Genetic studies of familial hemophagocytic lymphohistiocytosis

Abstract

The main contribution from this thesis is a refined knowledge of locus heterogeneity in familial hernophagocytic lympbohistiocytosis (FHL) and a further characterization of corresponding clinical and immunological phenotypes.

Familial hemophagocytic lymphohistiocytosis is a fatal autosomal recessive immune deficiency clinically characterized by fever, cytopenia, hepatosplenomegaly, and hemophagocytosis. A common feature of the pathophysiology is a defect in cytotoxic T lymphocyte (CTL) and natural killer (NK) cell cytotoxicity. Two different genes have been identified in FHL patients; the PRF1 gene and the MUNC13-4 gene. Perforin is a constituent of cytotoxic granules in CTLs and NK cells, released upon activation to cooperate with granzyme B (GZMB) in the induction of apoptosis in target cells. MUNC 13-4 functions in the same pathway by inducing degranulation of cytotoxic granules.

In Paper I the aim was to investigate the mutational spectrum and to elucidate the relative contribution of PRF1 gene mutations in FHL. Results from direct DNA sequencing and linkage analysis for homozygosity in consanguineous families suggests that in no more than 35-40% of the FHL patients the disease can be explained by mutations in PRF1 gene in our material. Since these results intimate locus heterogeneity we investigated the presence of mutations in the GZMB and granulysin (GNLY) genes (Paper Il). None of the sequence variations disclosed associated with the disease, and thus we did not find any support for genetic defects in these genes explaining FHL.

In order to characterize lymphocyte populations and PRF expression in FHL flow cytometry was used (Paper III). We observed diminished or reduced PRF expression correlating to mutations in the PRF1 gene. An interesting finding was a significant reduction in expression of circulating CD19+ B lymphocytes. A comprehensive study of genotype-phenotype correlations in large cohort of HLH patients revealed that PRF1 gene mutations were more frequent in the Middle East and Turkish populations (Paper IV). In addition, lymphadenopathy, jaundice and edema were found to be less frequent at presentation in patients without PRF1 gene mutations in the Nordic population compared to the non-Nordic population.