Cytotoxic lymphocytes protect an organism against viral pathogens and cancer by killing infected and transformed cells, through perforin-mediated mechanism. Mutations in perforin (PRF1) itself or in the secretory machinery responsible for its release (UNC13D, STX11, and STXBP2) are catastrophic, and lead to fatal immune dysregulation, familial haemophagocytic lymphohistiocytosis (FHL).
Traditionally, FHL has been associated with infant patients. However, it is now apparent that many patients remain disease-free for years, and then present with highly variable and often unexpected symptoms. They remain undiagnosed for a long time and, instead of receiving curative stem cell transplantation, they are treated symptomatically leading to high risk of severe neurological impairment, organ failure and/or death.
While the pathogenicity of frame-shift/nonsense mutations is rarely in doubt, the effect of missense mutations on protein function can vary enormously. Yet, over the last two decades, the pathogenicity of missense mutations was almost invariably assumed, and invasive stem cell transplantation was considered without verified FHL diagnosis. Sadly, transplantation without genetically proven FHL results in a 40% increased mortality compared to patients with proven FHL. Therefore, early and accurate diagnosis of the disease is essential to determine the most appropriate treatment option.
Due to the diversity of genetic causes of FHL, there was no test available to directly assess the effect of mutations on cytotoxic lymphocyte function, leading to delayed/erroneous diagnoses.
To address this significant clinical problem, we developed a novel and rapid single-platform experimental approach for testing the function of missense mutations in all four genes associated with FHL. Not only did we uncover unique cryptic cases of FHL that enabled an accurate diagnosis/treatment of patients, but in some instances, we also demonstrated that previously reported mutations were unlikely to cause the disease.
In addition to diagnosing patients, our unique approach will be paramount for assessing the prognosis of asymptomatic siblings and to guide genetic counselling advice for prospective parents.