LIF independent JAK signalling to chromatin in embryonic stem cells uncovered from an adult stem cell disease

Activating mutations in the tyrosine kinase JAK2 cause myeloproliferative neoplasms, clonal blood stem cell disorders with a propensity for leukaemic transformation. LIF signalling through JAK-STAT enables ES cell self-renewal. Here we show that mouse ES cells carrying the human JAK2V617F mutation could self-renew in chemically defined conditions without cytokines or small molecule inhibitors independently of JAK signalling through STAT3 or PI3K pathways. Phosphorylation of histone H3Y41 by JAK2 was recently shown to interfere with HP1α binding. Chromatin bound HP1α was lower in JAK2V617F ES cells but increased following JAK2 inhibition, coincident with a global reduction in H3Y41ph. JAK2 inhibition reduced Nanog, with a reduction in H3Y41ph and concomitant increase in HP1α at the Nanog promoter. Furthermore, Nanog was required for factor-independence of JAK2V617F ES cells. Taken together, these results uncover a previously unrecognised role for direct signalling to chromatin by JAK2 as an important mediator of ES cell self-renewal.