Prefibrotic Myelofibrosis (PreMF) Belongs to a Continuum of Epidemiological, Clinical and Histological Characteristics Featuring Primary Myelofibrosis (PMF)

WHO diagnostic criteria of myeloproliferative neoplasms (MPNs) identify a preMF variant including patients having absence or early grade fibrosis, dual myeloid and megakaryocytic dominance, and clusters of atypical, ectopic, variable in size megakaryocytes in the bone marrow (BM). Many issues on the variant are critical, such as its distinction from essential thrombocythemia, recognisability, biological identity and true place among MPNs. Here we describe the clinical presentation of patients with preMF collected among a large cohort of patients diagnosed with PMF according WHO and for whom long-term follow-up data were available. The aim is to investigate whether there is an underlying structure of the patients' characteristics that allows to assign them to a distinct variant of MPNs or to align them along a continuum in the realm of PMF.

We included in this study patients with PMF who were referred to the Center for the Study of Myelofibrosis of IRCCS Policlinico S. Matteo Foundation, Italy from 1990 to 2011. In all patients the BM biopsy taken at diagnosis was reviewed (pathologist: UM) and the diagnosis of preMF was established adopting the stringent criterion that, besides typical BM cellularity and megakaryocyte morphology, BM had less than grade 1 reticulin fibrosis (EUMNET criteria).

Of the 659 patients with PMF, 126 fulfilled the criteria we established for the diagnosis of preMF, thus accounting for 19.1% of the cohort. Female sex accounted for 59.5% of the preMF cohort while it accounted for 33.8% of PMF fibrotic type (P<0.0000). The mean age at diagnosis of patients with preMF was 39.2 years (range, 6 to 78 years), while it was 54.6, range 6 to 90 years (P<0.0000) in patients with PMF fibrosis type. Patients with preMF showed higher hemoglobin (14.0 vs. 11.9 g/dL; P=0.0004), higher platelet count (647 vs. 435 × 10⁹/L; P<0.0000), lower WBC count (9.0 vs. 10.7 x10⁹/L; P=0.02) and smaller spleen index (120 vs. 182; P<0.0000) than patients with MF fibrotic type. As a consequence, preMF displayed a greater frequency of patients with grade 0 IWG prognostic score ( 88.7% vs. 56.9%, P<0.0000). PreMF patients displayed a higher frequency of thrombotic events at the time of the diagnosis or in the year preceding the diagnosis (30.1% vs. 6.4%; P<0.0000). Both in prefibrotic and fibrotic type MF, the great majority of the thrombotic episodes were portal vein thrombosis or Budd-Chiari syndrome (89.4% in preMF and 79.4% in PMF fibrotic type). The proportion of JAK2 V617F mutated patients was not significantly different in the two categories of patients (65.2% vs. 62.1%; P=NS).

In order to assign patients with PMF to groups with different degrees of BM fibrosis, we further categorized MF fibrotic type into early MF (BM fibrosis grade 1) and advanced MF (BM fibrosis grade 2 or 3). Female frequency steadily varied from 59.5% to 43.2% and 25.6% in the 3 categories of BM fibrosis, respectively. Age varied from 39.2, 50.4, 57.0 years, respectively. Hemoglobin and platelet count steadily decreased (Hb=14.0, 13.3, 11.2 g/dL, respectively; platelet count=647, 554, 346 x10⁹/L, respectively). Spleen size steadily increased along with increasing of BM fibrosis (spleen index=120, 152, 200). All these parameters exhibited a statistically significant trend along the continuum of BM fibrosis grade (ANOVA, P<0.0001). During 993 cumulative person-years of follow-up, 4% of patients with preMF died. During 1372 cumulative person-years of follow-up, 15.4% of patients with early MF died. During 1446 cumulative person-years of follow-up, 24.5% of patients with advanced MF died. The cause of death was blast transformation in 80, 53 and 41.7% of cases, respectively. 94%, 80% and 54% of patients with preMF, early MF and advanced MF were alive at 10 years from the diagnosis. Overall, these results speak in favour to preMF belonging to a continuum of epidemiological, clinical and histological phenotypes in the realm of PMF. Splanchnic vein thrombosis, on the contrary, clusters in the category of preMF, suggesting that young age, female sex and a very indolent hematological phenotype are associated with biological characteristics that predispose to thrombosis.

The BM picture of preMF identifies a group of patients with PMF associated with a very indolent hematologic phenotype, female dominance and very young age at presentation characterized by high incidence of splanchnic vein thrombosis.

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