Long-term serial analysis of X-chromosome inactivation patterns and JAK2 V617F mutant levels in patients with essential thrombocythemia show that minor mutant-positive clones can remain stable for many years.

Essential thrombocythemia (ET) is heterogeneous with respect to natural history, X-chromosome inactivation patterns (XCIPs), and presence of the V617F mutation in Janus kinase 2 (JAK2). We studied 111 patients with ET; 39% were JAK2 mutant positive, and clone size (percentage mutant JAK2) was concordant with XCIP when constitutive T-cell patterns were taken into account. JAK2 mutant clones were present in both clonal and polyclonal cases as determined by XCIP, and the former had higher mutant JAK2 levels (median 26% versus 16%; P=.001). No change was observed in serial XCIP analysis of 14 polyclonal patients over a median follow-up of 61 months. Furthermore, 18 of 19 mutant-positive patients showed no significant change in mutant JAK2 level over a median follow-up of 47 months. These results suggest that, in many cases of ET, a small stable clone containing a JAK2 mutation can be maintained as a subpopulation for many years.