Immunodeficiency 122 (IMD122) — Comprehensive Disease Characterization

Disease: Immunodeficiency 122 (IMD122) Causal gene: POLD3 (DNA polymerase delta 3, accessory subunit) Identifiers: OMIM:620869 · MONDO:0971151 · DOID:0061088 · MedGen C5935632 (CUI) · UMLS C5935632 Evidence base: Ultra-rare Mendelian disorder described in only two unrelated patients (2023). All clinical content is derived from individual patient reports (case-level), not aggregated registries, plus in vitro functional and mouse model data.

Summary (Answer to the Research Question)

Immunodeficiency 122 is an ultra-rare autosomal-recessive syndromic combined immunodeficiency caused by biallelic hypomorphic missense variants in POLD3, which encodes the p66/p68 accessory, complex-stabilizing subunit of DNA polymerase δ (and a shared subunit of translesion polymerase ζ). Reduced polymerase δ function produces replicative stress, defective S-phase progression, and accumulation of DNA double-strand breaks, which impair proliferation and V(D)J/TCR recombination in lymphoid progenitors—producing severe combined immunodeficiency (SCID), frequently presenting as Omenn syndrome—and injure other highly proliferative lineages, causing neurodevelopmental delay, sensorineural hearing loss, and ectodermal anomalies. Onset is neonatal/congenital with a severe course; hematopoietic stem cell transplantation can reconstitute immunity but does not correct the cell-intrinsic replication defect in non-hematopoietic tissues.

1. Disease Information

• Overview: IMD122 is a monogenic inborn error of immunity in the category of combined immunodeficiencies with syndromic/associated features (per IUIS classification). It combines a profound T-cell (± B/NK) defect with neurodevelopmental, auditory, and ectodermal manifestations due to a defect in genome replication/maintenance.
• Key identifiers:
• OMIM phenotype: 620869 (Immunodeficiency 122)
• MONDO: MONDO:0971151
• DOID: 0061088; MedGen/UMLS: C5935632
• Orphanet: no dedicated code identified (newly described; would fall under "Combined immunodeficiency")
• ICD-11: maps to 4A00 (Primary immunodeficiencies) / ICD-10 D81.x (combined immunodeficiencies)
• MeSH: no specific descriptor; closest = Severe Combined Immunodeficiency (D016511); phenotype also = Omenn syndrome
• Synonyms / alternative names: IMD122; POLD3 deficiency; DNA polymerase delta 3 deficiency; syndromic SCID due to POLD3; autosomal-recessive polymerase δ (PolD) deficiency (POLD3 type); POLD3-related Omenn syndrome.
• Evidence source: Individual-patient (case report) level — two probands total (37030525; 38099988).

2. Etiology

• Causal factor (genetic): Biallelic (homozygous) pathogenic missense variants in POLD3 (AR). Both reported patients were offspring of consanguineous unions.
• "Here we report a homozygous POLD3 variant (NM_006591.3; p.Ile10Thr) in a Lebanese patient, the product of a consanguineous family, presenting with a syndromic severe combined immunodeficiency (SCID)" (37030525).
• Genetic risk factors: The causal variants themselves (p.Ile10Thr; p.K373T). No susceptibility loci or modifier genes identified (too few cases). Consanguinity is the principal risk context. POLD3 is strongly LoF-intolerant (gnomAD pLI 0.994, LOEUF 0.48), so only hypomorphic missense alleles—retaining partial function—cause viable disease; complete LoF is presumed lethal (mouse null embryonic-lethal, 29447390).
• Environmental risk factors: None established; disease is fully genetically determined. Infectious exposures are consequences (opportunistic infection due to immunodeficiency), not causes.
• Protective factors: None reported (genetic or environmental). Not applicable at the population level given rarity.
• Gene–environment interactions: Not characterized. Mechanistically, exogenous genotoxic/replication stressors (UV, chemotherapeutics, oxidative stress) would be predicted to aggravate the underlying replication defect (inferred, not demonstrated).

3. Phenotypes

Onset neonatal/congenital, severity severe, course progressive. Frequencies are qualitative (n=2). HPO terms derived from OMIM:620869 annotation.

Immune / laboratory abnormalities: - Severe combined immunodeficiency — HP:0000958 - Decreased total T cells — HP:0500093; decreased naive CD4+ (HP:0001744) and naive CD8+ (HP:0005359) proportions; oligoclonal T-cell expansion — HP:0010975 - Reduced NK cell count — HP:0000407; abnormal B-cell count — HP:0410243; decreased unswitched memory B cells — HP:0040218 - Decreased IgG — HP:0001019; abnormal IgM — HP:0004315; increased IgE — HP:0002716; eosinophilia — HP:0001999 - Thymic aplasia/hypoplasia — HP:0004430

Omenn-syndrome features (clinical signs/physical): - Erythroderma — HP:0410377; alopecia — HP:0008404; lymphadenopathy — HP:0002110; hepatomegaly — HP:0004429; splenomegaly — HP:0002783

Infections (symptoms): recurrent viral — HP:0002788; bacterial — HP:0001270; upper (HP:0001263) and lower (HP:0032126) respiratory infections; bronchiectasis — HP:0002007

Neurodevelopmental (behavioral/developmental): global developmental delay — HP:0001596; motor delay — HP:0002718; (progressive neurological regression in the Omenn patient)

Sensory: sensorineural hearing impairment — HP:0001880

Craniofacial / ectodermal (physical): frontal bossing — HP:0003212; abnormal facial shape — HP:0410378; enamel hypoplasia — HP:0011968; nail dystrophy — HP:0031430; dry skin — HP:0005403

Feeding/allergy: feeding difficulties — HP:0002240; food allergy — HP:0006297

Quality-of-life impact: profound—life-threatening infections, failure to thrive, developmental disability, deafness; without HSCT, SCID is uniformly fatal in infancy. Formal QoL instruments (EQ-5D/SF-36) not applicable/available.

4. Genetic / Molecular Information

• Causal gene: POLD3 — HGNC:20932; NCBI Gene 10714; Ensembl ENSG00000077514; cytoband 11q13.4 (GRCh38 chr11:74,493,851–74,669,117, + strand); transcript NM_006591.3; protein UniProt Q15054 (466 aa, "DNA polymerase delta subunit 3", p66/p68).
• Pathogenic variants (both germline, homozygous, missense): | Variant (cDNA/protein) | Patient | Classification | Consequence | |---|---|---|---| | NM_006591.3:c.29T>C, p.Ile10Thr | Lebanese, syndromic SCID (37030525) | Pathogenic (AR) | Abolishes POLD3 and POLD1/POLD2 protein expression → complex destabilization | | c.1118A>C, p.Lys373Thr (p.K373T) | Omenn syndrome (38099988) | Pathogenic (AR) | Normal PolD subunit levels but defective function → S-phase defect, ↑dsDNA breaks (rescued by WT POLD3) |
• Variant type/class: missense (both). No nonsense/frameshift/splice/structural variants reported (consistent with LoF intolerance).
• Allele frequency: both variants are private/ultra-rare, essentially absent from gnomAD; gene-level LoF constraint pLI 0.994, LOEUF 0.48, LoF o/e 0.33 (missense unconstrained, mis_z 0.76).
• Somatic vs germline: germline.
• Functional consequence: loss/reduction of function (hypomorphic). p.Ile10Thr → destabilization of the entire Pol δ heterocomplex; p.K373T → intrinsic functional impairment with preserved assembly.
• Modifier genes: none identified.
• Epigenetic information: none reported for this disorder.
• Chromosomal abnormalities: none (point mutations only). Downstream, POLD3 deficiency causes acquired genomic instability (chromosome breaks, micronuclei, aneuploidy — shown in mouse/cell models, 29447390).

5. Environmental Information

• Environmental factors: none causative. IMD122 is a monogenic disorder.
• Lifestyle factors: not applicable (congenital, infantile-lethal).
• Infectious agents: not causal but central to morbidity—patients suffer opportunistic and recurrent viral, bacterial, and respiratory infections secondary to the immunodeficiency (HP:0002788/HP:0001270/HP:0032126). Typical SCID pathogens (inferred from SCID literature): Pneumocystis jirovecii, CMV, adenovirus, RSV, candida; live-vaccine organisms (BCG, rotavirus) are contraindicated.

6. Mechanism / Pathophysiology

Ordered causal chain (initiating lesion → clinical manifestation):

1. Biallelic hypomorphic missense variant in POLD3 (p.Ile10Thr or p.K373T) → reduced functional DNA polymerase δ, either by destabilizing/degrading the POLD1–POLD2–POLD3(–POLD4) complex (p.Ile10Thr abolishes POLD1/POLD2/POLD3 expression) or by impairing complex function with preserved subunit levels (p.K373T). (Demonstrated in vitro.)
2. Reduced Pol δ activity → impaired high-fidelity DNA replication (leading- and lagging-strand synthesis, loss of PCNA-stimulated processivity) and reduced DNA repair capacity. (Demonstrated — UniProt-annotated functions; Conde POLD1/POLD2 data 31449058.)
3. → Replicative stress: defective S-phase entry/progression and accumulation of DNA double-strand breaks (γH2AX foci); the defect is rescued by wild-type POLD3, establishing causality. (Demonstrated — 38099988; analogous mouse data 29447390: extended S-phase, telomere loss, chromosome breaks, micronuclei, aneuploidy, DDR via ATR/ATM/53BP1/RIF1.) 4a. Immune branch: replication stress plus a defect in the early stages of TCR recombination → failure of thymocyte proliferation/V(D)J recombination → profound naive T-cell lymphopenia with restricted/oligoclonal TCR repertoire → SCID; residual autoreactive oligoclonal T-cell expansion with Th2 skewing → Omenn syndrome (erythroderma, eosinophilia, high IgE, hepatosplenomegaly, lymphadenopathy). (Demonstrated — 38099988.) 4b. Neurodevelopmental branch: replication stress in neural progenitors → impaired neurogenesis → global developmental delay / progressive neurological regression. (Inferred by analogy to POLD1/POLD2; 31449058.) 4c. Auditory/ectodermal branch: replication stress in cochlear and ectodermal (skin, hair, nail, dental enamel) progenitors → sensorineural hearing loss and ectodermal dysplasia-like features. (Inferred.)
4. → Clinical syndrome: neonatal-onset syndromic SCID/Omenn with neurodevelopmental delay, deafness, and ectodermal/craniofacial anomalies; death in infancy/early childhood without—and sometimes despite—HSCT.

Supporting detail: - Molecular pathways/processes: DNA replication (GO:0006260), DNA repair / double-strand-break repair (GO:0006302), replication-stress / DNA-damage response via ATR–ATM–53BP1–RIF1, cell-cycle S-phase (GO:0000082), V(D)J recombination (GO:0033151), T-cell differentiation (GO:0030217), apoptosis (GO:0006915). Reactome: "DNA strand elongation," "Polymerase switching," "Lagging strand synthesis." KEGG: DNA replication (hsa03030), Base/Nucleotide excision repair, Mismatch repair. - Protein dysfunction: loss/reduction of function of the Pol δ accessory subunit; POLD3 also co-forms DNA polymerase ζ (translesion synthesis, with REV3L/REV7) and participates in break-induced replication/homologous recombination—so its loss broadly compromises genome maintenance. - Cellular processes: cell-cycle dysregulation (extended/arrested S phase), genomic instability, apoptosis of stressed progenitors. - Immune involvement: this is a primary immunodeficiency (combined T/B/NK), not autoimmunity per se—though Omenn physiology produces immune dysregulation/Th2 inflammation. - Cell types (CL): T cells (CL:0000084), naive T cell (CL:0000898), thymocyte (CL:0000893), double-positive thymocyte (CL:0000809), B cell (CL:0000236), natural killer cell (CL:0000623), hematopoietic stem cell (CL:0000037), neural progenitor/neuron (CL:0000031/CL:0000540), keratinocyte (CL:0000312). - Molecular profiling: no transcriptomic/proteomic/metabolomic datasets published for POLD3 patients; functional readouts to date are flow-cytometric immunophenotyping, TCR-repertoire sequencing, cell-cycle/EdU S-phase assays, and γH2AX DSB-foci quantification.

7. Anatomical Structures Affected

• Organ level (primary): thymus (aplasia/hypoplasia, UBERON:0002370), bone marrow / hematopoietic system (UBERON:0002371), lymph nodes / spleen (lymphadenopathy, splenomegaly; UBERON:0000029/UBERON:0002106), brain / CNS (UBERON:0000955), inner ear / cochlea (UBERON:0001846/UBERON:0001844).
• Secondary involvement: lungs/airways (recurrent infection, bronchiectasis; UBERON:0002048), liver (hepatomegaly; UBERON:0002107), skin (erythroderma, dryness; UBERON:0002097), hair follicles (alopecia), nails, teeth/enamel.
• Body systems: immune/lymphatic, nervous, auditory, integumentary, respiratory (secondary), digestive (feeding difficulties).
• Tissue/cell level: lymphoid tissue and lymphocyte progenitors; neural tissue; cochlear/sensory epithelium; ectodermal epithelia (epidermis, hair, nail matrix, ameloblasts).
• Subcellular (GO Cellular Component): nucleus (GO:0005634), delta DNA polymerase complex (GO:0043625), zeta DNA polymerase complex (GO:0016035), replication fork (GO:0005657). POLD3 localizes to nucleus and cytoplasm (UniProt Q15054).
• Lateralization: systemic/bilateral (e.g., bilateral sensorineural hearing loss); not lateralized.

8. Temporal Development

• Onset: congenital/neonatal; immunologic and syndromic features manifest in early infancy. Onset pattern acute to subacute for infections (SCID presentation), with an insidious-progressive neurodevelopmental component.
• Progression: rapid for the immune defect; progressive neurological regression (documented post-HSCT in the Omenn patient). Disease course is progressive, not relapsing/episodic.
• Duration/stages: without curative therapy, SCID is fatal in the first 1–2 years; with HSCT, immune reconstitution is possible but non-immune (neuro/auditory) disease continues to progress → death reported at age 4 years despite HSCT at 6 months (38099988).
• Remission patterns: no spontaneous remission; partial, compartment-specific improvement with HSCT (immune only).
• Critical period / window of intervention: the neonatal–early-infancy window (ideally pre-infection HSCT, <3.5 months as in classic SCID) is critical for the immune compartment; however, the replication defect in CNS/cochlea likely has a developmental origin that transplantation cannot reverse.

9. Inheritance and Population

• Inheritance: Autosomal recessive (biallelic/homozygous). Both reported families consanguineous.
• Penetrance: presumed complete for biallelic pathogenic genotypes (n=2); expressivity variable between the two reports (classic SCID vs. Omenn presentation).
• Anticipation: not applicable (not a repeat-expansion disorder). Germline mosaicism: not reported.
• Founder effects / variant geography: both variants are private (one Lebanese, one in a separate pedigree reported from Europe); no founder haplotype established. Consanguinity (e.g., Middle Eastern/North African populations) increases the chance of homozygosity.
• Carrier frequency: not established; both alleles are essentially absent from gnomAD, so heterozygous carriers are vanishingly rare.
• Epidemiology: ultra-rare—only two patients reported worldwide (2023). Prevalence/incidence not quantifiable (far below 1/1,000,000). No sex predilection established (both reported probands male, but n too small to infer a sex ratio); AR inheritance predicts equal sex distribution. Age distribution: infants/young children.

10. Diagnostics

• Laboratory tests:
• Lymphocyte subsets by flow cytometry: low total/naive T cells (low TRECs), variable B and low NK cells; reduced naive CD4+/CD8+.
• Immunoglobulins: low IgG, abnormal IgM, elevated IgE; eosinophilia (CBC with differential).
• TCR-repertoire analysis: restricted/oligoclonal repertoire; evidence of defective early TCR recombination.
• Biomarkers: no specific circulating biomarker; cellular replication-stress markers (γH2AX DSB foci; defective EdU/S-phase incorporation in patient fibroblasts) are diagnostic functional supports (38099988).
• Imaging / functional: absent thymic shadow on chest imaging; audiometry/ABR for sensorineural hearing loss; brain MRI for neurodevelopmental assessment; chest CT for bronchiectasis.
• Biopsy/pathology: Omenn skin shows erythroderma; lymphoid tissue shows oligoclonal T-cell infiltration (consistent with Omenn; general).
• Genetic testing (diagnostic gold standard):
• Whole-exome (WES) or whole-genome (WGS) sequencing identified both cases; IEI/SCID/combined-immunodeficiency NGS gene panels should include POLD1, POLD2, POLD3.
• Single-gene/targeted confirmation and cascade/segregation testing in families; homozygosity mapping is useful in consanguineous pedigrees.
• CMA/karyotype/FISH/mtDNA/repeat-expansion testing: not indicated (point-mutation disorder).
• Clinical criteria / classification: meets SCID / Omenn syndrome diagnostic criteria (profound T-cell lymphopenia ± erythroderma, eosinophilia, high IgE, oligoclonal T cells); classified under IUIS "combined immunodeficiencies with associated/syndromic features."
• Differential diagnosis: other SCID/Omenn genes—RAG1/RAG2, DCLRE1C (Artemis), LIG4, NHEJ1, DNA-PKcs/PRKDC, IL2RG, JAK3, IL7R, ADA, RMRP (cartilage-hair hypoplasia); and the related POLD1- and POLD2-deficiency syndromes (replication-stress CID; 31449058). Distinguishing features: POLD3/POLD1/POLD2 disease uniquely couples SCID with replicative stress + neurodevelopmental delay + sensorineural hearing loss + ectodermal signs. Also distinguish POLD1 gain-of-function/MDPL (progeroid, mandibular hypoplasia, lipodystrophy) and POLD1/POLD2 proofreading-associated polyposis/cancer, which are mechanistically different and not part of IMD122.
• Screening: detectable by newborn SCID screening (TREC assay) given profound T-cell lymphopenia; carrier/prenatal/PGT testing feasible once familial variant known.

11. Outcome / Prognosis

• Survival/mortality: poor. SCID is fatal in infancy without HSCT; even with HSCT at 6 months, one patient died at age 4 years from progressive neurological regression (38099988). Disease-specific mortality is high.
• Morbidity/disability: severe—recurrent/opportunistic infections, failure to thrive, developmental disability, sensorineural deafness, bronchiectasis (chronic lung damage).
• Recovery potential: HSCT can reconstitute immunity (hematopoietic compartment) but does not correct the cell-intrinsic replication defect in CNS/cochlea/ectoderm, so neurodevelopmental and auditory outcomes remain poor.
• Prognostic factors: age at HSCT, pre-transplant infection/organ damage, and—unique to this disorder—severity of the non-hematopoietic replication defect (neuro/auditory), which transplantation cannot rescue.
• Prognostic biomarkers: degree of replication stress / genomic instability (γH2AX, S-phase defect) is a mechanistic correlate of severity (inferred).

12. Treatment

(No disease-specific approved therapy exists; management follows SCID/Omenn principles.)

• Definitive: Allogeneic hematopoietic stem cell transplantation (HSCT) — NCIT:C15431 (Hematopoietic Stem Cell Transplantation) / NCIT:C105867 (Allogeneic HSCT). Reconstitutes the immune system; performed at 6 months in the reported Omenn patient (38099988). Does not address non-immune manifestations.
• Supportive / prophylactic (standard SCID care, NCIT terms):
• Immunoglobulin replacement therapy (IVIG/SCIG) — NCIT:C603 (Immunoglobulin Therapy).
• Antimicrobial prophylaxis — anti-Pneumocystis (co-trimoxazole), antifungal, antiviral prophylaxis.
• Irradiated, CMV-safe, leukodepleted blood products; avoidance of live vaccines.
• Nutritional support for feeding difficulties; management of erythroderma; immunosuppression (e.g., steroids/cyclosporine) to control Omenn immune dysregulation pre-HSCT.
• Rehabilitation: hearing aids/cochlear implantation, early developmental/physical/speech therapy, audiology follow-up.
• Gene therapy / RNA / targeted / immunotherapy: none available or in trials for POLD3 deficiency. In vitro, WT POLD3 restoration rescues the cellular phenotype (38099988), providing proof-of-concept that gene correction could address the cell-intrinsic defect—experimental/hypothetical only.
• Pharmacogenomics: none specific; standard transplant-conditioning pharmacogenetics (e.g., busulfan, thiopurines) apply generically. Caution with genotoxic conditioning/chemotherapy given underlying DNA-repair/replication fragility (inferred; analogous to other DNA-repair SCIDs such as Artemis/LIG4, where reduced-toxicity conditioning is preferred).
• Clinical trials (NCT): none identified specific to POLD3/IMD122.

13. Prevention

• Primary prevention: not preventable at the individual level (monogenic, congenital). Genetic counseling for consanguineous couples and families with an affected child (25% recurrence risk per pregnancy).
• Secondary prevention / early detection: newborn SCID screening (TREC) enables presymptomatic detection and early pre-infection HSCT, the key opportunity to improve immune outcomes. Cascade testing of at-risk relatives.
• Genetic/reproductive prevention: carrier screening, prenatal diagnosis, and preimplantation genetic testing (PGT-M) once the familial variant is known.
• Tertiary prevention: infection prophylaxis, avoidance of live vaccines and non-irradiated blood products, early audiologic and developmental intervention, pulmonary care to limit bronchiectasis progression.
• Immunization/public-health/environmental measures: not applicable beyond standard infection-control for immunocompromised infants.

14. Other Species / Natural Disease

• Taxonomy / orthologs: POLD3 is conserved across eukaryotes. Mouse Pold3 (NCBI Gene 69745; Mus musculus, NCBI:txid10090); orthologs in rat, zebrafish; the yeast functional homolog is POL32 (Saccharomyces cerevisiae). POLD3/POL32 is part of Pol δ and Pol ζ across species.
• Natural disease in other species: no naturally occurring POLD3 disease reported in companion animals or wildlife (OMIA: none identified). Not a zoonosis; not transmissible.
• Comparative biology: the replication/genome-maintenance function of POLD3/POL32 is deeply evolutionarily conserved, making model systems highly informative. Complete loss is lethal from yeast to mouse, underscoring essentiality.
• Veterinary relevance: none established.

15. Model Organisms

• Mouse (Mus musculus, Pold3):
• Complete knockout (Pold3⁻/⁻) is embryonic lethal at E6.5 (29447390). "complete loss of Pold3 (Pold3-/-) resulted in early embryonic lethality at E6.5."
• Inducible KO / blastocyst outgrowth / ESCs: rapid DNA-damage response, massive apoptosis, telomere loss, chromosome breaks, extended S phase, replicative stress, micronucleation, aneuploidy; Pold3 acts via 53BP1, RIF1, ATR, ATM.
• Pold3⁺/⁻ (haploinsufficient) mice develop age-dependent impaired DSB repair, telomere shortening and chromosome breaks (spermatocytes).
• Phenotype recapitulation: strongly models the cell-intrinsic replication-stress/genomic-instability mechanism of human disease; limitation: null is embryonic-lethal, so it cannot model the viable, organ-specific human syndrome—hypomorphic knock-in alleles (e.g., patient-equivalent) would be needed to recapitulate SCID + neuro/auditory features.
• Cellular / in vitro models: patient-derived fibroblasts (S-phase defect, γH2AX DSB foci, rescued by WT POLD3 transduction — 38099988); patient immune cells (TCR-repertoire/recombination assays). Yeast pol32Δ is a classic genetic model of the accessory subunit.
• Other organisms: chicken DT40 and yeast systems have historically defined POLD3/POL32 roles in replication, translesion synthesis, and break-induced replication (background literature).
• Resources: MGI (Pold3), IMPC/IMSR, Alliance of Genome Resources, SGD (POL32), Cellosaurus (patient fibroblast lines).

Supported vs. Refuted Hypotheses

Supported: - IMD122 is caused by autosomal-recessive, biallelic hypomorphic POLD3 missense variants (37030525; 38099988). - The mechanism is reduced Pol δ function → replicative stress / S-phase defect / DSB accumulation → impaired lymphopoiesis and TCR recombination → SCID/Omenn, with parallel injury to neural, cochlear, and ectodermal lineages (38099988; 29447390; 31449058). - POLD3 is essential and LoF-intolerant; only hypomorphic alleles are viable (gnomAD; mouse null lethality).

Refuted / excluded: - Not caused by environmental, infectious, or acquired/somatic factors; infections are a consequence. - Not a chromosomal/structural or repeat-expansion disorder. - IMD122 is distinct from POLD1 gain-of-function MDPL progeroid syndrome and from POLD1/POLD2 proofreading-associated polyposis/cancer.

Limitations and Future Directions

• n=2 patients: frequencies, penetrance range, sex ratio, and full phenotypic spectrum are provisional; natural-history data are absent.
• No omics (transcriptomic/proteomic/metabolomic) profiling, no registry, no clinical trials.
• Unresolved: why certain tissues (cochlea, specific neural populations) are selectively vulnerable; whether reduced-toxicity HSCT conditioning improves outcomes; whether early gene-correction could rescue non-immune disease.
• Future: patient-equivalent hypomorphic knock-in mouse models; iPSC-derived neural/cochlear organoids; identification of additional patients via GeneMatcher to define the phenotypic spectrum and genotype–phenotype correlations.

Key References

• 37030525 — Mehawej et al., 2023. POLD3 deficiency is associated with severe combined immunodeficiency, neurodevelopmental delay, and hearing impairment. (First case; p.Ile10Thr.)
• 38099988 — Riestra et al., 2023. Human Autosomal Recessive DNA Polymerase Delta 3 Deficiency Presenting as Omenn Syndrome. (Second case; p.K373T; functional S-phase/DSB rescue.)
• 31449058 — Conde et al., 2019 (J Clin Invest). Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress. (POLD1/POLD2 parent syndrome.)
• 29447390 — Zhou et al., 2018. Pold3 is required for genomic stability and telomere integrity in embryonic stem cells and meiosis. (Mouse model.)
• Database sources: OMIM 620869; MONDO:0971151 (Monarch); UniProt Q15054; gnomAD (POLD3 constraint); HPO annotations (OMIM 620869).