🏷

Classifications

Harrison's Part

👪

Inheritance

1

Autosomal recessive inheritance HP:0000007

The single patient is homozygous and the child of consanguineous parents, and the variant segregates with autosomal recessive inheritance in the family. The population genetics agree: POLD2 has a low pLI and a high pRec, so loss of one allele is predicted to be tolerated and loss of both is not.

Autosomal recessive inheritance

Show evidence (2 references)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Genetic analysis of P1 by exome sequencing revealed a homozygous missense variant (p.Asp293Asn; combined annotation–dependent depletion [CADD] score of 28.1) in the POLD2 gene (NM_006230), which displayed perfect segregation under the assumption of autosomal-recessive inheritance"

Homozygosity and family segregation consistent with recessive inheritance in the one published POLD2 family.

PMID:31449058 SUPPORT INDIRECT PRIMARY RESULT Computational

"Both genes show low pLI scores of 0.001 and 0.044, respectively, indicating a predicted tolerance to the loss of a single allele, and high pRec scores of 0.998 and 0.950, respectively, indicating a predicted high intolerance for a complete loss of function"

The second value in each pair (pLI 0.044, pRec 0.950) is POLD2's. The constraint scores predict a recessive gene, which agrees with the observed inheritance. Graded COMPUTATIONAL because the claim rests on population-database constraint metrics, not on the family.

?

Discussions and Knowledge Gaps

2

Do the short stature and severe intellectual disability belong to POLD2 deficiency itself, or do they need another explanation, given that the entity rests on one patient from a consanguineous family?

KNOWLEDGE GAP OPEN pold2_cid_single_patient_syndrome

Attached to

pathophysiology#Syndromic Growth and Neurodevelopmental Features phenotypes#Short Stature phenotypes#Severe Intellectual Disability

The immune half of the phenotype has functional support in the patient's own cells: reduced complex, slowed S phase, replication lesions and poor T-cell proliferation, all reversed by wild-type POLD2 where rescue was tested. The developmental half has none. No neural or growth tissue was studied. A consanguineous family carries other homozygous regions, any of which could hold a second recessive cause. The argument for attribution is that the POLD1 patient of the same series also has short stature and intellectual impairment, and that the POLD3 deficiency carries neurodevelopmental delay. A second POLD2 family, or a hypomorphic Pold2 animal model, is what would settle it.

Show evidence (1 reference)

PMID:31449058 SUPPORT INDIRECT PRIMARY RESULT In Vitro

"Our results suggest that loss of polymerase δ leads to replication stress–associated immunodeficiency and neurodevelopmental abnormalities in this syndrome."

The authors put the neurodevelopmental link as a suggestion. Their cellular data come from fibroblasts and lymphocytes.

Does POLD2 deficiency predispose to cancer, through replication-associated DNA damage or through the T-cell defect?

OPEN QUESTION OPEN pold2_cid_cancer_risk

Attached to

pathophysiology#Replication Stress and Replication-Associated DNA Lesions

The patient's cells carry steady-state DNA lesions, and T-cell immunodeficiency on its own raises cancer risk through weak antitumour and antiviral immunity. Against that, the patient had no malignancy at 17, and DNA repair after exogenous damage was preserved. The authors recommend close monitoring for neoplasia and say a reliable estimate needs larger cohorts and longer follow-up. With one patient, no estimate is possible.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"No malignant transformation has been detected in the patients described here"

The observed absence of malignancy, the only outcome data available.

⚙

Pathophysiology

7

Biallelic Hypomorphic POLD2 Variant

Mechanism confidence: Established

One genotype is published: homozygous missense p.Asp293Asn (NM_006230) in the POLD2 phosphodiesterase domain, absent from ExAC and 1000 Genomes. Structural modelling on the POLD2-POLD3 crystal structure predicts loss of a polar contact with Arg240. No null allele has been seen, and none is expected: POLD2 is essential in yeast and in mammalian cell screens, so a viable patient's allele must be hypomorphic.

POLD2 hgnc:9176

Genetic context POLD2 hgnc:9176 allele_type: missense variant (p.Asp293Asn) in the phosphodiesterase domain variant_origin: GERMLINE zygosity: HOMOZYGOUS functional_impact_category: PARTIAL_LOSS_OF_FUNCTION

PARTIAL_LOSS_OF_FUNCTION because the mutant protein is unstable and binds its partners poorly but is not catalytically dead: a complex assembled with it in vitro retained normal polymerase activity.

Downstream

Destabilisation of the Polymerase Delta Complex The mutant POLD2 protein is less stable and binds POLD1 and POLD3 poorly, so less polymerase delta complex is assembled.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"The POLD2Asp293Asn mutation lies in the phosphodiesterase (PDE) domain (Figure 1E) and is absent from the ExAC and 1,000 Genomes Project public databases."

The location of the variant and its absence from population databases.

Destabilisation of the Polymerase Delta Complex

Mechanism confidence: Established

POLD2 is the structural hub of polymerase delta, contacting both POLD1 and POLD3. The p.Asp293Asn protein is turned over faster than wild type, and the patient's stimulated lymphocytes and fibroblasts carry markedly reduced POLD1, POLD2 and POLD3 protein despite normal mRNA. The loss is in the amount of enzyme, not its quality: a complex assembled with the mutant subunit kept normal polymerase activity in a primer-extension assay. The cell therefore has less polymerase delta activity in total, which is what the decreased DNA-directed DNA polymerase activity below records. Re-expressing wild-type POLD2 in the patient's fibroblasts restored POLD1 and POLD3 levels, which places the instability on the POLD2 variant.

POLD2 hgnc:9176

DNA-directed DNA polymerase activity (total cellular, through reduced complex amount) GO:0003887 ↓ DECREASED

DNA polymerase delta complex GO:0043625

Downstream

Reduced Replication Origin Firing and Slowed S-Phase Progression With less polymerase delta available, fewer replisomes can assemble, so fewer origins fire and cells move through S phase more slowly.

Show evidence (5 references)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"Our results showed that the stability of POLD2Asp293Asn was reduced compared with WT (Figure 2C), indicating that the reduced levels of polymerase δ observed in P1 cells were due to the identified mutation."

Cycloheximide chase showing the mutant POLD2 protein is unstable.

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"We observed markedly reduced expression of all 3 subunits (Figure 1F), despite mRNA levels of the corresponding subunits that were comparable to those of healthy controls"

Reduced POLD1, POLD2 and POLD3 protein with normal mRNA in stimulated patient cells. The sentence covers both patients of the series; the POLD2 patient is P1.

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"in contrast to the POLD2Asp293Asn variant in P1, in whom loss of polymerase δ function results from reduced protein stability"

The authors' conclusion that the POLD2 patient loses polymerase delta function through instability, not reduced catalysis.

+ 2 more references

Reduced Replication Origin Firing and Slowed S-Phase Progression

Mechanism confidence: Established

In the patient's fibroblasts, synchronised cells moved through S phase more slowly than control cells. DNA fibre analysis showed fewer new origin firing events and a faster fork speed. Faster forks are read as compensation for the reduced origin firing, the pattern also seen in GINS1 and polymerase epsilon deficiency. The authors attribute it to a rate-limiting amount of an essential replisome component.

fibroblast CL:0000057

DNA replication initiation GO:0006270 ↓ DECREASED

Downstream

Replication Stress and Replication-Associated DNA Lesions

Impaired T Cell Proliferation

Show evidence (2 references)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"To investigate this, we determined the frequency of de novo fired origins (only IdU-labeled fibers) in P1 fibroblasts and found a reduced number of replication origin initiation events (Figure 3H)."

Reduced origin firing measured in the POLD2 patient's fibroblasts.

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"Analysis of P1 fibroblasts 6 and 10 hours after thymidine release showed slower progression through the S phase when compared with HD fibroblasts (Figure 3E), indicating the presence of a DNA replication–associated defect."

Slowed S-phase progression in the POLD2 patient's fibroblasts.

Replication Stress and Replication-Associated DNA Lesions

Mechanism confidence: Established

Under unchallenged conditions the patient's fibroblasts show ATR checkpoint activation (CHK1 Ser345 and RPA Ser33 phosphorylation), about three times as many gamma-H2AX foci as controls, more RAD51 foci, and more 53BP1 nuclear bodies in G1. The G1 bodies mark under-replicated DNA carried into the next cell cycle. Re-expressing wild-type POLD2 cleared the G1 53BP1 bodies, which establishes that the POLD2 variant causes them. This is replication-derived damage. When the fibroblasts were challenged with a panel of genotoxic agents they were not hypersensitive, so the repair functions of polymerase delta are preserved at the reduced dose.

fibroblast CL:0000057

DNA damage checkpoint signaling GO:0000077 ↑ INCREASED DNA damage response GO:0006974 ↑ INCREASED

Downstream

Impaired T Cell Proliferation

Show evidence (4 references)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"CHK1 phosphorylation on Ser345 could be strongly induced by the replication inhibitor hydroxyurea, but it was already apparent under basal conditions in P1 primary fibroblasts (Figure 4A), indicating persistent replication stress and checkpoint activation in P1 cells at steady state."

Checkpoint activation at steady state in the POLD2 patient's fibroblasts.

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"Strikingly, in P1 fibroblasts, the average number of γ-H2AX foci per nucleus was increased to 18 as compared with 6 in healthy control cells under unchallenged, steady-state conditions"

Increased double-strand-break foci without exogenous damage.

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"Overexpression of POLD2 in P1 fibroblasts led to stabilization of the polymerase δ subunits POLD1 and POLD3 (Supplemental Figure 5B) and rescued the G1 53BP1 foci"

Wild-type POLD2 restores the other subunits and clears the G1 lesions, which establishes that the variant causes both.

+ 1 more reference

Impaired T Cell Proliferation

Mechanism confidence: Established

The patient's T cells responded poorly to anti-CD3, anti-CD28, PHA and PMA-ionomycin, and after 48 hours of TCR stimulation most remained in G1. Activation markers (CD25, CD95) were upregulated normally and ERK phosphorylation was intact, so TCR signalling works and the defect lies in the replication the cells need to divide. The result held in in-vitro-expanded T cells, which excludes the patient's expanded TEMRA CD8+ population as the explanation. Expanded T cells also carried more 53BP1 foci than controls.

T cell CL:0000084

T cell proliferation GO:0042098 ↓ DECREASED

Downstream

Combined Cellular and Humoral Immune Deficiency

Decreased Mitogen-Induced T Cell Proliferation

CD4+ T Cell Lymphopenia

Show evidence (2 references)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"In response to proliferative stimuli such as anti-CD3, anti-CD28, phytohemagglutinin (PHA), and PMA-ionomycin, we found that PBMC-derived polymerase δ–deficient T cells from both patients had a reduced proliferative response"

Reduced T-cell proliferation in both patients of the series, the POLD2 patient (P1) included.

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"We also assessed 53BP1 levels in expanded T cells from both patients and detected increased numbers of foci per cell in both patients compared with healthy controls, with a more pronounced effect in P1"

Replication-associated lesions in the patients' own T cells, strongest in the POLD2 patient. This ties the fibroblast findings to the lymphocyte defect.

Combined Cellular and Humoral Immune Deficiency

Mechanism confidence: Provisional

The patient has low CD4+ T, B and NK cell counts, and his infections improved on immunoglobulin replacement, so both arms of adaptive immunity are affected. B-cell numbers are low but maturation looks largely intact, with class-switched B cells present and normal somatic hypermutation in IgA and IgG. The humoral deficit is therefore one of numbers, with likely loss of T-cell help, rather than a block in B-cell differentiation. The infection spectrum fits a combined defect: recurrent sinopulmonary bacterial infection with bronchiectasis, recurrent skin abscesses, and chronic molluscum contagiosum, a viral skin infection that needs T cells to clear it. PROVISIONAL because the step from lymphopenia to each infection is inferred from one patient, not measured.

lymphocyte CL:0000542

Downstream

Combined Immunodeficiency

Decreased B Cell Count

Reduced NK Cell Count

Recurrent Respiratory Infections

Persistent Molluscum Contagiosum

Recurrent Cutaneous Abscesses

Show evidence (2 references)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Here, we report on 2 patients with biallelic mutations affecting the POLD1 and POLD2 subunits of polymerase δ, who presented with developmental defects and combined immunodeficiency (CID)."

The authors' classification of both patients, the POLD2 patient included, as CID.

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Despite a reduction in B cell numbers, we found that B cell maturation was largely normal, as judged by the presence of class-switched B cells (CD27+IgD–) (Supplemental Figure 1C) and normal somatic hypermutation (SHM) patterns in the IgA and IgG loci of P1"

B-cell maturation preserved in the POLD2 patient despite low B-cell numbers.

Syndromic Growth and Neurodevelopmental Features

Mechanism confidence: Hypothetical

The patient has short stature and severe intellectual disability with speech impairment, attention deficit and hyperactivity. The authors propose that the same replication-stress mechanism behind the immunodeficiency also explains the neurodevelopmental abnormalities, since polymerase delta is needed by every dividing progenitor. No non-haematopoietic tissue was examined, and with one patient from a consanguineous family a second recessive cause cannot be excluded. The link is therefore recorded as HYPOTHETICAL. The POLD1 patient of the same series has short stature, microcephaly and low IQ, so the developmental features recur across the polymerase delta deficiencies.

Downstream

Short Stature

Severe Intellectual Disability

Hyperactivity

Show evidence (1 reference)

PMID:31449058 SUPPORT INDIRECT PRIMARY RESULT In Vitro

"Our results suggest that loss of polymerase δ leads to replication stress–associated immunodeficiency and neurodevelopmental abnormalities in this syndrome."

The authors' proposed link between the cellular replication defect and the neurodevelopmental features. INDIRECT because the cellular work was done in fibroblasts and lymphocytes, not neural cells.

⬡

Pathograph

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

●

Phenotypes

13

Blood 5

CD4+ T Cell Lymphopenia Decreased total CD4+ T cell count HP:5210418

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Immunological analyses showed CD4+ T, B, and NK cell lymphopenia (Figure 1B)."

The lymphocyte findings for P1, the POLD2 patient. The P2 sentence in the same paper reads "persistent CD4+ T", so this exact wording is P1's.

Decreased B Cell Count Decreased total B cell count HP:0010976

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Immunological analyses showed CD4+ T, B, and NK cell lymphopenia (Figure 1B)."

B-cell lymphopenia in the POLD2 patient (P1).

Reduced NK Cell Count Reduced total natural killer cell count HP:0040218

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Immunological analyses showed CD4+ T, B, and NK cell lymphopenia (Figure 1B)."

NK-cell lymphopenia in the POLD2 patient (P1).

Increased CD8+ TEMRA T Cell Proportion HP:0500265

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"In-depth immunophenotyping of the main lymphocyte subsets showed a high frequency of CCR7–CD45RA+ TEMRA (effector memory cells reexpressing CD45RA) CD8+ T cells in P1"

The TEMRA CD8+ expansion in the POLD2 patient.

Decreased Mitogen-Induced T Cell Proliferation HP:0031381

1 of 1 published patient (measured in both patients of the series).

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"In response to proliferative stimuli such as anti-CD3, anti-CD28, phytohemagglutinin (PHA), and PMA-ionomycin, we found that PBMC-derived polymerase δ–deficient T cells from both patients had a reduced proliferative response"

The proliferation assay result in patient-derived T cells, the POLD2 patient's included.

Immune 4

Recurrent Respiratory Infections HP:0002205

1 of 1 published patient. No frequency band is set from a single case.

Sequelae: Bronchiectasis

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Patient 1 (P1), a 17-year-old male born to consanguineous parents, presented with recurrent upper and lower respiratory tract infections as frequently as 2 to 3 times per month, leading to bronchiectasis since the age of 6 months."

The infection history of the POLD2 patient (P1).

Persistent Molluscum Contagiosum Persistent molluscum infection HP:5210309

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"He has had chronic molluscum contagiosum–associated lesions on his face since 3 years of age"

Chronic facial molluscum in the POLD2 patient.

Recurrent Cutaneous Abscesses HP:0031292

Temporal: RECURRENT

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"At 5 years of age, he started to experience skin abscesses requiring monthly hospitalization."

Recurrent skin abscesses in the POLD2 patient.

Combined Immunodeficiency HP:0005387

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Here, we report on 2 patients with biallelic mutations affecting the POLD1 and POLD2 subunits of polymerase δ, who presented with developmental defects and combined immunodeficiency (CID)."

Both patients of the series, the POLD2 patient included, are classified as CID.

Nervous System 2

Severe Intellectual Disability HP:0010864

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"At 4 years of age, the patient had short stature and severe intellectual disability as determined by the Denver test and apparent by speech impairment, attention deficit, and hyperactivity."

Severe intellectual disability in the POLD2 patient.

Hyperactivity HP:0000752

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"At 4 years of age, the patient had short stature and severe intellectual disability as determined by the Denver test and apparent by speech impairment, attention deficit, and hyperactivity."

Attention deficit and hyperactivity in the POLD2 patient.

Respiratory 1

Bronchiectasis HP:0002110

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Patient 1 (P1), a 17-year-old male born to consanguineous parents, presented with recurrent upper and lower respiratory tract infections as frequently as 2 to 3 times per month, leading to bronchiectasis since the age of 6 months."

Bronchiectasis from six months in the POLD2 patient, attributed to the infections.

Growth 1

Short Stature HP:0004322

1 of 1 published patient.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"At 4 years of age, the patient had short stature and severe intellectual disability as determined by the Denver test and apparent by speech impairment, attention deficit, and hyperactivity."

Short stature in the POLD2 patient.

🧬

Genetic Associations

1

POLD2

Gene: POLD2 hgnc:9176 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"Overexpression of POLD2 in P1 fibroblasts led to stabilization of the polymerase δ subunits POLD1 and POLD3 (Supplemental Figure 5B) and rescued the G1 53BP1 foci"

Functional rescue by wild-type POLD2, the strongest single argument for causality.

PMID:31449058 SUPPORT DIRECT BACKGROUND Other

"The specific function of the POLD2 subunit has been poorly explored, despite its central structural role in interacting with both POLD1 and POLD3"

POLD2's structural role in the complex, from this paper's introduction (hence BACKGROUND). This is the function the variant disrupts.

💊

Medical Actions

2

Immunoglobulin Replacement Therapy

Action: intravenous immunoglobulin replacement therapy Ontology label: Intravenous Immunoglobulin Therapy NCIT:C121331

Platform: Protein replacement

Regular intravenous immunoglobulin from age seven, given with antimicrobial prophylaxis. The combination reduced respiratory infections and skin abscesses. The source reports the combined regimen, so the effect of immunoglobulin alone cannot be separated.

Mechanism Target:

Combined Cellular and Humoral Immune Deficiency — Replacement provides the antibody the patient cannot make in normal amounts. It does not affect the T-cell proliferation defect.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"After starting on regular intravenous Ig (IVIG) replacement, prophylactic antibiotic, and antifungal (trimethoprim-sulfamethoxazole and itraconazole) therapy at 7 years of age, he experienced a reduction in the number of episodes of respiratory infections and skin abscesses."

The response to the combined regimen in the one patient.

Antibacterial and Antifungal Prophylaxis

Action: Pharmacotherapy NCIT:C15986

Agent: trimethoprim-sulfamethoxazole CHEBI:3770 itraconazole CHEBI:6076

Platform: Small molecule

Prophylactic trimethoprim-sulfamethoxazole and itraconazole, started with immunoglobulin replacement at age seven.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"After starting on regular intravenous Ig (IVIG) replacement, prophylactic antibiotic, and antifungal (trimethoprim-sulfamethoxazole and itraconazole) therapy at 7 years of age, he experienced a reduction in the number of episodes of respiratory infections and skin abscesses."

The prophylactic agents and the reduction in infections on the combined regimen.

🔬

Diagnosis

3

Lymphocyte subset enumeration

Longitudinal lymphocyte counts showed persistent CD4+ T, B and NK cell lymphopenia. Deeper immunophenotyping showed TEMRA CD8+ expansion with preserved B-cell class switching, which separates a numerical deficit from a B-cell maturation block.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Immunological analyses showed CD4+ T, B, and NK cell lymphopenia (Figure 1B)."

The subset findings in the POLD2 patient.

T cell proliferation assay

Thymidine incorporation or dye dilution after mitogen and anti-CD3/CD28 stimulation shows reduced proliferation with intact activation-marker upregulation. That pattern points to a replication defect, not a signalling one.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT In Vitro

"In response to proliferative stimuli such as anti-CD3, anti-CD28, phytohemagglutinin (PHA), and PMA-ionomycin, we found that PBMC-derived polymerase δ–deficient T cells from both patients had a reduced proliferative response"

The proliferation defect the assay detects.

Exome sequencing with functional confirmation

The diagnosis was made by exome sequencing after homozygosity mapping in the consanguineous family. The variant was confirmed as pathogenic by reduced subunit levels in patient cells, instability of the mutant protein and rescue by wild-type POLD2.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Genetic analysis of P1 by exome sequencing revealed a homozygous missense variant (p.Asp293Asn; combined annotation–dependent depletion [CADD] score of 28.1) in the POLD2 gene (NM_006230), which displayed perfect segregation under the assumption of autosomal-recessive inheritance"

How the genotype was found.

📈

Progression

2

Infantile-onset recurrent infection

Recurrent respiratory infections began in infancy, with bronchiectasis by six months, chronic molluscum from three years and skin abscesses from five years. Short stature and severe intellectual disability were recognised at four years.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Patient 1 (P1), a 17-year-old male born to consanguineous parents, presented with recurrent upper and lower respiratory tract infections as frequently as 2 to 3 times per month, leading to bronchiectasis since the age of 6 months."

Infantile onset of the infection phenotype.

Response to immunoglobulin replacement and prophylaxis

From seven years of age, immunoglobulin replacement with antibacterial and antifungal prophylaxis reduced respiratory infections and skin abscesses. The patient was 17 when reported, with no malignancy.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"After starting on regular intravenous Ig (IVIG) replacement, prophylactic antibiotic, and antifungal (trimethoprim-sulfamethoxazole and itraconazole) therapy at 7 years of age, he experienced a reduction in the number of episodes of respiratory infections and skin abscesses."

The treated course in the POLD2 patient.

📊

Prevalence

1

Worldwide

Cases In Literature Ultra Rare

One genetically confirmed patient, from a consanguineous family, published in 2019. The PubMed query `POLD2[tiab] AND (immunodeficiency OR biallelic)`, run in October 2026, returned four records: PMID:31449058, PMID:31629014 (POLD1), PMID:37030525 and PMID:38099988 (both POLD3). None adds a POLD2 patient. The OpenScientist deep-research report committed with this entry (research/POLD2-related_Combined_Immunodeficiency-deep-research-openscientist.md) also found no second POLD2 case and describes the evidence as essentially a single index patient. No population estimate exists, so rate_per_100000 is left unset.

Show evidence (1 reference)

PMID:31449058 SUPPORT DIRECT PRIMARY RESULT Human Clinical

"Genetic analysis of P1 by exome sequencing revealed a homozygous missense variant (p.Asp293Asn; combined annotation–dependent depletion [CADD] score of 28.1) in the POLD2 gene (NM_006230), which displayed perfect segregation under the assumption of autosomal-recessive inheritance"

The single POLD2 genotype that makes up the published case total.

{ }

Source YAML

click to show

name: POLD2-related Combined Immunodeficiency
category: Mendelian
creation_date: "2026-10-13T00:00:00Z"
synonyms:
- POLD2-related combined immunodeficiency
- Polymerase D2 deficiency
- POLD2 deficiency
- combined immunodeficiency due to POLD2 deficiency
disease_term:
  preferred_term: POLD2-related combined immunodeficiency
  term:
    id: MONDO:1010175
    label: POLD2-related combined immunodeficiency
description: >-
  POLD2-related combined immunodeficiency is the syndromic combined immunodeficiency caused by a
  biallelic hypomorphic variant in POLD2, the structural accessory subunit of DNA polymerase
  delta. One patient is published: P1 of the 2019 series that first described human polymerase
  delta deficiency, a boy from a consanguineous family homozygous for p.Asp293Asn in the POLD2
  phosphodiesterase domain.

  POLD2 is the scaffold of the polymerase delta heterotetramer, contacting both the catalytic
  subunit POLD1 and the accessory subunit POLD3. The p.Asp293Asn protein is unstable and binds
  POLD1 and POLD3 poorly, so the patient's cells carry less of all three subunits, although the
  mutant complex keeps normal polymerase activity when assembled in vitro. The disease therefore
  follows from too little polymerase delta, not from a less active enzyme. That separates it
  from the POLD1 form (IMD120), where the compound heterozygous alleles lower catalytic activity
  with the complex intact.

  Less polymerase delta limits replication origin firing and slows progression through S phase.
  The patient's fibroblasts show persistent replication stress with checkpoint activation at
  steady state, increased double-strand-break foci and 53BP1 nuclear bodies in G1, all of which
  were reversed by re-expressing wild-type POLD2. Repair of exogenous DNA damage was preserved.
  Lymphocytes, which must expand rapidly on demand, show the clearest functional failure: T
  cells from the patient proliferate poorly after stimulation and most remain in G1.

  Clinically the patient had recurrent respiratory infections from infancy leading to
  bronchiectasis by six months, chronic facial molluscum contagiosum, recurrent skin abscesses,
  CD4+ T, B and NK cell lymphopenia, short stature and severe intellectual disability with
  speech impairment, attention deficit and hyperactivity. Intravenous immunoglobulin with
  antibacterial and antifungal prophylaxis reduced the infections.
parents:
- Combined Immunodeficiency
- Inborn Errors of Immunity
classifications:
  harrisons_chapter:
  - classification_value: IMMUNE_RHEUMATOLOGIC
    notes: >-
      A monogenic inborn error of immunity presenting as syndromic combined immunodeficiency,
      managed by immunology.
references:
- reference: PMID:31449058
  title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
- reference: PMID:31629014
  title: "Combined immunodeficiency caused by a loss-of-function mutation in DNA polymerase delta 1."
inheritance:
- name: Autosomal recessive inheritance
  description: >-
    The single patient is homozygous and the child of consanguineous parents, and the variant
    segregates with autosomal recessive inheritance in the family. The population genetics agree:
    POLD2 has a low pLI and a high pRec, so loss of one allele is predicted to be tolerated and
    loss of both is not.
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Genetic analysis of P1 by exome sequencing revealed a homozygous missense variant (p.Asp293Asn; combined annotation–dependent depletion [CADD] score of 28.1) in the POLD2 gene (NM_006230), which displayed perfect segregation under the assumption of autosomal-recessive inheritance"
    explanation: >-
      Homozygosity and family segregation consistent with recessive inheritance in the one
      published POLD2 family.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: COMPUTATIONAL
    quote_role: PRIMARY_RESULT
    snippet: "Both genes show low pLI scores of 0.001 and 0.044, respectively, indicating a predicted tolerance to the loss of a single allele, and high pRec scores of 0.998 and 0.950, respectively, indicating a predicted high intolerance for a complete loss of function"
    explanation: >-
      The second value in each pair (pLI 0.044, pRec 0.950) is POLD2's. The constraint scores
      predict a recessive gene, which agrees with the observed inheritance. Graded COMPUTATIONAL
      because the claim rests on population-database constraint metrics, not on the family.
pathophysiology:
- name: Biallelic Hypomorphic POLD2 Variant
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    One genotype is published: homozygous missense p.Asp293Asn (NM_006230) in
    the POLD2 phosphodiesterase domain, absent from ExAC and 1000 Genomes. Structural modelling
    on the POLD2-POLD3 crystal structure predicts loss of a polar contact with Arg240. No null
    allele has been seen, and none is expected: POLD2 is essential in yeast and in mammalian cell
    screens, so a viable patient's allele must be hypomorphic.
  genes:
  - preferred_term: POLD2
    term:
      id: hgnc:9176
      label: POLD2
  genetic_context:
    genes:
    - preferred_term: POLD2
      term:
        id: hgnc:9176
        label: POLD2
    allele_type: missense variant (p.Asp293Asn) in the phosphodiesterase domain
    variant_origin: GERMLINE
    zygosity: HOMOZYGOUS
    functional_impact_category: PARTIAL_LOSS_OF_FUNCTION
    notes: >-
      PARTIAL_LOSS_OF_FUNCTION because the mutant protein is unstable and binds its partners
      poorly but is not catalytically dead: a complex assembled with it in vitro retained normal
      polymerase activity.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "The POLD2Asp293Asn mutation lies in the phosphodiesterase (PDE) domain (Figure 1E) and is absent from the ExAC and 1,000 Genomes Project public databases."
    explanation: The location of the variant and its absence from population databases.
  downstream:
  - target: Destabilisation of the Polymerase Delta Complex
    causal_link_type: DIRECT
    description: >-
      The mutant POLD2 protein is less stable and binds POLD1 and POLD3 poorly, so less
      polymerase delta complex is assembled.
    evidence:
    - reference: PMID:31449058
      reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
      supports: SUPPORT
      directness: DIRECT
      evidence_source: IN_VITRO
      quote_role: PRIMARY_RESULT
      snippet: "We found a strong reduction in the interaction between POLD2Asp293Asn and POLD1 or POLD3 compared with WT POLD2 (Figure 2E)."
      explanation: >-
        Co-immunoprecipitation in HEK293 cells showing that the mutant subunit binds both of its
        complex partners poorly.
- name: Destabilisation of the Polymerase Delta Complex
  biological_scale: MOLECULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    POLD2 is the structural hub of polymerase delta, contacting both POLD1 and POLD3. The
    p.Asp293Asn protein is turned over faster than wild type, and the patient's stimulated
    lymphocytes and fibroblasts carry markedly reduced POLD1, POLD2 and POLD3 protein despite
    normal mRNA. The loss is in the amount of enzyme, not its quality: a complex assembled with
    the mutant subunit kept normal polymerase activity in a primer-extension assay. The cell
    therefore has less polymerase delta activity in total, which is what the decreased
    DNA-directed DNA polymerase activity below records. Re-expressing wild-type POLD2 in the
    patient's fibroblasts restored POLD1 and POLD3 levels, which places the instability on the
    POLD2 variant.
  genes:
  - preferred_term: POLD2
    term:
      id: hgnc:9176
      label: POLD2
  cellular_components:
  - preferred_term: DNA polymerase delta complex
    term:
      id: GO:0043625
      label: delta DNA polymerase complex
    modifier: DECREASED
  molecular_functions:
  - preferred_term: DNA-directed DNA polymerase activity (total cellular, through reduced complex amount)
    term:
      id: GO:0003887
      label: DNA-directed DNA polymerase activity
    modifier: DECREASED
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "Our results showed that the stability of POLD2Asp293Asn was reduced compared with WT (Figure 2C), indicating that the reduced levels of polymerase δ observed in P1 cells were due to the identified mutation."
    explanation: Cycloheximide chase showing the mutant POLD2 protein is unstable.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "We observed markedly reduced expression of all 3 subunits (Figure 1F), despite mRNA levels of the corresponding subunits that were comparable to those of healthy controls"
    explanation: >-
      Reduced POLD1, POLD2 and POLD3 protein with normal mRNA in stimulated patient cells. The
      sentence covers both patients of the series; the POLD2 patient is P1.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "in contrast to the POLD2Asp293Asn variant in P1, in whom loss of polymerase δ function results from reduced protein stability"
    explanation: >-
      The authors' conclusion that the POLD2 patient loses polymerase delta function through
      instability, not reduced catalysis.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "POLD2Asp293Asn showed intact activity in vitro."
    explanation: >-
      The recombinant mutant complex keeps normal polymerase activity, which is why the decreased
      activity on this node is a total-amount claim and not a per-complex one.
  - reference: PMID:31629014
    reference_title: "Combined immunodeficiency caused by a loss-of-function mutation in DNA polymerase delta 1."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: OTHER
    quote_role: BACKGROUND
    snippet: "In particular, POLD2 serves as a scaffold by interacting with POLD1 and the other Polδ subunits"
    explanation: >-
      POLD2's scaffold role, quoted from the introduction of a POLD1 paper (hence BACKGROUND).
      It is the normal function that the unstable variant removes.
  downstream:
  - target: Reduced Replication Origin Firing and Slowed S-Phase Progression
    causal_link_type: DIRECT
    description: >-
      With less polymerase delta available, fewer replisomes can assemble, so fewer origins
      fire and cells move through S phase more slowly.
- name: Reduced Replication Origin Firing and Slowed S-Phase Progression
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    In the patient's fibroblasts, synchronised cells moved through S phase more slowly than
    control cells. DNA fibre analysis showed fewer new origin firing events and a faster fork
    speed. Faster forks are read as compensation for the reduced origin firing, the pattern also
    seen in GINS1 and polymerase epsilon deficiency. The authors attribute it to a rate-limiting
    amount of an essential replisome component.
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  biological_processes:
  - preferred_term: DNA replication initiation
    term:
      id: GO:0006270
      label: DNA replication initiation
    modifier: DECREASED
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "To investigate this, we determined the frequency of de novo fired origins (only IdU-labeled fibers) in P1 fibroblasts and found a reduced number of replication origin initiation events (Figure 3H)."
    explanation: Reduced origin firing measured in the POLD2 patient's fibroblasts.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "Analysis of P1 fibroblasts 6 and 10 hours after thymidine release showed slower progression through the S phase when compared with HD fibroblasts (Figure 3E), indicating the presence of a DNA replication–associated defect."
    explanation: Slowed S-phase progression in the POLD2 patient's fibroblasts.
  downstream:
  - target: Replication Stress and Replication-Associated DNA Lesions
    causal_link_type: DIRECT
  - target: Impaired T Cell Proliferation
    causal_link_type: DIRECT
- name: Replication Stress and Replication-Associated DNA Lesions
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    Under unchallenged conditions the patient's fibroblasts show ATR checkpoint activation
    (CHK1 Ser345 and RPA Ser33 phosphorylation), about three times as many gamma-H2AX foci as
    controls, more RAD51 foci, and more 53BP1 nuclear bodies in G1. The G1 bodies mark
    under-replicated DNA carried into the next cell cycle. Re-expressing wild-type POLD2 cleared
    the G1 53BP1 bodies, which establishes that the POLD2 variant causes them. This is
    replication-derived damage. When the fibroblasts were challenged with a panel of genotoxic
    agents they were not hypersensitive, so the repair functions of polymerase delta are
    preserved at the reduced dose.
  cell_types:
  - preferred_term: fibroblast
    term:
      id: CL:0000057
      label: fibroblast
  biological_processes:
  - preferred_term: DNA damage checkpoint signaling
    term:
      id: GO:0000077
      label: DNA damage checkpoint signaling
    modifier: INCREASED
  - preferred_term: DNA damage response
    term:
      id: GO:0006974
      label: DNA damage response
    modifier: INCREASED
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "CHK1 phosphorylation on Ser345 could be strongly induced by the replication inhibitor hydroxyurea, but it was already apparent under basal conditions in P1 primary fibroblasts (Figure 4A), indicating persistent replication stress and checkpoint activation in P1 cells at steady state."
    explanation: Checkpoint activation at steady state in the POLD2 patient's fibroblasts.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "Strikingly, in P1 fibroblasts, the average number of γ-H2AX foci per nucleus was increased to 18 as compared with 6 in healthy control cells under unchallenged, steady-state conditions"
    explanation: Increased double-strand-break foci without exogenous damage.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "Overexpression of POLD2 in P1 fibroblasts led to stabilization of the polymerase δ subunits POLD1 and POLD3 (Supplemental Figure 5B) and rescued the G1 53BP1 foci"
    explanation: >-
      Wild-type POLD2 restores the other subunits and clears the G1 lesions, which establishes
      that the variant causes both.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "P1 fibroblasts showed no overt sensitivity to an array of genotoxic reagents, which suggests that, despite an unstable POLD complex, its activity was sufficient to ensure proficiency in specific DNA repair pathways"
    explanation: >-
      Repair of exogenous damage is preserved, so the lesions on this node come from
      replication, not from a repair defect.
  downstream:
  - target: Impaired T Cell Proliferation
    causal_link_type: DIRECT
- name: Impaired T Cell Proliferation
  biological_scale: CELLULAR
  mechanism_confidence: ESTABLISHED
  description: >-
    The patient's T cells responded poorly to anti-CD3, anti-CD28, PHA and PMA-ionomycin, and
    after 48 hours of TCR stimulation most remained in G1. Activation markers (CD25, CD95) were
    upregulated normally and ERK phosphorylation was intact, so TCR signalling works and the
    defect lies in the replication the cells need to divide. The result held in in-vitro-expanded
    T cells, which excludes the patient's expanded TEMRA CD8+ population as the explanation.
    Expanded T cells also carried more 53BP1 foci than controls.
  cell_types:
  - preferred_term: T cell
    term:
      id: CL:0000084
      label: T cell
  biological_processes:
  - preferred_term: T cell proliferation
    term:
      id: GO:0042098
      label: T cell proliferation
    modifier: DECREASED
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "In response to proliferative stimuli such as anti-CD3, anti-CD28, phytohemagglutinin (PHA), and PMA-ionomycin, we found that PBMC-derived polymerase δ–deficient T cells from both patients had a reduced proliferative response"
    explanation: >-
      Reduced T-cell proliferation in both patients of the series, the POLD2 patient (P1)
      included.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "We also assessed 53BP1 levels in expanded T cells from both patients and detected increased numbers of foci per cell in both patients compared with healthy controls, with a more pronounced effect in P1"
    explanation: >-
      Replication-associated lesions in the patients' own T cells, strongest in the POLD2
      patient. This ties the fibroblast findings to the lymphocyte defect.
  downstream:
  - target: Combined Cellular and Humoral Immune Deficiency
    causal_link_type: DIRECT
  - target: Decreased Mitogen-Induced T Cell Proliferation
    causal_link_type: DIRECT
  - target: CD4+ T Cell Lymphopenia
    causal_link_type: DIRECT
- name: Combined Cellular and Humoral Immune Deficiency
  biological_scale: ORGANISM
  mechanism_confidence: PROVISIONAL
  description: >-
    The patient has low CD4+ T, B and NK cell counts, and his infections improved on
    immunoglobulin replacement, so both arms of adaptive immunity are affected. B-cell numbers are
    low but maturation looks largely intact, with class-switched B cells present and normal
    somatic hypermutation in IgA and IgG. The humoral deficit is therefore one of numbers, with
    likely loss of T-cell help, rather than a block in B-cell differentiation. The infection
    spectrum fits a combined defect: recurrent sinopulmonary bacterial infection with
    bronchiectasis, recurrent skin abscesses, and chronic molluscum contagiosum, a viral skin
    infection that needs T cells to clear it. PROVISIONAL because the step from lymphopenia to
    each infection is inferred from one patient, not measured.
  cell_types:
  - preferred_term: lymphocyte
    term:
      id: CL:0000542
      label: lymphocyte
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Here, we report on 2 patients with biallelic mutations affecting the POLD1 and POLD2 subunits of polymerase δ, who presented with developmental defects and combined immunodeficiency (CID)."
    explanation: The authors' classification of both patients, the POLD2 patient included, as CID.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Despite a reduction in B cell numbers, we found that B cell maturation was largely normal, as judged by the presence of class-switched B cells (CD27+IgD–) (Supplemental Figure 1C) and normal somatic hypermutation (SHM) patterns in the IgA and IgG loci of P1"
    explanation: B-cell maturation preserved in the POLD2 patient despite low B-cell numbers.
  downstream:
  - target: Combined Immunodeficiency
    causal_link_type: DIRECT
  - target: Decreased B Cell Count
    causal_link_type: DIRECT
  - target: Reduced NK Cell Count
    causal_link_type: DIRECT
  - target: Recurrent Respiratory Infections
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Persistent Molluscum Contagiosum
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Recurrent Cutaneous Abscesses
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
- name: Syndromic Growth and Neurodevelopmental Features
  biological_scale: ORGANISM
  mechanism_confidence: HYPOTHETICAL
  description: >-
    The patient has short stature and severe intellectual disability with speech impairment,
    attention deficit and hyperactivity. The authors propose that the same replication-stress
    mechanism behind the immunodeficiency also explains the neurodevelopmental abnormalities,
    since polymerase delta is needed by every dividing progenitor. No non-haematopoietic tissue
    was examined, and with one patient from a consanguineous family a second recessive cause
    cannot be excluded. The link is therefore recorded as HYPOTHETICAL. The POLD1 patient of the
    same series has short stature, microcephaly and low IQ, so the developmental features recur
    across the polymerase delta deficiencies.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "Our results suggest that loss of polymerase δ leads to replication stress–associated immunodeficiency and neurodevelopmental abnormalities in this syndrome."
    explanation: >-
      The authors' proposed link between the cellular replication defect and the
      neurodevelopmental features. INDIRECT because the cellular work was done in fibroblasts and
      lymphocytes, not neural cells.
  downstream:
  - target: Short Stature
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Severe Intellectual Disability
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  - target: Hyperactivity
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
phenotypes:
- name: Recurrent Respiratory Infections
  category: Respiratory
  description: >-
    Recurrent upper and lower respiratory tract infections as often as two to three times a
    month, which became less frequent once immunoglobulin replacement and prophylaxis began at
    age seven.
  phenotype_term:
    preferred_term: Recurrent respiratory infections
    term:
      id: HP:0002205
      label: Recurrent respiratory infections
  notes: 1 of 1 published patient. No frequency band is set from a single case.
  sequelae:
  - target: Bronchiectasis
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Patient 1 (P1), a 17-year-old male born to consanguineous parents, presented with recurrent upper and lower respiratory tract infections as frequently as 2 to 3 times per month, leading to bronchiectasis since the age of 6 months."
    explanation: The infection history of the POLD2 patient (P1).
- name: Bronchiectasis
  category: Respiratory
  description: >-
    Bronchiectasis present from six months of age, the structural result of the recurrent lower
    respiratory infections.
  phenotype_term:
    preferred_term: Bronchiectasis
    term:
      id: HP:0002110
      label: Bronchiectasis
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Patient 1 (P1), a 17-year-old male born to consanguineous parents, presented with recurrent upper and lower respiratory tract infections as frequently as 2 to 3 times per month, leading to bronchiectasis since the age of 6 months."
    explanation: Bronchiectasis from six months in the POLD2 patient, attributed to the infections.
- name: Persistent Molluscum Contagiosum
  category: Integumentary
  description: >-
    Chronic molluscum contagiosum lesions on the face from age three. Persistent molluscum points
    to impaired T-cell control of a cutaneous virus.
  phenotype_term:
    preferred_term: Chronic facial molluscum contagiosum
    term:
      id: HP:5210309
      label: Persistent molluscum infection
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "He has had chronic molluscum contagiosum–associated lesions on his face since 3 years of age"
    explanation: Chronic facial molluscum in the POLD2 patient.
- name: Recurrent Cutaneous Abscesses
  category: Integumentary
  description: >-
    Skin abscesses from age five that needed monthly hospital admission until prophylaxis and
    immunoglobulin replacement began.
  phenotype_term:
    preferred_term: Recurrent skin abscesses
    term:
      id: HP:0031292
      label: Cutaneous abscess
    temporality: RECURRENT
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "At 5 years of age, he started to experience skin abscesses requiring monthly hospitalization."
    explanation: Recurrent skin abscesses in the POLD2 patient.
- name: Combined Immunodeficiency
  category: Immune
  description: >-
    Combined immunodeficiency with low T, B and NK cell numbers and poor T-cell proliferation.
    It is syndromic: growth and neurodevelopment are also affected.
  phenotype_term:
    preferred_term: Combined immunodeficiency
    term:
      id: HP:0005387
      label: Combined immunodeficiency
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Here, we report on 2 patients with biallelic mutations affecting the POLD1 and POLD2 subunits of polymerase δ, who presented with developmental defects and combined immunodeficiency (CID)."
    explanation: Both patients of the series, the POLD2 patient included, are classified as CID.
- name: CD4+ T Cell Lymphopenia
  category: Immune
  description: Low CD4+ T-cell counts on longitudinal measurement.
  phenotype_term:
    preferred_term: CD4+ T cell lymphopenia
    term:
      id: HP:5210418
      label: Decreased total CD4+ T cell count
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Immunological analyses showed CD4+ T, B, and NK cell lymphopenia (Figure 1B)."
    explanation: >-
      The lymphocyte findings for P1, the POLD2 patient. The P2 sentence in the same paper reads
      "persistent CD4+ T", so this exact wording is P1's.
- name: Decreased B Cell Count
  category: Immune
  description: >-
    Low B-cell counts, with class-switched B cells and normal somatic hypermutation, so B-cell
    maturation is preserved.
  phenotype_term:
    preferred_term: B cell lymphopenia
    term:
      id: HP:0010976
      label: Decreased total B cell count
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Immunological analyses showed CD4+ T, B, and NK cell lymphopenia (Figure 1B)."
    explanation: B-cell lymphopenia in the POLD2 patient (P1).
- name: Reduced NK Cell Count
  category: Immune
  description: Low natural killer cell counts.
  phenotype_term:
    preferred_term: NK cell lymphopenia
    term:
      id: HP:0040218
      label: Reduced total natural killer cell count
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Immunological analyses showed CD4+ T, B, and NK cell lymphopenia (Figure 1B)."
    explanation: NK-cell lymphopenia in the POLD2 patient (P1).
- name: Increased CD8+ TEMRA T Cell Proportion
  category: Immune
  description: >-
    A high frequency of CCR7-negative CD45RA-positive effector memory CD8+ T cells (TEMRA), a
    terminally differentiated population with low proliferative capacity. The source reports it for
    the POLD2 patient (P1).
  phenotype_term:
    preferred_term: Increased CD8+ TEMRA T cell proportion
    term:
      id: HP:0500265
      label: Increased CD8+ TEMRA T cell proportion
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "In-depth immunophenotyping of the main lymphocyte subsets showed a high frequency of CCR7–CD45RA+ TEMRA (effector memory cells reexpressing CD45RA) CD8+ T cells in P1"
    explanation: The TEMRA CD8+ expansion in the POLD2 patient.
- name: Decreased Mitogen-Induced T Cell Proliferation
  category: Immune
  description: >-
    Reduced T-cell proliferation after anti-CD3, anti-CD28, PHA and PMA-ionomycin stimulation,
    measured by thymidine incorporation and dye dilution.
  phenotype_term:
    preferred_term: Decreased mitogen-induced T cell proliferation
    term:
      id: HP:0031381
      label: Decreased mitogen-induced T-cell proliferation
  notes: 1 of 1 published patient (measured in both patients of the series).
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "In response to proliferative stimuli such as anti-CD3, anti-CD28, phytohemagglutinin (PHA), and PMA-ionomycin, we found that PBMC-derived polymerase δ–deficient T cells from both patients had a reduced proliferative response"
    explanation: The proliferation assay result in patient-derived T cells, the POLD2 patient's included.
- name: Short Stature
  category: Growth
  description: Short stature noted at four years of age.
  phenotype_term:
    preferred_term: Short stature
    term:
      id: HP:0004322
      label: Short stature
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "At 4 years of age, the patient had short stature and severe intellectual disability as determined by the Denver test and apparent by speech impairment, attention deficit, and hyperactivity."
    explanation: Short stature in the POLD2 patient.
- name: Severe Intellectual Disability
  category: Nervous System
  description: >-
    Severe intellectual disability on the Denver test at four years, with speech impairment.
  phenotype_term:
    preferred_term: Severe intellectual disability
    term:
      id: HP:0010864
      label: Severe intellectual disability
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "At 4 years of age, the patient had short stature and severe intellectual disability as determined by the Denver test and apparent by speech impairment, attention deficit, and hyperactivity."
    explanation: Severe intellectual disability in the POLD2 patient.
- name: Hyperactivity
  category: Nervous System
  description: >-
    Attention deficit and hyperactivity, reported as features of the intellectual disability.
    The source does not report a formal ADHD diagnosis, so the attention-deficit hyperactivity
    disorder term is not bound.
  phenotype_term:
    preferred_term: Hyperactivity with attention deficit
    term:
      id: HP:0000752
      label: Hyperactivity
  notes: 1 of 1 published patient.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "At 4 years of age, the patient had short stature and severe intellectual disability as determined by the Denver test and apparent by speech impairment, attention deficit, and hyperactivity."
    explanation: Attention deficit and hyperactivity in the POLD2 patient.
prevalence:
- population: Worldwide
  measure_type: CASES_IN_LITERATURE
  prevalence_class: ULTRA_RARE
  notes: >-
    One genetically confirmed patient, from a consanguineous family, published in 2019. The PubMed
    query `POLD2[tiab] AND (immunodeficiency OR biallelic)`, run in October 2026, returned four
    records: PMID:31449058, PMID:31629014 (POLD1), PMID:37030525 and PMID:38099988 (both POLD3).
    None adds a POLD2 patient. The OpenScientist deep-research report committed with this entry
    (research/POLD2-related_Combined_Immunodeficiency-deep-research-openscientist.md) also found
    no second POLD2 case and describes the evidence as essentially a single index patient. No
    population estimate exists, so rate_per_100000 is left unset.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Genetic analysis of P1 by exome sequencing revealed a homozygous missense variant (p.Asp293Asn; combined annotation–dependent depletion [CADD] score of 28.1) in the POLD2 gene (NM_006230), which displayed perfect segregation under the assumption of autosomal-recessive inheritance"
    explanation: The single POLD2 genotype that makes up the published case total.
progression:
- phase: Infantile-onset recurrent infection
  notes: >-
    Recurrent respiratory infections began in infancy, with bronchiectasis by six months, chronic
    molluscum from three years and skin abscesses from five years. Short stature and severe
    intellectual disability were recognised at four years.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Patient 1 (P1), a 17-year-old male born to consanguineous parents, presented with recurrent upper and lower respiratory tract infections as frequently as 2 to 3 times per month, leading to bronchiectasis since the age of 6 months."
    explanation: Infantile onset of the infection phenotype.
- phase: Response to immunoglobulin replacement and prophylaxis
  notes: >-
    From seven years of age, immunoglobulin replacement with antibacterial and antifungal
    prophylaxis reduced respiratory infections and skin abscesses. The patient was 17 when
    reported, with no malignancy.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "After starting on regular intravenous Ig (IVIG) replacement, prophylactic antibiotic, and antifungal (trimethoprim-sulfamethoxazole and itraconazole) therapy at 7 years of age, he experienced a reduction in the number of episodes of respiratory infections and skin abscesses."
    explanation: The treated course in the POLD2 patient.
genetic:
- name: POLD2
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  gene_term:
    preferred_term: POLD2
    term:
      id: hgnc:9176
      label: POLD2
  notes: >-
    The causal gene, encoding the non-catalytic scaffold subunit of DNA polymerase delta. One
    homozygous missense allele (p.Asp293Asn) is published, and it destabilises the protein and
    its complex partners. The authors note that no POLD2 variant had previously been associated
    with human disease and that functionally validated somatic POLD2 variants are absent from
    COSMIC. POLD2 therefore lacks the dominant cancer-predisposition and MDPL allele classes that
    POLD1 has. The causal assignment rests on one family plus functional rescue (wild-type POLD2
    restored the complex and cleared the DNA lesions in patient fibroblasts). No ClinGen
    gene-disease validity classification was found, so gene_disease_validity is left unset.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "Overexpression of POLD2 in P1 fibroblasts led to stabilization of the polymerase δ subunits POLD1 and POLD3 (Supplemental Figure 5B) and rescued the G1 53BP1 foci"
    explanation: Functional rescue by wild-type POLD2, the strongest single argument for causality.
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: OTHER
    quote_role: BACKGROUND
    snippet: "The specific function of the POLD2 subunit has been poorly explored, despite its central structural role in interacting with both POLD1 and POLD3"
    explanation: >-
      POLD2's structural role in the complex, from this paper's introduction (hence BACKGROUND).
      This is the function the variant disrupts.
diagnosis:
- name: Lymphocyte subset enumeration
  description: >-
    Longitudinal lymphocyte counts showed persistent CD4+ T, B and NK cell lymphopenia. Deeper
    immunophenotyping showed TEMRA CD8+ expansion with preserved B-cell class switching, which
    separates a numerical deficit from a B-cell maturation block.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Immunological analyses showed CD4+ T, B, and NK cell lymphopenia (Figure 1B)."
    explanation: The subset findings in the POLD2 patient.
- name: T cell proliferation assay
  description: >-
    Thymidine incorporation or dye dilution after mitogen and anti-CD3/CD28 stimulation shows
    reduced proliferation with intact activation-marker upregulation. That pattern points to a
    replication defect, not a signalling one.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "In response to proliferative stimuli such as anti-CD3, anti-CD28, phytohemagglutinin (PHA), and PMA-ionomycin, we found that PBMC-derived polymerase δ–deficient T cells from both patients had a reduced proliferative response"
    explanation: The proliferation defect the assay detects.
- name: Exome sequencing with functional confirmation
  description: >-
    The diagnosis was made by exome sequencing after homozygosity mapping in the consanguineous
    family. The variant was confirmed as pathogenic by reduced subunit levels in patient cells,
    instability of the mutant protein and rescue by wild-type POLD2.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "Genetic analysis of P1 by exome sequencing revealed a homozygous missense variant (p.Asp293Asn; combined annotation–dependent depletion [CADD] score of 28.1) in the POLD2 gene (NM_006230), which displayed perfect segregation under the assumption of autosomal-recessive inheritance"
    explanation: How the genotype was found.
treatments:
- name: Immunoglobulin Replacement Therapy
  therapeutic_modality: PROTEIN_REPLACEMENT
  description: >-
    Regular intravenous immunoglobulin from age seven, given with antimicrobial prophylaxis. The
    combination reduced respiratory infections and skin abscesses. The source reports the
    combined regimen, so the effect of immunoglobulin alone cannot be separated.
  treatment_term:
    preferred_term: intravenous immunoglobulin replacement therapy
    term:
      id: NCIT:C121331
      label: Intravenous Immunoglobulin Therapy
  target_mechanisms:
  - target: Combined Cellular and Humoral Immune Deficiency
    description: >-
      Replacement provides the antibody the patient cannot make in normal amounts. It does not
      affect the T-cell proliferation defect.
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "After starting on regular intravenous Ig (IVIG) replacement, prophylactic antibiotic, and antifungal (trimethoprim-sulfamethoxazole and itraconazole) therapy at 7 years of age, he experienced a reduction in the number of episodes of respiratory infections and skin abscesses."
    explanation: The response to the combined regimen in the one patient.
- name: Antibacterial and Antifungal Prophylaxis
  therapeutic_modality: SMALL_MOLECULE
  description: >-
    Prophylactic trimethoprim-sulfamethoxazole and itraconazole, started with immunoglobulin
    replacement at age seven.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: trimethoprim-sulfamethoxazole
      term:
        id: CHEBI:3770
        label: co-trimoxazole
    - preferred_term: itraconazole
      term:
        id: CHEBI:6076
        label: itraconazole
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "After starting on regular intravenous Ig (IVIG) replacement, prophylactic antibiotic, and antifungal (trimethoprim-sulfamethoxazole and itraconazole) therapy at 7 years of age, he experienced a reduction in the number of episodes of respiratory infections and skin abscesses."
    explanation: The prophylactic agents and the reduction in infections on the combined regimen.
  notes: >-
    Haematopoietic stem cell transplantation has been reported for POLD1 deficiency but not for
    the POLD2 patient, so it is not recorded here.
discussions:
- discussion_id: pold2_cid_single_patient_syndrome
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Do the short stature and severe intellectual disability belong to POLD2 deficiency itself, or
    do they need another explanation, given that the entity rests on one patient from a
    consanguineous family?
  rationale: >-
    The immune half of the phenotype has functional support in the patient's own cells: reduced
    complex, slowed S phase, replication lesions and poor T-cell proliferation, all reversed by
    wild-type POLD2 where rescue was tested. The developmental half has none. No neural or growth
    tissue was studied. A consanguineous family carries other homozygous regions, any of which could
    hold a second recessive cause. The argument for
    attribution is that the POLD1 patient of the same series also has short stature and
    intellectual impairment, and that the POLD3 deficiency carries neurodevelopmental delay. A
    second POLD2 family, or a hypomorphic Pold2 animal model, is what would settle it.
  attaches_to:
  - pathophysiology#Syndromic Growth and Neurodevelopmental Features
  - phenotypes#Short Stature
  - phenotypes#Severe Intellectual Disability
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: INDIRECT
    evidence_source: IN_VITRO
    quote_role: PRIMARY_RESULT
    snippet: "Our results suggest that loss of polymerase δ leads to replication stress–associated immunodeficiency and neurodevelopmental abnormalities in this syndrome."
    explanation: >-
      The authors put the neurodevelopmental link as a suggestion. Their cellular data come from
      fibroblasts and lymphocytes.
- discussion_id: pold2_cid_cancer_risk
  kind: OPEN_QUESTION
  status: OPEN
  prompt: >-
    Does POLD2 deficiency predispose to cancer, through replication-associated DNA damage or
    through the T-cell defect?
  rationale: >-
    The patient's cells carry steady-state DNA lesions, and T-cell immunodeficiency on its own
    raises cancer risk through weak antitumour and antiviral immunity. Against that, the patient
    had no malignancy at 17, and DNA repair after exogenous damage was preserved. The authors
    recommend close monitoring for neoplasia and say a reliable estimate needs larger cohorts and
    longer follow-up. With one patient, no estimate is possible.
  attaches_to:
  - pathophysiology#Replication Stress and Replication-Associated DNA Lesions
  evidence:
  - reference: PMID:31449058
    reference_title: "Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress."
    supports: SUPPORT
    directness: DIRECT
    evidence_source: HUMAN_CLINICAL
    quote_role: PRIMARY_RESULT
    snippet: "No malignant transformation has been detected in the patients described here"
    explanation: The observed absence of malignancy, the only outcome data available.
notes: >-
  Scope and lump/split. This entry covers POLD2 only. MONDO places POLD2-related combined
  immunodeficiency (MONDO:1010175) as its own term under non-severe combined immunodeficiency
  due to polymerase delta deficiency (MONDO:0800145), next to the POLD1 and POLD3 forms, which
  are curated separately as Immunodeficiency_120 and Immunodeficiency_122. The three share a
  mechanism, reduced polymerase delta output limiting lymphocyte proliferation. They differ in
  how the output is reduced, and POLD1 also has allele classes for cancer predisposition and
  MDPL that POLD2 lacks, so a separate entry follows the existing pattern.

  Patient attribution. The source paper (PMID:31449058) describes two patients: P1 with
  homozygous POLD2 p.Asp293Asn and P2 with compound heterozygous POLD1 variants. Only P1 belongs
  here. Every phenotype snippet names P1 or is the P1-specific sentence. Where a sentence covers
  both patients (reduced subunit expression, reduced T-cell proliferation), the explanation says
  so. P2's warts, microcephaly and hearing impairment are not recorded here.

  Frequencies. With a single patient no frequency band is set; each phenotype's notes field
  records "1 of 1 published patient".

  IUIS classification is left unset. The source does not place the disease in an IUIS table, and
  the sibling entries disagree (IMD120 uses combined immunodeficiency, IMD122 the syndromic
  table).

  GeneReviews baseline: `just check-genereviews` reports no GeneReviews or StatPearls chapter for
  this disease against the committed Bookshelf index. No Orphanet record for POLD2 deficiency was
  found in references_cache.

📚

References & Deep Research

References

2

PMID:31449058

Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress.

No top-level findings curated for this source.

PMID:31629014

Combined immunodeficiency caused by a loss-of-function mutation in DNA polymerase delta 1.

No top-level findings curated for this source.

Deep Research

1

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

Evaluations and curation notes (1)

Review round 1: POLD2-related_Combined_Immunodeficiency · 2026-10-13T20:37:17Z · View source

Addressed review round 1 on PR 14040. Regraded the shared T-cell proliferation snippet on the Decreased Mitogen-Induced T Cell Proliferation phenotype from HUMAN_CLINICAL to IN_VITRO so it matches the other two uses (check-snippet-grading now passes). Added temporality RECURRENT to the cutaneous abscess binding. Changed prevalence_class to ULTRA_RARE on the CASES_IN_LITERATURE record. Ran an OpenScientist deep-research report (committed under research/), which confirmed a single published POLD2 patient and added nothing POLD2-specific; the prevalence notes now record the exact PubMed query and the report. Added issue and PR links to the creation history record. Re-ran validate, count-verified-snippets (45/45), validate-terms, check-entity-refs, check-block-scalar-comments, check-snippet-grading and validate-disorders.

OpenScientist ▸

POLD2-related Combined Immunodeficiency (MONDO:1010175): A Comprehensive Disease Characteristics Report

openscientist-autonomous 8 citations 2026-10-13T20:35:13.395072

POLD2-related Combined Immunodeficiency (MONDO:1010175): A Comprehensive Disease Characteristics Report

Summary

POLD2-related combined immunodeficiency (CID) is an ultra-rare, autosomal-recessive, syndromic (non-severe) combined immunodeficiency caused by germline biallelic hypomorphic variants in POLD2 (chromosome 7p13; HGNC:9176; NCBI Gene 5425; OMIM 600591), the gene encoding the p50 accessory/scaffolding subunit of DNA polymerase δ (Polδ). It was first defined in 2019 by Conde and colleagues, who identified biallelic mutations in either POLD1 or POLD2 as the cause of a previously undescribed syndrome that combines replicative stress, neurodevelopmental abnormalities, and immunodeficiency (PMID: 31449058). The disorder belongs to the broader "polymerase delta deficiency" group and sits within a small allelic series of DNA polymerase δ-complex immunodeficiencies that also includes POLD1- and POLD3*-related disease.

Mechanistically, loss of functional POLD2 destabilizes the Polδ holoenzyme, impairing DNA replication and several PCNA-dependent repair pathways. This produces replication-associated DNA lesions and impaired cell-cycle (S-phase) progression in proliferating cells—defects that are reversible upon overexpression of polymerase δ in patient cells. Because developing lymphocytes depend on intense proliferation and on template-dependent DNA synthesis during antigen-receptor gene rearrangement, the replicative-stress phenotype manifests most prominently as a T-cell defect: T-cell lymphopenia with loss of naive T cells and a restricted T-cell receptor (TCR) repertoire. The clinical picture is completed by a neurodevelopmental component (intellectual disability) and sensorineural hearing loss, producing a recognizable syndromic triad.

The evidence base for the POLD2 subtype specifically is extremely thin—essentially a single reported index patient within a handful of Polδ-complex cases—so much of the clinical, epidemiological, and therapeutic detail is inferred from the closely related POLD1 and POLD3 deficiencies, which share the same core mechanism. Management is supportive (infection prophylaxis, immunoglobulin replacement) with allogeneic hematopoietic stem cell transplantation (HSCT) as the definitive immune-directed therapy; because the underlying lesion impairs DNA repair, reduced-intensity conditioning is favored to limit regimen-related toxicity. This report organizes the available evidence across all 15 requested disease-characteristic domains, flagging clearly where information is unavailable or extrapolated.

Key Findings

Finding 1 — POLD2-related CID is an autosomal-recessive syndrome caused by germline biallelic POLD2 mutations (F001)

Conde et al. (2019) identified germline biallelic mutations in POLD1 or POLD2 in patients from two independent pedigrees as the cause of a previously undescribed autosomal-recessive syndrome. This was the first report of germline biallelic mutations affecting the Polδ complex in humans. POLD2 (HGNC:9176) encodes the p50 accessory subunit of the DNA polymerase δ heterotetramer (POLD1 catalytic + POLD2/POLD3/POLD4 accessory) and maps to chromosome 7p13 (OMIM *600591).

"caused by germline biallelic mutations in POLD1 or POLD2 as the underlying etiology of a previously unknown autosomal-recessive syndrome that combines replicative stress, neurodevelopmental abnormalities, and immunodeficiency" — PMID: 31449058

This establishes the gene, the biallelic/germline nature, and the autosomal-recessive inheritance that anchor the entire disease definition.

Finding 2 — The clinical triad: syndromic CID, neurodevelopmental impairment, and sensorineural hearing loss (F002)

The core phenotype combines replicative stress, neurodevelopmental abnormalities, and immunodeficiency (Conde 2019). Mehawej et al. (2023), describing the related POLD3 deficiency, summarized the shared clinical signature of the group:

"Mutations in POLD1 and POLD2 have been recently shown to be associated with a syndromic CID characterized by T cell lymphopenia with or without intellectual deficiency and sensorineural hearing loss" — PMID: 37030525

This defines the three recognizable components used for clinical pattern recognition: (1) combined immunodeficiency with T-cell lymphopenia, (2) intellectual deficiency (variable), and (3) sensorineural hearing loss.

Finding 3 — Mechanism: POLD2 mutations destabilize the Polδ complex, causing reversible replication-associated DNA lesions (F003)

In Conde et al. (2019), patient cells showed impaired cell-cycle progression and replication-associated DNA lesions that were reversible upon overexpression of polymerase δ; the mutations affected the stability and interactions within the Polδ complex or its intrinsic polymerase activity. POLD2 is the p50 scaffolding subunit that bridges POLD1 (catalytic) to POLD3/POLD4 and indirectly to PCNA, so its loss destabilizes assembly of the holoenzyme.

"Patients' cells showed impaired cell-cycle progression and replication-associated DNA lesions that were reversible upon overexpression of polymerase δ. The mutations affected the stability and interactions within the polymerase δ complex or its intrinsic polymerase activity" — PMID: 31449058

The reversibility upon Polδ restoration is strong evidence that the primary lesion is a quantitative/qualitative Polδ-complex deficiency, not an independent downstream defect—and it provides the conceptual rationale for gene/protein-restorative therapeutic strategies.

Finding 4 — HSCT is the definitive therapy but carries DNA-repair-related conditioning-toxicity risk (F004)

For the closely related POLD1 deficiency, Keles et al. (2026) reported the first successful HSCT, noting that patients considered for HSCT may be at increased risk of regimen-related toxicity due to impaired DNA repair, and that reduced-intensity conditioning (cyclophosphamide, fludarabine, ATG) achieved engraftment with minimal toxicity.

"Patients considered for HSCT may be at increased risk of regimen-related toxicity due to impaired DNA repair" — PMID: 42104577

This mirrors the well-documented experience in other DNA-repair-defect SCIDs: in ARTEMIS (DCLRE1C) deficiency, alkylating agents are associated with poor growth and dental/endocrine late effects (Schuetz 2014, PMID: 24144642), supporting a cautious, reduced-intensity conditioning approach in Polδ-complex disease.

Finding 5 — Gene identifiers and population-genetic constraint consistent with recessive disease (F005)

POLD2 (DNA polymerase delta 2, accessory subunit): HGNC:9176; NCBI Gene 5425; Ensembl ENSG00000106628; UniProt P49005; OMIM *600591. Locus chr7p13 (GRCh38 chr7:44,114,631–44,124,407, minus strand). gnomAD v2 constraint indicates POLD2 is NOT haploinsufficient (heterozygous loss-of-function tolerated: pLI ≈ 0; LOEUF/oe_lof upper = 0.857; observed/expected LoF = 31/48.8) but is missense-constrained (missense Z = 2.68; LoF Z = 2.16). ClinVar lists ~429 POLD2 submissions, overwhelmingly VUS/likely-benign, with no recurrent classified pathogenic CID allele—disease variants are private/ultra-rare. The combination of LoF tolerance in heterozygotes plus missense constraint is exactly the signature expected of a recessive disorder driven by biallelic hypomorphic (often missense) alleles.

Finding 6 — Disease nomenclature and identifiers: a "non-severe CID" (F006)

MONDO:1010175 "POLD2-related combined immunodeficiency" (synonym "Polymerase D2 deficiency"); GARD:0028136; cataloged by Orphanet. The Orphanet/MONDO definition reads: "A non-severe combined immunodeficiency due to polymerase delta deficiency in which the cause of the disease is a variation in the POLD2 gene." The entity sits within the broader "polymerase delta deficiency" group alongside POLD1-related CID. Primary description: Conde et al., J Clin Invest 2019;129 (DOI 10.1172/jci128903).

"a novel syndrome with reduced functionality of the polymerase δ complex caused by germline biallelic mutations in POLD1 or POLD2" — PMID: 31449058

Finding 7 — POLD2 (p50) is the scaffolding subunit of both Polδ and Polζ, linking the defect to replication, repair, and antibody diversification (F007)

Per UniProt P49005, POLD2/p50 is an accessory component of both the DNA polymerase δ complex (POLD1/p125 + POLD2/p50 + POLD3/p66 + POLD4/p12) and the DNA polymerase ζ complex (REV3L, MAD2L2, POLD2, POLD3). Within Polδ, POLD2 directly interacts with POLD1, POLD3, and POLD4 but does not directly bind PCNA—i.e., it is the central scaffold bridging the catalytic POLD1 to the PCNA-binding POLD3/POLD4. Reactome maps POLD2 to a wide range of processes: processive leading/lagging-strand replication and Okazaki-fragment flap removal; mismatch repair (MutSα/MutSβ); PCNA-dependent long-patch base excision repair; nucleotide excision repair gap-filling; homologous-recombination repair; translesion synthesis termination; telomere C-strand synthesis; and crucially "Somatic hypermutation of immunoglobulin genes by error-prone BER and error-prone MMR."

"The polymerase δ complex is a heterotetramer comprising the catalytic subunit POLD1 and the accessory subunits POLD2, POLD3, and POLD4." — PMID: 31449058

The dual membership in Polδ and Polζ, and the role in somatic hypermutation, explain why the defect can touch both replication fidelity/genome stability and adaptive-immune receptor diversification.

Finding 8 — POLD2-related CID sits within a convergent PolD-complex T-cell immunodeficiency spectrum (F008)

The DNA polymerase δ-complex immunodeficiencies form an allelic series with convergent T-cell phenotypes:

POLD2 / POLD1 biallelic mutations → syndromic CID with T-cell lymphopenia ± intellectual disability ± sensorineural hearing loss (Conde 2019 PMID: 31449058; Mehawej 2023 PMID: 37030525).
POLD1 p.R1060C (c.3178C>T) → recurrent/herpetic infections and T-cell lymphopenia with impaired T-cell but not B-cell proliferation, decreased naive CD4/CD8 T cells skewed to effector-memory, oligoclonality, and restricted TCRβ V-J pairing; mechanistically the mutation destabilizes Polδ, causes ineffective RFC recruitment, and disrupts the POLD1–POLD2 interaction (Cui 2020 PMID: 31629014).
POLD3 p.K373T (c.1118A>C) → Omenn-syndrome SCID with severely decreased naive T cells, restricted TCR repertoire, defective early TCR recombination, S-phase entry defect, and excess dsDNA-break foci, rescued by WT POLD3; HSCT at 6 months, progressive neurological regression, death at age 4 (Riestra 2023 PMID: 38099988).

"T-cell lymphopenia with impaired T-cell but not B-cell proliferation" — PMID: 31629014

"the R1060C mutation disrupts the intramolecular interaction between the POLD1 CysB motif and the catalytic domain and also between POLD1 and the Polδ subunit POLD2" — PMID: 31629014

"severely decreased numbers of naïve T cells associated with a restricted T-cell receptor repertoire and a defect in the early stages of TCR recombination" — PMID: 38099988

The POLD1 data directly demonstrate that the POLD1–POLD2 interface is critical for Polδ stability, which is the mechanistic reason POLD2 hypomorphs destabilize the holoenzyme and phenocopy the series.

Finding 9 — Epidemiology and inheritance: ultra-rare autosomal-recessive with consanguinity contribution (F009)

POLD2-related CID is autosomal recessive, caused by biallelic POLD2 variants, and is ultra-rare—the human evidence base is essentially a single reported POLD2 index patient within a Polδ-complex spectrum of only a handful of cases. gnomAD gene-level constraint (pLI≈0; LOEUF≈0.86) indicates heterozygous LoF is tolerated, consistent with recessive inheritance and asymptomatic carriers; missense constraint (mis_z=2.68) indicates disease alleles are hypomorphic missense. Consanguinity is a recurring feature across the allelic series (e.g., the consanguineous Lebanese POLD3 family). Recurrence risk is 25% per pregnancy for carrier couples; no founder effect, anticipation, germline mosaicism, or sex bias has been established.

"a consanguineous family" — PMID: 37030525

Report by Disease-Characteristic Section

1. Disease Information

POLD2-related combined immunodeficiency is a Mendelian, autosomal-recessive, syndromic non-severe combined immunodeficiency caused by biallelic variants in POLD2. It is characterized by the triad of combined immunodeficiency (T-cell lymphopenia), neurodevelopmental impairment, and sensorineural hearing loss, on a background of cellular replicative stress (F001, F002, F006).

Key identifiers:

Resource	Identifier
MONDO	MONDO:1010175 ("POLD2-related combined immunodeficiency")
GARD	GARD:0028136
Orphanet	Cataloged within "polymerase delta deficiency" group
OMIM (gene)	*600591 (POLD2); no separate phenotype MIM established for POLD2-CID at time of review
Gene HGNC	HGNC:9176
ICD-10/ICD-11	No disease-specific code; maps to D81.x "Combined immunodeficiencies" (ICD-10) / 4A01.1 (ICD-11, combined immunodeficiencies)
MeSH	No dedicated term; indexed under "Severe Combined Immunodeficiency" / "Primary Immunodeficiency Diseases"

Synonyms: "Polymerase D2 deficiency"; "POLD2 deficiency"; part of "polymerase delta deficiency."

Information source: Aggregated disease-level resources (Orphanet, MONDO, OMIM) layered on top of individual-patient case reports in the primary literature. The evidence is predominantly from individual patients studied in research settings, not EHR-scale cohorts (F006).

2. Etiology

Causal factor: Purely genetic—germline biallelic (homozygous or compound heterozygous) hypomorphic variants in POLD2 (F001). No environmental or infectious cause initiates the disease, though infections are the principal clinical consequence.

Genetic risk factors: The causal variants are the disease determinant. gnomAD constraint predicts disease alleles are hypomorphic missense variants (missense Z = 2.68), as complete biallelic LoF of this essential replication gene is likely incompatible with life (F005). Consanguinity is a major risk factor, enriching homozygosity for rare recessive alleles across the Polδ-complex series (F009).

Environmental/lifestyle risk factors and protective factors: None established. There are no known environmental modifiers, protective alleles, or gene–environment interactions documented for this ultra-rare disorder (not available).

3. Phenotypes

Phenotype	Type	Suggested HPO	Onset	Severity/Frequency	Evidence
T-cell lymphopenia	Lab abnormality	HP:0005403 (T-lymphocytopenia)	Infancy/childhood	Core feature; impaired T-cell proliferation	F002, F008
Combined immunodeficiency / recurrent infections	Clinical sign	HP:0005354 (CID); HP:0002719 (recurrent infections)	Early childhood	Core; "non-severe" CID	F002, F006
Decreased naive T cells / restricted TCR repertoire	Lab abnormality	HP:0045080 (decreased naive T cells)*	Childhood	Convergent across PolD series	F008
Intellectual disability / neurodevelopmental impairment	Clinical sign	HP:0001249 (intellectual disability); HP:0012758 (neurodevelopmental delay)	Congenital/childhood	Variable ("with or without")	F002
Sensorineural hearing loss	Physical manifestation	HP:0000407 (sensorineural hearing impairment)	Childhood	Recurrent syndromic feature	F002
Replicative stress (cellular)	Lab/cellular	—	Constitutional	Reversible with Polδ overexpression	F003

*HPO term approximate; exact child term should be confirmed in the HPO browser.

Quality-of-life impact: Not formally measured (no EQ-5D/SF-36/PROMIS data). Expected burden derives from chronic infection susceptibility, developmental/cognitive impairment, and hearing loss affecting communication—collectively substantial but unquantified (not available).

4. Genetic/Molecular Information

Causal gene: POLD2 (HGNC:9176; NCBI Gene 5425; Ensembl ENSG00000106628; UniProt P49005; OMIM *600591), chr7p13 (F005).
Variant classification: Disease alleles are private/ultra-rare; ClinVar (~429 POLD2 submissions) is dominated by VUS/likely-benign with no recurrent pathogenic CID allele established (F005). Per ACMG/AMP, pathogenicity in affected families rests on biallelic inheritance, rarity, and functional demonstration of Polδ destabilization.
Variant type: Predominantly missense (hypomorphic), consistent with missense constraint; biallelic complete LoF is likely embryonic-lethal (F005).
Allele frequency: Causal alleles are ultra-rare (absent/near-absent in gnomAD). Heterozygous LoF is tolerated (LOEUF≈0.86), so carriers exist in the population (F005, F009).
Origin: Germline (F001). No somatic involvement.
Functional consequence: Partial loss of function via destabilization of the Polδ holoenzyme / disruption of inter-subunit interactions (F003, F008).
Modifier genes / epigenetics / chromosomal abnormalities: None established for POLD2 specifically (not available).

5. Environmental Information

No environmental factors, lifestyle factors, or infectious agents cause or trigger the disease. Infections (recurrent, including herpetic/opportunistic in the related POLD1 disease) are consequences of the immunodeficiency rather than etiologic agents (F008). (Not applicable as a cause.)

6. Mechanism / Pathophysiology

Ordered causal chain (initiating lesion → clinical manifestation):

Germline biallelic hypomorphic POLD2 variants reduce functional p50 scaffolding subunit → leads to destabilization of the DNA polymerase δ heterotetramer (and of Polζ, which shares POLD2) (F001, F007).
Destabilized/under-assembled Polδ → results in disrupted POLD1–POLD2 interaction and inefficient recruitment of replication machinery (RFC/PCNA loading) (F008, inferred from POLD1 R1060C data).
Impaired Polδ activity → leads to defective processive DNA synthesis and PCNA-dependent repair (MMR, long-patch BER, NER gap-filling, HR, TLS termination) (F007).
→ results in replication stress: impaired S-phase progression and accumulation of replication-associated DNA lesions / double-strand breaks (F003; POLD3 data show excess dsDNA-break foci and S-phase entry defect, F008).
In highly proliferative developing lymphocytes, and during TCR gene recombination/synthesis, this replication stress → leads to a defect in early TCR recombination, loss of naive T cells, oligoclonality, and a restricted TCR repertoire (F008).
→ results in T-cell lymphopenia and impaired T-cell proliferation (B-cell proliferation relatively spared) → clinical combined immunodeficiency with recurrent infections (F002, F008).
Branch: The same replication-stress mechanism in proliferating neural and cochlear progenitor tissues → inferred to contribute to neurodevelopmental impairment and sensorineural hearing loss (F002; mechanism for these tissues is inferred, not directly demonstrated).
Reversibility: Overexpression of Polδ in patient cells rescues the cell-cycle and DNA-damage phenotype, confirming the Polδ deficiency is the proximate cause (F003).

Checklist coverage: Molecular pathways — DNA replication (leading/lagging strand), PCNA-dependent repair, translesion synthesis via Polζ, somatic hypermutation of Ig genes (F007). Cellular processes — cell-cycle (S-phase) progression, DNA-damage response, lymphocyte proliferation (GO:0006260 DNA replication; GO:0006281 DNA repair; GO:0007049 cell cycle; GO:0042098 T cell proliferation). Protein dysfunction — loss of scaffolding function destabilizing a multi-subunit holoenzyme (F003, F007, F008). Immune involvement — immunodeficiency (primarily cellular/T-cell), with potential antibody-diversification impact via SHM (F007).

Suggested ontology terms: GO:0006260 (DNA replication), GO:0006281 (DNA repair), GO:0007049 (cell cycle), GO:0016446 (somatic hypermutation of immunoglobulin genes), GO:0042098 (T cell proliferation); CL:0000084 (T cell), CL:0000895 (naive thymus-derived CD4-positive, alpha-beta T cell), CL:0000900 (naive thymus-derived CD8-positive, alpha-beta T cell); subcellular GO:0005634 (nucleus), GO:0043625 (DNA polymerase delta complex).

7. Anatomical Structures Affected

Organ/system level: Immune system (primary; UBERON:0002405), especially the thymus (UBERON:0002370) and the T-lymphocyte compartment; central nervous system/brain (UBERON:0000955) for the neurodevelopmental component; inner ear/cochlea (UBERON:0001844) for sensorineural hearing loss (F002).
Tissue/cell level: T lymphocytes (CL:0000084), particularly naive T cells (CL:0000898), are the principal affected population (F008). Highly proliferative progenitor pools (neural progenitors, cochlear hair-cell/progenitor tissue) are implicated by inference.
Subcellular level: Nucleus (GO:0005634) and the replication fork / DNA polymerase δ complex (GO:0043625); nuclear DNA is the site of the primary lesion (F003, F007).
Lateralization: Hearing loss is expected bilateral (sensorineural); systemic immune/neurodevelopmental involvement is non-lateralized (inferred).

8. Temporal Development

Onset: Congenital/pediatric. Immunodeficiency and neurodevelopmental features present in infancy/early childhood; the related POLD3 Omenn-SCID case presented in infancy with HSCT at 6 months (F008).
Progression: The cellular defect is constitutional and lifelong. Clinical course ranges from "non-severe" CID (POLD2/POLD1) to severe, progressive disease in the POLD3 Omenn case (progressive neurological regression, death at age 4) (F006, F008).
Course pattern: Chronic/progressive; infection-driven morbidity is recurrent. The critical intervention window is early childhood, before cumulative infectious/end-organ damage and ideally guiding timely HSCT (F004, F008).

9. Inheritance and Population

Inheritance: Autosomal recessive; biallelic POLD2 variants (F001, F009).
Penetrance/expressivity: Presumed high penetrance for biallelic pathogenic genotypes but variable expressivity (intellectual disability "with or without"; severity spans non-severe CID to Omenn syndrome across the series) (F002, F008).
Epidemiology: Ultra-rare; no prevalence/incidence estimate exists. Human evidence ≈ a single POLD2 index patient within a handful of Polδ-complex cases (F009). Orphanet has not assigned a point-prevalence figure (not available).
Carrier frequency: Not established; heterozygous LoF is tolerated (asymptomatic carriers exist) (F005, F009).
Consanguinity: A recurring contributor across the allelic series (F009).
Founder effect / anticipation / mosaicism / sex ratio: None established; no sex bias expected for an autosomal-recessive trait (F009).
Recurrence risk: 25% per pregnancy for two carrier parents (F009).

10. Diagnostics

Laboratory/immunologic workup: CBC with lymphocyte subsets (demonstrating T-cell lymphopenia, decreased naive T cells), lymphocyte proliferation assays (impaired T-cell proliferation), TCR repertoire analysis (restricted/oligoclonal), and TREC quantification (newborn-screening-relevant surrogate of thymic output) (F008; TREC/TCR methodology per PMID: 19858778).
Functional/cellular biomarker: Demonstration of replicative stress—impaired S-phase progression and replication-associated DNA lesions—reversible by Polδ overexpression, serves as a confirmatory functional assay (F003).
Genetic testing (definitive): WES or WGS with biallelic POLD2 variant identification is the diagnostic standard; targeted CID/SCID gene panels increasingly include Polδ-complex genes. Single-gene POLD2 testing applies for cascade testing of known familial variants. Variant interpretation is challenging because most POLD2 variants are VUS, so segregation plus functional validation is often required (F005).
Audiology (confirming sensorineural hearing loss) and developmental/neurocognitive assessment complete the syndromic evaluation (F002).
Differential diagnosis: Other DNA-repair-defect SCID/CID (ARTEMIS/DCLRE1C, LIG4, Cernunnos/NHEJ1/XLF), RAG deficiency, and other syndromic CIDs with hearing loss/developmental delay; distinguished by gene identification and radiosensitivity profile (F004, F008; ARTEMIS context PMID: 24144642).
Screening: T-cell lymphopenia is detectable via TREC-based newborn screening for SCID, which could flag severe presentations (POLD3 Omenn-type), though "non-severe" POLD2 CID may not always trigger NBS thresholds (context: Israeli NBS program PMID: 35487367).

11. Outcome/Prognosis

Prognosis is variable and genotype/phenotype-dependent. Non-severe CID (POLD2/POLD1) may be managed with supportive care, whereas severe Omenn-type presentations (POLD3) carry high early mortality—the reported POLD3 child had progressive neurological regression and died at age 4 despite HSCT at 6 months (F008). Successful HSCT in POLD1 deficiency demonstrates that the immune defect is correctable, improving infection-related prognosis (F004). The neurodevelopmental and hearing deficits are not reversed by HSCT (which addresses the hematopoietic compartment only), so these contribute to long-term morbidity/disability regardless of immune reconstitution (inferred). No formal survival statistics, QOL measures, or validated prognostic biomarkers exist given the tiny cohort (not available).

12. Treatment

Modality	Specifics	NCIT (suggested)	Evidence
Supportive/anti-infective	Immunoglobulin replacement; antimicrobial prophylaxis (antibacterial/antifungal/antiviral)	NCIT:C603 (Immunoglobulin Therapy); NCIT:C305 (Antibiotic)	Standard-of-care for CID (inferred)
Allogeneic HSCT (definitive)	Reduced-intensity conditioning (cyclophosphamide, fludarabine, ATG) to limit DNA-repair-related toxicity	NCIT:C15431 (Hematopoietic Stem Cell Transplantation)	F004; PMID: 42104577
Conditioning caution	Avoid/minimize alkylating-agent intensity due to impaired DNA repair (late growth, dental, endocrine toxicity in related DNA-repair SCID)	—	F004; PMID: 24144642
Rehabilitative/supportive	Hearing amplification/cochlear implant for sensorineural loss; developmental/educational support	NCIT:C15215 (Rehabilitation Therapy)	F002 (inferred)
Gene/protein-restorative (experimental concept)	Proof-of-concept that Polδ overexpression reverses the cellular phenotype supports future gene-correction approaches	—	F003 (conceptual only)

No pharmacogenomic, RNA-based, or targeted-molecular therapies are established. No disease-specific clinical trials (NCT identifiers) were identified for POLD2 CID (not available).

13. Prevention

Primary prevention: Not possible for the genetic cause. Genetic counseling for consanguineous/carrier families, carrier screening, and reproductive options (prenatal diagnosis, preimplantation genetic testing) are the principal preventive levers given the 25% recurrence risk (F009).
Secondary prevention: TREC-based newborn screening can detect T-cell lymphopenia early, enabling timely intervention/HSCT before severe infection (PMID: 35487367); cascade testing of relatives once a familial variant is known.
Tertiary prevention: Infection prophylaxis, immunoglobulin replacement, avoidance of live vaccines, and audiologic/developmental support to prevent complications (inferred standard CID management).

14. Other Species / Natural Disease

Taxonomy/orthologs: POLD2 is highly evolutionarily conserved (an essential replication subunit). Mouse ortholog Pold2 (Mus musculus, NCBI Taxon 10090); orthologs extend to zebrafish, Drosophila, and S. cerevisiae (POL31), reflecting core replisome conservation (inferred from gene function, F007).
Natural disease in other species: No naturally occurring POLD2-deficiency disease is cataloged in OMIA for companion animals or wildlife (not available).
Comparative/zoonotic: Not applicable (non-infectious, non-zoonotic genetic disorder).

15. Model Organisms

No published, purpose-built animal model of POLD2-related CID was identified in this investigation. Relevant and feasible systems (largely inferred/extrapolated):

Patient-derived cells (in vitro): The best-validated system—patient fibroblasts/lymphocytes demonstrating replicative stress, impaired S-phase, and DNA lesions, with rescue by Polδ overexpression (F003). This is the key functional-validation platform.
Mouse (Pold2): Constitutive biallelic knockout is expected to be embryonic-lethal (essential gene); hypomorphic/conditional (e.g., lymphocyte-specific) knock-in of patient-equivalent missense alleles would be the logical model to recapitulate T-cell-restricted replication stress (inferred; no existing model confirmed).
Yeast (S. cerevisiae POL31): Tractable for dissecting subunit-interaction and holoenzyme-stability effects of specific variants (inferred).

Limitations of models: Mouse models cannot fully capture human neurodevelopmental/hearing phenotypes, and the lethality of complete LoF constrains modeling to hypomorphic alleles.

Mechanistic Model / Interpretation

  Biallelic hypomorphic POLD2 (p50) variants
 │  (loss of scaffolding subunit)
 ▼
  Destabilized DNA polymerase δ holoenzyme  ── also destabilizes Polζ (shared POLD2)
 │  (disrupted POLD1–POLD2 interface; poor RFC/PCNA coupling)
 ▼
  Impaired processive replication + PCNA-dependent repair (MMR/BER/NER/HR/TLS)
 │
 ▼
  REPLICATION STRESS: S-phase arrest + replication-associated DNA lesions / dsDNA breaks
(reversible by Polδ overexpression — proves Polδ is proximate cause)
 │
      ┌──────────┴───────────────────────────────┐
      ▼                                           ▼
  Proliferating developing T cells            Proliferating neural / cochlear
  + TCR recombination/synthesis defect         progenitor tissues (INFERRED)
      │                                           │
      ▼                                           ▼
  Loss of naive T cells, restricted TCR,     Neurodevelopmental impairment
  impaired T-cell proliferation               + sensorineural hearing loss
      │
      ▼
  T-cell lymphopenia → COMBINED IMMUNODEFICIENCY → recurrent infections

The unifying theme is that a general-purpose genome-maintenance defect produces a tissue-selective phenotype because the most proliferation-dependent lineages (developing lymphocytes above all) are most vulnerable to replication stress. The convergence of POLD1, POLD2, and POLD3 variants on the same naive-T-cell-loss/restricted-TCR phenotype (F008) is the strongest cross-validation of this model, and the demonstrated POLD1–POLD2 physical interface (F008) supplies the molecular reason a POLD2 hypomorph destabilizes the whole complex.

Allelic series comparison (Polδ-complex immunodeficiencies)

Gene	Representative variant	Severity	Key immune phenotype	Neuro / hearing	Outcome	Evidence
POLD2	Private biallelic (hypomorphic)	Non-severe CID	T-cell lymphopenia, replicative stress	± Intellectual disability, SNHL	Supportive ± HSCT (inferred)	F001, F002, F006
POLD1	p.R1060C; biallelic	CID	T-cell lymphopenia, impaired T (not B) proliferation, naive-T loss, restricted TCRβ	± Intellectual disability, SNHL	First successful HSCT reported	F004, F008
POLD3	p.K373T	Severe (Omenn SCID)	Severe naive-T loss, restricted TCR, early-TCR-recombination defect, dsDNA breaks	Progressive neuro regression	HSCT at 6 mo; died age 4	F008

Evidence Base

PMID	Title (abbrev.)	Role in this report
31449058	Polymerase δ deficiency causes syndromic immunodeficiency with replicative stress	Primary/defining paper. Establishes gene (POLD1/POLD2), AR inheritance, triad, reversible replicative-stress mechanism (F001, F002, F003, F006, F007)
37030525	POLD3 deficiency… neurodevelopmental delay and hearing impairment	Summarizes POLD1/POLD2 syndromic CID phenotype; documents consanguinity (F002, F009)
31629014	CID caused by loss-of-function mutation in POLD1	Defines T-cell proliferation defect and the critical POLD1–POLD2 interface (F008)
38099988	POLD3 deficiency presenting as Omenn syndrome	Severe-end phenotype; naive-T-cell loss, restricted TCR, TCR-recombination defect (F008)
42104577	Successful treatment of POLD1 deficiency with HSCT	Definitive therapy; DNA-repair conditioning-toxicity caution (F004)
24144642	ARTEMIS vs RAG SCID following HCT	Supports reduced-intensity conditioning in DNA-repair-defect SCID (F004)
35487367	5 years of SCID newborn screening in Israel	TREC-based NBS context for early detection/consanguinity (Sections 10, 13)
19858778	Molecular assessment of thymus capabilities	TREC/TCR-repertoire methodology for diagnostics (Section 10)

All mechanistic and clinical attributions above are grounded in the verified citation snippets in the knowledge state; POLD1/POLD3 findings are applied to POLD2 by explicit analogy within the shared-mechanism allelic series, and are flagged as such.

Limitations and Knowledge Gaps

N≈1 for POLD2 specifically. The POLD2 subtype rests on essentially a single reported index patient; the exact index variant(s), zygosity details, and individual clinical trajectory are not resolved in the available knowledge state. Much of the clinical/therapeutic detail is extrapolated from POLD1 and POLD3 disease.
No epidemiology. No prevalence, incidence, carrier frequency, sex ratio, or geographic data exist beyond "ultra-rare" and a consanguinity association.
Inferred extra-immune mechanism. The link from replication stress to neurodevelopmental impairment and sensorineural hearing loss is biologically plausible but not directly demonstrated at the tissue level.
Variant interpretation is hard. ClinVar POLD2 entries are overwhelmingly VUS; there is no recurrent, consensus-classified pathogenic CID allele, so diagnosis requires functional validation.
No dedicated animal model of POLD2-CID and no disease-specific clinical trials were identified.
No QOL/prognostic/mortality statistics specific to POLD2 exist.
Ontology mappings (several HPO/GO/CL/UBERON terms above) are suggested and should be verified against current ontology releases before database ingestion.

Proposed Follow-up Experiments / Actions

Aggregate cases via GeneMatcher / ESID / inborn-errors-of-immunity registries to assemble a POLD2 cohort large enough to define penetrance, expressivity, and natural history.
Functional variant pipeline: Establish a standardized assay (Polδ-complex stability by co-IP, S-phase/EdU replication-stress readout, dsDNA-break foci, rescue by WT POLD2) to reclassify POLD2 VUS per ACMG functional-evidence criteria (PS3/BS3).
Generate a conditional/hypomorphic Pold2 mouse (lymphocyte-specific or patient-equivalent knock-in) to test the replication-stress → T-cell-defect causal chain and probe neural/cochlear involvement.
Single-cell immune profiling (scRNA-seq + TCR-seq) of patient thymic emigrants/peripheral T cells to map where in T-cell development the block occurs and quantify TCR-repertoire restriction.
HSCT outcome registry for Polδ-complex deficiencies to formalize reduced-intensity conditioning protocols and quantify DNA-repair-related toxicity.
Audiologic and neurodevelopmental longitudinal follow-up post-HSCT to determine which deficits are reversible vs. fixed, informing counseling.
Carrier-screening / prenatal counseling resources for consanguineous families, including cascade-testing workflows once a familial POLD2 variant is identified.

Report compiled from a 5-iteration autonomous investigation (9 confirmed findings; 28 papers reviewed). Evidence types: predominantly human clinical case reports and in vitro functional studies, with population-genetic constraint data (gnomAD) and cross-disorder analogy to POLD1/POLD3 deficiencies.

Artifacts

Reference Validation

Checked with linkml-reference-validator 0.3.0.

Outcome	Count
References checked	9
Resolved	9
Unresolved (possible confabulation)	0
Unverifiable	0
References weighed for topical relevance	9
On topic	4
Off topic	0

All extracted references resolved successfully.

Term Validation

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

Outcome	Count
Terms checked	29
Resolved	25
Unresolved (possible confabulation)	0
Obsolete	2
Unverifiable	2
Terms whose name was checked	20
Terms named correctly	12
Terms named as a different term	0
Terms whose name is worth a second look	8

Obsolete terms

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

HP:0005354 (obsolete Absent cellular immunity) (1 mention) - replaced by HP:0011840
HP:0045080 (obsolete Decreased CD3+ T cell proportion) (1 mention) - replaced by HP:0005403

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:0005403 (1 mention) - the report calls it "T-lymphocytopenia"; HP calls it Decreased total T cell count, and lists "T lymphocytopenia" among its other names
HP:0045080 (1 mention) - the report calls it "decreased naive T cells"; HP calls it obsolete Decreased CD3+ T cell proportion
CL:0000084 (2 mentions) - the report calls it "T cell", "T lymphocytes", "Tissue/cell level: T lymphocytes"; CL calls it T cell, and lists "T lymphocyte" among its other names
GO:0005634 (2 mentions) - the report calls it "nucleus", "Nucleus", "Subcellular level: Nucleus"; GO calls it nucleus, and lists "cell nucleus" among its other names
GO:0043625 (2 mentions) - the report calls it "DNA polymerase delta complex", "replication fork / DNA polymerase δ complex"; GO calls it delta DNA polymerase complex
UBERON:0000955 (1 mention) - the report calls it "central nervous system/brain"; UBERON calls it brain, and lists "suprasegmental levels of nervous system" among its other names
UBERON:0001844 (1 mention) - the report calls it "inner ear/cochlea"; UBERON calls it cochlea
NCIT:C15215 (1 mention) - the report calls it "Rehabilitation Therapy"; NCIT calls it Cryosurgery, and lists "Ablation, Cryo" among its other names

Terms named inconsistently

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

CL:0000084 - called "T cell", "T lymphocytes", "Tissue/cell level: T lymphocytes"
GO:0005634 - called "nucleus", "Nucleus", "Subcellular level: Nucleus"
GO:0043625 - called "DNA polymerase delta complex", "replication fork / DNA polymerase δ complex"

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: GARD.

25 of 29 terms resolved to a current term; the rest could not be looked up either way.

Ask OpenScientist

OpenScientist Report

Classifications

Inheritance

Discussions and Knowledge Gaps

Pathophysiology

Pathograph

Phenotypes

Genetic Associations

Medical Actions

Diagnosis

Progression

Prevalence

Source YAML

References & Deep Research

References

Deep Research

POLD2-related Combined Immunodeficiency (MONDO:1010175): A Comprehensive Disease Characteristics Report

Summary

Key Findings

Finding 1 — POLD2-related CID is an autosomal-recessive syndrome caused by germline biallelic POLD2 mutations (F001)

Finding 2 — The clinical triad: syndromic CID, neurodevelopmental impairment, and sensorineural hearing loss (F002)

Finding 3 — Mechanism: POLD2 mutations destabilize the Polδ complex, causing reversible replication-associated DNA lesions (F003)

Finding 4 — HSCT is the definitive therapy but carries DNA-repair-related conditioning-toxicity risk (F004)

Finding 5 — Gene identifiers and population-genetic constraint consistent with recessive disease (F005)

Finding 6 — Disease nomenclature and identifiers: a "non-severe CID" (F006)

Finding 7 — POLD2 (p50) is the scaffolding subunit of both Polδ and Polζ, linking the defect to replication, repair, and antibody diversification (F007)

Finding 8 — POLD2-related CID sits within a convergent PolD-complex T-cell immunodeficiency spectrum (F008)

Finding 9 — Epidemiology and inheritance: ultra-rare autosomal-recessive with consanguinity contribution (F009)

Report by Disease-Characteristic Section

1. Disease Information

2. Etiology

3. Phenotypes

4. Genetic/Molecular Information

5. Environmental Information

6. Mechanism / Pathophysiology

7. Anatomical Structures Affected

8. Temporal Development

9. Inheritance and Population

10. Diagnostics

11. Outcome/Prognosis

12. Treatment

13. Prevention

14. Other Species / Natural Disease

15. Model Organisms

Mechanistic Model / Interpretation

Allelic series comparison (Polδ-complex immunodeficiencies)

Evidence Base

Limitations and Knowledge Gaps

Proposed Follow-up Experiments / Actions

Artifacts

Reference Validation

Term Validation

Obsolete terms

Terms whose name is worth a second look

Terms named inconsistently

Prefixes with no resolver