Transglutaminase2 Expression in Acute Myeloid Leukemia: Association with Adhesion Molecule Expression and Leukemic Blast Motility

In prior proteomic analysis on Acute myeloid leukemia (AML) cell lines evaluating the effects of several leukemogenic oncogenes we observed that transglutaminase2 (TG2) was expressed at greater levels as a consequence of oncogenic transformation. TG2 is a multi-domain, multi-functional enzyme with diverse biological functions, including extracellular matrix formation, integrin-mediated signalling, and signal transduction. It's normal roles remain obscure, but it is linked to the pathogenesis of celiac sprue, neurodegenerative diseases, and some cancers. In malignancy it is reported to be an anti-apoptotic mediator of hypoxia inducible factor (HIF) conferring a growth advantage to tumor cells. Expression has been associated with resistance to chemotherapy and apoptosis. We therefore assess the expression of TG2 protein in primary AML patient samples.

We analyzed 511 AML samples from patients with newly diagnosed AML using a custom made reverse phase proteomic array. This array included 11 normal bone marrow derived CD34+ samples as controls and had 140 paired same day blood and marrow samples and 49 paired diagnosis and relapse samples. The array was probed with antibodies against 203 targets including TG2 (Abcam, ab2386, UK). Supercurve algorithms were used to generate a single value from the five serial dilutions. Loading control and topographical normalization procedures accounted for protein concentration and background staining variations.

Expression of TG2 was statistically similar (p= 0.43) in paired blood and marrow samples and in protein prepared from fresh cells or from cryopreserved cells (p= 0.71). Expression was above, equal to or below that of normal CD34+ cells in 12%, 62%, and 27% of patients. Levels were significantly higher at relapse compared to diagnosis in the 49 paired samples (p = 0.003). Levels were higher in FAB M6 and M7 (P =<0.00001 and < 0.008) and lower in patients with inversion16. Higher TG2 expression was strongly inversely correlated with total WBC (r=.035, p < 0.0001) and the absolute blood blast count (r = −.30, p <0.0001). Patients with higher TG2 level had a shorter but not statistically significant overall survival in the entire cohort, and was not prognostic in subsets stratified based on cytogenetics or mutations (FLT3, NPM1, RAS). Likewise, patients with higher TG2 levels had shorter remission durations, but again this was not significant in the entire cohort or in subsets. Expression of TG2 was significantly correlated with 55 of 203 proteins. Notable among these were numerous integrin and adhesion proteins. Hierarchical clustering of these demonstrated that AML is characterized by two large cohorts, one in which TG2 is elevated and is positively correlated with CD49B, Integrinβ3, FAK, Fibronectin and IGFB2, a second in which TG2 is low and negatively correlated with high expression of Osteopontin, CD11 and, CD44 and a 3^rd in which only Caveolin1 is expressed. A Cytoscape interaction plot based on online databases of known protein-protein interactions revealed that TG2 has known interactions with Fibronectin, which it binds and post-translationally modifies, and integrinβ3. In combination this suggests that there is canonical interaction between TG2 and integrin and adhesion proteins active in AML. TG2 expression also correlated positively with numerous anti-apotptosis proteins.

TG2 is expressed in the majority of cases of AML at levels comparable to normal bone marrow CD34+ cells and levels became significantly higher at relapse suggesting that the protein expression signature associated with high TG2 levels may be selected for, or confer a subtle survival advantage to leukemic blasts. In support of this, while the level of TG2 was not statistically significantly prognostic for either overall survival or remission duration, patents with higher levels were somewhat more likely to relapse, and less likely to be alive beyond 3 years. TG2 has previously been linked to drug resistance in cancer and given the negative correlation between TG2 levels and peripheral blasts observed increased TG2 levels may lead to the protection of the leukemic stem cell due to increased adhesion/reduced motility. TG2 may therefore form part of a network of proteins that define poor outcome in AML patients and potentially offer a target to sensitize AML stem cells to drug treatment.

Off Label Use: Clofarabine in AML.