JAK/STAT signaling, stem cell subversion and human myeloproliferative neoplasms.

The JAK/STAT pathway has essential roles in several aspects of metazoan biology including haematopoiesis and stem cell function. Somatic mutations affecting this pathway occur in multiple tumour types and are especially common in human myeloproliferative neoplasms. The Green lab is studying the molecular and cellular mechanisms whereby aberrant JAK/STAT signalling subverts haematopoiesis and results in a myeloproliferative neoplasm.

JAK/STAT signalling and the MPNs.
The JAK/STAT pathway provides an attractive experimental paradigm for several reasons. It represents a relatively simple and direct link from cell surface/cytoplasmic signalling events to nuclear transcriptional programmes. In addition to the well-described canonical JAK/STAT pathway, recent experiments in flies revealed a non-canonical pathway which opens up chromatin independently of classical STAT-responsive cis-regulatory elements. In collaboration with the Kouzarides group we have recently provided mechanistic insights into this non-canonical pathway by identifying an unexpected role for JAK2 in the nucleus of haematopoietic cells whereby JAK2 phosphorylates histone H3 and modifies chromatin structure directly (see below). There is also a tantalising link between the JAK/STAT pathway, stem cell behaviour and cancer. JAK/STAT signalling is essential for maintenance of several Drosophila stem cell populations and influences the behaviour of mammalian HSCs, ES cells and neural stem cells. These observations may reflect a need for stem cells to maintain their genome in an accessible state, with dysregulation contributing to tumorigenesis.

The BCR-ABL negative MPNs represent a spectrum of clonal haematological disorders, with three main members: polycythaemia vera (PV), essential thrombocythaemia (ET) and idiopathic myelofibrosis (IMF). For many years the identity of molecular lesions remained obscure but in 2005 several groups, including our own, reported that the majority of patients harbor a single acquired gain-of-function mutation in JAK2. Although not always the initiating lesion, mutation of JAK2 clearly plays a central role in MPN pathogenesis with mouse models demonstrating that JAK2 is sufficient to give rise to an MPN. Functional studies have focused on JAK2/STAT5 signalling in the context of Type 1 cytokine receptors, especially EPOR and TPOR. However JAK2 interacts with multiple other cytokine receptors, including members of the gp130, gC, interferon, and IL3 families. JAK2 signalling therefore influences several downstream STATs with the net STAT transcriptional output depending on cellular context.

Recent highlights.
The discovery of the JAK2 mutation has revolutionised research into the MPNs and has allowed us to provide multiple insights into the molecular and cellular mechanisms by which JAK2 mutations subvert haematopoiesis and result in the various clinical phenotypes associated with human MPNs. These studies have had direct clinical impact with new approaches to classification and diagnosis already embedded in international guidelines. Moreover dissection of the molecular consequences of JAK2 mutations has also provided unexpected insights into fundamental biological mechanisms, including chromatin biology, cytokine signalling and cellular responses to DNA damage. Selected translational and fundamental highlights from the Green lab include:

JAK2 and MPL status identify sub-types of ET (Campbell et al Lancet 2005; Beer et al Blood 2009).
Unexpected discovery that AML arising in a patient with a JAK2-mutant MPN frequently lacks a JAK2 mutation (Campbell et al Blood 2006; Beer et al Blood 2010).
Development of PV but not ET is associated with clones homozygous for the JAK2 mutation and also with an unexpected defect in STAT1 signalling (Scott et al Blood 2006; Chen et al Cancer Cell 2010).
Discovery of JAK2 exon12 mutations which revealed a clinically distinct subtype of PV (Scott et al NEJM 2007).
Unexpected clonal complexity of early phase MPNs (Beer et al BJH 2009; Beer et al Blood 2010).
JAK2 mutation promotes survival of DNA damaged cells by inhibiting BCL-xL deamidation pathway (Zhao et al NEJM 2008).
Demonstration that JAK2 V617F mutation inhibits haematopoietic stem cell function in a conditional knock-in mouse model of ET (Li et al Blood 2010).
Unexpected nuclear role for JAK2 as a histone kinase which regulates transcription of target genes and can drive factor-independent ES cell self-renewal (Dawson et al Nature 2009; Griffiths et al Nature Cell Biol 2011).

MPN resources in Cambridge.
Over the past decade we have established a powerful combination of clinical resources and research expertise. Tony Green is Chief Investigator for the PT-1 suite of clinical studies (>1200 patients; largest randomized study of any MPN ; has been running since 1997; unique prospective dataset and samples). He also directs a specialist MPN clinic - the ability to study fresh samples from well characterized patients greatly facilitates much of our research. Importantly the interaction with the clinical service is 2-way: assays for JAK2V617F, JAK2 exon 12 and MPL mutations have all been transferred to our Regional Diagnostic Service and are already in routine use.

Haematopoiesis in Cambridge.
The Green lab is part of a consortium of Cambridge University groups, largely in the CIMR and adjacent buildings, that share a focus on normal and/or leukaemic haematopoiesis. Current programs include transcriptional networks of haematopoietic stem cells (Bertie Gottens), leukaemic stem cells (Brian Huntly), emergence of haematopoietic stem cells during development (Katrin Ottersbach), ribosome biology and bone marrow failure syndromes (Alan Warren, LMB), megakaryocyte and platelet biology (Willem Ouwehand and Cedric Ghevaert) and the pathogenesis of the myeloproliferative neoplasms (Tony Green). Particular strengths include close interactions with the Wellcome Trust Sanger Institute (eg Peter Campbell, George Vassiliou, David Adams, Pentao Liu), the Wellcome Trust/MRC Stem Cell Institute (eg Austin Smith, Brian Hendrich, Azim Surani) and with the Addenbrooke’s Department of Haematology.

Potential post-docs, clinical fellows and students: we are always keen to hear from good people. Please email me (alg48@cam.ac.uk) with a copy of your CV including the details of two referees.

Funding