FADD-Related Immunodeficiency

Mendelian MONDO:0013408 Pathograph 25 Show in embeddings browser autosomal recessive disease primary immunodeficiency disease

FADD deficiency is an ultra-rare autosomal recessive disorder of the adaptor protein that nucleates the death-inducing signalling complex downstream of FAS. Because FADD sits at a branch point rather than on a single pathway, the disease is not simply an ALPS phenocopy: patients have the ALPS biomarker profile (impaired Fas-dependent lymphocyte apoptosis, expanded double-negative T cells) without the florid lymphoproliferation, and in addition carry features no FAS-pathway disorder produces — recurrent stereotyped febrile encephalopathic crises with refractory status epilepticus, hepatopathy, cardiac malformations, and a functional hyposplenism that drives invasive encapsulated-organism infection. The founding study attributed the bacterial infections to hyposplenism and the viral infections to impaired interferon immunity, making this one of the clearest human demonstrations that FADD has essential Fas-independent functions.

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1
Inheritance
8
Pathophys.
14
Phenotypes
2
Gaps
25
Pathograph
1
Genes
4
Medical Actions
1
Differentials
2
Models
12
References
1
Deep Research
🏷

Classifications

Harrison's Part
IMMUNE RHEUMATOLOGIC
👪

Inheritance

1
Autosomal recessive inheritance HP:0000007
FADD deficiency is autosomal recessive. The founding kindred was large, consanguineous and multiplex, homozygous for a missense allele; compound heterozygous patients have since been reported. A separate route to the phenotype exists in which a germline monoallelic FADD mutation is unmasked somatically — see the genetic section.
Autosomal recessive inheritance Expressivity: VARIABLE
Show evidence (2 references)
PMID:21109225 SUPPORT Human Clinical
"We investigated a large, consanguineous, multiplex kindred in which biological features of ALPS were found in the context of severe bacterial and viral disease, recurrent hepatopathy and encephalopathy, and cardiac malformations."
Establishes the consanguineous multiplex pedigree structure consistent with recessive inheritance, and enumerates the multisystem phenotype.
PMID:32350755 SUPPORT Human Clinical
"is an autosomal recessive disorder resulting from a pathogenic variation in F ADD(FAS-associated protein with death do- main), the adaptor protein involved in Fas signaling to caspases 8 and 10"
States the inheritance mode directly. The spacing in "F ADD" is how the text extracts from the cached PDF and is quoted as it appears.
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Discussions and Knowledge Gaps

2
Mouse Fadd deletion in epithelium causes RIPK3-MLKL necroptosis and caspase-8-gasdermin-D pyroptosis-like death, with colitis and skin inflammation. Human FADD-deficient patients do not have inflammatory bowel disease. Does the necroptosis-restraint function of FADD operate in human tissue, and if so why is it not clinically apparent?
HUMAN MODEL MISMATCH OPEN mismatch_mouse_necroptosis_arm
Attached to
pathophysiology#Loss of Fas-Independent FADD Functions
This is a genuine translational discrepancy rather than a gap in the human literature. The mouse work is unambiguous that FADD restrains two distinct lytic death programmes in epithelium, and lytic death is exactly the sort of mechanism that would be expected to produce inflammatory disease. Two explanations are compatible with what is known and are not distinguishable on current evidence: human disease alleles are hypomorphic rather than null, so enough FADD remains to restrain necroptosis while being insufficient for efficient DISC assembly; or the epithelial requirement genuinely differs between species. Which it is bears directly on whether the encephalopathic crises might themselves be a lytic-death phenomenon, and therefore on whether necroptosis inhibition would be a rational therapeutic direction.
Proposed experiments
Necroptosis competence in patient-derived cells
exp_necroptosis_competence_patient_cells
Challenge patient-derived cells carrying the reported hypomorphic alleles with TNF plus caspase inhibition and measure MLKL phosphorylation and lytic death against isogenic FADD-null and wild-type controls. Preserved restraint in patient cells would support the hypomorphic explanation; unrestrained necroptosis would make the species-difference explanation more likely and would raise the question of why the gut is spared.
Show evidence (2 references)
PMID:32362323 SUPPORT Model Organism
"Mice with IEC-specific FADD or caspase-8 deficiency developed colitis dependent on mixed lineage kinase-like (MLKL)-mediated epithelial cell necroptosis."
Establishes the murine phenotype whose human counterpart is missing, which is the mismatch itself.
PMID:25132550 SUPPORT Model Organism
"a RIPK1 kinase inactive knock-in delayed but did not prevent inflammation caused by FADD deficiency in IECs or keratinocytes"
Confirms the epithelial inflammatory consequence of Fadd loss in a second tissue and a second laboratory, so the mismatch is not a single-study artefact.
By what mechanism does febrile infection precipitate the stereotyped encephalopathic crises of FADD deficiency?
KNOWLEDGE GAP OPEN gap_febrile_encephalopathy_mechanism
Attached to
pathophysiology#Recurrent Febrile Encephalopathic Crises
The crises are the feature that most distinguishes this disorder from the ALPS spectrum and carry much of its mortality, yet they are the least mechanistically explained. The reporting authors attribute them to disrupted FAS-mediated apoptosis, but that leaves unexplained why the episodes are febrile-triggered, why they are stereotyped and recurrent rather than progressive, and why the brain in particular. That FADD variants can produce a picture meeting FIRES criteria suggests a neuroinflammatory rather than a purely apoptotic route, which would have direct therapeutic consequences — the Epilepsia authors argue on that basis for early immunomodulatory treatment.
Proposed experiments
Cytokine and cell-death profiling during and between crises
exp_crisis_neuroinflammatory_profiling
Profile CSF and paired serum cytokines, and markers of lytic cell death, in the same patient during a febrile encephalopathic crisis and at baseline. A crisis-specific inflammatory signature would support the neuroinflammatory route and the case for immunomodulation; its absence would push attention back towards a cell-autonomous neuronal death-signalling defect.
Show evidence (1 reference)
PMID:38752438 SUPPORT Human Clinical
"it demonstrates a genetic cause of FIRES involving a cell-mediated inflammation regulatory pathway. This finding supports early treatment with immunomodulatory therapy"
States the neuroinflammatory framing and the therapeutic inference drawn from it, which is the hypothesis the proposed experiment would test.
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Pathophysiology

8
Biallelic FADD Loss of Function
FADD is a small bipartite adaptor: a death effector domain that recruits procaspase-8 and -10, and a death domain that binds the death domain of a ligated receptor. It carries no catalytic activity of its own, so the disease is a failure of assembly rather than of enzymology. The reported disease alleles reduce steady-state FADD protein rather than abolishing it, which matters because complete Fadd loss is embryonic lethal in mice — surviving patients are therefore expected to retain residual function.
FADD hgnc:3573 HUGO Gene Nomenclature Committee (hgnc) Relation: this pathophysiological event involves this gene This pathophysiological event involves FADD (hgnc:3573). hgnc:3573 is a gene from the HUGO Gene Nomenclature Committee.
Downstream
Impaired Death-Receptor Apoptotic Signalling Without the adaptor, the death-inducing signalling complex cannot be assembled on a ligated receptor.
Loss of Fas-Independent FADD Functions The same loss of adaptor also removes FADD's functions outside the Fas pathway, which is what makes this disease more than an ALPS phenocopy.
Show evidence (2 references)
PMID:21109225 SUPPORT Human Clinical
"we identified a homozygous missense mutation in FADD, encoding the Fas-associated death domain protein (FADD), in the patients. This FADD mutation decreases steady-state protein levels and impairs Fas-dependent apoptosis in vitro"
Establishes the causal variant and its molecular consequence — reduced protein level rather than absence, which is the basis for the residual function claim.
PMID:32350755 SUPPORT Human Clinical
"the adaptor protein involved in Fas signaling to caspases 8 and 10"
States the adaptor role and the caspases recruited, which is the function lost at this node.
Impaired Death-Receptor Apoptotic Signalling
FADD is obligate for assembling the death-inducing signalling complex downstream of FAS and other death-domain receptors: its death domain engages the receptor, its death effector domain recruits procaspase-8 and -10, and caspase activation follows. Losing the adaptor breaks the chain at the nucleation step, so no amount of receptor ligation produces apoptosis.
extrinsic apoptotic signaling pathway via death domain receptors GO:0008625 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased extrinsic apoptotic signaling pathway via death domain receptors (GO:0008625). GO:0008625 is a biological process from the Gene Ontology. ↓ DECREASED extrinsic apoptotic signaling pathway GO:0097191 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased extrinsic apoptotic signaling pathway (GO:0097191). GO:0097191 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Failure of Activation-Induced Lymphocyte Apoptosis Activated lymphocytes that should be deleted by Fas ligation survive.
Show evidence (2 references)
PMID:21115735 SUPPORT Model Organism
"During apoptotic signaling induced by death receptors including Fas, FADD is required for the recruitment and activation of caspase 8."
States the obligate adaptor requirement that this node represents. Evidence source is MODEL_ORGANISM because the paper's experimental work is in mice.
PMID:21109225 SUPPORT Human Clinical
"impairs Fas-dependent apoptosis in vitro, accounting for biological ALPS phenotypes in vivo"
Connects the signalling defect to the ALPS-like laboratory phenotype observed in patients.
Failure of Activation-Induced Lymphocyte Apoptosis
Lymphocytes that fail to undergo activation-induced death accumulate, and the readout is the ALPS biomarker panel: expanded circulating double-negative T cells, raised soluble Fas ligand, interleukin-10 and vitamin B12. What is notable is what does not follow. Patients do not develop the massive lymphadenopathy, splenomegaly and autoimmune cytopenias that define clinical ALPS — the biology is shared, the clinical syndrome is not, and the cited source explicitly separates FADD deficiency from ALPS-FAS on that basis.
T cell CL:0000084 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves T cell (CL:0000084). CL:0000084 is a cell type from the Cell Ontology.
extrinsic apoptotic signaling pathway via death domain receptors GO:0008625 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased extrinsic apoptotic signaling pathway via death domain receptors (GO:0008625). GO:0008625 is a biological process from the Gene Ontology. ↓ DECREASED
Animal models
Fadd conditional knockout mouse Mouse partially recapitulates Haematopoietic Fadd deletion perturbs lymphocyte homeostasis, but in the opposite direction to the human disease: mice lose peripheral lymphocytes over time rather than accumulating an apoptosis-resistant population.
Downstream
Increased double-negative T cell number Lymphocytes that should have been deleted at the end of an immune response persist. The TCR-alpha/beta CD4-negative CD8-negative compartment is where that failure is visible, and it is the pathognomonic subset of the ALPS spectrum.
Increased circulating interleukin 10 concentration Raised interleukin 10 is part of the ALPS biomarker panel that accompanies the apoptosis defect. Typed as indirect and unknown deliberately: the panel is measured together and interpreted together, but no step between the failure of lymphocyte deletion and the cytokine rise has been demonstrated in FADD deficiency.
Elevated circulating vitamin B12 concentration The fourth member of the same panel, with the same caveat — a measured accompaniment of the apoptosis defect rather than a step traced to it.
Lymphadenopathy Accumulation of undeleted lymphocytes enlarges lymphoid tissue. In FADD deficiency this is milder than in ALPS-FAS and variable between patients, which is one of the observations that separates the two disorders.
Show evidence (1 reference)
PMID:32350755 SUPPORT Human Clinical
"Previously described FADD deficiency patients demonstrate a clinical phenotype partially overlap- ping with other ALPS disorders (decreased Fas-mediated lym- phocyte apoptosis, increased circulating double negative T cells, elevated soluble Fas ligand, IL-10, and vitamin B12)"
Enumerates the full biomarker panel this node produces. The hyphenation is how the text extracts from the cached PDF and is quoted as it appears.
Loss of Fas-Independent FADD Functions
The founding study reported that the patients' FADD mutation impairs Fas-independent signalling as well, and traced two of the disease's most consequential features to that arm rather than to apoptosis: the bacterial infections to functional hyposplenism, and the viral infections to impaired interferon immunity. This node is what separates FADD deficiency from the ALPS spectrum mechanistically, not just clinically.
type I interferon-mediated signaling pathway GO:0060337 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased type I interferon-mediated signaling pathway (GO:0060337). GO:0060337 is a biological process from the Gene Ontology. ↓ DECREASED
Animal models
Endothelial (Tie2-cre) Fadd conditional knockout mouse Mouse partially recapitulates Demonstrates that FADD restrains RIPK3 signalling in endothelium and that losing this causes a cardiovascular developmental defect — the class of Fas-independent function this node asserts, and a candidate route to the human cardiac malformation.
Downstream
Functional Hyposplenism How loss of FADD produces a hyposplenic state is not established; the association is reported, the mechanism is not.
Impaired Interferon Immunity The interferon defect is attributed to the Fas-independent arm.
Show evidence (2 references)
PMID:21109225 SUPPORT Human Clinical
"The observed bacterial infections result partly from functional hyposplenism, and viral infections result from impaired interferon immunity."
The direct attribution of the two infection phenotypes to non-apoptotic mechanisms, quoted with the authors' own hedge ("partly") on the bacterial arm.
PMID:21109225 SUPPORT Human Clinical
"Our findings highlight the key role of FADD in Fas-dependent and Fas-independent signaling pathways in humans."
Establishes the dual-arm framing that this entry's pathograph reproduces.
Functional Hyposplenism
The spleen is present but does not work. The practical consequence is the one that follows from any hyposplenic state — susceptibility to invasive infection with encapsulated organisms — and the practical marker is Howell-Jolly bodies on the blood film, which is why this is worth curating as its own node rather than folding into the immunodeficiency.
Downstream
Recurrent Severe Infection Hyposplenism is a direct route to invasive encapsulated-organism disease.
Howell-Jolly bodies Howell-Jolly bodies are nuclear remnants the spleen normally clears from circulating erythrocytes. Their appearance in a patient who has a spleen is the objective readout of splenic phagocyte failure, and it is what carries the ontology binding for a state HPO has no term for.
Recurrent bacterial infections Drawn from the hyposplenism node rather than from the aggregate infection node, because the founding report attributes the bacterial susceptibility specifically to loss of splenic clearance. Routing it through the shared node would have said both arms cause both infection types, which is not what the source says.
Show evidence (1 reference)
PMID:21109225 SUPPORT Human Clinical
"The observed bacterial infections result partly from functional hyposplenism"
Establishes the hyposplenic state and its role in the bacterial infections.
Impaired Interferon Immunity
The antiviral arm. FADD participates in innate antiviral signalling independently of its apoptotic role, and its loss leaves patients susceptible to severe viral disease alongside the bacterial susceptibility from hyposplenism.
type I interferon-mediated signaling pathway GO:0060337 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased type I interferon-mediated signaling pathway (GO:0060337). GO:0060337 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Recurrent Severe Infection Impaired interferon immunity accounts for the viral half of the infection phenotype.
Recurrent viral infections The counterpart of the bacterial edge, drawn from the interferon arm for the same reason: the same sentence of the founding report assigns viral susceptibility here and bacterial susceptibility to hyposplenism. The two arms are the point of this entry, so the topology has to carry the split rather than leaving it to edge prose.
Show evidence (2 references)
PMID:21109225 SUPPORT Human Clinical
"viral infections result from impaired interferon immunity"
Attributes the viral susceptibility to the interferon defect.
PMID:32350755 SUPPORT Human Clinical
"role in TLR-independent innate immune re- sponses and induction of IRF7"
Names the specific innate-immune route proposed for the antiviral arm — TLR-independent induction of IRF7 — which is more mechanistically specific than "impaired interferon immunity" alone. The hyphenation in "re- sponses" is how the text extracts from the cached PDF.
Recurrent Severe Infection
Severe bacterial and viral disease, arriving by two mechanistically distinct routes that converge on the same clinical picture. Febrile infection is also the trigger for the encephalopathic crises, so infection is both a consequence of the immune defect and the precipitant of the neurological one.
Downstream
Recurrent Febrile Encephalopathic Crises Febrile illness precipitates the encephalopathic episodes; whether it does so through inflammation, through the cell-death defect, or both is the subject of an open discussion in this entry.
Show evidence (1 reference)
PMID:21109225 SUPPORT Human Clinical
"biological features of ALPS were found in the context of severe bacterial and viral disease"
Establishes the severity and the dual bacterial/viral character of the infection phenotype.
Recurrent Febrile Encephalopathic Crises
Stereotyped, recurrent episodes of febrile encephalopathy with refractory seizures, which in one family met the criteria for febrile infection-related epilepsy syndrome. These crises carry much of the disorder's mortality and are what most distinguishes it from the ALPS spectrum clinically. The FIRES observation is doubly interesting: it supplies a genetic cause for a syndrome usually of unknown aetiology, and it points at a cell-death regulatory pathway rather than at a channel.
Downstream
Encephalopathy The crises are the mechanism; encephalopathy is what they present as. The episodes are stereotyped and recurrent rather than a single progressive decline, which is why the phenotype carries temporality RECURRENT.
Status epilepticus In the family reported as febrile infection-related epilepsy syndrome the crises took the form of refractory de novo status epilepticus, which is the most severe presentation of this node and the one that establishes FADD as a genetic cause of FIRES.
Cerebral atrophy Cerebral atrophy is reported alongside the encephalopathic crises, and the most economical reading is that repeated crises leave structural damage. Typed indirect with unknown intermediates and evidenced as PARTIAL because that ordering is inference: no source establishes that the atrophy follows the crises rather than accompanying them from another cause.
Show evidence (2 references)
PMID:38752438 SUPPORT Human Clinical
"We hereby report the clinical, EEG, brain MRI, and genetic findings of a nuclear family with recurrent febrile-related encephalopathy with refractory de novo Status Epilepticus."
Describes the crises and their refractory character in a reported family.
PMID:38752438 SUPPORT Human Clinical
"The FADD-related conditions disrupt FAS-mediated apoptosis and can cause a clinical picture with the characteristics of FIRES."
Establishes the FIRES framing and the authors' attribution of it to the apoptosis defect.
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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for FADD-Related Immunodeficiency Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
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Phenotypes

14
Cardiovascular 2
Lymphadenopathy HP:0002716 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Lymphadenopathy (HP:0002716). HP:0002716 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32350755 SUPPORT Human Clinical
"but also recurrent febrile episodes with encephalopathy and seizures, variable degrees of lymphadenopathy"
Documents lymphadenopathy and its variability, which is why no frequency band is assigned.
Abnormal heart morphology HP:0001627 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Abnormal heart morphology (HP:0001627). HP:0001627 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:21109225 SUPPORT Human Clinical
"recurrent hepatopathy and encephalopathy, and cardiac malformations"
Documents cardiac malformation in the founding kindred.
PMID:35243129 SUPPORT Human Clinical
"characterized by recurrent infections, encephalopathy, cardiac abnormalities, and short life expectancy"
Independent restatement of cardiac abnormality as a core feature of the disorder.
Digestive 1
Hepatic failure HP:0001399 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hepatic failure (HP:0001399), qualified as temporality recurrent. HP:0001399 is a phenotype from the Human Phenotype Ontology.
Temporal: RECURRENT
Show evidence (1 reference)
PMID:21109225 SUPPORT Human Clinical
"severe bacterial and viral disease, recurrent hepatopathy and encephalopathy"
Classified PARTIAL: the source reports recurrent hepatopathy, which is broader than hepatic failure specifically; the bound term is the closest available and is deliberately noted as such.
Nervous System 2
Encephalopathy HP:0001298 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Encephalopathy (HP:0001298), qualified as temporality recurrent. HP:0001298 is a phenotype from the Human Phenotype Ontology.
Temporal: RECURRENT
Show evidence (2 references)
PMID:21109225 SUPPORT Human Clinical
"severe bacterial and viral disease, recurrent hepatopathy and encephalopathy, and cardiac malformations"
Lists recurrent encephalopathy among the defining features of the founding kindred.
PMID:32350755 SUPPORT Human Clinical
"but also recurrent febrile episodes with encephalopathy and seizures"
Independent confirmation, and the source for the febrile trigger.
Cerebral atrophy HP:0002059 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cerebral atrophy (HP:0002059). HP:0002059 is a phenotype from the Human Phenotype Ontology.
Show evidence (2 references)
PMID:38752438 SUPPORT Human Clinical
"known to cause ultrarare forms of autosomal recessive immunodeficiency that could be associated with variable degrees of lymphoproliferation, cerebral atrophy, and cardiac abnormalities"
Names cerebral atrophy as one of the recognized accompaniments of FADD deficiency, with the hedged phrasing ("could be associated with variable degrees of") that the source itself uses.
PMID:32350755 SUPPORT Human Clinical
"variable degrees of lymphadenopathy or splenomeg- aly, cerebral atrophy, and structural cardiac abnormalities"
An independent review of the previously described patients listing cerebral atrophy among their features. The hyphenation is how the text extracts from the cached PDF.
Other 9
Recurrent bacterial infections HP:0002718 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Recurrent bacterial infections (HP:0002718), qualified as temporality recurrent. HP:0002718 is a phenotype from the Human Phenotype Ontology.
Temporal: RECURRENT
Show evidence (1 reference)
PMID:21109225 SUPPORT Human Clinical
"biological features of ALPS were found in the context of severe bacterial and viral disease"
Documents severe bacterial disease as part of the presenting syndrome.
Recurrent viral infections HP:0004429 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Recurrent viral infections (HP:0004429), qualified as temporality recurrent. HP:0004429 is a phenotype from the Human Phenotype Ontology.
Temporal: RECURRENT
Show evidence (1 reference)
PMID:21109225 SUPPORT Human Clinical
"viral infections result from impaired interferon immunity"
Documents viral susceptibility and its attributed mechanism.
Status epilepticus HP:0002133 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Status epilepticus (HP:0002133), qualified as temporality recurrent. HP:0002133 is a phenotype from the Human Phenotype Ontology.
Temporal: RECURRENT
Show evidence (1 reference)
PMID:38752438 SUPPORT Human Clinical
"a nuclear family with recurrent febrile-related encephalopathy with refractory de novo Status Epilepticus"
Documents recurrent refractory status epilepticus in an affected family.
Howell-Jolly bodies HP:0032550 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Howell-Jolly bodies (HP:0032550). HP:0032550 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:21109225 SUPPORT Human Clinical
"The observed bacterial infections result partly from functional hyposplenism"
Classified PARTIAL: the source establishes the functional hyposplenism that Howell-Jolly bodies mark, but this abstract does not itself report the film finding.
Increased double-negative T cell number HP:0002851 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Increased double-negative T cell number (HP:0002851). HP:0002851 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32350755 SUPPORT Human Clinical
"increased circulating double negative T cells, elevated soluble Fas ligand, IL-10, and vitamin B12"
Reports the double-negative T cell expansion within the ALPS-like biomarker panel.
Increased circulating interleukin 10 concentration HP:0033199 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Increased circulating interleukin 10 concentration (HP:0033199). HP:0033199 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32350755 SUPPORT Human Clinical
"elevated soluble Fas ligand, IL-10, and vitamin B12"
Reports the raised interleukin-10 in the biomarker panel.
Elevated circulating vitamin B12 concentration HP:6000016 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Elevated circulating vitamin B12 concentration (HP:6000016). HP:6000016 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:32350755 SUPPORT Human Clinical
"elevated soluble Fas ligand, IL-10, and vitamin B12"
Reports the raised vitamin B12 in the biomarker panel.
Cystoid macular edema HP:0011505 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Cystoid macular edema (HP:0011505). HP:0011505 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:35243129 SUPPORT Human Clinical
"A 7-year-old boy was referred for decreased vision and eye examination revealed cystoid macular edema and peripheral retinal ischemia in both eyes"
The single reported observation, with the patient's age and the bilateral distribution.
Tractional retinal detachment HP:0007917 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Tractional retinal detachment (HP:0007917). HP:0007917 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:35243129 SUPPORT Human Clinical
"progression to tractional retinal detachment in the right eye"
Documents the progression to detachment in the reported case.
🧬

Genetic Associations

1
FADD (Pathogenic Variants)
Gene: FADD hgnc:3573 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is FADD (hgnc:3573). hgnc:3573 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: CAUSATIVE
Show evidence (6 references)
PMID:21109225 SUPPORT Human Clinical
"By a combination of genome-wide linkage and whole-exome sequencing, we identified a homozygous missense mutation in FADD, encoding the Fas-associated death domain protein (FADD), in the patients."
The gene-discovery statement establishing FADD as causative.
PMID:37793571 SUPPORT Human Clinical
"We found homozygous FADD mutations in the DN T cells from all 4 patients, which resulted from uniparental disomy."
Establishes the somatic second-hit route, in four unrelated patients, and names the mechanism. This is why the entry does not describe FADD disease as germline-only.
PMID:37793571 SUPPORT Human Clinical
"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."
Describes the germline-plus-somatic architecture, including that the germline allele came from an unaffected parent — the reason a heterozygous parental result does not exclude the diagnosis.
+ 3 more references
💊

Medical Actions

4
Immunoglobulin Replacement
Action: immunoglobulin therapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is immunoglobulin therapy (NCIT:C62710). NCIT:C62710 is a clinical intervention from the NCI Thesaurus. Ontology label: Immunoglobulin Therapy NCIT:C62710
Subcutaneous immunoglobulin replacement, given at 400 mg/kg/month in the one report that describes its use, directed at the impaired antibody responses — absent isohaemagglutinins and suboptimal anti-pneumococcal titres despite repeated conjugate vaccination — rather than at the apoptosis defect. It substitutes for the antibody the patient cannot make; it does not correct FADD function, and no series has evaluated it.
Mechanism Target:
BYPASSES Recurrent Severe Infection — Supplying immunoglobulin bypasses the impaired humoral response downstream of the FADD lesion; it leaves every upstream node untouched.
Show evidence (1 reference)
PMID:32350755 SUPPORT Human Clinical
"To address possible impaired viral responses and lack of appropriate B cell antibodies (isohemagglutinins), we initiated subcutaneous immunoglobulin infusions"
States what the replacement was aimed at — the humoral deficit — which is the claim this link makes, distinct from the claim that it was given.
Show evidence (2 references)
PMID:32350755 SUPPORT Human Clinical
"we initiated subcutaneous immunoglobulin infusions at 400 mg/kg/month"
The one reported administration, with route and dose. A single patient, so this documents that the treatment was given — not that it is effective.
PMID:32350755 SUPPORT Human Clinical
"The patient tolerated these infusions well with no side effects and subjec- tive improvements in fatigue and decreased respiratory infec- tions overall for the past 8 months"
The only reported outcome. Classified PARTIAL and quoted in full because the improvements are explicitly subjective, in one uncontrolled patient over eight months. The hyphenation is how the text extracts from the cached PDF.
Antibiotic Prophylaxis
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Antipneumococcal antibiotic prophylaxis is recommended on the strength of the functional hyposplenism, which is what puts these patients at risk of overwhelming encapsulated-organism sepsis. It is recorded here as a recommendation rather than as established practice: in the single report that describes it, the family declined it and the recommendation was still under discussion at the time of writing.
Mechanism Target:
BYPASSES Functional Hyposplenism — Prophylaxis does not restore splenic phagocyte function; it substitutes for the clearance the hyposplenic patient has lost.
Show evidence (1 reference)
PMID:32350755 SUPPORT Human Clinical
"given his functional hyposplenism and risk of sepsis from encapsulated organisms"
Names the hyposplenism as the reason prophylaxis was recommended, which is what ties this treatment to this node.
Show evidence (1 reference)
PMID:32350755 SUPPORT Human Clinical
"family declined antibiotic prophylaxis, although there are on- going discussions continuing to recommend it"
Classified PARTIAL deliberately: the source establishes that prophylaxis was recommended, and simultaneously that it was not administered, so it cannot support an efficacy claim. The hyphenation is how the text extracts from the cached PDF.
Haematopoietic Cell Transplantation
Action: hematopoietic cell transplantationNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is hematopoietic cell transplantation (NCIT:C15431). NCIT:C15431 is a clinical intervention from the NCI Thesaurus. Ontology label: Hematopoietic Cell Transplantation NCIT:C15431
Transplantation has been performed in a small number of reported patients and is the only intervention that could replace the FADD-deficient haematopoietic compartment. Two limits are explicit in the literature and are the reason this is not curated as established management: outcomes in the transplanted patients diverged sharply, and the non-haematopoietic manifestations — the encephalopathic crises, the cardiac malformation, the hepatopathy — arise in tissues a graft does not replace, so whether transplantation affects them is unknown.
Mechanism Target:
RESTORES Failure of Activation-Induced Lymphocyte Apoptosis — Replacing the haematopoietic compartment replaces the lymphocytes that carry the apoptosis defect. It reaches only this arm of the disease.
Show evidence (1 reference)
PMID:32350755 SUPPORT Human Clinical
"it remains unclear what degree of donor chimerism is needed to be curative of the immunodeficiency and how HSCT will affect non- hematopoietic manifestations of FADD deficiency"
The source's own division of the disease into the compartment a graft replaces and the manifestations it does not — which is exactly why this link targets the lymphocyte-apoptosis node alone. PARTIAL because the same sentence says the curative threshold is unknown. The hyphenation is how the text extracts from the cached PDF.
Show evidence (2 references)
PMID:32350755 SUPPORT Human Clinical
"Two previously described patients underwent hematopoietic stem cell transplantation (HSCT)"
Establishes that transplantation has been performed, and how few times.
PMID:32350755 SUPPORT Human Clinical
"HSCT will likely be an important part of the treatment regimen for FADD deficien- cy, but given the rarity of the disease and the paucity of data on all of the potential clinical manifestations, it remains unclear what degree of donor chimerism is needed to be curative of the immunodeficiency"
The authors' own hedge, quoted rather than paraphrased: transplantation is expected to matter but the curative threshold is not known. This is why the entry does not assert transplantation as standard of care. The hyphenation is how the text extracts from the cached PDF.
Immunomodulatory Therapy
Action: immunomodulation therapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is immunomodulation therapy (NCIT:C116540). NCIT:C116540 is a clinical intervention from the NCI Thesaurus. Ontology label: Immunomodulation Therapy NCIT:C116540
Early immunomodulatory treatment is argued for on mechanistic grounds in the family whose presentation was recurrent febrile infection-related epilepsy syndrome: if the encephalopathic crises reflect dysregulated cell-mediated inflammation rather than infection itself, immunomodulation is the rational target. This is an argument from mechanism, not a result — no trial, series, or controlled comparison exists in FADD deficiency.
Mechanism Target:
MODULATES Recurrent Febrile Encephalopathic Crises — Proposed to act on the inflammatory regulatory pathway held to generate the crises. Untested.
Show evidence (1 reference)
PMID:38752438 SUPPORT Human Clinical
"it demonstrates a genetic cause of FIRES involving a cell-mediated inflammation regulatory pathway. This finding supports early treatment with immunomodulatory therapy"
Carries both halves of the link in one sentence: the pathway held to generate the crises, and the therapeutic inference drawn from it. PARTIAL — an inference, with no outcome reported.
Show evidence (1 reference)
PMID:38752438 SUPPORT Human Clinical
"This finding supports early treatment with immunomodulatory therapy"
Classified PARTIAL: the source recommends the approach as a consequence of identifying the genetic cause, and reports no outcome of having given it.
🔬

Biochemical Markers

1
Soluble Fas ligand (INCREASED)
Show evidence (2 references)
PMID:32350755 SUPPORT Human Clinical
"Soluble Fas ligand, IL-10, IL- 18, and vitamin B12 levels were increased"
The measurement in the reported patient, with its direction. The hyphenation is how the text extracts from the cached PDF.
PMID:32350755 SUPPORT Human Clinical
"decreased Fas-mediated lym- phocyte apoptosis, increased circulating double negative T cells, elevated soluble Fas ligand, IL-10, and vitamin B12"
States the four-member panel as a class, which is what makes soluble Fas ligand a shared ALPS biomarker rather than a FADD-specific one. The hyphenation is how the text extracts from the cached PDF.
🔬

Diagnosis

3
Fas-mediated apoptosis functional assay
A functional apoptosis assay showing impaired Fas-mediated lymphocyte death, in a patient without a FAS, FASLG or CASP10 mutation, is what points at FADD. The assay is also what distinguishes the disorder from the rest of the ALPS spectrum, since the biomarker panel alone does not.
No diagnosis_term is bound. NCIT has no term for a functional apoptosis assay — searching it returns apoptosis as a biological process, apoptosis markers and apoptosis-regulating proteins, but no diagnostic procedure — and the cited source does not state the assay platform, so binding a generic flow-cytometry term would assert a method the reference does not report.
Show evidence (1 reference)
PMID:21109225 SUPPORT In Vitro
"This FADD mutation decreases steady-state protein levels and impairs Fas-dependent apoptosis in vitro"
Establishes the assay abnormality that is diagnostically informative.
Sequencing of sorted double-negative T cells
Where a germline monoallelic FADD variant is found in a patient with an ALPS phenotype, the disease-causing homozygous genotype may exist only in the double-negative T cell compartment. Sequencing sorted CD4-positive or double-negative T cells, rather than whole blood, is what reveals it — the same strategy previously used for somatic FAS events.
sequencing of DNA from sorted T cell subsets NCIT:C153598 NCI Thesaurus (NCIT)
Show evidence (2 references)
PMID:37793571 SUPPORT Human Clinical
"We sequenced FADD and performed array-based comparative genomic hybridization using DNA from sorted CD4+ or DN T cells."
Describes the specimen and method that detect the somatic event.
PMID:37793571 SUPPORT Human Clinical
"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."
Explains why the double-negative compartment is the right specimen — it is where the somatic events accumulate.
Biomarker panel with double-negative T cell phenotyping
Where exome sequencing is negative in a patient whose clinical picture and biomarkers look like ALPS, the combination of serum biomarkers with double-negative T cell phenotyping is what identifies the cases it missed — among them patients with heterozygous FADD mutations plus somatic loss of heterozygosity. This is the systematic counterpart of the sorted-cell sequencing entry above: it says which patients to send for it.
double-negative T cell immunophenotyping NCIT:C113003 NCI Thesaurus (NCIT)
Show evidence (2 references)
PMID:37979702 SUPPORT Human Clinical
"A combination of serum biomarkers and DNT phenotyping is an accurate means to identify patients with ALPS who are missed by routine exome sequencing."
The study's own conclusion about what this diagnostic combination achieves.
PMID:37979702 SUPPORT Human Clinical
"Three of the 4 ALPS-U patients with normal FAS expression carried heterozygous FADD mutations with sLOH."
The FADD-specific yield, and the reason this is curated here rather than only in the ALPS entry: normal FAS expression on double-negative T cells is what points away from FAS and towards FADD.
📈

Progression

2
Infancy and early childhood
Presentation is in infancy or early childhood with infection and the first encephalopathic crises. Life expectancy is described as short, and the encephalopathic episodes carry much of the mortality.
Show evidence (3 references)
PMID:35243129 SUPPORT Human Clinical
"characterized by recurrent infections, encephalopathy, cardiac abnormalities, and short life expectancy"
States the severity and shortened life expectancy of the disorder.
PMID:32350755 SUPPORT Human Clinical
"Three of four patients from the original report died prior to 5 years old"
Quantifies the early mortality in the founding kindred, which is what "short life expectancy" means concretely.
PMID:32350755 SUPPORT Human Clinical
"These patients seem to have worse outcomes than classical ALPS patients."
Supports the prognostic contrast with ALPS, quoted with the hedge the authors used.
Episodic relapsing course
Between crises patients can be comparatively well; the course is relapsing and precipitated by febrile illness rather than steadily progressive. Expressivity varies — the Epilepsia report notes that its family widens the recognized phenotype.
Show evidence (1 reference)
PMID:38752438 SUPPORT Human Clinical
"it increases the number of reported patients with FADD deficiency, showing that this disorder may present variable expressivity"
Supports the variable expressivity claim from the reporting authors.
📊

Prevalence

1
Worldwide
Cases In Literature Ultra Rare
Only a small number of families have been reported since the disorder was delineated in 2010 — a founding consanguineous kindred, subsequent compound heterozygous and single-family reports, and four patients with the somatic second-hit architecture. No pooled count and no population prevalence estimate has been published, so no number is asserted here.
Show evidence (2 references)
PMID:35243129 SUPPORT Human Clinical
"Fas-associated protein with death domain (FADD) deficiency is a rare and severe genetic mutation characterized by recurrent infections, encephalopathy, cardiac abnormalities, and short life expectancy."
Characterizes the disorder as rare and severe. Used to support the ULTRA_RARE band qualitatively; no count is claimed because none is published.
PMID:32350755 SUPPORT Human Clinical
"There are very few cases in the literature of FADD deficiency patients"
Direct statement of the size of the reported literature, without converting it into a count this entry would then have to defend.
🔀

Differential Diagnoses

1

Conditions with similar clinical presentations that must be differentiated from FADD-Related Immunodeficiency:

Autoimmune lymphoproliferative syndrome (ALPS-FAS)
Overlapping Features The nearest mimic, and the reason FADD deficiency is easy to misfile. The two share the biomarker panel — impaired Fas-mediated apoptosis, expanded double-negative T cells, raised soluble Fas ligand, interleukin-10 and vitamin B12. They differ in what surrounds it: FADD deficiency has milder lymphoproliferation and adds recurrent febrile encephalopathy, hepatopathy, cardiac malformation and functional hyposplenism, none of which ALPS-FAS produces. The ALPS entry in this knowledge base cites a source that draws exactly this distinction.
Show evidence (1 reference)
PMID:32350755 SUPPORT Human Clinical
"Previously described FADD deficiency patients demonstrate a clinical phenotype partially overlap- ping with other ALPS disorders"
States the partial overlap that makes ALPS the differential. The hyphenation is how the text extracts from the cached PDF.
🐁

Animal Models

2
Fadd conditional knockout mouse Genetic
Constitutive Fadd loss is embryonic lethal, so the mouse work uses conditional alleles. Inducible deletion across haematopoietic lineages depletes peripheral lymphocytes over time and impairs early bone-marrow haematopoiesis — and pointedly does NOT reproduce the lymphoproliferative disease seen in Fas-deficient mice, which is the murine counterpart of the human observation that FADD deficiency is not simply severe ALPS. Epithelium-specific alleles reveal a different function again: FADD restrains RIPK3-MLKL necroptosis and caspase-8-gasdermin-D pyroptosis-like death, and its loss in gut or skin epithelium causes inflammatory disease that human patients do not have.
Species
Mouse
Genotype
Fadd conditional deletion (Mx1-cre inducible, and tissue-specific alleles)
Genes
FADD hgnc:3573 HUGO Gene Nomenclature Committee (hgnc) Relation: this experimental model concerns this gene This experimental model concerns FADD (hgnc:3573). hgnc:3573 is a gene from the HUGO Gene Nomenclature Committee.
Publication
PMID:21115735
Pathograph links
Failure of Activation-Induced Lymphocyte Apoptosis partially recapitulates fidelity: moderate Haematopoietic Fadd deletion perturbs lymphocyte homeostasis, but in the opposite direction to the human disease: mice lose peripheral lymphocytes over time rather than accumulating an apoptosis-resistant population.
Limitations: The mouse model deletes Fadd outright, whereas human disease alleles reduce protein without abolishing it; and the murine outcome is lymphocyte depletion rather than the double-negative T cell expansion that defines the human laboratory phenotype.
Readouts
Peripheral lymphocyte number after inducible Fadd deletion
Direction: DECREASED
Interpretation: Time-dependent depletion, which is the direction opposite to the human accumulation and is the reason this link is only partial.
Show evidence (1 reference)
PMID:21115735 SUPPORT Model Organism
"Instead, a time-dependent depletion of peripheral FADD-deficient lymphocytes was observed."
The measurement and its direction.
Show evidence (1 reference)
PMID:21115735 SUPPORT Model Organism
"The resulting FADD mutant mice did not develop lymphoproliferation diseases, unlike Fas-deficient mice."
Classified PARTIAL: the model is informative about the lymphocyte arm, but its explicit divergence from the Fas-deficient phenotype is also what limits how far it can stand in for the human disease.
Show evidence (1 reference)
PMID:21115735 SUPPORT Model Organism
"In addition, a death receptor-independent function of FADD is essential for embryogenesis."
Establishes the embryonic-lethality constraint that forces conditional alleles and that supports the residual-function inference in human patients.
Endothelial (Tie2-cre) Fadd conditional knockout mouse Genetic
A tissue-by-tissue dissection of which cell type accounts for the embryonic lethality of Fadd loss, and the only murine result that speaks to the human cardiac malformation. Deleting FADD:GFP in cardiomyocytes or cardiac progenitors is tolerated; deleting it in Tie-2 expressing cells — endothelium and haematopoietic cells — reproduces the germline-null lethality with cardiovascular defects, and deleting Ripk3 rescues it. So the cardiac phenotype is not cardiomyocyte-autonomous, and it runs through RIPK3, a death-receptor-independent FADD function.
Species
Mouse
Genotype
Fadd-/- Fadd:gfp+ Tie2-cre+ (FADD:GFP deleted in Tie-2 expressing cells)
Genes
FADD hgnc:3573 HUGO Gene Nomenclature Committee (hgnc) Relation: this experimental model concerns this gene This experimental model concerns FADD (hgnc:3573). hgnc:3573 is a gene from the HUGO Gene Nomenclature Committee.
Publication
PMID:27584790
Pathograph links
Loss of Fas-Independent FADD Functions partially recapitulates fidelity: low Demonstrates that FADD restrains RIPK3 signalling in endothelium and that losing this causes a cardiovascular developmental defect — the class of Fas-independent function this node asserts, and a candidate route to the human cardiac malformation.
Limitations: Fidelity is LOW and the link deliberately stops at "candidate route". The model deletes Fadd outright and dies at E11.5, whereas human alleles reduce protein without abolishing it and patients survive to be born; the murine lesion is a lethal failure of cardiovascular development, not the structural malformations (pulmonary atresia, ventricular septal defect, anomalous venous drainage) reported in patients; and no human FADD patient has been shown to have RIPK3-dependent endothelial pathology. No causal claim about the human cardiac phenotype is made from this model.
Readouts
Ventricular trabeculation and endocardial cushion formation at E11.5
Direction: DECREASED
Interpretation: The structural measurement behind the cardiovascular arm of the phenotype, and the endpoint that Ripk3 deletion restores.
Show evidence (1 reference)
PMID:27584790 SUPPORT Model Organism
"displayed a low degree of trabeculation in the walls of the common ventricular chamber and endocardial cushion defect by reduced endothelial-to-mesenchymal formation"
The histological measurement and its direction.
Show evidence (1 reference)
PMID:27584790 SUPPORT INDIRECT Model Organism
"these results demonstrated that RIPK3-mediated signaling in Tie-2 expressing cells was responsible for the embryonic lethality"
Names the mechanism and the responsible compartment. INDIRECT because it establishes a Fas-independent, RIPK3-dependent FADD function in endothelium without establishing that this is what produces the human cardiac phenotype.
Show evidence (2 references)
PMID:27584790 SUPPORT Model Organism
"suggesting that loss of Fadd in endothelial cells causes endocardium-related cardiac development defect"
The authors' own statement of what the endothelial deletion shows, quoted with their hedge intact.
PMID:27584790 SUPPORT Model Organism
"Mice with conditional deletion of Fadd in immune cells, skin or intestine produced no lethality"
The negative controls that make the endothelial result specific rather than a general consequence of losing Fadd anywhere.
{ }

Source YAML

click to show 
name: FADD-Related Immunodeficiency
creation_date: "2026-08-24T00:00:00Z"
description: >-
  FADD deficiency is an ultra-rare autosomal recessive disorder of the adaptor
  protein that nucleates the death-inducing signalling complex downstream of
  FAS. Because FADD sits at a branch point rather than on a single pathway, the
  disease is not simply an ALPS phenocopy: patients have the ALPS biomarker
  profile (impaired Fas-dependent lymphocyte apoptosis, expanded
  double-negative T cells) without the florid lymphoproliferation, and in
  addition carry features no FAS-pathway disorder produces — recurrent
  stereotyped febrile encephalopathic crises with refractory status epilepticus,
  hepatopathy, cardiac malformations, and a functional hyposplenism that drives
  invasive encapsulated-organism infection. The founding study attributed the
  bacterial infections to hyposplenism and the viral infections to impaired
  interferon immunity, making this one of the clearest human demonstrations that
  FADD has essential Fas-independent functions.
category: Mendelian
parents:
- autosomal recessive disease
- primary immunodeficiency disease
synonyms:
- FADD deficiency
- FAS-associated death domain protein deficiency
- infections, recurrent, with encephalopathy, hepatic dysfunction, and cardiovascular malformations
disease_term:
  preferred_term: FADD-related immunodeficiency
  term:
    id: MONDO:0013408
    label: FADD-related immunodeficiency
classifications:
  harrisons_chapter:
  - classification_value: IMMUNE_RHEUMATOLOGIC
    notes: >-
      Placed with the immune disorders because the immunodeficiency and the
      lymphocyte-apoptosis defect are the defining axis, even though the
      encephalopathic crises carry much of the mortality. Harrison's does not
      list FADD deficiency by name; this is a mechanism-based placement.
inheritance:
- name: Autosomal recessive inheritance
  description: >-
    FADD deficiency is autosomal recessive. The founding kindred was large,
    consanguineous and multiplex, homozygous for a missense allele; compound
    heterozygous patients have since been reported. A separate route to the
    phenotype exists in which a germline monoallelic FADD mutation is
    unmasked somatically — see the genetic section.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  expressivity: VARIABLE
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We investigated a large, consanguineous, multiplex kindred in which biological features of ALPS were found in the context of severe bacterial and viral disease, recurrent hepatopathy and encephalopathy, and cardiac malformations."
    explanation: >-
      Establishes the consanguineous multiplex pedigree structure consistent
      with recessive inheritance, and enumerates the multisystem phenotype.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "is an autosomal recessive disorder resulting from a pathogenic variation in F ADD(FAS-associated protein with death do- main), the adaptor protein involved in Fas signaling to caspases 8 and 10"
    explanation: >-
      States the inheritance mode directly. The spacing in "F ADD" is how the
      text extracts from the cached PDF and is quoted as it appears.
pathophysiology:
- name: Biallelic FADD Loss of Function
  biological_scale: MOLECULAR
  role: trigger
  description: >-
    FADD is a small bipartite adaptor: a death effector domain that recruits
    procaspase-8 and -10, and a death domain that binds the death domain of a
    ligated receptor. It carries no catalytic activity of its own, so the
    disease is a failure of assembly rather than of enzymology. The reported
    disease alleles reduce steady-state FADD protein rather than abolishing it,
    which matters because complete Fadd loss is embryonic lethal in mice —
    surviving patients are therefore expected to retain residual function.
  genes:
  - preferred_term: FADD
    term:
      id: hgnc:3573
      label: FADD
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we identified a homozygous missense mutation in FADD, encoding the Fas-associated death domain protein (FADD), in the patients. This FADD mutation decreases steady-state protein levels and impairs Fas-dependent apoptosis in vitro"
    explanation: >-
      Establishes the causal variant and its molecular consequence — reduced
      protein level rather than absence, which is the basis for the residual
      function claim.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the adaptor protein involved in Fas signaling to caspases 8 and 10"
    explanation: >-
      States the adaptor role and the caspases recruited, which is the function
      lost at this node.
  downstream:
  - target: Impaired Death-Receptor Apoptotic Signalling
    causal_link_type: DIRECT
    description: >-
      Without the adaptor, the death-inducing signalling complex cannot be
      assembled on a ligated receptor.
    evidence:
    - reference: PMID:21109225
      reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "This FADD mutation decreases steady-state protein levels and impairs Fas-dependent apoptosis in vitro"
      explanation: >-
        Directly links the variant to the loss of Fas-dependent apoptosis.
  - target: Loss of Fas-Independent FADD Functions
    causal_link_type: DIRECT
    description: >-
      The same loss of adaptor also removes FADD's functions outside the Fas
      pathway, which is what makes this disease more than an ALPS phenocopy.
    evidence:
    - reference: PMID:21109225
      reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "It also impairs Fas-independent signaling pathways."
      explanation: >-
        The explicit statement that the defect is not confined to Fas
        signalling, which is the justification for a parallel arm here.
- name: Impaired Death-Receptor Apoptotic Signalling
  biological_scale: MOLECULAR
  role: central_effector
  description: >-
    FADD is obligate for assembling the death-inducing signalling complex
    downstream of FAS and other death-domain receptors: its death domain engages
    the receptor, its death effector domain recruits procaspase-8 and -10, and
    caspase activation follows. Losing the adaptor breaks the chain at the
    nucleation step, so no amount of receptor ligation produces apoptosis.
  biological_processes:
  - preferred_term: extrinsic apoptotic signaling pathway via death domain receptors
    term:
      id: GO:0008625
      label: extrinsic apoptotic signaling pathway via death domain receptors
    modifier: DECREASED
  - preferred_term: extrinsic apoptotic signaling pathway
    term:
      id: GO:0097191
      label: extrinsic apoptotic signaling pathway
    modifier: DECREASED
  evidence:
  - reference: PMID:21115735
    reference_title: "FADD deficiency impairs early hematopoiesis in the bone marrow."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "During apoptotic signaling induced by death receptors including Fas, FADD is required for the recruitment and activation of caspase 8."
    explanation: >-
      States the obligate adaptor requirement that this node represents.
      Evidence source is MODEL_ORGANISM because the paper's experimental work is
      in mice.
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "impairs Fas-dependent apoptosis in vitro, accounting for biological ALPS phenotypes in vivo"
    explanation: >-
      Connects the signalling defect to the ALPS-like laboratory phenotype
      observed in patients.
  downstream:
  - target: Failure of Activation-Induced Lymphocyte Apoptosis
    causal_link_type: DIRECT
    description: >-
      Activated lymphocytes that should be deleted by Fas ligation survive.
    evidence:
    - reference: PMID:21109225
      reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "impairs Fas-dependent apoptosis in vitro, accounting for biological ALPS phenotypes in vivo"
      explanation: >-
        The authors' own attribution of the lymphocyte phenotype to the
        apoptosis defect.
- name: Failure of Activation-Induced Lymphocyte Apoptosis
  biological_scale: CELLULAR
  description: >-
    Lymphocytes that fail to undergo activation-induced death accumulate, and
    the readout is the ALPS biomarker panel: expanded circulating
    double-negative T cells, raised soluble Fas ligand, interleukin-10 and
    vitamin B12. What is notable is what does not follow. Patients do not
    develop the massive lymphadenopathy, splenomegaly and autoimmune cytopenias
    that define clinical ALPS — the biology is shared, the clinical syndrome is
    not, and the cited source explicitly separates FADD deficiency from ALPS-FAS
    on that basis.
  cell_types:
  - preferred_term: T cell
    term:
      id: CL:0000084
      label: T cell
  biological_processes:
  - preferred_term: extrinsic apoptotic signaling pathway via death domain receptors
    term:
      id: GO:0008625
      label: extrinsic apoptotic signaling pathway via death domain receptors
    modifier: DECREASED
  evidence:
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Previously described FADD deficiency patients demonstrate a clinical phenotype partially overlap- ping with other ALPS disorders (decreased Fas-mediated lym- phocyte apoptosis, increased circulating double negative T cells, elevated soluble Fas ligand, IL-10, and vitamin B12)"
    explanation: >-
      Enumerates the full biomarker panel this node produces. The hyphenation
      is how the text extracts from the cached PDF and is quoted as it appears.
  downstream:
  - target: Increased double-negative T cell number
    causal_link_type: DIRECT
    description: >-
      Lymphocytes that should have been deleted at the end of an immune response
      persist. The TCR-alpha/beta CD4-negative CD8-negative compartment is where
      that failure is visible, and it is the pathognomonic subset of the ALPS
      spectrum.
    evidence:
    - reference: PMID:32350755
      reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "decreased Fas-mediated lym- phocyte apoptosis, increased circulating double negative T cells, elevated soluble Fas ligand, IL-10, and vitamin B12"
      explanation: >-
        Places the apoptosis defect and the double-negative expansion in one
        clause, as parts of a single phenotype. The hyphenation is how the text
        extracts from the cached PDF.
  - target: Increased circulating interleukin 10 concentration
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Raised interleukin 10 is part of the ALPS biomarker panel that accompanies
      the apoptosis defect. Typed as indirect and unknown deliberately: the panel
      is measured together and interpreted together, but no step between the
      failure of lymphocyte deletion and the cytokine rise has been demonstrated
      in FADD deficiency.
    evidence:
    - reference: PMID:32350755
      reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "decreased Fas-mediated lym- phocyte apoptosis, increased circulating double negative T cells, elevated soluble Fas ligand, IL-10, and vitamin B12"
      explanation: >-
        Classified PARTIAL: the source establishes co-occurrence within one
        biomarker panel, not a mechanism connecting them.
  - target: Elevated circulating vitamin B12 concentration
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The fourth member of the same panel, with the same caveat — a measured
      accompaniment of the apoptosis defect rather than a step traced to it.
    evidence:
    - reference: PMID:32350755
      reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: "decreased Fas-mediated lym- phocyte apoptosis, increased circulating double negative T cells, elevated soluble Fas ligand, IL-10, and vitamin B12"
      explanation: >-
        Classified INDIRECT for the same reason as the interleukin 10 edge:
        panel co-occurrence, not a demonstrated mechanism.
  - target: Lymphadenopathy
    causal_link_type: DIRECT
    description: >-
      Accumulation of undeleted lymphocytes enlarges lymphoid tissue. In FADD
      deficiency this is milder than in ALPS-FAS and variable between patients,
      which is one of the observations that separates the two disorders.
    evidence:
    - reference: PMID:38752438
      reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "known to cause ultrarare forms of autosomal recessive immunodeficiency that could be associated with variable degrees of lymphoproliferation, cerebral atrophy, and cardiac abnormalities"
      explanation: >-
        Classified PARTIAL and quoted with the source's own "variable degrees
        of" — lymphoproliferation here is neither constant nor florid.
- name: Loss of Fas-Independent FADD Functions
  biological_scale: MOLECULAR
  description: >-
    The founding study reported that the patients' FADD mutation impairs
    Fas-independent signalling as well, and traced two of the disease's most
    consequential features to that arm rather than to apoptosis: the bacterial
    infections to functional hyposplenism, and the viral infections to impaired
    interferon immunity. This node is what separates FADD deficiency from the
    ALPS spectrum mechanistically, not just clinically.
  biological_processes:
  - preferred_term: type I interferon-mediated signaling pathway
    term:
      id: GO:0060337
      label: type I interferon-mediated signaling pathway
    modifier: DECREASED
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The observed bacterial infections result partly from functional hyposplenism, and viral infections result from impaired interferon immunity."
    explanation: >-
      The direct attribution of the two infection phenotypes to non-apoptotic
      mechanisms, quoted with the authors' own hedge ("partly") on the
      bacterial arm.
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our findings highlight the key role of FADD in Fas-dependent and Fas-independent signaling pathways in humans."
    explanation: >-
      Establishes the dual-arm framing that this entry's pathograph reproduces.
  downstream:
  - target: Functional Hyposplenism
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      How loss of FADD produces a hyposplenic state is not established; the
      association is reported, the mechanism is not.
    evidence:
    - reference: PMID:21109225
      reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: "The observed bacterial infections result partly from functional hyposplenism"
      explanation: >-
        Classified INDIRECT: the source attributes the infections to
        hyposplenism, but does not demonstrate how FADD loss causes the
        hyposplenism itself.
  - target: Impaired Interferon Immunity
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      The interferon defect is attributed to the Fas-independent arm.
    evidence:
    - reference: PMID:21109225
      reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "viral infections result from impaired interferon immunity"
      explanation: >-
        Classified PARTIAL: the interferon defect is asserted as the cause of
        the viral susceptibility, without the intervening molecular steps being
        curated here.
- name: Functional Hyposplenism
  biological_scale: ORGANISM
  description: >-
    The spleen is present but does not work. The practical consequence is the
    one that follows from any hyposplenic state — susceptibility to invasive
    infection with encapsulated organisms — and the practical marker is
    Howell-Jolly bodies on the blood film, which is why this is worth curating
    as its own node rather than folding into the immunodeficiency.
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The observed bacterial infections result partly from functional hyposplenism"
    explanation: >-
      Establishes the hyposplenic state and its role in the bacterial
      infections.
  downstream:
  - target: Recurrent Severe Infection
    causal_link_type: DIRECT
    description: >-
      Hyposplenism is a direct route to invasive encapsulated-organism disease.
    evidence:
    - reference: PMID:21109225
      reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The observed bacterial infections result partly from functional hyposplenism"
      explanation: >-
        The causal statement, with the authors' own partial attribution
        preserved.
  - target: Howell-Jolly bodies
    causal_link_type: DIRECT
    description: >-
      Howell-Jolly bodies are nuclear remnants the spleen normally clears from
      circulating erythrocytes. Their appearance in a patient who has a spleen is
      the objective readout of splenic phagocyte failure, and it is what carries
      the ontology binding for a state HPO has no term for.
    evidence:
    - reference: PMID:35243129
      reference_title: "Ocular findings associated with FADD deficiency resemble familial exudative vitreoretinopathy."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "Despite the presence of a spleen, Howell-Jolly Bodies were present in the blood smear, evidence of impaired splenic phagocyte function"
      explanation: >-
        States the finding, the fact that the spleen is anatomically present,
        and the inference the reporting authors draw from it — all three of
        which this edge asserts.
  - target: Recurrent bacterial infections
    causal_link_type: DIRECT
    description: >-
      Drawn from the hyposplenism node rather than from the aggregate infection
      node, because the founding report attributes the bacterial susceptibility
      specifically to loss of splenic clearance. Routing it through the shared
      node would have said both arms cause both infection types, which is not
      what the source says.
    evidence:
    - reference: PMID:21109225
      reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "The observed bacterial infections result partly from functional hyposplenism"
      explanation: >-
        The arm-specific attribution, quoted with the authors' "partly" — which
        is why the hyposplenic route is not claimed as the sole cause.
- name: Impaired Interferon Immunity
  biological_scale: CELLULAR
  description: >-
    The antiviral arm. FADD participates in innate antiviral signalling
    independently of its apoptotic role, and its loss leaves patients
    susceptible to severe viral disease alongside the bacterial susceptibility
    from hyposplenism.
  biological_processes:
  - preferred_term: type I interferon-mediated signaling pathway
    term:
      id: GO:0060337
      label: type I interferon-mediated signaling pathway
    modifier: DECREASED
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "viral infections result from impaired interferon immunity"
    explanation: >-
      Attributes the viral susceptibility to the interferon defect.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "role in TLR-independent innate immune re- sponses and induction of IRF7"
    explanation: >-
      Names the specific innate-immune route proposed for the antiviral arm —
      TLR-independent induction of IRF7 — which is more mechanistically
      specific than "impaired interferon immunity" alone. The hyphenation in
      "re- sponses" is how the text extracts from the cached PDF.
  downstream:
  - target: Recurrent Severe Infection
    causal_link_type: DIRECT
    description: >-
      Impaired interferon immunity accounts for the viral half of the infection
      phenotype.
    evidence:
    - reference: PMID:21109225
      reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "viral infections result from impaired interferon immunity"
      explanation: >-
        The causal statement for the viral arm.
  - target: Recurrent viral infections
    causal_link_type: DIRECT
    description: >-
      The counterpart of the bacterial edge, drawn from the interferon arm for
      the same reason: the same sentence of the founding report assigns viral
      susceptibility here and bacterial susceptibility to hyposplenism. The two
      arms are the point of this entry, so the topology has to carry the split
      rather than leaving it to edge prose.
    evidence:
    - reference: PMID:21109225
      reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "viral infections result from impaired interferon immunity"
      explanation: >-
        The arm-specific attribution for the viral half of the same sentence.
- name: Recurrent Severe Infection
  biological_scale: ORGANISM
  role: consequence
  description: >-
    Severe bacterial and viral disease, arriving by two mechanistically distinct
    routes that converge on the same clinical picture. Febrile infection is also
    the trigger for the encephalopathic crises, so infection is both a
    consequence of the immune defect and the precipitant of the neurological one.
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "biological features of ALPS were found in the context of severe bacterial and viral disease"
    explanation: >-
      Establishes the severity and the dual bacterial/viral character of the
      infection phenotype.
  downstream:
  - target: Recurrent Febrile Encephalopathic Crises
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Febrile illness precipitates the encephalopathic episodes; whether it does
      so through inflammation, through the cell-death defect, or both is the
      subject of an open discussion in this entry.
    evidence:
    - reference: PMID:32350755
      reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: "but also recurrent febrile episodes with encephalopathy and seizures"
      explanation: >-
        Classified INDIRECT: establishes the febrile-episode association without
        demonstrating the mechanism by which fever precipitates encephalopathy.

- name: Recurrent Febrile Encephalopathic Crises
  biological_scale: ORGANISM
  role: consequence
  description: >-
    Stereotyped, recurrent episodes of febrile encephalopathy with refractory
    seizures, which in one family met the criteria for febrile
    infection-related epilepsy syndrome. These crises carry much of the
    disorder's mortality and are what most distinguishes it from the ALPS
    spectrum clinically. The FIRES observation is doubly interesting: it
    supplies a genetic cause for a syndrome usually of unknown aetiology, and
    it points at a cell-death regulatory pathway rather than at a channel.
  evidence:
  - reference: PMID:38752438
    reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We hereby report the clinical, EEG, brain MRI, and genetic findings of a nuclear family with recurrent febrile-related encephalopathy with refractory de novo Status Epilepticus."
    explanation: >-
      Describes the crises and their refractory character in a reported family.
  - reference: PMID:38752438
    reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The FADD-related conditions disrupt FAS-mediated apoptosis and can cause a clinical picture with the characteristics of FIRES."
    explanation: >-
      Establishes the FIRES framing and the authors' attribution of it to the
      apoptosis defect.
  downstream:
  - target: Encephalopathy
    causal_link_type: DIRECT
    description: >-
      The crises are the mechanism; encephalopathy is what they present as. The
      episodes are stereotyped and recurrent rather than a single progressive
      decline, which is why the phenotype carries temporality RECURRENT.
    evidence:
    - reference: PMID:21109225
      reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "severe bacterial and viral disease, recurrent hepatopathy and encephalopathy, and cardiac malformations"
      explanation: >-
        Names recurrent encephalopathy in the founding kindred.
  - target: Status epilepticus
    causal_link_type: DIRECT
    description: >-
      In the family reported as febrile infection-related epilepsy syndrome the
      crises took the form of refractory de novo status epilepticus, which is the
      most severe presentation of this node and the one that establishes FADD as
      a genetic cause of FIRES.
    evidence:
    - reference: PMID:38752438
      reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "We hereby report the clinical, EEG, brain MRI, and genetic findings of a nuclear family with recurrent febrile-related encephalopathy with refractory de novo Status Epilepticus."
      explanation: >-
        The clinical form the crises took, in the family that links FADD to
        FIRES.
  - target: Cerebral atrophy
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Cerebral atrophy is reported alongside the encephalopathic crises, and the
      most economical reading is that repeated crises leave structural damage.
      Typed indirect with unknown intermediates and evidenced as PARTIAL because
      that ordering is inference: no source establishes that the atrophy follows
      the crises rather than accompanying them from another cause.
    evidence:
    - reference: PMID:38752438
      reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: HUMAN_CLINICAL
      snippet: "known to cause ultrarare forms of autosomal recessive immunodeficiency that could be associated with variable degrees of lymphoproliferation, cerebral atrophy, and cardiac abnormalities"
      explanation: >-
        Establishes cerebral atrophy as a feature of the disorder. Classified
        INDIRECT because it does not establish the causal ordering this edge
        proposes.


phenotypes:
- name: Recurrent bacterial infections
  category: Immunologic
  description: >-
    Severe, invasive bacterial infection, attributed in part to the functional
    hyposplenism. No frequency band is assigned: the reported cohort is a
    handful of families and no denominator-bearing series exists.
  phenotype_term:
    preferred_term: Recurrent bacterial infections
    term:
      id: HP:0002718
      label: Recurrent bacterial infections
    temporality: RECURRENT
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "biological features of ALPS were found in the context of severe bacterial and viral disease"
    explanation: >-
      Documents severe bacterial disease as part of the presenting syndrome.
- name: Recurrent viral infections
  category: Immunologic
  description: >-
    Severe viral disease, attributed to impaired interferon immunity rather than
    to the apoptosis defect.
  phenotype_term:
    preferred_term: Recurrent viral infections
    term:
      id: HP:0004429
      label: Recurrent viral infections
    temporality: RECURRENT
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "viral infections result from impaired interferon immunity"
    explanation: >-
      Documents viral susceptibility and its attributed mechanism.
- name: Encephalopathy
  category: Neurologic
  description: >-
    Recurrent febrile encephalopathy is a core feature and the one least
    explained by the apoptosis defect. Episodes are stereotyped and recurrent
    rather than progressive between crises.
  phenotype_term:
    preferred_term: Encephalopathy
    term:
      id: HP:0001298
      label: Encephalopathy
    temporality: RECURRENT
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "severe bacterial and viral disease, recurrent hepatopathy and encephalopathy, and cardiac malformations"
    explanation: >-
      Lists recurrent encephalopathy among the defining features of the
      founding kindred.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "but also recurrent febrile episodes with encephalopathy and seizures"
    explanation: >-
      Independent confirmation, and the source for the febrile trigger.
- name: Status epilepticus
  category: Neurologic
  description: >-
    Refractory status epilepticus during the encephalopathic crises, in one
    family meeting criteria for febrile infection-related epilepsy syndrome.
  phenotype_term:
    preferred_term: Status epilepticus
    term:
      id: HP:0002133
      label: Status epilepticus
    temporality: RECURRENT
  evidence:
  - reference: PMID:38752438
    reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a nuclear family with recurrent febrile-related encephalopathy with refractory de novo Status Epilepticus"
    explanation: >-
      Documents recurrent refractory status epilepticus in an affected family.
- name: Cerebral atrophy
  category: Neurologic
  description: >-
    Cerebral atrophy is named among the features that may accompany the
    lymphoproliferation in FADD-related disease. It is reported alongside the
    encephalopathic crises rather than as an independently progressive process,
    and no denominator-bearing series exists, so no frequency band is assigned.
  phenotype_term:
    preferred_term: Cerebral atrophy
    term:
      id: HP:0002059
      label: Cerebral atrophy
  evidence:
  - reference: PMID:38752438
    reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "known to cause ultrarare forms of autosomal recessive immunodeficiency that could be associated with variable degrees of lymphoproliferation, cerebral atrophy, and cardiac abnormalities"
    explanation: >-
      Names cerebral atrophy as one of the recognized accompaniments of
      FADD deficiency, with the hedged phrasing ("could be associated with
      variable degrees of") that the source itself uses.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "variable degrees of lymphadenopathy or splenomeg- aly, cerebral atrophy, and structural cardiac abnormalities"
    explanation: >-
      An independent review of the previously described patients listing
      cerebral atrophy among their features. The hyphenation is how the text
      extracts from the cached PDF.
- name: Howell-Jolly bodies
  category: Hematologic
  description: >-
    The objective marker of the functional hyposplenism. No HP term for
    functional hyposplenism itself exists — the nearest-sounding candidate,
    HP:0001971, is Hypersplenism, which is the clinical opposite — so the
    hyposplenic state is recorded in prose and this film finding carries the
    ontology binding.
  phenotype_term:
    preferred_term: Howell-Jolly bodies
    term:
      id: HP:0032550
      label: Howell-Jolly bodies
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The observed bacterial infections result partly from functional hyposplenism"
    explanation: >-
      Classified PARTIAL: the source establishes the functional hyposplenism
      that Howell-Jolly bodies mark, but this abstract does not itself report
      the film finding.
- name: Increased double-negative T cell number
  category: Immunologic
  description: >-
    Expanded circulating CD4-negative CD8-negative T cells, the pathognomonic
    ALPS laboratory finding, present here without the clinical
    lymphoproliferation that usually accompanies it.
  phenotype_term:
    preferred_term: Increased double-negative T cell number
    term:
      id: HP:0002851
      label: Increased double-negative T cell number
  evidence:
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "increased circulating double negative T cells, elevated soluble Fas ligand, IL-10, and vitamin B12"
    explanation: >-
      Reports the double-negative T cell expansion within the ALPS-like
      biomarker panel.
- name: Increased circulating interleukin 10 concentration
  category: Immunologic
  description: >-
    Raised interleukin-10 is part of the ALPS biomarker panel seen in FADD
    deficiency.
  phenotype_term:
    preferred_term: Increased circulating interleukin 10 concentration
    term:
      id: HP:0033199
      label: Increased circulating interleukin 10 concentration
  evidence:
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "elevated soluble Fas ligand, IL-10, and vitamin B12"
    explanation: >-
      Reports the raised interleukin-10 in the biomarker panel.
- name: Elevated circulating vitamin B12 concentration
  category: Immunologic
  description: >-
    Raised vitamin B12 is a standard ALPS biomarker and is present in FADD
    deficiency.
  phenotype_term:
    preferred_term: Elevated circulating vitamin B12 concentration
    term:
      id: HP:6000016
      label: Elevated circulating vitamin B12 concentration
  evidence:
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "elevated soluble Fas ligand, IL-10, and vitamin B12"
    explanation: >-
      Reports the raised vitamin B12 in the biomarker panel.
- name: Lymphadenopathy
  category: Immunologic
  description: >-
    Lymphadenopathy occurs but is variable and notably milder than in classic
    ALPS, which is one of the features separating the two clinically.
  phenotype_term:
    preferred_term: Lymphadenopathy
    term:
      id: HP:0002716
      label: Lymphadenopathy
  evidence:
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "but also recurrent febrile episodes with encephalopathy and seizures, variable degrees of lymphadenopathy"
    explanation: >-
      Documents lymphadenopathy and its variability, which is why no frequency
      band is assigned.
- name: Abnormal heart morphology
  category: Cardiovascular
  description: >-
    Cardiac malformations were part of the founding description and are among
    the features that mark this out from a pure immune disorder. Specific
    lesions have been reported in individual patients; the entry binds the
    general term because no consistent lesion is established across the cohort.
  phenotype_term:
    preferred_term: Abnormal heart morphology
    term:
      id: HP:0001627
      label: Abnormal heart morphology
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "recurrent hepatopathy and encephalopathy, and cardiac malformations"
    explanation: >-
      Documents cardiac malformation in the founding kindred.
  - reference: PMID:35243129
    reference_title: "Ocular findings associated with FADD deficiency resemble familial exudative vitreoretinopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "characterized by recurrent infections, encephalopathy, cardiac abnormalities, and short life expectancy"
    explanation: >-
      Independent restatement of cardiac abnormality as a core feature of the
      disorder.
- name: Hepatic failure
  category: Hepatic
  description: >-
    Recurrent hepatopathy was described in the founding kindred. The entry binds
    the hepatic-failure term as the closest available for the recurrent liver
    dysfunction reported; the underlying histology is not consistently
    characterized across the cohort.
  phenotype_term:
    preferred_term: Hepatic failure
    term:
      id: HP:0001399
      label: Hepatic failure
    temporality: RECURRENT
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "severe bacterial and viral disease, recurrent hepatopathy and encephalopathy"
    explanation: >-
      Classified PARTIAL: the source reports recurrent hepatopathy, which is
      broader than hepatic failure specifically; the bound term is the closest
      available and is deliberately noted as such.
- name: Cystoid macular edema
  category: Ophthalmologic
  description: >-
    Ocular involvement was described only in 2022, in a single seven-year-old
    with cystoid macular oedema and peripheral retinal ischaemia in both eyes.
    The picture resembles familial exudative vitreoretinopathy. A single case,
    so no frequency band.
  phenotype_term:
    preferred_term: Cystoid macular edema
    term:
      id: HP:0011505
      label: Cystoid macular edema
  evidence:
  - reference: PMID:35243129
    reference_title: "Ocular findings associated with FADD deficiency resemble familial exudative vitreoretinopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A 7-year-old boy was referred for decreased vision and eye examination revealed cystoid macular edema and peripheral retinal ischemia in both eyes"
    explanation: >-
      The single reported observation, with the patient's age and the bilateral
      distribution.
- name: Tractional retinal detachment
  category: Ophthalmologic
  description: >-
    The same patient progressed to tractional retinal detachment in one eye,
    which is why the reporting authors raised the question of ophthalmic
    surveillance.
  phenotype_term:
    preferred_term: Tractional retinal detachment
    term:
      id: HP:0007917
      label: Tractional retinal detachment
  evidence:
  - reference: PMID:35243129
    reference_title: "Ocular findings associated with FADD deficiency resemble familial exudative vitreoretinopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "progression to tractional retinal detachment in the right eye"
    explanation: >-
      Documents the progression to detachment in the reported case.
biochemical:
- name: Soluble Fas ligand
  notes: >-
    Raised serum soluble Fas ligand is part of the ALPS biomarker panel that
    FADD deficiency shares, and it is measured in the reported patients. It is
    curated here rather than as a phenotype because HPO has no term for it — a
    search of HP returns nothing for Fas ligand — and the entry does not bind a
    misleading substitute. The other three panel members do have HP terms and
    are curated as phenotypes: double-negative T cell expansion, interleukin 10,
    and vitamin B12.
  presence: INCREASED
  evidence:
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Soluble Fas ligand, IL-10, IL- 18, and vitamin B12 levels were increased"
    explanation: >-
      The measurement in the reported patient, with its direction. The
      hyphenation is how the text extracts from the cached PDF.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "decreased Fas-mediated lym- phocyte apoptosis, increased circulating double negative T cells, elevated soluble Fas ligand, IL-10, and vitamin B12"
    explanation: >-
      States the four-member panel as a class, which is what makes soluble Fas
      ligand a shared ALPS biomarker rather than a FADD-specific one. The
      hyphenation is how the text extracts from the cached PDF.

genetic:
- name: FADD
  gene_term:
    preferred_term: FADD
    term:
      id: hgnc:3573
      label: FADD
  association: Pathogenic Variants
  relationship_type: CAUSATIVE
  notes: >-
    FADD is the only gene implicated. Reported alleles are missense and
    truncating, and act by reducing steady-state protein rather than abolishing
    it. Two genetic routes to the phenotype are now recognized and should not be
    conflated: the classical biallelic germline route, and a second in which a
    germline monoallelic FADD mutation inherited from a healthy parent is
    rendered homozygous in the pathogenic double-negative T cell compartment by
    a somatic event — uniparental disomy — so that the disease-causing genotype
    exists only in the affected cells. Sequencing peripheral blood in the second
    situation can miss the diagnosis. One allele recurs: p.Cys105Trp (c.315T>G),
    the homozygous genotype of the founding consanguineous kindred and of every
    subsequently reported homozygous patient up to 2020, and the genotype of the
    nuclear family whose presentation was recurrent febrile infection-related
    epilepsy syndrome.
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "By a combination of genome-wide linkage and whole-exome sequencing, we identified a homozygous missense mutation in FADD, encoding the Fas-associated death domain protein (FADD), in the patients."
    explanation: >-
      The gene-discovery statement establishing FADD as causative.
  - reference: PMID:37793571
    reference_title: "Combined germline and somatic human FADD mutations cause autoimmune lymphoproliferative syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We found homozygous FADD mutations in the DN T cells from all 4 patients, which resulted from uniparental disomy."
    explanation: >-
      Establishes the somatic second-hit route, in four unrelated patients, and
      names the mechanism. This is why the entry does not describe FADD disease
      as germline-only.
  - reference: PMID:37793571
    reference_title: "Combined germline and somatic human FADD mutations cause autoimmune lymphoproliferative syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "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."
    explanation: >-
      Describes the germline-plus-somatic architecture, including that the
      germline allele came from an unaffected parent — the reason a
      heterozygous parental result does not exclude the diagnosis.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a maternally inherited nonsense variant characterized by a 7 base-pair deletion (c.52_58delGACGAGC) predicted to severely truncate the protein, and a paternally inherited rare missense variant"
    explanation: >-
      A worked compound-heterozygous genotype pairing a truncating and a
      missense allele, establishing that allele class in trans.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All prior reports of patients with FADD deficiency were ho- mozygous for the pathogenic variation p.C105W."
    explanation: >-
      Establishes p.C105W as the recurring allele across every previously
      reported homozygous patient — which is what makes the compound
      heterozygous genotype in this report novel. The hyphenation is how the
      text extracts from the cached PDF.
  - reference: PMID:38752438
    reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Whole-exome sequencing (WES) revealed a homozygous p.C105W pathogenic variant of FADD gene"
    explanation: >-
      A further independent family homozygous for the same recurring allele,
      here presenting as recurrent febrile infection-related epilepsy syndrome
      — the observation behind this entry's variable-expressivity discussion.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Only a small number of families have been reported since the disorder was
    delineated in 2010 — a founding consanguineous kindred, subsequent compound
    heterozygous and single-family reports, and four patients with the somatic
    second-hit architecture. No pooled count and no population prevalence
    estimate has been published, so no number is asserted here.
  evidence:
  - reference: PMID:35243129
    reference_title: "Ocular findings associated with FADD deficiency resemble familial exudative vitreoretinopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Fas-associated protein with death domain (FADD) deficiency is a rare and severe genetic mutation characterized by recurrent infections, encephalopathy, cardiac abnormalities, and short life expectancy."
    explanation: >-
      Characterizes the disorder as rare and severe. Used to support the
      ULTRA_RARE band qualitatively; no count is claimed because none is
      published.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "There are very few cases in the literature of FADD deficiency patients"
    explanation: >-
      Direct statement of the size of the reported literature, without
      converting it into a count this entry would then have to defend.
progression:
- phase: Infancy and early childhood
  notes: >-
    Presentation is in infancy or early childhood with infection and the first
    encephalopathic crises. Life expectancy is described as short, and the
    encephalopathic episodes carry much of the mortality.
  evidence:
  - reference: PMID:35243129
    reference_title: "Ocular findings associated with FADD deficiency resemble familial exudative vitreoretinopathy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "characterized by recurrent infections, encephalopathy, cardiac abnormalities, and short life expectancy"
    explanation: >-
      States the severity and shortened life expectancy of the disorder.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Three of four patients from the original report died prior to 5 years old"
    explanation: >-
      Quantifies the early mortality in the founding kindred, which is what
      "short life expectancy" means concretely.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "These patients seem to have worse outcomes than classical ALPS patients."
    explanation: >-
      Supports the prognostic contrast with ALPS, quoted with the hedge the
      authors used.
- phase: Episodic relapsing course
  notes: >-
    Between crises patients can be comparatively well; the course is
    relapsing and precipitated by febrile illness rather than steadily
    progressive. Expressivity varies — the Epilepsia report notes that its
    family widens the recognized phenotype.
  evidence:
  - reference: PMID:38752438
    reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it increases the number of reported patients with FADD deficiency, showing that this disorder may present variable expressivity"
    explanation: >-
      Supports the variable expressivity claim from the reporting authors.
diagnosis:
- name: Fas-mediated apoptosis functional assay
  description: >-
    A functional apoptosis assay showing impaired Fas-mediated lymphocyte death,
    in a patient without a FAS, FASLG or CASP10 mutation, is what points at FADD.
    The assay is also what distinguishes the disorder from the rest of the ALPS
    spectrum, since the biomarker panel alone does not.
  notes: >-
    No diagnosis_term is bound. NCIT has no term for a functional apoptosis
    assay — searching it returns apoptosis as a biological process, apoptosis
    markers and apoptosis-regulating proteins, but no diagnostic procedure — and
    the cited source does not state the assay platform, so binding a generic
    flow-cytometry term would assert a method the reference does not report.
  evidence:
  - reference: PMID:21109225
    reference_title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "This FADD mutation decreases steady-state protein levels and impairs Fas-dependent apoptosis in vitro"
    explanation: >-
      Establishes the assay abnormality that is diagnostically informative.
- name: Sequencing of sorted double-negative T cells
  description: >-
    Where a germline monoallelic FADD variant is found in a patient with an
    ALPS phenotype, the disease-causing homozygous genotype may exist only in
    the double-negative T cell compartment. Sequencing sorted CD4-positive or
    double-negative T cells, rather than whole blood, is what reveals it — the
    same strategy previously used for somatic FAS events.
  diagnosis_term:
    preferred_term: sequencing of DNA from sorted T cell subsets
    term:
      id: NCIT:C153598
      label: DNA Sequencing
  evidence:
  - reference: PMID:37793571
    reference_title: "Combined germline and somatic human FADD mutations cause autoimmune lymphoproliferative syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We sequenced FADD and performed array-based comparative genomic hybridization using DNA from sorted CD4+ or DN T cells."
    explanation: >-
      Describes the specimen and method that detect the somatic event.
  - reference: PMID:37793571
    reference_title: "Combined germline and somatic human FADD mutations cause autoimmune lymphoproliferative syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "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."
    explanation: >-
      Explains why the double-negative compartment is the right specimen — it
      is where the somatic events accumulate.
- name: Biomarker panel with double-negative T cell phenotyping
  description: >-
    Where exome sequencing is negative in a patient whose clinical picture and
    biomarkers look like ALPS, the combination of serum biomarkers with
    double-negative T cell phenotyping is what identifies the cases it missed —
    among them patients with heterozygous FADD mutations plus somatic loss of
    heterozygosity. This is the systematic counterpart of the sorted-cell
    sequencing entry above: it says which patients to send for it.
  diagnosis_term:
    preferred_term: double-negative T cell immunophenotyping
    term:
      id: NCIT:C113003
      label: Immunological Flow Cytometry
  evidence:
  - reference: PMID:37979702
    reference_title: "Abnormal biomarkers predict complex FAS or FADD defects missed by exome sequencing."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "A combination of serum biomarkers and DNT phenotyping is an accurate means to identify patients with ALPS who are missed by routine exome sequencing."
    explanation: >-
      The study's own conclusion about what this diagnostic combination
      achieves.
  - reference: PMID:37979702
    reference_title: "Abnormal biomarkers predict complex FAS or FADD defects missed by exome sequencing."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Three of the 4 ALPS-U patients with normal FAS expression carried heterozygous FADD mutations with sLOH."
    explanation: >-
      The FADD-specific yield, and the reason this is curated here rather than
      only in the ALPS entry: normal FAS expression on double-negative T cells
      is what points away from FAS and towards FADD.
differential_diagnoses:
- name: Autoimmune lymphoproliferative syndrome (ALPS-FAS)
  description: >-
    The nearest mimic, and the reason FADD deficiency is easy to misfile. The
    two share the biomarker panel — impaired Fas-mediated apoptosis, expanded
    double-negative T cells, raised soluble Fas ligand, interleukin-10 and
    vitamin B12. They differ in what surrounds it: FADD deficiency has milder
    lymphoproliferation and adds recurrent febrile encephalopathy, hepatopathy,
    cardiac malformation and functional hyposplenism, none of which ALPS-FAS
    produces. The ALPS entry in this knowledge base cites a source that draws
    exactly this distinction.
  evidence:
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Previously described FADD deficiency patients demonstrate a clinical phenotype partially overlap- ping with other ALPS disorders"
    explanation: >-
      States the partial overlap that makes ALPS the differential. The
      hyphenation is how the text extracts from the cached PDF.
treatments:
- name: Immunoglobulin Replacement
  description: >-
    Subcutaneous immunoglobulin replacement, given at 400 mg/kg/month in the one
    report that describes its use, directed at the impaired antibody responses —
    absent isohaemagglutinins and suboptimal anti-pneumococcal titres despite
    repeated conjugate vaccination — rather than at the apoptosis defect. It
    substitutes for the antibody the patient cannot make; it does not correct
    FADD function, and no series has evaluated it.
  therapeutic_modality: PROTEIN_REPLACEMENT
  treatment_term:
    preferred_term: immunoglobulin therapy
    term:
      id: NCIT:C62710
      label: Immunoglobulin Therapy
  target_mechanisms:
  - target: Recurrent Severe Infection
    treatment_effect: BYPASSES
    description: >-
      Supplying immunoglobulin bypasses the impaired humoral response
      downstream of the FADD lesion; it leaves every upstream node untouched.
    evidence:
    - reference: PMID:32350755
      reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "To address possible impaired viral responses and lack of appropriate B cell antibodies (isohemagglutinins), we initiated subcutaneous immunoglobulin infusions"
      explanation: >-
        States what the replacement was aimed at — the humoral deficit — which
        is the claim this link makes, distinct from the claim that it was given.
  evidence:
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we initiated subcutaneous immunoglobulin infusions at 400 mg/kg/month"
    explanation: >-
      The one reported administration, with route and dose. A single patient, so
      this documents that the treatment was given — not that it is effective.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The patient tolerated these infusions well with no side effects and subjec- tive improvements in fatigue and decreased respiratory infec- tions overall for the past 8 months"
    explanation: >-
      The only reported outcome. Classified PARTIAL and quoted in full because
      the improvements are explicitly subjective, in one uncontrolled patient
      over eight months. The hyphenation is how the text extracts from the
      cached PDF.
- name: Antibiotic Prophylaxis
  description: >-
    Antipneumococcal antibiotic prophylaxis is recommended on the strength of
    the functional hyposplenism, which is what puts these patients at risk of
    overwhelming encapsulated-organism sepsis. It is recorded here as a
    recommendation rather than as established practice: in the single report that
    describes it, the family declined it and the recommendation was still under
    discussion at the time of writing.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
  target_mechanisms:
  - target: Functional Hyposplenism
    treatment_effect: BYPASSES
    description: >-
      Prophylaxis does not restore splenic phagocyte function; it substitutes
      for the clearance the hyposplenic patient has lost.
    evidence:
    - reference: PMID:32350755
      reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "given his functional hyposplenism and risk of sepsis from encapsulated organisms"
      explanation: >-
        Names the hyposplenism as the reason prophylaxis was recommended, which
        is what ties this treatment to this node.
  evidence:
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "family declined antibiotic prophylaxis, although there are on- going discussions continuing to recommend it"
    explanation: >-
      Classified PARTIAL deliberately: the source establishes that prophylaxis
      was recommended, and simultaneously that it was not administered, so it
      cannot support an efficacy claim. The hyphenation is how the text extracts
      from the cached PDF.
- name: Haematopoietic Cell Transplantation
  description: >-
    Transplantation has been performed in a small number of reported patients
    and is the only intervention that could replace the FADD-deficient
    haematopoietic compartment. Two limits are explicit in the literature and
    are the reason this is not curated as established management: outcomes in
    the transplanted patients diverged sharply, and the non-haematopoietic
    manifestations — the encephalopathic crises, the cardiac malformation, the
    hepatopathy — arise in tissues a graft does not replace, so whether
    transplantation affects them is unknown.
  therapeutic_modality: CELL_THERAPY
  treatment_term:
    preferred_term: hematopoietic cell transplantation
    term:
      id: NCIT:C15431
      label: Hematopoietic Cell Transplantation
  target_mechanisms:
  - target: Failure of Activation-Induced Lymphocyte Apoptosis
    treatment_effect: RESTORES
    description: >-
      Replacing the haematopoietic compartment replaces the lymphocytes that
      carry the apoptosis defect. It reaches only this arm of the disease.
    evidence:
    - reference: PMID:32350755
      reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "it remains unclear what degree of donor chimerism is needed to be curative of the immunodeficiency and how HSCT will affect non- hematopoietic manifestations of FADD deficiency"
      explanation: >-
        The source's own division of the disease into the compartment a graft
        replaces and the manifestations it does not — which is exactly why this
        link targets the lymphocyte-apoptosis node alone. PARTIAL because the
        same sentence says the curative threshold is unknown. The hyphenation is
        how the text extracts from the cached PDF.
  evidence:
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two previously described patients underwent hematopoietic stem cell transplantation (HSCT)"
    explanation: >-
      Establishes that transplantation has been performed, and how few times.
  - reference: PMID:32350755
    reference_title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "HSCT will likely be an important part of the treatment regimen for FADD deficien- cy, but given the rarity of the disease and the paucity of data on all of the potential clinical manifestations, it remains unclear what degree of donor chimerism is needed to be curative of the immunodeficiency"
    explanation: >-
      The authors' own hedge, quoted rather than paraphrased: transplantation is
      expected to matter but the curative threshold is not known. This is why
      the entry does not assert transplantation as standard of care. The
      hyphenation is how the text extracts from the cached PDF.
- name: Immunomodulatory Therapy
  description: >-
    Early immunomodulatory treatment is argued for on mechanistic grounds in the
    family whose presentation was recurrent febrile infection-related epilepsy
    syndrome: if the encephalopathic crises reflect dysregulated cell-mediated
    inflammation rather than infection itself, immunomodulation is the rational
    target. This is an argument from mechanism, not a result — no trial, series,
    or controlled comparison exists in FADD deficiency.
  therapeutic_modality: OTHER
  treatment_term:
    preferred_term: immunomodulation therapy
    term:
      id: NCIT:C116540
      label: Immunomodulation Therapy
  target_mechanisms:
  - target: Recurrent Febrile Encephalopathic Crises
    treatment_effect: MODULATES
    description: >-
      Proposed to act on the inflammatory regulatory pathway held to generate
      the crises. Untested.
    evidence:
    - reference: PMID:38752438
      reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: "it demonstrates a genetic cause of FIRES involving a cell-mediated inflammation regulatory pathway. This finding supports early treatment with immunomodulatory therapy"
      explanation: >-
        Carries both halves of the link in one sentence: the pathway held to
        generate the crises, and the therapeutic inference drawn from it.
        PARTIAL — an inference, with no outcome reported.
  evidence:
  - reference: PMID:38752438
    reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This finding supports early treatment with immunomodulatory therapy"
    explanation: >-
      Classified PARTIAL: the source recommends the approach as a consequence of
      identifying the genetic cause, and reports no outcome of having given it.
animal_models:
- name: Fadd conditional knockout mouse
  species: Mouse
  genotype: Fadd conditional deletion (Mx1-cre inducible, and tissue-specific alleles)
  category: Genetic
  description: >-
    Constitutive Fadd loss is embryonic lethal, so the mouse work uses
    conditional alleles. Inducible deletion across haematopoietic lineages
    depletes peripheral lymphocytes over time and impairs early bone-marrow
    haematopoiesis — and pointedly does NOT reproduce the lymphoproliferative
    disease seen in Fas-deficient mice, which is the murine counterpart of the
    human observation that FADD deficiency is not simply severe ALPS.
    Epithelium-specific alleles reveal a different function again: FADD restrains
    RIPK3-MLKL necroptosis and caspase-8-gasdermin-D pyroptosis-like death, and
    its loss in gut or skin epithelium causes inflammatory disease that human
    patients do not have.
  publication: PMID:21115735
  genes:
  - preferred_term: FADD
    term:
      id: hgnc:3573
      label: FADD
  modeled_mechanisms:
  - target: Failure of Activation-Induced Lymphocyte Apoptosis
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Haematopoietic Fadd deletion perturbs lymphocyte homeostasis, but in the
      opposite direction to the human disease: mice lose peripheral lymphocytes
      over time rather than accumulating an apoptosis-resistant population.
    limitations: >-
      The mouse model deletes Fadd outright, whereas human disease alleles
      reduce protein without abolishing it; and the murine outcome is lymphocyte
      depletion rather than the double-negative T cell expansion that defines
      the human laboratory phenotype.
    readouts:
    - name: Peripheral lymphocyte number after inducible Fadd deletion
      target: Failure of Activation-Induced Lymphocyte Apoptosis
      direction: DECREASED
      interpretation: >-
        Time-dependent depletion, which is the direction opposite to the human
        accumulation and is the reason this link is only partial.
      evidence:
      - reference: PMID:21115735
        reference_title: "FADD deficiency impairs early hematopoiesis in the bone marrow."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "Instead, a time-dependent depletion of peripheral FADD-deficient lymphocytes was observed."
        explanation: >-
          The measurement and its direction.
    evidence:
    - reference: PMID:21115735
      reference_title: "FADD deficiency impairs early hematopoiesis in the bone marrow."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "The resulting FADD mutant mice did not develop lymphoproliferation diseases, unlike Fas-deficient mice."
      explanation: >-
        Classified PARTIAL: the model is informative about the lymphocyte arm,
        but its explicit divergence from the Fas-deficient phenotype is also
        what limits how far it can stand in for the human disease.
  evidence:
  - reference: PMID:21115735
    reference_title: "FADD deficiency impairs early hematopoiesis in the bone marrow."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "In addition, a death receptor-independent function of FADD is essential for embryogenesis."
    explanation: >-
      Establishes the embryonic-lethality constraint that forces conditional
      alleles and that supports the residual-function inference in human
      patients.
- name: Endothelial (Tie2-cre) Fadd conditional knockout mouse
  species: Mouse
  genotype: Fadd-/- Fadd:gfp+ Tie2-cre+ (FADD:GFP deleted in Tie-2 expressing cells)
  category: Genetic
  description: >-
    A tissue-by-tissue dissection of which cell type accounts for the embryonic
    lethality of Fadd loss, and the only murine result that speaks to the human
    cardiac malformation. Deleting FADD:GFP in cardiomyocytes or cardiac
    progenitors is tolerated; deleting it in Tie-2 expressing cells — endothelium
    and haematopoietic cells — reproduces the germline-null lethality with
    cardiovascular defects, and deleting Ripk3 rescues it. So the cardiac
    phenotype is not cardiomyocyte-autonomous, and it runs through RIPK3, a
    death-receptor-independent FADD function.
  publication: PMID:27584790
  genes:
  - preferred_term: FADD
    term:
      id: hgnc:3573
      label: FADD
  modeled_mechanisms:
  - target: Loss of Fas-Independent FADD Functions
    relationship: PARTIALLY_RECAPITULATES
    fidelity: LOW
    description: >-
      Demonstrates that FADD restrains RIPK3 signalling in endothelium and that
      losing this causes a cardiovascular developmental defect — the class of
      Fas-independent function this node asserts, and a candidate route to the
      human cardiac malformation.
    limitations: >-
      Fidelity is LOW and the link deliberately stops at "candidate route".
      The model deletes Fadd outright and dies at E11.5, whereas human alleles
      reduce protein without abolishing it and patients survive to be born; the
      murine lesion is a lethal failure of cardiovascular development, not the
      structural malformations (pulmonary atresia, ventricular septal defect,
      anomalous venous drainage) reported in patients; and no human FADD patient
      has been shown to have RIPK3-dependent endothelial pathology. No causal
      claim about the human cardiac phenotype is made from this model.
    readouts:
    - name: Ventricular trabeculation and endocardial cushion formation at E11.5
      target: Loss of Fas-Independent FADD Functions
      direction: DECREASED
      interpretation: >-
        The structural measurement behind the cardiovascular arm of the
        phenotype, and the endpoint that Ripk3 deletion restores.
      evidence:
      - reference: PMID:27584790
        reference_title: "Lack of FADD in Tie-2 expressing cells causes RIPK3-mediated embryonic lethality."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "displayed a low degree of trabeculation in the walls of the common ventricular chamber and endocardial cushion defect by reduced endothelial-to-mesenchymal formation"
        explanation: >-
          The histological measurement and its direction.
    evidence:
    - reference: PMID:27584790
      reference_title: "Lack of FADD in Tie-2 expressing cells causes RIPK3-mediated embryonic lethality."
      supports: SUPPORT
      directness: INDIRECT
      evidence_source: MODEL_ORGANISM
      snippet: "these results demonstrated that RIPK3-mediated signaling in Tie-2 expressing cells was responsible for the embryonic lethality"
      explanation: >-
        Names the mechanism and the responsible compartment. INDIRECT because it
        establishes a Fas-independent, RIPK3-dependent FADD function in
        endothelium without establishing that this is what produces the human
        cardiac phenotype.
  evidence:
  - reference: PMID:27584790
    reference_title: "Lack of FADD in Tie-2 expressing cells causes RIPK3-mediated embryonic lethality."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "suggesting that loss of Fadd in endothelial cells causes endocardium-related cardiac development defect"
    explanation: >-
      The authors' own statement of what the endothelial deletion shows, quoted
      with their hedge intact.
  - reference: PMID:27584790
    reference_title: "Lack of FADD in Tie-2 expressing cells causes RIPK3-mediated embryonic lethality."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Mice with conditional deletion of Fadd in immune cells, skin or intestine produced no lethality"
    explanation: >-
      The negative controls that make the endothelial result specific rather
      than a general consequence of losing Fadd anywhere.
discussions:
- discussion_id: mismatch_mouse_necroptosis_arm
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  prompt: >-
    Mouse Fadd deletion in epithelium causes RIPK3-MLKL necroptosis and
    caspase-8-gasdermin-D pyroptosis-like death, with colitis and skin
    inflammation. Human FADD-deficient patients do not have inflammatory bowel
    disease. Does the necroptosis-restraint function of FADD operate in human
    tissue, and if so why is it not clinically apparent?
  attaches_to:
  - "pathophysiology#Loss of Fas-Independent FADD Functions"
  rationale: >-
    This is a genuine translational discrepancy rather than a gap in the human
    literature. The mouse work is unambiguous that FADD restrains two distinct
    lytic death programmes in epithelium, and lytic death is exactly the sort of
    mechanism that would be expected to produce inflammatory disease. Two
    explanations are compatible with what is known and are not distinguishable
    on current evidence: human disease alleles are hypomorphic rather than null,
    so enough FADD remains to restrain necroptosis while being insufficient for
    efficient DISC assembly; or the epithelial requirement genuinely differs
    between species. Which it is bears directly on whether the encephalopathic
    crises might themselves be a lytic-death phenomenon, and therefore on
    whether necroptosis inhibition would be a rational therapeutic direction.
  proposed_experiments:
  - experiment_id: exp_necroptosis_competence_patient_cells
    name: Necroptosis competence in patient-derived cells
    description: >-
      Challenge patient-derived cells carrying the reported hypomorphic alleles
      with TNF plus caspase inhibition and measure MLKL phosphorylation and
      lytic death against isogenic FADD-null and wild-type controls. Preserved
      restraint in patient cells would support the hypomorphic explanation;
      unrestrained necroptosis would make the species-difference explanation
      more likely and would raise the question of why the gut is spared.
  evidence:
  - reference: PMID:32362323
    reference_title: "FADD and Caspase-8 Regulate Gut Homeostasis and Inflammation by Controlling MLKL- and GSDMD-Mediated Death of Intestinal Epithelial Cells."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Mice with IEC-specific FADD or caspase-8 deficiency developed colitis dependent on mixed lineage kinase-like (MLKL)-mediated epithelial cell necroptosis."
    explanation: >-
      Establishes the murine phenotype whose human counterpart is missing, which
      is the mismatch itself.
  - reference: PMID:25132550
    reference_title: "RIPK1 maintains epithelial homeostasis by inhibiting apoptosis and necroptosis."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "a RIPK1 kinase inactive knock-in delayed but did not prevent inflammation caused by FADD deficiency in IECs or keratinocytes"
    explanation: >-
      Confirms the epithelial inflammatory consequence of Fadd loss in a second
      tissue and a second laboratory, so the mismatch is not a single-study
      artefact.
- discussion_id: gap_febrile_encephalopathy_mechanism
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    By what mechanism does febrile infection precipitate the stereotyped
    encephalopathic crises of FADD deficiency?
  attaches_to:
  - "pathophysiology#Recurrent Febrile Encephalopathic Crises"
  rationale: >-
    The crises are the feature that most distinguishes this disorder from the
    ALPS spectrum and carry much of its mortality, yet they are the least
    mechanistically explained. The reporting authors attribute them to
    disrupted FAS-mediated apoptosis, but that leaves unexplained why the
    episodes are febrile-triggered, why they are stereotyped and recurrent
    rather than progressive, and why the brain in particular. That FADD
    variants can produce a picture meeting FIRES criteria suggests a
    neuroinflammatory rather than a purely apoptotic route, which would have
    direct therapeutic consequences — the Epilepsia authors argue on that basis
    for early immunomodulatory treatment.
  proposed_experiments:
  - experiment_id: exp_crisis_neuroinflammatory_profiling
    name: Cytokine and cell-death profiling during and between crises
    description: >-
      Profile CSF and paired serum cytokines, and markers of lytic cell death,
      in the same patient during a febrile encephalopathic crisis and at
      baseline. A crisis-specific inflammatory signature would support the
      neuroinflammatory route and the case for immunomodulation; its absence
      would push attention back towards a cell-autonomous neuronal
      death-signalling defect.
  evidence:
  - reference: PMID:38752438
    reference_title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "it demonstrates a genetic cause of FIRES involving a cell-mediated inflammation regulatory pathway. This finding supports early treatment with immunomodulatory therapy"
    explanation: >-
      States the neuroinflammatory framing and the therapeutic inference drawn
      from it, which is the hypothesis the proposed experiment would test.
notes: >-
  On the treatments block. No FADD-specific therapy has been evaluated in any
  series, so every entry is scoped to what a cited source actually reports:
  immunoglobulin replacement and antibiotic prophylaxis are recorded as
  administered and as recommended-but-declined respectively, in one patient
  each; transplantation is recorded as performed in a small number of patients
  with the authors' own hedge about curative threshold quoted rather than
  paraphrased; and immunomodulation is recorded as an argument from mechanism
  with no reported outcome. Three of the four carry supports: PARTIAL for that
  reason. Antiseizure treatment is not curated — the reported patients receive
  it, but no cited source in this entry states so in quotable form.

  Hematopoietic transplantation in FADD deficiency is also described in Savic et
  al. 2015 (PMID:25794656), which is why that reference is listed below. It is
  not cited as evidence anywhere in this entry: the record caches with
  content_type unavailable and a zero-length body, so nothing in it can be
  quoted. The transplantation claims rest on PMID:32350755, which reviews those
  same patients in text that is available.

  Functional hyposplenism has no HP term. HPO was searched directly rather
  than inferred from the research report's suggestion: the only spleen-function
  hits are HP:0001746 Asplenia, which is anatomical absence and is wrong here
  (the reported patients have a spleen — PMID:35243129 notes Howell-Jolly
  bodies "despite the presence of a spleen"), and HP:0001971 Hypersplenism,
  which the report offered and which is the clinical opposite. So the
  hyposplenic state is described in prose and the ontology binding is carried
  by Howell-Jolly bodies (HP:0032550), the objective marker of it. The parent of
  the Hypersplenism term, HP:0025409 Abnormal spleen physiology, was also
  considered and passed over: it is the one spleen-function term that is neither
  inverted nor anatomical, but Hypersplenism is its only descendant, so it
  carries almost no information beyond the prose — and binding a near-vacuous
  parent alongside the specific finding that is already curated would add noise
  rather than precision. Soluble Fas
  ligand is handled the same way and for the same reason: a direct HPO search
  for "Fas" returns no relevant term.

  A note on this entry's deep-research report. Its NEC preflight passed and all
  9 references resolved, but seven of its suggested ontology terms were wrong on
  checking against OLS4, several dangerously: HP:0001971 was offered for
  "functional abnormality of the spleen" but is Hypersplenism; HP:0031913 was
  offered for Howell-Jolly bodies but is Rhombencephalosynapsis; HP:0011146 was
  offered for encephalopathy but is Dialeptic seizure; HP:0040218 was offered
  for double-negative T lymphocytosis but is Reduced total natural killer cell
  count; HP:0040214 was offered for elevated vitamin B12 but is Abnormal
  circulating insulin concentration. The report also asserted that "no somatic
  FADD variants have been implicated", which PMID:37793571 directly contradicts
  — that paper reports homozygous FADD mutations arising by uniparental disomy
  in the double-negative T cells of four unrelated patients. The somatic route
  is curated in the genetic section and in a dedicated diagnosis entry.

  Phenotype connectivity is 10 of 14, and the four unwired ones are unwired on
  purpose. Abnormal heart morphology, hepatic failure, cystoid macular oedema
  and tractional retinal detachment are all well evidenced as features of the
  disorder, but no source places them downstream of any node in this pathograph.
  For the cardiac phenotype the nearest thing to a mechanism is the Tie2-cre
  endothelial mouse curated below, and that link is deliberately held at
  fidelity LOW with the causal claim about human cardiac malformation explicitly
  refused — drawing a pathograph edge would make the assertion the model link
  declines to make. The ocular findings rest on a single 2022 patient, and the
  hepatopathy has no proposed mechanism at all in the cited literature. An edge
  in this graph is a causal claim; four phenotypes with no such claim available
  are better left visibly unconnected than wired on plausibility.

  No `datasets:` block: no disease-specific dataset was identified for this
  disorder.
references:
- reference: PMID:21109225
  title: "Whole-exome-sequencing-based discovery of human FADD deficiency."
- reference: PMID:32350755
  title: "Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency."
- reference: PMID:37793571
  title: "Combined germline and somatic human FADD mutations cause autoimmune lymphoproliferative syndrome."
- reference: PMID:38752438
  title: "FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review."
- reference: PMID:35243129
  title: "Ocular findings associated with FADD deficiency resemble familial exudative vitreoretinopathy."
- reference: PMID:21115735
  title: "FADD deficiency impairs early hematopoiesis in the bone marrow."
- reference: PMID:32362323
  title: "FADD and Caspase-8 Regulate Gut Homeostasis and Inflammation by Controlling MLKL- and GSDMD-Mediated Death of Intestinal Epithelial Cells."
- reference: PMID:25132550
  title: "RIPK1 maintains epithelial homeostasis by inhibiting apoptosis and necroptosis."
- reference: PMID:37199216
  title: "Expanding the clinical phenotype of FADD deficiency with a novel mutation and its role in Fas-mediated apoptotic pathway."
- reference: PMID:37979702
  title: "Abnormal biomarkers predict complex FAS or FADD defects missed by exome sequencing."
- reference: PMID:27584790
  title: "Lack of FADD in Tie-2 expressing cells causes RIPK3-mediated embryonic lethality."
- reference: PMID:25794656
  title: "A new case of Fas-associated death domain protein deficiency and update on treatment outcomes."
📚

References & Deep Research

References

12
PMID:21109225
Whole-exome-sequencing-based discovery of human FADD deficiency.
No top-level findings curated for this source.
PMID:32350755
Novel Compound Heterozygote Variations in FADD Identified to Cause FAS-Associated Protein with Death Domain Deficiency.
No top-level findings curated for this source.
PMID:37793571
Combined germline and somatic human FADD mutations cause autoimmune lymphoproliferative syndrome.
No top-level findings curated for this source.
PMID:38752438
FADD gene pathogenic variants causing recurrent febrile infection-related epilepsy syndrome: Case report and literature review.
No top-level findings curated for this source.
PMID:35243129
Ocular findings associated with FADD deficiency resemble familial exudative vitreoretinopathy.
No top-level findings curated for this source.
PMID:21115735
FADD deficiency impairs early hematopoiesis in the bone marrow.
No top-level findings curated for this source.
PMID:32362323
FADD and Caspase-8 Regulate Gut Homeostasis and Inflammation by Controlling MLKL- and GSDMD-Mediated Death of Intestinal Epithelial Cells.
No top-level findings curated for this source.
PMID:25132550
RIPK1 maintains epithelial homeostasis by inhibiting apoptosis and necroptosis.
No top-level findings curated for this source.
PMID:37199216
Expanding the clinical phenotype of FADD deficiency with a novel mutation and its role in Fas-mediated apoptotic pathway.
No top-level findings curated for this source.
PMID:37979702
Abnormal biomarkers predict complex FAS or FADD defects missed by exome sequencing.
No top-level findings curated for this source.
PMID:27584790
Lack of FADD in Tie-2 expressing cells causes RIPK3-mediated embryonic lethality.
No top-level findings curated for this source.
PMID:25794656
A new case of Fas-associated death domain protein deficiency and update on treatment outcomes.
No top-level findings curated for this source.

Deep Research

1
Claude Code ▸
1. Disease Information
claude-haiku-4-5-20251001, claude-sonnet-5 9 citations 2026-08-24T16:19:06.936769

1. Disease Information

Overview: FADD-related immunodeficiency (also termed FADD deficiency or Immunodeficiency-90 with encephalopathy, functional hyposplenia, and hepatic dysfunction, IMD90) is an ultra-rare autosomal recessive primary immunodeficiency/immune dysregulation disorder caused by biallelic loss-of-function variants in FADD (Fas-associated protein with death domain), the central adaptor protein of the death-inducing signaling complex (DISC) in extrinsic apoptosis. It presents a phenotype that partially overlaps with autoimmune lymphoproliferative syndrome (ALPS) — impaired Fas-mediated lymphocyte apoptosis and elevated double-negative T cells — but is clinically distinct, combining recurrent severe bacterial and viral infections, functional hyposplenism, recurrent hepatopathy, and characteristic stereotyped febrile encephalopathic episodes with refractory seizures, sometimes meeting criteria for febrile infection-related epilepsy syndrome (FIRES). It was first molecularly characterized in 2010 via combined genome-wide linkage analysis and whole-exome sequencing (Bolze et al., Am J Hum Genet 2010;87(6):873–881, PMID:21109225) omim.org.

Key identifiers: - OMIM phenotype: #613759 (IMMUNODEFICIENCY 90 WITH ENCEPHALOPATHY, FUNCTIONAL HYPOSPLENIA, AND HEPATIC DYSFUNCTION; IMD90) omim.org - OMIM gene: *602457 (FAS-ASSOCIATED VIA DEATH DOMAIN; FADD) omim.org - MONDO: MONDO:0013408 thegencc.org - HGNC: 3573 (gene symbol FADD, also known as MORT1, GIG3) genecards.org - Gene location: chromosome 11q13.3 atlasgeneticsoncology.org - Orphanet, GARD (NIH rare disease portal): "FADD-related immunodeficiency" rarediseases.info.nih.gov; also listed under NORD rarediseases.org - NCBI GTR condition: C3151062 ncbi.nlm.nih.gov - Protein: UniProt Q13158, 208 amino acids, ~23.3 kDa genecards.org

Synonyms: FADD deficiency; MORT1 deficiency; IMD90; ALPS-like disease due to FADD deficiency.

Evidence basis: All published clinical information derives from aggregated case reports/case series (fewer than ~15 patients described worldwide across at least 8 published articles as of 2023–2024), not large cohort or EHR-derived data — a hallmark of an ultra-rare Mendelian disorder figshare.com dataset cited via search.

2. Etiology

Primary cause: Biallelic (homozygous or compound heterozygous) pathogenic variants in FADD, encoding the sole adaptor protein that couples activated death receptors (FAS/CD95, TNFR1) to initiator caspases-8/-10, causing partial loss of FADD protein function/expression.

Reported causal variants

Variant Zygosity Patients Source
c.315C>G / p.C105W (missense) Homozygous Original 3 affected members of a large consanguineous Pakistani kindred (not found in 282 Pakistani controls) Bolze et al. 2010, PMID:21109225 omim.org
c.313T>C / p.C105R (missense, novel) Compound heterozygous (with a truncating allele) 1 US infant (BJH-reported family) Setia et al. 2023, PMID:37199216
c.52_58delGACGAGC (7-bp deletion, frameshift/truncating) Compound heterozygous (paternal allele c.313T>C) Same infant as above Setia et al. 2023
Other compound heterozygous variants Compound heterozygous 2 patients (Journal of Clinical Immunology report) PMID for "Novel Compound Heterozygote Variations..." J Clin Immunol 2020, PMC7253512
Homozygous p.C105W Homozygous 1 patient presenting as FIRES Giovannini et al. 2024, Epilepsia 65(7):e119–e124, DOI:10.1111/epi.18008

The C105 residue recurs across independent reports (C105W and C105R), suggesting it is a mechanistic hotspot in the death-effector domain (DED) region critical for DISC assembly. Both novel variants reported in 2023 are absent from gnomAD in the homozygous state, consistent with severe deleteriousness under purifying selection [pmc.ncbi.nlm.nih.gov summary via WebFetch].

Risk factors: - Genetic: Consanguinity is a major risk factor — the founding kindred was a large consanguineous Pakistani family; homozygosity for rare deleterious FADD alleles is markedly more likely in consanguineous unions. - Environmental/infectious: Because FADD deficiency causes functional hyposplenism and impaired interferon-mediated antiviral immunity, environmental infectious exposures (particularly encapsulated bacteria such as Streptococcus pneumoniae, and viruses including HHV-6) act as major disease-modifying/triggering factors for both the infectious and encephalopathic components of the phenotype. - No protective genetic or environmental factors have been reported in the literature — the condition is too rare for population-level GWAS or protective-variant studies.

Gene–environment interaction: The stereotyped febrile encephalopathic episodes are typically precipitated by febrile illness/infection (in at least one reported case, immediately following MMR vaccination), implicating an infection- or immune-activation-triggered inflammatory/necroptotic cascade in genetically susceptible (FADD-deficient) neural or immune tissue as the proximate mechanism of the encephalopathy — consistent with a FIRES-like presentation.

3. Phenotypes

FADD deficiency phenotypes span immunologic, neurologic, hepatic, cardiac, splenic, and (recently recognized) ocular systems.

Phenotype Type Onset Frequency (qualitative, small case series) Suggested HPO term
Recurrent severe bacterial infections (esp. invasive pneumococcal disease) Clinical sign/symptom Infancy Frequent HP:0002718 (Recurrent infections); HP:0006515 (Recurrent pneumonia)
Recurrent severe viral infections Clinical sign/symptom Infancy Frequent HP:0002719
Functional hyposplenism (with Howell-Jolly bodies on smear) Laboratory abnormality Infancy/childhood Frequent HP:0001971 (Functional abnormality of the spleen); HP:0031913 (Howell-Jolly bodies)
Recurrent hepatopathy (portal inflammation, fibrosis) Clinical sign / laboratory Variable Frequent HP:0001395 (Hepatic fibrosis); HP:0001392 (Abnormal liver physiology)
Recurrent stereotyped febrile encephalopathic episodes with refractory seizures (some meeting FIRES criteria) Symptom/clinical sign Infancy–early childhood, episodic Frequent, often the most severe/lethal feature HP:0011146 (Encephalopathy); HP:0002373 (Febrile seizures); HP:0002373; HP:0032437 (encephalopathy episodes)
Cerebral atrophy Imaging finding Progressive with episodes Reported in several cases HP:0002059
Cardiac malformations (ventricular septal defect, pulmonary artery atresia) Structural anomaly Congenital Variable, reported subset HP:0001629 (VSD); HP:0004935 (Pulmonary artery atresia)
Variable lymphadenopathy/splenomegaly Clinical sign Variable Variable — milder than classic ALPS HP:0002716 (Lymphadenopathy); HP:0001744 (Splenomegaly)
Increased double-negative (CD3+TCRαβ+CD4−CD8−) T cells Laboratory abnormality — Consistent finding HP:0040218 (double-negative T-lymphocytosis, ALPS-associated)
Elevated soluble Fas ligand (sFasL) Laboratory abnormality — Consistent (biomarker, no dedicated HPO term)
Elevated IL-10, IL-18 Laboratory abnormality — Reported —
Elevated vitamin B12 Laboratory abnormality — Consistent, ALPS-like biomarker HP:0040214 (elevated vitamin B12)
Ocular findings resembling familial exudative vitreoretinopathy (FEVR-like retinal vascular changes) Clinical sign Reported in 1 recent case Newly described (2022) HP:0000501 (Glaucoma)/HP:0025580 (Vitreoretinopathy) — closest match
Absence of appropriate isohemagglutinins despite normal immunoglobulin levels Laboratory abnormality — Reported HP:0025406

Clinical course pattern: Episodic/relapsing-remitting encephalopathic crises superimposed on a chronic background of infection susceptibility and hepatopathy; disease course is frequently rapidly fatal in early childhood (see Outcome section) but is variable-severity, as newer reports (2023–2024) describe milder or more indolent presentations and longer survival with active management [WebFetch of GARD/PMC summaries].

Quality of life impact: No formal QOL instrument data (EQ-5D, SF-36) exist for this ultra-rare condition; qualitative reports describe severe impact from recurrent hospitalization, status epilepticus, and, in survivors, chronic fatigue and recurrent respiratory infections managed with immunoglobulin replacement (subjective improvement in fatigue reported with subcutaneous IgG in the 2023 case).

4. Genetic/Molecular Information

  • Causal gene: FADD (HGNC:3573; OMIM *602457), chromosome 11q13.3.
  • Variant classes reported: missense (p.C105W, p.C105R) at a conserved cysteine residue implicated in DED structure/function, and a frameshift/truncating 7-bp deletion (c.52_58delGACGAGC) predicted to severely truncate the protein.
  • Population frequency: No homozygous carriers of the reported pathogenic alleles are present in gnomAD, consistent with severe deleteriousness; the original p.C105W allele was absent from 282 ethnically matched Pakistani controls.
  • Functional consequence: Reduced FADD protein expression and profoundly impaired Fas-mediated apoptosis in patient-derived cells ("profound deficiency in appropriate cell death" on functional apoptosis assay) — consistent with partial/hypomorphic loss of function rather than complete null (complete FADD loss is embryonic lethal in mice — see Model Organisms section), implying that surviving human patients likely carry hypomorphic alleles that retain some residual function.
  • Modifier genes: None specifically established; phenotypic variability (e.g., presence/absence of cardiac malformations, ocular findings, severity of encephalopathy) across the small number of reported patients suggests possible modifier or stochastic effects, but this is not formally studied.
  • Somatic vs. germline: All reported FADD deficiency variants are germline; no somatic FADD variants have been implicated (contrasting with somatic FAS variants seen in some ALPS).
  • Epigenetics/chromosomal abnormalities: Not reported as a mechanism for FADD deficiency; not applicable. (Note: somatic 11q13.3 amplification involving FADD has separately been studied as an oncogenic driver in head and neck squamous cell carcinoma — a distinct cancer biology context, not part of the germline immunodeficiency disorder.)

Protein structure: FADD is a 208-amino-acid, ~23 kDa bipartite adaptor protein comprising an N-terminal death effector domain (DED, six-α-helix death-fold) and a C-terminal death domain (DD). Upon FAS/TNFR ligation, the receptor DD engages FADD's DD via homophilic interaction; FADD's DED then recruits procaspase-8/-10 DEDs to nucleate the death-inducing signaling complex (DISC), driving caspase-8/-10 activation and apoptosis execution [ncbi.nlm.nih.gov/PMC10970579; nature.com NMR structure Eberstadt et al.].

5. Environmental Information

  • Infectious triggers: Febrile infectious illness is the principal recognized trigger for the encephalopathic crises; HHV-6 IgG elevation was documented in one recent case as the only infectious workup abnormality during an encephalopathic episode. Vaccination (MMR) preceded a fatal encephalopathic episode in an affected sibling in one family, though causality versus coincidental febrile trigger cannot be established from a single case.
  • No specific toxin, occupational, or lifestyle risk factors have been identified — expected given the pediatric-onset, purely monogenic nature of the disease.
  • Bacterial pathogens of particular relevance: encapsulated organisms, especially Streptococcus pneumoniae, causing invasive pneumococcal disease exploiting the functional hyposplenic state.

6. Mechanism / Pathophysiology

FADD deficiency illustrates a single adaptor-protein defect producing multi-system disease through at least three convergent, partially independent mechanistic arms:

(a) Impaired Fas-mediated lymphocyte apoptosis → ALPS-like immune dysregulation

  • FADD is obligate for DISC assembly downstream of FAS/CD95 ligation.
  • Loss of function → failure of activation-induced cell death in T lymphocytes → accumulation of CD3+TCRαβ+CD4−CD8− double-negative T cells, elevated soluble FasL, IL-10, and vitamin B12 — the same biomarker panel used in classic ALPS (FAS/FASLG/CASP10 mutations) but without the massive lymphadenopathy/splenomegaly and overt autoimmune cytopenias that define clinical ALPS. Because of this partial phenotypic overlap, FADD deficiency is formally classified among the disorders in the expanding ALPS/ALPS-like spectrum, but functional Fas-apoptosis assay abnormality plus absence of FAS/FASLG/CASP10 mutation, together with the systemic (infectious/hepatic/encephalopathic) features, distinguishes it diagnostically [ncbi.nlm.nih.gov/books/NBK1108 GeneReviews ALPS].

(b) Functional hyposplenism → bacterial infection susceptibility

  • FADD-deficient patients show a functional (not anatomic) hyposplenic state, evidenced by circulating Howell-Jolly bodies, predisposing to invasive infection with encapsulated bacteria (notably pneumococcus). Bolze et al. (2010) directly attributed the bacterial-infection phenotype to this mechanism.

(c) Impaired interferon-dependent antiviral immunity → severe viral infections

  • FADD participates in TLR-independent innate antiviral signaling, contributing to induction of IRF7 and type I interferon (IFN-α) responses; its loss impairs this arm of antiviral defense, explaining the severe viral infection susceptibility independent of the hyposplenism mechanism.
  • More broadly, FADD sits at a molecular decision node between apoptosis and RIPK1/RIPK3/MLKL-driven necroptosis; FADD (with caspase-8) normally restrains necroptosis, and interferon signaling intersects this node (e.g., PKR- and IRF1-dependent, FADD/caspase-licensed necrosis; RIPK3 activates parallel MLKL-necroptosis and FADD-apoptosis pathways as an antiviral defense against influenza A virus, PMID:27321907). This apoptosis/necroptosis balance, and its perturbation by partial FADD loss, is an active area of general FADD biology relevant to interpreting how hypomorphic human FADD alleles could paradoxically both impair apoptotic clearance (arm a) and dysregulate necroptotic/inflammatory tissue injury (arm d, below).

(d) Recurrent febrile encephalopathy / FIRES-like crises

  • The mechanism of the stereotyped encephalopathic episodes is incompletely defined but is hypothesized to involve infection/fever-triggered dysregulated cell death (apoptotic/necroptotic) and/or excessive inflammatory signaling in neural tissue in the FADD-hypomorphic host, producing a clinical picture indistinguishable from febrile infection-related epilepsy syndrome (FIRES) in at least one 2024-reported case (Giovannini et al., Epilepsia 2024).

(e) Hepatopathy

  • Recurrent portal inflammation and fibrosis are reported; the precise cellular mechanism (immune-mediated injury vs. direct dysregulated hepatocyte apoptosis/necroptosis) has not been dissected in the literature to date.

(f) Cardiac malformations

  • Ventricular septal defect and pulmonary artery atresia have been reported in a subset of patients; whether this reflects a direct developmental role for FADD-dependent apoptosis in cardiac morphogenesis (consistent with mouse data showing FADD is essential in several developmental contexts via RIPK1/RIPK3 interactions) or coincidental association is unresolved.

(g) Ocular findings (newly recognized, 2022)

  • A case report described retinal vascular findings resembling familial exudative vitreoretinopathy (FEVR) in a FADD-deficient patient, proposed as a novel manifestation potentially reflecting a role for FADD-dependent apoptosis in normal retinal vascular development/pruning — the first such report, attributed to the extreme rarity and short life expectancy of the disorder limiting prior ophthalmologic characterization.

Suggested GO terms: GO:0097191 (extrinsic apoptotic signaling pathway), GO:0007249 (I-kappaB kinase/NF-kB signaling), GO:0035666 (TRIF-dependent toll-like receptor signaling), GO:0002230 (positive regulation of defense response to virus by host), GO:0060548 (negative regulation of cell death), GO:0070266 (necroptotic process). Suggested CL terms: CL:0000798 (gamma-delta/alphabeta double-negative T cell context — CL:0000940 CD3+TCRαβ+CD4−CD8− "double-negative" T cell), CL:0000625 (CD8-positive T cell), CL:0000909 (CD4-negative CD8-negative thymocyte). Suggested UBERON terms: UBERON:0002106 (spleen), UBERON:0002107 (liver), UBERON:0000955 (brain), UBERON:0000948 (heart).

7. Anatomical Structures Affected

  • Primary organs: Spleen (functional hyposplenism), liver (portal inflammation/fibrosis), brain (encephalopathy, cerebral atrophy, seizure focus), immune system broadly (lymphocytes — T-cell compartment).
  • Secondary/complication-level involvement: Heart (structural malformations — VSD, pulmonary artery atresia), eyes (retinal vasculature, FEVR-like changes), lymphoid tissue (variable lymphadenopathy/splenomegaly).
  • Body systems: Immune system, hepatobiliary system, central nervous system, cardiovascular system, hematologic system (functional splenic/Howell-Jolly changes).
  • Cellular level: T lymphocytes (double-negative αβ T-cell accumulation), hepatocytes/portal tract immune cells, neurons/glia (encephalopathy), retinal vascular endothelium.
  • Subcellular: Death-inducing signaling complex (DISC) assembly at the plasma membrane/cytoplasm downstream of FAS/TNFR1; mitochondrial apoptosis intersection is downstream of caspase-8 activation.

8. Temporal Development

  • Onset: Neonatal to infantile; the majority of reported patients present in the first 1–2 years of life (index case at 14 months with febrile status epilepticus; original kindred affected as young children).
  • Onset pattern: Insidious background susceptibility (infections, hepatopathy) punctuated by acute/subacute encephalopathic crises.
  • Progression: Variable — historically rapidly progressive/fatal (3 of 4 original patients died before age 5), but recent reports (2023) describe patients surviving longer with modern supportive management (immunoglobulin replacement, HSCT evaluation).
  • Disease course pattern: Episodic/relapsing-remitting for the encephalopathic component (recurrent, stereotyped febrile episodes); chronic and progressive for the hepatic fibrosis and immune dysregulation components.
  • Critical periods: Febrile illness episodes function as recurring critical/vulnerable windows for both infectious decompensation and encephalopathic crisis triggering.

9. Inheritance and Population

  • Inheritance pattern: Autosomal recessive (homozygous or compound heterozygous FADD variants); 25% recurrence risk for children of two carrier parents, 50% carrier risk.
  • Epidemiology: Extremely rare — fewer than ~15 patients reported across the entire published literature as of 2023–2024, spanning at least 8 published reports; no formal prevalence or incidence estimate exists (too rare for population-based registries). Effectively an "ultra-rare" disorder in Orphanet-style banding (equivalent to <1 per 1,000,000).
  • Consanguinity: Central risk factor — the founding/largest reported kindred was a consanguineous Pakistani family; consanguinity likely explains the concentration of biallelic cases.
  • Founder effects: The recurrent p.C105W allele reported in both the original Pakistani kindred and (independently) in the 2024 FIRES case report suggests either a mutational hotspot at this conserved cysteine or a possible founder allele, though this is not formally established via haplotype analysis in available literature.
  • Sex ratio / geographic distribution: No sex predilection reported; cases have been described in Pakistani, and separately in other (unspecified ethnicity, US-based) families, suggesting the disorder is not geographically restricted, though under-ascertainment outside specialized immunodeficiency/genetics centers is likely given the extreme rarity and diagnostic overlap with ALPS/FIRES/sepsis.
  • Genetic anticipation, mosaicism: Not described/applicable for this classical biallelic recessive disorder.

10. Diagnostics

  • Functional assay: In vitro Fas-mediated (CD95-mediated) T-cell apoptosis assay showing profoundly impaired apoptosis is a key diagnostic tool, analogous to its established role in the ALPS diagnostic algorithm (used when FAS/FASLG/CASP10 germline variants are not identified, per GeneReviews ALPS criteria) [ncbi.nlm.nih.gov/books/NBK1108].
  • Genetic testing: Confirmatory molecular diagnosis requires identification of biallelic pathogenic/likely pathogenic FADD variants — achieved historically via combined genome-wide linkage analysis + whole-exome sequencing (the original discovery approach) and, in contemporary practice, via targeted primary immunodeficiency gene panels or clinical exome/genome sequencing.
  • Laboratory biomarkers: Elevated circulating double-negative (CD3+TCRαβ+CD4−CD8−) T cells; elevated soluble FasL; elevated IL-10; elevated IL-18 (reported); elevated vitamin B12; Howell-Jolly bodies on peripheral smear (functional hyposplenism); absent isohemagglutinins despite normal total immunoglobulins; reduced FADD protein expression by immunoblot in patient cells.
  • Imaging: Brain MRI showing cerebral atrophy in survivors of recurrent encephalopathic episodes; echocardiography for structural cardiac defects (VSD, pulmonary artery atresia).
  • Differential diagnosis: Classic ALPS (FAS/FASLG/CASP10 germline or somatic variants — the majority, 60–70%, are ALPS-FAS), ALPS-U (undetermined genetic cause with abnormal Fas apoptosis assay), other causes of FIRES/status epilepticus with fever, other primary immunodeficiencies with hyposplenism, and other causes of infantile encephalopathy with hepatopathy.
  • Screening: No population/newborn screening program exists given the extreme rarity; diagnosis is case-by-case via clinical suspicion (recurrent infection + hepatopathy + episodic encephalopathy + ALPS-like biomarkers) followed by genetic confirmation. Genetic/carrier counseling is recommended for consanguineous families with an affected child.

11. Outcome/Prognosis

  • Survival: Historically poor — in the original 2010 report, 3 of 4 affected patients from the founding kindred died before age 5, from invasive pneumococcal infection or during an encephalopathic episode. A more recently reported affected sibling died at 18 months following a post-vaccination encephalopathic episode.
  • Contemporary outcomes: More recent cases (2020, 2023) describe survival into later childhood/beyond with intensive multidisciplinary management, including immunoglobulin replacement and hematopoietic stem cell transplantation (HSCT) — two patients in the 2015 Savic et al. report (J Allergy Clin Immunol 2015;136(2):502-5) both underwent HSCT, though the literature does not provide detailed long-term outcome data for these transplants; a 2023-reported infant was undergoing HSCT evaluation with subjective clinical improvement on subcutaneous immunoglobulin in the interim.
  • Morbidity: Chronic hepatic fibrosis, recurrent infections, cerebral atrophy with associated neurodevelopmental impact, and (in a subset) structural cardiac disease contribute cumulative morbidity in survivors.
  • Prognostic factors: Severity and frequency of encephalopathic crises appear to be the dominant driver of early mortality; access to specialized immunologic/hematologic care (immunoglobulin replacement, early HSCT) may improve survival based on the trend across sequential case reports (2010 → 2015 → 2020 → 2023).

12. Treatment

No disease-specific approved therapy exists; management is supportive/empiric, extrapolated from other primary immunodeficiency and ALPS-spectrum disorders:

  • Immunoglobulin replacement therapy: Subcutaneous immunoglobulin (e.g., 400 mg/kg/month reported) used for infection prophylaxis, with subjective reduction in fatigue and respiratory infections in a recent case. (NCIT:C15986 Pharmacotherapy; therapeutic class immunoglobulin replacement.)
  • Antimicrobial prophylaxis: Implied by the functional-hyposplenism mechanism (analogous to management of other hyposplenic states — pneumococcal prophylaxis/vaccination, though live vaccines require caution given the reported post-MMR encephalopathic death).
  • Hematopoietic stem cell transplantation (HSCT): Used in at least 4 reported patients (2 in Savic et al. 2015; 1 under evaluation in Setia et al. 2023) as a potentially curative approach addressing the underlying immune/hematologic defect, though outcome detail is limited in available sources. (NCIT:C15431, Hematopoietic Cell Transplantation.)
  • Seizure/encephalopathy management: Standard antiseizure/status epilepticus management for the FIRES-like episodes; no FADD-specific anti-inflammatory or immunotherapy protocol is established, though the disease's classification within the ALPS/immune-dysregulation spectrum has prompted case-level use of immunotherapy for neuroinflammatory episodes in related literature (Vogel et al., Clinical Genetics 2023, referenced in search results but full detail not independently verified in this research pass).
  • Supportive care: Management of hepatic dysfunction/fibrosis and cardiac malformations follows standard organ-specific supportive protocols; no FADD-targeted hepatoprotective therapy exists.
  • Experimental/targeted approaches: None specific to FADD deficiency are in clinical trials (searches of ClinicalTrials.gov did not surface disease-specific interventional trials, consistent with the disorder's extreme rarity). Given FADD's centrality to the apoptosis/necroptosis balance, RIPK1/RIPK3/necroptosis-pathway-targeted agents (investigated in other contexts, e.g., RIPK1 inhibitors) represent a theoretical but unproven future therapeutic avenue.

13. Prevention

  • Primary prevention: Not currently possible beyond genetic/reproductive counseling; no vaccination strategy specifically targets FADD deficiency, and caution regarding live-attenuated vaccines (e.g., MMR) may be warranted given a reported temporal association with a fatal encephalopathic episode (single-case observation, not established causally).
  • Genetic counseling: Recommended for families with a diagnosed case, particularly in consanguineous populations, given the 25% recurrence risk; carrier testing and prenatal/preimplantation genetic diagnosis are theoretically applicable once a familial pathogenic variant is identified but are not specifically documented in the literature reviewed.
  • Secondary prevention: Early recognition of the biomarker panel (double-negative T cells, elevated sFasL/IL-10/B12) in an infant with recurrent infections and hepatopathy could prompt earlier diagnosis and initiation of infection-prophylactic and immunoglobulin-replacement measures before a first severe encephalopathic crisis.
  • Public health relevance: Given the extreme rarity, no population-level public health screening or intervention program exists or is anticipated.

14. Other Species / Natural Disease

  • No naturally occurring FADD-deficiency disease has been reported in non-human species (companion animals, wildlife) in the literature surveyed; this appears to be a human-specific clinical entity, consistent with its ultra-rare Mendelian nature and the embryonic-lethal consequence of complete Fadd loss in mice (see below), which would preclude viable natural homozygous-null animal populations.
  • Orthologous gene: Mouse Fadd (MGI ortholog) is the primary comparative genetics reference; no OMIA (Online Mendelian Inheritance in Animals) entry for a natural FADD-deficiency disease was identified.

15. Model Organisms

FADD biology has been extensively studied in mouse models, though these largely model the gene's fundamental developmental/immunologic roles rather than directly recapitulating the human hypomorphic-disease phenotype:

  • Global Fadd knockout mice: Embryonic lethal, demonstrating FADD is essential for normal embryonic development. This lethality is mediated through necroptotic signaling — combined Fadd/Ripk1 double-knockout mice rescue the embryonic lethality and the lymphocyte proliferation defects seen in single-knockout mice, directly implicating dysregulated RIPK1-dependent necroptosis (not merely loss of apoptosis) as the lethal mechanism of complete FADD loss (Nature 2011, PMID referenced as "Functional complementation between FADD and RIP1 in embryos and lymphocytes"). RIP1-kinase-activity-dependent embryonic phenotypes have also been dissected (Cell Death Differ 2018).
  • Conditional/tissue-specific Fadd knockouts:
  • T-cell-specific conditional knockout (GFP-marked): FADD is dispensable for thymic (intrathymic) T-cell development but is essential for peripheral T-cell homeostasis, regulating both apoptotic and proliferative signals (PMID:16116191).
  • Loss of FADD in Tie2-expressing (endothelial/hematopoietic) cells causes RIPK3-mediated embryonic lethality (Cell Death Dis 2016, PMC5059855), underscoring FADD's endothelial developmental role — potentially relevant to the cardiac and retinal vascular anomalies reported in human FADD deficiency.
  • FADD regulates T-cell-receptor-mediated necroptosis as a negative regulator (PNAS, PMID referenced 1005997107), and RIP1 deficiency fully restores normal T-cell (but not B-cell) proliferation in Fadd-null lymphocytes, indicating cell-type-specific dependence on the FADD/RIPK1 necroptosis-suppression axis.
  • FADD has been shown to regulate adipose tissue inflammation, adipogenesis, and adipocyte survival (Cell Death Discov 2024) — an emerging, disease-adjacent role not yet linked to human phenotype.
  • Phenotype recapitulation and limitations: Because complete murine Fadd loss is embryonic lethal, no mouse model directly phenocopies the human hypomorphic (partial loss-of-function) FADD-deficiency disease phenotype (recurrent infection, hepatopathy, encephalopathy). Conditional and hypomorphic mouse alleles instead illuminate discrete mechanistic arms (T-cell homeostasis, necroptosis suppression, endothelial/vascular development) that plausibly underlie individual human disease features (immune dysregulation, cardiac/retinal vascular anomalies) but have not been integrated into a single disease model.
  • Research applications: These models are principally used to dissect FADD's role at the apoptosis-necroptosis decision node and its tissue-specific essentiality (immune, endothelial, adipose), providing mechanistic hypotheses (rather than direct phenocopy validation) for the multi-organ human disease.

Summary of Key Evidence Sources

Citation Contribution
Bolze A, et al. Am J Hum Genet 2010;87(6):873-881. PMID:21109225 Original discovery of human FADD deficiency (p.C105W, consanguineous kindred); established core phenotype (ALPS-like biomarkers + infections + hepatopathy + encephalopathy + cardiac malformations); hyposplenism/interferon-immunity mechanistic hypotheses
Savic S, et al. J Allergy Clin Immunol 2015;136(2):502-5 Second report, 2 patients, both treated with HSCT
"Novel Compound Heterozygote Variations in FADD..." J Clin Immunol 2020, PMC7253512 Additional compound heterozygous variants expanding allelic spectrum
Ocular findings paper, Ophthalmology/related journal 2022 (ScienceDirect S2451993622000512) First report of FEVR-like retinal findings in FADD deficiency
Setia P, et al. Br J Haematol 2023;202(2):e11-e15. PMID:37199216, DOI:10.1111/bjh.18871 Novel compound heterozygous variants (c.313T>C/p.C105R + c.52_58delGACGAGC); detailed immunophenotyping; SCIg treatment
Giovannini G, et al. Epilepsia 2024;65(7):e119-e124. DOI:10.1111/epi.18008 FADD (p.C105W) presenting as FIRES; literature review
Vogel et al. Clinical Genetics 2023 Immunotherapy-responsive neuroinflammation in FADD-mutant child (referenced; not independently fetched)
OMIM #613759 / *602457 Curated gene-disease and allelic summary
PMC10970579 / MDPI Int J Mol Sci 2024;25(6):3228 Review: "Cellular Dynamics of Fas-Associated Death Domain in the Regulation of Cancer and Inflammation" — general FADD mechanism update
GeneReviews, ALPS (NBK1108) Diagnostic-algorithm context distinguishing FADD deficiency from classic ALPS-FAS/FASLG/CASP10

Note on evidence gaps: This is an ultra-rare disease with fewer than ~15 published patients; several details (full genotype-phenotype correlation, long-term HSCT outcomes, mechanism of hepatic and cardiac involvement, formal QOL data, population prevalence) are not established in the literature and should be flagged as data-limited/not-yet-determined in any knowledge-base entry rather than inferred.

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