Hematopoietic Cell Expression of OPAL1, an Outcome Predictor in Pediatric Acute Lymphoblastic Leukemia.

Background: A novel sequence, NM_hypothetical protein FLJ20154, has been identified in our previous microarray study as one of the most powerful predictors of CCR in B precursor acute lymphoblastic leukemia (ALL). We have cloned, characterized, and named this gene as Outcome Predictor in Acute Leukemia 1(OPAL1). Our preliminary data indicates that OPAL1 distinguishes those ALL patients with t(12;21) who will achieve CCR, and predicts for CCR in children who have adverse prognostic features (normal karyotype, higher age, higher white blood cell count at presentation). To further investigate the role of OPAL1 in ALL, we first determined OPAL1 expression in cells from different hematopoietic lineages, compared the OPAL1 mRNA expression levels of normal hematopoietic cells with ALL patients with t(12;21) or t(1;19).

Methods: Peripheral blood mononuclear cells from 46 healthy individuals (age range: 2–18 years) were collected and sorted into three groups: B enriched (CD19+), T enriched (CD3+) and others (mostly were nutrophil cells); along with pre-treatment ALL samples with high blast count (>90%) from patient with t(12;21) or t(1;19) were used for study. mRNA expression levels of OPAL1 were determined by real-time quantitative PCR. The OPAL1 expression values were calculated by using the formula: Value=1/2^^ΔCt, ΔCt=Ct_target−Ct_control.. The generated values were grouped into four multitude ranges categories: −10⁻², 10⁻¹, 10⁰, 10¹.

Results: Most of the relative expression ratio of OPAL1 (normalized to GAPDH) for the B enriched (29/45, 64.5%) or T enriched cells (6/12, 50%) in healthy individuals fell in the 10⁰ level, while 75% (9/12) of the other residual mononuclear cells (mostly neutrophils cells), their OPAL1 relative expression ratio were in the 10⁻¹ level which is ten times lower than those seen in normal T and B cells. OPAL1 expression ratio were significantly decreased in ALL patients with t(12;21) and t(1;19) compared with normal B enriched cells, p=0.0079 and p= 0.0006, respectively. Forty five percent (9/20) of t(12;21) patients their OPAL1expression ratio were at 10⁻¹ level, and 40% (8/20) at 10⁻² level. As for patients with t(1;19) or higher risk patients group (three patients in this group were bcr/abl positive), 60% (9/15) of the OPAL1 expression ratio were at 10⁻² level. These results strongly suggested that the correlation of higher OPAL1 expression is associated with “good risk” genetic subtypes of ALL

Summary: Although currently the biologic functions of OPAL1 remained to be determined, the high expression level of OPAL1 in normal T or B lymphocytes compares with non-T, non-B cells; low expression in ALL leukemia blast cells suggested that OPAL1 may play a role in lymphocyte differentiation, especially in the development and maturation of the normal B cell. The reverse relationship between OPAL1 expression level and the patient genetic risk factors indicates that OPAL1 might be another strong predictor for treatment outcome along with other well known clinical indicators. Hence, the levels of OPAL1 may be useful in identifying ALL patients with a potential good outcome, who may be able to benefit from less intensive treatment regimens and still achieve long term remission.