As part of an international collaborative effort, scientists in Finland, Norway, Sweden, and Austria have discovered the origins of a disease characterized by immune dysregulation called ‘familial hemophagocytic lymphohistiocytosis’ (HLH). Published in the high-ranked scientific journal Blood, the study provides a basis for both a deeper understanding of the biology of HLH and the exploration of new therapeutic approaches.
Occurring usually in early childhood, familial or genetically-determined HLH is one of the most dramatic hematologic disorders. It is characterized by the inability of specific immune cells, namely T lymphocytes and natural killer (NK) cells, to kill an infected (e.g., virus-infected) target cell. As a consequence, the body secretes biological messengers, so-called ‘cytokines’, that generate massive immune activation and hyperinflammation throughout the entire body. “If untreated, the hyperinflammation associated with HLH can be lethal in a short period of time”, says Kaan Boztug, MD, Scientific Director of St. Anna CCRI and senior author of the study.
Until recently, four subtypes of familial HLH had been known, caused by mutations in genes involved in regulating the immune defense. “Now we have identified a new subtype of this disease, caused by inherited mutations in a novel disease-causing gene that encodes the protein RhoG”, comments Janna Saarela MD, co-senior author of the study and Director of the Centre for Molecular Medicine Norway (NCMM) and Research Director at Institute for Molecular Medicine Finland, FIMM, University of Helsinki. The researchers show how deficiency of RhoG specifically impairs the cell-killing function of T lymphocytes and NK cells. This results in their uncontrolled activation and ultimately causes HLH.
In particular, RhoG deficiency impairs the process of exocytosis in specific immune cells and disables their killing ability. Immune cells like T and NK cells use exocytosis to release cytotoxic molecules to attack and kill infected or tumor cells. When RhoG deficiency stops this function in immune cells, they cannot kill their target cells as intended.
RhoG regulates lymphocyte cytotoxicity
In their study, the scientists investigated a Finnish patient, an infant who developed severe HLH at the age of four months. While the disease was associated with impaired cytotoxicity of T and NK cells, no mutations were found in known HLH-associated genes. Further genetic analysis conducted in/at? the Institute for Molecular Medicine Finland (FIMM), University of Helsinki, revealed deleterious mutations in the gene encoding RhoG. By experimental ablation of RhoG, the scientists confirmed the previously unknown role of RhoG in the cytotoxic function of human lymphocytes. RhoG interacts with an exocytosis protein called Munc13-4 and is essential for the anchoring of cytotoxic granules to the plasma membrane. This docking is a critical step in exocytosis. It is required for further fusion of the vesicles with the plasma membrane and the release of the cytotoxic granules. Despite a drastic and specific effect on cytotoxic function, RhoG deficiency does not affect other functions of immune cells that play an important role for the disease development.
More accurate diagnosis process for patients
Based on the understanding of the underlying molecular mechanism of familial HLH, the researchers are looking forward to an improved prognosis and targeted treatment of the disease in the long-term. As a short-term consequence, the now discovered RhoG deficiency can help HLH patients by enabling a genetic diagnosis.
“Tight connections between clinical diagnostics and research helps defining molecular diagnosis for rare disease patients suffering from yet unidentified disorders. It also provides opportunity for discovery of novel disease genes and understanding their pathomechanisms, which may impact disease management and prognosis”, comments Professor Saarela.
This study is an exciting breakthrough that brings up new important scientific questions. “The discovery of RhoG deficiency has opened up new insights into the molecular functions of this protein and revealed highly relevant questions. We have found that RhoG regulates both, the ‘cell skeleton’ and the exocytosis machinery. Now we are very keen to know how RhoG coordinates their activity in space and time”, says Dr. Artem Kalinichenko, PhD, Senior Postdoctoral researcher at St. Anna CCRI and Ludwig Boltzmann Institute for Rare and Undiagnosed Diseases (LBI-RUD).
Collaborative research on rare diseases
This scientific work was possible thanks to a collaboration between St. Anna CCRI with the LBI-RUD, the CeMM Research Center for Molecular Medicine of the Austrian Academy of Sciences, the Medical University of Vienna, Karolinska Institute, and the two Nordic EMBL partnership institutes for molecular medicine, the Institute for Molecular Medicine Finland, FIMM, University of Helsinki, and the Centre for Molecular Medicine Norway, NCMM, University of Oslo. Special thanks go to the co-senior authors Kaan Boztug, St. Anna CCRI, Yenan T. Bryceson (Karolinska Institute, Stockholm, Sweden) and Mikko R.J. Seppänen (Rare Diseases Center, Children’s Hospital, University of Helsinki, Finland) as well as the clinical doctors responsible for diagnosis and treatment of the patient, Terhi Tapiainen (Oulu University Hospital), Mervi Taskinen and Kim Wettenranta (Helsinki University Hospital).
Prof. Saarela is the Director of the Centre for Molecular Medicine Norway (NCMM). Her research group is shared between NCMM and the Institute of Molecular Medicine Finland (FIMM), where she was previously assistant director and head of the FIMM Technology Centre. FIMM and NCMM share many similarities in terms of research expertise and interests. The Saarela group’s research focuses on understanding the biological pathways and pathogenic mechanisms behind rare and common immune diseases.
Original publication: RhoG deficiency abrogates cytotoxicity of human lymphocytes and causes hemophagocytic lymphohistiocytosis. Artem Kalinichenko, Giovanna Perinetti Casoni*, Loic Dupre*, Luca C. Trotta*, Jakob Huemer, Donatella Galgano, Yolla German, Ben Haladik, Julia Pazmandi, Marini Thian, Özlem Yüce Petronczki, Samuel C.C Chiang, Mervi H Taskinen, Anne Hekkala, Saila Kauppila, Outi Lindgren, Terhi Tapiainen, Michael J. Kraakman, Kim Vettenranta, Alexis J. Lomakin, Janna Saarela§ , Mikko R J Seppänen§, Yenan T Bryceson§, Kaan Boztug§‡. * these authors contributed equally, § these authors contributed equally. Blood. 2021 Apr 15;137(15):2033-2045. doi: 10.1182/blood.2020008738.
Funding: This work was supported by European Research Council through an ERC Consolidator Grant “iDysChart” (Kaan Boztug) and the Vienna Science and technology Fund (WWTF) through project LS14-031 (Kaan Boztug); Austrian Academy of Science (ÖAW) through DOC fellowship program 25365 (Jakob Huemer) and 25225 (Marini Thian); Finnish Foundation for Pediatric Research and Pediatric Research Center, Helsinki University Hospital (Mikko RJ. Seppänen), and Swedish Research Council, Cancer Foundation, Children's Cancer Foundation, and Knut and Alice Wallenberg Foundation to YenanT. Bryceson.
Research Director Janna Saarela
Institute for Molecular Medicine Finland, HiLIFE, University of Helsinki