Revisiting autoimmune lymphoproliferative syndrome caused by Fas ligand mutations.

Fas ligand (FasL) is expressed by activated T cells and induces death in target cells upon binding to Fas. Loss-of-function FAS or FASLG mutations cause autoimmune-lymphoproliferative syndrome (ALPS) characterized by expanded double-negative T cells (DNT) and elevated serum biomarkers. While most ALPS patients carry heterozygous FAS mutations, FASLG mutations are rare and usually biallelic. Only 2 heterozygous variants were reported, associated with an atypical clinical phenotype. We revisited the significance of heterozygous FASLG mutations as a cause of ALPS. Clinical features and biomarkers were analyzed in 24 individuals with homozygous or heterozygous FASLG variants predicted to be deleterious. Cytotoxicity assays were performed with patient T cells and biochemical assays with recombinant FasL. Homozygous FASLG variants abrogated cytotoxicity and resulted in early-onset severe ALPS with elevated DNT, raised vitamin B <sub>12</sub> , and usually no soluble FasL. In contrast, heterozygous variants affected FasL function by reducing expression, impairing trimerization, or preventing Fas binding. However, they were not associated with elevated DNT and vitamin B <sub>12</sub> , and they did not affect FasL-mediated cytotoxicity. The dominant-negative effects of previously published variants could not be confirmed. Even Y166C, causing loss of Fas binding with a dominant-negative effect in biochemical assays, did not impair cellular cytotoxicity or cause vitamin B <sub>12</sub> and DNT elevation. Heterozygous loss-of-function mutations are better tolerated for FASLG than for FAS, which may explain the low frequency of ALPS-FASLG