Objective:To investigate calreticulin (CALR) gene mutations classification in BCR-ABL1 negative myeloproliterative neoplasms (MPN), and its relationship with clinical manifestations.

Methods:Genomic DNA polymerase chain reaction (PCR) amplification product Sanger sequencing method was used to detect the mutation of exon 9 of CALR gene in 236 patients with BCR-ABL1 negative MPN (excluding polycythemia vera and negative CALR mutations) in Ruijin Hospital of Shanghai Jiao Tong University School of Medicine from November 2015 to November 2018. The mutations were classified into 52 bp deletion (type 1) mutation, 5 bp insertion (type 2) mutation and other mutation types according to PCR sequencing analysis. The clinical characteristics of the carriers with two kinds of mutations in 198 patients with essential thrombocythemia (ET) and 38 primary myelofibrosis (PMF) were compared. For the types of mutations that could not be determined, they were classified according to the α-helix propensity score of the mutant protein peptide chain or the degree of retention of the negatively charged amino acid residues, and the differences between the two classification methods were also compared.

Results:Among 236 patients, the CALR gene type 1 or type 2 mutation was detected in 206 cases (87.3%), including 173 ET patients (99 cases of type 1 mutation and 74 cases of type 2 mutation) and 33 PMF patients (28 cases of type 1 mutation and 5 cases of type 2 mutation). The CALR non-type 1 or non-type 2 mutation was detected in 30 cases, including 25 ET patients and 5 PMF patients. Among 173 ET patients with CALR gene mutation, the white blood cell count (WBC) of patients with type 1 mutation was higher than that of patients with type 2 mutation [(8.6±2.7)×10⁹/L vs. (7.6±2.4)×10⁹/L, t = 2.45, P = 0.015]. Among 33 PMF patients with CALR gene mutation, the age of patients with type 1 mutation was older than that of patients with type 2 mutation [(58±13) years old vs. (41±16) years old, t = 2.51, P = 0.018]. According to the α-helix propensity score of mutant protein peptide chain and the degree of retention of the negatively charged amino acid residues, 27 kinds of non-type 1 or non-type 2 mutations were classified by using sequencing method, and there were differences between the two methods. According to the α-helix propensity score of the mutant protein peptide chain, the proportion of type 1/type 1-like mutation in PMF patients was higher than that in ET patients [78.9% (30/38) vs. 56.6% (112/198), P < 0.01]. According to the degree of retention of negatively charged amino acid residues in the mutant protein peptide chain, the isoelectric point (pI) value of the mutant protein peptide chain was higher than that of the wild type sequence. The pI value of the type 1-like mutant protein peptide chain was higher than that of the type 2-like mutation (11.79±0.15 vs. 10.02±0.42, t = 11.51, P < 0.01).

Conclusions:Type 1 mutated ET patients may be closely related to the high risk of myelofibrosis transformation. The results of the classification of CALR mutations are different according to the α-helix propensity score of the mutant protein peptide chain and the degree of retention of the negatively charged amino acid residues. Further study is necessary to identify the pathogenesis of MPN caused by CALR mutation, and to determine the relationship between mutation type and prognosis of disease.