Mechanisms of therapy resistance in acute lymphoblastic leukemia

Abstract: Acute lymphoblastic leukemia (ALL) is a highly aggressive pediatric cancer that can affect both B cells and T cells. The advent of new therapies has increased the cure rates for both B-ALL and T-ALL patients. However, some patients still experience relapse with a variable response to the treatment and display poor survival. Thus,identification of novel predictive biomarkers that can predict therapy resistance may help to stratify this group of patients. This could also aid in developing an effective treatment strategy.Glucocorticoids are widely used along with the chemotherapeutic regimens for treating ALL patients. The response to glucocorticoids can predict long-term remission outcome. To understand the mechanisms of resistance to glucocorticoids, such as dexamethasone, we generated dexamethasone-resistant B-ALL cell lines in paper I. One such resistant cell line was found to possess increased FLT3 expression levels with FLT3-ITD and FLT3-R845G mutations that led to the activation of oncogenic RTK signaling. Further, second-generation FLT3 inhibitors, such as AC220 and crenolanib, suppressed this signaling both in vitro and in vivo.We continued exploring the dexamethasone resistance mechanisms in paper II using a different approach. We observed that dexamethasone exposure caused upregulation of Aurora kinase and its various downstream effector kinases such as JAK, p38, mTOR, and S6K. These kinases lead to β-catenin stabilization through phosphorylation-dependent inactivation of GSK-3β either directly or indirectly. Indeed, we observed partial restoration of dexamethasone sensitivity with a combination of dexamethasone and inhibitors targeting either these kinases or β-catenin.The expression of BCL2 varies in T-ALL depending on its stage of maturation, thereby T-ALL displays a heterogenous response to the BCL2-specific inhibitor venetoclax. We thus studied the mechanisms of venetoclax resistance using a panel of T-ALL cell lines in paper III. We observed that all the venetoclax-resistant T-ALL cell lines displayed non-universal changes in the expression of BCL2 family members and cancer stem cell markers, along with specific enrichment of cytokine signaling pathways. However, further investigations are warranted to identify additional mechanisms of venetoclax resistance in T-ALL.Combination therapy is usually the choice of treatment to overcome monotherapy resistance. With this in mind, in paper IV we identified that inhibiting BCL2 by venetoclax synergizes with PLK1 inhibition by volasertib in T-ALL cell lines and PDX models. We observed that BCL2L13 and PMAIP1 genes get upregulated upon PLK1 inhibition, probably through transcriptional regulation by FOXOs in interaction with β-catenin. Thus, the pro-apoptotic functions exhibited by BCL2L13 and PMAIP1 probably synergize with BCL2 inhibition in T-ALL, with the help of sustained β-catenin levels. Moreover, we also identified upregulation of oxidative phosphorylation (OXPHOS) in T-ALL PDXs that didn’t display synergy, which could be treated with a combination of venetoclax and oligomycin. However, additional experiments will be required to verify the above results.