Several Somatic Mutations of JAK2 Exon 12 Are Found in Patients with a JAK2 (V617F)-Negative Myeloproliferative Disorder That Is Mainly Characterized by Erythrocytosis.

About 95% of patients with polycythemia vera (PV) carry the unique V617F mutation in JAK2 exon 14, which encodes a portion of the JH2 auto-inhibitory domain of the Jak2 kinase. Mutations in exon 12 have been recently reported in JAK2 (V617F)-negative patients with PV or idiopathic erythrocytosis. We searched for exon 12 mutations in 168 patients with JAK2 (V617F)-negative myeloproliferative disorders. The 2001 WHO criteria were employed for diagnosis. Of the 168 patients studied, 47 had sporadic PV, 11 had familial PV, 75 had essential thrombocythemia (ET), and 35 had primary myelofibrosis (PM). Seventeen patients with PV, including 15/47 sporadic cases and 2/11 familial cases, were found to carry deletions (n=15) or duplications (n=2) of exon 12 in circulating granulocytes but not in T-lymphocytes. None of the 110 patients with ET or PM was found to be positive. Mutations were detected by sequencing, and were then confirmed by sub-cloning in bacteria in 7/17 cases. Four of the 8 mutations detected were novel, while the most frequent ones were N542–E543del and E543–D544del. Mutations spanned from base 1606 to 1640, and the two duplications modified the rest of the sequence by adding 33 bp. In terms of protein, deletions predicted aminoacid changes spanning from phenylalanine 537 to aspartic acid 544, while duplications predicted changes from phenylalanine 547 onwards within the JH2 pseudokinase domain. Three categories of molecular lesions were identified:

• those involving a K539L substitution;

• those involving the E543del; and

• aminoacid duplications involving a substitution of phenylalanine 547.

At clinical onset, 16/17 (94%) patients carrying a JAK2 exon 12 mutation had low serum erythropoietin (Epo) levels, indicating a combination of absolute erythrocytosis and suppressed endogenous Epo production. Moreover, 12/17 patients had erythrocytosis associated with normal white blood cell and platelet counts, i.e., isolated erythrocytosis. This frequency (71%) was significantly higher than that observed in 92 patients diagnosed with JAK2 (V617F)-positive PV at the Department of Hematology, IRCCS Policlinico San Matteo, Pavia, Italy (P<0.001). Most of these latter patients, in fact, had erythrocytosis combined with leukocytosis (WBC>12 x 10⁹/L) and/or thrombocytosis (PLT>400 x 10⁹/L), and only 22% of them had isolated erythrocytosis. Both patients with familial PV carrying an exon 12 mutation had an affected sibling with JAK2 (V617F)-positive PV. While the former showed isolated erythrocytosis, their JAK2 (V617F)-positive siblings had also thrombocytosis.

In conclusion:

1. several somatic mutations of JAK2 exon 12 - mostly 6 bp deletions - can be found in patients with a myeloproliferative disorder that is mainly characterized by erythrocytosis associated with low serum Epo levels;

2. a genetic predisposition to acquisition of different JAK2 mutations is inherited in families with myeloproliferative disorders, and the mutation type (exon 12 vs exon 14) contributes to determining their variable clinical phenotype.