Masked polycythaemia vera is genetically intermediate between JAK2V617F mutated essential thrombocythaemia and overt polycythaemia vera

Polycythaemia vera (PV) can be defined as a myeloproliferative neoplasm (MPN) with erythrocytosis and JAK2V617F (or an equivalent marker of clonality). While these simple criteria are sufficient for most cases of overt PV, an estimated 10–15% of PV patients do not have an elevated haemoglobin concentration (Hb) and may mimic JAK2-positive essential thrombocythaemia (ET). This poses a problem for accurate diagnosis and may also have clinical significance, since patients with ‘masked’ PV (mPV) might receive less intense treatment leading to shortened survival compared with overt PV. Several studies have focussed on alternative criteria for erythrocytosis to improve diagnostic accuracy. These data have led to the adoption of haematocrit (Hct) thresholds and lower Hb thresholds in the recent revision of the World Health Organization (WHO) criteria. Both the British Committee for Standards in Haematology (BCSH) and the 2008 WHO incorporated minor criteria that enable a diagnosis of PV without a JAK2mutation: one of these is the presence of erythropoietin (EPO)-independent erythroid burst-forming units (BFU-E). EPO-independent BFU-E are strongly associated with the presence of a JAK2 mutation especially with the loss of heterozygosity (LOH) for JAK2. Copy-number-neutral LOH in PV occurs through uniparental disomy, which in the large majority of cases results in replacement of the normal JAK2 allele by the mutated JAK2 allele through mitotic recombination. Whereas EPO-independent BFU-E are found in some JAK2-positive ET cases, the finding of an expanded LOH clone (⩾20% of BFU-E) is unusual in ET. We hypothesized that patients with mPV would show a JAK2 clonal structure more closely resembling overt PV, and that this might have diagnostic utility. For the purposes of this study, we defined mPV as a JAK2-positive MPN with Hb below the 2008 WHO cutoff for PV and either Hb above the sex-specific normal range or Hb above the middle of the normal range despite iron deficiency or bone marrow (BM) panmyelosis. Patients with WHO-2008-defined PV (n= 20) and JAK2-positive ET (n= 12) were selected for comparison. All patients consented to tissue banking, and the use of archived samples was approved by the relevant ethics committee. BFU-E were grown from either peripheral blood (PB) or BM mononuclear cells, plated in methylcellulose supplemented with no or low-dose EPO (0.02 IU ml 1 recombinant erythropoietin-α, Janssen–Cilag). Where possible at least 30 individual BFU-E per patient were genotyped by single-nucleotide primer extension assay for JAK2V617F. A total of 2984 colonies (28–127 per patient) were genotyped. The clinical and laboratory characteristics of the three cohorts are summarized in Table 1. Median age at diagnosis for ET, mPV and overt PV was 65.5, 63.0 and 62.5 years respectively. There was a stepwise increase in Hb and Hct, and a decrease in platelet counts from ET to mPV to overt PV, consistent with previous reports. Significant differences were observed in serum ferritin and uric acid concentrations, and a similar trend was seen for leukocyte counts, lactate dehydrogenase activity and for PB JAK2V617F allele burden. JAK2 LOH was detected in 5 ET patients (42%, Figure 1), always in a minority (o10%) of BFU-E, concordant with a previous report. In overt PV patients JAK2 LOH was identified in 19 patients (95%),