Frequency of JAK2 Exon 12 Mutations in JAK2 Exon 14 V617F-Negative Patients: High Frequency in ET Patients.

The identification of the G to T point (V617F) mutation in exon 14 of the JH2 domain of the Janus kinase 2 gene (JAK2) has recently simplified the diagnosis of many patients affected with a variety of myeloproliferative disorders (MPDs). More recently JAK2 exon 12 mutations have been described in these disorders; although the role of these mutations is as yet unclear. There have been only two investigations which have identified the presence of exon 12 mutations in clinical diagnosis among polycythaemia vera (PV) (Scott et al NEJM 2007, Pardanani et al Leukaemia 2007). A total of 725 patients diagnosed clinically with a variety of MPDs were investigated for both the frequently occurring exon 14 V617F and for the rarer exon 12 mutations.

Methods: Peripheral blood samples from all patients were screened for the V617F mutation by gene-scaning on an ABI 3130 fragment analyser. 478 of these patients were further screened for the exon 12 mutant allele by denaturing high performance liquid chromatography (Transgenomic WAVE®) and direct sequencing.

Results: Of 725 patients screened for the V617F mutation 204 (28%) were found to be heterozygous (HET) and 30 (4%) homozygously (HM) mutated while 491 (68%) were wildtype (WT). 478 were further tested for the exon 12 mutation and eight were found to carry mutations. Two of the patients had previously described mutations while three novel mutations were described in the remaining six patients. Only one of these was in the PV group while the majority of exon 12 mutations clustered in the essential thrombocythemia (ET) (4/8) and Budd Chiari patients (2/8). All exon 12 mutations were identified at very low level.

Conclusions: Combined rapid gene-scanning and WAVE analysis allows for the characterisation of a large cohort of patients diagnosed clinically to have a variety of MPDs. This helps with the correct diagnosis of a frequently difficult group of patients. Detection of exon 12 mutations should be used in patients who are WT for V617F, including patients in the ET and the Budd Chiari syndrome groups. Both V617F and exon 12 mutation analysis tests should be included in primary screening in the routine management of MPD patients.