CD99 Is Highly Expressed in Acute Myeloid Leukemia (AML) and Presents a Viable Therapeutic Target

Acute myeloid leukemia (AML) is a heterogeneous hematologic malignancy characterized by clonal proliferation and block of differentiation of myeloid precursors. Overall survival for patients with AML remains dismal (<50% for younger patients and <10% for older patients) due to high relapse rate. In search for novel therapeutic targets in AML, we compared gene expression data of normal hematopoietic vs AML cells from 7 datasets (GSE13159, GSE13164, GSE7186, GSE1159, GSE995, GSE31174 and TCGA Leukemia) available on Oncomine. We identified CD99 to be significantly upregulated in AML cells compared with normal cells in all data sets with available measurements of CD99 expression (median ranking 155, p = 0.013); other genes among the top 10 genes identified in this analysis were FLT3 (median ranking 102, p<0.001) and WT1 (median ranking 120, p<0.001); both are known to play a role in AML. CD99 was significantly over-expressed (p<0.001) in 542 AML patients as compared with PBMCs from 74 healthy donors from the GSE13159 dataset. In the GSE13164 dataset, CD99 was significantly over-expressed in 257 AML patients as compared with PBMCs from 58 healthy donors. Consistently, in the GSE7186 dataset, CD99 was significantly over-expressed (p<0.001) in 23 AML patients as compared with 6 normal bone marrow samples and in the GSE1159 dataset, CD99 was significantly over-expressed (p=0.001) in 285 AML patients as compared with 5 normal bone marrow, and 3 normal blood samples. We also analyzed CD99 expression in cells obtained from 23 patients with AML and sorted according to their CD34 and CD38 expression levels (GSE3077 dataset). We found that CD99 expression was significantly higher (p<0.001) in the CD34+CD38+ and CD34+CD38- subpopulation compared with CD34-CD38- and CD34-CD38+; suggesting a possible role of CD99 in AML stem cells.

Interestingly, analysis of three datasets (GSE22848, GSE6891, GSE15434) via R2: Genomics Analysis and Visualization Platform showed a correlation between CD99 expression and the presence of FLT3-ITD mutation. In the GSE22848 dataset, CD99 was significantly over-expressed (p=0.007) in 48 patients with FLT3-ITD as compared with 189 patients with FLT3 wildtype. In the GSE6891 dataset, 126 patients with FLT3-ITD had a significant over-expression of CD99 (p=0.006) as compared with 334 patients with FLT3 wildtype and in the GSE15434 dataset 90 FLT3-ITD positive patients had significantly higher levels of CD99 (p<0.001) as compared with 161 patients with the wildtype gene.

CD99 (E2, MIC2), a 32-kD cell surface glycoprotein, is known to be involved in the transendothelium migration of neutrophils, T-cell adhesion, and T-cell death by a caspase-independent pathway. In cancer cells, CD99 was found to be highly expressed on the cell surface of Ewing's sarcoma tumors and in gliomas. Importantly, CD99 expression levels were found to be correlated with tumor invasiveness and with lower survival rates. In order to examine the role of CD99 in AML, we assessed CD99 expression by flow cytometry in nine AML cell lines (KG-1, KG-1A, MOLM13, MV4-11, Kasumi-1, THP-1, NB4, U937, UOC-M1); we found CD99 to be expressed in all cell lines. To determine whether CD99 is a potential therapeutic target in AML, we treated leukemia cells with anti-CD99 mAb (mAb 0662) at 5 µg/mL and analyzed cell viability 48 hours post-treatment. We found significant decrease in cell viability; 15% in MV4-11 cells (p=0.02), 32% in MOLM13 cells (p=0.002) and 18% in THP-1 cells (p<0.001) as compared with untreated controls measured by Alamar blue assay. Furthermore, inhibiting CD99 led to a decrease in migration of MV4-11, MOLM13 and THP-1 cells when analyzed using a trans-well migration assay.

In conclusion, CD99 is highly expressed in AML, and this expression is significantly higher in less differentiated leukemia cells and in patients with FLT3-ITD mutation. Functional studies using CD99 antibodies revealed a possible role of this gene in cell survival and cell migration. Further studies are needed to establish CD99 as a potential therapeutic target and further investigations are ongoing to determine the mechanism by which CD99 regulates cell survival in AML.