Bmi-1 Protein Expression in CD34 Positive Acute Myelogenous Leukemia Cells as a Prognostic Factor.

Background: Bmi-1 is a novel protein marker, one of the Polycomb group proteins which are involved in self-renewal, senescence, apoptosis and cell cycle regulation of both normal and leukemic haematopoietic stem cells (HSCs). Bmi-1 expression is restricted to primitive bone marrow progenitors and has a critical role in regulating the proliferative output of this cell population. The downstream target of bmi-1 gene is Ink4a locus encoding p16^Ink4a and p19^Arf repressed in a Bmi1 dependent manner. Enforced expression of p16^Ink4a and p19^Arf in HSCs leads to senescence and apoptosis, respectively. Recent data suggest bmi-1 to be responsible for immortalization of acute leukemia cells in murine model and one of the potential targets for therapeutic approaches.

Aims: In this study we analyze expression of bmi-1 protein in acute myelogenous leukemia (AML) cells in humans. We tried to establish the prognostic significance and survival prediction due to bmi-1 expression in AML cells.

Patients: 43 patients (F/M -18/25, median age 55) with AML (FAB subtypes M0,M1,M2,M4 and M5 in 3, 11, 19, 5 and 5 cases, respectively) diagnosed in our unit in two recent years and 8 healthy bone marrow volunteer donors were included into the study. All patients received anthracyclines and cytarabine based induction treatment according the guidelines of Polish Leukemia Study Group. Eight patients died during the follow up observation (early deaths related to treatment toxicity and infection complications were previously excluded).

Methods: Bone marrow specimens were collected from patients during routine diagnostic procedure. Mononuclear cells were permeabilised and incubated with anti-bmi1 antibody (Upstate), followed by surface staining with anti-CD34 (Dako).

Results: We estimated expression of bmi-1 in CD34+ AML cells and its correlation with overall survival (patients who did not complete induction therapy and deaths not related to AML were excluded from the study). On the other hand the differences in bmi expression in CD34+ population between AML subtypes were evaluated. We found significant negative correlation (Spearman test) between bmi-1 expression in CD34+ and overall survival (r= −0.39, p=0,04). The U Mann-Whitney test revealed significant difference in CD34+bmi-1+ cells expression between M0 with M1 patients on one side and M4 with M5 patients on the other side (p=0,02). The mentioned result is restricted to double positive cells, the difference in CD34+ cells alone was not significant. The difference in bmi1 expression in CD34+ cells between AML patients and healthy controls was statistically significant (p=0,003).

Conclusions: Bmi-1 has been known for its role in repression Ink4a locus encoding p16^Ink4a and p19^Arf which are capable of inducing cell cycle arrest, cellular senescence and apoptosis. In this context bmi-1 is responsible for immortalization of HSCs. In this study we tried to establish the potential role of bmi-1 expression in CD34 positive AML cells, the differences according to FAB classification and its prognostic value. The follow up observation is still open. The further analysis is obviously required to determine if bmi-1 could be a potential target for future therapeutic approaches.