RESUMEN
Autoimmune lymphoproliferative syndrome (ALPS) is a rare genetic disorder featuring chronic lymphadenopathy, splenomegaly, cytopenias, and increased lymphoma risk. Differentiating ALPS from immunodeficiencies with overlapping symptoms is challenging. This study evaluated the performance and the diagnostic yield of a 15-gene NGS panel for ALPS at Cincinnati Children's Hospital Medical Center. Samples from 802 patients submitted for ALPS NGS panel were studied between May 2014 and January 2023. A total of 62 patients (7.7%) had a definite diagnosis: 52/62 cases (84%) showed 37 unique pathogenic/likely pathogenic germline FAS variants supporting ALPS diagnosis (6.5%, 52/802). The ALPS diagnostic yield increased to 30% in patients who additionally fulfilled abnormal ALPS immunology findings criteria. 17/37 (46%) diagnostic FAS variants were novel variants reported for the first time in ALPS. 10/802 cases (1.2%) showed diagnostic findings in five genes (ADA2, CTLA4, KRAS, MAGT1, NRAS) which are related to autoimmune lymphoproliferative immunodeficiency (ALPID). Family studies enabled the reclassification of variants of unknown significance (VUS) and also the identification of at-risk family members of FAS-positive patients, which helped in the follow-up diagnosis and treatment. Alongside family studies, complete clinical phenotypes and abnormal ALPS immunology and Fas-mediated apoptosis results helped clarify uncertain genetic findings. This study describes the largest cohort of genetic testing for suspected ALPS in North America and highlights the effectiveness of the ALPS NGS panel in distinguishing ALPS from non-ALPS immunodeficiencies. More comprehensive assessment from exome or genome sequencing could be considered for undefined ALPS-U patients or non-ALPS immunodeficiencies after weighing cost, completeness, and timeliness of different genetic testing options.
Asunto(s)
Síndrome Linfoproliferativo Autoinmune , Pruebas Genéticas , Humanos , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/genética , Pruebas Genéticas/métodos , Femenino , Masculino , Niño , Preescolar , Lactante , Adolescente , Receptor fas/genética , Secuenciación de Nucleótidos de Alto Rendimiento , Predisposición Genética a la Enfermedad , Hospitales Pediátricos , Mutación/genéticaAsunto(s)
Mutación , Humanos , Linfoma de Burkitt/genética , Linfoma de Burkitt/patología , Masculino , Infecciones por Virus de Epstein-Barr/genética , Proteínas Tirosina Quinasas/genética , Linfadenopatía/genética , Linfadenopatía/patología , Síndromes de Inmunodeficiencia/genética , Síndrome Linfoproliferativo Autoinmune/genética , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Femenino , Trastornos Linfoproliferativos/genética , Trastornos Linfoproliferativos/patología , Enfermedades de Inmunodeficiencia Primaria/genéticaRESUMEN
Autoimmune lymphoproliferative syndrome (ALPS) is a primary disorder of lymphocyte homeostasis, leading to chronic lymphoproliferation, autoimmune cytopenia, and increased risk of lymphoma. The genetic landscape of ALPS includes mutations in FAS, FASLG, and FADD, all associated with apoptosis deficiency, while the role of CASP10 defect in the disease remains debated. In this study, we aimed to assess the impact of CASP10 variants on ALPS pathogenesis. We benefit from thousands of genetic analysis datasets performed in our Institute's genetic platform to identify individuals carrying CASP10 variants previously suspected to be involved in ALPS outcome: p.C401LfsX15, p.V410I and p.Y446C, both at heterozygous and homozygous state. Clinical and laboratory features of the six included subjects were variable but not consistent with ALPS. Two individuals were healthy. Comprehensive analyses of CASP10 protein expression and FAS-mediated apoptosis were conducted and compared to healthy controls and ALPS patients with FAS mutations. Missense CASP10 variants (p.V410I and p.Y446C), which are common in the general population, did not disrupt CASP10 expression, nor FAS-mediated apoptosis. In contrast, homozygous p.C401LfsX15 CASP10 variant lead to a complete abolished CASP10 expression but had no impact on FAS-mediated apoptosis function. At heterozygous state, this p.C401LfsX15 variant lead to a reduced CASP10 protein levels but remained associated with a normal FAS-mediated apoptosis function. These findings demonstrate that CASPASE 10 is dispensable for FAS-mediated apoptosis. In consequences, CASP10 defect unlikely contribute to ALPS pathogenesis, since they did not result in an impairment of FAS-mediated apoptosis nor in clinical features of ALPS in human. Moreover, the absence of FAS expression up-regulation in subjects with CASP10 variants rule out any compensatory mechanisms possibly involved in the normal apoptosis function observed. In conclusion, this study challenges the notion that CASP10 variants contribute to the development of ALPS.
Asunto(s)
Apoptosis , Síndrome Linfoproliferativo Autoinmune , Caspasa 10 , Mutación , Receptor fas , Humanos , Caspasa 10/genética , Caspasa 10/metabolismo , Síndrome Linfoproliferativo Autoinmune/genética , Masculino , Femenino , Mutación/genética , Apoptosis/genética , Receptor fas/genética , Receptor fas/metabolismo , Adulto , Niño , Adolescente , Persona de Mediana EdadRESUMEN
Multiple myeloma is a rare disease in pediatrics, where about 30 cases are described under 15 years old. It is even rarer when atypical multiple myeloma occurs in the context of autoimmunity. This case describes a 9-year-old female with autoimmune lymphoproliferative-like disease and combined immune deficiency that developed acute kidney failure with monoclonal peak associated with RAC2 and TNFRSF9 variants. An adapted protocol from the backbone adult multiple myeloma standard of care with the addition of an allogeneic hematopoietic stem cell transplant was used. The patient, now nearly a year posttransplant, shows 100% chimerism with no sign of relapse.
Asunto(s)
Trasplante de Células Madre Hematopoyéticas , Humanos , Femenino , Niño , Lesión Renal Aguda/etiología , Lesión Renal Aguda/patología , Mieloma Múltiple/complicaciones , Mieloma Múltiple/patología , Síndrome Linfoproliferativo Autoinmune/complicaciones , Síndrome Linfoproliferativo Autoinmune/patología , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/genéticaAsunto(s)
Enfermedades Autoinmunes , Síndrome Linfoproliferativo Autoinmune , Trastornos Linfoproliferativos , Humanos , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/genética , Linfocitos T , Mutación , Apoptosis , Trastornos Linfoproliferativos/diagnósticoRESUMEN
BACKGROUND: Lymphoproliferation and autoimmune cytopenias characterise autoimmune lymphoproliferative syndrome. Other conditions sharing these manifestations have been termed autoimmune lymphoproliferative syndrome-like diseases, although they are frequently more severe. The aim of this study was to define the genetic, clinical, and immunological features of these disorders to improve their diagnostic classification. METHODS: In this prospective cohort study, patients were referred to the Center for Chronic Immunodeficiency in Freiburg, Germany, between Jan 1, 2008 and March 5, 2022. We enrolled patients younger than 18 years with lymphoproliferation and autoimmune cytopenia, lymphoproliferation and at least one additional sign of an inborn error of immunity (SoIEI), bilineage autoimmune cytopenia, or autoimmune cytopenia and at least one additional SoIEI. Autoimmune lymphoproliferative syndrome biomarkers were determined in all patients. Sanger sequencing followed by in-depth genetic studies were recommended for patients with biomarkers indicative of autoimmune lymphoproliferative syndrome, while IEI panels, exome sequencing, or genome sequencing were recommended for patients without such biomarkers. Genetic analyses were done as decided by the treating physician. The study was registered on the German Clinical Trials Register, DRKS00011383, and is ongoing. FINDINGS: We recruited 431 children referred for autoimmune lymphoproliferative syndrome evaluation, of whom 236 (55%) were included on the basis of lymphoproliferation and autoimmune cytopenia, 148 (34%) on the basis of lymphoproliferation and another SoIEI, 33 (8%) on the basis of autoimmune bicytopenia, and 14 (3%) on the basis of autoimmune cytopenia and another SoIEI. Median age at diagnostic evaluation was 9·8 years (IQR 5·5-13·8), and the cohort comprised 279 (65%) boys and 152 (35%) girls. After biomarker and genetic assessments, autoimmune lymphoproliferative syndrome was diagnosed in 71 (16%) patients. Among the remaining 360 patients, 54 (15%) had mostly autosomal-dominant autoimmune lymphoproliferative immunodeficiencies (AD-ALPID), most commonly affecting JAK-STAT (26 patients), CTLA4-LRBA (14), PI3K (six), RAS (five), or NFκB (three) signalling. 19 (5%) patients had other IEIs, 17 (5%) had non-IEI diagnoses, 79 (22%) were unresolved despite extended genetics (ALPID-U), and 191 (53%) had insufficient genetic workup for diagnosis. 16 (10%) of 161 patients with a final diagnosis had somatic mutations. Alternative classification of patients fulfilling common variable immunodeficiency or Evans syndrome criteria did not increase the proportion of genetic diagnoses. INTERPRETATION: The ALPID phenotype defined in this study is enriched for patients with genetic diseases treatable with targeted therapies. The term ALPID might be useful to focus diagnostic and therapeutic efforts by triggering extended genetic analysis and consideration of targeted therapies, including in some children currently classified as having common variable immunodeficiency or Evans syndrome. FUNDING: Deutsche Forschungsgemeinschaft under Germany's Excellence Strategy. TRANSLATION: For the German translation of the abstract see Supplementary Materials section.
Asunto(s)
Anemia Hemolítica Autoinmune , Síndrome Linfoproliferativo Autoinmune , Inmunodeficiencia Variable Común , Trombocitopenia , Masculino , Femenino , Niño , Humanos , Preescolar , Adolescente , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/genética , Síndrome Linfoproliferativo Autoinmune/terapia , Estudios Prospectivos , Biomarcadores , Proteínas Adaptadoras Transductoras de Señales/genéticaRESUMEN
BACKGROUND: The autoimmune lymphoproliferative syndrome (ALPS) is a noninfectious and nonmalignant lymphoproliferative disease frequently associated with autoimmune cytopenia resulting from defective FAS signaling. We previously described germline monoallelic FAS (TNFRSF6) haploinsufficient mutations associated with somatic events, such as loss of heterozygosity on the second allele of FAS, as a cause of ALPS-FAS. These somatic events were identified by sequencing FAS in DNA from double-negative (DN) T cells, the pathognomonic T-cell subset in ALPS, in which the somatic events accumulated. OBJECTIVE: We sought to identify whether a somatic event affecting the FAS-associated death domain (FADD) gene could be related to the disease onset in 4 unrelated patients with ALPS carrying a germline monoallelic mutation of the FADD protein inherited from a healthy parent. METHODS: We sequenced FADD and performed array-based comparative genomic hybridization using DNA from sorted CD4+ or DN T cells. RESULTS: We found homozygous FADD mutations in the DN T cells from all 4 patients, which resulted from uniparental disomy. FADD deficiency caused by germline heterozygous FADD mutations associated with a somatic loss of heterozygosity was a phenocopy of ALPS-FAS without the more complex symptoms reported in patients with germline biallelic FADD mutations. CONCLUSIONS: The association of germline and somatic events affecting the FADD gene is a new genetic cause of ALPS.
Asunto(s)
Síndrome Linfoproliferativo Autoinmune , Proteína de Dominio de Muerte Asociada a Fas , Humanos , Apoptosis/genética , Enfermedades Autoinmunes/genética , Síndrome Linfoproliferativo Autoinmune/genética , Hibridación Genómica Comparativa , ADN , Receptor fas/genética , Proteína de Dominio de Muerte Asociada a Fas/genética , Proteína de Dominio de Muerte Asociada a Fas/metabolismo , Células Germinativas/patología , MutaciónRESUMEN
BACKGROUND: Elevated TCRαß+CD4-CD8- double-negative T cells (DNT) and serum biomarkers help identify FAS mutant patients with autoimmune lymphoproliferative syndrome (ALPS). However, in some patients with clinical features and biomarkers consistent with ALPS, germline or somatic FAS mutations cannot be identified on standard exon sequencing (ALPS-undetermined: ALPS-U). OBJECTIVE: We sought to explore whether complex genetic alterations in the FAS gene escaping standard sequencing or mutations in other FAS pathway-related genes could explain these cases. METHODS: Genetic analysis included whole FAS gene sequencing, copy number variation analysis, and sequencing of FAS cDNA and other FAS pathway-related genes. It was guided by FAS expression analysis on CD57+DNT, which can predict somatic loss of heterozygosity (sLOH). RESULTS: Nine of 16 patients with ALPS-U lacked FAS expression on CD57+DNT predicting heterozygous "loss-of-expression" FAS mutations plus acquired somatic second hits in the FAS gene, enriched in DNT. Indeed, 7 of 9 analyzed patients carried deep intronic mutations or large deletions in the FAS gene combined with sLOH detectable in DNT; 1 patient showed a FAS exon duplication. Three patients had reduced FAS expression, and 2 of them harbored mutations in the FAS promoter, which reduced FAS expression in reporter assays. Three of the 4 ALPS-U patients with normal FAS expression carried heterozygous FADD mutations with sLOH. CONCLUSION: A combination of serum biomarkers and DNT phenotyping is an accurate means to identify patients with ALPS who are missed by routine exome sequencing.
Asunto(s)
Síndrome Linfoproliferativo Autoinmune , Receptor fas , Humanos , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/genética , Biomarcadores , Variaciones en el Número de Copia de ADN , Secuenciación del Exoma , Receptor fas/genética , Proteína de Dominio de Muerte Asociada a Fas/genética , MutaciónRESUMEN
Chronic nonmalignant lymphoproliferation and autoimmune cytopenia are relevant manifestations of immunohematologic diseases of childhood. Their diagnostic classification is challenging but important for therapy. Autoimmune lymphoproliferative syndrome (ALPS) is a genetically defined inborn error of immunity combining these manifestations, but it can explain only a small proportion of cases. Diagnostic categories such as ALPS-like disease, common variable immunodeficiency, or Evans syndrome have therefore been used. Advances in genetics and increasing availablity of targeted therapies call for more therapy-oriented disease classification. Moreover, recent discoveries in the (re)analysis of genetic conditions affecting FAS signaling ask for a more precise definition of ALPS. In this review, we propose the term autoimmune lymphoproliferative immunodeficiencies for a disease phenotype that is enriched for patients with genetic diseases for which targeted therapies are available. For patients without a current molecular diagnosis, this term defines a subgroup of immune dysregulatory disorders for further studies. Within the concept of autoimmune lymphoproliferative immunodeficiencies, we propose a revision of the ALPS classification, restricting use of this term to conditions with clear evidence of perturbation of FAS signaling and resulting specific biologic and clinical consequences. This proposed approach to redefining ALPS and other lymphoproliferative conditions provides a framework for disease classification and diagnosis that is relevant for the many specialists confronted with these diseases.
Asunto(s)
Anemia Hemolítica Autoinmune , Enfermedades Autoinmunes , Síndrome Linfoproliferativo Autoinmune , Inmunodeficiencia Variable Común , Enfermedades del Sistema Inmune , Trastornos Linfoproliferativos , Humanos , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/genética , Síndrome Linfoproliferativo Autoinmune/terapia , Fenotipo , Receptor fas/genética , Trastornos Linfoproliferativos/diagnóstico , Trastornos Linfoproliferativos/genética , Trastornos Linfoproliferativos/terapiaRESUMEN
PURPOSE: Protein kinase C δ (PKCδ) deficiency is a rare genetic disorder identified as a monogenic cause of systemic lupus erythematosus in 2013. Since the first cases were described, the phenotype has expanded to include children presenting with autoimmune lymphoproliferative syndrome-related syndromes and infection susceptibility similar to chronic granulomatous disease or combined immunodeficiency. We review the current published data regarding the pathophysiology, clinical presentation, investigation and management of PKCδ deficiency. METHODS: Literature review was performed using MEDLINE. RESULTS: Twenty cases have been described in the literature with significant heterogeneity. CONCLUSION: The variation in clinical presentation delineates the broad and critical role of PKCδ in immune tolerance and effector functions against pathogens.
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Síndrome Linfoproliferativo Autoinmune , Lupus Eritematoso Sistémico , Niño , Humanos , Proteína Quinasa C-delta/genética , Lupus Eritematoso Sistémico/diagnóstico , Lupus Eritematoso Sistémico/genética , Síndrome Linfoproliferativo Autoinmune/genética , Tolerancia Inmunológica , Variación Biológica PoblacionalRESUMEN
As a disorder of immune dysregulation, autoimmune lymphoproliferative syndrome (ALPS) stems from pathogenic variants in the first apoptosis signal-mediated apoptosis (Fas) and Fas-ligand pathway that result in elevations of CD3+ TCRαß+ CD4- CD8- T cells along with chronic lymphoproliferation, a heightened risk for malignancy, and importantly for the rheumatologist, increased risk of autoimmunity. While immune cytopenias are the most encountered autoimmune phenomena, there is increasing appreciation for ocular, musculoskeletal, pulmonary and renal inflammatory manifestations similar to more common rheumatology diseases. Additionally, ALPS-like conditions that share similar clinical features and opportunities for targeted therapy are increasingly recognized via genetic testing, highlighting the need for rheumatologists to be facile in the recognition and diagnosis of this spectrum of disorders. This review will focus on clinical and laboratory features of both ALPS and ALPS-like disorders with the intent to provide a framework for rheumatologists to understand the pathophysiologic drivers and discriminate between diagnoses.
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Enfermedades Autoinmunes , Síndrome Linfoproliferativo Autoinmune , Neoplasias , Humanos , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/genética , Receptor fas/genética , AutoinmunidadRESUMEN
Autoimmune Lymphoproliferative Syndrome (ALPS) is an autoimmune disease that has been reported in over 2200 patients. It is a rare, genetic disease where pathogenic variants occur in the extrinsic pathway of apoptosis. Various mutations in different genes, such as FAS, FASL, and CASP10, can result in ALPS. Most commonly, pathogenic variants occur in the FAS receptor. This malfunctioning pathway allows for the abnormal accumulation of lymphocytes, namely CD3 + TCRαß+CD4 - CD8- (double negative (DN) T) cells, which are a hallmark of the disease. This disease usually presents in childhood with lymphadenopathy and splenomegaly as a result of lymphoproliferation. Over time, these patients may develop cytopenias or lymphomas because of irregularities in the immune system. Current treatments include glucocorticoids, mycophenolate mofetil, sirolimus, immunoglobulin G, and rituximab. These medications serve to manage the symptoms and there are no standardized recommendations for the management of ALPS. The only curative therapy is a bone marrow transplant, but this is rarely done because of the complications. This review serves to broaden the understanding of ALPS by discussing the mechanism of immune dysregulation, how the symptoms manifest, and the mechanisms of treatment. Additionally, we discuss the epidemiology, comorbidities, and medications relating to ALPS patients across the United States using data from Cosmos.
Asunto(s)
Enfermedades Autoinmunes , Síndrome Linfoproliferativo Autoinmune , Trastornos Linfoproliferativos , Humanos , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/genética , Síndrome Linfoproliferativo Autoinmune/terapia , Enfermedades Autoinmunes/tratamiento farmacológico , Receptor fas/genética , Receptor fas/uso terapéutico , Esplenomegalia/tratamiento farmacológico , Esplenomegalia/genética , Esplenomegalia/patología , Mutación , Sirolimus/uso terapéutico , Trastornos Linfoproliferativos/genética , Trastornos Linfoproliferativos/patologíaRESUMEN
Autoimmune lymphoproliferative syndrome (ALPS) is a disease of lymphocyte homeostasis caused by FAS-mediated apoptotic pathway dysfunction and is characterized by non-malignant lymphoproliferation with an increased number of TCRαß+CD4-CD8- double-negative T cells (αßDNTs). Conversely, RAS-associated leukoproliferative disease (RALD), which is caused by gain-of-functional somatic variants in KRAS or NRAS, is considered a group of diseases with a similar course. Herein, we present a 7-year-old Japanese female of RALD harboring NRAS variant that aggressively progressed to juvenile myelomonocytic leukemia (JMML) with increased αßDNTs. She eventually underwent hematopoietic cell transplantation due to acute respiratory distress which was caused by pulmonary infiltration of JMML blasts. In general, αßDNTs have been remarkably increased in ALPS; however, FAS pathway gene abnormalities were not observed in this case. This case with RALD had repeated shock/pre-shock episodes as the condition progressed. This shock was thought to be caused by the presence of a high number of αßDNTs. The αßDNTs observed in this case revealed high CCR4, CCR6, and CD45RO expressions, which were similar to Th17. These increased Th17-like αßDNTs have triggered the inflammation, resulting in the pathogenesis of shock, because Th17 secretes pro-inflammatory cytokines such as interleukin (IL)-17A and granulocyte-macrophage colony-stimulating factor. The presence of IL-17A-secreting αßDNTs has been reported in systemic lupus erythematosus (SLE) and Sjögren's syndrome. The present case is complicated with SLE, suggesting the involvement of Th17-like αßDNTs in the disease pathogenesis. Examining the characteristics of αßDNTs in RALD, JMML, and ALPS may reveal the pathologies in these cases.
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Síndrome Linfoproliferativo Autoinmune , Lupus Eritematoso Sistémico , Trastornos Linfoproliferativos , Femenino , Humanos , Niño , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/genética , Linfocitos T CD4-Positivos , Receptores de Antígenos de Linfocitos T alfa-beta/genéticaAsunto(s)
Enfermedades Autoinmunes , Síndrome Linfoproliferativo Autoinmune , Trastornos Linfoproliferativos , Humanos , Niño , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/tratamiento farmacológico , Síndrome Linfoproliferativo Autoinmune/genética , Sirolimus/efectos adversos , Receptor fas/genética , Trastornos Linfoproliferativos/diagnóstico , Trastornos Linfoproliferativos/tratamiento farmacológico , ApoptosisAsunto(s)
Enfermedades Autoinmunes , Síndrome Linfoproliferativo Autoinmune , Humanos , Síndrome Linfoproliferativo Autoinmune/genética , Enfermedades Autoinmunes/genética , Fenotipo , Mutación , Receptor fas/genética , Apoptosis/genética , Proteína de Dominio de Muerte Asociada a Fas/genética , Proteína de Dominio de Muerte Asociada a Fas/metabolismoAsunto(s)
Síndrome Linfoproliferativo Autoinmune , Linfohistiocitosis Hemofagocítica , Humanos , Síndrome Linfoproliferativo Autoinmune/complicaciones , Síndrome Linfoproliferativo Autoinmune/diagnóstico , Síndrome Linfoproliferativo Autoinmune/genética , Linfohistiocitosis Hemofagocítica/diagnóstico , Caspasa 10RESUMEN
BACKGROUND: 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. OBJECTIVE: We revisited the significance of heterozygous FASLG mutations as a cause of ALPS. METHODS: 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. RESULTS: Homozygous FASLG variants abrogated cytotoxicity and resulted in early-onset severe ALPS with elevated DNT, raised vitamin B12, 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 B12, 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 B12 and DNT elevation. CONCLUSION: Heterozygous loss-of-function mutations are better tolerated for FASLG than for FAS, which may explain the low frequency of ALPS-FASLG.