Human nude SCID is an autosomal recessive inborn error of immunity caused by biallelic loss-of-function variants in FOXN1, the winged-helix (forkhead) transcription factor mutated at the nude locus of mice and rats. FOXN1 is expressed in thymic epithelium and in terminally differentiating skin epithelium, and its loss produces a single-gene lesion with two anatomically separate consequences. In the thymic anlage, thymic epithelial cells fail to differentiate, so no functional thymic microenvironment forms; the resulting athymia abolishes thymopoiesis and produces profound T-cell lymphopenia with numerically preserved B and NK cells, presenting in the first months of life as severe, recurrent, life-threatening infection. In the skin, loss of the same factor disrupts the growth-differentiation balance of keratinocytes in hair follicles and nail bed, producing congenital alopecia universalis and nail dystrophy (characteristically proximal arciform leukonychia and koilonychia). Because the defect is in thymic stroma rather than in hematopoietic stem cells, hematopoietic cell transplantation does not reconstitute thymopoiesis unless the graft itself carries mature donor T cells, and cultured thymus tissue implantation is the mechanism-directed treatment. The disease is ultra-rare; most reported patients descend from a single R255X founder pedigree in Acerno, southern Italy. Cross-references: OMIM:601705, Orphanet ORPHA:169095. Two adjacent FOXN1-related entities are deliberately NOT covered by this entry. (1) FOXN1 haploinsufficiency (OMIM 600838), an autosomal dominant condition in which heterozygous carriers are ascertained through low TRECs on newborn screening and show T-cell lymphopenia that improves with age, usually with normal or near-normal hair and only subtle nail dystrophy; IUIS lists it as a separate row from the AR disease. (2) Compound heterozygous hypomorphic FOXN1 genotypes, which can produce selective thymic hypoplasia with entirely normal hair and nails. Both are described in the genetic and notes sections here only to keep the boundary of this entry explicit.
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name: T-cell Immunodeficiency, Congenital Alopecia, and Nail Dystrophy
creation_date: '2026-08-26T00:00:00Z'
category: Genetic
synonyms:
- nude SCID
- FOXN1 deficiency
- alymphoid cystic thymic dysgenesis
- winged-helix-nude
- winged helix deficiency
- severe T-cell immunodeficiency-congenital alopecia-nail dystrophy syndrome
description: >-
Human nude SCID is an autosomal recessive inborn error of immunity caused by
biallelic loss-of-function variants in FOXN1, the winged-helix (forkhead)
transcription factor mutated at the nude locus of mice and rats. FOXN1 is
expressed in thymic epithelium and in terminally differentiating skin
epithelium, and its loss produces a single-gene lesion with two anatomically
separate consequences. In the thymic anlage, thymic epithelial cells fail to
differentiate, so no functional thymic microenvironment forms; the resulting
athymia abolishes thymopoiesis and produces profound T-cell lymphopenia with
numerically preserved B and NK cells, presenting in the first months of life
as severe, recurrent, life-threatening infection. In the skin, loss of the
same factor disrupts the growth-differentiation balance of keratinocytes in
hair follicles and nail bed, producing congenital alopecia universalis and
nail dystrophy (characteristically proximal arciform leukonychia and
koilonychia). Because the defect is in thymic stroma rather than in
hematopoietic stem cells, hematopoietic cell transplantation does not
reconstitute thymopoiesis unless the graft itself carries mature donor T
cells, and cultured thymus tissue implantation is the mechanism-directed
treatment. The disease is ultra-rare; most reported patients descend from a
single R255X founder pedigree in Acerno, southern Italy. Cross-references:
OMIM:601705, Orphanet ORPHA:169095.
Two adjacent FOXN1-related entities are deliberately NOT covered by this
entry. (1) FOXN1 haploinsufficiency (OMIM 600838), an autosomal dominant
condition in which heterozygous carriers are ascertained through low TRECs on
newborn screening and show T-cell lymphopenia that improves with age, usually
with normal or near-normal hair and only subtle nail dystrophy; IUIS lists it
as a separate row from the AR disease. (2) Compound heterozygous hypomorphic
FOXN1 genotypes, which can produce selective thymic hypoplasia with entirely
normal hair and nails. Both are described in the genetic and notes sections
here only to keep the boundary of this entry explicit.
disease_term:
preferred_term: FOXN1 deficiency (nude SCID)
term:
id: MONDO:0011132
label: T-cell immunodeficiency, congenital alopecia, and nail dystrophy
parents:
- Severe combined immunodeficiency
- Primary immunodeficiency
- Congenital athymia
classifications:
harrisons_chapter:
- classification_value: IMMUNE_RHEUMATOLOGIC
notes: >-
Nude SCID is an inborn error of immunity presenting as severe combined
immunodeficiency, so its primary clinical home is the immunology grouping.
evidence:
- reference: PMID:33464451
reference_title: "Expanding the Nude SCID/CID Phenotype Associated with FOXN1 Homozygous, Compound Heterozygous, or Heterozygous Mutations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Human nude SCID is a rare autosomal recessive inborn error of immunity
(IEI) characterized by congenital athymia, alopecia, and nail dystrophy.
explanation: >-
Characterizes the disease as an inborn error of immunity, supporting
placement in the immunology/rheumatology Part.
- classification_value: GENETICS_ENVIRONMENT_DISEASE
notes: >-
Monogenic autosomal recessive disorder of a single transcription factor,
with a documented founder mutation and carrier screening in an extended
pedigree.
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Nude severe combined immunodeficiency is a rare inherited disease caused
by autosomal recessive loss-of-function mutations in FOXN1.
explanation: >-
Establishes the disease as a Mendelian, autosomal recessive single-gene
condition.
iuis_category:
classification_value: combined immunodeficiency with syndromic features
notes: >-
IUIS 2022 phenotypic classification, Table 2 section 3 (thymic defects
with additional congenital anomalies). The row cited here is the
autosomal recessive "FOXN1 deficiency" entry (FOXN1, AR, OMIM 601705),
which is this disease. The immediately following IUIS row, "FOXN1
haploinsufficiency" (FOXN1, AD, OMIM 600838), is a separate and distinct
entity - heterozygous carriers with severe T-cell lymphopenia at birth
that normalises by adulthood, nail dystrophy, and no congenital athymia -
and is NOT covered by this entry.
evidence:
- reference: PMID:35748970
reference_title: "Human Inborn Errors of Immunity: 2022 Update on the Classification from the International Union of Immunological Societies Expert Committee."
supports: SUPPORT
evidence_source: OTHER
snippet: "FOXN1 deficiency FOXN1 AR 601705"
explanation: >-
The IUIS Table 2 thymic-defects row assigns FOXN1 deficiency to the
combined immunodeficiency with syndromic features table, and records the
autosomal recessive inheritance and OMIM 601705 identity that separate
it from the adjacent AD haploinsufficiency row.
inheritance:
- name: Autosomal recessive
inheritance_term:
preferred_term: Autosomal recessive inheritance
term:
id: HP:0000007
label: Autosomal recessive inheritance
description: >-
Nude SCID is caused by biallelic loss-of-function variants in FOXN1.
Heterozygous carriers of the R255X founder allele are healthy with respect
to the full triad, which is what allowed a 6.52% carrier frequency to
persist in the Acerno pedigree.
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Nude severe combined immunodeficiency is a rare inherited disease caused
by autosomal recessive loss-of-function mutations in FOXN1.
explanation: States the autosomal recessive, loss-of-function basis of the disease.
- reference: PMID:8911612
reference_title: "Congenital Alopecia and nail dystrophy associated with severe functional T-cell immunodeficiency in two sibs."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We report on two sisters affected by congenital alopecia, nail dystrophy,
and a severe T-cell immunodeficiency, presumably inherited as an
autosomal-recessive disorder.
explanation: >-
The original clinical report of the syndrome in two affected sibs, from
which autosomal recessive inheritance was inferred.
pathophysiology:
- name: FOXN1 Transcription Factor Loss of Function
biological_scale: MOLECULAR
description: >-
Biallelic loss-of-function variants abolish FOXN1 transcription-factor
activity. FOXN1 is a winged-helix (forkhead) DNA-binding transcription
factor; the nonsense and frameshift alleles that cause human nude SCID
remove or inactivate the DNA-binding domain, so the FOXN1 target gene
program is not transcribed. This is the initiating lesion, and every
downstream node - thymic and cutaneous alike - follows from the absence of
this single transcriptional output rather than from any defect intrinsic to
hematopoietic cells.
genes:
- preferred_term: FOXN1
term:
id: hgnc:12765
label: FOXN1
genetic_context:
variant_origin: GERMLINE
zygosity: HOMOZYGOUS
functional_impact_category: LOSS_OF_FUNCTION
description: >-
Germline biallelic loss-of-function FOXN1 alleles; the prototype and most
frequently reported allele is the nonsense variant R255X.
molecular_functions:
- preferred_term: FOXN1 winged-helix transcription factor activity
term:
id: GO:0003700
label: DNA-binding transcription factor activity
modifier: LOSS_OF_FUNCTION
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
This gene encodes a transcription factor essential for the development of
the thymus, the primary lymphoid organ that supports T-cell development
and selection.
explanation: >-
Identifies FOXN1 as a transcription factor and establishes that its
transcriptional output is what thymic development depends on.
- reference: PMID:7969402
reference_title: "New member of the winged-helix protein family disrupted in mouse and rat nude mutations."
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: >-
Here we show that one of the genes from this critical region, designated
whn, encodes a new member of the winged-helix domain family of
transcription factors, and that it is disrupted on mouse nu and rat rnuN
alleles.
explanation: >-
Identifies the nude gene as a winged-helix transcription factor, the
molecular identity of FOXN1 and the origin of the "winged-helix-nude"
name.
- reference: PMID:7969402
reference_title: "New member of the winged-helix protein family disrupted in mouse and rat nude mutations."
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: >-
Mutant transcripts do not encode the characteristic DNA-binding domain,
strongly suggesting that the whn gene is the nude gene.
explanation: >-
Establishes that the nude alleles act by removing the DNA-binding domain,
i.e. by abolishing transcription-factor function.
- reference: PMID:15180707
reference_title: "Ancestral founder mutation of the nude (FOXN1) gene in congenital severe combined immunodeficiency associated with alopecia in southern Italy population."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The first described human FOXN1 mutation was a C792T transition in exon 5
resulting in the nonsense mutation R255X, and was detected in two probands
originated from a small community in southern Italy.
explanation: >-
Documents the prototype human loss-of-function allele as a nonsense
mutation.
downstream:
- target: Failed Thymic Epithelial Cell Differentiation
causal_link_type: DIRECT
description: >-
FOXN1 is the master regulator of thymic epithelial identity; without its
transcriptional program the thymic epithelial compartment does not
differentiate.
evidence:
- reference: PMID:31447097
reference_title: "Heterozygous FOXN1 Variants Cause Low TRECs and Severe T Cell Lymphopenia, Revealing a Crucial Role of FOXN1 in Supporting Early Thymopoiesis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "FOXN1 is the master regulatory gene of thymic epithelium development."
explanation: >-
Directly links loss of FOXN1 to failure of the thymic epithelial
compartment.
- target: Impaired Keratinocyte Terminal Differentiation
causal_link_type: DIRECT
description: >-
The same transcription factor is expressed in terminally differentiating
skin epithelium, so its loss simultaneously disrupts the keratinocyte
growth-differentiation balance in hair follicle and nail bed. This is the
parallel, non-immunological arm of the same molecular lesion.
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
In the skin and its appendages FOXN1 is expressed in epithelial cells
that have stopped proliferating and are in the process of terminal
differentiation
explanation: >-
Establishes the cutaneous expression domain through which the same
FOXN1 loss produces the ectodermal arm of the phenotype.
- name: Failed Thymic Epithelial Cell Differentiation
biological_scale: CELLULAR
description: >-
Without FOXN1, thymic epithelial cells fail to differentiate into the
cortical and medullary subsets that constitute the thymic microenvironment.
Cortical TECs normally mediate positive selection of T-cell precursors and
medullary TECs negative selection; neither compartment forms. This is a
stromal, non-hematopoietic defect, which is the single most important
mechanistic fact about the disease for treatment selection.
cell_types:
- preferred_term: thymic epithelial cell
term:
id: CL:0002293
label: epithelial cell of thymus
- preferred_term: cortical thymic epithelial cell
term:
id: CL:0002364
label: cortical thymic epithelial cell
- preferred_term: medullary thymic epithelial cell
term:
id: CL:0002365
label: medullary thymic epithelial cell
biological_processes:
- preferred_term: thymic epithelial cell differentiation
term:
id: GO:0030855
label: epithelial cell differentiation
modifier: DECREASED
- preferred_term: thymus epithelium morphogenesis
term:
id: GO:0097536
label: thymus epithelium morphogenesis
modifier: DECREASED
locations:
- preferred_term: thymus
term:
id: UBERON:0002370
label: thymus
evidence:
- reference: PMID:31566583
reference_title: "FOXN1 compound heterozygous mutations cause selective thymic hypoplasia in humans."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We report on 2 patients with compound heterozygous mutations in forkhead
box N1 (FOXN1), a transcription factor essential for thymic epithelial
cell (TEC) differentiation.
explanation: >-
Identifies TEC differentiation as the FOXN1-dependent cellular process
that fails.
- reference: PMID:31566583
reference_title: "FOXN1 compound heterozygous mutations cause selective thymic hypoplasia in humans."
supports: SUPPORT
evidence_source: IN_VITRO
snippet: >-
Characterization of the functional changes due to the Foxn1 mutations
revealed a 5-amino acid segment at the end of the DNA-binding domain
essential for the development of TECs but not keratinocytes.
explanation: >-
Functional dissection showing that distinct parts of the FOXN1 DNA-binding
domain are required for the thymic and cutaneous arms, which is why
hypomorphic genotypes can dissociate the two.
- reference: PMID:36648576
reference_title: >-
Clinical Practice Guidelines for the Immunological Management of Chromosome 22q11.2 Deletion Syndrome and Other Defects in Thymic Development.
supports: SUPPORT
evidence_source: OTHER
snippet: >-
the growth and differentiation of skin epithelial cells, including hair
and nails
explanation: >-
In the thymic-defect guideline's FOXN1 section, this clause names the
cutaneous half of the FOXN1-dependent epithelial program, alongside the
thymic epithelial cells named in the preceding clause.
downstream:
- target: Athymia
causal_link_type: DIRECT
description: >-
With no differentiated thymic epithelium there is no thymic organ to form;
the result is congenital absence of a functional thymus.
evidence:
- reference: PMID:31447097
reference_title: "Heterozygous FOXN1 Variants Cause Low TRECs and Severe T Cell Lymphopenia, Revealing a Crucial Role of FOXN1 in Supporting Early Thymopoiesis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
FOXN1 deficiency leads to thymic aplasia, alopecia, and nail dystrophy,
accounting for the nude/severe combined immunodeficiency (nu/SCID)
phenotype in humans and mice.
explanation: >-
Links loss of the FOXN1-dependent thymic epithelium to thymic aplasia.
- name: Athymia
biological_scale: TISSUE
description: >-
Congenital absence of a functional thymus. This is thymic aplasia proper -
the organ never forms - as distinguished from the thymic hypoplasia seen in
hematopoietic-intrinsic SCID, where an epithelial rudiment is present but is
not populated by thymocytes.
locations:
- preferred_term: thymus
term:
id: UBERON:0002370
label: thymus
biological_processes:
- preferred_term: thymus development
term:
id: GO:0048538
label: thymus development
modifier: DECREASED
evidence:
- reference: PMID:20978268
reference_title: "First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
FOXN1 deficiency is a primary immunodeficiency characterized by athymia,
alopecia totalis, and nail dystrophy.
explanation: Names athymia as the defining thymic lesion of FOXN1 deficiency.
- reference: PMID:18339010
reference_title: "FOXN1 homozygous mutation associated with anencephaly and severe neural tube defect in human athymic Nude/SCID fetus."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We identified a human fetus homozygous for a mutation in FOXN1 gene who
lacked the thymus and also had abnormal skin, anencephaly and spina bifida.
explanation: >-
Direct anatomical confirmation, in a homozygous human fetus, that the
thymus is absent rather than merely small.
downstream:
- target: Absent Thymopoiesis
causal_link_type: DIRECT
description: >-
Thymopoiesis requires the thymic microenvironment; with no thymus, T-cell
precursors cannot undergo lineage commitment, positive selection, or
negative selection.
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
This gene encodes a transcription factor essential for the development
of the thymus, the primary lymphoid organ that supports T-cell
development and selection.
explanation: >-
States that the thymus is the organ supporting T-cell development, so
its absence abolishes that process.
- name: Absent Thymopoiesis
biological_scale: CELLULAR
description: >-
T-cell precursors arriving from the bone marrow have no thymic epithelial
niche to interact with, so intrathymic T-cell differentiation does not
occur. The hematopoietic stem cell compartment itself is normal - the block
is entirely extrinsic to it.
cell_types:
- preferred_term: T cell
term:
id: CL:0000084
label: T cell
biological_processes:
- preferred_term: T cell differentiation in thymus
term:
id: GO:0033077
label: T cell differentiation in thymus
modifier: DECREASED
evidence:
- reference: PMID:36648576
reference_title: >-
Clinical Practice Guidelines for the Immunological Management of Chromosome 22q11.2 Deletion Syndrome and Other Defects in Thymic Development.
supports: SUPPORT
evidence_source: OTHER
snippet: >-
leading to profound TCL. Affected individuals also have alopecia and
dysplastic nails
explanation: >-
The guideline states that autosomal recessive FOXN1 deficiency blocks the
interaction of T-cell precursors with cortical and medullary thymic
epithelium, producing profound T-cell lymphopenia.
- reference: PMID:20978268
reference_title: "First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Subject 1 presented with disseminated Bacillus Calmette-Guérin infection
and oligoclonal T cells with no naive markers.
explanation: >-
Absence of naive-marker T cells is the direct cellular readout of absent
thymic output.
downstream:
- target: Profound T-Cell Lymphopenia
causal_link_type: DIRECT
description: >-
With no thymic output, circulating T cells are absent or reduced to a few
oligoclonal, transplacentally acquired or peripherally expanded cells.
evidence:
- reference: PMID:31447097
reference_title: "Heterozygous FOXN1 Variants Cause Low TRECs and Severe T Cell Lymphopenia, Revealing a Crucial Role of FOXN1 in Supporting Early Thymopoiesis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
FOXN1 deficiency leads to thymic aplasia, alopecia, and nail dystrophy,
accounting for the nude/severe combined immunodeficiency (nu/SCID)
phenotype in humans and mice.
explanation: >-
Connects thymic aplasia to the SCID immunophenotype.
- name: Profound T-Cell Lymphopenia
biological_scale: ORGANISM
description: >-
The immunophenotype is T-negative with numerically preserved B and NK cells
(T-B+NK+). B cells are present but antibody production is compromised
because T-cell help is absent, so humoral as well as cellular immunity
fails despite a normal B-cell count. This is why the entity is a combined
immunodeficiency rather than a pure T-cell defect.
cell_types:
- preferred_term: T cell
term:
id: CL:0000084
label: T cell
biological_processes:
- preferred_term: T cell homeostasis
term:
id: GO:0043029
label: T cell homeostasis
modifier: DECREASED
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
To date nine cases have been reported presenting with the clinical triad
of absent thymus resulting in severe T-cell immunodeficiency, congenital
alopecia universalis and nail dystrophy.
explanation: >-
States that the severe T-cell immunodeficiency is the consequence of the
absent thymus.
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Although B-cells are typically present in normal numbers, antibody
production is compromised in the absence of T-cell help
explanation: >-
Documents the B+ immunophenotype and explains why preserved B-cell numbers
do not confer functional humoral immunity.
downstream:
- target: Severe Recurrent Infection
causal_link_type: DIRECT
description: >-
Loss of T-cell-mediated immunity, compounded by the failure of
T-dependent antibody responses, leaves the infant defenceless against
viral, fungal, opportunistic and encapsulated-bacterial pathogens and
against live vaccines.
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
All reported patients presented in the first months of life with severe,
recurrent, life-threatening infections
explanation: >-
Links the T-cell defect to the presenting infectious phenotype and its
timing.
- name: Severe Recurrent Infection
biological_scale: ORGANISM
description: >-
Severe, recurrent, life-threatening infection beginning in the first months
of life is the presenting feature and, untreated, the cause of death.
Disseminated BCG disease has been reported where BCG was given before the
immunodeficiency was recognised.
biological_processes:
- preferred_term: defense response
term:
id: GO:0006952
label: defense response
modifier: DECREASED
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Without these the prognosis is poor due to early-onset life threatening
infections.
explanation: >-
Establishes early-onset life-threatening infection as the cause of the
poor untreated prognosis.
- reference: PMID:15180707
reference_title: "Ancestral founder mutation of the nude (FOXN1) gene in congenital severe combined immunodeficiency associated with alopecia in southern Italy population."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
In this community, four additional children affected with congenital
alopecia died in early childhood because of severe infections.
explanation: >-
Documents early-childhood death from severe infection in affected members
of the founder pedigree.
- name: Impaired Keratinocyte Terminal Differentiation
biological_scale: CELLULAR
description: >-
FOXN1 is expressed in skin epithelial cells that have exited the cell cycle
and are undergoing terminal differentiation, where it controls expression of
kinases governing cell survival, metabolism and cell-cycle progression. Its
loss disrupts the balance between growth and differentiation in these cells.
Notably, hair follicles are present in normal numbers - the defect is in the
differentiated product, not in follicle formation.
cell_types:
- preferred_term: keratinocyte
term:
id: CL:0000312
label: keratinocyte
biological_processes:
- preferred_term: keratinocyte differentiation
term:
id: GO:0030216
label: keratinocyte differentiation
modifier: DECREASED
- preferred_term: hair follicle development
term:
id: GO:0001942
label: hair follicle development
modifier: DECREASED
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
In the skin and its appendages FOXN1 is expressed in epithelial cells that
have stopped proliferating and are in the process of terminal
differentiation
explanation: >-
Locates FOXN1 function in terminally differentiating skin epithelium.
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
As a consequence, loss-of-function mutations disrupt the balance between
normal growth and differentiation of these cells
explanation: >-
States the cellular consequence of FOXN1 loss in keratinocytes.
- reference: PMID:7969402
reference_title: "New member of the winged-helix protein family disrupted in mouse and rat nude mutations."
supports: SUPPORT
evidence_source: MODEL_ORGANISM
snippet: >-
Mutations at the nude locus of mice and rats disrupt normal hair growth
and thymus development, causing nude mice and rats to be immune-deficient.
explanation: >-
The animal nude phenotype couples the same two arms - hair and thymus -
confirming a shared single-gene origin.
downstream:
- target: Congenital Alopecia Universalis
causal_link_type: DIRECT
description: >-
Defective terminal differentiation of follicular keratinocytes yields
structurally abnormal hair shafts from numerically normal follicles,
manifesting as alopecia present from birth.
evidence:
- reference: PMID:31447097
reference_title: "Heterozygous FOXN1 Variants Cause Low TRECs and Severe T Cell Lymphopenia, Revealing a Crucial Role of FOXN1 in Supporting Early Thymopoiesis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
FOXN1 deficiency leads to thymic aplasia, alopecia, and nail dystrophy,
accounting for the nude/severe combined immunodeficiency (nu/SCID)
phenotype in humans and mice.
explanation: Links FOXN1 deficiency to alopecia.
- target: Nail Dystrophy
causal_link_type: DIRECT
description: >-
The same keratinocyte differentiation defect in the nail bed produces the
characteristic nail plate abnormalities.
evidence:
- reference: PMID:31447097
reference_title: "Heterozygous FOXN1 Variants Cause Low TRECs and Severe T Cell Lymphopenia, Revealing a Crucial Role of FOXN1 in Supporting Early Thymopoiesis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
FOXN1 deficiency leads to thymic aplasia, alopecia, and nail dystrophy,
accounting for the nude/severe combined immunodeficiency (nu/SCID)
phenotype in humans and mice.
explanation: Links FOXN1 deficiency to nail dystrophy.
phenotypes:
- category: Immunologic
name: Profound T-Cell Lymphopenia
description: >-
T cells are absent or severely reduced, with B and NK cells numerically
preserved (T-B+NK+). Newborn screening shows very low or absent T-cell
receptor excision circles.
phenotype_term:
preferred_term: Decreased total T cell count
term:
id: HP:0005403
label: Decreased total T cell count
frequency: VERY_FREQUENT
evidence:
- reference: PMID:20978268
reference_title: "First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Subject 2 had respiratory failure, human herpes virus 6 infection, cytopenias, and no circulating T cells."
explanation: >-
Documents complete absence of circulating T cells in a FOXN1-deficient
infant.
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
To date nine cases have been reported presenting with the clinical triad
of absent thymus resulting in severe T-cell immunodeficiency, congenital
alopecia universalis and nail dystrophy.
explanation: >-
Severe T-cell immunodeficiency is one of the three defining features
present in every reported case, supporting a very frequent band.
- category: Immunologic
name: Severe Combined Immunodeficiency Phenotype
description: >-
The combination of absent T-cell immunity with functionally absent
T-dependent antibody responses constitutes severe combined immunodeficiency,
notwithstanding normal B-cell numbers.
phenotype_term:
preferred_term: Severe combined immunodeficiency
term:
id: HP:0004430
label: Severe combined immunodeficiency
evidence:
- reference: PMID:33464451
reference_title: "Expanding the Nude SCID/CID Phenotype Associated with FOXN1 Homozygous, Compound Heterozygous, or Heterozygous Mutations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Human nude SCID is a rare autosomal recessive inborn error of immunity
(IEI) characterized by congenital athymia, alopecia, and nail dystrophy.
explanation: Establishes the SCID classification of the disease.
- category: Immunologic
name: Severe Recurrent Infection from Birth
description: >-
All reported patients present in the first months of life with severe,
recurrent, life-threatening infections, including viral, fungal and
opportunistic organisms, encapsulated bacteria, and disseminated disease
after live vaccines such as BCG.
phenotype_term:
preferred_term: Recurrent severe infections
term:
id: HP:0002719
label: Recurrent infections
temporality: RECURRENT
severity: SEVERE
frequency: VERY_FREQUENT
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
All reported patients presented in the first months of life with severe,
recurrent, life-threatening infections
explanation: >-
"All reported patients" supports the very frequent (80-100%) band as well
as the phenotype itself.
- reference: PMID:20978268
reference_title: "First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Subject 1 presented with disseminated Bacillus Calmette-Guérin infection
and oligoclonal T cells with no naive markers.
explanation: >-
Illustrates disseminated live-vaccine (BCG) disease as a presenting
infection.
- category: Imaging
name: Athymia
description: >-
Congenital absence of the thymus, confirmed anatomically in a homozygous
fetus and inferred clinically from absent thymic shadow with absent naive
T cells.
phenotype_term:
preferred_term: Aplasia of the thymus
term:
id: HP:0005359
label: Aplasia of the thymus
frequency: VERY_FREQUENT
notes: >-
Aplasia, not hypoplasia: in FOXN1 deficiency the thymic epithelial
primordium fails to differentiate, so the organ does not form. This is the
distinction drawn in the Severe_Combined_Immunodeficiency entry, where
HP:0000778 Hypoplasia of the thymus is used because a rudiment is present
but unpopulated.
evidence:
- reference: PMID:18339010
reference_title: "FOXN1 homozygous mutation associated with anencephaly and severe neural tube defect in human athymic Nude/SCID fetus."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We identified a human fetus homozygous for a mutation in FOXN1 gene who
lacked the thymus and also had abnormal skin, anencephaly and spina bifida.
explanation: Anatomical confirmation of absent thymus in a homozygous human fetus.
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
To date nine cases have been reported presenting with the clinical triad
of absent thymus resulting in severe T-cell immunodeficiency, congenital
alopecia universalis and nail dystrophy.
explanation: >-
Absent thymus is one of the three features present in the reported case
series, supporting the very frequent band.
- category: Dermatologic
name: Congenital Alopecia Universalis
description: >-
Alopecia present from birth affecting scalp, eyebrows and eyelashes. Hair
follicles are numerically normal but produce structurally abnormal hair.
phenotype_term:
preferred_term: Congenital alopecia universalis
term:
id: HP:0002289
label: Alopecia universalis
onset:
onset_category: CONGENITAL
frequency: VERY_FREQUENT
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Dermatological features include congenital alopecia affecting the scalp,
eyebrows and eyelashes
explanation: Documents the distribution of the congenital alopecia.
- reference: PMID:8911612
reference_title: "Congenital Alopecia and nail dystrophy associated with severe functional T-cell immunodeficiency in two sibs."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We report on two sisters affected by congenital alopecia, nail dystrophy,
and a severe T-cell immunodeficiency, presumably inherited as an
autosomal-recessive disorder.
explanation: The original report establishing congenital alopecia as a core feature.
notes: >-
Not invariant. In a large international survey of FOXN1 genotypes, alopecia
and nail dystrophy were "not always present", so the very frequent band
reflects the classic triad rather than a universal finding.
- category: Dermatologic
name: Nail Dystrophy
description: >-
Dystrophic nails present from birth, most frequently proximal arciform
leukonychia and koilonychia, though canaliform dystrophy and Beau's lines
have also been described.
phenotype_term:
preferred_term: Nail dystrophy
term:
id: HP:0008404
label: Nail dystrophy
onset:
onset_category: CONGENITAL
frequency: VERY_FREQUENT
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The latter most frequently features proximal arciform leukonychia and
koilonychia
explanation: Specifies the characteristic nail changes.
- reference: PMID:20978268
reference_title: "First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
FOXN1 deficiency is a primary immunodeficiency characterized by athymia,
alopecia totalis, and nail dystrophy.
explanation: Nail dystrophy is one of the three characterizing features.
- category: Dermatologic
name: Leukonychia
description: >-
Proximal arciform leukonychia (white banding of the nail plate) is the most
frequent specific nail change.
phenotype_term:
preferred_term: Proximal arciform leukonychia
term:
id: HP:0001820
label: Leukonychia
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The latter most frequently features proximal arciform leukonychia and
koilonychia
explanation: Names proximal arciform leukonychia as a most-frequent nail finding.
- category: Dermatologic
name: Koilonychia
description: >-
Spoon-shaped (concave) nails, reported alongside leukonychia as the most
frequent nail dystrophy pattern.
phenotype_term:
preferred_term: Koilonychia
term:
id: HP:0001598
label: Concave nail
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The latter most frequently features proximal arciform leukonychia and
koilonychia
explanation: Names koilonychia as a most-frequent nail finding.
- category: Immunologic
name: Omenn Syndrome Features
description: >-
A substantial minority of patients develop Omenn syndrome, an inflammatory
state driven by expansion of autoreactive oligoclonal T cells in the setting
of SCID, presenting with erythroderma, hepatosplenomegaly, lymphadenopathy,
diarrhoea, failure to thrive, eosinophilia and elevated IgE.
phenotype_term:
preferred_term: Erythroderma (Omenn-like presentation)
term:
id: HP:0001019
label: Erythroderma
frequency: FREQUENT
evidence:
- reference: PMID:33464451
reference_title: "Expanding the Nude SCID/CID Phenotype Associated with FOXN1 Homozygous, Compound Heterozygous, or Heterozygous Mutations."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Nail dystrophy and alopecia, that represent the hallmarks of the syndrome,
were not always present, while almost 50% of the patients developed Omenn
syndrome.
explanation: >-
Quantifies Omenn syndrome at almost 50% of the surveyed cohort, supporting
the frequent (30-79%) band, and simultaneously documents that the
dermatological hallmarks are not invariant.
- category: Neurologic
name: Neural Tube Defect
description: >-
Neural tube defects have been described in two fetuses from the Acerno
kindred, one with anencephaly and spina bifida. FOXN1 is expressed in mouse
developing choroid plexus, suggesting a role in neurulation. This is a rare
finding restricted to a single pedigree and should not be expected in every
patient.
phenotype_term:
preferred_term: Neural tube defect
term:
id: HP:0045005
label: Neural tube defect
frequency: VERY_RARE
evidence:
- reference: PMID:18339010
reference_title: "FOXN1 homozygous mutation associated with anencephaly and severe neural tube defect in human athymic Nude/SCID fetus."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We identified a human fetus homozygous for a mutation in FOXN1 gene who
lacked the thymus and also had abnormal skin, anencephaly and spina bifida.
explanation: >-
Documents anencephaly and spina bifida in a homozygous FOXN1 fetus.
- reference: PMID:18339010
reference_title: "FOXN1 homozygous mutation associated with anencephaly and severe neural tube defect in human athymic Nude/SCID fetus."
supports: SUPPORT
directness: INDIRECT
evidence_source: MODEL_ORGANISM
snippet: >-
Moreover, we found that FOXN1 gene is expressed in mouse developing
choroid plexus. These observations suggest that FOXN1 may be involved in
neurulation in humans.
explanation: >-
Provides the model-organism expression evidence behind the proposed role
in neurulation. Indirect: the quote reports mouse choroid-plexus
expression and an inference, not an observed human neural tube defect,
which rests on the fetal case in the companion evidence item.
- category: Neurologic
name: Anencephaly
description: >-
Anencephaly was present in one homozygous FOXN1 fetus from the Acerno
kindred, together with spina bifida.
phenotype_term:
preferred_term: Anencephaly
term:
id: HP:0002323
label: Anencephaly
frequency: VERY_RARE
evidence:
- reference: PMID:18339010
reference_title: "FOXN1 homozygous mutation associated with anencephaly and severe neural tube defect in human athymic Nude/SCID fetus."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We identified a human fetus homozygous for a mutation in FOXN1 gene who
lacked the thymus and also had abnormal skin, anencephaly and spina bifida.
explanation: Documents anencephaly in a homozygous FOXN1 fetus.
genetic:
- name: FOXN1
gene_term:
preferred_term: FOXN1
term:
id: hgnc:12765
label: FOXN1
association: CAUSATIVE
relationship_type: CAUSATIVE
notes: >-
Biallelic loss-of-function FOXN1 variants cause nude SCID. Reported alleles
include the founder nonsense variant R255X (c.792C>T, exon 5), R320W, and
S188fs. R255X accounts for most reported patients, all descending from a
single 19th-century ancestral couple in Acerno, southern Italy; the same
allele was later found in a Portuguese child of consanguineous parents.
Scope note on other FOXN1 genotypes, which are NOT this disease. Monoallelic
FOXN1 loss-of-function (FOXN1 haploinsufficiency, autosomal dominant,
OMIM 600838) is ascertained through low TRECs at newborn screening and
causes T-cell lymphopenia that improves with age, with normal or nearly
normal hair and only subtle nail dystrophy (spoon nails), and generally no
congenital athymia. Compound heterozygous hypomorphic genotypes can produce
selective thymic hypoplasia with a T-/loB+NK+ SCID presentation and
completely normal hair and nails. IUIS lists FOXN1 haploinsufficiency as its
own row, distinct from the AR FOXN1 deficiency row that this entry curates.
Historical note on the founding human report. Frank et al., Nature 1999
(PMID:10206641, "Exposing the human nude phenotype") identified FOXN1 as the
gene mutated in the human nude phenotype and is listed in the top-level
references block. Its cached PubMed record carries no abstract text, so no
exact-quote snippet can be drawn from it, and it is deliberately not used as
an evidence item anywhere in this entry.
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Nude severe combined immunodeficiency is a rare inherited disease caused
by autosomal recessive loss-of-function mutations in FOXN1.
explanation: >-
Establishes FOXN1 as the causative gene and loss of function as the
mechanism.
- reference: PMID:15180707
reference_title: "Ancestral founder mutation of the nude (FOXN1) gene in congenital severe combined immunodeficiency associated with alopecia in southern Italy population."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The first described human FOXN1 mutation was a C792T transition in exon 5
resulting in the nonsense mutation R255X, and was detected in two probands
originated from a small community in southern Italy.
explanation: Identifies the prototype human allele and its geographic origin.
- reference: PMID:15180707
reference_title: "Ancestral founder mutation of the nude (FOXN1) gene in congenital severe combined immunodeficiency associated with alopecia in southern Italy population."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
The three haplotypes identified, 3/R255X/3, 3/R255X/2 and 3/R255X/1, are
consistent with a single ancestral origin for the mutation R255X.
explanation: >-
Haplotype analysis establishing R255X as a single ancestral founder
allele.
- name: FOXN1 (heterozygous - distinct entity, not this disease)
gene_term:
preferred_term: FOXN1
term:
id: hgnc:12765
label: FOXN1
association: MODIFIER
relationship_type: MODIFIER
notes: >-
Recorded for boundary clarity only. Monoallelic FOXN1 loss-of-function is a
separate autosomal dominant entity (FOXN1 haploinsufficiency, OMIM 600838,
its own IUIS row) and is NOT the disease curated here. It is included in
this section so that a curator or reader encountering a heterozygous FOXN1
result is not led to this entry by default. Its immunophenotype is milder
and age-limited, and the ectodermal features are minimal.
evidence:
- reference: PMID:31447097
reference_title: "Heterozygous FOXN1 Variants Cause Low TRECs and Severe T Cell Lymphopenia, Revealing a Crucial Role of FOXN1 in Supporting Early Thymopoiesis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
We identified several newborns with low levels of T cell receptor excision
circles (TRECs) and T cell lymphopenia at birth, who carried heterozygous
loss-of-function FOXN1 variants.
explanation: >-
Documents the heterozygous newborn-screening phenotype that defines the
separate haploinsufficiency entity.
- reference: PMID:31447097
reference_title: "Heterozygous FOXN1 Variants Cause Low TRECs and Severe T Cell Lymphopenia, Revealing a Crucial Role of FOXN1 in Supporting Early Thymopoiesis."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Longitudinal analysis showed persistent T cell lymphopenia during infancy,
often associated with nail dystrophy.
explanation: >-
Characterizes the milder, age-limited heterozygous phenotype, distinct
from the congenital athymia of the recessive disease.
- reference: PMID:36648576
reference_title: >-
Clinical Practice Guidelines for the Immunological Management of Chromosome 22q11.2 Deletion Syndrome and Other Defects in Thymic Development.
supports: SUPPORT
evidence_source: OTHER
snippet: >-
Congenital athymia is generally not present, and individuals have normal
or almost normal appearing hair with only subtle nail dystrophy (spoon
nails).
explanation: >-
The guideline states explicitly that heterozygous FOXN1 carriers lack the
athymia and the ectodermal hallmarks that define this entry's disease.
prevalence:
- population: Worldwide
measure_type: CASES_IN_LITERATURE
prevalence_class: ULTRA_RARE
notes: >-
Ultra-rare. As of the 2017 disease review, nine cases had been reported in
the literature, six of them from the single Acerno founder pedigree in
southern Italy. Newborn screening and next-generation sequencing have since
identified additional and atypical cases, so the true incidence is thought
to be higher than the historical case count implies. Orphanet code
ORPHA:169095; no ORPHA structured cache entry was built for this entry.
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Nude severe combined immunodeficiency is a rare inherited disease caused by autosomal
recessive loss-of-function mutations in FOXN1. This gene encodes a transcription factor
essential for the development of the thymus, the primary lymphoid organ that supports T-cell
development and selection. To date nine cases have been reported presenting with the clinical
triad of absent thymus resulting in severe T-cell immunodeficiency, congenital alopecia
universalis and nail dystrophy. ... Six patients originated from Acerno in southern Italy; all
had the same homozygous founder mutation (R255X)
explanation: >-
The review reports nine FOXN1-associated nude SCID cases, including six from Acerno with the
same founder mutation. The quoted count is the literature available to that review.
- reference: PMID:33464451
reference_title: "Expanding the Nude SCID/CID Phenotype Associated with FOXN1 Homozygous, Compound Heterozygous, or Heterozygous Mutations."
supports: SUPPORT
directness: INDIRECT
evidence_source: HUMAN_CLINICAL
snippet: >-
However, the recent introduction of newborn screening for IEIs and
high-throughput sequencing has led to the identification of novel and
atypical cases.
explanation: >-
Supports this record's caveat that ascertainment has improved, so
published case totals understate true occurrence. Indirect: the quote
speaks to the completeness of case finding rather than reporting any
occurrence figure of its own.
- population: Acerno, southern Italy (R255X carrier screening)
measure_type: CARRIER_FREQUENCY
prevalence_class: ABOVE_1_IN_1000
rate_per_100000: 6520.0
notes: >-
Carrier frequency of the R255X founder allele, not disease prevalence:
55 heterozygous carriers among 843 screened inhabitants (6.52%), all linked
in a seven-generation pedigree descending from one ancestral couple.
evidence:
- reference: PMID:15180707
reference_title: "Ancestral founder mutation of the nude (FOXN1) gene in congenital severe combined immunodeficiency associated with alopecia in southern Italy population."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
This analysis led us to identify 55 heterozygous carriers (6.52%) of the
R255X mutation out of 843 inhabitants screened.
explanation: >-
Directly reports the carrier count, denominator and percentage behind this
carrier-frequency record.
treatments:
- name: Cultured Thymus Tissue Implantation
description: >-
Implantation of cultured allogeneic postnatal thymus tissue (obtained from
infants undergoing corrective cardiac surgery) into the quadriceps muscle.
This is the mechanism-directed treatment for nude SCID: because FOXN1 is
expressed in thymic epithelium and not in hematopoietic cells, supplying a
functional thymic stromal environment - rather than replacing the patient's
own normal hematopoietic compartment - is what restores thymopoiesis. HLA
matching is not required. Reconstitution is slow, typically taking six to
twelve months, so supportive infection prophylaxis must bridge that
interval. Autoimmunity, principally thyroid, is a recognised late
complication.
therapeutic_modality: SURGERY
treatment_term:
preferred_term: cultured thymus tissue implantation
term:
id: NCIT:C15289
label: Organ Transplantation
target_phenotypes:
- preferred_term: Aplasia of the thymus
term:
id: HP:0005359
label: Aplasia of the thymus
- preferred_term: Decreased total T cell count
term:
id: HP:0005403
label: Decreased total T cell count
target_mechanisms:
- target: Athymia
treatment_effect: RESTORES
description: >-
The implant supplies the thymic epithelial microenvironment the patient
never formed, directly addressing the athymia node rather than any
hematopoietic step.
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Given that FOXN1 is expressed in TECs and not haematopoietic cells,
establishing a functional thymic stromal environment is expected to
provide more complete and long-lasting immune reconstitution
explanation: >-
States the mechanistic rationale for treating the stromal defect
directly.
- target: Absent Thymopoiesis
treatment_effect: RESTORES
description: >-
Once a thymic microenvironment is present, host T-cell precursors undergo
normal intrathymic development, producing naive T cells and TREC-positive
recent thymic emigrants.
evidence:
- reference: PMID:20978268
reference_title: "First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
In summary, thymus transplantation led to T-cell reconstitution and
function in these FOXN1 deficient infants.
explanation: >-
Demonstrates restored thymopoiesis in the two FOXN1-deficient infants
treated.
evidence:
- reference: PMID:20978268
reference_title: "First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: "Two infants with FOXN1 deficiency were transplanted with cultured postnatal thymus tissue."
explanation: >-
The first application of cultured thymus tissue specifically in FOXN1
deficiency.
- reference: PMID:20978268
reference_title: "First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Both subjects developed functional immunity. Subjects 1 and 2 have
1053/mm(3) and 1232/mm(3) CD3(+) cells
explanation: >-
Quantifies the reconstituted T-cell compartment achieved in both treated
FOXN1-deficient infants.
- reference: PMID:34362576
reference_title: "Experience with cultured thymus tissue in 105 children."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Treatment with CTT led to development of naive T cells with a 1-year
survival rate of 77% and a median follow-up time of 7.6 years.
explanation: >-
Provides the pooled efficacy and survival estimate across the full
congenital-athymia cohort, which includes FOXN1 deficiency.
- reference: PMID:34362576
reference_title: "Experience with cultured thymus tissue in 105 children."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Immune reconstitution sufficient to prevent infections and support
survival typically develops 6 to12 months after administration of CTT.
explanation: >-
Establishes the months-long reconstitution interval that supportive care
must bridge.
- reference: PMID:20978268
reference_title: "First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases."
supports: SUPPORT
directness: DIRECT
evidence_source: HUMAN_CLINICAL
snippet: "Subject 2 developed autoimmune thyroid disease 1.6 years after transplantation."
explanation: >-
Supports this treatment's stated late complication: a directly observed
case of post-implantation thyroid autoimmunity in one of the two treated
FOXN1-deficient infants.
notes: >-
The IUIS/thymic-defect literature reports these outcomes for congenital
athymia as a whole (complete DiGeorge, CHARGE, FOXN1 deficiency and others).
The FOXN1-specific experience remains two published infants; the 105-child
cohort figures are cohort-level, not FOXN1-specific, and are cited as such.
- name: Hematopoietic Cell Transplantation
description: >-
Allogeneic HCT does not correct the underlying lesion, because FOXN1 is
expressed in thymic stroma and not in hematopoietic cells - transplanting
normal stem cells into a patient with no thymus leaves them with nowhere to
differentiate. Where HCT has helped in nude SCID, the benefit is attributed
to mature donor T cells carried in the graft rather than to restored
thymopoiesis, which is why the literature restricts it to HLA-matched
genoidentical sibling donors, and why it is preferred chiefly when
thymus tissue is unavailable or when rapid T-cell recovery is needed for a
pre-existing systemic viral infection. Outcomes reported for athymia treated
with HCT are poor.
therapeutic_modality: CELL_THERAPY
treatment_term:
preferred_term: hematopoietic cell transplantation
term:
id: NCIT:C15431
label: Hematopoietic Cell Transplantation
target_mechanisms:
- target: Profound T-Cell Lymphopenia
treatment_effect: MODULATES
description: >-
A genoidentical graft can supply mature donor T cells that partially
populate the periphery. It does not act on the athymia or the thymopoiesis
block upstream of it, so the effect is compensatory rather than
restorative.
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
whereas the other was alive and infection-free when assessed 6 years
later likely due to the presence of mature donor T-cells
explanation: >-
Describes the surviving HCT recipient among the two treated; the review
attributes the outcome to mature donor T cells present in the bone
marrow graft rather than to restored thymic function — the mechanistic
point behind typing this treatment MODULATES rather than RESTORES.
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Options for treating the underlying immune deficiency include HLA-matched
genoidentical haematopoietic cell transplantation containing mature donor
T-cells or thymus tissue transplantation.
explanation: >-
Identifies the two definitive options and specifies that the HCT option
depends on mature donor T cells in the graft.
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Given that FOXN1 is expressed in TECs and not haematopoietic cells,
establishing a functional thymic stromal environment is expected to
provide more complete and long-lasting immune reconstitution
explanation: >-
States why replacing the hematopoietic compartment is not expected to
provide complete reconstitution in a thymic stromal defect.
- reference: PMID:34362576
reference_title: "Experience with cultured thymus tissue in 105 children."
supports: SUPPORT
directness: INDIRECT
evidence_source: HUMAN_CLINICAL
snippet: >-
only 7 of 17 patients (41%) were reported as surviving by Janda et al.
explanation: >-
Reports the survival figure for congenital athymia treated with
hematopoietic stem cell transplantation, the comparator against which
cultured thymus tissue is assessed. Indirect because this is the
congenital-athymia cohort broadly, not a FOXN1-only series.
- reference: PMID:36648576
reference_title: >-
Clinical Practice Guidelines for the Immunological Management of Chromosome 22q11.2 Deletion Syndrome and Other Defects in Thymic Development.
supports: REFUTE
evidence_source: OTHER
snippet: >-
did not prove effective for children affected with biallelic FOXN1
deficiency
explanation: >-
The thymic-defect guideline states that bone marrow transplantation was
not effective in biallelic FOXN1 deficiency, refuting HCT as a
mechanism-correcting therapy for this disease.
- name: Infection Prophylaxis and Protective Isolation
description: >-
Supportive management that bridges the interval before and after definitive
treatment: protective isolation in a laminar-flow room, prophylaxis against
Pneumocystis jirovecii, fungal and viral infection, and anti-mycobacterial
treatment for any infant already immunised with BCG. Live vaccines are
contraindicated. Blood products, if required, must be CMV-negative,
irradiated and leucocyte-depleted.
therapeutic_modality: SMALL_MOLECULE
treatment_term:
preferred_term: antimicrobial prophylaxis
term:
id: NCIT:C15986
label: Pharmacotherapy
target_mechanisms:
- target: Severe Recurrent Infection
treatment_effect: INHIBITS
description: >-
Prophylaxis and isolation act on the infectious consequence of the
immunodeficiency; they do not touch the thymic lesion upstream.
evidence:
- reference: PMID:34362576
reference_title: "Experience with cultured thymus tissue in 105 children."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Currently, the only available treatment for these patients, outside of a
clinical trial, is supportive care, which may include strict
infection-prevention measures including immunoglobulin replacement,
prophylactic antimicrobials, and protective isolation.
explanation: >-
Names the supportive components and their role in congenital athymia
outside definitive therapy.
evidence:
- reference: PMID:34362576
reference_title: "Experience with cultured thymus tissue in 105 children."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Currently, the only available treatment for these patients, outside of a
clinical trial, is supportive care, which may include strict
infection-prevention measures including immunoglobulin replacement,
prophylactic antimicrobials, and protective isolation.
explanation: >-
Establishes supportive infection prevention as the standing management for
congenital athymia.
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
This involves isolation in a laminar flow room, prophylaxis against
explanation: >-
The disease review specifies laminar-flow isolation and infection
prophylaxis as the supportive regimen for nude SCID.
- name: Immunoglobulin Replacement Therapy
description: >-
Immunoglobulin replacement compensates for the absent T-dependent antibody
response. B cells are numerically normal in nude SCID, so the humoral defect
is functional rather than quantitative; replacement addresses that
functional gap and can be discontinued once thymic reconstitution restores
T-cell help.
therapeutic_modality: OTHER
treatment_term:
preferred_term: immunoglobulin replacement therapy
term:
id: NCIT:C62710
label: Immunoglobulin Therapy
target_mechanisms:
- target: Profound T-Cell Lymphopenia
treatment_effect: MODULATES
description: >-
Replacement immunoglobulin substitutes for the antibody responses the
patient cannot mount without T-cell help. It compensates for a downstream
consequence and does not alter the thymic lesion.
evidence:
- reference: PMID:20978268
reference_title: "First use of thymus transplantation therapy for FOXN1 deficiency (nude/SCID): a report of 2 cases."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Both subjects developed antigen-specific proliferative responses and
have discontinued immunoglobulin replacement.
explanation: >-
Shows that immunoglobulin replacement was the standing substitute for
T-dependent antibody responses and became unnecessary once thymic
function was restored.
evidence:
- reference: PMID:34362576
reference_title: "Experience with cultured thymus tissue in 105 children."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Currently, the only available treatment for these patients, outside of a
clinical trial, is supportive care, which may include strict
infection-prevention measures including immunoglobulin replacement,
prophylactic antimicrobials, and protective isolation.
explanation: >-
Names immunoglobulin replacement as a component of supportive care in
congenital athymia.
- name: Genetic Counseling and Carrier Testing
description: >-
FOXN1 sequencing establishes the diagnosis, guides therapeutic management,
and enables counselling. In the Acerno pedigree, targeted PCR for the known
R255X allele supported population-level carrier screening.
therapeutic_modality: BEHAVIORAL
treatment_term:
preferred_term: genetic counseling
term:
id: NCIT:C15240
label: Genetic Counseling
evidence:
- reference: PMID:28077132
reference_title: "FOXN1 deficient nude severe combined immunodeficiency."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
Diagnosis relies on testing for FOXN1 mutations, which allows genetic
counselling and guides therapeutic management.
explanation: >-
States the role of molecular diagnosis in counselling and management.
- reference: PMID:15180707
reference_title: "Ancestral founder mutation of the nude (FOXN1) gene in congenital severe combined immunodeficiency associated with alopecia in southern Italy population."
supports: SUPPORT
evidence_source: HUMAN_CLINICAL
snippet: >-
In this study, we report on the screening for this mutation in 30% of the
village population.
explanation: >-
Documents founder-allele carrier screening at population scale in the
affected community.
references:
- reference: PMID:10206641
title: "Exposing the human nude phenotype."