Our insight into the molecular basis of myeloproliferative neoplasms (MPN) took a landmark stride in 2005 with the identification of JAK2 mutations in nearly all polycythemia vera and the majority of essential thrombocythemia (ET) and primary myelofibrosis (PMF) cases1,2,3. The subsequent identification of novel somatic activating mutations in myeloproliferative leukemia virus oncogene (MPL) in 2006 provided an additional mechanistic explanation for JAK–STAT activation in MPN4. Differential distribution of MPL mutations according to MPN subtype has since been discerned: MPL-mutated ET is uncommon, with a variable but typically cited incidence of less than 5% (5–9), while that of MPL-mutated PMF is at least twice as frequent5. Phenotypically, older age and lower hemoglobin levels have been remarked in MPL-mutated vs unmutated ET cohorts6,7. Further, MPL-mutated ET cohorts have higher reported rates of fibrotic progression than their MPL wild-type counterparts: in the order of 33.3% (vs 7.5% in MPL-unmutated) in one series8,9. Together, these observations suggest the possibility that some instances of MPL-mutated ET might actually represent prefibrotic PMF. This distinction is not insignificant as prognosis and management may vary correspondingly10. The current study was designed as a histopathological reappraisal and retrospective assessment of phenotypic and prognostic correlates in consecutive cases designated as MPL-mutated ET.

After board approval, patients were recruited from the institutional databases of the Mayo Clinic, Rochester, MN, USA. Study inclusion criteria required the availability of bone marrow biopsy slides obtained at diagnosis or within 1 year of diagnosis for central review by one of the authors (C.A.H.), an experienced hematopathologist. Central pathology review included assessment of bone marrow cellularity and extent of trilineage proliferation, megakaryocyte morphology, and grading of reticulin fibrosis. The review process was completely blinded to clinical, laboratory, and outcomes data. All diagnoses were in accordance with the 2016 World Health Organization (WHO) criteria11; the diagnosis of prefibrotic PMF specifically was based on strict morphological criteria as the stipulation for blinded review precluded access to clinical data. Screening for driver mutation status was performed using conventional methods. Data abstracted corresponded to the time of diagnosis, or within 1 year, and included MPL mutation type, peripheral blood smear parameters, karyotype, and presence of additional non-driver mutations based on availability of next-generation sequencing-derived information. Corresponding data were collected from MPL-mutated patients with PMF for purpose of comparison. Risk stratification was consistent with conventional prognostic models: international prognostic score for ET (IPSET)12 and dynamic international prognostic scoring system for PMF (DIPSS-plus)13. Overall survival (OS) was defined by the time from date of referral to date of death (uncensored) or last contact (censored); myelofibrosis-free survival (MFFS) considered myelofibrotic transformation as the uncensored variable. Differences in the distribution of continuous variables between categories were compared using the Mann–Whitney or Kruskal–Wallis test. Categorical variables were compared using the χ2 test. Survival and time-to-event curves were prepared using the Kaplan–Meier method and compared by the log-rank test. P-values <0.05 were considered significant. The JMP® Pro 13.0.0 software package was used for all analyses (SAS Institute, Cary, NC, USA).

A total of 665 patients with ET were annotated for their driver mutational status; 18 (2.7%) were reported out as being MPL-mutated; by comparison, among 867 patients with PMF, 47 (5.4%) were signed out as MPL-mutated. Among the 18 cases with MPL-mutated ET, bone marrow sides were available for central pathology review in 14 patients; all but 4 of these were treatment-naïve with the latter on treatment with hydroxyurea (n = 3) or anagrelide (n = 1) at the time of referral. Informative cases were subsequently reassigned the diagnosis of either prefibrotic PMF (n = 8; 57%) or were felt to be morphologically consistent with true WHO-defined ET (n = 6; 43%); exposure to therapeutic agents at the time of bone marrow sampling was balanced between them (n = 2 each). Comparison of these two distinct histopathological patterns, i.e. true ET vs reassigned prefibrotic PMF, was respectively characterized by lower (median 35%, range 30–50) vs higher (median 65%, range 40–80) bone marrow cellularity (P < 0.001), ET (n = 6) vs PMF (n = 8) consistent megakaryocyte morphology (P < 0.001) and presence of trilineage proliferation (0% vs 100%; P < 0.001); in contrast, the degree of reticulin fibrosis was similar between the two (P = 0.1) (Supplemental Table 1). The reassigned prefibrotic PMF (n = 8), vs confirmed ET (n = 6), cases presented platelet counts consistently <1000 × 109/l (100% vs 67%; P = 0.06), a higher frequency of increased serum levels of lactate dehydrogenase (LDH) (60% vs 0%; P = 0.02), higher likelihood of having hemoglobin levels below the sex-adjusted reference range values (29% vs 0%; P = 0.1), leukoerythroblastosis (14% vs 0%; P = 0.2), constitutional symptoms (13% vs 0%; P = 0.2), and a higher incidence of thrombosis history at presentation (38% vs 0%; P = 0.04) (Table 1). Interestingly, reassigned prefibrotic PMF also displayed a narrower MPL mutational spectrum compared to those confirmed as ET (MPLW515L/K incidence 100% vs 60%; P = 0.04). The incidences of abnormal karyotype and high molecular risk mutations (ASXL1, SRSF2, and U2AF1) were similarly low between the two groups (Table 1). Over a median follow-up of 8 (reassigned prefibrotic PMF) and 10 years (true MPL-mutated ET), we documented a higher incidence of thrombosis after diagnosis in prefibrotic PMF (25% vs 0%; P = 0.1) but similar rates of leukemic transformation (0% for both) and fibrotic progression (38% vs 33%; P = 0.87).

Table 1 Comparison of presenting features and outcomes in three MPL-mutated cohorts: MPL-mutated essential thrombocythemia having undergone central pathology review and either confirmed as essential thrombocythemia (n = 6) or re-classified as primary myelofibrosis (n = 8) and MPL-mutated primary myelofibrosis (n = 54)
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When all 665 ET patients were assessed for myelofibrosis-free survival, MPL-mutated cases (prior to central review) displayed significantly worse outcome compared to patients with other driver mutations with a myelofibrosis transformation rate of 33% compared to 11%, 15%, and 21% in triple negative, JAK2, and CALR-mutated cohorts, respectively (P = 0.04; Fig. 1a). Median overall survival rates in confirmed MPL-mutated ET, ET re-classified as PMF, and MPL-mutated PMF were not reached, 11.6 and 5.3 years, respectively (confirmed ET vs PMF, P = 0.01; ET re-classified vs PMF, P = 0.04; confirmed ET vs ET re-classified as PMF, P = 0.54) (Fig. 1b).

Fig. 1
figure 1

1a Myelofibrosis-free survival data in 665 patients with essential thrombocythemia stratified by driver mutational status. 1b Survival data in 14 centrally re-reviewed patients with MPL-mutated essential thrombocythemia; morphologically confirmed vs re-classified as primary myelofibrosis vs MPL-mutated primary myelofibrosis

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The current study suggests that the majority of routinely assigned cases of MPL-mutated ET probably represent prefibrotic PMF when morphologically scrutinized. We fully acknowledge the limitations inherent to this report including its retrospective nature and the limited number of informative cases, and our data require further validation. These conditions notwithstanding, we have documented, even prior to central pathology review, a significantly higher rate of fibrotic progression in MPL-mutated ET compared to patients with other driver mutations. While not all reports are consistent in this regard14, our data remain aligned with the majority of large scale, mature studies on the subject8,9. After central pathology review, the similar rates of fibrotic progression between morphologically confirmed MPL-mutated ET and those reassigned as prefibrotic PMF further suggest the latter to be biologically more akin to PMF. Correspondingly, when clinical correlates were considered in concert with morphologic assessment, patients re-classified as MPL-mutated prefibrotic PMF presented more frequent features consistent with their morphological re-allocation including presence of hemoglobin below sex-adjusted norms and LDH concentrations above reference range, all of which were consistently and conspicuously absent in those conserving their true MPL-mutated ET designation. Although rates of leukemic transformation and overall survival estimates did not differ substantially between the two groups, previous data have disclosed a markedly relevant influence of accurate morphological diagnosis on survival in ET10 and we believe this distinction remains an important one. Consequently, the exceeding infrequency of true MPL-mutated ET should at the very least confront clinicians with the possibility that some, if not most, of these cases correspond to prefibrotic PMF and prompt closer consideration and diagnostic revision when warranted.