Phenotypic correlations of CALR mutation variant allele frequency in patients with myelofibrosis
Paola Guglielmelli, Chiara Maccari, Benedetta Sordi, Manjola Balliu, Alessandro Atanasio, Carmela Mannarelli, Giulio Capecchi, Ilaria Sestini, Giacomo Coltro, Giuseppe Gaetano Loscocco, Giada Rotunno, Eva Angori, Filippo C. Borri, Ayalew Tefferi, Alessandro M. Vannucchi
Abstract
Somatic mutations in calreticulin ( CALR ), encoding for the reticulum-endoplasmic-associated Ca 2+ -binding chaperone protein calreticulin, located at chromosome 19p13.13, represent the second most frequent driver mutation in myeloproliferative neoplasms (MPN) [ 1 , 2 ]. CALR mutation is harbored by 20–30% of patients with essential thrombocythemia (ET) and 25–35% of prefibrotic primary myelofibrosis (pre-PMF) and overt PMF [ 3 ]. There are two main types of CALR mutation: Type 1, a del52 bp deletion, and similar abnormalities (defined as Type 1-like based on similar structural characteristics), and Type 2, a 5 bp insertion, and Type 2-like mutations, all located in exon 9 and causing a reading frameshift. Most patients have CALR variant allele frequency (VAF) below 50% (conventionally defined as heterozygous), as opposite to a few with higher VAF (above 50%, homozygous). Homozygosity, that may be associated with uniparental disomy (UPD)/copy neutral loss-of-heterozigosity (CN-LOH) at chr19 [ 4 ], indicates the presence of cells with UPD/CN-LOH in a variable context of heterozygous cells. CALR homozygosity is hallmarked by myeloperoxidase (MPO) deficiency in myeloid cells due to abnormal proteosomal degradation of immature protein [ 5 ]. In patients with JAK2 V617F mutation, phenotypic and prognostic correlates of higher vs. lower VAF were reported across the spectrum of MPN [ 6 ], and variably included correlation of higher VAF with higher red blood cell values and neutrophil counts, lower platelet counts, larger spleen, pruritus, venous thrombosis [ 7 ], and transformation to secondary forms of MF [ 8 ]. Remarkably, JAK2 V617F VAF in the lowest quartile was associated with shorter overall survival (OS) and leukemia-free survival (LFS) in patients with PMF [ 9 , 10 ]. Conversely, little information is available regarding the phenotypic and/or prognostic impact of CALR VAF. The aim of this study was to characterize the hematological and clinical correlates of CALR VAF in patients with PMF and post-essential thrombocythemia myelofibrosis (PET-MF).