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The role of cytosolic calreticulin in the promotion of oncogenic signaling in acute lymphoblastic leukemia Lam, Shing Tat Theodore

Abstract

Acute lymphoblastic leukemia (ALL), a malignancy of the bone marrow that is mainly caused by the uncontrolled proliferation of B- or T-lymphoblasts, is the most common form of pediatric cancer in Canada. Pediatric ALL is currently highly treatable due to advances in contemporary risk-directed ALL chemotherapy. However, some patients will be unresponsive to treatment and prognosis remains poor for patients undergoing relapse. Adverse side effects from drug toxicity used in chemotherapy also remain a major challenge. Recently, the lectin chaperones calreticulin (CALR) and calnexin (CANX), along with their co-chaperone, PDIA3, are increasingly implicated in the biological processes of various human cancers, along with their main role of facilitating protein folding in the endoplasmic reticulum (ER). Here, I investigated the role of CALR and PDIA3 in the promotion of pro-survival JAK/STAT signaling in human T-lymphoblasts. Previously, PDIA3 was found to be required for CALR localization in the cytosol. I generated CALR-/- and PDIA3-/- T-lymphoblasts using CRISPR-Cas9 to assess the effects of deficiency in total and cytosolic CALR on JAK/STAT signaling. CALR-/- and PDIA3-/- cells exhibited reduction in STAT3 phosphorylation compared to wildtype cells. This suggested that cytosolic CALR played a role in the regulation of STAT3 phosphorylation. Unexpectedly, I found that STAT5b mRNA was reduced in CALR-/- and PDIA3-/- Jurkat T-lymphoblasts, leading to an observed deficiency in STAT5 protein expression. This further emphasized the biological significance of CALR and PDIA3 in their extra-ER functions. To assess the clinical significance of CALR, CANX and PDIA3 expression in ALL, I analyzed publicly available transcriptome databases of pediatric ALL, which revealed the potential clinical utility of CALR and CANX mRNA expression in diagnostic tumour samples as biomarkers for predicting treatment response. Elevated CALR and CANX was correlated with the presence of minimal residual diseases (MRD) at the end of induction treatment in T-ALL and B-ALL respectively. As MRD remains a powerful prognostic indicator for risk stratification, the linkage between diagnostic tumour CALR and CANX mRNA expression and MRD status may provide important insights in treatment response for physicians at the time of diagnosis.

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Attribution-NonCommercial-NoDerivatives 4.0 International