Approximately 20% of patients with myeloproliferative neoplasms (MPN) harbor mutations in the gene calreticulin (CALR), with 80% of those mutations classified as either type I or type II. While type II CALR-mutant proteins retain many of the Ca2+ binding sites present in the wild-type protein, type I CALR-mutant proteins lose these residues. The functional consequences of this differential loss of Ca2+ binding sites remain unexplored. Here, we show that the loss of Ca2+ binding residues in the type I mutant CALR protein directly impairs its Ca2+ binding ability, which in turn leads to depleted endoplasmic reticulum (ER) Ca2+ and subsequent activation of the IRE1α/XBP1 pathway of the unfolded protein response. Genetic or pharmacologic inhibition of IRE1α/XBP1 signaling induces cell death in type I mutant but not type II mutant or wild-type CALR-expressing cells, and abrogates type I mutant CALR-driven MPN disease progression in vivo. Significance: Current targeted therapies for CALR-mutated MPNs are not curative and fail to differentiate between type I- versus type II-driven disease. To improve treatment strategies, it is critical to identify CALR mutation type–specific vulnerabilities. Here we show that IRE1α/XBP1 represents a unique, targetable dependency specific to type I CALR-mutated MPNs. This article is highlighted in the In This Issue feature, p. 265

This manuscript of research highlight focused on one paper recently published in Nature Metabolism entitled “Mitochondrial Long Non‐coding RNA GAS5 Tunes TCA Metabolism in Response to Nutrient Stress” from Lin Aifu's group in Zhejiang University. In this manuscript, we discussed the novel findings in Lin's paper and concluded that the metabolon is emerging as a novel cellular structure that regulates specific metabolic pathways.

Calcium homeostasis is critical for cell proliferation, and emerging evidence shows that cancer cells exhibit altered calcium signals to fulfill their need for proliferation. However, it remains unclear whether there are oncogene-specific calcium homeostasis regulations that can expose novel therapeutic targets. Here, from RNAi screen, we report that adenosylhomocysteinase like protein 1 (AHCYL1), a suppressor of the endoplasmic reticulum (ER) calcium channel protein inositol trisphosphate receptor (IP3R), is selectively upregulated and critical for cell proliferation and tumor growth potential of human NRAS-mutated melanoma, but not for melanoma expressing BRAF V600E. Mechanistically, AHCYL1 deficiency results in decreased ER calcium levels, activates the unfolded protein response (UPR), and triggers downstream apoptosis. In addition, we show that AHCYL1 transcription is regulated by activating transcription factor 2 (ATF2) in NRAS-mutated melanoma. Our work provides evidence for oncogene-specific calcium regulations and suggests AHCYL1 as a novel therapeutic target for RAS mutant-expressing human cancers, including melanoma. Implications: Our findings suggest that targeting the AHCYL1–IP3R axis presents a novel therapeutic approach for NRAS-mutated melanomas, with potential applicability to all cancers harboring RAS mutations, such as KRAS-mutated human colorectal cancers.

Most calreticulin (CALR) mutations in myeloproliferative neoplasms are classified as either type 1, a 52–base pair deletion (CALRdel52); or type 2, a 5–base pair insertion (CALRins5). Both are gain-of-function (GOF) mutations that generate an identical mutant C-terminal tail, which mediates the binding to, and activation of, the thrombopoietin receptor myeloproliferative leukemia protein (MPL). We recently reported that despite this shared GOF, CALRdel52 but not CALRins5 mutations cause loss of calcium binding function, leading to activation of, and dependency on, the inositol-requiring enzyme 1/X-box binding protein 1 pathway of the unfolded protein response (UPR). This led us to ask whether CALRins5 mutations activate and depend on a different UPR pathway, and whether this is likewise mediated by a mutation type–specific loss-of-function (LOF). Here, we show that CALRins5 mutations lead to activation of the activating transcription factor 6 (ATF6) pathway of the UPR due to loss of CALR chaperone function. This LOF is caused by interference of the CALRins5 mutant C terminus with key chaperone residue H170. Furthermore, we show that CALRins5 cells are partially dependent on ATF6 for cytokine-independent growth, and identify B-cell lymphoma extra large as a transcriptional target of ATF6 that promotes type 2 CALR-mutant cell survival.

Mutations in calreticulin (CALR) represent the second most common oncogenic driver of myeloproliferative neoplasms. CALR mutations result in the introduction of a neomorphic C-terminal tail that is absent from the normal proteome, which has positioned it as a compelling target for immunotherapeutic intervention. Early studies have demonstrated mutant CALR can elicit T-cell responses, laying the foundation for efforts to exploit this vulnerability through vaccines, antibody-based strategies, and T-cell–based therapies. Here, we summarize the current understanding of normal and mutant CALR biology, discuss progress in developing CALR-directed immunotherapies, and highlight the challenges and opportunities for translating these approaches into the clinic.