Detection of Three Different MPLW515 Mutations in 10.1% of All JAK2 V617 Unmutated ET and 9.3% of All JAK2 V617F Unmutated OMF: A Study of 387 Patients.

Recently MPLW515 mutations have been reported in osteomyelofibrosis (OMF) and essential thrombocythemia (ET). To further evaluate this marker for routine diagnostics of CMPD we have analyzed 387 patients (pts) by a melting curve assay and subsequent sequencing of pts with aberrant curves. 97 pts had a diagnosis of ET (79 with unmutated JAK2V617 (V617wt) and 18 with JAK2V617F), 164 had V617wt undefined CMPD with thrombocytosis, 101 had OMF (54 with V617wt and 47 with V617F) and 25 had V617wt/exon12wt polycythemia vera (PV). Overall we detected 3 different mutations in 16 pts: W515L (n=10), W515K (n=5) and a so far undescribed W515S mutation in 1 pt. In 8/97 ET MPLW515 mutations were detected. All 8 pts were V617wt. Thus, the frequency of W515 mutations in V617wt ET was 8/79 (10.1%). In addition, 3/164 (1.8%) CMPD with thrombocytosis were mutated (subsequently classified ET). In OMF MPLW515 mutations were detected in 5 of 54 V617wt pts (9.3%). No MPLW515mut was found in 18 ET and 47 OMF with JAK2V617F and in the 25 V617wt/exon12wt polycythemia vera (PV). In 4 pts the mutation was homozygous (3 W515K, 1 W515L) and all 4 pts were at advanced stage of their disease: 3 OMF 2-6 years (y) after initial diagnosis and 1 ET in transformation 9 y after diagnosis. Mutation/wt ratio in the remaining pts were 10–100% (median: 40%) in ET and 40% in OMF. The female/male ratio in the mutated cases was 9/7. The age was between 30 and 80 y (median 64 y) and thus was in the same range as in V617F mutated ET/OMF (median 65.3 y; n=489) but significantly higher than in V617wt ET/OMF (median 55.6 y; n=410; p<0.01). Median peripheral blood counts of the W515 mutated cases were: WBC 6,700/μl, Hb 120 g/L, platelets 755,000/μl in ET and WBC 5,200/μl, Hb 80 g/L, platelets 175,000/μl in OMF. These values are similar to those in the JAK2V617F control group (WBC 9,150/μl, Hb 142 g/L, platelets 719,000/μl in ET and and WBC 11,400/μl, Hb 105 g/L, platelets 195,000/μl in OMF. All pts with available data (not all parameters available for all cases) showed dysplastic and enhanced megakaryopoiesis and elevated erythrocyte count. 50% had spenomegaly and mild hepatomegaly. Cardiac problems were described in 4 pts. Karyotype was available in 8 pts and all had normal karyotypes. In particular, it has to be mentioned that even in 1p34, where the MPL gene resides, no aberration was observed. This has not been anticipated as in JAK2 mutated cases there is a high correlation to trisomy 9 or aberration of 9p (where the JAK2 gene resides). In conclusion, MPLW515 mutated ET and OMF can not be discriminated from JAK2V617F mutated ET or OMF based on clinical parameters. However, MPLW515 may help to support the diagnosis of pts being suspicious for ET or OMF and showing no JAK2V617F mutation. In addition, during follow up homozygousity can indicate progression of disease.