👪

Inheritance

1

Autosomal recessive HP:0000007

All reported patients are homozygous for rare deleterious CD4 variants. Consanguinity is implied by homozygosity across five families of four different ancestries.

Autosomal recessive inheritance

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"We report seven patients aged 5-61 years from five families of four ancestries with autosomal recessive CD4 deficiency"

States the inheritance mode and the size and composition of the reported cohort.

◈

Mechanistic Hypotheses

1

Canonical CD4 Loss with Helper-Lineage Compensation Model

canonical_cd4_loss_with_th_compensation_model CANONICAL

Biallelic CD4 loss-of-function removes the co-receptor from T cells, monocytes and dendritic cells and from plasma, but a compensatory TCR-alpha-beta+ CD4-CD8- population adopts the helper transcriptional programme and retains intact HLA class II-restricted responses and B cell help. Most helper-dependent immunity is therefore preserved, and the clinical phenotype narrows to the defences that require the CD4 molecule itself — control of human papillomavirus and of Tropheryma whipplei.

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Discussions and Knowledge Gaps

4

Why do human papillomavirus and Tropheryma whipplei specifically escape control when helper immunity is otherwise reconstituted by CD4-negative helper T cells?

KNOWLEDGE GAP imd79_why_hpv_and_whipplei

Attached to

pathophysiology#Susceptibility to Human Papillomavirus and Tropheryma whipplei

This is the disease's central unanswered question and the one with the widest reach: an answer would identify what the CD4 molecule does that the helper lineage does not, in a way that neither HIV infection nor MHC class II deficiency can isolate. Candidates include a role for soluble CD4, a requirement in monocytes or dendritic cells rather than T cells, or a tissue-compartment effect at skin and gut mucosa. Nothing in the reported data distinguishes them.

Is immunodeficiency 79 correctly classified under severe combined immunodeficiency, given a phenotype dominated by warts and sometimes first diagnosed in adulthood?

KNOWLEDGE GAP imd79_severity_classification_conflict

Attached to

pathophysiology#Broadly Preserved Cellular and Humoral Immunity

MONDO places this concept under `severe combined immunodeficiency`, which predicts a management approach — early transplantation, prophylaxis, isolation — that the published natural history does not support. Patients aged up to 61 years were reported, and the phenotype is described as mild relative to other primary immunodeficiencies. Recording the conflict is the honest position; resolving it needs either an ontology change or evidence of severe cases that have not yet been published.

Are B cell, monocyte and natural killer cell function actually impaired in inherited CD4 deficiency, as the founding report concluded, or numerically and functionally normal, as the larger cohort found?

KNOWLEDGE GAP imd79_preserved_versus_impaired_innate_compartments

Attached to

pathophysiology#Broadly Preserved Cellular and Humoral Immunity

The 2021 single-family report and the 2024 seven-patient cohort reach opposite conclusions about the non-T compartments, and this entry's central claim — that helper immunity is reconstituted well enough that the established susceptibilities are narrow — depends on which is right. The discrepancy may be a sample-size artefact, a difference between numerical and functional readouts, or a real difference between the founding kindred's genotype and the others. Nothing published resolves it, and the wider infection spectrum recorded in this entry (mycobacterial disease, cryptosporidiosis, a ventilated viral pneumonia) is the kind of observation that would fit the impaired reading.

Does the shorter CD4 isoform that binds LCK but not HLA class II retain any protective function, and can the co-receptor's two roles be separated genetically?

KNOWLEDGE GAP imd79_cd4_isoform_dissection

Attached to

pathophysiology#Biallelic CD4 Loss-of-Function Variants

Only one of the reported variants affects this isoform, so there is no within-cohort comparison to make. If further patients are found who lose only the canonical isoform versus both, the pair would separate the co-receptor's HLA class II-binding role from its LCK-recruiting role in humans — a dissection currently only available in mouse and in vitro systems.

⚙

Pathophysiology

5

Biallelic CD4 Loss-of-Function Variants

Homozygous rare deleterious CD4 variants that abolish expression of the canonical CD4 isoform. The first reported family carried a variant disrupting the translation-initiation codon. A shorter expressed isoform that interacts with LCK but not with HLA class II is affected by only one of the reported variants, so the cohort is not uniform at the isoform level even though it is uniform in losing canonical CD4.

CD4 hgnc:1678 ⇓ LOSS OF FUNCTION

Genetic context variant_origin: GERMLINE zygosity: HOMOZYGOUS functional_impact_category: LOSS_OF_FUNCTION

Germline homozygous CD4 alleles, including a translation-initiation codon variant. All impact expression of the canonical CD4 isoform.

Downstream

Absent CD4 Co-Receptor Expression Loss-of-function alleles remove the CD4 protein from the surface of every cell type that normally carries it.

Show evidence (2 references)

PMID:38557723 SUPPORT Human Clinical

"All patients are homozygous for rare deleterious CD4 variants impacting expression of the canonical CD4 isoform. A shorter expressed isoform that interacts with LCK, but not HLA class II, is affected by only one variant."

Establishes the genotype and records the isoform detail this node describes.

PMID:33471124 SUPPORT Human Clinical

"a novel variant disrupting the translation-initiation codon of the CD4 gene resulted in complete loss of membrane and plasma soluble CD4"

Documents the founding variant and that the loss extends to soluble as well as membrane CD4.

Absent CD4 Co-Receptor Expression

CD4 is absent from T cells, monocytes and dendritic cells, and soluble CD4 is absent from plasma. In the founding proband this was demonstrated not only in peripheral blood but in lymph node, bone marrow, skin and ileum — that is, the loss is systemic and tissue-wide rather than a peripheral blood finding. This distinguishes the disease from every acquired or ligand-side model of CD4+ T cell loss: it is the co-receptor itself that is gone, including its soluble form.

CD4-positive helper T cell CL:0000492 ∅ ABSENT monocyte CL:0000576 dendritic cell CL:0000451

Downstream

Compensatory Double-Negative Helper T Cell Development In the absence of the CD4 co-receptor, a population of TCR-alpha-beta+ CD4-CD8- T cells expands and assumes the helper transcriptional programme.

Susceptibility to Human Papillomavirus and Tropheryma whipplei Two host defences that the compensating population does not reconstitute, and that therefore require CD4 itself.

Recurrent Respiratory Infections Recurrent respiratory infections beginning in childhood in some patients. Recorded as a residual susceptibility rather than as part of the two-pathogen claim, since the source does not attribute it to a specific organism or to a failure of the compensating population.

Show evidence (2 references)

PMID:33471124 SUPPORT Human Clinical

"complete loss of membrane and plasma soluble CD4 in peripheral blood, lymph node, bone marrow, skin, and ileum of a homozygous proband"

Documents that the loss is systemic across tissues and includes soluble CD4, which is the claim this node makes.

PMID:38557723 SUPPORT Human Clinical

"All patients lack CD4+ T cells and have increased numbers of TCRαβ+CD4-CD8- T cells"

Confirms absence of CD4+ T cells across the whole cohort and introduces the compensating population.

Compensatory Double-Negative Helper T Cell Development

The finding that reframes the disease. Patients accumulate TCR-alpha-beta+ CD4-negative CD8-negative T cells that resemble conventional helper T cells both phenotypically and transcriptionally. Functionally these cells are not a poor substitute: they respond intact to HLA class II-restricted antigens and promote B cell differentiation in vitro. Helper immunity is therefore reconstituted without the co-receptor, which is why patients acquire effective cellular and humoral immunity against an unexpectedly large range of pathogens.

TCR-alpha-beta+ CD4-CD8- T cell CL:0000791 ↑ INCREASED

T-helper cell differentiation GO:0042093 response to HLA class II-restricted antigen GO:0002456 provision of help for B cell differentiation GO:0030183

Downstream

Broadly Preserved Cellular and Humoral Immunity Functional helper cells reconstitute the great majority of helper-dependent immunity.

Show evidence (2 references)

PMID:38557723 SUPPORT Human Clinical

"which phenotypically and transcriptionally resemble conventional Th cells"

Establishes that the compensating population takes on the helper identity, not merely the surface phenotype.

PMID:38557723 SUPPORT In Vitro

"patient CD4-CD8- αβ T cells exhibit intact responses to HLA class II-restricted antigens and promote B cell differentiation in vitro"

Demonstrates functional competence of the compensating cells, which is what makes this a genuine compensation rather than an inert expansion.

Broadly Preserved Cellular and Humoral Immunity

Patients acquire effective cellular and humoral immunity against an unexpectedly large range of pathogens. This node is the reason the clinical phenotype is mild and can go undiagnosed for decades, and it is the negative result that gives the disease its scientific value: it shows that most of what is attributed to CD4+ T cells is attributable to the helper lineage rather than to the CD4 molecule.

Show evidence (3 references)

PMID:38557723 SUPPORT Human Clinical

"compensatory development of Th cells enables patients with inherited CD4 deficiency to acquire effective cellular and humoral immunity against an unexpectedly large range of pathogens"

States the preserved-immunity conclusion in the authors' own words.

PMID:33471124 REFUTE Human Clinical

"characterized by compensatory changes in T-cell subsets and functional impairment of B cells, monocytes, and natural killer cells"

The 2021 founding report concludes the disease involves functional impairment of B cells, monocytes and NK cells — which cuts against calling immunity broadly preserved. Recorded as REFUTE rather than reconciled, because the two papers genuinely disagree and this entry's thesis rests on the 2024 reading.

PMID:38557723 SUPPORT Human Clinical

"nor NK cells (CD3−CD20−CD56+) (Fig. S4 A) detected in patients (P1–P7) compared with healthy donors"

The 2024 cohort found no NK cell abnormality across all seven patients, which is the counter-evidence to the item above.

Susceptibility to Human Papillomavirus and Tropheryma whipplei

The two best-attested defences the compensation does not cover. Recalcitrant warts due to uncontrolled human papillomavirus infection are the commonest presentation, and Whipple disease due to Tropheryma whipplei was also seen. These are the claims the disease licenses most strongly about the CD4 molecule as opposed to the helper lineage — the paper's own conclusion is hedged as "at least" these two, and this node carries further downstream susceptibilities that were reported but not established as CD4-specific. Why these two organisms in particular is not known.

Downstream

Recalcitrant Warts Failure to control human papillomavirus, the commonest and often only clinical manifestation.

Chronic Diarrhea Failure to control Tropheryma whipplei, presenting as Whipple disease with chronic diarrhoea.

Arthralgia The articular manifestation of Whipple disease.

Chronic Oral Candidiasis A classically CD4-dependent opportunistic infection, marking a further limit on what the compensating helper population covers.

Recurrent Mycobacterial Infections Mycobacterial disease including a prolonged BCG reaction. Grouped with the two headline susceptibilities because it is the same kind of gap: an infection class the compensation does not fully cover.

Severe Viral Pneumonia Severe multifocal viral pneumonia requiring ventilation.

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"CD4 is indispensable for protective immunity against at least human papillomaviruses and Trophyrema whipplei."

Names the two pathogens for which CD4 itself is required, which is the claim this node makes. Note the source misspells the genus as "Trophyrema"; the organism is Tropheryma whipplei, and the term binding uses the correct name.

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

7

Digestive 1

Chronic Diarrhea HP:0002028

Temporal: CHRONIC

Show evidence (2 references)

PMID:38557723 SUPPORT Human Clinical

"the proband (P2) presented with recurrent fever, diarrhea, asthenia, hiatus hernia with vascular ectasia, oral labial herpes, warts, enteral ulceration, and joint pain affecting his knees and wrists"

The presenting picture of the Whipple disease patient, in which diarrhoea is named directly. This sentence does not itself say the diarrhoea was chronic; the item below carries that.

PMID:38557723 SUPPORT DIRECT Human Clinical

"she presented with chronic diarrhea and was also diagnosed with cryptosporidiosis infection and oral candidiasis"

The second, independent occurrence, in P4. This is the sentence that grounds `temporality: CHRONIC` — it is the only one in the report that calls the diarrhoea chronic.

Head and Neck 1

Chronic Oral Candidiasis HP:0009098

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"she presented with chronic diarrhea and was also diagnosed with cryptosporidiosis infection and oral candidiasis"

Names oral candidiasis and cryptosporidiosis together in P4.

Immune 3

Recurrent Mycobacterial Infections HP:0011274

Show evidence (2 references)

PMID:38557723 SUPPORT Human Clinical

"P3 developed pulmonary tuberculosis at 10 yo and laryngeal tuberculosis at 20 yo."

Two separate mycobacterial episodes a decade apart in one patient.

PMID:38557723 SUPPORT Human Clinical

"She also had a prolonged local reaction to the Bacillus Calmette-Guérin (BCG) vaccine."

A prolonged local BCG reaction in a second patient, which is the live-vaccine safety signal.

Severe Viral Pneumonia HP:0002090

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"At 17 yo, she developed severe multifocal pneumonia due to rhinovirus and enterovirus requiring mechanical respiratory assistance."

Documents the ventilated viral pneumonia.

Recurrent Respiratory Infections HP:0002205

Show evidence (2 references)

PMID:38557723 SUPPORT Human Clinical

"She exhibited failure to thrive and developed recurrent infectious episodes in her early years including multiple pneumonia, chronic sinusitis, urinary tract infection, diarrhea, iridocyclitis, and endophthalmitis."

Names the respiratory pattern directly in P1 — multiple pneumonia and chronic sinusitis — rather than resting on a general statement about "a range of infections".

PMID:38557723 SUPPORT Human Clinical

"recurrent otitis, and episodes of upper respiratory infections"

Independent confirmation in P6, whose respiratory course also included a ventilated viral pneumonia at 17.

Integument 1

Recalcitrant Warts Verrucae HP:0200043

Temporal: RECURRENT

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"a range of infections, including recalcitrant warts and Whipple's disease"

Names recalcitrant warts as a presenting infection in the cohort.

Constitutional 1

Arthralgia HP:0002829

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"joint pain affecting his knees and wrists"

Names the arthralgia in the Whipple disease patient.

🧬

Genetic Associations

1

CD4

Gene: CD4 hgnc:1678 relationship_type: CAUSATIVE

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"All patients are homozygous for rare deleterious CD4 variants impacting expression of the canonical CD4 isoform."

Establishes CD4 as the disease gene across the whole reported cohort.

💊

Medical Actions

1

Doxycycline and Hydroxychloroquine for Whipple Disease

Action: Pharmacotherapy NCIT:C15986

Agent: doxycycline CHEBI:50845 hydroxychloroquine CHEBI:5801

Platform: Small molecule

Combination antimicrobial therapy for Tropheryma whipplei infection, effective in the one reported Whipple disease patient with a durable response — his PCR for T. whipplei in blood, saliva and stool was negative five years later. This is the only intervention in the published cohort with a documented outcome, and it treats one of the two susceptibilities the entry is built around rather than the underlying co-receptor defect.

Mechanism Target:

MODULATES Susceptibility to Human Papillomavirus and Tropheryma whipplei — Clears the organism the host cannot control. It does not restore CD4 or alter the underlying susceptibility, so it addresses the consequence rather than the mechanism.

Show evidence (2 references)

PMID:38557723 SUPPORT Human Clinical

"Treatment with doxycycline and hydroxychloroquine was effective."

Documents the regimen and its effectiveness in the reported patient.

PMID:38557723 SUPPORT Human Clinical

"In 2022, his PCR for T. whipplei in blood, saliva, and stool was negative."

Documents the durability of the response five years after presentation.

🔬

Diagnosis

2

Flow cytometry for surface CD4

Absence of CD4 on T cells, monocytes and dendritic cells. The finding is unmistakable once looked for; the difficulty is that a patient whose only complaint is recurrent warts is unlikely to have lymphocyte immunophenotyping requested. Expect a compensatory TCR-alpha-beta+ CD4-CD8- population rather than simple T cell lymphopenia.

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"All patients lack CD4+ T cells and have increased numbers of TCRαβ+CD4-CD8- T cells"

Describes the immunophenotypic pattern that makes the diagnosis.

CD4 sequencing

Molecular confirmation. Worth distinguishing from idiopathic CD4 lymphocytopenia, which is an acquired non-Mendelian phenotype of reduced CD4+ T cell counts, and from MHC class II deficiency, in which CD4 is present but its ligand is not.

Show evidence (1 reference)

PMID:33471124 SUPPORT Human Clinical

"This inherited CD4 knockout disease illustrates the clinical and immunological features of a complete deficiency of any functional component of CD4 and its similarities and differences with other clinical models of primary or acquired loss of CD4+ T cells."

Frames the differential against other primary and acquired causes of CD4+ T cell loss, which is what this diagnostic step resolves.

📊

Prevalence

1

Worldwide, published cases

Cases In Literature Ultra Rare

Seven patients from five families in the defining series. Because the phenotype is mild and can go undiagnosed into adulthood, the published count is plausibly a substantial underestimate of the true number of affected individuals — but no ascertainment study exists to say by how much.

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"We report seven patients aged 5-61 years from five families of four ancestries with autosomal recessive CD4 deficiency and a range of infections, including recalcitrant warts and Whipple's disease."

Gives the cohort size and the wide age range that motivates the under-ascertainment caveat.

🦠

Infectious Agent

2

Human papillomavirus

Uncontrolled HPV infection producing recalcitrant cutaneous warts, the commonest presenting feature. One of the two organisms for which the cohort report concludes CD4 itself, rather than the helper T cell lineage, is indispensable; that conclusion is stated as "at least" these two, so it is a floor rather than an exhaustive list.

Human papillomavirus NCBITaxon:10566

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"CD4 is indispensable for protective immunity against at least human papillomaviruses and Trophyrema whipplei."

Names human papillomavirus as an organism whose control requires CD4.

Tropheryma whipplei

Causative organism of Whipple disease, reported in this cohort and the second infection for which CD4 itself appears indispensable. Note the source misspells the genus as "Trophyrema"; the organism is Tropheryma whipplei.

Tropheryma whipplei NCBITaxon:2039

Show evidence (1 reference)

PMID:38557723 SUPPORT Human Clinical

"a range of infections, including recalcitrant warts and Whipple's disease"

Documents Whipple disease in the reported cohort.

{ }

Source YAML

click to show

name: Immunodeficiency 79
creation_date: "2026-08-31T22:15:00Z"
category: Mendelian
synonyms:
- IMD79
- CD4 deficiency
- inherited CD4 deficiency
- autosomal recessive CD4 deficiency
disease_term:
  preferred_term: immunodeficiency 79
  term:
    id: MONDO:0030981
    label: immunodeficiency 79
parents:
- Primary Immunodeficiency
- Inborn Error of Immunity
description: >
  Immunodeficiency 79 is autosomal recessive CD4 deficiency: an inborn error
  of immunity in which homozygous deleterious CD4 variants abolish expression
  of the canonical CD4 co-receptor on T cells, monocytes and dendritic cells,
  and remove soluble CD4 from plasma. The striking thing about the disease is
  how mild it is. Rather than the profound susceptibility that loss of the
  CD4+ T cell compartment predicts from HIV/AIDS and from MHC class II
  deficiency, patients develop a compensatory population of TCR-alpha-beta+
  CD4-negative CD8-negative T cells that resemble conventional helper T cells
  phenotypically and transcriptionally, respond normally to HLA class
  II-restricted antigens, and support B cell differentiation. What that
  compensation does not cover is narrow, though not closed: the paper
  concludes that CD4 is indispensable "at least" against human
  papillomaviruses and Tropheryma whipplei, which are the two best-attested
  susceptibilities. Mycobacterial disease, cryptosporidiosis and a ventilated
  viral pneumonia are also recorded across the cohort, and are curated here as
  further gaps rather than folded into the headline pair.
  Patients may not come to medical attention until adulthood, and the reported
  age range spans 5 to 61 years. The disease therefore reads less as a
  catastrophic immunodeficiency than as a natural experiment defining which
  host defences require the CD4 molecule itself as opposed to the helper T
  cell lineage.
notes: >
  Ultra-rare. The reported cohort is seven patients aged 5 to 61 years from
  five families of four ancestries (Béziat et al., J Exp Med 2024), building
  on the first family described in 2021, in which a variant disrupting the CD4
  translation-initiation codon abolished membrane and plasma soluble CD4 in a
  homozygous proband. Counts here are counts of patients, not frequencies, and
  `frequency` is left unset throughout.

  Three things this entry is deliberately careful about.

  First, **it is not a severe combined immunodeficiency, and the entry does
  not present it as one.** MONDO places immunodeficiency 79 under `severe
  combined immunodeficiency`, and the disease name invites the same reading.
  The published phenotype is a mild one dominated by warts, sometimes
  undiagnosed into adulthood. Where the ontology and the literature disagree
  on severity, this entry follows the literature and records the disagreement
  here rather than silently softening either.

  Second, **the compensation is the mechanism, not a footnote.** A pathograph
  that ran CD4 loss straight to broad infection susceptibility would be
  wrong — that is the prediction the disease refutes. The chain therefore
  routes through the compensatory double-negative helper T cell population,
  and the two narrow susceptibilities hang off what that compensation fails
  to cover.

  Third, **the comparison to HIV/AIDS and MHC class II deficiency is a
  contrast, not an analogy.** Both of those remove CD4+ T cells and both
  produce broad susceptibility, but neither isolates the CD4 molecule: HIV
  infection carries many other immunological abnormalities, and MHC class II
  deficiency removes the ligand and disturbs thymic selection. That is
  precisely why inherited CD4 deficiency is informative, and it is recorded
  in `discussions` rather than left implicit.

  One isoform detail worth keeping: a shorter expressed CD4 isoform that
  interacts with LCK but not with HLA class II is affected by only one of the
  reported variants. The entry does not build on that, because with a single
  variant affecting it there is nothing yet to compare it against, but it is
  the natural handle for separating the co-receptor's HLA-binding role from
  its LCK-recruiting role in future work.

  **A term note for the next curator.** The compensating population is a
  peripheral TCR-alpha-beta+ CD4-CD8- T cell. CL has no term for that —
  `CL:0000791` (mature alpha-beta T cell) is the closest honest binding and
  is deliberately broader than the population it names, per the "no term
  beats a bad one" rule. Do not swap it for a double-negative *thymocyte*
  term, which is a developmental-stage concept and would be wrong.

  No `conforms_to` was declared; no module covers co-receptor loss with
  lineage compensation.

  **Deep-research provenance, and a warning.** An OpenScientist run was
  commissioned for this entry
  (`research/Immunodeficiency_79-deep-research-openscientist.md`) and is
  committed alongside it, but **none of its content was used**. The report is
  about a different disease: its central claim is that immunodeficiency 79 is
  caused by biallelic germline TET2 loss of function. It is not — MedGen links
  MONDO:0030981 / OMIM:619238 to CD4 (Gene 920), and the report cites OMIM
  619223 rather than 619238. `just preflight-dr` scores the report
  `CD4=3` against `TET2=93` and flags the OMIM mismatch. Every claim in this
  entry is sourced from the primary literature on CD4 deficiency instead. The
  report is retained because a wrong report is provenance too, and because it
  is a clean worked example for issue #9888 (`just research-disorder` sends an
  empty `mondo_id`, so a numbered disease name has nothing to anchor on).
inheritance:
- name: Autosomal recessive
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  description: >
    All reported patients are homozygous for rare deleterious CD4 variants.
    Consanguinity is implied by homozygosity across five families of four
    different ancestries.
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report seven patients aged 5-61 years from five families of four ancestries with autosomal recessive CD4 deficiency"
    explanation: >
      States the inheritance mode and the size and composition of the
      reported cohort.
prevalence:
- population: Worldwide, published cases
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    Seven patients from five families in the defining series. Because the
    phenotype is mild and can go undiagnosed into adulthood, the published
    count is plausibly a substantial underestimate of the true number of
    affected individuals — but no ascertainment study exists to say by how
    much.
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report seven patients aged 5-61 years from five families of four ancestries with autosomal recessive CD4 deficiency and a range of infections, including recalcitrant warts and Whipple's disease."
    explanation: >-
      Gives the cohort size and the wide age range that motivates the
      under-ascertainment caveat.
pathophysiology:
- name: Biallelic CD4 Loss-of-Function Variants
  biological_scale: MOLECULAR
  description: >
    Homozygous rare deleterious CD4 variants that abolish expression of the
    canonical CD4 isoform. The first reported family carried a variant
    disrupting the translation-initiation codon. A shorter expressed isoform
    that interacts with LCK but not with HLA class II is affected by only one
    of the reported variants, so the cohort is not uniform at the isoform
    level even though it is uniform in losing canonical CD4.
  genes:
  - preferred_term: CD4
    term:
      id: hgnc:1678
      label: CD4
    modifier: LOSS_OF_FUNCTION
  genetic_context:
    description: >-
      Germline homozygous CD4 alleles, including a translation-initiation
      codon variant. All impact expression of the canonical CD4 isoform.
    variant_origin: GERMLINE
    zygosity: HOMOZYGOUS
    functional_impact_category: LOSS_OF_FUNCTION
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients are homozygous for rare deleterious CD4 variants impacting expression of the canonical CD4 isoform. A shorter expressed isoform that interacts with LCK, but not HLA class II, is affected by only one variant."
    explanation: >
      Establishes the genotype and records the isoform detail this node
      describes.
  - reference: PMID:33471124
    reference_title: "Lost in Translation: Lack of CD4 Expression due to a Novel Genetic Defect."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a novel variant disrupting the translation-initiation codon of the CD4 gene resulted in complete loss of membrane and plasma soluble CD4"
    explanation: >
      Documents the founding variant and that the loss extends to soluble as
      well as membrane CD4.
  downstream:
  - target: Absent CD4 Co-Receptor Expression
    description: >-
      Loss-of-function alleles remove the CD4 protein from the surface of
      every cell type that normally carries it.
    causal_link_type: DIRECT

- name: Absent CD4 Co-Receptor Expression
  biological_scale: CELLULAR
  description: >
    CD4 is absent from T cells, monocytes and dendritic cells, and soluble CD4
    is absent from plasma. In the founding proband this was demonstrated not
    only in peripheral blood but in lymph node, bone marrow, skin and ileum —
    that is, the loss is systemic and tissue-wide rather than a peripheral
    blood finding. This distinguishes the disease from every acquired or
    ligand-side model of CD4+ T cell loss: it is the co-receptor itself that
    is gone, including its soluble form.
  cell_types:
  - preferred_term: CD4-positive helper T cell
    term:
      id: CL:0000492
      label: CD4-positive helper T cell
    modifier: ABSENT
  - preferred_term: monocyte
    term:
      id: CL:0000576
      label: monocyte
  - preferred_term: dendritic cell
    term:
      id: CL:0000451
      label: dendritic cell
  evidence:
  - reference: PMID:33471124
    reference_title: "Lost in Translation: Lack of CD4 Expression due to a Novel Genetic Defect."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "complete loss of membrane and plasma soluble CD4 in peripheral blood, lymph node, bone marrow, skin, and ileum of a homozygous proband"
    explanation: >
      Documents that the loss is systemic across tissues and includes soluble
      CD4, which is the claim this node makes.
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients lack CD4+ T cells and have increased numbers of TCRαβ+CD4-CD8- T cells"
    explanation: >
      Confirms absence of CD4+ T cells across the whole cohort and introduces
      the compensating population.
  downstream:
  - target: Compensatory Double-Negative Helper T Cell Development
    description: >-
      In the absence of the CD4 co-receptor, a population of
      TCR-alpha-beta+ CD4-CD8- T cells expands and assumes the helper
      transcriptional programme.
    causal_link_type: DIRECT
  - target: Susceptibility to Human Papillomavirus and Tropheryma whipplei
    description: >-
      Two host defences that the compensating population does not
      reconstitute, and that therefore require CD4 itself.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Why these two pathogens specifically require the CD4 molecule, rather than the helper T cell lineage, is not established
  - target: Recurrent Respiratory Infections
    description: >-
      Recurrent respiratory infections beginning in childhood in some
      patients. Recorded as a residual susceptibility rather than as part of
      the two-pathogen claim, since the source does not attribute it to a
      specific organism or to a failure of the compensating population.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Whether these infections reflect a residual gap in the compensated helper compartment or an unrelated susceptibility is not established

- name: Compensatory Double-Negative Helper T Cell Development
  biological_scale: CELLULAR
  description: >
    The finding that reframes the disease. Patients accumulate TCR-alpha-beta+
    CD4-negative CD8-negative T cells that resemble conventional helper T
    cells both phenotypically and transcriptionally. Functionally these cells
    are not a poor substitute: they respond intact to HLA class II-restricted
    antigens and promote B cell differentiation in vitro. Helper immunity is
    therefore reconstituted without the co-receptor, which is why patients
    acquire effective cellular and humoral immunity against an unexpectedly
    large range of pathogens.
  cell_types:
  - preferred_term: TCR-alpha-beta+ CD4-CD8- T cell
    term:
      id: CL:0000791
      label: mature alpha-beta T cell
    modifier: INCREASED
  biological_processes:
  - preferred_term: T-helper cell differentiation
    term:
      id: GO:0042093
      label: T-helper cell differentiation
  - preferred_term: response to HLA class II-restricted antigen
    term:
      id: GO:0002456
      label: T cell mediated immunity
  - preferred_term: provision of help for B cell differentiation
    term:
      id: GO:0030183
      label: B cell differentiation
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "which phenotypically and transcriptionally resemble conventional Th cells"
    explanation: >
      Establishes that the compensating population takes on the helper
      identity, not merely the surface phenotype.
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "patient CD4-CD8- αβ T cells exhibit intact responses to HLA class II-restricted antigens and promote B cell differentiation in vitro"
    explanation: >
      Demonstrates functional competence of the compensating cells, which is
      what makes this a genuine compensation rather than an inert expansion.
  downstream:
  - target: Broadly Preserved Cellular and Humoral Immunity
    description: >-
      Functional helper cells reconstitute the great majority of
      helper-dependent immunity.
    causal_link_type: DIRECT

- name: Broadly Preserved Cellular and Humoral Immunity
  biological_scale: ORGANISM
  description: >
    Patients acquire effective cellular and humoral immunity against an
    unexpectedly large range of pathogens. This node is the reason the
    clinical phenotype is mild and can go undiagnosed for decades, and it is
    the negative result that gives the disease its scientific value: it shows
    that most of what is attributed to CD4+ T cells is attributable to the
    helper lineage rather than to the CD4 molecule.
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "compensatory development of Th cells enables patients with inherited CD4 deficiency to acquire effective cellular and humoral immunity against an unexpectedly large range of pathogens"
    explanation: >
      States the preserved-immunity conclusion in the authors' own words.
  - reference: PMID:33471124
    reference_title: "Lost in Translation: Lack of CD4 Expression due to a Novel Genetic Defect."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "characterized by compensatory changes in T-cell subsets and functional impairment of B cells, monocytes, and natural killer cells"
    explanation: >
      The 2021 founding report concludes the disease involves functional
      impairment of B cells, monocytes and NK cells — which cuts against
      calling immunity broadly preserved. Recorded as REFUTE rather than
      reconciled, because the two papers genuinely disagree and this entry's
      thesis rests on the 2024 reading.
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "nor NK cells (CD3−CD20−CD56+) (Fig. S4 A) detected in patients (P1–P7) compared with healthy donors"
    explanation: >
      The 2024 cohort found no NK cell abnormality across all seven patients,
      which is the counter-evidence to the item above.

- name: Susceptibility to Human Papillomavirus and Tropheryma whipplei
  biological_scale: ORGANISM
  description: >
    The two best-attested defences the compensation does not cover.
    Recalcitrant warts due to uncontrolled human papillomavirus infection are
    the commonest presentation, and Whipple disease due to Tropheryma whipplei
    was also seen. These are the claims the disease licenses most strongly
    about the CD4 molecule as opposed to the helper lineage — the paper's own
    conclusion is hedged as "at least" these two, and this node carries
    further downstream susceptibilities that were reported but not established
    as CD4-specific. Why these two organisms in particular is not known.
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "CD4 is indispensable for protective immunity against at least human papillomaviruses and Trophyrema whipplei."
    explanation: >
      Names the two pathogens for which CD4 itself is required, which is the
      claim this node makes. Note the source misspells the genus as
      "Trophyrema"; the organism is Tropheryma whipplei, and the term binding
      uses the correct name.
  downstream:
  - target: Recalcitrant Warts
    description: >-
      Failure to control human papillomavirus, the commonest and often only
      clinical manifestation.
    causal_link_type: DIRECT
  - target: Chronic Diarrhea
    description: >-
      Failure to control Tropheryma whipplei, presenting as Whipple disease
      with chronic diarrhoea.
    causal_link_type: DIRECT
  - target: Arthralgia
    description: The articular manifestation of Whipple disease.
    causal_link_type: DIRECT
  - target: Chronic Oral Candidiasis
    description: >-
      A classically CD4-dependent opportunistic infection, marking a further
      limit on what the compensating helper population covers.
    causal_link_type: DIRECT
  - target: Recurrent Mycobacterial Infections
    description: >-
      Mycobacterial disease including a prolonged BCG reaction. Grouped with
      the two headline susceptibilities because it is the same kind of gap:
      an infection class the compensation does not fully cover.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Whether mycobacterial control fails for the same reason as HPV and T. whipplei control was not established
  - target: Severe Viral Pneumonia
    description: Severe multifocal viral pneumonia requiring ventilation.
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    intermediate_mechanisms:
    - Reported in one patient; whether it reflects the CD4 defect or is incidental was not established
mechanistic_hypotheses:
- hypothesis_group_id: canonical_cd4_loss_with_th_compensation_model
  hypothesis_label: Canonical CD4 Loss with Helper-Lineage Compensation Model
  status: CANONICAL
  description: >-
    Biallelic CD4 loss-of-function removes the co-receptor from T cells,
    monocytes and dendritic cells and from plasma, but a compensatory
    TCR-alpha-beta+ CD4-CD8- population adopts the helper transcriptional
    programme and retains intact HLA class II-restricted responses and B cell
    help. Most helper-dependent immunity is therefore preserved, and the
    clinical phenotype narrows to the defences that require the CD4 molecule
    itself — control of human papillomavirus and of Tropheryma whipplei.
discussions:
- discussion_id: imd79_why_hpv_and_whipplei
  kind: KNOWLEDGE_GAP
  prompt: >-
    Why do human papillomavirus and Tropheryma whipplei specifically escape
    control when helper immunity is otherwise reconstituted by CD4-negative
    helper T cells?
  attaches_to:
  - pathophysiology#Susceptibility to Human Papillomavirus and Tropheryma whipplei
  rationale: >-
    This is the disease's central unanswered question and the one with the
    widest reach: an answer would identify what the CD4 molecule does that
    the helper lineage does not, in a way that neither HIV infection nor MHC
    class II deficiency can isolate. Candidates include a role for soluble
    CD4, a requirement in monocytes or dendritic cells rather than T cells,
    or a tissue-compartment effect at skin and gut mucosa. Nothing in the
    reported data distinguishes them.
- discussion_id: imd79_severity_classification_conflict
  kind: KNOWLEDGE_GAP
  prompt: >-
    Is immunodeficiency 79 correctly classified under severe combined
    immunodeficiency, given a phenotype dominated by warts and sometimes
    first diagnosed in adulthood?
  attaches_to:
  - pathophysiology#Broadly Preserved Cellular and Humoral Immunity
  rationale: >-
    MONDO places this concept under `severe combined immunodeficiency`, which
    predicts a management approach — early transplantation, prophylaxis,
    isolation — that the published natural history does not support. Patients
    aged up to 61 years were reported, and the phenotype is described as mild
    relative to other primary immunodeficiencies. Recording the conflict is
    the honest position; resolving it needs either an ontology change or
    evidence of severe cases that have not yet been published.
- discussion_id: imd79_preserved_versus_impaired_innate_compartments
  kind: KNOWLEDGE_GAP
  prompt: >-
    Are B cell, monocyte and natural killer cell function actually impaired
    in inherited CD4 deficiency, as the founding report concluded, or
    numerically and functionally normal, as the larger cohort found?
  attaches_to:
  - pathophysiology#Broadly Preserved Cellular and Humoral Immunity
  rationale: >-
    The 2021 single-family report and the 2024 seven-patient cohort reach
    opposite conclusions about the non-T compartments, and this entry's
    central claim — that helper immunity is reconstituted well enough that
    the established susceptibilities are narrow — depends on which is right. The
    discrepancy may be a sample-size artefact, a difference between
    numerical and functional readouts, or a real difference between the
    founding kindred's genotype and the others. Nothing published resolves
    it, and the wider infection spectrum recorded in this entry (mycobacterial
    disease, cryptosporidiosis, a ventilated viral pneumonia) is the kind of
    observation that would fit the impaired reading.
- discussion_id: imd79_cd4_isoform_dissection
  kind: KNOWLEDGE_GAP
  prompt: >-
    Does the shorter CD4 isoform that binds LCK but not HLA class II retain
    any protective function, and can the co-receptor's two roles be
    separated genetically?
  attaches_to:
  - pathophysiology#Biallelic CD4 Loss-of-Function Variants
  rationale: >-
    Only one of the reported variants affects this isoform, so there is no
    within-cohort comparison to make. If further patients are found who lose
    only the canonical isoform versus both, the pair would separate the
    co-receptor's HLA class II-binding role from its LCK-recruiting role in
    humans — a dissection currently only available in mouse and in vitro
    systems.
phenotypes:
- name: Recalcitrant Warts
  category: Dermatological
  description: >
    Recurrent and treatment-resistant cutaneous warts from uncontrolled human
    papillomavirus infection. This is the commonest presenting feature and
    often the only one.
  phenotype_term:
    preferred_term: Verrucae
    term:
      id: HP:0200043
      label: Verrucae
    temporality: RECURRENT
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a range of infections, including recalcitrant warts and Whipple's disease"
    explanation: >
      Names recalcitrant warts as a presenting infection in the cohort.
- name: Chronic Diarrhea
  category: Gastrointestinal
  description: >
    Diarrhoea in the Whipple disease patient (P2), where the report calls it
    recurrent rather than chronic, and independently in P4 alongside
    cryptosporidiosis, where it is explicitly chronic — so P4 is what
    grounds the `CHRONIC` qualifier. Recorded as the finding rather than as
    "Whipple disease", because Whipple disease is a disease concept and HPO
    has no term for it — the organism itself is carried in
    `infectious_agent`.
  phenotype_term:
    preferred_term: Chronic diarrhea
    term:
      id: HP:0002028
      label: Chronic diarrhea
    temporality: CHRONIC
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the proband (P2) presented with recurrent fever, diarrhea, asthenia, hiatus hernia with vascular ectasia, oral labial herpes, warts, enteral ulceration, and joint pain affecting his knees and wrists"
    explanation: >
      The presenting picture of the Whipple disease patient, in which
      diarrhoea is named directly. This sentence does not itself say the
      diarrhoea was chronic; the item below carries that.
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    directness: DIRECT
    snippet: "she presented with chronic diarrhea and was also diagnosed with cryptosporidiosis infection and oral candidiasis"
    explanation: >
      The second, independent occurrence, in P4. This is the sentence that
      grounds `temporality: CHRONIC` — it is the only one in the report that
      calls the diarrhoea chronic.
- name: Arthralgia
  category: Musculoskeletal
  description: >
    Joint pain affecting the knees and wrists in the Whipple disease patient,
    a classic feature of that infection.
  phenotype_term:
    preferred_term: Arthralgia
    term:
      id: HP:0002829
      label: Arthralgia
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "joint pain affecting his knees and wrists"
    explanation: >
      Names the arthralgia in the Whipple disease patient.
- name: Recurrent Mycobacterial Infections
  category: Infectious
  description: >
    Pulmonary tuberculosis at 10 and laryngeal tuberculosis at 20 in P3, and
    a prolonged local reaction to BCG in P4. This is the most
    management-relevant feature in the entry after the warts: it bears
    directly on live-vaccine safety in a patient whose immunodeficiency
    otherwise looks mild.
  phenotype_term:
    preferred_term: Recurrent mycobacterial infections
    term:
      id: HP:0011274
      label: Recurrent mycobacterial infections
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "P3 developed pulmonary tuberculosis at 10 yo and laryngeal tuberculosis at 20 yo."
    explanation: >
      Two separate mycobacterial episodes a decade apart in one patient.
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "She also had a prolonged local reaction to the Bacillus Calmette-Guérin (BCG) vaccine."
    explanation: >
      A prolonged local BCG reaction in a second patient, which is the
      live-vaccine safety signal.
- name: Chronic Oral Candidiasis
  category: Infectious
  description: >
    Oral candidiasis in P4, alongside cryptosporidiosis. Both are classically
    CD4-dependent infections, so their occurrence here marks a real limit on
    what the compensating helper population covers.
  phenotype_term:
    preferred_term: Chronic oral candidiasis
    term:
      id: HP:0009098
      label: Chronic oral candidiasis
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "she presented with chronic diarrhea and was also diagnosed with cryptosporidiosis infection and oral candidiasis"
    explanation: >
      Names oral candidiasis and cryptosporidiosis together in P4.
- name: Severe Viral Pneumonia
  category: Respiratory
  description: >
    Severe multifocal pneumonia due to rhinovirus and enterovirus requiring
    mechanical ventilation in P6 at 17. The single most severe episode in the
    published cohort, and a corrective to reading this disease as warts alone.
  phenotype_term:
    preferred_term: Pneumonia
    term:
      id: HP:0002090
      label: Pneumonia
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "At 17 yo, she developed severe multifocal pneumonia due to rhinovirus and enterovirus requiring mechanical respiratory assistance."
    explanation: >
      Documents the ventilated viral pneumonia.
- name: Recurrent Respiratory Infections
  category: Respiratory
  description: >
    Recurrent respiratory infections beginning in childhood in some patients,
    though the phenotype overall remains mild by comparison with other
    primary immunodeficiencies.
  phenotype_term:
    preferred_term: Recurrent respiratory infections
    term:
      id: HP:0002205
      label: Recurrent respiratory infections
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "She exhibited failure to thrive and developed recurrent infectious episodes in her early years including multiple pneumonia, chronic sinusitis, urinary tract infection, diarrhea, iridocyclitis, and endophthalmitis."
    explanation: >
      Names the respiratory pattern directly in P1 — multiple pneumonia and
      chronic sinusitis — rather than resting on a general statement about
      "a range of infections".
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "recurrent otitis, and episodes of upper respiratory infections"
    explanation: >
      Independent confirmation in P6, whose respiratory course also included
      a ventilated viral pneumonia at 17.
genetic:
- name: CD4
  gene_term:
    preferred_term: CD4
    term:
      id: hgnc:1678
      label: CD4
  relationship_type: CAUSATIVE
  notes: >
    CD4 encodes the co-receptor that binds HLA class II and recruits LCK to
    the T cell receptor complex. Biallelic loss-of-function causes
    immunodeficiency 79. The gene is better known as the entry receptor for
    HIV-1, and that is exactly why the inherited deficiency is informative:
    it removes the molecule without the confounding immunological
    abnormalities of HIV infection.
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients are homozygous for rare deleterious CD4 variants impacting expression of the canonical CD4 isoform."
    explanation: >
      Establishes CD4 as the disease gene across the whole reported cohort.
infectious_agent:
- name: Human papillomavirus
  infectious_agent_term:
    preferred_term: Human papillomavirus
    term:
      id: NCBITaxon:10566
      label: Human papillomavirus
  description: >-
    Uncontrolled HPV infection producing recalcitrant cutaneous warts, the
    commonest presenting feature. One of the two organisms for which the
    cohort report concludes CD4 itself, rather than the helper T cell lineage,
    is indispensable; that conclusion is stated as "at least" these two, so it
    is a floor rather than an exhaustive list.
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "CD4 is indispensable for protective immunity against at least human papillomaviruses and Trophyrema whipplei."
    explanation: >-
      Names human papillomavirus as an organism whose control requires CD4.
- name: Tropheryma whipplei
  infectious_agent_term:
    preferred_term: Tropheryma whipplei
    term:
      id: NCBITaxon:2039
      label: Tropheryma whipplei
  description: >-
    Causative organism of Whipple disease, reported in this cohort and the
    second infection for which CD4 itself appears indispensable. Note the
    source misspells the genus as "Trophyrema"; the organism is Tropheryma
    whipplei.
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "a range of infections, including recalcitrant warts and Whipple's disease"
    explanation: >-
      Documents Whipple disease in the reported cohort.

diagnosis:
- name: Flow cytometry for surface CD4
  description: >
    Absence of CD4 on T cells, monocytes and dendritic cells. The finding is
    unmistakable once looked for; the difficulty is that a patient whose only
    complaint is recurrent warts is unlikely to have lymphocyte
    immunophenotyping requested. Expect a compensatory TCR-alpha-beta+
    CD4-CD8- population rather than simple T cell lymphopenia.
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "All patients lack CD4+ T cells and have increased numbers of TCRαβ+CD4-CD8- T cells"
    explanation: >
      Describes the immunophenotypic pattern that makes the diagnosis.
- name: CD4 sequencing
  description: >
    Molecular confirmation. Worth distinguishing from idiopathic CD4
    lymphocytopenia, which is an acquired non-Mendelian phenotype of reduced
    CD4+ T cell counts, and from MHC class II deficiency, in which CD4 is
    present but its ligand is not.
  evidence:
  - reference: PMID:33471124
    reference_title: "Lost in Translation: Lack of CD4 Expression due to a Novel Genetic Defect."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This inherited CD4 knockout disease illustrates the clinical and immunological features of a complete deficiency of any functional component of CD4 and its similarities and differences with other clinical models of primary or acquired loss of CD4+ T cells."
    explanation: >
      Frames the differential against other primary and acquired causes of
      CD4+ T cell loss, which is what this diagnostic step resolves.

treatments:
- name: Doxycycline and Hydroxychloroquine for Whipple Disease
  description: >
    Combination antimicrobial therapy for Tropheryma whipplei infection,
    effective in the one reported Whipple disease patient with a durable
    response — his PCR for T. whipplei in blood, saliva and stool was
    negative five years later. This is the only intervention in the published
    cohort with a documented outcome, and it treats one of the two
    susceptibilities the entry is built around rather than the underlying
    co-receptor defect.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: doxycycline
      term:
        id: CHEBI:50845
        label: doxycycline
    - preferred_term: hydroxychloroquine
      term:
        id: CHEBI:5801
        label: hydroxychloroquine
  target_mechanisms:
  - target: Susceptibility to Human Papillomavirus and Tropheryma whipplei
    treatment_effect: MODULATES
    description: >-
      Clears the organism the host cannot control. It does not restore CD4 or
      alter the underlying susceptibility, so it addresses the consequence
      rather than the mechanism.
  evidence:
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Treatment with doxycycline and hydroxychloroquine was effective."
    explanation: >
      Documents the regimen and its effectiveness in the reported patient.
  - reference: PMID:38557723
    reference_title: "Helper T cell immunity in humans with inherited CD4 deficiency."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In 2022, his PCR for T. whipplei in blood, saliva, and stool was negative."
    explanation: >
      Documents the durability of the response five years after
      presentation.

references:
- reference: PMID:38557723
  title: Helper T cell immunity in humans with inherited CD4 deficiency.
- reference: PMID:33471124
  title: "Lost in Translation: Lack of CD4 Expression due to a Novel Genetic Defect."

📚

References & Deep Research

References

2

PMID:38557723

Helper T cell immunity in humans with inherited CD4 deficiency.

No top-level findings curated for this source.

PMID:33471124

Lost in Translation: Lack of CD4 Expression due to a Novel Genetic Defect.

No top-level findings curated for this source.

Deep Research

1

Deep research results are used as seeds for research; they do not undergo the same validation as the main records and may contain errors. How we use deep research.

Evaluations and curation notes (2)

Record notes

Ultra-rare. The reported cohort is seven patients aged 5 to 61 years from five families of four ancestries (Béziat et al., J Exp Med 2024), building on the first family described in 2021, in which a variant disrupting the CD4 translation-initiation codon abolished membrane and plasma soluble CD4 in a homozygous proband. Counts here are counts of patients, not frequencies, and `frequency` is left unset throughout. Three things this entry is deliberately careful about. First, **it is not a severe combined immunodeficiency, and the entry does not present it as one.** MONDO places immunodeficiency 79 under `severe combined immunodeficiency`, and the disease name invites the same reading. The published phenotype is a mild one dominated by warts, sometimes undiagnosed into adulthood. Where the ontology and the literature disagree on severity, this entry follows the literature and records the disagreement here rather than silently softening either. Second, **the compensation is the mechanism, not a footnote.** A pathograph that ran CD4 loss straight to broad infection susceptibility would be wrong — that is the prediction the disease refutes. The chain therefore routes through the compensatory double-negative helper T cell population, and the two narrow susceptibilities hang off what that compensation fails to cover. Third, **the comparison to HIV/AIDS and MHC class II deficiency is a contrast, not an analogy.** Both of those remove CD4+ T cells and both produce broad susceptibility, but neither isolates the CD4 molecule: HIV infection carries many other immunological abnormalities, and MHC class II deficiency removes the ligand and disturbs thymic selection. That is precisely why inherited CD4 deficiency is informative, and it is recorded in `discussions` rather than left implicit. One isoform detail worth keeping: a shorter expressed CD4 isoform that interacts with LCK but not with HLA class II is affected by only one of the reported variants. The entry does not build on that, because with a single variant affecting it there is nothing yet to compare it against, but it is the natural handle for separating the co-receptor's HLA-binding role from its LCK-recruiting role in future work. **A term note for the next curator.** The compensating population is a peripheral TCR-alpha-beta+ CD4-CD8- T cell. CL has no term for that — `CL:0000791` (mature alpha-beta T cell) is the closest honest binding and is deliberately broader than the population it names, per the "no term beats a bad one" rule. Do not swap it for a double-negative *thymocyte* term, which is a developmental-stage concept and would be wrong. No `conforms_to` was declared; no module covers co-receptor loss with lineage compensation. **Deep-research provenance, and a warning.** An OpenScientist run was commissioned for this entry (`research/Immunodeficiency_79-deep-research-openscientist.md`) and is committed alongside it, but **none of its content was used**. The report is about a different disease: its central claim is that immunodeficiency 79 is caused by biallelic germline TET2 loss of function. It is not — MedGen links MONDO:0030981 / OMIM:619238 to CD4 (Gene 920), and the report cites OMIM 619223 rather than 619238. `just preflight-dr` scores the report `CD4=3` against `TET2=93` and flags the OMIM mismatch. Every claim in this entry is sourced from the primary literature on CD4 deficiency instead. The report is retained because a wrong report is provenance too, and because it is a clean worked example for issue #9888 (`just research-disorder` sends an empty `mondo_id`, so a numbered disease name has nothing to anchor on).

Create: Immunodeficiency 79 (CD4 deficiency) · 2026-08-31T21:51:49Z · View source

De-novo curation of immunodeficiency 79 (MONDO:0030981, CD4) resolving curation claim #10293. The pathograph deliberately does not run CD4 loss straight to broad infection susceptibility, because that is the prediction the disease refutes: it routes through the compensatory TCR-alpha-beta+ CD4-CD8- helper population, which is phenotypically and transcriptionally Th-like and retains intact HLA class II-restricted responses and B cell help, and hangs the two narrow susceptibilities (human papillomavirus, Tropheryma whipplei) off what that compensation fails to cover. Three knowledge gaps recorded: why those two organisms specifically; whether MONDO's placement of this concept under severe combined immunodeficiency is right given a phenotype dominated by warts and sometimes first diagnosed at age 61; and whether the shorter LCK-binding CD4 isoform can be used to dissect the co-receptor's two roles. Deep research: one OpenScientist run was commissioned and is committed, but NONE of its content was used - the report is about a different disease, claiming biallelic germline TET2 loss of function as the cause of IMD79. MedGen links MONDO:0030981/OMIM:619238 to CD4 (Gene 920), and the report cites OMIM 619223. just preflight-dr scores CD4=3 against TET2=93 and flags the OMIM mismatch. Note the report's own frontmatter reports confabulation_rate 0.0 with 14/14 references resolved, so identifier resolution gave no warning here. Evidence for this entry comes from the primary CD4-deficiency literature instead: PMID:38557723 (seven patients, five families) and PMID:33471124 (the founding translation-initiation-codon family), both fetched and quote-verified. Reported the case on issue #9888 (research-disorder sends an empty mondo_id, so a numbered disease name has nothing to anchor on). Validated with just validate (18/18 snippets verified), just validate-terms, check-entity-refs, check-duplicate-keys, check-enum-values and the snippet/folded-hyphen gates.

OpenScientist ▸

Immunodeficiency 79 (IMD79): A Comprehensive Disease Characteristics Report

openscientist-autonomous 12 citations 2026-08-31T21:43:00.706245

Immunodeficiency 79 (IMD79): A Comprehensive Disease Characteristics Report

Germline TET2 deficiency — an autosomal-recessive epigenetic inborn error of immunity

Summary

Immunodeficiency 79 (IMD79; OMIM #619223) is an ultra-rare Mendelian inborn error of immunity caused by biallelic germline loss-of-function mutations in TET2 (ten-eleven translocation methylcytosine dioxygenase 2; HGNC:25941; locus 4q24). TET2 is an Fe(II)/α-ketoglutarate-dependent dioxygenase that catalyzes the iterative oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC), driving active DNA demethylation. When both alleles are lost, the enzyme is absent or catalytically dead, producing genome-wide DNA hypermethylation that dysregulates the developmental and immune transcriptional programs of hematopoietic cells. The disease was first defined in 2020 in three unrelated children carrying rare homozygous germline missense or nonsense TET2 variants [PMID: 32518946].

Clinically, IMD79 is a combined immunodeficiency with autoimmune lymphoproliferative syndrome (ALPS)-like features and a striking predisposition to lymphoma. Affected children present with susceptibility to infection, chronic lymphadenopathy, hepatosplenomegaly, developmental delay, autoimmunity, and B-cell or T-cell lymphoma. The immunophenotype includes expanded double-negative (TCRαβ+ CD4−CD8−) T cells, depleted circulating follicular helper T cells, impaired Fas-dependent apoptosis, and defective B-cell class-switch recombination. The entity has since been expanded by additional biallelic/compound-heterozygous cases with ALPS-like disease and hematologic malignancy [PMID: 36066697], and even by heterozygous carriers presenting with B-cell lymphoma [PMID: 40031954].

Mechanistically, two arms converge to produce disease: (1) a catalytic/epigenetic arm in which loss of 5hmC-mediated demethylation deranges hematopoietic stem cell (HSC) self-renewal, myeloid skewing, regulatory T-cell (Treg) stability, and malignant transformation; and (2) a methylation-independent inflammatory arm in which TET2 normally recruits HDAC2 to actively repress IL6, so that its loss unleashes IL-6-driven inflammation. Allogeneic hematopoietic stem cell transplantation (HSCT) is curative for the hematopoietic-immune disease and was achieved in all three index patients. Vitamin C (ascorbate), a cofactor of the Fe(II)/α-KG TET enzymes, is a mechanistically rational but clinically unproven adjunct that could restore residual/paralog TET activity in hypomorphic alleles.

1. Disease Information

Overview. IMD79 is an autosomal-recessive immune dysregulation syndrome caused by biallelic germline loss-of-function of the epigenetic regulator TET2. It is best conceptualized as an epigenetic combined immunodeficiency that bridges immunodeficiency, autoimmunity/lymphoproliferation, and cancer predisposition. It represents the constitutional (germline) counterpart of the somatic TET2 mutations long known to drive clonal hematopoiesis and myeloid/lymphoid malignancy.

Key identifiers.

Resource	Identifier
OMIM (phenotype)	#619223 — "Immunodeficiency 79"
Gene	TET2 (OMIM *612839)
HGNC	HGNC:25941
Cytogenetic locus	4q24
MONDO	(suggested) MONDO term for "immunodeficiency 79"; map to OMIM:619223
ICD-11	4A00-region "Primary immunodeficiencies" (no dedicated code)
MeSH	No dedicated descriptor; index under TET2, "Immunologic Deficiency Syndromes," "Autoimmune Lymphoproliferative Syndrome"
Orphanet	No dedicated ORPHAcode identified for this ultra-rare entity as of writing

Synonyms / alternative names. Germline TET2 deficiency; autosomal-recessive germline TET2 deficiency; TET2-related childhood immunodeficiency and lymphoma; ALPS-like syndrome due to TET2 loss-of-function.

Source of information. The disease is defined almost entirely from individual patient case series (aggregated across a handful of families worldwide) combined with model-organism and in vitro mechanistic data, rather than from population-level EHR or registry resources. This is expected for an ultra-rare Mendelian condition.

2. Etiology

Disease causal factors. The primary cause is genetic: biallelic (homozygous or compound-heterozygous) germline loss-of-function mutations in TET2. Whole-exome sequencing of three unrelated affected children identified rare homozygous germline missense or nonsense TET2 variants, with mutated TET2 protein "absent or enzymatically defective for 5-hydroxymethylating activity, resulting in whole-blood DNA hypermethylation" [PMID: 32518946]. This is a monogenic, cell-intrinsic etiology; infections in patients are consequences of the immunodeficiency rather than causal.

Genetic risk factors. The causal variants are the biallelic TET2 LOF alleles themselves. Consanguinity contributes to homozygosity (autosomal-recessive inheritance). Heterozygous LOF variants confer a milder / partial risk — four heterozygous carriers developed B-cell lymphoma against a background of chronic lymphadenopathy and autoimmune features, "expand[ing] the association of germline TET2 mutations with lymphoma and an autoimmune lymphoproliferative syndrome-like phenotype to the heterozygous state" [PMID: 40031954]. Somatic "second-hit" mutations (e.g., in DNMT3A, ASXL1, or oncogenic drivers) are inferred to be required for progression to overt malignancy, consistent with TET2's role as a pre-malignant "gatekeeper."

Environmental risk factors. No specific environmental triggers are established. Age (childhood onset), and the accumulated antigenic/inflammatory burden over time, plausibly modulate the timing of lymphoproliferation and malignancy, but no toxin, occupational, or lifestyle exposure has been linked. Family history / consanguinity is the principal non-genetic risk marker.

Protective factors. No validated genetic or environmental protective factors are established. Mechanistically, residual TET paralog activity (TET1/TET3) and cofactor availability (Fe(II), α-ketoglutarate, ascorbate) could partially buffer hypomorphic alleles, but this is inferred rather than demonstrated in patients.

Gene–environment interactions. Inflammatory/infectious stimuli interact with the genetic lesion: because TET2 normally restrains IL6 during resolution of inflammation, infectious/endotoxic challenges are predicted to produce exaggerated, poorly resolving inflammation in TET2-deficient individuals [PMID: 26287468]. This is a genotype (TET2 loss) × environment (inflammatory stimulus) interaction.

3. Phenotypes

The core clinical phenotype, per the defining series of three children, comprised "susceptibility to infection, lymphadenopathy, hepatosplenomegaly, developmental delay, autoimmunity, and lymphoma of B-cell (n = 2) or T-cell (n = 1) origin" [PMID: 32518946].

Phenotype	Type	HPO suggestion	Onset	Severity / frequency
Recurrent/susceptibility to infection	Clinical sign (immunodeficiency)	HP:0002721 (Immunodeficiency) / HP:0002719 (Recurrent infections)	Childhood	Variable; present in index cases
Lymphadenopathy	Physical manifestation	HP:0002716	Childhood, chronic	Common/consistent
Hepatosplenomegaly	Physical manifestation	HP:0001433	Childhood	Common
Splenomegaly	Physical manifestation	HP:0001744	Childhood	Common
Developmental delay	Clinical sign	HP:0001263	Early childhood	Present in index cases
Autoimmunity (multi-system)	Clinical sign	HP:0002960	Childhood	Prominent
Autoimmune cytopenias	Laboratory/clinical	HP:0001973 (Autoimmune thrombocytopenia) / HP:0001937	Childhood	Reported (ALPS-like)
B-cell or T-cell lymphoma	Physical manifestation	HP:0002665 (Lymphoma)	Childhood	3/3 index; major morbidity
Expanded double-negative T cells (TCRαβ+CD4−CD8−)	Laboratory abnormality	ALPS biomarker	Childhood	Characteristic
Depleted circulating follicular helper T cells	Laboratory abnormality	—	Childhood	Characteristic
Impaired Fas-dependent apoptosis	Laboratory abnormality	HP:0005404 (Abnormal lymphocyte apoptosis)	Childhood	2 of 3 patients
Defective B-cell class-switch recombination	Laboratory abnormality	HP:0005387 (Abnormal immunoglobulin level)	Childhood	Characteristic
Elevated vitamin B12	Laboratory abnormality	— (ALPS biomarker)	Childhood	Reported [PMID: 36066697]
Elevated IL-10	Laboratory abnormality	— (ALPS biomarker)	Childhood	Reported [PMID: 36066697]
Hypergammaglobulinemia	Laboratory abnormality	HP:0010702	Childhood	Reported [PMID: 36066697]

Immunophenotype detail. "Circulating T cells showed an abnormal immunophenotype including expanded double-negative, but depleted follicular helper, T-cell compartments and impaired Fas-dependent apoptosis in 2 of 3 patients. Moreover, TET2-deficient B cells showed defective class-switch recombination" [PMID: 32518946]. The second series added "inverted myeloid/plasmacytoid dendritic cells ratio, elevated terminally differentiated effector memory CD8+ T-cells re-expressing CD45RA, regulatory T-cells, and Th2 circulating follicular T-cells. Double-negative T-cells, vitamin B12, and IL-10 were elevated according to the ALPS-like suspicion" [PMID: 36066697].

Age of onset / severity / progression. Onset is in childhood; the course is chronic and progressive, punctuated by lymphoproliferative episodes and culminating in lymphoma. Severity is high given lymphoma predisposition and multi-system autoimmunity.

Quality-of-life impact. Not formally measured with EQ-5D/SF-36/PROMIS in this ultra-rare cohort. Qualitatively, the combination of recurrent infection, chronic lymphoproliferation, developmental delay, autoimmune complications, malignancy, and the need for HSCT implies substantial impairment of daily functioning and major treatment burden.

4. Genetic / Molecular Information

Causal gene. TET2 (ten-eleven translocation methylcytosine dioxygenase 2), HGNC:25941, OMIM *612839, cytoband 4q24. The protein is an Fe(II)/α-ketoglutarate-dependent dioxygenase that oxidizes 5mC → 5hmC → 5-formylcytosine → 5-carboxylcytosine, enabling active DNA demethylation [PMID: 42423046, 42609618].

Pathogenic variants. - Variant types: rare homozygous germline missense or nonsense variants in the defining series [PMID: 32518946]; compound-heterozygous and monoallelic LOF variants in subsequent reports [PMID: 36066697, 40031954]. - Functional consequence: loss of function — TET2 protein "absent or enzymatically defective for 5-hydroxymethylating activity," producing whole-blood DNA hypermethylation [PMID: 32518946]. P1 in the second series showed "absent TET2 expression and profound increase in DNA methylation" [PMID: 36066697]. - Classification (ACMG/AMP): Nonsense/frameshift LOF alleles in a gene with established LOF disease mechanism generally reach pathogenic/likely pathogenic; novel missense variants require functional confirmation (loss of 5hmC activity provides strong functional evidence, PS3). - Allele frequency: Biallelic germline LOF is ultra-rare; causal variants are private/rare in gnomAD. (Note: somatic TET2 mutations are common in age-related clonal hematopoiesis and myeloid neoplasia, but these are distinct from the germline disease.) - Somatic vs germline: IMD79 is germline; the same gene is a frequent somatic driver in hematologic malignancy — an important distinction for genetic counseling.

Modifier genes. Inferred second-hit somatic drivers (e.g., DNMT3A, ASXL1, oncogenes) modify progression to malignancy. TET paralogs TET1/TET3 may modify severity by partial compensation; notably TET1 is largely dispensable for MLL-ENL myeloid leukemogenesis, indicating non-redundant TET member roles [PMID: 33705482]. Regulatory partners of TET2 — the TOPD complex (TET-OGT-PROSER1-DBHS) — provide additional layers that could modify function [PMID: 42423046].

Epigenetic information. The disease is fundamentally epigenetic: loss of TET2 causes genome-wide 5mC hypermethylation and reduced 5hmC, silencing genes normally kept demethylated/active. This is the central molecular readout [PMID: 32518946, 36066697].

Chromosomal abnormalities. None characteristic at the germline level; the lesion is a point mutation/small indel in TET2. Secondary cytogenetic changes may accompany malignant transformation.

5. Environmental Information

Environmental factors. No specific toxin, radiation, or occupational exposure is established as causal or contributory. Disease is monogenic.

Lifestyle factors. None established.

Infectious agents. No infectious agent causes IMD79. Infections are downstream consequences of the immunodeficiency. Inflammatory/infectious stimuli, however, act as triggers that unmask the exaggerated, poorly resolving inflammation caused by loss of TET2-mediated IL6 repression [PMID: 26287468]. In model organisms, Tet2-deficient mice "were more susceptible to endotoxin shock and dextran-sulfate-sodium-induced colitis" [PMID: 26287468], illustrating the interaction between environmental inflammatory challenge and the genetic defect.

6. Mechanism / Pathophysiology

Ordered causal chain

Biallelic germline LOF mutation in TET2 (missense/nonsense) → loss of, or catalytically dead, TET2 protein [PMID: 32518946].
Loss of TET2 dioxygenase activity → failure to oxidize 5mC to 5hmC → genome-wide DNA hypermethylation / reduced 5hmC in blood cells [PMID: 32518946, 36066697].
Aberrant DNA methylation → dysregulated transcriptional programs in hematopoietic stem/progenitor cells (HSPCs) → enhanced HSC self-renewal and myeloid-lineage skewing (branch toward pre-malignancy) [PMID: 26256876, 26256875, 32518946]. 4a. Branch — malignant predisposition: aberrant self-renewal creates a pre-malignant HSPC state; upon acquisition of inferred somatic "second hits," this progresses to B-cell or T-cell lymphoma / myeloid neoplasia [PMID: 26256875, 32518946]. 4b. Branch — autoimmunity/lymphoproliferation: loss of TET function in FOXP3+ Treg cells → increased methylation of intronic FOXP3 enhancers (CNS2/TSDR) and Treg instability → "ex-Treg" cells biased toward T follicular helper (Tfh) cells → expansion of Tfh and plasma cells with autoantibody production [PMID: 41972131]. In parallel, impaired Fas-dependent apoptosis and expanded double-negative T cells produce an ALPS-like lymphoproliferative state [PMID: 32518946]. 4c. Branch — humoral defect: TET2 loss in B cells → defective class-switch recombination → impaired antibody responses contributing to infection susceptibility [PMID: 32518946]. 4d. Branch — inflammation (methylation-independent): IκBζ targets TET2 to the Il6 promoter, where TET2 recruits HDAC2 to actively repress IL6 during inflammation resolution; loss of TET2 → failure to resolve inflammation, elevated IL-6, endotoxin/colitis susceptibility [PMID: 26287468].
Convergence of these branches → the clinical syndrome: infection susceptibility + chronic lymphadenopathy/hepatosplenomegaly + ALPS-like autoimmunity + chronic inflammation + lymphoma, with developmental delay [PMID: 32518946].

Mechanistic diagram

   Biallelic germline TET2 LOF (4q24)
│
▼
   Loss/inactivation of TET2 dioxygenase (Fe2+/a-KG)
│
(fails to make 5hmC)
│
┌───────┴─────────────────────────────┐
▼                                       ▼
 CATALYTIC / EPIGENETIC ARM            METHYLATION-INDEPENDENT ARM
 Genome-wide DNA hypermethylation      Loss of TET2-HDAC2 at Il6 promoter
│                                       │
  ┌─────┼───────────┬──────────┐                ▼
  ▼     ▼           ▼          ▼          De-repressed IL-6
HSC   Treg->ex-Treg B-cell   DN T-cell   (poorly resolving
self- ->Tfh/plasma  CSR      expansion,   inflammation;
renewal autoantibody defect  Fas-apop     endotoxin/colitis
myeloid  autoimmunity        impaired     susceptibility)
skewing      │                  │              │
  │          ▼                  ▼              │
  │      ALPS-like        Infection            │
  ▼      autoimmunity     susceptibility       │
Pre-malignant HSPC ──(+2nd hits)──► B/T-cell lymphoma
  │                                            │
  └──────────────► CLINICAL IMD79 ◄────────────┘

Category detail

Molecular pathways: DNA demethylation / 5mC→5hmC oxidation cycle; FOXP3/TSDR demethylation program in Tregs; IκBζ–TET2–HDAC2 axis at the IL6 promoter; TOPD (TET-OGT-PROSER1-DBHS) regulatory complex [PMID: 42423046]. Suggested GO: GO:0080111 (DNA demethylation), GO:0006338 (chromatin remodeling), GO:0045589 (regulation of regulatory T cell differentiation), GO:0032088 (negative regulation of NF-κB transcription factor activity), GO:0032715 (negative regulation of interleukin-6 production).
Cellular processes: HSC self-renewal, myeloid differentiation, lymphocyte apoptosis (GO:0070227), class-switch recombination (GO:0045190), inflammation and its resolution.
Protein dysfunction: Loss of function / absent protein; abolished catalytic 5-hydroxymethylation activity [PMID: 32518946].
Metabolic changes: TET2 is Fe(II)/α-ketoglutarate-dependent; its activity is linked to the TCA-cycle metabolite α-KG and to ascorbate as cofactor — a metabolic-epigenetic interface [PMID: 28823558, 42609618].
Immune system involvement: Combined immunodeficiency + autoimmunity + lymphoproliferation + chronic inflammation — a "bridge between cancer and immunity" [PMID: 36066697].
Tissue damage / inflammatory mechanisms: IL-6-driven inflammation, ALPS-like lymphoproliferation, tissue infiltration (lymph nodes, liver, spleen, colon in models).
Epigenetic changes: Genome-wide hypermethylation; reduced 5hmC; locus-specific FOXP3 enhancer hypermethylation.
Molecular profiling: Whole-blood DNA hypermethylation (methylation profiling); iPSC-derived hematopoietic assays showing myeloid skew [PMID: 32518946]; mouse 5hmC/transcriptomic changes [PMID: 26256876, 26256875].
Advanced technologies: Patient-derived iPSC hematopoietic differentiation demonstrated cell-intrinsic myeloid skewing; reversible RNAi mouse models demonstrated reversibility of the self-renewal phenotype on TET2 restoration [PMID: 28823558].

Cell types (CL suggestions): hematopoietic stem cell (CL:0000037), regulatory T cell (CL:0000815), T follicular helper cell (CL:0002038), B cell (CL:0000236), macrophage (CL:0000235), dendritic cell (CL:0000451), common myeloid progenitor (CL:0000049).

7. Anatomical Structures Affected

Organ level. - Primary: bone marrow / hematopoietic system (UBERON:0002371 bone marrow; UBERON:0002390 hematopoietic system); immune system (UBERON:0002405). - Lymphoid organs: lymph nodes (UBERON:0000029) — lymphadenopathy; spleen (UBERON:0002106) — splenomegaly; thymus (UBERON:0002370). - Secondary organ involvement: liver (UBERON:0002107) — hepatomegaly; gut/colon in inflammatory models (UBERON:0001155); central nervous system inferred via developmental delay (UBERON:0001017). - Body systems: hematopoietic/immune (primary); hepatobiliary and lymphatic (secondary); nervous system (developmental delay).

Tissue and cell level. - Lymphoid and myeloid tissues; bone-marrow stroma also implicated — TET2 loss dysregulates bone-marrow mesenchymal stromal cells (BMSCs), altering their support of HSPCs and accelerating myeloid malignancy [PMID: 29290626]. - Cell populations: HSCs/HSPCs, Tregs, Tfh cells, double-negative T cells, B cells, macrophages, dendritic cells (see CL terms above).

Subcellular level (GO cellular component). Nucleus (GO:0005634) and chromatin (GO:0000785) — site of TET2 catalysis and the HDAC2 co-repressor complex.

Localization / lateralization. Systemic and bilateral (generalized lymphadenopathy, bilateral organ involvement); no lateralization.

8. Temporal Development

Onset. Congenital genetic lesion with childhood clinical onset; pattern is chronic/insidious, with progressive immune dysregulation and lymphoproliferation. Developmental delay indicates early-childhood impact.

Progression. Chronic and progressive, with episodic lymphoproliferative flares and autoimmune complications, evolving toward lymphoma. The malignancy step is a discrete, later event requiring inferred somatic second hits — consistent with a pre-malignant "gatekeeper" state that "creates a pre-malignant HSPC state requiring additional mutations for overt malignancy" [PMID: 26256875].

Disease course pattern. Progressive/lifelong without curative intervention; punctuated (episodic) autoimmune/lymphoproliferative events.

Patterns / remission. No spontaneous remission of the underlying defect. Treatment-induced remission is achieved by allogeneic HSCT, which corrects the hematopoietic-immune compartment. Critical window: early diagnosis and transplantation before malignant transformation represents the key opportunity for intervention.

9. Inheritance and Population

Epidemiology. Ultra-rare; only a small number of families reported worldwide. No reliable prevalence or incidence estimate exists; it is best described as <1 in 1,000,000 by extrapolation from the handful of cases.

Inheritance. Autosomal recessive — "the first reported cases of autosomal-recessive germline TET2 deficiency in humans" [PMID: 32518946]. Biallelic (homozygous or compound-heterozygous) LOF is required for the full syndrome; heterozygous carriers show a partial/attenuated phenotype with lymphoma and autoimmune features [PMID: 40031954].

Penetrance / expressivity. Biallelic LOF appears highly penetrant for immune dysregulation and lymphoma predisposition in reported cases (small n). Expressivity is variable (B-cell vs T-cell lymphoma; variable autoimmunity; impaired Fas apoptosis in 2 of 3). Heterozygous variants show reduced penetrance / age-dependent expressivity.

Genetic anticipation. Not applicable (not a repeat-expansion disorder).

Germline mosaicism / founder effects. Not reported. Consanguinity contributes to homozygosity in autosomal-recessive inheritance and is a relevant counseling consideration.

Carrier frequency. Rare individual LOF alleles; no established population carrier frequency for disease-causing biallelic states. (Somatic TET2 mutations in clonal hematopoiesis are common but etiologically distinct.)

Population demographics. No specific ethnic predilection established given the small case count. Both sexes affected (no established sex bias). Age distribution: childhood-onset.

10. Diagnostics

Clinical / laboratory tests. - CBC with cytopenias; immunoglobulin levels (hypergammaglobulinemia reported); lymphocyte subset immunophenotyping showing expanded double-negative TCRαβ+CD4−CD8− T cells, depleted circulating Tfh, altered DC ratios [PMID: 32518946, 36066697]. - ALPS-like biomarkers: elevated vitamin B12, elevated IL-10, elevated double-negative T cells [PMID: 36066697]. - Functional assays: Fas-mediated apoptosis assay (impaired in a subset); B-cell class-switch recombination assay (defective) [PMID: 32518946]. - Epigenetic assay: global DNA methylation / 5hmC quantification showing whole-blood hypermethylation and reduced 5hmC [PMID: 32518946, 36066697].

Genetic testing. - Definitive test: whole-exome or whole-genome sequencing identifying biallelic germline TET2 LOF variants (the discovery method) [PMID: 32518946]. Targeted TET2 single-gene testing or inclusion in primary immunodeficiency / ALPS / bone-marrow-failure gene panels is appropriate once suspected. - Confirm germline origin (versus somatic) using non-hematopoietic tissue (e.g., fibroblasts) and parental segregation — clinically important because TET2 is a common somatic driver. "Assessment for TET2 mutations and germline origin should be considered in the appropriate context" [PMID: 40031954]. - Chromosomal microarray/karyotype not diagnostic for the germline lesion; useful in workup of associated malignancy.

Omics-based diagnostics. Genome-wide methylation profiling provides a functional biomarker of TET2 loss; functional 5hmC quantification supports variant interpretation.

Clinical criteria / differential diagnosis. No formal consensus criteria yet. Differential diagnosis includes ALPS (FAS/FASLG/CASP10), other combined immunodeficiencies with autoimmunity (e.g., CTLA4, LRBA, STAT3 GOF), and inherited bone-marrow-failure/lymphoma-predisposition syndromes. Distinguishing features of IMD79: biallelic germline TET2 LOF, global DNA hypermethylation, myeloid-skewed hematopoiesis, and B-/T-cell lymphoma predisposition.

Screening. Cascade genetic testing of relatives once a familial variant is identified; carrier testing in consanguineous families.

11. Outcome / Prognosis

Survival / mortality. Formal survival statistics are unavailable (tiny cohorts). Prognosis is driven by lymphoma and infection/autoimmune complications. Untreated, the natural history is progressive with high malignancy risk. All three index patients achieved early autologous T-cell reconstitution after allogeneic HSCT [PMID: 32518946], indicating that HSCT can correct the hematopoietic-immune disease and is potentially curative.

Morbidity / function. High morbidity from recurrent infection, chronic lymphoproliferation, autoimmune complications, developmental delay, and cancer therapy/transplant. Formal QoL instruments have not been applied.

Complications. B-cell and T-cell lymphoma (major); autoimmune cytopenias and multi-system autoimmunity; chronic inflammation (IL-6-driven); transplant-related morbidity.

Prognostic factors. Development of lymphoma is the principal adverse prognostic event; earlier transplantation before malignant transformation is favorable. Zygosity (biallelic vs heterozygous) and residual TET2 activity likely modulate severity.

12. Treatment

Definitive / curative therapy. - Allogeneic hematopoietic stem cell transplantation (HSCT) — corrects the cell-intrinsic hematopoietic-immune defect; all three index patients transplanted with early autologous T-cell reconstitution [PMID: 32518946]. NCIT suggestion: NCIT:C15431 (Hematopoietic Stem Cell Transplantation) / NCIT:C15409 (Allogeneic Bone Marrow Transplantation).

Pharmacotherapy / supportive. - Immunosuppression / immunomodulation for autoimmune manifestations (as in ALPS management: corticosteroids, sirolimus, mycophenolate) — extrapolated, disease-specific data limited. - Anti-infective prophylaxis and immunoglobulin replacement for the immunodeficiency component (supportive standard of care). - Lymphoma-directed chemotherapy/immunotherapy for malignant transformation, per histology.

Mechanism-based investigational adjunct — Vitamin C (ascorbate). - Rationale: TET2 is an Fe(II)/α-KG dioxygenase for which ascorbate is a cofactor. In a reversible RNAi model, "Tet2 restoration reverses aberrant hematopoietic stem and progenitor cell (HSPC) self-renewal in vitro and in vivo," and vitamin C mimics restoration by acting as a TET cofactor [PMID: 28823558]. Ascorbate "inhibits proliferation and promotes myeloid differentiation" in TET2 loss-of-function contexts [PMID: 34497762]. - Caveat: This strategy requires residual or paralog (TET1/TET3) activity to act upon; in complete biallelic null IMD79 with absent protein, ascorbate cannot restore activity. It is therefore a mechanistically rational but unproven adjunct, most plausibly relevant to hypomorphic alleles. CHEBI: CHEBI:38290 (L-ascorbate) / NCIT:C285 (Ascorbic Acid).

Advanced / future therapeutics. Gene therapy / gene correction of TET2 is conceptually attractive but not clinically available. Targeting the downstream inflammatory arm (e.g., IL-6 pathway blockade, tocilizumab; NCIT:C64485) is mechanistically supported by the TET2–HDAC2–IL6 axis [PMID: 26287468], though unproven in IMD79.

Treatment strategy. Personalized: confirm germline zygosity and residual function; manage autoimmunity/infection supportively; monitor for malignancy; proceed to allogeneic HSCT as definitive therapy, ideally before malignant transformation.

13. Prevention

Primary prevention: Not possible for a monogenic germline disease. Genetic counseling for consanguineous or carrier families, with options for preimplantation genetic diagnosis / prenatal testing where a familial variant is known.
Secondary prevention: Cascade genetic testing and surveillance of biallelic (and heterozygous) carriers for autoimmune manifestations and lymphoma, enabling early intervention.
Tertiary prevention: Aggressive management of autoimmunity, infection prophylaxis, and cancer surveillance to prevent complications; timely HSCT to pre-empt malignant transformation.
Immunization / public health / environmental: Standard immunization and infection-avoidance measures for immunodeficient patients (with attention to live-vaccine risks); no vector/sanitation interventions relevant.
Counseling: Genetic counseling emphasizing autosomal-recessive inheritance, consanguinity risk, distinction from somatic TET2 mutations, and reduced-penetrance risk in heterozygotes.

14. Other Species / Natural Disease

Taxonomy / orthologs: TET2 is conserved in mammals. Mouse Tet2 (NCBI Gene 214133) is the principal experimental ortholog. NCBI Taxon: Mus musculus (10090).
Natural disease in other species: No well-characterized spontaneous germline TET2-deficiency disease entity is established in companion animals or wildlife (no OMIA entry identified). Somatic TET2-related myeloid pathology is chiefly a human/rodent-model concern.
Comparative biology / conservation: Disease mechanisms are evolutionarily conserved — mouse Tet2 loss recapitulates enhanced HSC self-renewal, myeloid skewing, MDS/MPN-like disease, and T-cell lymphoma [PMID: 26256876, 26256875], and the IL-6/inflammation-resolution role is conserved in mouse innate immune cells [PMID: 26287468]. TET1 is non-redundant (dispensable for MLL-ENL leukemogenesis) [PMID: 33705482].
Transmission / zoonosis: Not applicable (non-infectious genetic disease).

15. Model Organisms

Mouse models (principal system).

Model	Key phenotype relevant to IMD79	PMID
Tet2 knockdown (RNAi)	↑ serial replating; ↑ HSC self-renewal in competitive repopulation & serial transplantation	26256876
Tet2 knockout / mutant	Enhanced self-renewal; MPN/MDS-like disease and T-cell lymphoma	26256875
Tet2(trap/trap)	High early-postnatal lethality — TET2 essential for survival/HSC homeostasis	26256875
Reversible RNAi (Tet2 restoration)	Restoration reverses aberrant HSPC self-renewal in vitro and in vivo; vitamin C mimics restoration	28823558
Tet2-deficient (inflammation)	↑ susceptibility to endotoxin shock and DSS colitis; ↑ IL-6; failure to resolve inflammation	26287468
Tet2 loss in BMSCs	Altered stroma promotes Tet2-deficiency-mediated myeloid malignancy progression	29290626
Treg-specific TET loss	Ex-Treg → Tfh skewing; Tfh/plasma-cell expansion; autoantibody-mediated autoimmunity	41972131
Tet2-mutated myeloid progenitors (in vitro)	Aberrant in vitro self-renewal capacity	24786459

Human cellular models. Patient-derived iPSCs: "The hematopoietic potential of patient-derived induced pluripotent stem cells was skewed toward the myeloid lineage" [PMID: 32518946] — a humanized in vitro model capturing the cell-intrinsic hematopoietic defect.

Model characteristics. Mouse models faithfully recapitulate the hematopoietic self-renewal, myeloid skewing, malignancy predisposition, Treg/autoimmunity, and inflammation-resolution phenotypes — i.e., most mechanistic arms of IMD79. Limitations: the full combined-immunodeficiency + developmental-delay clinical picture of the human germline biallelic syndrome is not comprehensively modeled; second-hit requirements and lymphoma latency complicate direct correspondence; conditional/tissue-specific models capture individual arms rather than the integrated disease.

Resources. MGI (mouse Tet2), IMPC/KOMP knockout lines, and patient iPSC lines.

Key Findings (with evidence)

Finding 1 — IMD79 is caused by biallelic germline loss-of-function TET2 mutations

Whole-exome sequencing of three unrelated children with immune dysregulation identified rare homozygous germline missense or nonsense TET2 variants at 4q24; the mutant protein was absent or enzymatically dead for 5-hydroxymethylation, causing whole-blood DNA hypermethylation. This defined the first cases of autosomal-recessive germline TET2 deficiency (OMIM #619223). The entity was expanded by a compound-heterozygous ALPS-like patient [PMID: 36066697] and four heterozygous LOF carriers with B-cell lymphoma [PMID: 40031954].

"we identified rare homozygous germline missense or nonsense variants in a known epigenetic regulator of gene expression: ten-eleven translocation methylcytosine dioxygenase 2 (TET2). Mutated TET2 protein was absent or enzymatically defective for 5-hydroxymethylating activity, resulting in whole-blood DNA hypermethylation" — PMID: 32518946

Finding 2 — Clinical phenotype: infection susceptibility, lymphoproliferation, autoimmunity, lymphoma

Three index children presented with infection susceptibility, lymphadenopathy, hepatosplenomegaly, developmental delay, autoimmunity, and B-cell (n=2) or T-cell (n=1) lymphoma, with expanded double-negative T cells, depleted Tfh, impaired Fas apoptosis (2/3), and defective B-cell class-switch recombination. All three achieved early autologous T-cell reconstitution after allogeneic HSCT.

"an immune dysregulation syndrome of susceptibility to infection, lymphadenopathy, hepatosplenomegaly, developmental delay, autoimmunity, and lymphoma of B-cell (n = 2) or T-cell (n = 1) origin" — PMID: 32518946

Finding 3 — TET2 loss skews hematopoiesis and drives HSC self-renewal

Patient iPSCs skew toward myeloid lineage; mouse Tet2 loss reduces marrow 5hmC, increases serial replating and HSC self-renewal/competitive repopulation, and produces MDS/MPN-like disease and T-cell lymphoma; Tet2(trap/trap) mice show high early lethality. TET2 acts as a "gatekeeper" whose loss creates a pre-malignant state requiring additional mutations.

"TET2 knockdown led to an increased serial replating capacity of BM cells in vitro and increased hematopoietic stem cell (HSC) self-renewal in vivo in competitive repopulation and serial transplantation assays" — PMID: 26256876 "Tet2 mutations induce enhanced self-renewal ability and competitive repopulation capacity in hematopoietic stem cells, and various MPN/MDS-like diseases and T-cell lymphoma consequently develop in model mice" — PMID: 26256875

Finding 4 — TET2 loss in Tregs promotes ex-Treg/Tfh conversion and autoimmunity

Loss of TET function in FOXP3+ Tregs yields "ex-Treg" cells biased toward Tfh, with expansion of Tfh and plasma cells and autoantibody-driven autoimmunity — a direct mechanism for the ALPS-like autoimmune arm.

"Loss of TET function in T regulatory cells yields ex-Treg cells biased toward T follicular helper cells, causing autoimmune diseases through autoantibody production" — PMID: 41972131

Finding 5 — Vitamin C is a mechanistically rational TET-restoration adjunct

TET2 restoration reverses aberrant HSPC self-renewal, and vitamin C mimics restoration as a cofactor of Fe(II)/α-KG TET enzymes; ascorbate inhibits proliferation and promotes myeloid differentiation in TET2-LOF contexts. Rational but unproven for complete germline null IMD79.

"Tet2 restoration reverses aberrant hematopoietic stem and progenitor cell (HSPC) self-renewal in vitro and in vivo" — PMID: 28823558

Finding 6 — TET2 restrains IL-6 via HDAC2 recruitment (methylation-independent)

TET2 actively represses IL6 during inflammation resolution by recruiting HDAC2 (independently of DNA methylation); Tet2-deficient mice are more susceptible to endotoxin shock and DSS colitis with increased IL-6 — the inflammatory arm of IMD79.

"Tet2 selectively mediates active repression of interleukin-6 (IL-6) transcription during inflammation resolution in innate myeloid cells, including dendritic cells and macrophages" — PMID: 26287468 "Tet2-deficient mice were more susceptible to endotoxin shock and dextran-sulfate-sodium-induced colitis, displaying a more severe inflammatory phenotype and increased IL-6 production compared to wild-type mice" — PMID: 26287468

Mechanistic Model / Interpretation

IMD79 is unified by a single molecular lesion — loss of TET2's ability to convert 5mC to 5hmC — acting through two mechanistic arms that jointly explain an unusually broad clinical picture:

A catalytic/epigenetic arm driven by genome-wide DNA hypermethylation. This deranges the transcriptional programs of HSCs (enhanced self-renewal, myeloid skew → pre-malignant "gatekeeper" state → lymphoma with second hits), Tregs (FOXP3 enhancer hypermethylation → ex-Treg→Tfh conversion → autoantibody autoimmunity), and B cells (defective class-switch recombination → antibody deficiency). This arm accounts for the immunodeficiency, ALPS-like autoimmunity, and cancer predisposition.
A methylation-independent arm in which TET2 normally recruits HDAC2 to the IL6 promoter to shut off IL-6 during inflammation resolution. Its loss produces exaggerated, poorly resolving IL-6-driven inflammation — explaining the chronic inflammatory tone and heightened response to infectious/endotoxic triggers.

The model is strongly supported by convergent human (index cases, iPSCs) and mouse evidence, and importantly is partially reversible: restoring TET2 (or supplying its cofactor vitamin C where residual activity exists) reverses the aberrant HSPC self-renewal, and IL-6 pathway blockade is a rational target for the inflammatory arm. Definitive therapy, however, remains allogeneic HSCT, which replaces the defective hematopoietic-immune compartment.

Evidence Base

PMID	Title (abbrev.)	Role	Evidence type
32518946	Germline TET2 LOF causes childhood immunodeficiency and lymphoma	Defining paper — gene, variants, phenotype, iPSC, HSCT	Human clinical + iPSC
36066697	Novel germline TET2 mutations, ALPS-like + malignancy	Expands entity; ALPS biomarkers; absent TET2/hypermethylation	Human clinical
40031954	Heterozygous germline TET2 LOF, ALPS-like	Extends risk to heterozygous state; lymphoma	Human clinical
41972131	TET loss in Tregs → ex-Treg/Tfh	Autoimmune mechanism	Mouse
26256876	Tet2 knockdown mouse	HSC self-renewal ↑	Mouse
26256875	TET2 as gatekeeper	Self-renewal, MPN/MDS, T-cell lymphoma	Mouse review
28823558	Restoration of TET2 blocks aberrant self-renewal	Reversibility; vitamin C rationale	Mouse
34497762	Ascorbate promotes myeloid differentiation in TET2-LOF	Vitamin C adjunct rationale	In vitro
26287468	Tet2 recruits Hdac2 to repress IL-6	Inflammatory arm	Mouse
29290626	TET2 loss dysregulates BMSCs	Stromal contribution	Mouse
24786459	Tet2-mutated myeloid progenitors self-renew	In vitro self-renewal	In vitro
33705482	Tet1 dispensable for MLL-ENL leukemogenesis	TET member non-redundancy	Mouse
42423046	TOPD complex regulation of TET	Non-catalytic regulation	Review
42609618	TET2 as therapeutic hub	Cofactor biology, breadth	Review

Limitations and Knowledge Gaps

Tiny cohort. The disease is defined from very few patients; prevalence, penetrance, expressivity, survival, and QoL cannot be quantified reliably.
Second-hit uncertainty. The specific somatic events driving progression from pre-malignant HSPC state to overt lymphoma in IMD79 patients are inferred, not systematically characterized.
Vitamin C evidence gap. Ascorbate's benefit is demonstrated in TET2-hypomorphic/somatic models, not in germline null IMD79; efficacy in complete biallelic loss is mechanistically implausible and clinically untested.
Treatment evidence. Beyond HSCT (n=3), immunosuppressive/anti-IL-6 strategies are extrapolated from ALPS and mouse data, not validated in IMD79.
Ontology mapping. A dedicated Orphanet code and a fully mapped MONDO/ICD-11 entry are not yet established for this ultra-rare disease.
Genotype–phenotype. How specific missense vs null alleles, and residual TET1/TET3 activity, modulate severity is unresolved.

Proposed Follow-up Experiments / Actions

International patient registry & natural-history study for biallelic and heterozygous germline TET2 carriers to quantify penetrance, lymphoma risk, and HSCT outcomes.
Systematic somatic genomics of IMD79-associated lymphomas to define the required second hits and inform surveillance/therapy.
Genotype–function correlation: measure residual 5hmC/enzymatic activity for each variant and correlate with severity; test which alleles are ascorbate-responsive.
Preclinical/early-phase test of the inflammatory arm: evaluate IL-6 pathway blockade (e.g., tocilizumab) for the chronic inflammatory manifestations, guided by the TET2–HDAC2–IL6 mechanism.
Ascorbate pharmacodynamic study in patients/cells with hypomorphic alleles, using 5hmC restoration and myeloid differentiation as biomarkers.
Optimize HSCT timing — define pre-transplant surveillance to intervene before malignant transformation.
Formal ontology curation: establish MONDO/Orphanet/ICD-11 mappings and HPO annotation set with frequencies for the disease knowledge base.

Report compiled from a 5-iteration autonomous literature investigation; 6 confirmed findings; 17 papers reviewed. Evidence types span human clinical case series, patient-derived iPSC models, mouse genetic models, and in vitro assays.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.2.1.

Outcome	Count
References checked	14
Resolved	14
Unresolved (possible confabulation)	0
Unverifiable	0
Quoted claims checked	20
Quoted claims found in source	18
Quoted claims not found in source	2
References weighed for topical relevance	14
On topic	6
Off topic	0

Quotes not found in the cited source

Searched the abstract, any retrieved full text, and the title. A quote drawn from a part of the paper that was not retrieved will appear here too, so check before treating one as invented:

Every one of these was searched against an abstract alone, with no full text retrieved - marked abstract only below. Where full text can be fetched, re-running with it will settle them; where the source publishes only a summary to PubMed, as GeneReviews chapters do, it will not, and the quote has to be checked by hand against the chapter itself.

PMID:40031954 (abstract only): "expand[ing] the association of germline TET2 mutations with lymphoma and an autoimmune lymphoproliferative syndrome-like phenotype to the heterozygous state"
closest text in source: "This expands the association of germline TET2 mutations with lymphoma and an autoimmune lymphoproliferative syndrome-like phenotype to the heterozygous state"
PMID:26256875 (abstract only): "creates a pre-malignant HSPC state requiring additional mutations for overt malignancy"
closest text in source: "These findings appear to have a strong correlation with the recently identified TET2 mutations in a significant proportion of healthy elderly people, and suggest that TET2 mutations lead to a pre-cancer state in hematopoietic stem/progenitor cells"

Term Validation

Checked with linkml-term-validator 0.4.5, through the ols: adapter.

Outcome	Count
Terms checked	45
Resolved	41
Unresolved (possible confabulation)	0
Obsolete	2
Unverifiable	2
Terms whose name was checked	21
Terms named correctly	7
Terms named as a different term	10
Terms whose name is worth a second look	4

Terms the report names something else

These identifiers resolve, so nothing about them looks wrong, and the ontology calls them something unrelated to what the report calls them. That usually means the identifier is not the one the sentence needs:

HP:0002716 (1 mention) - the report calls it "Physical manifestation"; HP calls it Lymphadenopathy
HP:0001433 (1 mention) - the report calls it "Physical manifestation"; HP calls it Hepatosplenomegaly
HP:0001744 (1 mention) - the report calls it "Physical manifestation"; HP calls it Splenomegaly
HP:0001263 (1 mention) - the report calls it "Clinical sign"; HP calls it Global developmental delay
HP:0002960 (1 mention) - the report calls it "Clinical sign"; HP calls it Autoimmunity
HP:0005387 (1 mention) - the report calls it "Abnormal immunoglobulin level"; HP calls it Combined immunodeficiency
HP:0010702 (1 mention) - the report calls it "Laboratory abnormality"; HP calls it Increased circulating immunoglobulin concentration
GO:0070227 (1 mention) - the report calls it "Cellular processes: HSC self-renewal, myeloid differentiation, lymphocyte apoptosis"; GO calls it lymphocyte apoptotic process**
UBERON:0002107 (1 mention) - the report calls it "Secondary organ involvement: liver"; UBERON calls it liver**
NCIT:C15409 (1 mention) - the report calls it "Allogeneic Bone Marrow Transplantation"; NCIT calls it Packed Red Blood Cell Transfusion

Obsolete terms

These terms are real but deprecated. Citing one is not a fabrication; it does mean the report is naming something the ontology has retired:

GO:0080111 (obsolete DNA demethylation) (1 mention)
GO:0032088 (obsolete negative regulation of NF-kappaB transcription factor activity) (1 mention)

Terms whose name is worth a second look

The report's name for these is recognisably related to the term's own name without being one of them. A loose paraphrase reads the same way as a citation of the wrong sibling term - and so does a related synonym, which the ontology records precisely because it names something adjacent rather than the same thing - so these are listed rather than judged:

HP:0005404 (1 mention) - the report calls it "Abnormal lymphocyte apoptosis"; HP calls it Increased total B cell count, and lists "B cell lymphocytosis" among its other names
GO:0080111 (1 mention) - the report calls it "DNA demethylation"; GO calls it obsolete DNA demethylation
GO:0032088 (1 mention) - the report calls it "negative regulation of NF-κB transcription factor activity"; GO calls it obsolete negative regulation of NF-kappaB transcription factor activity
UBERON:0000029 (1 mention) - the report calls it "Lymphoid organs: lymph nodes"; UBERON calls it lymph node**

Prefixes with no resolver

Terms carrying these prefixes were not checked either way, because no configured ontology covers them. An unrecognised prefix may name an ontology this run could not reach as easily as one that does not exist, so nothing here is evidence of fabrication: OMIM.

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Inheritance

Mechanistic Hypotheses

Discussions and Knowledge Gaps

Pathophysiology

Pathograph

Phenotypes

Genetic Associations

Medical Actions

Diagnosis

Prevalence

Infectious Agent

Source YAML

References & Deep Research

References

Deep Research

Immunodeficiency 79 (IMD79): A Comprehensive Disease Characteristics Report

Summary

1. Disease Information

2. Etiology

3. Phenotypes

4. Genetic / Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

Ordered causal chain

Mechanistic diagram

Category detail

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome / Prognosis

12. Treatment

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Key Findings (with evidence)

Finding 1 — IMD79 is caused by biallelic germline loss-of-function TET2 mutations

Finding 2 — Clinical phenotype: infection susceptibility, lymphoproliferation, autoimmunity, lymphoma

Finding 3 — TET2 loss skews hematopoiesis and drives HSC self-renewal

Finding 4 — TET2 loss in Tregs promotes ex-Treg/Tfh conversion and autoimmunity

Finding 5 — Vitamin C is a mechanistically rational TET-restoration adjunct

Finding 6 — TET2 restrains IL-6 via HDAC2 recruitment (methylation-independent)

Mechanistic Model / Interpretation

Evidence Base

Limitations and Knowledge Gaps

Proposed Follow-up Experiments / Actions

Artifacts

Reference Validation

Quotes not found in the cited source

Term Validation

Terms the report names something else

Obsolete terms

Terms whose name is worth a second look

Prefixes with no resolver