APOC2 Is Upregulated in Acute Myeloid Leukemia and Presents a Novel Therapeutic Target

Acute Myeloid Leukemia (AML), is a devastating hematologic malignancy that affects the hematopoietic stem cells. Certain cytogenetic and molecular abnormalities have been implicated in disease pathogenesis, which result in abnormal proliferation and differentiation of myeloid blasts. Current therapy provides five years' overall survival (OS) in 35%-40% of young patients and 5-15% of old patients. Thus, there is an urgent need to investigate new therapeutic targets.

To identify therapeutic targets in AML, we analyzed gene expression datasets (GSE1159, GSE7186, GSE13159 and GSE13164) for genes that are differentially overexpressed in AML compared with healthy donor cells. We found that Apolipoprotein CII (APOC2) was consistently expressed at a significantly higher level in AML than in control samples (P<0.0001). APOC2 is component of low density lipoprotein which activates lipase and thus contributes to lipoproteins metabolism. However, the role of APOC2 in cancer is unknown.

Here, we report that APOC2 mRNA is significantly overexpressed in 542 AML patients compared with 74 healthy peripheral blood mononuclear cells (PBMC) samples (GSE13159, 2.9 fold, P<0.0001). In GSE13164, APOC2 median mRNA level was higher in 257 AML samples compared with 58 healthy PBMC samples (1.7 fold, P<0.0001). Consistently, APOC2 median mRNA level was upregulated in 285 AML samples compared with 8 control samples in the GSE1159 dataset (6 fold, P<0.001). In the GSE7186, APOC2 showed a higher median mRNA level in 23 AML patients compared with 6 normal bone marrow samples (32 fold, P<0.0001).

To understand the role of APOC2 and its deregulation in AML, we analyzed its expression according to cytogenetic and mutational status in patients with AML. APOC2 is significantly upregulated in t(11q23)/MLL-rearranged AML. APOC2 median mRNA level was higher in 38 patients with MLL rearrangement than that in 504 patients without MLL rearrangement (GSE13159, 9 fold, P<0.0001). APOC2 median mRNA level was also higher in 47 patients with MLL rearrangement than that in 190 patients without MLL rearrangement (GSE17855, 6.6 fold, P<0.0001). Similar findings were observed in three other data sets (GSE1159, 19-fold, P<0.0001; TCGA, 6 fold, P=0.0003; GSE13164, 6.7 fold, P<0.0001). In addition, patients with FLT3 mutation had 5-7 fold higher APOC2 median mRNA expression compared with patients carrying the wild type FLT3 (GSE1159, 5.4 fold, P=0.0006; TCGA data, 6.9 fold, P=0.0007). APOC2 levels were also higher in patients with t(15;17) translocation than that in patients without t(15;17) (GSE1159, 4.9 fold, P=0.0024; TCGA, 36.5 fold, P=0.0098).

We also examined the association between APOC2 overexpression and clinical outcome of patients with AML excluding t(15;17) patients. Patients in the TCGA data were dichotomized based on their APOC2 mRNA expression Z-score (RNA Seq V2 RSEM) into high (Z-score ≥1) and low (Z-Score <1). Patients with high APOC2 levels had significantly shorter OS than patients with low APOC2 expression (median OS 4.05 vs 10.3 months, P=0.002).

To assess the function of APOC2 in AML, we established an ectopic lentiviral expression system to generate stable cells overexpressing APOC2 (APOC2-OE). In three different AML cell lines (U937, MOLM13 and THP-1), APOC2 overexpression caused significant increase in cell proliferation compared with empty vector control (2 fold, P<0.0001). Similarly, in primary blasts from three patients with AML, overexpression of APOC2 increased cell number of primary blasts (3 fold, P=0.0364). Given that APOC2 is a secreted protein, we examined the effect of secreted APOC2 on the growth of AML cell lines. Naïve cell lines (U937, MOLM13 and THP-1) were treated with supernatant collected from their respective APOC2-OE cells or empty vector cells. Naïve cells treated with supernatant from APOC2-OE cells showed a significant increase of cell proliferation compared with cells treated with supernatant from empty vector cells (30 to 70% increase, P<0.01).

In conclusion, APOC2 is highly expressed in AML, specifically in patients with MLL-rearrangement and in patients with FLT3 -ITD mutation. We also demonstrated that secreted APOC2 promotes cell proliferation in AML cells. More studies are ongoing to characterize the functional role of APOC2 overexpression in leukemogenesis and to investigate the mechanisms by which this gene is deregulated in MLL-rearrangements AML.