To more accurately map the del q
To more accurately map the del(7q) and to search for other submicroscopic aberrations, we performed BAC-based array CGH analysis on BM from the accelerated phase. This confirmed an interstitial deletion in chromosome bands 7q22.1-q35 of maximal size 50.0Mb (Fig. 2D). The minimal region of deletion encompassed BAC-probes RP11-494L11 to RP5-819O4 mapping from 98,055,374bp to 144,989,208bp and the maximal region with BAC-probes RP5-1145A22 to RP11-302C22 mapping from 97,163,382bp to 147,171,825bp. There were no additional copy number changes. Confirmatory interphase nuclei FISH analysis was positive in 91% of the BM PD 173074 (Fig. 2D). Retrospective FISH analysis of BM from the time of ET diagnosis was negative for del(7q) (0/1000 cells counted). Finally, we quantified the load of del(7q) by FISH on FACS-sorted stem and progenitor cells in the acceleration phase. We found 39% del(7q) positive cells in the stem cell containing compartment (CD34+CD38−) and 78% of committed myeloid progenitors (CD34+CD38+) were positive. Our patient highlights several important features of disease progression of MPN. Foremost, the del(7q) represents a key event in this patient. While the leukemogenic potential and the poor prognosis associated with this aberration are described, little is known about associated molecular changes and the kinetics of disease progression [7,8]. The almost three years long lag phase was characterized by a stable normalization of blood cell counts and a significant decrease in JAK2+ allele burden. Although JAK2+ does not affect survival or leukemic transformation, significant changes in the allele burden can be a harbinger of transformation . While retrospective FISH analysis of BM from ET diagnosis was negative for the del(7q), the exact time for acquisition of this aberration remains speculative. The remarkable long-lasting period with normalized peripheral blood cell values after cessation of cytoreductive therapy, may be the result of a slow and gradual displacement of the MPN clone and residual normal hematopoiesis by the del(7q) clone. Considering the del(7q) in relation to the JAK2 and TET2 mutations, we did not obtain enough cells for molecular analysis of sorted subpopulations. However, the JAK2 and the del(7q) most likely occurred in two independent stem cells that already harbored all three TET2 mutations. This is reflected by the fact that the del(7q) was present in 91% of analyzed cells from the acceleration phase whereas the JAK2+ was quantified only at the 0.2% level. The stable TET2 mutational status from ET to overt AML indicates that loss of TET2 function was not implicated in disease progression in this case but rather represents an early priming event in the hematopoietic progenitors. Presence of multiple TET2 mutations, two loss-of-function and one missense variant, has not been described previously in ET patients . The A1505T might be a rare normal germline variant, but involved tissue was not available for confirmation of this. Needless to say, more patients are needed to evaluate whether the rare cases of ET transformation, could harbor similar complex TET2 mutations and to address the possible clonal heterogeneity . It might be speculated if the patient had an early prefibrotic stage of myelofibrosis at the time of initial diagnosis, which per se implies an increased risk of myelofibrotic and leukemogenic transformation . However, there was no strong evidence of this in the bone marrow biopsies. Additionally, this does not diminish the important information obtained in the patient regarding the malignant progression described. In conclusion, AML transformation in the reported patient was driven by the del(7q) and preceded by a notable lag phase of three years of sustained normalization of peripheral blood cell counts, which may have concealed pending transformation. The del(7q) most likely emerged in a JAK2 wild type clone and the disease progression appeared uninfluenced by an otherwise complex but stable TET2 mutational status.