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Monogenic primary immunodeficiencies reveal new concepts of immune regulation

Rapid development of next generation sequencing (NGS) technologies and decreased costs of the whole exome and genome sequencing coupled with modern biochemistry and cellular biology methodology, have made identification of novel disease-causing gene mutations routine. Primary immunodeficiency diseases (PIDDs) are a clinically heterogeneous group of clinical entities caused by mutations in the genes which control the immune responses. Most of the PIDDs are monogenic rare disorders that follow Mendelian inheritance. (Read more)

If the PIDDs are so rare, why would we want to investigate them? The answer is simple – because we want to help our patients. Even though being rare, PIDDs are causing significant mortality and morbidity. Finding the molecular cause of the disease enables early protective interventions, such as IvIg therapy, genetic counselling for the family, and importantly, reveals potential targets for the development of the curative gene therapies for these diseases.

Moreover, PIDDs are often associated with autoimmune or autoinflammatory conditions and even increased risk for hematological malignancies, thus patients with monogenic PIDD offer unique monogenic natural human knock-down/knock-in models to study the role of the target gene in these much more common diseases.

But what is most fascinating from the view of an immunologist: monogenic PIDDs can teach us new insights how the human immune system works and lead to discoveries of completely novel immune regulators. An example of such unexpected PIDD gene is MAGT1, a magnesium transporter, in which loss of function mutations cause X-linked immunodeficiency (X-linked magnesium defect, Epstein-Barr virus infection, and neoplasia; or XMEN). This disease was discovered in my former postdoc laboratory at National Institutes of Health in the USA. MAGT1 gene mutations were originally found from two brothers with clinical history of repeated respiratory and gastrointestinal infections together with abnormally high Ebstein-Barr virus levels and led to discovery that MAGT1 and free intracellular magnesium plays essential role in T cell receptor signalling and defence against EBV infection by NK cells (Nature 2011;475(7357):471-476, Science 2013;341(6142):186-191).

Even though we might have the most state of the art NGS methodologies in use, it is obvious, especially in case of rare diseases, that genomics and bioinformatics alone are insufficient to conclusively associate the gene variant with the disease. Additional biochemical and molecular validation is required. The main questions to answer are: How does the mutation affect the protein biochemically? How does the given variant affect to the function of the cellular pathways in which the protein participates in? Do these alterations explain the cellular phenotype and clinical symptoms of the patient?

Currently available laboratory techniques including efficient cell isolation and cultivation, improved electroporation techniques to introduce DNA into cells, altering gene expression with interfering RNA, genomic editing with tools such as the clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system and single cell RNA sequencing techniques have made experimental studies with human cell lines and primary cells feasible. Expensive and laborious animal experiments with genetically altered mice are no longer an absolute requirement to study a human disease. My aim is to set up a research line into the Pediatric Research Center to functionally validate mutations found from PIDD patients. Although massive clinical genomics programs in many large centers around the world are searching for new PIDDs, Finland offers unique possibilities for PIDD research. The prevalence of PIDDs seems to be unexpectedly high in our population. According to recent report by Selenius and colleagues the prevalence of common variable immunodeficiency, CVID, is ~ 6.9/100 000 adult individuals. Finland has organized health care system, well documented and easy accessible patient records and usually easy access to patient samples. Furthermore, Finnish patients are usually willing to participate in medical research. And of course, we have a superb scientific community.

Although PIDD causing mutations have been identified in more than 350 genes, still many of the patients with clinical and laboratory evidence for PIDD do not have mutations in previously identified PIDD genes, and thus lack a specific diagnosis. Identification of the molecular cause of the disease has a great significance to the patients and their families. Moreover, identification of the causative PIDD mutations reveals new concepts of the molecular basis of human immune responses and potentially novel regulators of the immune system.