Myeloproliferative neoplasms are chronic myeloid diseases that can reflect an early stage of tumorigenesis. Patients with these neoplasms are identified early in the disease trajectory because of the over-production of mature cells types. JAK2 is mutated in the vast majority of patients with myeloproliferative neoplasms and TET2 is mutated in ∼10% of patients. These neoplasms offer a unique opportunity to assess disease course over a long period of time, because differences in patient variables (such as age, sex, therapy and genetic background) can be controlled for, thus enabling direct comparison of genetically distinct subclones that develop within a patient.

Despite the enormous progress in our understanding of the accumulation of somatic mutations and the associated genetic complexity of cancer, it is not known whether the order in which mutations in a tumour are acquired have any relevance to disease characteristics. David Kent and coauthors addressed this question by studying mutation order in a set of myeloproliferative neoplasms in which approximately 10% of patients progress to more-severe disease—acute leukaemia. They provide compelling evidence that mutation order influences the biology and outcome of the disease.

The researchers screened the exons of 246 patients with myeloproliferative disorders (consisting of polycythemia vera, essential thrombocythemia, and myelofibrosis disorders) harbouring the JAK2 V617F mutation, and identified a group of 24 patients in whom mutation order could be established. As Kent explains: “We analysed mutation order by growing individual red cell colonies from patient blood samples (each colony was derived from a single cell) and we established whether no genes, one gene or two genes were mutated. Each patient is a composite of a non-mutant, single mutant (either the JAK2 or TET2 mutation alone), or double mutant (both JAK2 and TET2 mutations).” The researchers then surveyed the clinical characteristics of these patients and the molecular and cellular features of the stem and progenitor cells present in patient samples to look for differences between patients who acquired a 'JAK2-first' mutation and those that acquired a 'TET2-first' mutation.

Patients in whom the JAK2 mutation was acquired first had a greater likelihood of having polycythemia vera than essential thrombocythemia, an increased risk of thrombosis and greater sensitivity to the JAK2-inhibitor ruxolitinib on colony formation in vitro, compared with patients showing the TET2-first mutational profile. Furthermore, the order of the mutations influenced cell profilerative response, as progenitor expansion of individual double-mutation haematopoeitic stem cells and progenitor cells was considerably different in TET2-first patients and JAK2-first patients. The proliferation of progenitor cells was enhanced by acquisition of a JAK2 mutation on a TET2 normal background, but not on a TET2-mutant background.

“This study shows for the first time that the order in which mutations are acquired impacts on the clinical and biological features of disease. Patients who acquire a JAK2 mutation prior to a TET2 mutation show signs of disease approximately 10 years earlier, have an increased risk of thrombotic events, and are more responsive to JAK inhibitor therapy in cell culture experiments,” explains Kent. The researchers subsequently showed that individual stem cells and progenitor cells with the same genetic mutations (for example, double mutant for TET2 and JAK2) had completely distinct molecular and cellular characteristics that were dependent on the order in which the mutations were acquired.

Credit: NPG

Kent highlights the true significance of these striking research findings: “This work establishes a paradigm that mutation order matters in cancer.” Importantly, the approach from this study could help to improve our understanding of how and when cancer cells develop mutations. Crucially, the order in which genetic faults appear can affect how patients respond to different therapies.

The results of this important research illustrate that the order in which mutations are acquired are likely to be relevant in many cancers and could provide useful insights into the origins of cancer. By knowing how patients might respond differently to drugs as a result of the mutational order, we might be able to personalize treatment more efficiently and cost-effectively for patients.

In the future, Kent and his colleagues plan to further define the following: “First, we would like to extend the search to other mutations in the myeloproliferative neoplasms, then to other myeloid malignancies, and ideally other researchers would also search in other cancer types. Second, we hope that a prospective clinical trial could be established to determine whether order impacts on progression to more-severe disease or dictates response to JAK-inhibitor therapy.”