Novel molecular mechanism of cellular transformation by a mutant molecular chaperone in myeloproliferative neoplasms

Abstract

Deregulation of the cytokine-receptor signaling pathway plays a significant role in tumorigenesis. Such deregulation is frequently caused by alterations in the genes involved in the signaling pathway. At the end of 2013, recurrent somatic mutations in the calreticulin (CALR) gene that encodes a molecular chaperone were identified in a subset of patients with Philadelphia-chromosome negative myeloproliferative neoplasms (MPN). The present review focuses on the role of CALR mutations in the oncogenic transformations observed in MPN. All the CALR mutations were found to generate a + 1 frameshift in the reading frame on exon 9, which encodes the carboxy (C)-terminus end of CALR, and thus conferred a common mutant-specific sequence in all the CALR mutants. The mutant CALR (but not the wild-type) constitutively activates the thrombopoietin (TPO) receptor, myeloproliferative leukemia protein (MPL), even in the absence of TPO to induce cellular transformation. Preferential interaction between the mutant CALR and MPL is achieved by a presumptive conformational change induced by the mutant-specific C-terminus domain, which allows N-domain binding to MPL. Even though mutant CALR is expressed on the cell surface and is secreted out of cells, it only presents autocrine capacity for MPL activation. These findings define a novel molecular mechanism by which the mutant molecular chaperone constitutively activates the cytokine receptor to induce cellular transformation.

Keywords: Autocrine; JAK2; calreticulin; myeloproliferative neoplasm; thrombopoietin receptor.

Figure 1

(a) Mutant and wild‐type calreticulin (CALR). Mutations found in patients with myeloproliferative neoplasms are either deletions or insertions in a specific region in exon 9 of the CALR gene. These mutations cause a + 1 frameshift in the reading frame, which results in all mutant CALR proteins possessing the same amino acid sequence at the carboxyl (C)‐terminal. Amino acid sequences of the C‐terminus of wild‐type CALR and the two most important mutants, CALR del52 (type 1) and ins5 (type 2). Arrowheads indicate the boundary between the amino acid sequences unaffected and affected by the frameshift mutation. (b) Domain structures of wild‐type (CALR ^wt) and mutant CALR (CALR ^mut). CALR proteins consist of the following domains: a signal peptide (SP), amino‐terminal N‐domain (N), proline rich P‐domain (P), carboxy‐terminal C‐domain (C) that includes an endoplasmic reticulum retention signal protein, KDEL, in the wild type. Model for the preferential myeloproliferative leukemia protein (MPL)‐binding of mutant CALR is presented: the P‐domain blocks binding of the N‐domain to MPL in wild‐type CALR, whereas in mutant CALR, this capacity of the P‐domain is blocked by the mutant‐specific C‐terminus domain.

Figure 2

Model for the constitutive activation of the thrombopoietin (TPO) receptor, MPL, by mutant calreticulin (CALR) in myeloproliferative neoplasm (MPN) cells harboring the CALR mutation. In normal hematopoiesis (right), the downstream activation of myeloproliferative leukemia protein (MPL) is regulated by the concentration of TPO to control hematopoiesis. In CALR‐mutant cells (left), mutant CALR constitutively activates the downstream molecules of MPL and induces oncogenic transformation in a MPL‐dependent manner. Activation of MPL by mutant CALR may not occur on the cell surface (see text).

Figure 3

Cell‐autonomous activation of myeloproliferative leukemia protein (MPL) by mutant calreticulin (CALR). Although mutant CALR is expressed on the cell surface and is secreted out of cells, it does not activate the MPL expressed in normal cells that do not express mutant CALR.

Figure 4

Location of mutant calreticulin (CALR)‐dependent activation of myeloproliferative leukemia protein (MPL) in the cell. Mutant CALR and MPL are likely to be engaged in the endoplasmic reticulum (ER), where wild‐type CALR functions as a molecular chaperone to properly fold glycosylated proteins. Although premature activation of the downstream molecules of MPL before CALR reaches the cell surface has been proposed, the location of MPL activation by mutant CALR in the cell has, so far, not been determined.