The Belgian federal government supports fundamental research by conferring Quinquennial Awards to two Belgian researchers. This year, the francophone winner was Stefan Constantinescu, a University of Louvain professor and member of both the UCL de Duve Institute and the Brussels Branch of the Ludwig Institute for Cancer Research. He was recognised for highly advanced scientific work on the molecular mechanisms that induce myeloproliferative disorders and that can lead to potentially highly aggressive blood cancers.
Myeloproliferative disorders are chronic clonal haematological malignancies in which the bone marrow produces too many mature myeloid cells. This provokes an increase of circulating cells which in turn increases the risk of thrombosis and especially of developing aggressive leukaemia. Prof. Constantinescu and his team focus on the mechanisms that drive this overproduction of myeloid cells.
Myeloproliferative disorders – excluding chronic myelogenous leukaemia, which has been elucidated – mainly include three diseases distinguished by the type of excess cells. In Vaquez disease or polycythaemia vera, the type is red blood cells (haemoglobin and haematocrit); in essential thrombocythaemia, it’s platelets; and in primitive myelofibrosis, or myeloid splenomegaly, it’s medullary fibrosis linked to the overproduction of cells in bone marrow.
Understanding blood
‘To understand the malfunctioning of red blood cell production,’ Prof. Constantinescu explains, ‘you first have to understand haematopoiesis. Blood stem cells in the bone marrow must at some point differentiate into either lymphoid stem cells or myeloid stem cells. The latter, after several intermediary phases of proliferation, again differentiate and give rise to either red blood cells (erythrocyte lineage), monocytes and macrophages (monocyte lineage), platelets (thrombocyte lineage), or granulocytes (polynuclear lineage). But at each intermediary stage, while the cells that receive the signal to survive and differentiate go on to specialise, the others undergo apoptosis.’ We thus produce more progenitors subject to ‘orders’ than we do other actors, such as cytokines – including erythropoietin (EPO) and thrombopoietin (TPO) – to differentiate and multiply, or even to self-destruct in the absence of cytokines.
But in myeloproliferative neoplasms, apoptosis doesn’t occur. ‘In these blood cancers, a mutated stem cell overproduces blood cells.’ It’s already known that some cytokines are involved in this process, either to activate or inhibit it. ‘Cytokines participate in cell communication by attaching to a matching receptor on the cell’s external membrane. This is how they transmit messages to the cell. They awaken the STAT (signal transducers and activators of transcription) system, after phosphorylation by tyrosine kinases (Janus kinases, or JAK). This activated mechanism provokes in the cell nucleus the transcription of specific target genes and therefore determines which genes are activated.’
When anarchy takes over…
‘Initially,’ Prof. Constantinescu continues, ‘a stem cell acquires a mutation among those that induce myeloproliferative disorders. It becomes a clone. When it becomes capable, it produces myeloid cells that – as progenitors of red blood cells, platelets, even neutrophilic granulocytes – can survive, multiply and differentiate without the signal emitted by cytokines.’ The mutation transmitted to the progenitors causes the system to fail; cytokines no longer have control over proliferation and cell apoptosis. ‘Multiplication occurs without stimulation, thus independent of the activity of cytokines such as EPO, TPO or GCSF. In 2005, we postulated that JAK2 was involved, because when it is overexpressed we observed an opposite effect on the mechanisms of the disease. And indeed, in 2005, we helped discover that this excessive cell proliferation is linked to a mutation in the gene that codes for the tyrosine kinase JAK2 protein: the mutation JAK2 V617F. This unique mutation is at the origin of 70% of myeloproliferative neoplasms, and is involved in 97% of polycythaemia vera cases and 64% of essential thrombocythaemia cases. JAK2 inhibitors exist but act only on the kinase system, not on the pseudokinase system, and that’s one of the subjects of our research.’
Genetic mutations
For patients who don’t have the JAK2 V617F mutation but do have the disease, it’s necessary to look further upstream, including for the mutation that operates on a receptor whose role is to activate JAK2. ‘In the case of mutation, the system is continuously active and generates so-called “wild” JAK2. And we discovered the W515 mutation on the thrombopoietin receptor which can induce thrombocytosis, and observed that this mutation was the second cause of myeloproliferative neoplasms, which we do not find in polycythaemia vera but do find in essential thrombocythaemia and myelofibrosis. Our work in this field consists of understanding why the loss of tryptophan that we observed makes the machine malfunction so severely.’
Prof. Constantinescu continues, ‘And then there’s the discovery of the calreticulin mutation by one of our collaborators in Vienna: it’s a protein that normally plays an important role in metabolising calcium in the reticulum and, like a chaperon protein, helps other proteins to fold. In the case of mutation, we observed a malfunctioning of calreticulin characterised by a pathological activation of cytokine receptors; moreover, the mutated calreticulin “exits” the endoplasmic reticulum, which it normally cannot do, and brings with it the receptor for TPO, which thus becomes active. Finally, JAK2 is involved here, too, because it’s located downstream from the TPO receptor. This shows that it plays a role at all levels of myeloproliferative neoplasms. Many questions remain, and we’re working on them.’
They’re doing so with the support of numerous sponsors, including the Ludwig Institute for Cancer Research, the de Duve Institute, the UCL Health Sciences Sector, the FNRS, Télévie, PAI Belspo and ARC Belgique, The Atlantic Philantropies (New York), the EU Marie Curie programme, the NIH, the Belgian Cancer Foundation, Salus Sanguinis, the non-profit Les Avions de Sébastien and the MPN Foundation (Chicago).
Carine Maillard
A glance at Stefan Constantinescu's bio
1964: Born
1988: PhD in Medicine, University of Bucharest
1991: PhD in Virology, Carol Davila University of Medicine and Pharmacy, Bucharest (equivalence 2002: PhD in Biomedical Sciences, UCL)
1992-94: Postdoctorate, University of Tennessee College of Medicine (Memphis)
1995-2000: Postdoctorate and Researcher, Whitehead Institute for Biomedical Research,
Massachusetts Institute of Technology (Cambridge) (with Prof. Harvey Lodish)
2000: Head of Laboratory, Ludwig Institute; Associate Member, Institute of Cell Pathology
2003: FNRS Researcher; Associate Professor, UCL
2005: Associate Member, Ludwig Institute
2010: Member, Ludwig Institute
2011: FRS-FNRS Senior Research Associate; Professor, UCL
2015: Full-time Professor of Cell and Molecular Biology, UCL; Member and Coordinator of the Cell Signalling Centre, de Duve Institute; FRS-FNRS Honorary Research Director
2013: Associate Member, Belgian Royal Academy of Medicine