Myeloproliferative neoplasms (MPN) are a heterogeneous group of hematopoietic stem cell-derived disorders, including polycythemia vera (PV), essential thrombocytemia (ET) and primary myelofibrosis (PMF), spontaneously evolving in acute myeloid leukemia. The genetic basis of these diseases are somatic driver mutations, occurring on Janus kinase 2 (JAK2), calreticulin (CALR) and myeloproliferative leukemia (MPL) genes, all deregulating the JAK2/STAT signaling pathway. Whereas JAK2 and MPL mutations are related to essential pathways of myelopoiesis, CALR mutations affect a multifunctional protein, apparently not connected with hematopoiesis.
CALR mutations occur in ET or PMF subjects, but are rare or non-existent in PV. They consist of a wide-variety of deletions and insertions in exon 9, all resulting in a 1-bp frameshift, which removes the ER retrieval motif KDEL and converts the first 31 bases of CALR 3¿UTR into coding sequence. We recently detected a novel exon 9 deletion in an evolutionarily conserved CALR 3¿UTR region, designated c.1254+10_+33del24, in two siblings diagnosed with JAK2V617F-negative PV. This mutation occurred 10 bp downstream the CALR stop codon, thus not altering the coding sequence, but leading to overexpression of CALR and to erythrocytosis.
The scope of this project is to elucidate the role of CALR 3¿UTR sequence in normal myelopoiesis and MPNs, inspecting regulation orchestrated by miRNA and complex regulative functions of 3¿UTR, such as mRNA or protein localization and translation efficiency. As additional degree of complexity, RNA editing of CALR mRNA is studied in relationship with 3¿UTR functions, since it can affect them modifying RNA complementarity without changing genomic sequence. By exploring these unknown aspects of MPNs in primary samples from normal donors and MPN patients, we may shed light on new mechanisms of hematopoiesis, which are deregulated in MPNs, revealing useful cues for the design of novel targeted treatments.
Research on hematological disorders has served as a pioneer for all branches of cancer research. Advances obtained in these diseases, such as chemotherapy, transplantation and targeted therapies were extended to other malignancies. MPNs reflect an early stage of tumorigenesis, which is inaccessible in most cancers. In addition to phenotypic mimicry, each type of myeloproliferative neoplasm is capable of evolving into another type, making diagnosis, risk assessment, and therapeutic choices difficult. Indeed, myeloproliferation masks a clone of transformed hematopoietic stem cells capable of expansion and transformation to an aggressive form of bone marrow failure or acute leukemia[1].
Understanding the involvement of CALR and its 3'UTR region in physiological hematopoiesis and MPNs could be helpful in deciphering the mechanisms linked to onset of MPNs and their progression to acute myeloid leukemia.
Identifying a role for mRNA editing in the context of MPNs could contribute to depict a novel scenario, in which post-transcriptional modifications of mRNA and their possible variants due to genetic alterations could orchestrate a different regulation of gene expression in MPNs. Particularly the alteration of 3'UTR sequences due to frameshift mutations could explain aberrant regulative state involving new RBPs that could became candidate for target therapy; on the other hand, anomalies of mRNA editing in MPNs could influences the recognition of consensus sequences for RBPs.
Studying the mutations of our patients¿ cohort in the context of mRNA editing may clarify further aspects of the physiologic role of CALR in hematopoiesis and the intricate pathogenic mechanisms of MPNs, revealing a higher regulation level that could determine the phenotypic differences between PV, ET and PMF and disease progression. This complex network of regulation, which may act at physiological state, could be the hidden part of the iceberg, determining the differences between diverse clinics, although operating on the same genetic lesion. Moreover, this new area of research has immense potential with regard to targeted pharmacological interventions.