Calreticulin-mediated protein folding in health and disease

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Abstract Calreticulin (CRT) is a calcium-binding endoplasmic reticulum (ER) chaperone that is important for the folding of many N-linked glycoproteins. It is best known for its function in the assembly of major histocompatibility complex (MHC) class I (MHC-I) molecules, which present peptide antigens to CD8+ T cells for protective immunity against cancers and infections. Recent studies show that somatic frameshift mutations that alter the C-terminus of CRT are one class of driver mutations in myeloproliferative neoplasms (MPN). A 52 base-pair deletion mutant CRTDel52 and a 5 base-pair insertion mutant CRTIns5 are recurrent mutations in essential thrombocythemia (ET), primary myelofibrosis (PMF) and post-ET myelofibrosis (MF). It is our overall hypothesis that the changes to the sequence and charge of the C-terminal domains of CRT induced by the mutations enable both megakaryocyte transformation and immune system evasion in MPN. Although oncogenic transformation in megakaryocytes requires both mutated CRT and the thrombopoietin receptor/myeloproliferative leukemia protein (Mpl), the molecular mechanisms of mutant CRT-mediated constitutive activation of Mpl are unknown. CRT is known to be important for ER calcium storage and cellular calcium signaling, but precisely how MPN mutations alter these processes is not understood. The human MHC-I locus is highly polymorphic. While it is known that MPN-linked mutant CRTs are ineffectively incorporated into the MHC-I peptide-loading complex (PLC), it is unknown whether MPN CRT mutants have differential influences on the assembly of various MHC-I allotypes. Studies are proposed here to address these gaps in knowledge. We present evidence that disulfide-linked dimers of CRTDel52 are important for Mpl activation. The nature of CRTDel52 dimers and their complexes with Mpl will be elucidated using structural and mutagenesis studies. The effects of CRT deficiency, haploinsufficiency and mutation upon cellular calcium signaling will be examined. Based on the knowledge of variable dependencies on CRT for cell-surface expression of MHC-I allotypes, we will examine whether MPN CRT mutants impair MHC class I assembly for some allotypes, but alter assembly and peptide display for some other MHC-I allotypes. Such alterations could be exploited for immunotherapeutic targeting of MPN. Finally, mutant CRT expression is also detectable in blood monocytes of MPN patients, leading to predictions of alterations to antigen presenting cell (APC) calcium signaling and phagocytosis, which will be examined. Taken together, these studies will further our understanding of how mutations of a ubiquitous ER chaperone drive cell transformation, and influence the fundamental immunological processes of antigen presentation and phagocytosis.