Location:         Cell Biology Program, SickKids Research Institute,

                           Peter Gilgan Centre for Research and Learning, Toronto

Affiliations:     Division of Haematology/Oncology, Hospital for Sick Children

                           Departments of Biochemistry and Paediatrics, University of Toronto


My group has been continuously funded by the Canadian Institutes of Health Research (CIHR) since 2006.

We currently hold two 5-year project grants:

1) Role of VPS33B/VPS16B in megakaryocyte granule formation – awarded 03/2023

2) Role of NBEAL2 in alpha granule formation and maturation in megakaryocytes and platelets – awarded 03/2022

Current and past funding support has also come from the Canadian Hemophilia Society, Heart and Stroke Canada, and private sources.

Our students, fellows and other trainees have been supported by a wide range of sources.



As a SickKids hematologist, my primary responsibility is to help children with problems linked to excessive bleeding and/or blood clotting. Those problems range from acute to chronic, and as with many conditions that affect children, they are often inherited.

As a professor in the Departments of Biochemistry and Paediatrics at the University of Toronto, and a Senior Scientist in the Cell Biology Program of the SickKids Research Institute, I oversee a research program with interconnected aims that include:

1) Advancing the diagnosis, treatment and clinical management of patients with bleeding/clotting disorders.

2) Advancing knowledge of the physiological, developmental, cellular, genetic and molecular aspects of bleeding/clotting and other pediatric disorders.

3) Advancing basic understanding of the physiological systems associated with blood clotting (i.e. hemostasis), with an emphasis on its primary cellular effectors: blood platelets.

4) Collaborating with colleagues at SickKids and elsewhere in the pursuit of studies aimed at gaining a better and more complete understanding of health and disease.

Over the years, several prominent themes have emerged within our research program. Some are focused around particular diseases. Others emphasize aspects of the underlying biology of disease, and of the inner workings of the physiological systems, organs, tissues, cells and molecules that normally keep us healthy. There have also been opportunities to learn key lessons concerning the role of science in advancing health, including examples of how progress is made on many fronts. A few of those themes and lessons are outlined here and on the following pages.


Overactive Clotting: Sickle Cell Disease (SCD), Hemoglobin and Red Blood Cells

SCD is the most prominent clotting (prothrombotic) disorder treated at SickKids. It is associated with hemoglobin, the oxygen carrier in red blood cells (RBCs). More specifically, SCD is linked to variants in the beta globin component of hemoglobin, HBB, which cause RBCs to become unstable and change shape within the bloodstream. As a result, SCD patients are at high risk for serious health problems. From a young age, they often experience painful acute attacks – known as crises – that can require a visit the emergency ward, and sometimes a hospital stay. Over time, SCD patients can experience damaged joints, persistent anemia, and life-threatening events such as strokes. They are also at high risk for severe reactions to common childhood infections. While SCD treatments have advanced considerably over the years, the situation remains far from satisfactory for many patients. Working with clinicians at SickKids, we have used our advanced laboratory methods to measure aspects of hemostasis in children with SCD. Recently, these studies revealed a link between homozygosity for the S variant of HBB and high baseline hemostatic activity, which persists in a subgroup of patients when they are in or out of crisis.1


Clotting Factor Deficiencies: Von Willebrand’s Disease (VWD) and Hemophilia

VWD is a relatively common hereditary condition affecting children managed by the SickKids Bleeding Disorders clinic. It is caused by loss or abnormal function of Von Willebrand factor (VWF), a large glycoprotein released by damaged cells lining blood vessels that initiates protective clot formation (i.e. hemostasis) at wound sites. We have been involved with several studies examining the biology of VWF and its involvement in a variety of pathological situations.2-5

VWF is the bloodstream carrier of clotting Factor 8 (F8), a protein produced by liver cells that is missing/abnormal in patients with hemophilia A. These patients are mostly boys because the F8 gene is on the X chromosome, as co-incidentally is F9, where variants are associated with hemophilia B. We have collaborated extensively with clinical colleagues at SickKids and elsewhere to monitor hemostatic activity in pediatric hemophilia patients, assess the effects of treatment, examine variations (i.e. heterogeneity) in treatment requirements among patients, and explore the potential benefits of non-medical interventions such as moderate exercise.6-9 We have also studied patients with variants affecting other clotting factors, such as F12.10


Hemophilia – A Global Success Story In Progress

The importance of research in advancing treatment of bleeding disorders is illustrated by the care received by hemophilia patients at SickKids, across Canada and in many places around the world. Decades of advances in biomedical research have unravelled the physiology of hemostasis, and advanced methods for patient diagnosis (e.g. gene analysis) and monitoring (e.g. precise measurement of clotting factor levels). As a result, many hemophilia patients – including those with no prior family history – are identified as infants, and immediately offered treatment with appropriate clotting factors mass-produced by pharmaceutical corporations using molecular biology-based technologies. Starting factor replacement therapy early in life greatly lowers patient risks of acute bleeding and chronic symptoms such as joint damage. This treatment is, nevertheless, aimed at management not cure, and it is not without costs, inconveniences and risks. Factors are expensive, and while Canada’s socialized medicine system provides them to all patients that is not true everywhere. Factors must also be injected – in some patients every other day – and many patients develop immune reactions that can be difficult and expensive to resolve. These challenges have continued to drive advances in knowledge and treatment, which include the development of engineered factors that persist much longer in the bloodstream than natural factors. This allows some patients to go several weeks between injections, with reduced risk of immune responses.


Platelet Problems: Inherited Disorders

Childhood bleeding problems often involve platelets, the blood cells primarily responsible for initiating and co-ordinating hemostasis. The smallest cells in the body, platelets are produced in the bone marrow by the largest cells, megakaryocytes, which uniquely among blood-forming (hematopoietic) progenitors release thousands of cellular offspring into the bloodstream. Bleeding problems can arise when platelets are present in unusually low numbers, a condition known as thrombocytopenia. In children, this condition is often associated with acquired autoimmunity causing abnormally rapid removal of circulating platelets (i.e. immune thrombocytopenia). There are also hereditary forms of thrombocytopenia, which are often associated with abnormalities in platelet structure. For example, abnormally large platelets (macrothrombocytopenia) are seen in patients heterozygous for variants in the non-muscle myosin protein MYH9, several of which we have characterized.11,12 In contrast, unusually small platelets (microthrombocytopenia) occurs in patients with Wiskott-Aldrich syndrome having variants in the X-linked WAS gene, and in ARPC1B deficiency, a condition affecting actin dynamics that we identified in collaboration with several SickKids physicians and researchers.13


Platelet Dense Granule Deficiency

Several hereditary platelet disorders have been linked to abnormalities in the structure, function and number of the transport/secretory vesicles that occupy much of the platelet interior. These include dense granules, which carry an assortment of small molecules (e.g. serotonin) and ions (e.g. calcium). Dense granules are decreased/absent in patients with various forms of Hermansky-Pudlak syndrome, which is associated with variants in several distinct HPS family genes. We have examined how dense granules relate to early human development,14 and we have endeavored to improve methods for rapidly determining dense granule number and structure.15,16 Recently, we worked with several collaborators to employ live uptake experiments and advanced analytical and electron microscopy methods to reveal that dense granules represent a unique and physiologically important reservoir of free zinc in the bloodstream.17


Platelet Alpha Granule Deficiencies

The most abundant platelet vesicles are the protein-packed a-granules, which are a major research focus of our group. In studies of conditions where these granules are defective or absent, we have identified several genes/proteins involved in granule production, which takes place within platelet precursor megakaryocytes. Those proteins include VPS16B and VPS33B, associated with ARC syndrome,18-21 a multisystem disorder where among other symptoms patient platelets lack a-granules. In a culmination of years of work by many lab members, we were recently successful in purifying the VPS16B-33B complex, and characterizing its composition and structure. This complex was revealed to have unique properties that indicate critical functions in intracellular protein trafficking.18

Another protein we have definitively linked to a-granule production is NBEAL2, which is abnormal/absent in gray platelet syndrome (GPS).22 In a series of studies using animal models and human cells, we have uncovered key roles for NBEAL2 in the trafficking and stability of a-granule cargo proteins, and we have identified several proteins, other molecules and cell structures that NBEAL2 interacts with.23-27 These studies have advanced our knowledge of a-granule production, and they also gave us the tools to delve deep into megakaryocyte cell biology. Most recently, we were able to use some of those tools to determine the role of “proton pumps” in facilitating the massive amount of complex protein trafficking that is required for megakaryocytes to generate platelets.28


Keys to Success

Up to this point we have been looking at disorders where the key genes/proteins involved have been identified and characterized. Experience with conditions such as GPS22,29 showed us that gaining such information typically requires a massive effort on multiple fronts, as groups of researchers working in different places labor over many years studying patients and families to discern inheritance patterns (e.g. X-linked versus autosomal), characterize abnormalities (e.g. missing/abnormal proteins, unusual platelet features), and painstakingly track down genes, proteins and variants associated with particular disorders. These efforts are far from complete, since many of the patients we see in clinic have problems that have not been definitively linked to a biological cause. But as the hemophilia example shows, the payoffs from finding such links can be priceless in terms of advancing our ability to diagnose patients and apply treatments – many of which have been facilitated by advances in understanding the biological basis of disease and the basic mechanisms of the physiological, cellular and molecular systems we rely on to stay healthy.

Modern biomedical science is a global enterprise, and we have collaborators around the world involved with projects already mentioned and others investigating a wide range of topics. We also have many interactions with colleagues in the Cell Biology Program, other programs in the SickKids Research Institute, and the main hospital. Those interactions are always positive and collegial – which not all researchers can take for granted – and this spirit of co-operation can have outstanding results. An example occurred a few years back with an effort to solve a mystery concerning a young SickKids patient whose symptoms had puzzled a long list of physicians who had become involved with his case in frequent visits for a range of problems that included abnormal immune responses – such as severe symptoms from normally mild infections – gastrointestinal issues and platelet abnormalities. To make a long story short, we ultimately tracked the problems to loss of expression of ARPC1B, a component of the Arp2/3 complex that generates branched actin filaments within cells. This came as a surprize, because ARPC1B was considered to be an indispensable protein. It turned out that it is possible to live without ARPC1B because its gene is linked to a related gene that codes for ARPC1A, which can substitute for ARPC1B, but does not do a very good job in gut and blood cells.

This work received considerable media interest, and it was recognized by the inaugural Janet Rossant Research Innovation Prize. It also stimulated exploration of ARPC1B deficiency, which has since been identified in patients around the world. Most importantly, it showed the SickKids care team how to deal with the problem that sparked the entire effort. The index patient received a bone marrow transplant, and at last report was doing well.

References Cited

  1. Sussman, R.G. et al. Res Pract Thromb Haemost 8, 102374 (2024).
  2. Riedl Khursigara, M. et al. J Thromb Haemost 18, 1484-1494 (2020).
  3. Bowman, M.L. et al. J Thromb Haemost 15, 1403-1411 (2017).
  4. Noone, D.G. et al. Kidney Int 90, 123-34 (2016).
  5. Labarque, V., Stain, A.M., Blanchette, V., Kahr, W.H. & Carcao, M.D. Haemophilia 19, 602-6 (2013).
  6. Mathews, N. et al. Res Pract Thromb Haemost 6, e12800 (2022).
  7. Kumar, R. et al. Blood 140, 1156-1166 (2022).
  8. Sholzberg, M. et al. Haemophilia 23, e162-e165 (2017).
  9. Kumar, R. et al. Thromb Haemost 115, 1120-8 (2016).
  10. Pluthero, F.G., Ryan, C., Williams, S., Brandao, L.R. & Kahr, W.H. Br J Haematol 152, 111-2 (2011).
  11. Pecci, A. et al. Hum Mutat 35, 236-47 (2014).
  12. Kahr, W.H. et al. Thromb Haemost 102, 1241-50 (2009).
  13. Kahr, W.H. et al. Nat Commun 8, 14816 (2017).
  14. Urban, D. et al. Haematologica 102, e36-e38 (2017).
  15. Pluthero, F.G. & Kahr, W.H.A. Platelets 34, 2157808 (2023).
  16. Al-Huniti, A. & Kahr, W.H. Transfus Med Rev 34, 277-285 (2020).
  17. Kahr, W.H.A. et al. Res Pract Thromb Haemost 8, 102352 (2024).
  18. Liu, R.J.Y. et al. J Biol Chem 299, 104718 (2023).
  19. Penon-Portmann, M. et al. J Thromb Haemost 20, 1712-1719 (2022).
  20. Urban, D. et al. Blood 120, 5032-40 (2012).
  21. Lo, B. et al. Blood 106, 4159-66 (2005).
  22. Kahr, W.H. et al. Nat Genet 43, 738-40 (2011).
  23. Pluthero, F.G. & Kahr, W.H.A. J Thromb Haemost 19, 318-322 (2021).
  24. Lo, R.W. et al. Blood 136, 715-725 (2020).
  25. Lo, R.W., Li, L., Leung, R., Pluthero, F.G. & Kahr, W.H.A. Arterioscler Thromb Vasc Biol 38, 2435-2447 (2018).
  26. Pluthero, F.G., Di Paola, J., Carcao, M.D. & Kahr, W.H.A. Platelets 29, 632-635 (2018).
  27. Kahr, W.H. et al. Blood 122, 3349-58 (2013).
  28. Lu, C.Y. et al. J Thromb Haemost (2024).
  29. Fabbro, S. et al. Blood 117, 3430-4 (2011).



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