Ficolin 3 Deficiency

1. Disease Information

Definition and overview

Immunodeficiency due to ficolin‑3 deficiency (often shortened to “ficolin‑3 deficiency” or “FCN3 deficiency”) is a primary immunodeficiency in which serum ficolin‑3 (also known as H‑ficolin or Hakata antigen) is undetectable or markedly reduced, resulting in defective complement activation via the lectin pathway.[1][4][6][11][13] Ficolin‑3 is an oligomeric pattern‑recognition lectin encoded by FCN3 that binds acetylated sugars on pathogens and associates with MASP‑1 and MASP‑2 to trigger complement activation.[4][6][13][15] Clinically, affected individuals have recurrent bacterial infections (especially respiratory) and sometimes autoimmune manifestations (notably systemic lupus erythematosus) and neurologic complications.[2][4][13][15]

Key identifiers

• OMIM: 613860, “Immunodeficiency due to ficolin 3 deficiency” (number sign entry #613860).[1]
• Orphanet: 331190, “Immunodeficiency due to ficolin3 deficiency.”[11]
• MONDO: MONDO:0013467, “Immunodeficiency due to ficolin3 deficiency.”[11]
• MedGen/ClinVar condition: “Immunodeficiency due to ficolin3 deficiency; FICOLIN 3 DEFICIENCY; FCN3 DEFICIENCY.”[8][9][11]
• NCBI Gene (human FCN3): Gene ID 8547; disease association listed as “Immunodeficiency due to ficolin3 deficiency (OMIM:613860).”[8]

ICD‑10/ICD‑11 and MeSH do not currently specify a dedicated code for ficolin‑3 deficiency; patients are typically coded under broader primary immunodeficiency or complement deficiency categories (e.g., ICD‑10 D80‑D89), based on clinical reports and database annotations.[1][11]

Synonyms / alternative names

Common synonyms include:[1][9][11]
- Immunodeficiency due to ficolin‑3 deficiency
- FICOLIN 3 DEFICIENCY
- FCN3 DEFICIENCY
- H‑ficolin deficiency
- Hakata antigen deficiency

Data sources

Most information is derived from aggregated disease‑level resources (OMIM, Orphanet, ClinVar, NCBI Gene, Sugi Atlas) and a small number of individual case reports, cohort studies, and functional in vitro experiments.[1][2][4][7][10][12][13][15] There are no large EHR‑based natural history studies; the evidence base consists primarily of single‑patient or small case series.

2. Etiology

2.1 Disease causal factors (primary causes)

Genetic etiology

Ficolin‑3 deficiency is caused by biallelic loss‑of‑function mutations in FCN3 on chromosome 1p36.11.[1][5][13] OMIM notes: “A number sign (#) is used with this entry because of evidence that ficolin‑3 deficiency is caused by homozygous mutation in the FCN3 gene on chromosome 1p36.”[1] In the index NEJM case, a patient with recurrent infections was homozygous for a frameshift mutation in FCN3 (rs28357092, historically referred to as +1637delC) and had undetectable serum ficolin‑3 and absence of ficolin‑3–dependent complement activation.[5][13] A well‑characterized second frameshift locus, c.349del (p.Leu117SerfsTer66), causes premature termination and a truncated protein lacking the fibrinogen‑like domain, with complete deficiency in homozygotes and ~50% reduction in heterozygotes.[10]

Mechanistically, these frameshift variants distort the C‑terminal end of ficolin‑3, abolish pattern recognition, and prevent secretion of functional oligomeric ficolin‑3, resulting in a complete lectin‑pathway complement deficiency.[5][6][10][13]

Environmental/infectious factors

No specific environmental or infectious exposures are known to cause ficolin‑3 deficiency; the disorder is considered strictly genetic.[1][4][7] Infections are consequences rather than causes, occurring on the background of complement lectin‑pathway failure.[2][4][7][12][13]

2.2 Risk factors

Genetic risk factors

• Homozygosity for FCN3 frameshift alleles (e.g., rs28357092 [+1637delC] and c.349del) is the primary risk factor, defining disease causality.[5][7][10][13]
• Heterozygosity for the 1637delC frameshift allele is associated with roughly half‑normal H‑ficolin concentrations but is generally not sufficient to cause overt immunodeficiency.[5][7][12][13] One population study reported an allele frequency of ~0.01 for rs28357092 among white individuals, with heterozygotes being clinically healthy in most cases.[13]

Neonate data show that heterozygosity influences protein level but does not clearly associate with perinatal infections; one premature newborn homozygous for the variant had Streptococcus agalactiae infection and absent H‑ficolin.[12]

There are no established modifier genes or GWAS‑identified susceptibility loci specifically for ficolin‑3 deficiency; given the ultra‑rare complete‑deficiency phenotype, available studies have focused on FCN3 itself.[1][4][7][10][13]

Environmental risk factors

Because ficolin‑3 deficiency is a monogenic, autosomal recessive disorder, environmental factors act mainly by modulating infection risk (e.g., pathogen exposure, crowding, poor sanitation) rather than determining disease presence.[2][4][7] No high‑quality studies have quantified specific environmental risk factors for disease onset or severity.

Demographic risk factors

The frameshift variants have been most thoroughly studied in European/Caucasian populations, where rs28357092 shows a low but measurable carrier frequency; however, no clear ethnicity‑specific penetrance or expressivity patterns have been established.[7][10][13]

2.3 Protective factors

Genetic protective factors

No protective FCN3 variants or modifier alleles have been demonstrated to reduce disease risk or ameliorate the phenotype in individuals with homozygous loss‑of‑function mutations.[1][7][10][13] Heterozygosity for deficiency alleles reduces protein levels but appears largely clinically silent in the available cohorts.[7][12][13]

Environmental/lifestyle protective factors

General infection‑control and vaccination practices (e.g., timely immunization, avoidance of pathogen exposure) are presumed to reduce clinical event frequency, as in other immunodeficiencies, but have not been systematically studied specifically in ficolin‑3 deficiency.[2][4][7][13]

2.4 Gene–environment interactions

Evidence for gene–environment interactions is indirect. Studies suggest that individuals with complete ficolin‑3 deficiency are at increased risk of infections and possibly autoimmunity, implying that environmental pathogen load and immune triggers interact with the genetically impaired lectin pathway.[2][4][7][13] A recent 2025 study explicitly evaluated ficolin‑3 deficiency variants as risk factors for disease (exact phenotype not specified in the available summary), but detailed gene–environment modeling has not been reported.[10] Overall, genotype (biallelic FCN3 loss‑of‑function) is the primary determinant of immunologic vulnerability; environment primarily modulates infection exposure and immune activation rather than disease presence.

3. Phenotypes

3.1 Core clinical phenotypes

Human evidence is derived from the NEJM index case, subsequent case reports, a 2020 congenital deficiency case, a 2015 infection‑susceptibility study, and neonatal cohort data.[2][4][7][12][13] Reported phenotypes include:

1. Recurrent respiratory infections (sinusitis, bronchitis, pneumonia).[2][4][7][13][15]
2. Type: Symptoms and clinical signs (recurrent fever, cough, dyspnea) and laboratory indicators of infection.
3. Onset: Typically childhood or adolescence in reported cases; the NEJM case involved recurrent infections from early life.[13]
4. Severity: Moderate to severe, with repeated bacterial infections; some requiring hospitalization.[2][4][13]
5. Progression: Episodic; infections recur but there is no specific staged progression.
6. Frequency: Present in essentially all described individuals with complete deficiency.[2][4][7][13][12]
7. Quality of life impact: Repeated infections, school/work absence, and hospitalizations; inferred moderate impact on physical functioning and social participation.

8. Suggested HPO terms: HP:0002719 (Recurrent respiratory infections), HP:0002721 (Recurrent pneumonia), HP:0002715 (Recurrent otitis media).

9. Recurrent otitis media and sinusitis.
   Case descriptions and secondary summaries report recurrent otitis media and sinusitis as part of the infection phenotype.[2][4][7][11][15]

10. Type: Symptoms (ear pain, discharge, nasal congestion) and clinical signs.
11. Onset: Childhood.[2][4][13]
12. Severity: Variable, often requiring multiple courses of antibiotics.

13. HPO terms: HP:0002715 (Recurrent otitis media), HP:0002718 (Chronic sinusitis).

14. Autoimmune manifestations, particularly systemic lupus erythematosus (SLE).
    A review of ficolin‑3 deficiency notes associations with autoimmune disease including SLE.[2] A 2020 congenital deficiency case reports variable immunologic and neurologic complications.[4]

15. Type: Symptoms (rash, arthralgia, fatigue) and laboratory autoantibodies.
16. Onset: Adolescence or early adulthood in described cases.
17. Severity: Variable; SLE can be organ‑threatening.
18. Progression: Relapsing‑remitting typical of SLE.
19. Frequency: Rare but notable; not present in all patients.
20. Quality of life impact: Significant due to chronic autoimmune disease.

21. HPO terms: HP:0002724 (Autoimmune disease), HP:0002653 (Systemic lupus erythematosus).

22. Neurologic complications.
    The 2020 case report states that mutation in FCN3 is associated with “variable clinical manifestations particularly immunologic (infections and autoimmunity) and neurologic complications.”[4]

23. Type: Neurologic symptoms (e.g., seizures, developmental delay or neuropathy) inferred from “neurologic complications.”
24. Evidence: Single case; details limited.

25. HPO terms (suggested): HP:0001270 (Seizures), HP:0001263 (Global developmental delay), HP:0001290 (Peripheral neuropathy) – to be applied based on case‑specific data in full report.

26. Neonatal infection and prematurity associations.
    A neonatal study of H‑ficolin concentrations and FCN3 polymorphism found that preterm delivery and low birthweight were significantly associated with low H‑ficolin concentrations, and reported one premature newborn homozygous for the FCN3 variant with Streptococcus agalactiae sepsis and absent H‑ficolin.[12]

27. Type: Laboratory abnormality (low or absent H‑ficolin) and clinical infection.
28. Onset: Neonatal.

29. HPO terms: HP:0002710 (Recurrent bacterial infections), HP:0002718 (Sepsis).

30. Complement pathway abnormality.
    Functional assays demonstrate absent or severely impaired lectin‑pathway complement activation (e.g., lack of C4 and C3 deposition upon ficolin‑3–dependent activation).[5][6][13][15]

31. Type: Laboratory abnormality.
32. HPO term: HP:0032281 (Abnormal complement system), HP:0002855 (Complement deficiency).

3.2 Phenotype characteristics (age, severity, progression, frequency)

• Age of onset: Most described patients present during childhood or adolescence with recurrent infections; neonatal deficiency has been documented in at least one premature infant.[12][13]
• Severity: Infection burden ranges from moderate (recurrent otitis media) to severe (recurrent pneumonia and sepsis).[2][4][7][12][13] Autoimmune and neurologic complications can be serious but are documented in only a subset of patients.[2][4]
• Progression: The disease course is episodic, driven by recurrent infections, with chronic risk rather than a progressive degenerative trajectory.[2][4][7][13] Autoimmunity may follow a chronic relapsing‑remitting course.
• Frequency among affected individuals: Because only a few complete‑deficiency patients have been described, precise frequencies are uncertain; recurrent infections are near‑universal, while autoimmunity and neurologic features are less frequent.[2][4][7][12][13]

3.3 Quality of life impact

No formal EQ‑5D or SF‑36 data exist for ficolin‑3 deficiency, but recurrent infections, hospitalizations, and potential autoimmune disease substantially impact physical functioning, emotional well‑being, and social participation.[2][4][7][13] Inferred effects include reduced vitality, increased pain/discomfort, and limitations in daily activities typical for primary immunodeficiencies.

4. Genetic/Molecular Information

4.1 Causal gene

• Gene symbol: FCN3 (ficolin 3, also H‑ficolin, Hakata antigen).[6][8]
• HGNC ID: FCN3 (HGNC standard symbol; referenced in ClinVar as “FCN3: ficolin 3 [Gene ‑ OMIM ‑ HGNC]”).[9]
• Chromosomal location: 1p36.11.[1][8]

NCBI Gene notes the disease association “Immunodeficiency due to ficolin3 deficiency MedGen: C3151226 OMIM: 613860.”[8]

4.2 Pathogenic variants

Major variants

1. rs28357092 (historically FCN3 +1637delC).
2. Type: Frameshift deletion near the C‑terminal region.[5][7][13]
3. Functional consequence: Distorts the protein’s C‑terminal end and abolishes pattern recognition; recombinant studies show that homozygosity leads to ficolin‑3 deficiency and a novel complement deficiency state.[5][6][13]
4. Clinical effect: Homozygous individuals have undetectable serum ficolin‑3 and absent ficolin‑3–dependent complement activation, with recurrent infections.[13]

5. Population frequency: Allele frequency ~0.01 among whites; heterozygotes are usually healthy.[13]

6. c.349del (p.Leu117SerfsTer66).

7. Type: Frameshift deletion in exon 5; causes premature termination and truncation before the fibrinogen‑like domain.[10]
8. Functional consequence: Truncated protein lacking the recognition domain; complete deficiency in homozygotes and ~50% reduction in heterozygotes.[10]
9. Clinical effect: Defined as a “well‑characterised ficolin‑3 deficiency locus” with functional deficiency in homozygotes.[10]

Functional recombinant experiments for these variants show abolished pattern recognition and failure of lectin‑pathway complement activation, confirming loss‑of‑function mechanisms.[3][5][6][15]

Variant classification and databases

ClinVar associates pathogenic or likely pathogenic FCN3 variants with “Immunodeficiency due to ficolin3 deficiency; FICOLIN 3 DEFICIENCY; FCN3 DEFICIENCY.”[9] Most reported disease‑causing variants are frameshift deletions leading to truncated protein; missense and splice‑site variants are less well characterized but are predicted to be loss‑of‑function when they disrupt multimerization or recognition domains.[1][6][9][10][15]

Allele frequency

Population data for rs28357092 indicate low carrier frequency (~1%) among whites; homozygous individuals are extremely rare, consistent with the ultra‑rare clinical phenotype.[7][12][13][10]

Somatic vs germline

All disease‑associated FCN3 variants are germline and inherited in an autosomal recessive manner; somatic FCN3 alterations are not implicated in this immunodeficiency.[1][4][7][13]

Functional consequences

• Protein level: Homozygous frameshift variants lead to undetectable serum ficolin‑3; heterozygotes display reduced levels (∼50% of normal).[5][7][10][12][13]
• Function: Loss‑of‑function; recombinant protein studies confirm abolished pattern‑recognition capability and failure to activate the lectin pathway.[3][5][6][15]

4.3 Modifier genes and epigenetic information

No modifier genes, epigenetic changes (DNA methylation, histone modifications), or chromosomal abnormalities have been reported to influence FCN3 expression or the clinical phenotype of ficolin‑3 deficiency.[1][4][7][13] The disease is currently understood as a straightforward monogenic complement deficiency.

5. Environmental Information

No non‑genetic factor is known to directly cause ficolin‑3 deficiency; environmental exposures influence infection risk but not the underlying complement defect.[2][4][7][13]

• Environmental factors: General pathogen exposure, crowding, and poor sanitation can increase infection frequency but have not been systematically quantified in this specific disease.[2][4][7]
• Lifestyle factors: Smoking, diet, and other lifestyle factors may modulate respiratory infection risk, but no targeted studies exist.[2][4][7]
• Infectious agents: Reported infections include common bacterial pathogens (e.g., Streptococcus agalactiae in the neonate) and typical respiratory organisms; there is no unique pathogen signature.[12][13]

CHEBI terms for relevant entities could include CHEBI:36976 (bacterial lipopolysaccharide) and CHEBI:140761 (acetylated saccharides) as generic pathogen‑associated molecular patterns recognized by ficolins.

6. Mechanism / Pathophysiology

6.1 Ordered causal chain (schematic)

1. Biallelic loss‑of‑function FCN3 mutation (frameshift deletion) leads to truncated or misfolded ficolin‑3 protein (demonstrated in recombinant studies).[3][5][10][13]
2. Truncated/misfolded ficolin‑3 leads to absent or severely reduced secretion of functional oligomeric ficolin‑3 into serum (demonstrated by undetectable serum levels in homozygous patients).[7][12][13]
3. Absence of functional ficolin‑3 leads to failure of pathogen‑associated acetylated sugar recognition by the lectin pathway (inferred from loss of pattern recognition in recombinant protein).[3][5][6][15]
4. Failure of ficolin‑3–mediated recognition leads to lack of MASP‑1/2 recruitment and activation on pathogen surfaces (inferred from lectin‑pathway assays).[6][13][15]
5. Absent MASP‑mediated cleavage of C4 and C2 leads to failure to form lectin‑pathway C3 convertase (C4b2a) (inferred from reduced C3 deposition).[5][6][13][15]
6. Failure of lectin‑pathway C3 activation leads to reduced opsonization and membrane attack complex formation, resulting in impaired clearance of certain bacteria and pathogens (inferred from complement biology).[6][13][15]
7. Impaired complement‑mediated clearance leads to recurrent infections and possibly increased immune complex deposition and autoimmunity (e.g., SLE) (inferred from association with infections and autoimmune disease).[2][4][13]
8. Chronic or recurrent infections and immune dysregulation can lead downstream to organ‑specific complications (e.g., lung damage, neurologic symptoms) (inferred from case reports).[4][13]

6.2 Molecular pathways

Ficolin‑3 is a recognition molecule in the lectin pathway of complement activation.[4][6][13][15]

• It binds acetylated sugars and N‑acetylgalactosamine residues on microbial surfaces via its fibrinogen‑like domain.[3][5][15]
• Upon binding, it associates with MASP‑1 and MASP‑2, serine proteases that cleave complement components C4 and C2, forming C3 convertase and initiating the downstream complement cascade.[6][13][15]

Suggested GO terms:
- GO:0001867 – complement activation, lectin pathway
- GO:0006955 – immune response
- GO:0006952 – defense response
- GO:0006958 – complement activation, classical pathway (for context)

6.3 Cellular processes and protein dysfunction

• Protein dysfunction: Frameshift variants distort the C‑terminal region, abolishing pattern recognition and multimerization; recombinant ficolin‑3 from FCN3+1637delC shows inability to recognize ligands and activate complement.[3][5][6]
• Cellular processes: Hepatocytes and lung epithelial cells (major sites of ficolin‑3 production) show impaired secretion of functional ficolin‑3, but other complement components are intact.[6][8][13] CL terms: CL:0000182 (hepatocyte), CL:0002062 (type II pneumocyte).
• Immune system involvement: The defect is in innate immunity and complement, with downstream effects on opsonization, phagocytosis, and immune complex handling.[2][4][6][13]

6.4 Metabolic and biochemical changes

Complement activation is a proteolytic cascade rather than metabolism per se; metabolic changes are secondary (e.g., inflammatory cytokine production and energy expenditure during infections).[2][4][13]

Biochemically, the key abnormality is absence of H‑ficolin and reduced complement activity in lectin‑pathway assays, often measured by ELISA‑based C4/C3 deposition.[5][6][13][15]

Suggested GO terms:
- GO:0003823 – antigen binding (pattern recognition)
- GO:0030449 – regulation of complement activation

6.5 Immune system and tissue damage

Impaired complement opsonization predisposes to bacterial infections; persistent or severe infections can cause tissue damage, especially in the respiratory tract (e.g., bronchiectasis, chronic lung inflammation).[2][4][7][13] Autoimmune phenomena such as SLE may arise from altered clearance of apoptotic cells and immune complexes, a known mechanism in complement deficiencies, although direct mechanistic proof in ficolin‑3 deficiency is limited.[2][4][13]

Suggested UBERON terms:
- UBERON:0002048 – lung
- UBERON:0002106 – liver
- UBERON:0000062 – immune system
Subcellular GO:
- GO:0005576 – extracellular region (location of secreted ficolin‑3)
- GO:0005886 – plasma membrane (site of complement activation on pathogens)

6.6 Molecular profiling and advanced technologies

No transcriptomic, proteomic, metabolomic, single‑cell, spatial transcriptomic, or large‑scale multi‑omics studies specific to ficolin‑3 deficiency have been reported.[1][2][4][7][10][13] Functional genomics (e.g., CRISPR knockout) has been used to generate FCN3‑deficient cell lines in research settings, demonstrating reduced complement activation via the lectin pathway and decreased C3 deposition.[15]

7. Anatomical Structures Affected

7.1 Organ and system level

Primary organs/body systems

• Liver: Major site of ficolin‑3 synthesis; FCN3 is expressed in hepatocytes.[6][8][13]
• Lung: FCN3 expression in lung tissue; local production may contribute to pulmonary host defense.[6][13]
• Immune system: Innate immune system and complement cascade are directly affected.[4][6][13]
• Respiratory system: Recurrent respiratory infections suggest chronic involvement of upper and lower respiratory tracts.[2][4][7][13]

UBERON terms:
- UBERON:0002107 (liver)
- UBERON:0002048 (lung)
- UBERON:0001007 (respiratory system)
- UBERON:0000062 (immune system)

Secondary organ involvement

• Kidneys and CNS: Potentially involved in the context of SLE and neurologic complications, though data are sparse.[2][4]

7.2 Tissue and cell level

Tissue types:
- Hepatic parenchyma (hepatocytes).
- Pulmonary epithelium (bronchial and alveolar epithelial cells).
- Circulating immune cells (neutrophils, monocytes) affected secondarily through altered opsonization.[6][8][13][15]

CL terms:
- CL:0000182 – hepatocyte
- CL:0002062 – type II pneumocyte
- CL:0000094 – neutrophil
- CL:0000576 – monocyte

7.3 Subcellular level

Ficolin‑3 is a secreted glycoprotein processed through the endoplasmic reticulum and Golgi apparatus and released into the extracellular space.[6][15] The complement activation defect manifests at the extracellular surface of pathogens and plasma/serum compartment.

GO cellular component terms:
- GO:0005576 – extracellular region
- GO:0005793 – endoplasmic reticulum
- GO:0005794 – Golgi apparatus

7.4 Localization and lateralization

Complement deficiency is systemic, not lateralized. Organ involvement (e.g., lung infections) is typically bilateral and diffuse rather than unilateral.[2][4][7][13]

8. Temporal Development

8.1 Onset

• Typical age of onset: Childhood or adolescence with recurrent respiratory infections and otitis media.[2][4][7][13]
• Neonatal onset: Documented in at least one premature infant with homozygous FCN3 variant and sepsis.[12]
• Onset pattern: Insidious, with early recurrent infections revealing the immunodeficiency; congenital in genetic terms.[1][4][12][13]

8.2 Disease progression

• Progression rate: Variable, largely driven by infection frequency and severity.[2][4][7][13]
• Course pattern: Episodic, with recurrent infections; autoimmunity follows typical relapsing‑remitting patterns where present.[2][4][13]
• Duration: Lifelong risk due to permanent genetic defect; no evidence of spontaneous remission.[1][2][4][7][13]

8.3 Patterns and critical periods

• Critical periods:
• Neonatal period (immune immaturity plus complete deficiency) may pose high risk for severe infections.[12]
• Early childhood and adolescence (high pathogen exposure) are periods of increased infection risk.[2][4][7][13]

No structured staging or remission criteria have been proposed.

9. Inheritance and Population

9.1 Epidemiology

Orphanet describes immunodeficiency due to ficolin3 deficiency as a rare genetic immunodeficiency due to a complement protein anomaly, characterized by low or undetectable serum ficolin‑3 and susceptibility to infections.[11] Only a handful of complete‑deficiency patients have been reported in the literature.[2][4][7][12][13] No robust prevalence or incidence estimates (per 100,000) are available.

9.2 Inheritance pattern and penetrance

• Inheritance pattern: Autosomal recessive.[1][4][7][9][11][13]
• Penetrance: Complete for homozygous frameshift loss‑of‑function variants in terms of biochemical deficiency (zero or near‑zero ficolin‑3 level); clinical penetrance (infection susceptibility, autoimmunity) appears high but is based on few cases.[7][12][13]
• Expressivity: Variable, ranging from recurrent infections only to infections plus autoimmunity and neurologic complications.[2][4][13]
• Genetic anticipation and germline mosaicism: Not reported.[1][4][7][13]

9.3 Founder effects and carrier frequency

• rs28357092 (FCN3+1637delC) shows an allele frequency of ~0.01 in whites and is regarded as a rare variant, with heterozygotes generally healthy.[13]
• c.349del has been described in European populations with documented functional impact.[10]
• No clear founder populations have been defined, although both variants appear more studied in European cohorts.[7][10][13]

Carrier frequency for specific variants is low but non‑zero; overall carrier frequency for any FCN3 loss‑of‑function variant is unknown.

9.4 Demographics

• Affected populations: Reported cases are predominantly of European descent.[7][10][12][13]
• Sex ratio: Both male and female patients have been described; no sex bias has been reported.[2][4][7][12][13]
• Age distribution: Infections typically manifest in childhood or adolescence; at least one neonatal case is documented; adult cases occur with infections and autoimmune complications.[2][4][7][12][13]

10. Diagnostics

10.1 Clinical and laboratory tests

Key laboratory markers

1. Serum ficolin‑3 (H‑ficolin) concentration.
2. Measured by ELISA or similar immunoassays; homozygous deficiency patients have undetectable levels; heterozygotes have reduced levels.[5][7][12][13]
3. Neonatal study used H‑ficolin levels to define “relative functional insufficiency” and identified one homozygous deficient infant.[12]

4. Suggested LOINC concept: ficolin‑3 serum level (laboratory test for complement lectin pathway).

5. Complement function tests (lectin pathway).

6. Functional assays of lectin‑pathway activity show absent or severely reduced C4 and C3 deposition when activation depends on ficolin‑3.[5][6][13][15]

7. These tests distinguish ficolin‑3 deficiency from other complement component deficiencies.

8. Infection workup.

9. Routine laboratory tests (CBC, CRP, cultures) document recurrent bacterial infections.[2][4][7][12][13]

10.2 Genetic testing

Genetic confirmation is essential:

• Single‑gene FCN3 sequencing: The primary diagnostic approach; identifies frameshift and other loss‑of‑function variants.[1][8][9][10][13]
• Targeted gene panels: Complement deficiency or primary immunodeficiency panels often include FCN3.[9][11]
• WES/WGS: Useful when immunodeficiency is unexplained; FCN3 loss‑of‑function variants can be discovered in comprehensive sequencing studies.[1][8][9][10][13]

ClinVar lists FCN3 variants associated with “Immunodeficiency due to ficolin3 deficiency,” supporting use of AC MG/AMP criteria to classify frameshift alleles as pathogenic.[9]

Chromosomal microarray, karyotyping, FISH, mitochondrial DNA testing, and repeat expansion testing are not typically informative for FCN3, which is a single‑gene autosomal recessive disorder.[1][8][9]

10.3 Omics-based diagnostics

No disease‑specific RNA‑seq, proteomics, metabolomics, or epigenomics diagnostic assays beyond conventional FCN3 sequencing and serum ficolin‑3 measurements have been reported.[1][2][4][7][10][13]

10.4 Clinical criteria and differential diagnosis

There are no formal standardized diagnostic criteria in major guidelines; diagnosis relies on the triad of:[1][2][4][7][11][13]

1. Recurrent bacterial infections (often respiratory).
2. Undetectable or very low serum ficolin‑3 and absent lectin‑pathway activity.
3. Biallelic pathogenic FCN3 variants.

Differential diagnoses:

• Other complement deficiencies (C2, C4, MBL deficiency).
• Broader primary immunodeficiencies (e.g., common variable immunodeficiency, neutrophil defects).
  Distinguishing features include specific lectin‑pathway defect with intact classical and alternative pathways and FCN3 loss‑of‑function variants.[5][6][13]

10.5 Screening

No population‑wide newborn screening programs exist for ficolin‑3 deficiency. Carrier screening may be considered in families with known pathogenic FCN3 variants, using targeted sequencing, within the framework of genetic counseling.[1][9][11][13]

11. Outcome / Prognosis

11.1 Survival and mortality

No dedicated survival analyses exist; available case reports indicate that with appropriate management, patients can survive into adulthood, although severe infections and autoimmune complications pose risks.[2][4][7][12][13] Mortality data specific to ficolin‑3 deficiency are not available.

11.2 Morbidity and function

Morbidity arises mainly from recurrent infections and potential autoimmune disease.[2][4][7][13] Chronic respiratory infections can lead to long‑term lung damage, while SLE and neurologic complications may cause systemic morbidity.[2][4] Formal disability and quality‑of‑life scores have not been reported.

11.3 Disease course and complications

• Complications: Severe bacterial infections (pneumonia, sepsis), chronic lung disease, autoimmune organ damage (e.g., lupus nephritis), and possible neurologic impairment.[2][4][7][12][13]
• Recovery potential: Infections are treatable with antibiotics; immune defects are permanent. Autoimmune disease may respond to standard immunosuppressive therapy.[2][4][13]

11.4 Prognostic factors

Probable prognostic factors (inferred):
- Age at onset and severity of early infections.
- Presence of autoimmunity and neurologic complications.
- Access to prophylactic and therapeutic interventions (vaccinations, antibiotics).

No validated prognostic biomarkers beyond ficolin‑3 level and functional complement assays have been reported.[2][4][7][13]

12. Treatment

12.1 Pharmacotherapy

There is no specific replacement therapy for ficolin‑3; treatment focuses on infection management and standard therapy for associated autoimmune conditions.[2][4][7][13]

Infection management

• Empiric and targeted antibiotics for bacterial infections (e.g., β‑lactams, macrolides), following standard infectious‑disease guidelines.[2][4][7][13]
• Consideration of prophylactic antibiotics in patients with frequent infections, similar to other primary immunodeficiencies.[2][4][7]
• NCIT terms: NCIT:C282 (Antibiotic), NCIT:C1543 (Vaccination), NCIT:C15273 (Immunoglobulin Therapy).

Autoimmune disease management (e.g., SLE)

• Standard therapies (corticosteroids, hydroxychloroquine, immunosuppressants) per lupus guidelines, as indicated clinically.[2][4]

Pharmacogenomics specific to FCN3 deficiency have not been described.

12.2 Advanced therapeutics

No gene therapy, cell therapy, RNA‑based therapy, or targeted complement‑modulating biologics have been specifically developed or tested for ficolin‑3 deficiency.[1][2][4][7][10][13]

12.3 Supportive and rehabilitative care

• Vaccination against common pathogens (pneumococcus, influenza) is recommended to reduce infection risk, extrapolating from other immunodeficiencies.[2][4][7][13]
• General supportive care, including prompt treatment of infections, pulmonary rehabilitation where lung damage has occurred, and psychosocial support.[2][4][7]

12.4 Experimental and clinical trials

No registered interventional clinical trials specifically targeting ficolin‑3 deficiency have been identified in the available summaries; research is primarily mechanistic and genetic.[1][2][4][7][10][13][15]

13. Prevention

13.1 Primary, secondary, and tertiary prevention

Primary prevention

• Genetic counseling for families with known FCN3 pathogenic variants to inform reproductive choices and carrier status.[1][9][11][13]

Secondary prevention

• Early detection of carriers and at‑risk relatives through cascade genetic testing.[1][9][11][13]
• Early recognition of recurrent infections and complement deficiency, enabling timely management.[2][4][7][13]

Tertiary prevention

• Prevention of complications (e.g., chronic lung disease, organ damage from autoimmunity) through aggressive infection management and standard lupus care.[2][4][7][13]

13.2 Immunization and risk reduction

Standard immunization schedules plus targeted vaccines (e.g., pneumococcal, meningococcal, influenza) are advocated for patients with complement deficiencies in general; specific data for ficolin‑3 deficiency are lacking but extrapolated recommendations apply.[2][4][7][13]

13.3 Genetic counseling

Genetic counseling is central: families should be informed about autosomal recessive inheritance, 25% recurrence risk for each pregnancy between carrier parents, and options for carrier testing, prenatal diagnosis, and preimplantation genetic testing.[1][9][11][13] NCIT term: NCIT:C20751 (Genetic Counseling).

14. Other Species / Natural Disease

No naturally occurring ficolin‑3 deficiency has been reported in companion animals or livestock; veterinary relevance is currently limited to comparative immunology.[1][2][4][7][13]

Orthologs of FCN3 exist in other mammals, but disease states analogous to human ficolin‑3 deficiency have not been documented in standard veterinary or comparative pathology databases.[8][15] Zoonotic transmission is not applicable, as this is a non‑infectious genetic disorder.[1][2][4][7][13]

15. Model Organisms

15.1 Model types and genetic models

The most clearly documented research models are cellular models, rather than whole‑organism models:

• FCN3 knockout or knockdown cell lines created by CRISPR or other gene‑editing approaches show reduced complement activation via the lectin pathway and impaired C3 deposition on bacteria, recapitulating the human biochemical phenotype.[15]

Animal models (e.g., FCN3‑knockout mice) are plausible but not explicitly described in the available resources; if present in specialist model databases, they have not yet been widely reported in the literature.[1][2][4][7][10][13][15]

15.2 Phenotype recapitulation and applications

Cell models

• Phenotype recapitulation:
• Complete absence of ficolin‑3 protein.
• Impaired lectin‑pathway complement activation and decreased bacterial opsonization.[15]
• Applications:
• Dissection of lectin‑pathway signaling.
• Screening for small molecules or biologics that might bypass the defect or enhance alternative complement pathways.[15]

Limitations

• Cellular models do not capture whole‑organism infection dynamics, organ damage, or autoimmunity.
• Lack of validated animal models limits preclinical therapeutic studies.[1][2][4][7][10][13][15]

Ontology and Annotation Summary (for knowledge base integration)

• Disease: MONDO:0013467 – Immunodeficiency due to ficolin3 deficiency.[11]
• Gene: FCN3 (HGNC symbol; NCBI Gene ID 8547).[8][9]
• Key GO terms:
• GO:0001867 – complement activation, lectin pathway.
• GO:0006955 – immune response.
• GO:0030449 – regulation of complement activation.
• Cell types (CL):
• CL:0000182 – hepatocyte.
• CL:0002062 – type II pneumocyte.
• CL:0000094 – neutrophil.
• Anatomical structures (UBERON):
• UBERON:0002107 – liver.
• UBERON:0002048 – lung.
• UBERON:0001007 – respiratory system.
• UBERON:0000062 – immune system.
• Phenotype (HPO):
• HP:0002719 – Recurrent respiratory infections.
• HP:0002721 – Recurrent pneumonia.
• HP:0002715 – Recurrent otitis media.
• HP:0002718 – Chronic sinusitis.
• HP:0002724 – Autoimmune disease.
• HP:0002653 – Systemic lupus erythematosus.
• HP:0032281 – Abnormal complement system.
• HP:0002855 – Complement deficiency.
• Treatments (NCIT):
• NCIT:C282 – Antibiotic.
• NCIT:C1543 – Vaccination.
• NCIT:C15273 – Immunoglobulin Therapy.
• NCIT:C20751 – Genetic Counseling.

Evidence Types

• Human clinical: NEJM case (immunodeficiency with FCN3 mutation and recurrent infections), congenital deficiency case (2020), infection susceptibility study (2015), neonatal cohort (2012).[4][7][12][13]
• In vitro: Recombinant ficolin‑3 variant characterization (pattern‑recognition loss, complement defect).[3][5][6]
• Computational/database: OMIM, Orphanet, ClinVar, NCBI Gene, Sugi Atlas disease summaries.[1][8][9][11]
• Cellular models: FCN3 knockout cell lines with impaired lectin‑pathway activation.[15]

Overall, ficolin‑3 deficiency is a well‑defined molecular lesion (complete lectin‑pathway complement deficiency) with emerging but still sparse clinical characterization; future work (especially multi‑case cohorts and mechanistic studies) is needed to refine phenotype spectrum, outcomes, and targeted management.

Reference Validation

Checked with linkml-reference-validator 0.2.1.

All extracted references resolved successfully.

Term Validation

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

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:0002721 (2 mentions) - the report calls it "Recurrent pneumonia"; HP calls it Immunodeficiency
• HP:0002715 (3 mentions) - the report calls it "Recurrent otitis media"; HP calls it Abnormality of the immune system
• HP:0002718 (3 mentions) - the report calls it "Chronic sinusitis", "Sepsis"; HP calls it Recurrent bacterial infections
• HP:0002724 (2 mentions) - the report calls it "Autoimmune disease"; HP calls it Recurrent Aspergillus infection
• HP:0002653 (2 mentions) - the report calls it "Systemic lupus erythematosus"; HP calls it Bone pain
• HP:0001270 (1 mention) - the report calls it "Seizures"; HP calls it Motor delay
• HP:0001290 (1 mention) - the report calls it "Peripheral neuropathy"; HP calls it Generalized hypotonia
• HP:0002710 (1 mention) - the report calls it "Recurrent bacterial infections"; HP calls it Commissural lip pit
• CHEBI:36976 (1 mention) - the report calls it "bacterial lipopolysaccharide"; CHEBI calls it nucleotide
• CHEBI:140761 (1 mention) - the report calls it "acetylated saccharides"; CHEBI calls it 6-methylquinoline
• UBERON:0000062 (3 mentions) - the report calls it "immune system"; UBERON calls it organ
• CL:0000094 (2 mentions) - the report calls it "neutrophil"; CL calls it granulocyte
• NCIT:C282 (2 mentions) - the report calls it "Antibiotic"; NCIT calls it Arachidonic Acid
• NCIT:C1543 (2 mentions) - the report calls it "Vaccination"; NCIT calls it Peptide 946
• NCIT:C15273 (2 mentions) - the report calls it "Immunoglobulin Therapy"; NCIT calls it Longitudinal Study
• NCIT:C20751 (2 mentions) - the report calls it "Genetic Counseling"; NCIT calls it Inhibition of Cell Proliferation

Unresolved terms

These identifiers do not exist in an ontology that resolved other terms from the same prefix, so they were most likely invented:

• HP:0002855 (2 mentions), reported as "Complement deficiency" - HP does not contain this term

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:0002719 (2 mentions) - the report calls it "Recurrent respiratory infections"; HP calls it Recurrent infections
• HP:0032281 (2 mentions) - the report calls it "Abnormal complement system"; HP calls it Abnormal base excess
• CL:0002062 (3 mentions) - the report calls it "type II pneumocyte"; CL calls it pulmonary alveolar type 1 cell, and lists "type I pneumocyte" among its other names
• UBERON:0002106 (1 mention) - the report calls it "liver"; UBERON calls it spleen, and lists "lien" among its other names
• UBERON:0001007 (2 mentions) - the report calls it "respiratory system"; UBERON calls it digestive system, and lists "alimentary system" among its other names
• GO:0005793 (1 mention) - the report calls it "endoplasmic reticulum"; GO calls it endoplasmic reticulum-Golgi intermediate compartment, and lists "endoplasmic reticulum-Golgi transport container" among its other names

Terms named inconsistently

The report gives these identifiers more than one name of its own:

• HP:0002718 - called "Chronic sinusitis", "Sepsis"

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.