Homozygous calreticulin mutations in patients with myelofibrosis lead to acquired myeloperoxidase deficiency

Abstract

Key Points• Acquired MPO deficiency in patients with MPN is uniquely associated with homozygous CALR mutations.• In line with a posttranscriptional defect, MPO deficiency results from reduced MPO protein levels, but not from decreased MPO mRNA.The pathogenesis of acquired myeloperoxidase (MPO) deficiency, a rare phenomenon observed in patients with Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs), is unknown. MPO is a glycoprotein (GP) chaperoned by calreticulin (CALR) in the endoplasmic reticulum. Mutations in CALR are frequently found in patients with myelofibrosis (MF) and essential thrombocythemia (ET) with nonmutated Janus kinase 2 (JAK2). We hypothesized that acquired MPO deficiency in MPN could be associated with the presence of CALR mutations. A cohort of 317 patients with MPN (142 polycythemia vera [PV], 94 ET, and 81 MF) was screened for MPO deficiency. MPO deficiency was observed in 6/81 MF patients (7.4%), but not in PV or ET patients. Susceptibility to infections had been documented in 2/6 (33%) MPO-deficient patients. Five out of 6 patients with MPO deficiency carried a homozygous CALR mutation and were also deficient in eosinophilic peroxidase (EPX). In contrast, 1 patient with MF, a JAK2-V617F mutation, and MPO deficiency, carried 2 previously reported MPO mutations and showed normal EPX activity. Patients with homozygous CALR mutations had reduced MPO protein, but normal MPO messenger RNA (mRNA) levels supporting a posttranscriptional defect in MPO production. Finally, we demonstrate in vitro that in the absence of CALR, immature MPO protein precursors are degraded in the proteasome. Therefore, 4 decades after the first description of acquired MPO deficiency in MPN, we provide the molecular correlate associated with this phenomenon and evidence that CALR mutations can affect the biosynthesis of GPs. (Blood. 2016;127(25):3253-3259)

Genetic basis and molecular pathophysiology of classical myeloproliferative neoplasms

“…33 In contrast to JAK2V617F, the VAF for mutant CALR in ET is high, on average around 40%. 10 In rare cases of ET, the VAF can be lower, but almost never below 15%.…”

Progenitor genotyping reveals a complex clonal architecture in a subset of CALR‐mutated myeloproliferative neoplasms

“…Mutant CALR allele frequencies that are suggestive of homozygosity are rare in MPN patients; only three examples of apparent homozygosity were detected in 286 CALR ‐mutated cases (from a cohort of 1215 patients with ET or MF) (Klampfl et al , ). Most mutation‐homozygous individuals have primary or post‐thrombocythaemic MF (Rumi et al , ; Theocharides et al , ); the case reported here (Patient B), and one other (Klampfl et al , ), were diagnosed with ET. Mutation homozygosity in Patient B was not the result of an independent mutation event within her remaining wildtype CALR allele, but rather by mitotic recombination that produced a region of uniparental disomy (UPD) within chromosome 19 that included the CALR locus (Fig B).…”

“…This event would have preceded diagnosis, as most of the clonogenic progenitors assessed at that time were already mutation‐homozygous (Fig A). The clinical consequences of CALR mutation homozygosity remain unclear, although a subgroup of MF patients (6 of 81), most of whom were CALR mutation‐homozygous, were shown to have acquired myeloperoxidase (MPO) deficiency (Theocharides et al , ). However, this disorder was absent in 94 patients with ET, so it is unlikely to be caused by CALR mutation‐homozygous alone.…”

JAGN1 is required for fungal killing in neutrophil extracellular traps: Implications for severe congenital neutropenia

“…The proMPO protein is then exported to the Golgi whereupon further processing takes place prior to the storage of MPO in azurophilic granules . Recent studies have revealed that homozygous mutations in the calreticulin gene ( CALR ) lead to acquired MPO deficiency; these individuals have reduced MPO protein, but normal MPO mRNA levels, supporting a posttranscriptional defect in MPO production . Further studies are required to understand how JAGN1 regulates MPO expression in neutrophils, but our findings do not support a role of JAGN1 in modulating N ‐glycosylation of MPO.…”