML cells more prone to
aneuploidy and clonal evolution than b2a2 progenitors and may therefore explain the cytogenetic results of CML patients.