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Comparison of the gene regulatory programs of blood stem cells and megakaryocytes

Final Report Summary - MEGAGEN (Comparison of the gene regulatory programs of blood stem cells and megakaryocytes)

Executive Summary:

MEGAGEN - Discovery of novel regulators of the birth of the blood platelet

Blood stem cells produce all other types of blood cells. They remain active throughout a person's lifetime, providing a constant supply of new blood cells. Genes are translated into proteins in every cell and the specific translation of certain genes is critical for the cell identity and normal function. Serious problems, such as certain types of leukaemia, can arise when this tightly regulated phenomenon is disturbed in blood cells.

Megakaryocytes are bone marrow cells derived from blood stem cells responsible for the production of blood 'thrombocytes' – the platelets which are necessary for normal blood clotting. The aim of MEGAGEN was to improve the insight into gene regulation leading to megakaryocyte formation from stem cells. This involved chromatin immunoprecipitation combined with massive parallel sequencing (ChIP-Seq) for multiple transcription factors on cord-blood derived megakaryocytes. We found that binding, of all 5 transcription factors assayed, to the same bit of the DNA turns on a megakaryocyte-specific protein repertoire. We have a list of 151 candidate target genes bound by all 5 transcription factors. This list contains many known regulators of blood formation, but also many genes for which no role in blood formation has been demonstrated before. For eight of the latter genes we were able to show functionally validate them as regulators of haematopoiesis in zebrafish. This comprehensive functional annotation of the megakaryocyte genome was a world-first and was published in Developmental Cell in 2011. This data was also critical to the discovery of the underlying molecular mechanism by which a common sequence variant at 7q22.3 is associated with the volume and function of platelets.

Platelets are the first cells to arrive at a site of injury. They stick to the damaged blood vessel wall and release proteins that help to form a network of fibres in which further blood cells are captured and a clot is formed. Thereafter platelets will deliver signals to the damaged cells to start the healing process. However, the same platelets can also do harm if they are too active or there are too many of them. A clot may be formed in arteries of the heart or the arteries bringing blood to our brain leading to a heart attack or stroke, respectively.

Ultimately, the ongoing work on the list of target genes will lead to a better understanding of the molecular mechanisms that make these cells function normally. Increasing the knowledge on platelet formation will contribute to the improvement of human health, because platelets are known to be major players in cardiovascular disease. Treatments for people at risk are not ideal. For example, a drug targeting a key platelet protein (GPIb) was shown to cause a shortage of platelets leading to excessive bleeding. A better understanding of how platelets are formed by the blood stem cell may one day lead to the development of a new class of drugs or we may be able to make 'smarter platelets' in the laboratory which are better in repairing the damaged vessel wall.

Furthermore, strokes and heart attacks are the major morbidity associated with myeloproliferative neoplasms (MPNs). Only about 50% of the genetic basis of MPNs is known and current treatments like JAK2 inhibitors do not decrease morbidity.

In addition, patients with a low platelet count, often induced by treatment of cancer, currently receive platelets harvested from donors. The global market for platelet provision is $300m. 250,000 platelet units are issued in the UK each year (£58m cost); a major burden financially and logistically on the healthcare system. Also, with every transfusion, there is the risk of transmission of blood-borne infectious diseases. Therefore, ultimately, we would want to replace these donor-derived products with safer platelets produced in the laboratory. While sufficient numbers of megakaryocytes can be grown in the lab, efficient production of platelets from these cells is currently hampered by a lack of knowledge of the process of platelet formation. Current research is aimed at improving this knowledge and therefore the data feeds directly into this translational research. It is likely that amongst the key regulators of megakaryocyte growth uncovered by our work, there will be proteins or processes that can be modified to improve the production of platelets in the laboratory for transfusion in patients.