Mutations in the Exon 12 of the JAK2 Gene Appeared To Be a Rare Event Essentially Clustered in V617F JAK2 Negative PV Patients.
Authors: Kouroupi Eirini , Parquet Nathalie , Zoi Christine , Kiladjian Jean-Jacques , Grigoraki Vassiliki , Zoi Katerina , Scott Linda , Dosquet Christine , Fenaux Pierre , Loukopoulos Dimitris , Chomienne Christine , Cassinat Bruno 
Published: April 2020

Abstract
Since the description in 2005 of the recurrent V617F JAK2 gene mutation in MPD patients, we have tested more than 1500 patients each year and confirmed the relative frequency of this mutation in distinct MPD subclasses and its usefulness in classyfying diseases such early PV, MIF, ET and thrombosis such Budd Chiari syndrome.
In the case of erythrocytosis we notably observed that among patients with true polycythemia (red cell mass measurement increased by 25%), the presence of the mutation significantly distinguished between PV patients (95% V617F positive) and patients with Idiopathic Erythrocytosis (IE, 0% positive); In ET patients about 50% were positive ; In thrombosis, we demonstrated that the V617F mutation is present in about half of the patients with deep vein thrombosis (Budd Chiari syndrome or portal vein thrombosis), reinforcing the guess that occurrence of thrombosis in these patients is caused by an ignored MPD.
The description in 2007 of novel recurrent mutations in the exon 12 of the JAK2 gene led us to test whether these new alterations could help in classifying some of the V617F negative patients. We have tested patients with PV (n= 25) of whom 10 had a post PV myelofibrosis, deep vein thrombosis (n= 20) and IE (n= 21). PV patients fulfilled either PVSG or WHO criteria. IE patients were diagnosed on the basis of an elevation of the hematocrit (median hematocrit of 54% (range: 49% to 56%)), a raised red cell mass (median excess of red cell mass was +35% (range: +25% to +104%); no identifiable cause of secondary erythrocytosis (serum Epo level was under or within the normal range in all patients); and absence of PV according to PVSG or WHO criteria (no splenomegaly, median WBC (×109/L) of 6180 (range: 3500–8300) ; median platelet count (x109/L) of 240 (range: 174–358). DNA extracted from peripheral blood has been tested using the allele specific PCR described in Scott et al., NEJM 2007, allowing to detect all of the described mutations. We failed to identify any mutation in patients with IE or deep vein thrombosis. However, 2 female patients diagnosed with PV were identified to harbour the N542-E543del mutation. With the exception of their relative younger age (41 and 45 respectively) and a very elevated hematocrit (58 and 59%) it was not possible to find any common points to these 2 patients that could allow individualizing patients susceptible to harbour exon 12 mutations. One patient had very low Epo level and splenomegaly, while the other patient had increased WBC but no splenomegaly. None of them had myelofibrosis. In conclusion, only 2 out of 25 PV patients harboured JAK2 exon 12 mutations, thus around 5% of PV patients that fulfilled PV criteria are still negative for any mutation. The fact that IE patients are always negative let the question unsolved of whether this entity is a MPD or not. The search for exon 12 JAK2 gene mutation should be restricted to patients which fulfil PV diagnosis criteria and do not exhibit the “classical” V617F mutation. In these cases, the presence of the mutation is very useful in order to well classify these patients as MPD patients.