Identifying modifier genes in Autoimmune Lymphoproliferative Syndrome

Abstract Autoimmune Lymphoproliferative Syndrome (ALPS) is an inherited disorder of defective lymphocyte apoptosis, characterized by chronic lymphadenopathy and/or splenomegaly, autoimmune cytopenias, and the expansion of a unique population of CD4-CD8- α/β T cells. ALPS is predominately caused by mutations in Fas with incomplete penetrance. In order to identify modifier genes and understand the mechanism of incomplete penetrance of disease in ALPS, we performed a linkage study using five large families with multiple ALPS patients and healthy Fas mutation-positive relatives (HMPRs) in each family. We identified a putative modifier locus shared by two of the families and a few potential linkage intervals shared by two of the other families. To screen large numbers of family members for an apoptosis defect, we developed a fast assay for testing lymphocyte Fas-mediated apoptosis susceptibility ex vivo, which we have named the FasT Kill assay. The conventional in vitro apoptosis assay takes over a week to obtain results whereas our assay using multi-parameter flow cytometry to specifically analyze apoptosis of the Fas-sensitive effector memory T cells in PBMCs can yield a diagnosis within a single workday. Using the FasT Kill assay, we could rapidly identify patients with apoptosis defects for the diagnosis of ALPS and compare the severity of apoptosis defects among ALPS patients or HMPRs. The results of these genetic analyses will be discussed.