Selective Expression of the Immunoregulatory Lectin, Galectin-1, in Precursor B Cell Acute Lymphoblastic Leukemias (ALLs) with MLL Rearrangements

Translocations of the mixed lineage leukemia (MLL) gene on chromosome 11q23 are found in over 70% of infant leukemias and associated with a poor prognosis. MLL translocations generate a new chimeric gene, in which N-terminal portion of MLL is fused to C-terminal sequence from multiple different partners. Given the variety of MLL fusion partners, molecular techniques such as FISH or Southern blotting are not able to detect all genetic abnormalities involving MLL. In addition, results from these time-consuming techniques are not always available at initial diagnosis. To identify molecular markers of MLL translocation that might be used in rapid diagnostic assays, we first compared the transcriptional profiles of primary precursor B cell acute lymphoblastic leukemias (ALLs) with and without MLL translocations. Galectin-1 (Gal1) transcripts were significantly more abundant in MLL-rearranged ALLs (MLL-ALL) from two extensive and independent datasets. Consistent with these findings, Gal1 protein was significantly more abundant in MLL-ALL cell lines than in non-MLL ALL lines by western blotting. These observations were of interest because Gal1 is a secreted immunoregulatory glycan-binding protein that induces the apoptosis of cytotoxic T cells and T helper1 cells and fosters an immunosuppressive tumor microenvironment. To assess the diagnostic utility of Gal1 expression in identifying the MLL-ALL subtype, we performed Gal1 immunostaining on a large series of primary ALLs with known MLL status. All 10 MLL-rearranged precursor-B ALLs had abundant Gal1 expression (10/10, 100%); in marked contrast, only 1 of 38 pre-B ALLs without a cytogenetically detectable MLL translocation expressed Gal1 (3%), p<0.001. Since intracellular flow cytometry is routinely used in the diagnostic evaluation of ALL, we evaluated Gal1 expression in ALL subtypes with a recently developed Gal1 monoclonal antibody and intracellular flow cytometry. With this technique, Gal1 protein expression was high in MLL-ALL cell lines and low/undetectable in ALL lines without the MLL translocation. Since deregulated gene expression in MLL-rearranged leukemias may be related to the altered histone methyltransferase activity of MLL fusion protein complex, we analyzed histone H3 lysine 79 (H3K79) dimethylation in the Gal1 promoter region using chromatin immunoprecipitation. Gal1 promoter H3K79 dimethylation 5 fold higher in the MV-4-11 cell line bearing MLL-AF4 fusion was ≈ gene than in a pre-B ALL line without the MLL translocation, suggesting that this epigenetic modification may be a mechanism for Gal1 overexpression in MLL. We conclude that:

1. Gal1 is specifically overexpressed in MLL-ALLs compared to non-MLL ALLs;

2. Gal1 overexpression is likely driven by the altered histone methyltransferase activity of the MLL fusion protein complex; and 3) Gal1 is a promising diagnostic marker in these MLL-ALLs.