A new JAK2 gene mutation in patients with polycythemia vera and splanchnic vein thrombosis

To the editor:

Venous thrombosis is common in white individuals, affecting 1 out of 1000 individuals every year. The pathogenesis of venous thrombosis is multifactorial, involving acquired and genetic factors. Very recently, the JAK2 V617F mutation, an acquired somatic event occurring in most patients with polycythemia vera and in about half of the patients with essential thrombocythemia or myelofibrosis,^1,,–4  has been found in a high proportion of patients with Budd-Chiari syndrome (BCS)⁵  or in noncirrhotic patients with portal and mesenteric venous thrombosis, a heterogeneous group of disorders.⁶ 

We now report a new mutation in the JAK2 gene locus in 2 unrelated noncirrhotic patients with polycythemia vera and presenting with splanchnic venous thrombosis diagnosed and followed at the Gastroenterology Unit of the “A. Cardarelli” Hospital, Naples, Italy. Patients were a 39-year-old woman with portal and mesenteric venous thrombosis (Table 1, no. 1) and a 14-year-old girl with Budd-Chiari syndrome (Table 1, no. 2). Approval was obtained from the “A. Cardarelli” Hospital and the Scientific Institute for Research, Hospitalization, and Health Care (Istituto Cura a Carattere Scientifico [IRCCS]) “Casa Sollievo della Sofferenza” institutional review board for these studies. Informed consent was obtained in accordance with the Declaration of Helsinki. Venous thromboses were diagnosed by Doppler ultrasonography, spiral computed tomography, or magnetic resonance imaging as required during the routine diagnostic work-up. Polycythemia vera was diagnosed according to established criteria.⁷  DNA was extracted from peripheral blood leukocytes according to standard protocols, and amplifications of all coding regions of the JAK2 gene and intron/exon boundaries were achieved using sense and antisense oligonucleotides designed and numbered on the basis of the known sequence of the JAK2 gene locus (GenBank accession no. AL161450). Then, amplified DNA fragments were subjected to direct cycle-sequence analysis using an ABI PRISM 3100 Genetic Analyzer sequencer (PE Biosystems, Foster City, CA). Gene sequencing of the JAK2 gene locus showed a heterozygous GAAATG deletion in exon 12 at DNA position 51324–51329 (numbered according to GenBank accession no. AL161450), leading to an Arg→Lys (AGA>AAA) substitution at amino acid position 541 and Asn (542)–Glu (543) deletion (Figure 1). The deletion abolishes a site of action for the MboII endonuclease (Figure 1). No other acquired or genetic thrombophilic risk factor or cytogenetic abnormality was identified in either patient. The currently reported mutation is similar to one previously described (ie, N542-E543del),⁸  and molecular modeling suggested that residues 541 through 543 lie within a region linking the predicted SH2 and JH2 domains of JAK2.⁹  Although it is unclear how the present mutation and others occurring in exon 12 may affect JAK2 functioning, the Asn (542)–Glu (543) deletion has been shown as a gain-of-function mutation leading to ligand-independent signaling through JAK2.⁸ 

In V617F-negative patients with a myeloproliferative disorder and a normal karyotype, acquired somatic mutations have been found in exon 12,⁹  suggesting that this region may be a “hot-spot” for gain-of-function mutations. Because a portion of patients presenting with splanchnic venous thrombosis did not suffer from an overt myeloproliferative disorder,⁶  present findings suggest that, in addition to JAK2 V617F, screening for exon 12 mutations may be useful to recognize patients who should be carefully observed for the subsequent development of overt disease.