The Clinical Phenotype of Patients with Essential Thrombocythemia Harboring MPL 515W>L/K Mutation.

A 515W>L/K mutation in MPL (MPL^mut) has been described in 5–10% of patients (pts) with myelofibrosis (Pikman Y, PloS Med 2006), possibly associated with JAK2617V>F allele. Among 217 subjects with primary myelofibrosis, the 18 MPL^mut pts presented a more severe anemic phenotype than MPL wild-type (MPL^WT) pts (Guglielmelli P, BJH 2007). MPL 515W>L mutation has been reported also in 4 pts with essential thrombocythemia (ET) (1%) (Pardanani A, Blood 2006). We have collected 13 MPL^mut pts in an unselected series of 273 ET pts according to WHO criteria (4.7% of total) to evaluate whether MPL mutation associated with unique clinical characteristics and eventually whether MPL^mut ET pts should be re-classified as having WHO pre-fibrotic myelofibrosis. A novel quantitative real-time PCR assay for 515W>L and 515W>K allele in granulocyte DNA has been designed; detection limit was 0.01% for W>L allele and 0.1% for W>K allele. Six pts were 515W>L and seven were 515W>K; 1 pt with W>L and 4 pts with W>K allele had only mutant allele. Mean mutant allele burden was 44(+/−25)% and 59(+/−21)% for W>L and W>K, respectively. Seven MPL^mut pts (53%) also harbored JAK2617V>F allele, as compared to 164/260 of MPL^WT (63%); they were 2/6 pts with 515W>L and 5/7 with 515W>K. Mean 617V>F allele burden was significantly lower in MPL^mut (11+/− 9%) than MPL^WT pts (24+/− 17%; P=0.03). There was no difference in age or gender, but median disease duration was longer in MPL^mut pts (110 vs 57 months, P=0.001). At diagnosis, platelet count was significantly higher in MPL^mut pts (1,113+/− 438x10⁹/L vs 864+/− 302x10⁹/L; P=0.02), while hemoglobin, serum ferritin, LDH level, and leukocyte count were not statistically different. Frequency of pts presenting endogenous erythroid colonies or PRV-1 over-expression was similar among MPL^mut (37% and 44%, respectively) or MPL^WT pts (48% and 43%). There was no difference in splenomegaly, systemic symptoms, major thrombosis (30% vs 21%) or hemorrhages (8% vs 6%) between MPL^mut and MPL^WT pts. However, pts with microvessel disease were significantly more frequent among MPL^mut (77% vs 34%, P=0.002). Bone marrow (BM) biopsy at diagnosis of MPL^mut pts was reviewed in a blinded fashion among 30 random biopsies from MPL^WT ET pts. There was no significant difference in total cellularity, erythoid or myeloid lineage beteween MPL^WT and MPL^mut pts. Megakaryocyte hyperplasia was prominent in MPL^mut pts, with Mks being either scattered or in loose clusters similarly to MPL^WT pts. However, in addition to typical large Mks, MPL^mut pts displayed a discrete number of small-size vWF and/or CD61-pos Mks. BM fibrosis quantification (revised EUMNET criteria) revealed grade 0–1 fibrosis in all but one MPL^mut pts, who presented grade 1–2. Finally, there was no evidence of leukoerythroblastosis in blood smears. One pt died after 11 yrs of major thrombosis, none evolved to myelofibrosis. Overall, these data indicate that prevalence of MPL mutation in ET may be higher (5%) than previously reported, but the mutation per se does not associate with a unique clinical phenotype, a part for a higher platelet count and greater occurrence of microvessel symptoms. Increased number of small-size Mks was observed in BM biopsy, but the whole hystologic pattern, as well as long-term stability of disease, did not support alternative diagnosis of pre-fibrotic myelofibrosis.