Lung Disease, Immunodeficiency, and Chromosome Breakage Syndrome (LICS) — Comprehensive Disease Characterization

MONDO: MONDO:0014984 · OMIM: #617241 · Gene: NSMCE3 (NDNL2 / MAGEG1) · Category: Mendelian, autosomal recessive

Summary

Lung Disease, Immunodeficiency, and Chromosome Breakage Syndrome (LICS) is an ultra-rare, autosomal-recessive Mendelian disorder caused by biallelic missense mutations in NSMCE3 (also called NDNL2 / MAGEG1), a subunit of the SMC5/6 genome-maintenance complex. The disease was first defined in 2016 by whole-exome sequencing of four children from two unrelated kindreds who died of severe pulmonary disease in infancy following viral pneumonia, with combined T- and B-cell immunodeficiency and constitutional chromosome breakage PMID: 27427983. LICS belongs to an emerging "SMC5/6 complexopathy" spectrum alongside NSMCE2- and SMC5-related microcephalic primordial dwarfism.

The central molecular lesion is destabilization of the SMC5/6 complex: the pathogenic variants p.Leu264Phe (c.790C>T) and p.Pro209Leu (c.626C>T) abolish NSE3's binding to its partner subunits within the NSE1/NSE3/NSE4 module. Because NSE3 directly binds double-stranded DNA and this module is required to load SMC5/6 onto chromatin, the variants collapse a DNA-binding/loading function, producing defective homologous recombination, sensitivity to replication stress, chromosome rearrangements, and micronuclei. The resulting cellular phenotype is cell-intrinsic.

Clinically, the defining and most lethal feature is viral-triggered diffuse alveolar damage / pediatric acute respiratory distress syndrome (PARDS) that progresses to irreversible interstitial fibrosis, refractory to corticosteroids and to hematopoietic stem-cell transplantation (HSCT). The immunodeficiency is variable (T-cell predominant) and does not fully explain the lung disease. Death typically occurs in infancy to early childhood, though extended survival to 6.5 years and a fatal case at 11 years have been reported. A plausible but inferred amplifying mechanism links constitutional chromosome instability (micronuclei/chromatin bridges) to cGAS-STING/type-I-interferon signalling driving pulmonary fibrosis — this branch has not been directly demonstrated in LICS lung tissue. No curative therapy exists; management is supportive with genetic counselling for at-risk families.

Section 1 — Disease Information

Overview. LICS is a Mendelian chromosome-instability (chromosome-breakage) syndrome characterized by the triad of (1) severe, often fatal early-childhood lung disease (ARDS/PARDS), (2) combined (predominantly T-cell) immunodeficiency, and (3) spontaneous chromosome breakage/rearrangements with micronuclei. It is caused by destabilization of the SMC5/6 complex through biallelic NSMCE3 mutations PMID: 27427983.

Key identifiers.

Resource Identifier
MONDO MONDO:0014984
OMIM #617241 (Lung disease, immunodeficiency, and chromosome breakage syndrome, LICS)
Gene (OMIM) NSMCE3 608243
HGNC gene NSMCE3
Cytogenetic locus 15q13
Inheritance Autosomal recessive

Synonyms / alternative names. LICS; Lung disease–immunodeficiency–chromosome breakage syndrome; SMC5/6 complex deficiency due to NSMCE3; NDNL2-related / MAGEG1-related chromosome breakage syndrome.

Information source. Knowledge is derived from aggregated disease-level resources (OMIM, Orphanet) and a small number of primary case reports/series totaling roughly a dozen genetically confirmed patients worldwide. This is not EHR/individual-patient-registry derived — the literature base is case reports and small kindred studies.

Section 2 — Etiology

Primary cause (genetic). LICS is a monogenic disorder caused by biallelic (homozygous or compound-heterozygous) missense mutations in NSMCE3, which encodes a non-SMC element of the SMC5/6 complex essential for the DNA-damage response and chromosome segregation PMID: 27427983.

Genetic risk factors. The disease-causing alleles are the risk factors themselves: - c.790C>T, p.Leu264Phe (rs199905054; homozygous in the original and 2021 cohorts; OMIM variant 608243.0001) - c.626C>T, p.Pro209Leu (NM_138704.3; compound-heterozygous; OMIM variant 608243.0002)

Consanguinity and founder effects increase homozygosity risk, consistent with the recessive inheritance and the recurrence of p.L264F across kindreds. No established modifier genes are documented, though variable immunophenotype suggests unidentified modifiers or stochastic/environmental contributions.

Environmental risk / trigger factors. Although the underlying defect is constitutional, viral respiratory infection is the principal environmental trigger of the lethal lung disease. In the original cohort, fatal pulmonary disease followed viral pneumonia PMID: 27427983; multiple viral infections were implicated in subsequent cases.

Protective factors. None established. No protective variants or lifestyle/dietary factors are known for this ultra-rare disorder.

Gene–environment interaction. The core GxE axis is germline SMC5/6 destabilization × viral infection → ARDS. SMC5/6 is itself a broad-spectrum antiviral restriction factor (Section 6); loss of complex function may impair control of viral episomes while the genome-instability phenotype sensitizes the host to infection-driven immunopathology. This interaction is plausible but mechanistically incompletely resolved.

Section 3 — Phenotypes

LICS phenotypes cluster into three domains. Suggested HPO terms are provided; frequencies are qualitative given the small cohorts.

Phenotype Type Onset Severity / course Frequency Suggested HPO
Severe lung disease / ARDS (diffuse alveolar damage, PARDS) Clinical sign / imaging-histopathology Infancy–early childhood Severe, rapidly progressive, irreversible; often fatal Core (defining); near-universal in symptomatic cases HP:0002878 (Respiratory failure); HP:0002090 (Pneumonia); HP:0006536 (Obstructive lung disease)
Interstitial pulmonary fibrosis Physical/histopathological After viral insult Progressive Frequent HP:0002206 (Pulmonary fibrosis)
Combined immunodeficiency (T-cell predominant) Laboratory abnormality Infancy Variable (severe to limited) Variable HP:0005354 (Combined immunodeficiency)
Decreased T-cell count / proliferation Laboratory abnormality Infancy Variable Common in original cohort HP:0005403 (Decreased T cell count); HP:0012177
Impaired antibody responses Laboratory abnormality Infancy Variable Common HP:0004313 (Decreased antibody level)
Recurrent/viral respiratory infections Symptom Infancy Variable; may be absent Variable HP:0002788 (Recurrent respiratory infections)
Chromosome breakage / rearrangements, micronuclei, marker chromosomes Laboratory (cytogenetic) Congenital (constitutional) Stable cellular trait Universal HP:0040012 (Chromosome breakage)
Hypoglycemia Laboratory abnormality Infancy Variable Reported (2021+) HP:0001943 (Hypoglycemia)
Hepatomegaly Clinical sign Infancy Variable Reported (2021+) HP:0002240 (Hepatomegaly)

Age of onset: neonatal to early-childhood; severity: severe for the lung phenotype, variable for immunodeficiency; progression: lung disease is progressive and irreversible once triggered.

Quality of life. Given infant/early-childhood lethality, the dominant QoL burden is respiratory failure requiring intensive care; formal QoL instruments (EQ-5D/SF-36) are not applicable/not reported.

Supporting evidence: "Four children from two unrelated kindreds died of severe pulmonary disease during infancy following viral pneumonia with evidence of combined T and B cell immunodeficiency" PMID: 27427983; "the immunological abnormalities can be limited and do not fully explain the LICS phenotype" PMID: 33741030.

Section 4 — Genetic / Molecular Information

Causal gene. NSMCE3 (NDNL2 / MAGEG1), OMIM 608243, cytogenetic locus 15q13, encoding a non-SMC element 3 subunit of the SMC5/6 complex. NSE3/MAGEG1 is the founding member of the MAGE (melanoma-associated antigen) protein family and partners with the NSE4 kleisin (EID-family) subunit PMID: 22536443.

Pathogenic variants.

Variant (cDNA) Protein Zygosity dbSNP / OMIM allele Functional consequence
c.790C>T p.Leu264Phe Homozygous (original + 2021 cohorts) rs199905054 / 608243.0001 Abolishes NSE3–NSE4 binding
c.626C>T p.Pro209Leu Compound-heterozygous NM_138704.3 / 608243.0002 Abolishes NSE3 binding to both NSE4 and NSE1

Variant classification: Pathogenic (ACMG/AMP) — biallelic, co-segregating, functionally validated. Variant type: missense. Allele frequency: rare; p.L264F (rs199905054) present at very low frequency in population databases, consistent with a recessive hypomorph. Origin: germline. Functional consequence: loss of function via complex destabilization — patient cells show undetectable NSE3 protein with secondarily decreased SMC5 and SMC6, increased genotoxin sensitivity, and defective homologous recombination; non-homologous end joining and V(D)J recombination are normal PMID: 27427983.

Chromosomal abnormalities (acquired, in patient cells). Karyotyping of peripheral lymphocytes showed multiple de novo supernumerary marker chromosomes, chromosome rearrangements, and micronuclei — hallmarks of constitutional chromosome instability rather than a single recurrent rearrangement.

Modifier genes / epigenetics. No validated modifiers or disease-specific epigenetic marks are reported. The variable immunophenotype hints at modifiers but none are mapped.

Complexopathy context. Three SMC5/6 subunits now map to distinct recessive disorders: NSMCE3 → LICS; NSMCE2 (MMS21) → primordial dwarfism/insulin resistance/gonadal failure PMID: 25105364; SMC5 (in-frame Arg372 deletion) → microcephalic primordial dwarfism with chromosomal instability and insulin resistance PMID: 36627765.

Section 5 — Environmental Information

Environmental factors. No chemical/radiation/occupational exposures are implicated in disease causation (the cause is genetic). However, patient cells are hypersensitive to genotoxic/replication-stress agents in vitro, meaning DNA-damaging exposures would be expected to be poorly tolerated.

Lifestyle factors. Not applicable (disease manifests in infancy/early childhood).

Infectious agents. Viral respiratory infection is the dominant disease trigger. Fatal pulmonary disease followed viral pneumonia in the index cases PMID: 27427983; multiple viruses have been implicated across cases. No single causative pathogen is required — the host SMC5/6 defect converts common viral infection into lethal immunopathology. Mechanistically relevant, SMC5/6 restricts episomal DNA viruses (e.g., HBV cccDNA) PMID: 42793236.

Section 6 — Mechanism / Pathophysiology

Ordered causal chain

  1. Biallelic NSMCE3 missense mutation (p.L264F and/or p.P209L) → results in loss/abolition of NSE3 binding to NSE4 (and NSE1 for P209L) within the NSE1/NSE3/NSE4 module PMID: 27427983.
  2. Loss of intra-module interactions → leads to destabilization of the whole SMC5/6 complex (undetectable NSE3; secondary loss of SMC5 and SMC6) PMID: 27427983.
  3. Because the NSE1/NSE3/NSE4 module binds double-stranded DNA and is required for chromatin loading, complex destabilization → leads to reduced SMC5/6 association with chromatin PMID: 26446992.
  4. Impaired SMC5/6 chromatin function → results in defective homologous recombination and reduced tolerance of replication stress / DNA damage PMID: 27427983.
  5. Defective HR/replication-fork maintenance → leads to chromosome rearrangements, supernumerary marker chromosomes, and micronuclei (constitutional chromosome instability) PMID: 27427983.
  6. Branch A (immune): chromosome instability + loss of SMC5/6 → results in impaired lymphocyte development/function → T-cell–predominant combined immunodeficiency (variable) PMID: 27427983.
  7. Branch B (shared apoptosis): SMC5/6 dysfunction → triggers p53-dependent apoptosis (demonstrated for SMC5 depletion in zebrafish, rescued by p53 co-deletion) PMID: 36627765 — likely contributing to cell loss across tissues.
  8. Trigger: viral respiratory infection in this sensitized host → leads to diffuse alveolar damage / PARDS.
  9. Diffuse alveolar damage with hyaline membranes, alveolar septal thickening → progresses to early interstitial fibrosis and metaplastic squamous alveolar epithelium (autopsy findings) → results in irreversible respiratory failure and death PMID: 33741030.
  10. Inferred amplifying branch (not demonstrated in LICS): micronuclei/chromatin bridges → cytosolic DNA → cGAS → 2′3′-cGAMP → STING activation → chronic type-I-interferon/pro-inflammatory signalling → fibrotic remodeling of lung PMID: 36876871; PMID: 42559700. Caveat: a single-cell cGAMP-reporter study found genotoxic-stress cGAS activation is a rare event not driven primarily by micronuclei enrichment PMID: 42629426.

Mechanistic detail by category

Section 7 — Anatomical Structures Affected

Section 8 — Temporal Development

Section 9 — Inheritance and Population

Section 10 — Diagnostics

Cytogenetics / chromosome breakage testing. Karyotype of peripheral lymphocytes reveals chromosome rearrangements, supernumerary marker chromosomes, and increased micronuclei; cells show hypersensitivity to replication stress/DNA-damaging agents and defective homologous recombination (with normal NHEJ and V(D)J recombination) PMID: 27427983. Micronucleus/chromosome-breakage assays and genotoxin-sensitivity testing are informative.

Immunologic work-up. Decreased T-cell numbers, decreased T-cell proliferative responses, and impaired antibody responses with normal B-lymphocyte numbers — a combined, T-cell-predominant immunodeficiency; may be limited in some patients PMID: 27427983; PMID: 33741030.

Laboratory: Hypoglycemia reported in later cases; monitor hepatic status (hepatomegaly).

Imaging / histopathology. Chest imaging shows diffuse interstitial/alveolar disease (ARDS/PARDS). Lung histopathology: classic diffuse alveolar damage with hyaline membranes, extensive alveolar septal thickening, early interstitial fibrosis, and metaplastic squamous alveolar epithelium — without evidence of ongoing infection at autopsy PMID: 33741030.

Genetic testing (definitive). Whole-exome (or whole-genome) sequencing established the diagnosis in the index families PMID: 27427983. Recommended approach: WES/WGS or a chromosome-instability/primary-immunodeficiency gene panel including NSMCE3; confirmatory single-gene NSMCE3 sequencing for known familial variants; functional confirmation (SMC5/6 protein levels, HR assays). Chromosomal microarray/karyotype supports the breakage phenotype but does not identify the point mutation.

Clinical criteria / differential diagnosis. No formal consensus criteria exist. Differentials: other chromosome-breakage/instability syndromes — Nijmegen breakage syndrome, ataxia-telangiectasia, Fanconi anemia, Bloom syndrome, ICF syndrome — and SMC5/6 complexopathies (NSMCE2, SMC5 — which instead present with microcephalic primordial dwarfism and insulin resistance, distinguishing them from LICS). The distinctive LICS combination is lethal viral-triggered lung disease + chromosome breakage + variable combined immunodeficiency.

Screening. Carrier/cascade testing for known familial NSMCE3 variants; prenatal/preimplantation genetic testing feasible in families with an identified genotype.

Section 11 — Outcome / Prognosis

Section 12 — Treatment

No curative therapy exists. Care is supportive and largely ineffective at reversing the lung disease.

Section 13 — Prevention

Section 14 — Other Species / Natural Disease

Section 15 — Model Organisms

Mechanistic Model / Interpretation

 Biallelic NSMCE3 missense (p.L264F / p.P209L)
        |  abolishes NSE3-NSE4 (+/-NSE1) binding
        v
 SMC5/6 complex DESTABILIZED (down NSE3, SMC5, SMC6)
        |  NSE1/NSE3/NSE4 module normally binds dsDNA & loads complex
        v
 reduced SMC5/6 chromatin loading  ->  defective HR + replication-stress intolerance
        |
        v
 Constitutional CHROMOSOME INSTABILITY (rearrangements, marker chromosomes, micronuclei)
        |---------------+----------------------------+
        v               v                            v
 p53-dependent    Combined (T-cell-pred.)     [INFERRED] micronuclei -> cGAS-STING
 apoptosis        immunodeficiency (variable)  -> type-I IFN -> pro-fibrotic signalling
        |               |                            |
        +---------------+-------------+--------------+
                                      v
            VIRAL RESPIRATORY INFECTION (trigger; SMC5/6 is antiviral RF)
                                      v
            Diffuse alveolar damage / PARDS -> interstitial fibrosis
                                      v
            Irreversible respiratory failure -> death (infancy-early childhood)

The unifying interpretation is that a scaffolding/assembly loss-of-function in SMC5/6, driven by NSE3 destabilization, produces genome instability that is cell-intrinsic across tissues. Two phenotypic domains (immunodeficiency and chromosome breakage) are directly explained by impaired HR/genome maintenance in dividing cells. The lethal lung phenotype is only partially explained by the immunodeficiency (it is not corrected by HSCT, and immunodeficiency can be clinically limited), implying an additional cell-intrinsic lung-epithelial/fibrotic mechanism — for which the micronuclei → cGAS-STING → type-I-IFN → fibrosis axis is an attractive but unproven, inferred candidate. Viral infection is the universal clinical trigger, consistent with SMC5/6's dual role as both genome guardian and antiviral restriction factor.

Evidence Base

PMID Title (abbrev.) Role in this report
27427983 Destabilized SMC5/6 complex leads to chromosome breakage syndrome with severe lung disease Primary/defining paper — identifies biallelic NSMCE3 mutations, complex destabilization, clinical triad, cellular phenotypes
33741030 New insights in phenotype and treatment of LICS Follow-up cohort (n=5): variable immunodeficiency, lung histopathology (DAD/fibrosis), HSCT failure, survival to 6.5 y
40728043 Fatal lung disease caused by NSMCE3 mutation in an 11-year-old boy Extends age range of fatal disease to 11 years
26446992 Chromatin association of SMC5/6 depends on NSE3 binding to DNA Mechanistic link — NSE3 DNA-binding module required for chromatin loading; mutants hypersensitive
22536443 Analysis of the Nse3/MAGE-binding domain of Nse4/EID proteins NSE3/MAGEG1 identity and NSE4-kleisin partnership
36627765 In-frame deletion of SMC5 — primordial dwarfism, instability, insulin resistance Complexopathy spectrum; p53-dependent apoptosis in zebrafish
25105364 Hypomorphism in human NSMCE2 — primordial dwarfism and insulin resistance Complexopathy spectrum (differential diagnosis)
42793236 The Multifunctional SMC5/6 Complex...Antiviral Restriction, and Human Disease SMC5/6 as broad-spectrum antiviral restriction factor
41825673 Mechanism insights into Smc5/6 in HBV inhibition NSE1/NSE3 subcomplex suffices for transcriptional silencing
36592669 Critical HBx residues for Smc5/6 interaction Viral evasion of SMC5/6 (host–virus axis)
36876871 cGAS-STING signalling in cancer / chromosomal instability Supports inferred CIN→micronuclei→cGAS-STING axis
42559700 Hyper-aging in systemic sclerosis — genome instability, inflammaging, fibrosis Links cGAS-STING/type-I-IFN to lung fibrosis (inferred branch)
42629426 cGAMP reporter — cGAS activation by DNA damage Counter-evidence tempering the inferred micronuclei→cGAS branch

Limitations and Knowledge Gaps

  1. Extreme rarity. Conclusions rest on ~a dozen patients across a few kindreds; no prevalence/incidence, penetrance, or carrier-frequency estimates are statistically robust.
  2. Lung mechanism unresolved. The cell-intrinsic driver of lethal lung disease is not established. HSCT failure and limited immunodeficiency in some patients show the immune defect cannot fully explain the pulmonary phenotype.
  3. cGAS-STING branch is inferred, not demonstrated. No study has shown cGAS-STING/type-I-IFN activation in LICS lung tissue, and single-cell data caution that micronuclei may not be the dominant cGAS activator PMID: 42629426.
  4. No animal model of the lung phenotype. Existing yeast/zebrafish/cell models capture genome instability and apoptosis but not viral-triggered lung immunopathology.
  5. No modifiers mapped despite clear variable expressivity of immunodeficiency.
  6. No therapeutics. All attempted treatments (corticosteroids, HSCT, supportive care) have failed to alter the lethal course.

Proposed Follow-up Experiments / Actions

  1. Generate a conditional/humanized Nsmce3 (p.L264F) mouse and challenge with respiratory virus to test whether lung epithelial SMC5/6 loss + viral trigger recapitulates diffuse alveolar damage/fibrosis.
  2. Directly test the cGAS-STING branch in LICS tissue: measure cytosolic DNA, cGAMP, STING pathway activation, and type-I-IFN signatures in patient-derived lung organoids/airway epithelium and autopsy tissue; test STING/IFN inhibition for anti-fibrotic effect.
  3. Dissect cell-intrinsic lung epithelial vulnerability using patient iPSC-derived alveolar type II cells under replication stress and viral infection.
  4. Map modifiers of immunophenotype via deeper genomic/transcriptomic profiling across the (small) patient set and unaffected carriers.
  5. Define the antiviral-restriction contribution of NSE3 loss to viral control in respiratory-relevant cells, linking SMC5/6's dual genome-guardian/antiviral roles to the infection trigger.
  6. Establish an international LICS registry to improve natural-history, prognosis, penetrance, and genotype–phenotype data.

Evidence source legend: findings from PMID: 27427983 and PMID: 33741030 are human clinical; PMID: 26446992, PMID: 41825673, and PMID: 22536443 are in vitro / model organism (yeast); PMID: 36627765 includes zebrafish model data; the cGAS-STING/fibrosis links (PMID: 36876871, PMID: 42559700, PMID: 42629426) are extrapolated from other disease contexts and flagged as inferred.