Autoimmune diseases develop in about 5% of human beings and arise with a delayed stochastic penetrance when lymphocytes directed towards self components, such as chromatin in lupus or insulin-producing cells in diabetes, are generated. This occurs when self-tolerance checkpoints of the immune system are bypassed as a consequence of inherited mutations of key genes involved in activation, survival or death of lymphocytes.
The autoimmune lymphoproliferative syndrome (ALPS) is a model of self-tolerance checkpoints defects as autoimmune manifestations occur in 2/3 of ALPS patients. Mutations in the TNFRSF6 gene (encoding FAS, also known as the CD95 receptor) is the most common cause of ALPS. Patients carrying a homozygous TNFRSF6 mutation have an early-onset, severe phenotype, whereas subjects with heterozygous TNFRSF6 mutations present ALPS of variable intensity. T cells from patients with homozygous or heterozygous TNFRSF6 mutations respectively exhibit a complete or partial functional impairment in an in vitro FAS-induced apoptosis assay. Moreover, we recently described somatic heterozygous mutations in TNFRSF6 in patients with a clinical ALPS phenotype in which activated T cells showed normal sensitivity to FAS-induced apoptosis in vitro. The inability to detect an apoptosis defect was due to spontaneous in vitro apoptosis of the mutant T cells. However, more than 80% of these patients' DN T cells were mutated. These patients displayed mosaic expression of a somatic TNFRSF6 mutation, which provided the affected cells a selective advantage and accounted for the observed lymphoproliferation and autoimmunity.
Heterozygous germline TNFRSF6 mutations are not always associated with clinical expression (partial clinical penetrance) since some mutation-positive relatives (MPRs) remain asymptomatic despite an in vitro FAS-induced apoptosis impairment on their T cells. This observation led us to postulate that a second event is necessary for disease expression in patients with TNFRSF6 mutations when an incomplete penetrance is observed.
We analyzed double-negative (DN) T lymphocytes from seven ALPS patients with low penetrance mutations for FAS expression and defects in the second TNFRSF6 allele and compared with data from healthy, mutation-positive relatives and controls. Missense mutations (in three patients) and loss of heterozygosity (LOH, in four patients) were found in the second TNFRSF6 allele. The LOH observed in 4 of our patients with germline TNFRSF6 ECD mutations resulted from loss of the wild-type allele and concomitant duplication of the mutant one, i.e. telomeric UPD. This type of mechanism is usually observed in genetic disorders of development, where it can be involved in trisomy or monosomy rescue during embryonic development. In 6 patients, alteration or loss of the second TNFRSF6 allele resulted in the absence of FAS cell surface expression on DN T lymphocytes.
In these seven patients the disease develop as a consequence of somatic genetic events of TNFRSF6 in a subpopulation of lymphocytes combined with inherited heterozygous mutations of the same gene. This finding is the first demonstration that combined germline and somatic mutations of the same gene is leading to a non-malignant autoimmune disease in humans.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.