Genetic and cellular studies of familial hemophagocytic lymphohistiocytosis
Abstract: Familial hemophagocytic lymphohistiocytosis (FHL) is a rare autosomal recessive and genetically heterogeneous disorder of immune dysregulation with an incidence of 1/50000 live births that is inevitably fatal without appropriate treatment. The disease is characterized by fever, hepatosplenomegaly, cytopenias, hyperferritinemia, hypertriglyceridemia, hypofibrinogenemia and, sometimes, hemophagocytosis in bone marrow and/or other organs such as liver, spleen or lymph nodes. Three genes have so far been linked to the disease: PRF1, UNC13D and STX11. In this thesis the mutational spectrum and clinical implications of UNC13D and STX11 mutations in a well characterized cohort of patients were studied. Moreover, functional cellular studies with focus on natural killer (NK) cell activity and cytotoxic lymphocyte degranulation was performed in patients with mutations affecting these three genes. In addition, genotype-phenotype correlations in a large cohort of patients was studied. The frequency of bi-allelic STX11 mutations in our cohort of PRF1-negative families was 14%. Some affected patients had a remarkably less severe disease course than most FHL patients, including long periods of remission without therapy. However, a few patients developed secondary MDS/AML. Although this could be attributed to the treatment provided including etoposide, it is also possible that mutations affecting cytotoxic functions may affect the surveillance of transformed cells (paper I). The localization and function of the protein syntaxin-11 encoded by the gene STX11 was previously unknown. We report that the protein is expressed in cytotoxic T cells as well as NK cells, and that NK cells from patients with biallelic STX11 mutations fail to degranulate when encountering susceptible target cells. The same pattern is seen in patients carrying UNC13D mutations whereas patients carrying PRF1 mutations have a normal degranulation pattern. Notably, when stimulated with IL-2, syntaxin-11 deficient cells regained their cytotoxic capacity and this was also observed in a patient carrying a bi-allelic UNC13D mutation (paper II). We identified six different UNC13D mutations affecting altogether 9/38 individuals (24%) in 6/34 (18%) unrelated PRF1/STX11-negative families. Four novel mutations were revealed. The age at onset varied from birth to 14 years in the patients carrying bi-allelic UNC13D mutations, high-lighting that FHL should be considered not only in infants but also in adolescents, and possibly young adults, presenting with fever, splenomegaly, cytopenia, hyperferritinemia, and/or CNS symptoms (paper III). Since hemophagocytic lymphohistiocytosis (HLH) is a heterogeneous disease with regard to genotype and phenotype, we studied 76 patients with HLH in order to search for genotype-phenotype correlations. Patients carrying PRF1 mutations had significantly higher risk of early onset (age <6 months) compared with patients carrying mutations in STX11 [adjusted odds ratio 8.23 (95% CI=1.20-56.40), p=0.035]. Moreover, patients with STX11 mutations had a decreased risk of pathological CSF at diagnosis compared to patients without any known biallelic mutation [adjusted odds ratio 26.37 (95% CI=1.90-366.81), p=0.015] (paper IV).
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