Clinical and Haematological Features of Genetic Mutations in MPN Patients with Diagnosis of Prefibrotic Idiopathic Myelofibrosis (MF0)

Background: Diagnosis of a prefibrotic state (MF0) presents histological diagnostic difficulties. MF0 has a worst prognostic impact than Essential Thrombocythemia (TE) as regard the thrombotic risk and a higher risk towards Idiopathic Myelofibrosis (MI) and acute leukemia evolution. For this reason it is very useful to identify this group of patients.

Aims: The aim of this study was to evaluate clinical and hematological impact of the mutational status in patients with an MF0 diagnosis.

Methods: A retrospective chart review was performed from January 2010 to July 2015 in a single center in 317 patients affected by Ph negative chronic myeloproliferative neoplasms. Polycythemia vera cases were excluded. Thirty-three patients have been classified as MF0 on the base of the bone marrow histological examination. Onset features include cytogenetics, blood counts, peripheral CD34-positive cells, spleen and liver size, thrombotic events (prior to and post-diagnosis) and thrombotic risk. Fragment analysis was performed to study exon 9 CALR mutation, Real-time PCR was applied for exon 14 JAK2 V617F mutation and Sanger sequencing was used to identify exon 10 MPL mutations for 505 and 515 codons. According to genetic results, patients were identified into four groups according to a positivity for JAK2, MPL, CALR and for triple negativity.

Results: The 33 patients with MF0 were: 11 (33%) JAK2 positive, 10 (30%) CALR positive, (10) 30% triple negative and 2 (7%) MPL positive. The latter group was not further considered for analysis due to the low number of cases. Eighty percent of CALR positive patients had a deletion on the exon 9 of the gene (8 del52bp and 2 del46bp), while 20% had the type 2 mutation (ins5bp). The average of JAK2 allelic burden was 20%. The 3 groups of patients were comparable for age, white blood counts and hemoglobin values. There was a female prevalence (23 vs 10 males). Platelet count was higher (median 875.500 10³ µl, interquartile range 303.000 10³ µl , p = 0.008) in CALR positive patients compared to JAK2 positive (median 569.000 10³ µl, interquartile range 433.000 10³ µl) and to triple negative patients (median PLT count 629.500 10³ µl, interquartile range 378.250 10³ µl). Noteworthy, bone marrow cytogenetic exam was normal in all patients. JAK2 positive patients had a larger spleen compared to the other two groups. Pre-diagnosis thrombotic events were exclusive for JAK2 positive patients and absent in the other groups. Triple negative patients do not have a negative prognostic impact for the thrombotic risk.

Conclusions: CALR deletion could be considered as an MF0 marker and should be included in diagnostic work-up. JAK2 positivity in MF0 is associated with a high thrombotic risk.