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Classifications

Harrison's Part

NIH Highlighted Topic

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Inheritance

1

Autosomal recessive inheritance HP:0000007

CMMRD requires two defective alleles of a single mismatch repair gene (homozygous or compound heterozygous). Both parents are obligate heterozygous carriers and are managed as Lynch syndrome. Consanguinity and endogamy raise the local frequency substantially. Penetrance is very high but not formally complete: cumulative cancer incidence reaches 90% by age 18 years, and 97% of patients in the largest cohort had developed cancer, so it is recorded as INCOMPLETE with a penetrance_percentage of 90 at age 18 rather than COMPLETE. Expressivity is highly variable by gene and by variant class - MLH1 and MSH2 biallelic variants give the earliest onset and worst survival, PMS2 and MSH6 are later and milder, and hypomorphic alleles can delay first cancer into adulthood, producing a Lynch-like presentation.

Autosomal recessive inheritance Penetrance: INCOMPLETE Penetrance %: 90% cumulative cancer incidence by age 18 years Expressivity: VARIABLE

Show evidence (2 references)

PMID:24737826 SUPPORT Human Clinical

"Constitutional mismatch repair deficiency (CMMRD) syndrome is a distinct childhood cancer predisposition syndrome that results from biallelic germline mutations in one of the four MMR genes, MLH1, MSH2, MSH6 or PMS2."

States the defining biallelic germline genotype across all four MMR genes.

PMID:38552658 SUPPORT Human Clinical

"The cumulative cancer incidence by age 18 years was 90% (95% CI 80-99)."

Quantifies the near-complete penetrance recorded in this Inheritance block.

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Subtypes

4

Mismatch repair cancer syndrome 1 (MLH1-related) MONDO:0010159

MLH1 hgnc:7127

Biallelic MLH1 variants (OMIM 276300). Together with MSH2 this is the severe end of the spectrum: earliest cancer onset and the poorest survival. Historically some cases were reported as Turcot syndrome before the genetic basis was understood.

Show evidence (2 references)

PMID:38552658 SUPPORT Human Clinical

"MLH1 or MSH2 variants caused earlier cancer onset than PMS2 or MSH6 variants, and inferior survival"

Establishes MLH1 as one of the two severe gene-defined forms.

PMID:42348008 SUPPORT Human Clinical

"Other previously published cases of CMMRD were likely misclassified as Turcot syndrome"

Supports the historical Turcot syndrome labelling noted in this subtype description.

Mismatch repair cancer syndrome 2 (MSH2-related) MONDO:0030840

MSH2 hgnc:7325

Biallelic MSH2 variants (OMIM 619096). The least commonly reported and the most severe gene-defined form, with the earliest cancer onset and the poorest survival of the four.

Show evidence (2 references)

PMID:38552658 SUPPORT Human Clinical

"MLH1 or MSH2 variants caused earlier cancer onset than PMS2 or MSH6 variants, and inferior survival"

Places MSH2 with MLH1 in the early-onset, inferior-survival group.

PMID:39910726 SUPPORT Human Clinical

"PMS2 is the most affected gene, followed by MSH6, MLH1, and MSH2."

Ranks MSH2 last by reported case frequency, supporting "least commonly reported".

Mismatch repair cancer syndrome 3 (MSH6-related) MONDO:0030841

MSH6 hgnc:7329

Biallelic MSH6 variants (OMIM 619097). The second most frequently reported molecular form. Later onset than MLH1/MSH2 but earlier than PMS2, and the broadest cutaneous spectrum along with PMS2.

Show evidence (3 references)

PMID:39910726 SUPPORT Human Clinical

"PMS2 is the most affected gene, followed by MSH6, MLH1, and MSH2."

Ranks MSH6 second among the gene-defined forms by reported case frequency.

PMID:39910726 SUPPORT Human Clinical

"with the age of onset progressively decreasing from PMS2 to MSH6, to MLH1 and MSH2"

Supports placing MSH6 onset between PMS2 (latest) and MLH1/MSH2 (earliest).

PMID:39910726 SUPPORT Human Clinical

"PMS2 and MSH6 pathogenic variants are linked to the broadest spectrum of cutaneous manifestations"

Supports the broad cutaneous spectrum attributed to MSH6 alongside PMS2.

Mismatch repair cancer syndrome 4 (PMS2-related) MONDO:0030843

PMS2 hgnc:9122

Biallelic PMS2 variants (OMIM 619101). The most commonly reported molecular form, with the latest onset and the best survival of the four. Diagnosis is technically complicated by the PMS2 pseudogenes, so locus-specific methods are required. Gastrointestinal tumours predominate in the later-onset PMS2 group whereas CNS tumours predominate in early-onset cases.

Show evidence (2 references)

PMID:41333974 SUPPORT Human Clinical

"The PMS2-related subtype (PMS2-CMMRD) is the most common molecular form of CMMRD, exhibiting variable severity and both early and late-onset clinical presentations."

Establishes PMS2 as the most common molecular form with a bimodal onset distribution.

PMID:41333974 SUPPORT Human Clinical

"Significant differences in tumour distribution were observed, with CNS tumours being most prevalent in the early-onset group, while GI tumours were more common in the later-onset group."

Supports the CNS-early / GI-late tumour distribution described for this subtype.

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Mechanistic Hypotheses

2

Constitutional Mutator plus Somatic Polymerase Proofreading Loss Model

constitutional_mutator_ultrahypermutation_model CANONICAL

Evidence balance 1 support

Biallelic germline MMR loss removes post-replicative mismatch correction from every cell of the body, so microsatellite instability and an elevated base-substitution rate are present constitutionally rather than only in tumour tissue. Proliferative compartments (neural/glial progenitors, intestinal crypt stem cells, lymphoid precursors) accumulate driver mutations first, which is why CNS, gastrointestinal and haematological tumours dominate the spectrum. In most high-grade tumours an early somatic mutation in the exonuclease (proofreading) domain of POLE or POLD1 is frequently selected, and the combined loss of proofreading and mismatch repair produces an ultra-hypermutant genome that accumulates mutations in a rapid burst. This two-step model explains both the distinctive pathognomonic mutational signature used diagnostically and the extreme neoantigen load that makes these tumours checkpoint-inhibitor responsive.

Show evidence (1 reference)

PMID:25642631 SUPPORT Human Clinical

"The ensuing mutation signatures and numbers are unique and diagnostic of childhood germ-line bMMRD"

Supports the combined germline-MMR plus somatic-polymerase model as producing a distinctive, diagnostic mutational output.

Hypomorphic Allele / Residual Repair Model of Late-Onset CMMRD

hypomorphic_allele_late_onset_model EMERGING

Evidence balance 2 support

A subset of biallelic MMR carriers present in adulthood with a Lynch-like tumour spectrum rather than in childhood. The proposed explanation is that the variants are hypomorphic - a leaky splice variant or a missense change that retains partial repair capacity - so residual mismatch repair delays the accumulation of driver mutations without preventing it. If correct, constitutional microsatellite instability testing rather than genotype alone is what discriminates such patients from true Lynch syndrome, and it predicts that the phenotype should scale with residual repair activity. This is currently supported by case-level functional and constitutional-MSI data, not by a systematic genotype-function series.

Show evidence (2 references)

PMID:38789506 SUPPORT Human Clinical

"A hypomorphic effect of these and other variants found in additional late onset CMMRD cases, identified by literature review, likely explains a LS-like phenotype."

States the hypomorphic-residual-function hypothesis for late-onset, Lynch-mimicking CMMRD.

PMID:38789506 SUPPORT Human Clinical

"The apparent genotype-phenotype conflict was resolved by detection of constitutional microsatellite instability in both patients, a hallmark feature of CMMRD."

Supports constitutional MSI, not genotype, as the discriminating test predicted by this model.

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Discussions and Knowledge Gaps

2

Why do CNS, gastrointestinal and lymphoid compartments transform preferentially in CMMRD when mismatch repair is absent from every tissue?

KNOWLEDGE GAP OPEN cmmrd_normal_tissue_mmr_kinetics

Attached to

pathophysiology#Constitutional Microsatellite Instability and Mutator Phenotype pathophysiology#Accelerated Multi-Organ Tumorigenesis

Constitutional instability is systemic, yet the tumour spectrum is not. Constitutional instability scores differ by tissue (gastrointestinal > blood > brain) and rise with age within an individual, so the tissue-specific transformation risk is not simply a function of measured instability. The determinants - proliferative index of the stem compartment, tissue-specific dependence on MMR versus backup pathways, immune surveillance, or the tissue-specific probability of acquiring the somatic POLE/POLD1 hit - have not been disentangled. Getting this right would sharpen surveillance intervals, which are currently uniform rather than risk-weighted by tissue.

Proposed experiments

Clone-resolved mutation burden across matched normal tissues in CMMRD

exp_cmmrd_tissue_resolved_mutation_burden

Measure somatic mutation burden and microsatellite indel load at single-cell or clone-resolved depth in matched normal brain, intestinal crypt and haematopoietic compartments from the same CMMRD individual, and test whether the compartment with the highest measured instability is the compartment that transforms first.

Show evidence (1 reference)

PMID:36240479 SUPPORT Human Clinical

"In normal cells, MMRDness scores differed between tissues (GI > blood > brain)"

Design rationale - a tissue gradient in constitutional instability is already measurable, so a clone-resolved comparison across the same tissues is feasible.

Longitudinal constitutional-instability scoring against organ of first cancer

exp_cmmrd_longitudinal_instability_vs_first_cancer

Score constitutional genomic instability serially in blood and, where available, gastrointestinal tissue from a prospective CMMRD surveillance cohort, and test whether the instability trajectory predicts the organ and timing of the first cancer well enough to justify tissue-weighted rather than uniform surveillance intervals.

Show evidence (1 reference)

PMID:36240479 SUPPORT Human Clinical

"Importantly, increased MMRDness score was associated with younger age of first cancer presentation in individuals with CMMRD"

Design rationale - instability score already predicts age at first cancer, so the proposed extension to organ of first cancer is a tractable next step.

Show evidence (1 reference)

PMID:36240479 SUPPORT Human Clinical

"In normal cells, MMRDness scores differed between tissues (GI > blood > brain), increased over time in the same individual, and revealed genotype-phenotype associations within the mismatch repair genes."

Documents the tissue-dependent and age-dependent instability that this gap asks to be reconciled with the observed tumour spectrum.

How much residual mismatch repair activity is compatible with an adult-onset, Lynch-like phenotype, and where does the CMMRD/Lynch management boundary fall for biallelic carriers of hypomorphic variants?

KNOWLEDGE GAP OPEN cmmrd_hypomorphic_penetrance_boundary

Attached to

pathophysiology#Biallelic Germline Mismatch Repair Gene Inactivation

Late-onset CMMRD is currently recognised case by case, after constitutional MSI testing contradicts an assumed Lynch diagnosis. There is no calibrated relationship between residual repair activity and age at first cancer, so a biallelic carrier of two leaky variants cannot be assigned to a surveillance schedule on genotype. The two schedules differ substantially (annual brain MRI and gastrointestinal endoscopy from early childhood in CMMRD; adult gastrointestinal and gynaecological surveillance in Lynch), so the cost of guessing is high in both directions.

Proposed experiments

Calibration of residual mismatch repair activity against age at first cancer

exp_cmmrd_hypomorphic_residual_activity_calibration

Assay residual mismatch repair activity quantitatively across a series of candidate hypomorphic MMR variants (leaky splice and missense alleles) and correlate the measured activity with constitutional instability score and age at first cancer, to establish whether residual activity predicts the CMMRD-versus-Lynch management boundary.

Show evidence (1 reference)

PMID:38789506 SUPPORT Human Clinical

"MLH1 c.306G>A was shown to cause leaky exon 3 skipping."

Design rationale - a leaky, partially functional allele has already been demonstrated functionally, so a graded activity series is experimentally reachable.

Constitutional MSI screening of adults with biallelic MMR variants of uncertain significance

exp_cmmrd_adult_vus_constitutional_msi_screen

Systematically apply constitutional microsatellite instability testing to adult patients carrying biallelic or compound-heterozygous MMR variants of uncertain significance who are currently managed as Lynch syndrome, to estimate how often late-onset CMMRD is being missed.

Show evidence (1 reference)

PMID:38789506 SUPPORT Human Clinical

"CMMRD testing in carriers of compound heterozygous or homozygous MMR VUS may find similar cases and novel hypomorphic variants."

The authors propose exactly this screen; the citation records the precedent and the expected yield.

Show evidence (1 reference)

PMID:38789506 SUPPORT Human Clinical

"Individualised management of mono- and bi-allelic carriers of hypomorphic MMR variants is needed until we better characterise the associated phenotypes."

The authors state the gap directly - the phenotypes are not yet characterised well enough to set a management rule.

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Pathophysiology

8

Biallelic Germline Mismatch Repair Gene Inactivation

Both alleles of MLH1, MSH2, MSH6 or PMS2 carry a pathogenic variant in the germ line (homozygous or compound heterozygous). The corresponding heterodimer - MutSalpha (MSH2/MSH6) for mismatch recognition or MutLalpha (MLH1/PMS2) for the downstream incision step - is absent or non-functional in every nucleated cell, so post-replicative correction of polymerase slippage and base-base mismatches fails constitutionally. This is the point at which CMMRD diverges from Lynch syndrome, where the second allele is intact until a somatic hit occurs in one tissue.

Genetic context MLH1 hgnc:7127 MSH2 hgnc:7325 MSH6 hgnc:7329 PMS2 hgnc:9122 allele_type: GERMLINE variant_origin: GERMLINE zygosity: HOMOZYGOUS functional_impact_category: LOSS_OF_FUNCTION

Biallelic germline loss-of-function variants in one MMR gene. Zygosity is recorded as HOMOZYGOUS because the schema enum has no BIALLELIC value; compound heterozygosity (two different pathogenic alleles in trans) is equally common and equally causal, and COMPOUND_HETEROZYGOUS is the correct value for those patients. What matters mechanistically is that neither allele is functional. Frameshift and truncating variants give earlier cancers and worse outcomes than missense variants within the same gene.

mismatch repair complex GO:0032300

mismatch repair GO:0006298 ⇓ LOSS OF FUNCTION DNA-templated DNA replication maintenance of fidelity GO:0045005 ↓ DECREASED

nucleus GO:0005634

Downstream

Constitutional Microsatellite Instability and Mutator Phenotype Loss of the MutSalpha/MutLalpha correction step leaves polymerase slippage errors uncorrected, so insertion-deletion loops accumulate at microsatellites in normal as well as neoplastic tissue.

Impaired Immunoglobulin Class-Switch Recombination and Somatic Hypermutation The same MutSalpha/MutLalpha heterodimers act as a backup pathway for the processing of activation-induced cytidine deaminase-induced lesions during class-switch recombination and somatic hypermutation, so their constitutional loss also perturbs antibody maturation.

Show evidence (3 references)

PMID:24737826 SUPPORT Human Clinical

"Constitutional mismatch repair deficiency (CMMRD) syndrome is a distinct childhood cancer predisposition syndrome that results from biallelic germline mutations in one of the four MMR genes, MLH1, MSH2, MSH6 or PMS2."

Establishes the biallelic germline MMR lesion as the initiating event.

PMID:38789506 SUPPORT Human Clinical

"Lynch syndrome (LS) and constitutional mismatch repair deficiency (CMMRD) are distinct cancer syndromes caused, respectively, by mono- and bi-allelic germline mismatch repair (MMR) variants."

States the mono- versus biallelic distinction that separates this node from the corresponding Lynch syndrome node.

PMID:38552658 SUPPORT Human Clinical

"Frameshift or truncating variants within the same gene caused earlier cancers and inferior outcomes compared with missense variants (p<0·0001)."

Supports the variant-class effect recorded in this node's genetic context.

Constitutional Microsatellite Instability and Mutator Phenotype

Uncorrected replication errors accumulate as base substitutions and insertion-deletion loops, most conspicuously at short tandem repeats. Unlike Lynch syndrome, where microsatellite instability appears only after somatic biallelic MMR inactivation in the tumour, in CMMRD the instability signature is present in normal blood, gastrointestinal and brain tissue, increases with age in the same individual, and its magnitude tracks with how early the first cancer appears. Constitutional MSI is therefore both the mechanistic core of the syndrome and its most discriminating diagnostic assay.

intestinal crypt stem cell CL:0002250 neural stem cell CL:0000047 hematopoietic stem cell CL:0000037

DNA repair GO:0006281 ↓ DECREASED

nucleus GO:0005634

Experimental models

CMMRD patient-derived lymphoblastoid cell lines (MSI and methylation-tolerance assay) CELL LINE Microsatellite instability is measured in normal patient-derived cells rather than in tumour, which is exactly the constitutional claim this node makes and the feature that separates CMMRD from Lynch syndrome.

Animal models

MMR knockout mouse panel (Msh2, Msh6, Mlh1, Pms2) Mouse recapitulates Null mice for each of the four genes develop a systemic mutator phenotype and are cancer-predisposed, which is the constitutional consequence this node asserts.

Downstream

Somatic Polymerase Proofreading Loss and Ultra-Hypermutation In a mutator background, somatic variants in the POLE or POLD1 exonuclease domain arise and are selected early, compounding the defect.

Loss of MMR-Dependent Damage Signalling and Alkylating Agent Tolerance The MutSalpha/MutLalpha complexes also transduce the damage signal that converts O6-methylguanine lesions into cytotoxicity, so their loss produces tolerance rather than repair of methylating-agent damage.

Accelerated Multi-Organ Tumorigenesis Frameshift mutations accumulating in coding microsatellites of tumour-suppressor and growth-control genes drive transformation in whichever proliferative compartment reaches the driver threshold first.

Show evidence (2 references)

PMID:36240479 SUPPORT Human Clinical

"Importantly, increased MMRDness score was associated with younger age of first cancer presentation in individuals with CMMRD"

Shows that the magnitude of constitutional instability is quantitatively related to the clinical phenotype, supporting this node as the mechanistic core rather than an incidental marker.

PMID:38552658 SUPPORT Human Clinical

"All cancers showed high mutation and microsatellite indel burdens, and pathognomonic mutational signatures."

Confirms the mutator phenotype across the full IRRDC tumour series.

Somatic Polymerase Proofreading Loss and Ultra-Hypermutation

An early somatic mutation in the exonuclease (proofreading) domain of POLE or POLD1 is frequently acquired and selected in CMMRD high-grade tumours - every ultra-hypermutated tumour in the index series carried one. Loss of proofreading on top of absent mismatch repair removes both layers of replication-error correction, and mutations then accumulate in a rapid burst rather than gradually. The resulting ultra-hypermutant genome (in excess of 100, and in brain tumours often over 250, mutations per megabase) is the highest burden recorded in human cancer and carries a strand-biased signature attributable to the mutant polymerase.

Genetic context allele_type: SOMATIC variant_origin: SOMATIC functional_impact_category: LOSS_OF_FUNCTION

Somatic exonuclease-domain variants in POLE or POLD1 acquired within the tumour, on the constitutional biallelic MMR background. These are tumour-restricted second events, not part of the inherited genotype.

DNA replication proofreading GO:0045004 ⇓ LOSS OF FUNCTION

3'-5' exonuclease activity GO:0008408 ⇓ LOSS OF FUNCTION

Animal models

Germline MMR-deficient de novo mouse glioma model (Msh2 or Msh6 loss) Mouse fails to recapitulate Loss of Msh2 or Msh6 in mouse glioma did not produce high tumour mutation burden or microsatellite instability, and did not confer response to PD-1 blockade. The hypermutation that makes human CMMRD gliomas checkpoint-responsive therefore does not follow from MMR loss alone in this system.

Downstream

Neoantigen Generation and T Cell Infiltration The extreme coding mutation burden yields a correspondingly extreme load of predicted neoepitopes.

Accelerated Multi-Organ Tumorigenesis The mutation burst rapidly supplies the driver alterations needed for high-grade transformation, compressing the interval from predisposition to malignancy.

Show evidence (3 references)

PMID:25642631 SUPPORT Human Clinical

"High-grade bMMRD brain tumors exhibited massive numbers of substitution mutations (>250/Mb), which was greater than all childhood and most cancers"

Quantifies the ultra-hypermutant burden in CMMRD high-grade brain tumours.

PMID:28805995 SUPPORT Human Clinical

"Biallelic mismatch repair deficiency (bMMRD) in tumours is frequently associated with somatic mutations in the exonuclease domains of DNA polymerases POLE or POLD1, and results in a characteristic mutational profile."

Establishes the somatic POLE/POLD1 proofreading hit as a recurrent feature of CMMRD tumours.

PMID:28805995 SUPPORT Human Clinical

"We identified a germline homozygous nonsense variant, p.R802*, in the PMS2 gene. Additionally, by genome sequencing of these tumours, we found extremely high somatic mutation rates (237/Mb and 123/Mb), as well as somatic mutations in the proofreading domain of POLE polymerase"

Worked example pairing a germline biallelic PMS2 genotype with somatic POLE proofreading loss and the resulting mutation rate.

Loss of MMR-Dependent Damage Signalling and Alkylating Agent Tolerance

Beyond error correction, MutSalpha/MutLalpha are required to convert persistent O6-methylguanine mispairs into a cytotoxic signal. Without them the lesion is tolerated rather than lethal, so MMR-deficient tumours are intrinsically resistant to methylating chemotherapy such as temozolomide, and continued exposure selects further mutation rather than killing the tumour. This is a therapeutically decisive consequence: standard glioma protocols cannot be applied reflexively in CMMRD.

DNA damage response GO:0006974 ⚠ ABNORMAL

Experimental models

CMMRD patient-derived lymphoblastoid cell lines (MSI and methylation-tolerance assay) CELL LINE Tolerance of patient lymphoblastoid cells to methylating and thiopurine agents is the functional assay for the loss of MMR-dependent damage signalling asserted by this node.

Show evidence (1 reference)

PMID:34992263 SUPPORT Human Clinical

"MMRD and PPD cancers are commonly lethal due to the inherent resistance to chemo-irradiation."

Establishes intrinsic chemo-radioresistance as a defining clinical property of replication-repair-deficient cancers.

Impaired Immunoglobulin Class-Switch Recombination and Somatic Hypermutation

MMR heterodimers act as a backup pathway for repair of activation-induced cytidine deaminase-induced lesions during class-switch recombination and somatic hypermutation. In CMMRD this produces measurable B-cell abnormalities - reduced somatic hypermutation frequency, skewed IGH subclass usage, reduced class-switched memory B cells and plasmablasts. Crucially, these are biological rather than clinical findings: a systematic cohort study found no consistent laboratory pattern and no warning signs of primary immunodeficiency, so CMMRD should not be curated or managed as an immunodeficiency syndrome. The node is retained because the B-cell phenotype is real and mechanistically informative about MMR function in vivo.

B cell CL:0000236

somatic hypermutation of immunoglobulin genes GO:0016446 ↓ DECREASED isotype switching GO:0045190 ⚠ ABNORMAL

Animal models

MMR knockout mouse panel (Msh2, Msh6, Mlh1, Pms2) Mouse recapitulates The same knockout panel is where the requirement for MMR proteins in B cell maturation was established, which is the mechanistic basis for the immunoglobulin abnormalities seen in CMMRD patients.

Show evidence (2 references)

PMID:30013564 SUPPORT Human Clinical

"results of next generation sequencing-based analyses of antigen-selected B-cell receptor rearrangements showed a significantly reduced frequency of SHM and an increased number of rearranged immunoglobulin heavy chain (IGH) transcripts that use IGHG3, IGHG1, and IGHA1 subclasses"

Documents the measurable B-cell somatic-hypermutation and class-switch abnormality.

PMID:30013564 REFUTE Human Clinical

"Importantly, none of the patients showed any clinical warning signs of PID (infections, immune dysregulation, inflammation, failure to thrive, etc.)."

Refutes the stronger claim that this node amounts to a clinical primary immunodeficiency; it is recorded here so the biological finding is not over-read.

Neoantigen Generation and T Cell Infiltration

The ultra-hypermutant coding genome produces neoepitopes at a density several times higher than in melanoma, lung cancer or sporadic MSI-high gastrointestinal cancer. These are presented on MHC and drive CD8-positive T cell recognition, converting even conventionally immune-cold tumours such as high-grade glioma into immune-infiltrated lesions. Microsatellite insertion-deletion load contributes independently of total mutation burden, which is why MS-indels predict response in tumours whose overall burden is only moderately elevated.

CD8-positive, alpha-beta T cell CL:0000625

antigen processing and presentation GO:0019882 ↑ INCREASED T cell mediated cytotoxicity GO:0001913 ↑ INCREASED

Downstream

Adaptive Immune Resistance in Hypermutant Tumours Sustained T cell infiltration and interferon signalling drive compensatory checkpoint engagement, which is what checkpoint blockade releases.

Show evidence (2 references)

PMID:27001570 SUPPORT Human Clinical

"bMMRD GBM harbored mean neoantigen loads seven to 16 times higher than those in immunoresponsive melanomas, lung cancers, or microsatellite-unstable GI cancers (P < .001)."

Quantifies the neoantigen load underlying this node.

PMID:34992263 SUPPORT Human Clinical

"High mutation burden predicted response for ultra-hypermutant cancers (>100 mutations per Mb) enriched for combined MMRD + PPD, while MS-indels predicted response in MMRD tumors with lower mutation burden (10-100 mutations per Mb)."

Distinguishes the two independent antigenic mechanisms modelled by this node.

Adaptive Immune Resistance in Hypermutant Tumours

Despite an extreme neoantigen load, CMMRD tumours progress, because PD-1/PD-L1 engagement suppresses the infiltrating effector T cells. This node is the therapeutic target: PD-1 blockade releases the pre-existing anti-tumour response and produces durable remissions in tumours - notably recurrent high-grade glioma - that do not respond to checkpoint inhibition in the unselected paediatric population. Responses are characteristically delayed, and pseudo-progression (flare) is common and should not be read as failure. Resistance after initial response is associated with rising CTLA-4 expression, which is the rationale for CTLA-4-directed salvage.

T cell CL:0000084

negative regulation of T cell mediated immunity GO:0002710 ↑ INCREASED

Show evidence (2 references)

PMID:37823831 SUPPORT Human Clinical

"CTLA4 expression increased over time, and subsequent CTLA4 inhibition resulted in response/stable disease in 75%."

Documents the evolving checkpoint-resistance mechanism and its therapeutic reversal.

PMID:34992263 SUPPORT Human Clinical

"Pseudo-progression (flare) was common and was associated with immune activation in the tumor microenvironment and systemically."

Supports the flare phenomenon described in this node.

Accelerated Multi-Organ Tumorigenesis

The constitutional mutator state converts every proliferative compartment into a site of accelerated clonal evolution, so tumours arise early, in multiple organs, and repeatedly over a lifetime rather than as a single event. In the IRRDC cohort 339 cancers occurred in 97% of 201 patients, the median interval between successive cancers was under two years, and neoplasms arose in fifteen different organs. CNS tumours dominate and carry the worst survival; gastrointestinal and haematological malignancies follow. Low-grade lesions are not indolent - the great majority progress to high grade within a few years if not resected, which is what makes structured surveillance rather than symptom-driven presentation the decisive management variable.

glial cell CL:0000125 intestinal crypt stem cell CL:0002250 hematopoietic stem cell CL:0000037

Animal models

Germline MMR-deficient de novo mouse glioma model (Msh2 or Msh6 loss) Mouse partially recapitulates Germline MMR deficiency accelerated progression from low-grade to high-grade glioma and shortened survival, reproducing the accelerated-tumorigenesis consequence in the CNS compartment that dominates the human tumour spectrum.

Show evidence (3 references)

PMID:38552658 SUPPORT Human Clinical

"339 cancers were reported in 194 (97%) of 201 patients."

Quantifies the multi-tumour burden modelled by this node.

PMID:38552658 SUPPORT Human Clinical

"CNS tumours were the most frequent (173 [51%] cancers), followed by gastrointestinal (75 [22%]), haematological (61 [18%]), and other cancer types (30 [9%])."

Gives the organ distribution of the tumours arising from this node.

PMID:33945292 SUPPORT Human Clinical

"Of the 64 low-grade tumors detected, the cumulative likelihood of transformation from low-to high-grade was 81% for GI cancers within 8 years and 100% for gliomas in 6 years."

Supports the statement that low-grade lesions in CMMRD reliably progress rather than remaining indolent.

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Pathograph

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Phenotypes

11

Blood 2

Haematological Malignancy Lymphoma HP:0002665

Show evidence (2 references)

PMID:38552658 SUPPORT Human Clinical

"CNS tumours were the most frequent (173 [51%] cancers), followed by gastrointestinal (75 [22%]), haematological (61 [18%]), and other cancer types (30 [9%])."

Haematological malignancies are 18% of the 339 cancers in this cohort. No `frequency` band is asserted, for the same reason as the gastrointestinal record: cancer counts do not give the patient-level fraction.

PMID:33945292 SUPPORT Human Clinical

"By contrast, only 16% of hematologic malignancies were detected asymptomatically (P < .001)."

Supports the statement that haematological disease escapes the surveillance protocol.

Leukaemia Leukemia HP:0001909

Show evidence (1 reference)

PMID:42348008 SUPPORT Human Clinical

"two daughters were diagnosed with leukemia at the ages of one and two years, respectively, and a third daughter developed NHL at the age of three"

Documents very early-onset leukaemia in the founding CMMRD family reports.

Digestive 1

Gastrointestinal Carcinoma Colon cancer HP:0003003

Show evidence (2 references)

PMID:38552658 SUPPORT Human Clinical

"CNS tumours were the most frequent (173 [51%] cancers), followed by gastrointestinal (75 [22%]), haematological (61 [18%]), and other cancer types (30 [9%])."

Gastrointestinal tumours are 22% of the 339 cancers in this cohort. No `frequency` band is asserted: 75 gastrointestinal cancers among 201 patients bounds the patient-level fraction at 37% at most, which straddles the OCCASIONAL/FREQUENT boundary, and the cohort does not report the number of patients affected.

PMID:26391938 SUPPORT Human Clinical

"Colonoscopy showed numerous polyps and a colorectal mass lesion, of which a biopsy revealed adenocarcinoma"

Worked paediatric case of CMMRD colorectal adenocarcinoma arising on a polyposis background.

Integument 1

Multiple Cafe-au-Lait Macules VERY_FREQUENT Multiple cafe-au-lait spots HP:0007565

Show evidence (2 references)

PMID:24440087 SUPPORT Human Clinical

"All children with CMMRD had café-au-lait spots and 11/14 came from consanguineous families."

Reports cafe-au-lait spots in every child in the consortium series, supporting the VERY_FREQUENT band.

PMID:39910726 SUPPORT Human Clinical

"Patients with CMMRD present with café-au-lait macules that are fewer in number and larger than in patients with neurofibromatosis type 1."

Supports the described morphological difference from the NF1 pattern.

Neoplasm 1

Multiple Metachronous Primary Neoplasms HP:0002664

Temporal: RECURRENT

Show evidence (3 references)

PMID:38552658 SUPPORT Human Clinical

"Median time between cancer diagnoses for patients with more than one cancer was 1·9 years (IQR 0·8-3·9)."

Establishes that a substantial subgroup develops more than one primary and quantifies the interval between them.

PMID:38552658 SUPPORT Human Clinical

"339 cancers were reported in 194 (97%) of 201 patients."

The cancer count exceeds the patient count, which is the cohort-level signal of multiple primaries per patient.

PMID:38552658 SUPPORT Human Clinical

"Neoplasms developed in 15 organs and included early-onset adult cancers."

Supports the multi-organ rather than single-site distribution of the successive primaries.

Other 6

Central Nervous System Tumour FREQUENT Brain neoplasm HP:0030692

Show evidence (2 references)

PMID:38552658 SUPPORT Human Clinical

"CNS tumours were the most frequent (173 [51%] cancers), followed by gastrointestinal (75 [22%]), haematological (61 [18%]), and other cancer types (30 [9%])."

CNS tumours are 51% of the 339 cancers in a cohort where 97% of 201 patients developed cancer. The cohort reports cancer counts rather than the number of patients with at least one CNS tumour, and patients frequently have more than one cancer, so the patient-level fraction cannot be read off directly. FREQUENT (30-79%) is the conservative band that the reported counts support; VERY_FREQUENT would be an over-read.

PMID:38552658 SUPPORT Human Clinical

"Patients with CNS tumours had the poorest overall survival rates"

Supports the prognostic statement in the description.

High-Grade Glioma and Glioblastoma FREQUENT Glioblastoma multiforme HP:0012174

Show evidence (1 reference)

PMID:27001570 SUPPORT Human Clinical

"Biallelic mismatch repair deficiency (bMMRD) is a highly penetrant childhood cancer syndrome often resulting in GBM characterized by a high mutational burden."

States that glioblastoma is a frequent ("often resulting in") outcome of CMMRD, supporting a FREQUENT rather than an obligate band.

Small Intestinal Neoplasm Neoplasm of the small intestine HP:0100833

Show evidence (1 reference)

PMID:38789506 SUPPORT Human Clinical

"During follow up endoscopies, duodenal adenomatous polyps were found and resected."

Documents small-bowel (duodenal) neoplasia in a molecularly confirmed CMMRD patient.

Colorectal Polyposis HP:0200063

Show evidence (2 references)

PMID:26391938 SUPPORT Human Clinical

"Colonoscopy showed numerous polyps and a colorectal mass lesion"

Documents numerous colorectal polyps in a molecularly confirmed CMMRD patient.

PMID:33945292 SUPPORT Human Clinical

"Of the 64 low-grade tumors detected, the cumulative likelihood of transformation from low-to high-grade was 81% for GI cancers within 8 years"

Supports the malignant-transformation statement for low-grade gastrointestinal lesions.

Hypopigmented Skin Patches HP:0001053

Show evidence (1 reference)

PMID:39910726 SUPPORT Human Clinical

"Additional dermatological findings include hypopigmented patches and intertriginous freckling."

Records hypopigmented patches among the characteristic cutaneous findings.

Intertriginous Freckling Axillary freckling HP:0000997

Show evidence (2 references)

PMID:39910726 SUPPORT Human Clinical

"Additional dermatological findings include hypopigmented patches and intertriginous freckling."

Records intertriginous freckling among the characteristic cutaneous findings.

PMID:27779754 SUPPORT Human Clinical

"Evaluation of the clinical findings of genetically proven CMMRD patients shows that not only multiple café-au-lait macules but also any of the diagnostic features of NF1 may be present in a CMMRD patient."

Supports the statement that NF1 diagnostic features, freckling included, may be present in CMMRD.

🧬

Genetic Associations

5

MLH1

Gene: MLH1 hgnc:7127 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (1 reference)

PMID:39910726 SUPPORT Human Clinical

"PMS2 is the most affected gene, followed by MSH6, MLH1, and MSH2."

Gives the relative reporting frequency recorded in this record.

MSH2

Gene: MSH2 hgnc:7325 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:39910726 SUPPORT Human Clinical

"PMS2 is the most affected gene, followed by MSH6, MLH1, and MSH2."

Gives the relative reporting frequency recorded in this record.

PMID:38552658 SUPPORT Human Clinical

"MLH1 or MSH2 variants caused earlier cancer onset than PMS2 or MSH6 variants, and inferior survival"

Supports the severity ranking recorded in the notes.

MSH6

Gene: MSH6 hgnc:7329 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:39910726 SUPPORT Human Clinical

"PMS2 is the most affected gene, followed by MSH6, MLH1, and MSH2."

Gives the relative reporting frequency recorded in this record.

PMID:38789506 SUPPORT Human Clinical

"Two suspected LS patients with first cancer diagnosis aged 27 or 38 years were found to be homozygous for an MMR (likely) pathogenic variant, MSH6 c.3226C>T (p.(Arg1076Cys)), or variant of uncertain significance (VUS), MLH1 c.306G>A (p.(Glu102=))."

Worked example of a homozygous MSH6 missense variant producing late-onset, Lynch-mimicking CMMRD.

PMS2

Gene: PMS2 hgnc:9122 relationship_type: CAUSATIVE variant_origin: GERMLINE

Show evidence (2 references)

PMID:41333974 SUPPORT Human Clinical

"The PMS2-related subtype (PMS2-CMMRD) is the most common molecular form of CMMRD, exhibiting variable severity and both early and late-onset clinical presentations."

Establishes PMS2 as the most commonly reported causal gene.

PMID:33622763 SUPPORT Human Clinical

"Genetic testing may not be informative and is complicated by pseudogenes associated with the most commonly associated gene, PMS2."

Supports the pseudogene caveat recorded in the notes.

POLE

Gene: POLE hgnc:9177 relationship_type: MODIFIER variant_origin: SOMATIC

Show evidence (1 reference)

PMID:28805995 SUPPORT Human Clinical

"Biallelic mismatch repair deficiency (bMMRD) in tumours is frequently associated with somatic mutations in the exonuclease domains of DNA polymerases POLE or POLD1, and results in a characteristic mutational profile."

Establishes somatic POLE/POLD1 proofreading loss as a recurrent modifier in CMMRD tumours.

💊

Medical Actions

8

PD-1 Immune Checkpoint Blockade

Action: Pharmacotherapy NCIT:C15986

Agent: nivolumab NCIT:C68814 pembrolizumab NCIT:C106432

Nivolumab or pembrolizumab. The landmark targeted therapy for CMMRD: the ultra-high mutation and neoantigen burden makes these tumours checkpoint-responsive even in settings - recurrent high-grade glioma, CNS disease - where unselected paediatric trials had failed. Responses are frequently delayed, so best overall response substantially exceeds initial objective response, and pseudo-progression (flare) must not be mistaken for failure. Registered as a phase 1/2 pilot trial (NCT02992964) in children with tumour mutational burden at or above 5 mutations/Mb and/or mismatch repair deficiency.

Mechanism Target:

INHIBITS Adaptive Immune Resistance in Hypermutant Tumours — Anti-PD-1 antibodies block the PD-1/PD-L1 interaction that suppresses the neoantigen-driven T cell response already present in these tumours, releasing pre-existing anti-tumour immunity rather than creating it.

Show evidence (1 reference)

PMID:37126021 SUPPORT Human Clinical

"Delayed immune responses contributed to best overall response of 50%, improving on initial objective responses (20%) and leading to 2-year overall survival (OS) of 50%"

Quantifies the response and survival benefit of releasing this checkpoint node.

Show evidence (4 references)

PMID:37126021 SUPPORT Human Clinical

"Nivolumab resulted in durable responses and prolonged survival for the first time in a pediatric trial of refractory hypermutated cancers including malignant gliomas."

Primary prospective trial conclusion supporting PD-1 blockade in this population.

PMID:37126021 SUPPORT Human Clinical

"Four children, including three with refractory malignant gliomas are in complete remission at a median follow-up of 37 months"

Documents the durability of the responses recorded in the description.

PMID:34992263 SUPPORT Human Clinical

"Durable objective responses were observed in most patients, culminating in a 3 year survival of 41.4%."

Independent international registry series supporting durable benefit from PD-1 inhibition.

+ 1 more reference

CTLA-4 Blockade After Anti-PD-1 Failure

Action: Pharmacotherapy NCIT:C15986

Agent: anti-CTLA-4 monoclonal antibody NCIT:C128036

Anti-CTLA-4 antibody added to checkpoint-inhibitor-based therapy continued beyond progression. Progression on anti-PD-1 monotherapy is not the end of the immune-directed option: CTLA-4 expression rises over time, and CTLA-4 inhibition produces response or stable disease in most such patients. Continuing checkpoint-inhibitor-based salvage after a second progression prolongs survival, most clearly in tumours with extreme mutation burden. Local and systemic immune adverse events are frequent, and are more frequent in biallelic mismatch repair deficiency than in Lynch syndrome.

Mechanism Target:

INHIBITS Adaptive Immune Resistance in Hypermutant Tumours — CTLA-4 blockade targets the checkpoint axis that becomes dominant as PD-1-directed resistance evolves, re-engaging the same neoantigen-driven T cell response.

Show evidence (1 reference)

PMID:37823831 SUPPORT Human Clinical

"CTLA4 expression increased over time, and subsequent CTLA4 inhibition resulted in response/stable disease in 75%."

Directly supports CTLA-4 blockade acting on the evolved checkpoint-resistance node.

Show evidence (2 references)

PMID:37823831 SUPPORT Human Clinical

"After second progression/recurrence (n = 55), continuing ICI-based salvage prolonged survival to 11.6 months (n = 38; P < 0.001), particularly for those with extreme mutation burden (P = 0.03)."

Quantifies the survival benefit of continuing immune-directed therapy past anti-PD-1 failure.

PMID:37823831 SUPPORT Human Clinical

"Local (flare) and systemic immune adverse events were frequent (biallelic mismatch-repair deficiency > Lynch syndrome)."

Supports the toxicity statement and its CMMRD-versus-Lynch gradient.

Reirradiation

Action: radiation therapy Ontology label: Radiation Therapy NCIT:C15313

Repeat irradiation of a progressive replication-repair-deficient high-grade glioma, given as part of salvage therapy after failure of anti-PD-1 monotherapy. Response is reported to track with tumour genomics rather than radiation dose alone: tumours that respond lack the deleterious post-radiation indel signature (ID8) that marks radiation-refractory disease, which makes mutational-signature analysis a candidate selection tool rather than a post-hoc description.

Show evidence (1 reference)

PMID:37823831 SUPPORT Human Clinical

"Response to reirradiation was explained by an absence of deleterious postradiation indel signatures (ID8)."

Supports reirradiation as an active salvage arm and identifies the genomic correlate of response.

RAS-MAPK Pathway Inhibition as Immune-Directed Salvage

Action: Pharmacotherapy NCIT:C15986

Agent: mitogen-activated protein kinase inhibitor NCIT:C2149

Targeted inhibition of the RAS-MAPK pathway used alongside continued checkpoint inhibition after anti-PD-1 failure. Reported to reinvigorate both peripheral immune responses and radiologic responses, placing it with CTLA-4 blockade as an immune-directed rather than purely cytotoxic salvage arm. The evidence is a consortium case series, not a randomised comparison, and no specific agent is named in the source, so the agent is bound at drug-class level.

Mechanism Target:

INHIBITS Adaptive Immune Resistance in Hypermutant Tumours — RAS-MAPK-pathway inhibition restores the peripheral immune response that adaptive resistance had suppressed, acting on the same node as the checkpoint antibodies rather than on tumour proliferation alone.

Show evidence (1 reference)

PMID:37823831 SUPPORT Human Clinical

"RAS-MAPK-pathway inhibition led to the reinvigoration of peripheral immune and radiologic responses."

Supports RAS-MAPK inhibition relieving adaptive immune resistance rather than acting only as a cytostatic agent.

Structured Cancer Surveillance Protocol

Action: cancer screening Ontology label: Cancer Screening NCIT:C15406

The consensus international surveillance protocol - annual brain MRI, regular upper and lower gastrointestinal endoscopy, whole-body MRI and clinical examination - beginning at molecular diagnosis. This is the single most effective intervention in CMMRD: five-year survival is 90% when the cancer is found asymptomatically against 50% when it presents symptomatically, and adherence to full surveillance is associated with four-year survival of 79% against 15% for those not under surveillance. Its effectiveness is unevenly distributed: nearly all gastrointestinal and other solid tumours and three-quarters of brain cancers are caught asymptomatically, but only a small minority of haematological malignancies are. These are non-randomised comparisons and are vulnerable to lead-time and ascertainment bias.

Show evidence (5 references)

PMID:33945292 SUPPORT Human Clinical

"Patient outcome measured by adherence to the surveillance protocol revealed 4-year OS of 79% (95% CI, 54.8 to 90.9) for patients undergoing full surveillance, 55% (95% CI, 28.5 to 74.5) for partial surveillance, and 15% (95% CI, 5.2 to 28.8) for those not under surveillance (P < .0001)."

Quantifies the dose-response relationship between surveillance adherence and survival.

PMID:33945292 SUPPORT Human Clinical

"For patients undergoing surveillance, all GI and other solid tumors, and 75% of brain cancers were detected asymptomatically."

Supports the uneven yield of the protocol across tumour types described here.

PMID:24440087 SUPPORT Human Clinical

"The surveillance protocol detected 39 lesions which included asymptomatic malignant gliomas and gastrointestinal carcinomas."

Early consortium evidence that the protocol detects lesions before they become symptomatic.

+ 2 more references

Endoscopic Surveillance and Polypectomy

Action: colonoscopy Ontology label: Colonoscopy NCIT:C16450

Regular upper and lower gastrointestinal endoscopy with resection of adenomas. Because low-grade gastrointestinal lesions in CMMRD transform at a very high rate within a few years, polypectomy is therapeutic rather than merely diagnostic.

Show evidence (1 reference)

PMID:33945292 SUPPORT Human Clinical

"Of the 64 low-grade tumors detected, the cumulative likelihood of transformation from low-to high-grade was 81% for GI cancers within 8 years and 100% for gliomas in 6 years."

Supports resection rather than observation of low-grade gastrointestinal lesions.

Surgical Resection

Action: surgical procedure Ontology label: Surgical Procedure NCIT:C15329

Maximal safe resection of CNS tumours and segmental or more extensive bowel surgery according to polyp and tumour burden. Surveillance-detected lesions are typically resectable in full, which is a large part of why surveillance changes survival.

Show evidence (1 reference)

PMID:24440087 SUPPORT Human Clinical

"All tumours were amenable to complete resection and all patients undergoing surveillance are alive."

Supports complete surgical resection of surveillance-detected lesions.

Genetic Counselling and Cascade Family Testing

Action: genetic counseling Ontology label: Genetic Counseling NCIT:C15240

Both parents of a CMMRD proband are obligate heterozygous MMR variant carriers and require Lynch syndrome counselling and adult surveillance, as do many other relatives. Siblings have a one-in-four risk of CMMRD. Family history is often uninformative - Lynch-spectrum adult cancers were documented in only a minority of CMMRD families - so an unremarkable pedigree must not be used to argue against the diagnosis.

Show evidence (2 references)

PMID:24440087 SUPPORT Human Clinical

"CMMRD is a highly penetrant syndrome where family history of cancer may not be contributory."

Supports the warning that an unremarkable family history does not exclude CMMRD.

PMID:24440087 SUPPORT Human Clinical

"While childhood CMMRD related tumours were observed in all families, Lynch related tumours in adults were observed in only 2/14 families"

Quantifies how often the parental Lynch phenotype is actually visible in the pedigree.

🔬

Diagnosis

4

Germline Multigene Panel Testing of MLH1, MSH2, MSH6 and PMS2

Sequencing plus deletion/duplication analysis of all four MMR genes, with confirmation that the two variants are in trans by parental testing. All four genes are tested, not only the one whose protein is absent by immunohistochemistry. PMS2 requires locus-specific methods because of its pseudogenes. Biallelic pathogenic/likely pathogenic variants remain the diagnostic standard.

Show evidence (1 reference)

PMID:33622763 SUPPORT Human Clinical

"The criteria incorporate germline mismatch repair results, ancillary tests and clinical manifestation to determine a diagnosis."

States that germline MMR results are the backbone of the consensus diagnostic criteria.

Mismatch Repair Immunohistochemistry in Tumour and Normal Tissue

Loss of MLH1, MSH2, MSH6 or PMS2 protein staining. In CMMRD, and unlike Lynch syndrome, the loss is also present in adjacent NORMAL tissue, which is the single most accessible discriminator between the two syndromes. Missense variants that produce a stable but non-functional protein can retain staining, so normal immunohistochemistry does not exclude the diagnosis.

Show evidence (2 references)

PMID:24440087 SUPPORT Human Clinical

"Furthermore, screening of normal tissue by immunohistochemistry correlated with genetic confirmation of CMMRD."

Supports normal-tissue immunohistochemistry as a CMMRD-specific diagnostic step.

PMID:24440087 SUPPORT Human Clinical

"Tumour immunohistochemistry was 100% sensitive and specific in diagnosing mismatch repair (MMR) deficiency of the corresponding gene while microsatellite instability was neither sensitive nor specific as a diagnostic tool (p<0.0001)."

Establishes the performance of immunohistochemistry and, importantly, that conventional MSI panels are unreliable in this paediatric setting.

Constitutional Microsatellite Instability Testing (LOGIC / low-pass genome sequencing)

Quantitative genomic-instability scoring on blood or saliva DNA. Because CMMRD produces microsatellite instability in normal tissue, a functional constitutional assay can make the diagnosis without a tumour and can resolve cases where genotype is ambiguous (variants of uncertain significance, hypomorphic alleles). The LOGIC assay was 100% sensitive and specific in childhood cancers and clearly outperformed the conventional MSI panel, immunohistochemistry and tumour mutational burden. It remains a specialised rather than a universally available test.

Show evidence (2 references)

PMID:36240479 SUPPORT Human Clinical

"Overall, LOGIC was 100% sensitive and specific in detecting MMRD in childhood cancers (N = 376)."

Quantifies the diagnostic performance of the constitutional-instability assay.

PMID:36240479 SUPPORT Human Clinical

"LOGIC was able to distinguish CMMRD from other cancer predisposition syndromes using blood and saliva DNA"

Supports the tumour-free, blood/saliva-based diagnostic route described here.

C4CMMRD Clinical Scoring System

A three-point scoring system that weights the patient's malignancy and additional features (pigmentary skin findings, brain malformations, pilomatricomas, a second childhood malignancy, a Lynch-spectrum tumour in a relative, parental consanguinity), with three points triggering diagnostic evaluation. It is a case-finding trigger, not a diagnosis: contemporary practice requires molecular or functional confirmation.

Show evidence (1 reference)

PMID:24737826 SUPPORT Human Clinical

"According to the scoring system, CMMRD should be suspected in any cancer patient who reaches a minimum of three points by adding the points of the malignancy and the additional features."

States the three-point threshold recorded in this description.

📈

Progression

5

First cancer in childhood

Age: Median 8.9 years (IQR 5.9-12.6)

CMMRD is usually recognised when the first malignancy appears, at a median of 8.9 years. Onset is earliest in the MLH1- and MSH2-related forms and latest in the PMS2-related form, so this median sits across a genotype-stratified distribution rather than describing a single natural history.

Show evidence (1 reference)

PMID:38552658 SUPPORT Human Clinical

"Median age at diagnosis of CMMRD or a related cancer was 8·9 years (IQR 5·9-12·6), and median follow-up from diagnosis was 7·2 years (3·6-14·8)."

Gives the median age at first cancer or CMMRD diagnosis in the IRRDC cohort.

Near-complete cancer penetrance by adulthood

Age: By 18 years

Cumulative cancer incidence reaches 90% by age 18, which is what makes lifelong surveillance from molecular diagnosis rather than from first symptom the standard of care. This is a registry cohort ascertained largely through cancer, so the figure is an upper estimate for the penetrance of an unselected biallelic genotype.

Show evidence (1 reference)

PMID:38552658 SUPPORT Human Clinical

"The cumulative cancer incidence by age 18 years was 90% (95% CI 80-99)."

Quantifies cumulative cancer incidence by the end of childhood.

Metachronous second and subsequent cancers

Age: Median 1.9 years after the preceding cancer

Survivors of a first cancer face a short interval to the next one: the median time between successive cancer diagnoses is 1.9 years, and neoplasms arise across 15 organs. Surveillance therefore does not stop after the first tumour is treated.

Show evidence (2 references)

PMID:38552658 SUPPORT Human Clinical

"Median time between cancer diagnoses for patients with more than one cancer was 1·9 years (IQR 0·8-3·9)."

Quantifies the interval between successive primary cancers.

PMID:38552658 SUPPORT Human Clinical

"Neoplasms developed in 15 organs and included early-onset adult cancers."

Supports the multi-organ distribution of the successive primaries.

Ten-year survival stratified by tumour group

Age: 10 years from cancer diagnosis

Outcome is determined mainly by which tumour group presents. CNS tumours carry by far the worst prognosis, then haematological, then gastrointestinal, with other solid tumours near-uniformly survived. This ordering is the prognostic counterpart of the tumour-spectrum frequencies curated in the phenotypes block.

Show evidence (1 reference)

PMID:38552658 SUPPORT Human Clinical

"Patients with CNS tumours had the poorest overall survival rates (39% [95% CI 30-52] at 10 years from diagnosis; log-rank p<0·0001 across four cancer types)"

Gives 10-year overall survival for the worst-prognosis tumour group and establishes that the four groups differ significantly.

Survival stratified by causative gene

Age: 15 years of age

Overall survival at age 15 falls in the order PMS2 > MSH6 > MLH1 > MSH2, and within a gene, frameshift or truncating variants do worse than missense variants. This gene and variant-class gradient is what distinguishes the four subtype entries prognostically, and it persists even after surveillance and checkpoint-inhibitor interventions improve survival overall.

Show evidence (4 references)

PMID:38552658 SUPPORT Human Clinical

"MLH1 or MSH2 variants caused earlier cancer onset than PMS2 or MSH6 variants, and inferior survival"

Establishes the direction of the gene-defined survival gradient.

PMID:38552658 SUPPORT Human Clinical

"overall survival at age 15 years 63% [95% CI 55-73] for PMS2"

Anchors the best-prognosis end of that gradient with a figure. The same sentence in the source reports 49% for MSH6, 19% for MLH1 and 0% for MSH2, completing the ordering described in the notes.

PMID:38552658 SUPPORT Human Clinical

"Frameshift or truncating variants within the same gene caused earlier cancers and inferior outcomes compared with missense variants (p<0·0001)."

Supports the within-gene variant-class gradient described here.

+ 1 more reference

📊

Prevalence

2

Worldwide

Birth Prevalence 0.1 per 100,000 <1 in 1,000,000

Birth prevalence estimated at approximately 1 per 1,000,000. Substantially higher in populations with high rates of consanguinity or endogamy, and in closed communities within countries whose overall consanguinity rate is low.

Show evidence (2 references)

PMID:42348008 SUPPORT Human Clinical

"The birth prevalence is estimated to be 1 in a million"

States the worldwide birth-prevalence estimate recorded here.

PMID:38789506 SUPPORT Human Clinical

"is caused by bi-allelic germline MMR PVs and is much rarer than LS, with an estimated birth incidence of one in a million"

Independent statement of the same one-in-a-million birth incidence, contrasted against Lynch syndrome carrier frequency.

Consanguineous and endogamous communities

Unknown Unknown

No population-based rate is available for high-consanguinity settings; the IRRDC cohort documents enrichment rather than measuring a rate, so no numeric value is asserted here.

Show evidence (1 reference)

PMID:38552658 SUPPORT Human Clinical

"Endogamy among minorities and closed communities contributed to high homozygosity within countries with low consanguinity."

Supports geographic and community clustering, but reports ascertainment within a referral consortium rather than a population denominator - hence PARTIAL and no rate.

⚖️

Clinical Burden

High

Near-complete cancer penetrance by age 18, a median of under two years between successive primary cancers, and 10-year survival of 39% for the commonest tumour group. Management is lifelong intensive multi-modality surveillance from molecular diagnosis onward rather than treatment of a single event.

Show evidence (2 references)

PMID:38552658 SUPPORT Human Clinical

"The cumulative cancer incidence by age 18 years was 90% (95% CI 80-99)."

Supports near-complete cancer penetrance by adulthood.

PMID:33945292 SUPPORT Human Clinical

"Five-year overall survival (OS) was 90% (95% CI, 78.6 to 100) and 50% (95% CI, 39.2 to 63.7) when cancer was detected asymptomatically and symptomatically, respectively"

Supports the mortality burden and the dependence of outcome on intensive surveillance.

🔀

Differential Diagnoses

4

Conditions with similar clinical presentations that must be differentiated from Constitutional Mismatch Repair Deficiency:

Lynch Syndrome

Overlapping Features The monoallelic disease caused by variants in the same four genes. Distinguished by inheritance (dominant versus recessive), age (adult versus childhood), tumour spectrum (gastrointestinal/genitourinary versus CNS-predominant), and decisively by the absence of constitutional microsatellite instability and of normal-tissue MMR protein loss. A late-onset CMMRD patient with hypomorphic alleles can present exactly like Lynch syndrome, so constitutional MSI testing is what resolves the two.

Show evidence (1 reference)

PMID:38789506 SUPPORT Human Clinical

"Lynch syndrome (LS) and constitutional mismatch repair deficiency (CMMRD) are distinct cancer syndromes caused, respectively, by mono- and bi-allelic germline mismatch repair (MMR) variants. LS predisposes to mainly gastrointestinal and genitourinary cancers in adulthood. CMMRD predisposes to..."

States the mono- versus biallelic and adult- versus childhood-onset distinction.

Neurofibromatosis Type 1

Overlapping Features The most consequential misdiagnosis. CMMRD patients may fulfil formal NF1 diagnostic criteria on pigmentary findings alone, and NF1 shares several tumour types (high-grade glioma, acute myeloid leukaemia, rhabdomyosarcoma) with CMMRD. A child labelled NF1 who develops a CMMRD-spectrum tumour, or who has no explanatory NF1/SPRED1 variant, should be tested for CMMRD - the surveillance implications differ completely.

Show evidence (2 references)

PMID:27779754 SUPPORT Human Clinical

"This phenotypic overlap may lead to misdiagnosis of CMMRD patients as having NF1, which impedes adequate management of the patients and their families."

States the misdiagnosis risk and its clinical cost.

PMID:38552658 SUPPORT Human Clinical

"Frequent dermatological manifestations (117 [93%] of 126 patients with complete data) led to a clinical overlap with neurofibromatosis type 1 (35 [28%] of 126)."

Quantifies how often the NF1 overlap actually arises in a large CMMRD cohort.

Legius Syndrome

Overlapping Features SPRED1-related. Shares cafe-au-lait macules and freckling with both NF1 and CMMRD but lacks the CMMRD tumour spectrum and constitutional microsatellite instability.

Show evidence (1 reference)

PMID:41333974 SUPPORT Human Clinical

"As clinical phenotype in CMMRD overlaps with other rare genetic diseases, such as neurofibromatosis type 1 (NF1) and Legius syndrome"

Names Legius syndrome among the phenotypic mimics of CMMRD.

Polymerase Proofreading-Associated Polyposis

Overlapping Features Germline heterozygous POLE or POLD1 exonuclease-domain variants causing multiple colorectal adenomas and early-onset colorectal cancer. The mechanistic mirror image of the somatic proofreading loss modelled here: in CMMRD the proofreading defect is acquired in the tumour on top of a constitutional mismatch repair defect, whereas in PPAP it is the constitutional lesion and mismatch repair is intact. That difference is what the laboratory sees - PPAP tumours are hypermutated but microsatellite stable and retain normal MMR protein staining, while CMMRD shows constitutional microsatellite instability and MMR protein loss in normal tissue. Inheritance is dominant rather than recessive.

Show evidence (2 references)

PMID:23263490 SUPPORT Human Clinical

"we identified specific heterozygous POLE or POLD1 germline variants in several multiple-adenoma and/or CRC cases but in no controls"

Establishes PPAP as a distinct germline proofreading-deficiency cancer predisposition, and its heterozygous (dominant) genetics.

PMID:23263490 SUPPORT Human Clinical

"the tumors from mutation carriers were microsatellite stable but tended to acquire base substitution mutations"

Gives the discriminating laboratory finding - microsatellite stability - that separates PPAP from CMMRD.

🔬

Clinical Trials

2

NCT02992964 PHASE_I TERMINATED

Pilot study of nivolumab in paediatric patients with recurrent or refractory hypermutant cancers, the first prospective paediatric trial restricted to tumours with mutational burden at or above 5 mutations/Mb and/or mismatch repair deficiency. Registered on ClinicalTrials.gov as phase 1/2 and terminated after limited enrolment, but it generated the published efficacy cohort that established checkpoint blockade in CMMRD. Recorded here as PHASE_I because the dismech phase enum has no combined phase 1/2 value.

Target Phenotypes: Glioblastoma multiforme HP:0012174 Brain neoplasm HP:0030692

Show evidence (2 references)

clinicaltrials:NCT02992964 SUPPORT Human Clinical

"This study is to assess clinical and radiological benefits of treatment with Nivolumab in children with hypermutated cancers, including those with bMMRD syndrome."

The trial record states the CMMRD (bMMRD) population and the checkpoint-blockade intervention.

PMID:37126021 SUPPORT Human Clinical

"We report the first prospective pediatric trial (NCT02992964) using nivolumab exclusively for refractory nonhematologic cancers harboring tumor mutation burden (TMB) ≥5 mutations/megabase (mut/Mb) and/or mismatch repair deficiency (MMRD)."

Publication of the trial, giving its eligibility criteria and identifier.

NCT05770102 PHASE_II RECRUITING

DETERMINE treatment arm 02: atezolizumab (anti-PD-L1) in adult, paediatric and teenage/young adult patients whose cancers are TMB-high, MSI-high, or who have proven CMMRD. A UK national umbrella-basket platform trial testing whether an approved checkpoint inhibitor extends to rare molecularly defined indications; it is the currently open trial that admits CMMRD patients by predisposition rather than by tumour type. Registered as phase 2/3; recorded here as PHASE_II because the dismech phase enum has no combined value.

Target Phenotypes: Neoplasm HP:0002664

Show evidence (1 reference)

clinicaltrials:NCT05770102 SUPPORT Human Clinical

"Atezolizumab works in patients with these types of cancers which have certain changes in the cancer cells called high tumour mutational burden (TMB) or high microsatellite instability (MSI) or proven (previously diagnosed) constitutional mismatch repair deficiency (CMMRD)."

The trial record states that proven CMMRD is an eligibility route into this checkpoint-inhibitor arm.

🧫

Experimental Models

1

CMMRD patient-derived lymphoblastoid cell lines (MSI and methylation-tolerance assay) CELL_LINE

Lymphoblastoid cells from CMMRD patients and MMR-proficient controls, assayed for microsatellite instability by PCR and for tolerance to methylating and thiopurine agents. This is the human ex vivo system in which the two defining constitutional consequences of biallelic MMR loss - instability in normal (non-tumour) cells, and loss of the MMR-dependent damage response - are measured directly, and it is also the basis of a diagnostic assay for the 30% of patients whose gene screening is uninformative.

Organism

human NCBITaxon:9606

Cell source

Patient-derived (EBV-immortalized peripheral blood lymphocytes)

Publication

PMID:26116798

Pathograph links

Constitutional Microsatellite Instability and Mutator Phenotype measures fidelity: high Microsatellite instability is measured in normal patient-derived cells rather than in tumour, which is exactly the constitutional claim this node makes and the feature that separates CMMRD from Lynch syndrome.

Limitations: EBV-immortalized lymphoblastoid cells are a proliferating lymphoid compartment and cannot report the tissue gradient in constitutional instability that the tumour-spectrum knowledge gap concerns; instability measured in this compartment says nothing about brain or intestine.

Readouts

Microsatellite instability in lymphoblastoid cells

PCR-based MSI analysis of patient versus control lymphoblastoid lines.

Direction: INCREASED

Interpretation: Instability present in non-neoplastic patient cells is the direct measurement of the constitutional mutator phenotype.

Show evidence (1 reference)

PMID:26116798 SUPPORT In Vitro

"In the training set, we identified parameters, based on MSI and LC tolerance to methylation, that detected patients with CMMRD vs controls with 100% sensitivity and 100% specificity."

The measurement separates patients from controls without error in the training set.

Show evidence (1 reference)

PMID:26116798 SUPPORT In Vitro

"Using these assays, we defined experimental parameters that allowed discrimination of a series of 14 patients with CMMRD from 52 controls (training set)."

Supports treating this cell system as informative for the constitutional instability node, at a cohort size that establishes the parameters rather than illustrating them.

Loss of MMR-Dependent Damage Signalling and Alkylating Agent Tolerance measures fidelity: high Tolerance of patient lymphoblastoid cells to methylating and thiopurine agents is the functional assay for the loss of MMR-dependent damage signalling asserted by this node.

Limitations: The assay demonstrates tolerance in a lymphoid cell line; it does not establish the clinical consequence for alkylating-agent chemotherapy in CMMRD tumours, which is inferred rather than measured here.

Readouts

Lymphoblastoid cell tolerance to methylating agents

Survival of patient versus control lymphoblastoid cells exposed to methylating or thiopurine agents.

Direction: INCREASED

Interpretation: Failure to die after methylation damage is the functional signature of an absent MMR-dependent damage response.

Show evidence (1 reference)

PMID:26116798 SUPPORT In Vitro

"The presence of MSI and tolerance to methylation in LCs identified patients with CMMRD with 100% sensitivity and specificity."

Reports methylation tolerance as a measured, discriminating property of patient cells.

Show evidence (1 reference)

PMID:26116798 SUPPORT In Vitro

"Among 23 patients suspected of having CMMRD, 6 had MSI and LC tolerance to methylation (CMMRD highly probable), 15 had neither MSI nor LC tolerance to methylation (unlikely to have CMMRD), and 2 were considered doubtful for CMMRD based on having only 1 of the 2 features."

Methylation tolerance and instability can dissociate in individual patients, which is why this link is curated against the damage-signalling node separately from the instability node rather than folded into it.

Show evidence (1 reference)

PMID:26116798 SUPPORT In Vitro

"We examined MSI by PCR analysis and tolerance to methylating or thiopurine agents (functional characteristics of MMR-deficient tumor cells) in lymphoblastoid cells (LCs) from 3 patients with CMMRD and 5 individuals with MMR-proficient LCs (controls)."

Describes the model system and its two readouts against MMR-proficient controls.

🐁

Animal Models

2

MMR knockout mouse panel (Msh2, Msh6, Mlh1, Pms2)

Germline knockout lines for each of the four genes that cause CMMRD in humans. Because the knockouts are homozygous null they match the biallelic constitutional state, unlike the heterozygous lines used to model Lynch syndrome, and they establish that loss of each gene alone is sufficient for a systemic mutator phenotype and cancer predisposition. The same panel supplies the only in vivo evidence for the class-switch-recombination arm of the syndrome.

Species

Mouse

Genotype

Homozygous germline knockout of Msh2, Msh6, Mlh1 or Pms2

Publication

PMID:26708047

Pathograph links

Constitutional Microsatellite Instability and Mutator Phenotype recapitulates fidelity: moderate Null mice for each of the four genes develop a systemic mutator phenotype and are cancer-predisposed, which is the constitutional consequence this node asserts.

Limitations: The review is framed around modelling Lynch syndrome and sporadic MMR-deficient colorectal cancer rather than CMMRD, and reports the mutator and cancer-predisposition phenotypes without establishing the CNS-predominant tumour distribution that defines the human syndrome. Treat the model as informative for the mutator phenotype itself, not for the tissue-specific transformation risk that the tumour-spectrum knowledge gap asks about.

Readouts

Mutator phenotype and cancer predisposition in MMR-null mice

Mutation-rate and tumour-incidence phenotypes reported across the Msh2, Msh6, Mlh1 and Pms2 null lines.

Direction: INCREASED

Interpretation: Each of the four genes that cause human CMMRD produces the mutator phenotype on its own when knocked out, matching the genetic architecture of the syndrome.

Show evidence (1 reference)

PMID:26708047 SUPPORT Model Organism

"mouse lines with knockout mutations in Msh2, Msh6, Mlh1 and Pms2 that form the MutSα (Msh2-Msh6) and MutLα (Mlh1-Pms2) complexes and play major roles in the repair of RERs in eukaryotic cells and the MMR-dependent DDR, display strong mutator and cancer predisposition phenotypes"

Reports the mutator and cancer-predisposition phenotypes for exactly the four CMMRD genes.

Show evidence (1 reference)

PMID:26708047 SUPPORT Model Organism

"mouse models with mutations in all known MSH and MLH genes have been generated to study their roles in genome maintenance and tumor suppression"

Supports treating the knockout panel as informative for the genome instability node.

Impaired Immunoglobulin Class-Switch Recombination and Somatic Hypermutation recapitulates fidelity: moderate The same knockout panel is where the requirement for MMR proteins in B cell maturation was established, which is the mechanistic basis for the immunoglobulin abnormalities seen in CMMRD patients.

Limitations: The abstract reports an essential function in B cell maturation without separating class-switch recombination from somatic hypermutation or quantifying the immunoglobulin deficit, and mouse B cell development is not a substitute for the human immunological phenotype, which in patients is a laboratory abnormality rather than overt clinical immunodeficiency.

Show evidence (1 reference)

PMID:26708047 SUPPORT Model Organism

"These studies also revealed essential functions for some of the MMR genes in B cell maturation and fertility."

In vivo support for the B cell maturation arm of the syndrome.

Show evidence (1 reference)

PMID:26708047 SUPPORT Model Organism

"the generation and analysis of mouse lines with knockout mutations in all of the known MMR genes has provided insight into how loss of individual MMR genes affects genome stability and contributes to cancer susceptibility"

Establishes the knockout panel as the standard in vivo system for MMR loss.

Germline MMR-deficient de novo mouse glioma model (Msh2 or Msh6 loss)

De novo mouse models of germline and somatic MMR-deficient high-grade glioma, built to test which parts of the human MMR-deficient glioma phenotype follow from MMR loss alone. The answer was largely negative, which is what makes this model informative: germline MMR deficiency accelerated progression from low-grade to high-grade glioma and shortened survival, but did so through the tumour immune microenvironment rather than by producing the hypermutation and checkpoint sensitivity that define human CMMRD gliomas.

Species

Mouse

Genotype

De novo high-grade glioma with germline or somatic Msh2 or Msh6 loss

Publication

PMID:41433099

Pathograph links

Somatic Polymerase Proofreading Loss and Ultra-Hypermutation fails to recapitulate fidelity: low Loss of Msh2 or Msh6 in mouse glioma did not produce high tumour mutation burden or microsatellite instability, and did not confer response to PD-1 blockade. The hypermutation that makes human CMMRD gliomas checkpoint-responsive therefore does not follow from MMR loss alone in this system.

Limitations: The mouse models carry MMR loss without the somatic POLE or POLD1 exonuclease-domain lesion that this node asserts is the source of ultra-hypermutation in human CMMRD tumours, so the negative result is consistent with the two-step model curated here rather than a refutation of it - but it does mean the model cannot be used to study the hypermutant state or its checkpoint sensitivity. The same paper reports the corresponding discordance in human GBM, so this is not a purely murine artefact.

Readouts

Tumour mutation burden and microsatellite instability in MMR-null mouse glioma

TMB and MSI measured in de novo mouse gliomas after Msh2 or Msh6 loss, with anti-PD-1 response as the functional readout.

Direction: UNCHANGED

Interpretation: A real negative: MMR loss alone leaves mutation burden and microsatellite status unchanged, so the ultra-hypermutant phenotype requires the additional somatic proofreading lesion.

Show evidence (1 reference)

PMID:41433099 SUPPORT Model Organism

"Surprisingly, loss of Msh2 or Msh6 did not lead to high TMB, MSI, nor did it confer a response to anti-programmed cell death 1 (anti-PD-1) in GBM."

The negative result itself.

Show evidence (1 reference)

PMID:41433099 SUPPORT Model Organism

"human GBM showed discordance between MMR gene mutations and the TMB and MSI"

Shows the same discordance in human tumours, so the failure is not attributable to species alone.

Accelerated Multi-Organ Tumorigenesis partially recapitulates fidelity: moderate Germline MMR deficiency accelerated progression from low-grade to high-grade glioma and shortened survival, reproducing the accelerated-tumorigenesis consequence in the CNS compartment that dominates the human tumour spectrum.

Limitations: Only the CNS arm is modelled, so the multi-organ distribution the node asserts is not reproduced, and the effect was driven by the tumour immune microenvironment rather than being tumour-cell intrinsic. That is a mechanistically different route to accelerated progression from the cell-intrinsic mutation accumulation curated upstream of this node.

Readouts

Progression from low-grade to high-grade glioma and survival

Grade progression and overall survival in germline MMR-deficient versus MMR-proficient tumour-bearing mice.

Direction: INCREASED

Interpretation: Faster grade progression with shorter survival is the model's counterpart of accelerated tumorigenesis, restricted to the CNS.

Show evidence (1 reference)

PMID:41433099 SUPPORT Model Organism

"Germline MMRd promoted the progression from low-grade to HGG and reduced survival compared with MMR-proficient (MMRp) tumor-bearing mice."

Reports the acceleration and the survival cost.

Show evidence (1 reference)

PMID:41433099 SUPPORT Model Organism

"This effect was not tumor cell intrinsic but was associated with MMRd in the tumor immune microenvironment, driving immunosuppressive myeloid programs, reduced lymphoid infiltration, and CD8+ T cell exhaustion."

Identifies the route by which the model accelerates tumorigenesis, and so the respect in which it only partially matches the human node.

Show evidence (1 reference)

PMID:41433099 SUPPORT Model Organism

"Here, we developed de novo mouse models of germline and somatic MMR-deficient (MMRd) HGGs."

Establishes the model system and that it covers the germline (CMMRD-like) as well as the somatic case.

{ }

Source YAML

click to show

name: Constitutional Mismatch Repair Deficiency
creation_date: "2026-08-27T00:00:00Z"
description: >-
  Constitutional mismatch repair deficiency (CMMRD) is an autosomal recessive childhood
  cancer predisposition syndrome caused by biallelic - that is, two-defective-copy - germline
  pathogenic variants in one of
  the four DNA mismatch repair genes MLH1, MSH2, MSH6 or PMS2. Because both alleles are
  defective in the germ line, mismatch repair fails in every normal tissue rather than only
  in a tumour that has acquired a somatic second hit. That is the mechanistic boundary
  against Lynch syndrome (see `Lynch_Syndrome.yaml`, MONDO:0005835), which is the
  monoallelic disease caused by variants in the same four genes: Lynch carriers have intact
  repair until a somatic second hit occurs in a susceptible epithelium and present with
  adult-onset colorectal, endometrial and other gastrointestinal/genitourinary cancers,
  whereas CMMRD patients carry constitutional microsatellite instability from birth and
  present in the first two decades with CNS tumours, haematological malignancy and early
  gastrointestinal carcinoma, together with cafe-au-lait macules and other pigmentary
  findings that mimic neurofibromatosis type 1. The constitutional mutator state is
  frequently compounded in high-grade tumours by early somatic loss of POLE/POLD1
  proofreading, producing an ultra-hypermutant genome (over 100 mutations per megabase, and
  over 250 in high-grade brain tumours) that exceeds every other childhood cancer and most
  adult cancers, and is the basis for the syndrome's landmark sensitivity to PD-1 immune
  checkpoint blockade. The parents of a CMMRD patient are obligate heterozygous
  carriers and are themselves managed as Lynch syndrome; the two entries are therefore
  cross-referenced rather than merged.
categories:
- Hereditary Cancer Syndrome
- Childhood Cancer Predisposition Syndrome
- DNA Repair Disorder
disease_term:
  preferred_term: constitutional mismatch repair deficiency syndrome
  term:
    id: MONDO:0031219
    label: mismatch repair cancer syndrome
synonyms:
- CMMRD
- constitutional mismatch repair deficiency syndrome
- biallelic mismatch repair deficiency
- bMMRD
- childhood cancer syndrome with biallelic mismatch repair deficiency
classifications:
  harrisons_chapter:
  - classification_value: ONCOLOGY_HEMATOLOGY
  nih_research_priority:
  - classification_value: NIH_HT_68_childhood_adolescent_young_adult_aya_cancer
    notes: >-
      One of the most penetrant childhood cancer predisposition syndromes known, with a 90%
      cumulative cancer incidence by age 18 and a median first-cancer age below 10 years.
  - classification_value: NIH_HT_42_rare_cancers_across_cancer_control_continuum
    notes: >-
      Ultra-rare (birth prevalence about 1 in a million) and spanning the full control
      continuum: molecular diagnosis, structured international surveillance, and a
      tumour-agnostic immunotherapy indication.
inheritance:
- name: Autosomal recessive inheritance
  inheritance_term:
    preferred_term: Autosomal recessive inheritance
    term:
      id: HP:0000007
      label: Autosomal recessive inheritance
  penetrance: INCOMPLETE
  penetrance_percentage: "90% cumulative cancer incidence by age 18 years"
  expressivity: VARIABLE
  description: >-
    CMMRD requires two defective alleles of a single mismatch repair gene (homozygous or
    compound heterozygous). Both parents are obligate heterozygous carriers and are managed
    as Lynch syndrome. Consanguinity and endogamy raise the local frequency substantially.
    Penetrance is very high but not formally complete: cumulative cancer incidence reaches
    90% by age 18 years, and 97% of patients in the largest cohort had developed cancer, so
    it is recorded as INCOMPLETE with a penetrance_percentage of 90 at age 18 rather than
    COMPLETE. Expressivity is highly variable by gene and by variant class - MLH1 and MSH2
    biallelic variants give the earliest onset and worst survival, PMS2 and MSH6 are later
    and milder, and hypomorphic alleles can delay first cancer into adulthood, producing a
    Lynch-like presentation.
  evidence:
  - reference: PMID:24737826
    reference_title: "Diagnostic criteria for constitutional mismatch repair deficiency syndrome: suggestions of the European consortium 'care for CMMRD' (C4CMMRD)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Constitutional mismatch repair deficiency (CMMRD) syndrome is a distinct childhood cancer predisposition syndrome that results from biallelic germline mutations in one of the four MMR genes, MLH1, MSH2, MSH6 or PMS2.
    explanation: States the defining biallelic germline genotype across all four MMR genes.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The cumulative cancer incidence by age 18 years was 90% (95% CI 80-99).
    explanation: Quantifies the near-complete penetrance recorded in this Inheritance block.
prevalence:
- population: Worldwide
  measure_type: BIRTH_PREVALENCE
  prevalence_class: BELOW_1_IN_1000000
  rate_per_100000: 0.1
  notes: >-
    Birth prevalence estimated at approximately 1 per 1,000,000. Substantially higher in
    populations with high rates of consanguinity or endogamy, and in closed communities
    within countries whose overall consanguinity rate is low.
  evidence:
  - reference: PMID:42348008
    reference_title: "The impact of international care networks on the clinical management of constitutional mismatch repair deficiency (CMMRD): a review of recent developments."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The birth prevalence is estimated to be 1 in a million
    explanation: States the worldwide birth-prevalence estimate recorded here.
  - reference: PMID:38789506
    reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: is caused by bi-allelic germline MMR PVs and is much rarer than LS, with an estimated birth incidence of one in a million
    explanation: Independent statement of the same one-in-a-million birth incidence, contrasted against Lynch syndrome carrier frequency.
- population: Consanguineous and endogamous communities
  measure_type: UNKNOWN
  prevalence_class: UNKNOWN
  notes: >-
    No population-based rate is available for high-consanguinity settings; the IRRDC cohort
    documents enrichment rather than measuring a rate, so no numeric value is asserted here.
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Endogamy among minorities and closed communities contributed to high homozygosity within countries with low consanguinity.
    explanation: >-
      Supports geographic and community clustering, but reports ascertainment within a
      referral consortium rather than a population denominator - hence PARTIAL and no rate.
has_subtypes:
- name: CMMRD1
  display_name: Mismatch repair cancer syndrome 1 (MLH1-related)
  subtype_term:
    preferred_term: mismatch repair cancer syndrome 1
    term:
      id: MONDO:0010159
      label: mismatch repair cancer syndrome 1
  description: >-
    Biallelic MLH1 variants (OMIM 276300). Together with MSH2 this is the severe end of the
    spectrum: earliest cancer onset and the poorest survival. Historically some cases were
    reported as Turcot syndrome before the genetic basis was understood.
  genes:
  - preferred_term: MLH1
    term:
      id: hgnc:7127
      label: MLH1
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: MLH1 or MSH2 variants caused earlier cancer onset than PMS2 or MSH6 variants, and inferior survival
    explanation: Establishes MLH1 as one of the two severe gene-defined forms.
  - reference: PMID:42348008
    reference_title: "The impact of international care networks on the clinical management of constitutional mismatch repair deficiency (CMMRD): a review of recent developments."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Other previously published cases of CMMRD were likely misclassified as Turcot syndrome"
    explanation: Supports the historical Turcot syndrome labelling noted in this subtype description.
- name: CMMRD2
  display_name: Mismatch repair cancer syndrome 2 (MSH2-related)
  subtype_term:
    preferred_term: mismatch repair cancer syndrome 2
    term:
      id: MONDO:0030840
      label: mismatch repair cancer syndrome 2
  description: >-
    Biallelic MSH2 variants (OMIM 619096). The least commonly reported and the most severe
    gene-defined form, with the earliest cancer onset and the poorest survival of the four.
  genes:
  - preferred_term: MSH2
    term:
      id: hgnc:7325
      label: MSH2
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: MLH1 or MSH2 variants caused earlier cancer onset than PMS2 or MSH6 variants, and inferior survival
    explanation: Places MSH2 with MLH1 in the early-onset, inferior-survival group.
  - reference: PMID:39910726
    reference_title: "Constitutional Mismatch Repair Deficiency: Scoping Review of a Cancer-Predisposition Syndrome With Distinctive Cutaneous Findings."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: PMS2 is the most affected gene, followed by MSH6, MLH1, and MSH2.
    explanation: Ranks MSH2 last by reported case frequency, supporting "least commonly reported".
- name: CMMRD3
  display_name: Mismatch repair cancer syndrome 3 (MSH6-related)
  subtype_term:
    preferred_term: mismatch repair cancer syndrome 3
    term:
      id: MONDO:0030841
      label: mismatch repair cancer syndrome 3
  description: >-
    Biallelic MSH6 variants (OMIM 619097). The second most frequently reported molecular
    form. Later onset than MLH1/MSH2 but earlier than PMS2, and the broadest cutaneous
    spectrum along with PMS2.
  genes:
  - preferred_term: MSH6
    term:
      id: hgnc:7329
      label: MSH6
  evidence:
  - reference: PMID:39910726
    reference_title: "Constitutional Mismatch Repair Deficiency: Scoping Review of a Cancer-Predisposition Syndrome With Distinctive Cutaneous Findings."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: PMS2 is the most affected gene, followed by MSH6, MLH1, and MSH2.
    explanation: Ranks MSH6 second among the gene-defined forms by reported case frequency.
  - reference: PMID:39910726
    reference_title: "Constitutional Mismatch Repair Deficiency: Scoping Review of a Cancer-Predisposition Syndrome With Distinctive Cutaneous Findings."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: with the age of onset progressively decreasing from PMS2 to MSH6, to MLH1 and MSH2
    explanation: Supports placing MSH6 onset between PMS2 (latest) and MLH1/MSH2 (earliest).
  - reference: PMID:39910726
    reference_title: "Constitutional Mismatch Repair Deficiency: Scoping Review of a Cancer-Predisposition Syndrome With Distinctive Cutaneous Findings."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: PMS2 and MSH6 pathogenic variants are linked to the broadest spectrum of cutaneous manifestations
    explanation: Supports the broad cutaneous spectrum attributed to MSH6 alongside PMS2.
- name: CMMRD4
  display_name: Mismatch repair cancer syndrome 4 (PMS2-related)
  subtype_term:
    preferred_term: mismatch repair cancer syndrome 4
    term:
      id: MONDO:0030843
      label: mismatch repair cancer syndrome 4
  description: >-
    Biallelic PMS2 variants (OMIM 619101). The most commonly reported molecular form, with
    the latest onset and the best survival of the four. Diagnosis is technically complicated
    by the PMS2 pseudogenes, so locus-specific methods are required. Gastrointestinal
    tumours predominate in the later-onset PMS2 group whereas CNS tumours predominate in
    early-onset cases.
  genes:
  - preferred_term: PMS2
    term:
      id: hgnc:9122
      label: PMS2
  evidence:
  - reference: PMID:41333974
    reference_title: "Genotype-phenotype correlations in PMS2-associated constitutional mismatch repair deficiency: a systematic literature review"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The PMS2-related subtype (PMS2-CMMRD) is the most common molecular form of CMMRD, exhibiting variable severity and both early and late-onset clinical presentations.
    explanation: Establishes PMS2 as the most common molecular form with a bimodal onset distribution.
  - reference: PMID:41333974
    reference_title: "Genotype-phenotype correlations in PMS2-associated constitutional mismatch repair deficiency: a systematic literature review"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Significant differences in tumour distribution were observed, with CNS tumours being most prevalent in the early-onset group, while GI tumours were more common in the later-onset group.
    explanation: Supports the CNS-early / GI-late tumour distribution described for this subtype.
progression:
- phase: First cancer in childhood
  age_range: Median 8.9 years (IQR 5.9-12.6)
  notes: >-
    CMMRD is usually recognised when the first malignancy appears, at a median of 8.9
    years. Onset is earliest in the MLH1- and MSH2-related forms and latest in the
    PMS2-related form, so this median sits across a genotype-stratified distribution
    rather than describing a single natural history.
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Median age at diagnosis of CMMRD or a related cancer was 8·9 years (IQR 5·9-12·6), and median follow-up from diagnosis was 7·2 years (3·6-14·8).
    explanation: Gives the median age at first cancer or CMMRD diagnosis in the IRRDC cohort.
- phase: Near-complete cancer penetrance by adulthood
  age_range: By 18 years
  notes: >-
    Cumulative cancer incidence reaches 90% by age 18, which is what makes lifelong
    surveillance from molecular diagnosis rather than from first symptom the standard of
    care. This is a registry cohort ascertained largely through cancer, so the figure is
    an upper estimate for the penetrance of an unselected biallelic genotype.
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The cumulative cancer incidence by age 18 years was 90% (95% CI 80-99).
    explanation: Quantifies cumulative cancer incidence by the end of childhood.
- phase: Metachronous second and subsequent cancers
  age_range: Median 1.9 years after the preceding cancer
  notes: >-
    Survivors of a first cancer face a short interval to the next one: the median time
    between successive cancer diagnoses is 1.9 years, and neoplasms arise across 15
    organs. Surveillance therefore does not stop after the first tumour is treated.
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Median time between cancer diagnoses for patients with more than one cancer was 1·9 years (IQR 0·8-3·9).
    explanation: Quantifies the interval between successive primary cancers.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Neoplasms developed in 15 organs and included early-onset adult cancers.
    explanation: Supports the multi-organ distribution of the successive primaries.
- phase: Ten-year survival stratified by tumour group
  age_range: 10 years from cancer diagnosis
  notes: >-
    Outcome is determined mainly by which tumour group presents. CNS tumours carry by far
    the worst prognosis, then haematological, then gastrointestinal, with other solid
    tumours near-uniformly survived. This ordering is the prognostic counterpart of the
    tumour-spectrum frequencies curated in the phenotypes block.
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Patients with CNS tumours had the poorest overall survival rates (39% [95% CI 30-52] at 10 years from diagnosis; log-rank p<0·0001 across four cancer types)
    explanation: Gives 10-year overall survival for the worst-prognosis tumour group and
      establishes that the four groups differ significantly.
- phase: Survival stratified by causative gene
  age_range: 15 years of age
  notes: >-
    Overall survival at age 15 falls in the order PMS2 > MSH6 > MLH1 > MSH2, and within a
    gene, frameshift or truncating variants do worse than missense variants. This gene and
    variant-class gradient is what distinguishes the four subtype entries prognostically,
    and it persists even after surveillance and checkpoint-inhibitor interventions improve
    survival overall.
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: MLH1 or MSH2 variants caused earlier cancer onset than PMS2 or MSH6 variants, and inferior survival
    explanation: Establishes the direction of the gene-defined survival gradient.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: overall survival at age 15 years 63% [95% CI 55-73] for PMS2
    explanation: >-
      Anchors the best-prognosis end of that gradient with a figure. The same sentence in
      the source reports 49% for MSH6, 19% for MLH1 and 0% for MSH2, completing the
      ordering described in the notes.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Frameshift or truncating variants within the same gene caused earlier cancers and inferior outcomes compared with missense variants (p<0·0001).
    explanation: Supports the within-gene variant-class gradient described here.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The greater deleterious effects of MLH1 and MSH2 variants as compared with PMS2 and MSH6 variants persisted despite overall improvements in survival after surveillance or immune checkpoint inhibitor interventions.
    explanation: Supports the statement that the genotype gradient survives the two
      interventions that improve outcome.
clinical_burden:
  burden_level: HIGH
  rationale: >-
    Near-complete cancer penetrance by age 18, a median of under two years between
    successive primary cancers, and 10-year survival of 39% for the commonest tumour
    group. Management is lifelong intensive multi-modality surveillance from molecular
    diagnosis onward rather than treatment of a single event.
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The cumulative cancer incidence by age 18 years was 90% (95% CI 80-99).
    explanation: Supports near-complete cancer penetrance by adulthood.
  - reference: PMID:33945292
    reference_title: Survival Benefit for Individuals With Constitutional Mismatch Repair Deficiency Undergoing Surveillance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Five-year overall survival (OS) was 90% (95% CI, 78.6 to 100) and 50% (95% CI, 39.2 to 63.7) when cancer was detected asymptomatically and symptomatically, respectively"
    explanation: Supports the mortality burden and the dependence of outcome on intensive
      surveillance.
mechanistic_hypotheses:
- hypothesis_group_id: constitutional_mutator_ultrahypermutation_model
  hypothesis_label: Constitutional Mutator plus Somatic Polymerase Proofreading Loss Model
  status: CANONICAL
  description: >-
    Biallelic germline MMR loss removes post-replicative mismatch correction from every
    cell of the body, so microsatellite instability and an elevated base-substitution rate
    are present constitutionally rather than only in tumour tissue. Proliferative
    compartments (neural/glial progenitors, intestinal crypt stem cells, lymphoid
    precursors) accumulate driver mutations first, which is why CNS, gastrointestinal and
    haematological tumours dominate the spectrum. In most high-grade tumours an early
    somatic mutation in the exonuclease (proofreading) domain of POLE or POLD1 is
    frequently selected, and the combined loss of proofreading and mismatch repair produces an
    ultra-hypermutant genome that accumulates mutations in a rapid burst. This two-step
    model explains both the distinctive pathognomonic mutational signature used
    diagnostically and the extreme neoantigen load that makes these tumours
    checkpoint-inhibitor responsive.
  evidence:
  - reference: PMID:25642631
    reference_title: Combined hereditary and somatic mutations of replication error repair genes result in rapid onset of ultra-hypermutated cancers.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The ensuing mutation signatures and numbers are unique and diagnostic of childhood germ-line bMMRD
    explanation: Supports the combined germline-MMR plus somatic-polymerase model as producing a distinctive, diagnostic mutational output.
- hypothesis_group_id: hypomorphic_allele_late_onset_model
  hypothesis_label: Hypomorphic Allele / Residual Repair Model of Late-Onset CMMRD
  status: EMERGING
  description: >-
    A subset of biallelic MMR carriers present in adulthood with a Lynch-like tumour
    spectrum rather than in childhood. The proposed explanation is that the variants are
    hypomorphic - a leaky splice variant or a missense change that retains partial repair
    capacity - so residual mismatch repair delays the accumulation of driver mutations
    without preventing it. If correct, constitutional microsatellite instability testing
    rather than genotype alone is what discriminates such patients from true Lynch
    syndrome, and it predicts that the phenotype should scale with residual repair
    activity. This is currently supported by case-level functional and constitutional-MSI
    data, not by a systematic genotype-function series.
  evidence:
  - reference: PMID:38789506
    reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: A hypomorphic effect of these and other variants found in additional late onset CMMRD cases, identified by literature review, likely explains a LS-like phenotype.
    explanation: States the hypomorphic-residual-function hypothesis for late-onset, Lynch-mimicking CMMRD.
  - reference: PMID:38789506
    reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The apparent genotype-phenotype conflict was resolved by detection of constitutional microsatellite instability in both patients, a hallmark feature of CMMRD.
    explanation: Supports constitutional MSI, not genotype, as the discriminating test predicted by this model.
pathophysiology:
- name: Biallelic Germline Mismatch Repair Gene Inactivation
  biological_scale: MOLECULAR
  role: trigger
  conforms_to: "genome_instability_mutation#Genome-Maintenance Defect or Replication Stress"
  mechanism_confidence: ESTABLISHED
  description: >-
    Both alleles of MLH1, MSH2, MSH6 or PMS2 carry a pathogenic variant in the germ line
    (homozygous or compound heterozygous). The corresponding heterodimer - MutSalpha
    (MSH2/MSH6) for mismatch recognition or MutLalpha (MLH1/PMS2) for the downstream
    incision step - is absent or non-functional in every nucleated cell, so post-replicative
    correction of polymerase slippage and base-base mismatches fails constitutionally. This
    is the point at which CMMRD diverges from Lynch syndrome, where the second allele is
    intact until a somatic hit occurs in one tissue.
  locations:
  - preferred_term: nucleus
    term:
      id: GO:0005634
      label: nucleus
  protein_complexes:
  - preferred_term: mismatch repair complex
    term:
      id: GO:0032300
      label: mismatch repair complex
  biological_processes:
  - preferred_term: mismatch repair
    modifier: LOSS_OF_FUNCTION
    term:
      id: GO:0006298
      label: mismatch repair
  - preferred_term: DNA-templated DNA replication maintenance of fidelity
    modifier: DECREASED
    term:
      id: GO:0045005
      label: DNA-templated DNA replication maintenance of fidelity
  genetic_context:
    genes:
    - preferred_term: MLH1
      term:
        id: hgnc:7127
        label: MLH1
    - preferred_term: MSH2
      term:
        id: hgnc:7325
        label: MSH2
    - preferred_term: MSH6
      term:
        id: hgnc:7329
        label: MSH6
    - preferred_term: PMS2
      term:
        id: hgnc:9122
        label: PMS2
    allele_type: GERMLINE
    variant_origin: GERMLINE
    zygosity: HOMOZYGOUS
    functional_impact_category: LOSS_OF_FUNCTION
    description: >-
      Biallelic germline loss-of-function variants in one MMR gene. Zygosity is recorded as
      HOMOZYGOUS because the schema enum has no BIALLELIC value; compound heterozygosity
      (two different pathogenic alleles in trans) is equally common and equally causal, and
      COMPOUND_HETEROZYGOUS is the correct value for those patients. What matters
      mechanistically is that neither allele is functional. Frameshift and truncating
      variants give earlier cancers and worse outcomes than missense variants within the
      same gene.
  downstream:
  - target: Constitutional Microsatellite Instability and Mutator Phenotype
    description: >-
      Loss of the MutSalpha/MutLalpha correction step leaves polymerase slippage errors
      uncorrected, so insertion-deletion loops accumulate at microsatellites in normal as
      well as neoplastic tissue.
    evidence:
    - reference: PMID:36240479
      reference_title: Genomic Microsatellite Signatures Identify Germline Mismatch Repair Deficiency and Risk of Cancer Onset.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: In normal cells, MMRDness scores differed between tissues (GI > blood > brain), increased over time in the same individual, and revealed genotype-phenotype associations within the mismatch repair genes.
      explanation: >-
        Demonstrates that the mismatch-repair-deficiency signature is measurable in normal
        (non-tumour) tissue, which is what distinguishes the constitutional state from the
        tumour-restricted MMR loss of Lynch syndrome.
  - target: Impaired Immunoglobulin Class-Switch Recombination and Somatic Hypermutation
    description: >-
      The same MutSalpha/MutLalpha heterodimers act as a backup pathway for the processing
      of activation-induced cytidine deaminase-induced lesions during class-switch
      recombination and somatic hypermutation, so their constitutional loss also perturbs
      antibody maturation.
    evidence:
    - reference: PMID:30013564
      reference_title: No Overt Clinical Immunodeficiency Despite Immune Biological Abnormalities in Patients With Constitutional Mismatch Repair Deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: The mismatch repair (MMR) machinery, consisting of homologs of MutSα, MutLα, and MutSβ (MSH2/MSH6, MLH1/PMS2, and MSH2/MSH3, respectively) and other proteins, is involved in CSR
      explanation: States the mechanistic link from the same MMR heterodimers to class-switch recombination.
  evidence:
  - reference: PMID:24737826
    reference_title: "Diagnostic criteria for constitutional mismatch repair deficiency syndrome: suggestions of the European consortium 'care for CMMRD' (C4CMMRD)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Constitutional mismatch repair deficiency (CMMRD) syndrome is a distinct childhood cancer predisposition syndrome that results from biallelic germline mutations in one of the four MMR genes, MLH1, MSH2, MSH6 or PMS2.
    explanation: Establishes the biallelic germline MMR lesion as the initiating event.
  - reference: PMID:38789506
    reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Lynch syndrome (LS) and constitutional mismatch repair deficiency (CMMRD) are distinct cancer syndromes caused, respectively, by mono- and bi-allelic germline mismatch repair (MMR) variants."
    explanation: >-
      States the mono- versus biallelic distinction that separates this node from the
      corresponding Lynch syndrome node.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Frameshift or truncating variants within the same gene caused earlier cancers and inferior outcomes compared with missense variants (p<0·0001).
    explanation: Supports the variant-class effect recorded in this node's genetic context.
- name: Constitutional Microsatellite Instability and Mutator Phenotype
  biological_scale: CELLULAR
  role: central_effector
  conforms_to: "genome_instability_mutation#Mutator Phenotype and Chromosomal Instability"
  mechanism_confidence: ESTABLISHED
  description: >-
    Uncorrected replication errors accumulate as base substitutions and insertion-deletion
    loops, most conspicuously at short tandem repeats. Unlike Lynch syndrome, where
    microsatellite instability appears only after somatic biallelic MMR inactivation in the
    tumour, in CMMRD the instability signature is present in normal blood, gastrointestinal
    and brain tissue, increases with age in the same individual, and its magnitude tracks
    with how early the first cancer appears. Constitutional MSI is therefore both the
    mechanistic core of the syndrome and its most discriminating diagnostic assay.
  locations:
  - preferred_term: nucleus
    term:
      id: GO:0005634
      label: nucleus
  cell_types:
  - preferred_term: intestinal crypt stem cell
    term:
      id: CL:0002250
      label: intestinal crypt stem cell
  - preferred_term: neural stem cell
    term:
      id: CL:0000047
      label: neural stem cell
  - preferred_term: hematopoietic stem cell
    term:
      id: CL:0000037
      label: hematopoietic stem cell
  biological_processes:
  - preferred_term: DNA repair
    modifier: DECREASED
    term:
      id: GO:0006281
      label: DNA repair
  downstream:
  - target: Somatic Polymerase Proofreading Loss and Ultra-Hypermutation
    description: >-
      In a mutator background, somatic variants in the POLE or POLD1 exonuclease domain
      arise and are selected early, compounding the defect.
    evidence:
    - reference: PMID:25642631
      reference_title: Combined hereditary and somatic mutations of replication error repair genes result in rapid onset of ultra-hypermutated cancers.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: All ultra-hypermutated bMMRD cancers acquired early somatic driver mutations in DNA polymerase
      explanation: Establishes the causal ordering - the germline mutator state precedes and enables the somatic polymerase hit.
  - target: Loss of MMR-Dependent Damage Signalling and Alkylating Agent Tolerance
    description: >-
      The MutSalpha/MutLalpha complexes also transduce the damage signal that converts
      O6-methylguanine lesions into cytotoxicity, so their loss produces tolerance rather
      than repair of methylating-agent damage.
    evidence:
    - reference: PMID:34992263
      reference_title: Genomic predictors of response to PD-1 inhibition in children with germline DNA replication repair deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: MMRD and PPD cancers are commonly lethal due to the inherent resistance to chemo-irradiation.
      explanation: Supports the therapeutic consequence of losing MMR-dependent damage signalling.
  - target: Accelerated Multi-Organ Tumorigenesis
    description: >-
      Frameshift mutations accumulating in coding microsatellites of tumour-suppressor and
      growth-control genes drive transformation in whichever proliferative compartment
      reaches the driver threshold first.
    evidence:
    - reference: PMID:38552658
      reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: All cancers showed high mutation and microsatellite indel burdens, and pathognomonic mutational signatures.
      explanation: Links the microsatellite indel burden directly to the tumours that develop across the cohort.
  evidence:
  - reference: PMID:36240479
    reference_title: Genomic Microsatellite Signatures Identify Germline Mismatch Repair Deficiency and Risk of Cancer Onset.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Importantly, increased MMRDness score was associated with younger age of first cancer presentation in individuals with CMMRD
    explanation: >-
      Shows that the magnitude of constitutional instability is quantitatively related to
      the clinical phenotype, supporting this node as the mechanistic core rather than an
      incidental marker.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: All cancers showed high mutation and microsatellite indel burdens, and pathognomonic mutational signatures.
    explanation: Confirms the mutator phenotype across the full IRRDC tumour series.
- name: Somatic Polymerase Proofreading Loss and Ultra-Hypermutation
  biological_scale: MOLECULAR
  role: amplifier
  mechanism_confidence: ESTABLISHED
  description: >-
    An early somatic mutation in the exonuclease (proofreading) domain of POLE or POLD1 is
    frequently acquired and selected in CMMRD high-grade tumours - every ultra-hypermutated
    tumour in the index series carried one. Loss of proofreading on
    top of absent mismatch repair removes both layers of replication-error correction, and
    mutations then accumulate in a rapid burst rather than gradually. The resulting
    ultra-hypermutant genome (in excess of 100, and in brain tumours often over 250,
    mutations per megabase) is the highest burden recorded in human cancer and carries a
    strand-biased signature attributable to the mutant polymerase.
  molecular_functions:
  - preferred_term: 3'-5' exonuclease activity
    modifier: LOSS_OF_FUNCTION
    term:
      id: GO:0008408
      label: 3'-5' exonuclease activity
  biological_processes:
  - preferred_term: DNA replication proofreading
    modifier: LOSS_OF_FUNCTION
    term:
      id: GO:0045004
      label: DNA replication proofreading
  genetic_context:
    allele_type: SOMATIC
    variant_origin: SOMATIC
    functional_impact_category: LOSS_OF_FUNCTION
    description: >-
      Somatic exonuclease-domain variants in POLE or POLD1 acquired within the tumour, on
      the constitutional biallelic MMR background. These are tumour-restricted second
      events, not part of the inherited genotype.
  downstream:
  - target: Neoantigen Generation and T Cell Infiltration
    description: >-
      The extreme coding mutation burden yields a correspondingly extreme load of predicted
      neoepitopes.
    evidence:
    - reference: PMID:27001570
      reference_title: Immune Checkpoint Inhibition for Hypermutant Glioblastoma Multiforme Resulting From Germline Biallelic Mismatch Repair Deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: bMMRD GBM harbored mean neoantigen loads seven to 16 times higher than those in immunoresponsive melanomas, lung cancers, or microsatellite-unstable GI cancers (P < .001).
      explanation: Quantifies the neoantigen output of the ultra-hypermutant genome relative to known immunoresponsive tumours.
  - target: Accelerated Multi-Organ Tumorigenesis
    description: >-
      The mutation burst rapidly supplies the driver alterations needed for high-grade
      transformation, compressing the interval from predisposition to malignancy.
    evidence:
    - reference: PMID:25642631
      reference_title: Combined hereditary and somatic mutations of replication error repair genes result in rapid onset of ultra-hypermutated cancers.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: We suggest a new mechanism of cancer progression in which mutations develop in a rapid burst after ablation of replication repair.
      explanation: States the burst-accumulation mechanism linking dual repair loss to rapid tumour progression.
  evidence:
  - reference: PMID:25642631
    reference_title: Combined hereditary and somatic mutations of replication error repair genes result in rapid onset of ultra-hypermutated cancers.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: High-grade bMMRD brain tumors exhibited massive numbers of substitution mutations (>250/Mb), which was greater than all childhood and most cancers
    explanation: Quantifies the ultra-hypermutant burden in CMMRD high-grade brain tumours.
  - reference: PMID:28805995
    reference_title: Germline PMS2 and somatic POLE exonuclease mutations cause hypermutability of the leading DNA strand in biallelic mismatch repair deficiency syndrome brain tumours.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Biallelic mismatch repair deficiency (bMMRD) in tumours is frequently associated with somatic mutations in the exonuclease domains of DNA polymerases POLE or POLD1, and results in a characteristic mutational profile.
    explanation: Establishes the somatic POLE/POLD1 proofreading hit as a recurrent feature of CMMRD tumours.
  - reference: PMID:28805995
    reference_title: Germline PMS2 and somatic POLE exonuclease mutations cause hypermutability of the leading DNA strand in biallelic mismatch repair deficiency syndrome brain tumours.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: We identified a germline homozygous nonsense variant, p.R802*, in the PMS2 gene. Additionally, by genome sequencing of these tumours, we found extremely high somatic mutation rates (237/Mb and 123/Mb), as well as somatic mutations in the proofreading domain of POLE polymerase
    explanation: Worked example pairing a germline biallelic PMS2 genotype with somatic POLE proofreading loss and the resulting mutation rate.
- name: Loss of MMR-Dependent Damage Signalling and Alkylating Agent Tolerance
  biological_scale: CELLULAR
  role: consequence
  conforms_to: "genome_instability_mutation#Failure of DNA Damage Surveillance and Repair"
  mechanism_confidence: ESTABLISHED
  description: >-
    Beyond error correction, MutSalpha/MutLalpha are required to convert persistent
    O6-methylguanine mispairs into a cytotoxic signal. Without them the lesion is tolerated
    rather than lethal, so MMR-deficient tumours are intrinsically resistant to methylating
    chemotherapy such as temozolomide, and continued exposure selects further mutation
    rather than killing the tumour. This is a therapeutically decisive consequence: standard
    glioma protocols cannot be applied reflexively in CMMRD.
  biological_processes:
  - preferred_term: DNA damage response
    modifier: ABNORMAL
    term:
      id: GO:0006974
      label: DNA damage response
  evidence:
  - reference: PMID:34992263
    reference_title: Genomic predictors of response to PD-1 inhibition in children with germline DNA replication repair deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: MMRD and PPD cancers are commonly lethal due to the inherent resistance to chemo-irradiation.
    explanation: Establishes intrinsic chemo-radioresistance as a defining clinical property of replication-repair-deficient cancers.
- name: Impaired Immunoglobulin Class-Switch Recombination and Somatic Hypermutation
  biological_scale: CELLULAR
  role: consequence
  mechanism_confidence: ESTABLISHED
  description: >-
    MMR heterodimers act as a backup pathway for repair of activation-induced cytidine
    deaminase-induced lesions during class-switch recombination and somatic hypermutation.
    In CMMRD this produces measurable B-cell abnormalities - reduced somatic hypermutation
    frequency, skewed IGH subclass usage, reduced class-switched memory B cells and
    plasmablasts. Crucially, these are biological rather than clinical findings: a
    systematic cohort study found no consistent laboratory pattern and no warning signs of
    primary immunodeficiency, so CMMRD should not be curated or managed as an
    immunodeficiency syndrome. The node is retained because the B-cell phenotype is real and
    mechanistically informative about MMR function in vivo.
  cell_types:
  - preferred_term: B cell
    term:
      id: CL:0000236
      label: B cell
  biological_processes:
  - preferred_term: somatic hypermutation of immunoglobulin genes
    modifier: DECREASED
    term:
      id: GO:0016446
      label: somatic hypermutation of immunoglobulin genes
  - preferred_term: isotype switching
    modifier: ABNORMAL
    term:
      id: GO:0045190
      label: isotype switching
  evidence:
  - reference: PMID:30013564
    reference_title: No Overt Clinical Immunodeficiency Despite Immune Biological Abnormalities in Patients With Constitutional Mismatch Repair Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: results of next generation sequencing-based analyses of antigen-selected B-cell receptor rearrangements showed a significantly reduced frequency of SHM and an increased number of rearranged immunoglobulin heavy chain (IGH) transcripts that use IGHG3, IGHG1, and IGHA1 subclasses
    explanation: Documents the measurable B-cell somatic-hypermutation and class-switch abnormality.
  - reference: PMID:30013564
    reference_title: No Overt Clinical Immunodeficiency Despite Immune Biological Abnormalities in Patients With Constitutional Mismatch Repair Deficiency.
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "Importantly, none of the patients showed any clinical warning signs of PID (infections, immune dysregulation, inflammation, failure to thrive, etc.)."
    explanation: >-
      Refutes the stronger claim that this node amounts to a clinical primary
      immunodeficiency; it is recorded here so the biological finding is not over-read.
- name: Neoantigen Generation and T Cell Infiltration
  biological_scale: TISSUE
  role: consequence
  conforms_to: "immune_checkpoint_blockade#Neoantigen Generation"
  mechanism_confidence: ESTABLISHED
  description: >-
    The ultra-hypermutant coding genome produces neoepitopes at a density several times
    higher than in melanoma, lung cancer or sporadic MSI-high gastrointestinal cancer. These
    are presented on MHC and drive CD8-positive T cell recognition, converting even
    conventionally immune-cold tumours such as high-grade glioma into immune-infiltrated
    lesions. Microsatellite insertion-deletion load contributes independently of total
    mutation burden, which is why MS-indels predict response in tumours whose overall burden
    is only moderately elevated.
  cell_types:
  - preferred_term: CD8-positive, alpha-beta T cell
    term:
      id: CL:0000625
      label: CD8-positive, alpha-beta T cell
  biological_processes:
  - preferred_term: antigen processing and presentation
    modifier: INCREASED
    term:
      id: GO:0019882
      label: antigen processing and presentation
  - preferred_term: T cell mediated cytotoxicity
    modifier: INCREASED
    term:
      id: GO:0001913
      label: T cell mediated cytotoxicity
  downstream:
  - target: Adaptive Immune Resistance in Hypermutant Tumours
    description: >-
      Sustained T cell infiltration and interferon signalling drive compensatory checkpoint
      engagement, which is what checkpoint blockade releases.
    evidence:
    - reference: PMID:34992263
      reference_title: Genomic predictors of response to PD-1 inhibition in children with germline DNA replication repair deficiency.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: both mechanisms were associated with increased immune infiltration even in 'immunologically cold' tumors such as gliomas, contributing to the favorable response
      explanation: Links the mutational mechanisms to immune infiltration and to checkpoint-blockade responsiveness.
  evidence:
  - reference: PMID:27001570
    reference_title: Immune Checkpoint Inhibition for Hypermutant Glioblastoma Multiforme Resulting From Germline Biallelic Mismatch Repair Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: bMMRD GBM harbored mean neoantigen loads seven to 16 times higher than those in immunoresponsive melanomas, lung cancers, or microsatellite-unstable GI cancers (P < .001).
    explanation: Quantifies the neoantigen load underlying this node.
  - reference: PMID:34992263
    reference_title: Genomic predictors of response to PD-1 inhibition in children with germline DNA replication repair deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: High mutation burden predicted response for ultra-hypermutant cancers (>100 mutations per Mb) enriched for combined MMRD + PPD, while MS-indels predicted response in MMRD tumors with lower mutation burden (10-100 mutations per Mb).
    explanation: Distinguishes the two independent antigenic mechanisms modelled by this node.
- name: Adaptive Immune Resistance in Hypermutant Tumours
  biological_scale: TISSUE
  role: consequence
  conforms_to: "immune_checkpoint_blockade#Adaptive Immune Resistance"
  mechanism_confidence: ESTABLISHED
  description: >-
    Despite an extreme neoantigen load, CMMRD tumours progress, because PD-1/PD-L1
    engagement suppresses the infiltrating effector T cells. This node is the therapeutic
    target: PD-1 blockade releases the pre-existing anti-tumour response and produces
    durable remissions in tumours - notably recurrent high-grade glioma - that do not
    respond to checkpoint inhibition in the unselected paediatric population. Responses are
    characteristically delayed, and pseudo-progression (flare) is common and should not be
    read as failure. Resistance after initial response is associated with rising CTLA-4
    expression, which is the rationale for CTLA-4-directed salvage.
  cell_types:
  - preferred_term: T cell
    term:
      id: CL:0000084
      label: T cell
  biological_processes:
  - preferred_term: negative regulation of T cell mediated immunity
    modifier: INCREASED
    term:
      id: GO:0002710
      label: negative regulation of T cell mediated immunity
  evidence:
  - reference: PMID:37823831
    reference_title: "Combined Immunotherapy Improves Outcome for Replication-Repair-Deficient (RRD) High-Grade Glioma Failing Anti-PD-1 Monotherapy: A Report from the International RRD Consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: CTLA4 expression increased over time, and subsequent CTLA4 inhibition resulted in response/stable disease in 75%.
    explanation: Documents the evolving checkpoint-resistance mechanism and its therapeutic reversal.
  - reference: PMID:34992263
    reference_title: Genomic predictors of response to PD-1 inhibition in children with germline DNA replication repair deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Pseudo-progression (flare) was common and was associated with immune activation in the tumor microenvironment and systemically.
    explanation: Supports the flare phenomenon described in this node.
- name: Accelerated Multi-Organ Tumorigenesis
  biological_scale: ORGANISM
  role: consequence
  conforms_to: "genome_instability_mutation#Accelerated Clonal Evolution"
  mechanism_confidence: ESTABLISHED
  description: >-
    The constitutional mutator state converts every proliferative compartment into a site of
    accelerated clonal evolution, so tumours arise early, in multiple organs, and repeatedly
    over a lifetime rather than as a single event. In the IRRDC cohort 339 cancers occurred
    in 97% of 201 patients, the median interval between successive cancers was under two
    years, and neoplasms arose in fifteen different organs. CNS tumours dominate and carry
    the worst survival; gastrointestinal and haematological malignancies follow. Low-grade
    lesions are not indolent - the great majority progress to high grade within a few years
    if not resected, which is what makes structured surveillance rather than symptom-driven
    presentation the decisive management variable.
  cell_types:
  - preferred_term: glial cell
    term:
      id: CL:0000125
      label: glial cell
  - preferred_term: intestinal crypt stem cell
    term:
      id: CL:0002250
      label: intestinal crypt stem cell
  - preferred_term: hematopoietic stem cell
    term:
      id: CL:0000037
      label: hematopoietic stem cell
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: 339 cancers were reported in 194 (97%) of 201 patients.
    explanation: Quantifies the multi-tumour burden modelled by this node.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: CNS tumours were the most frequent (173 [51%] cancers), followed by gastrointestinal (75 [22%]), haematological (61 [18%]), and other cancer types (30 [9%]).
    explanation: Gives the organ distribution of the tumours arising from this node.
  - reference: PMID:33945292
    reference_title: Survival Benefit for Individuals With Constitutional Mismatch Repair Deficiency Undergoing Surveillance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Of the 64 low-grade tumors detected, the cumulative likelihood of transformation from low-to high-grade was 81% for GI cancers within 8 years and 100% for gliomas in 6 years.
    explanation: Supports the statement that low-grade lesions in CMMRD reliably progress rather than remaining indolent.
phenotypes:
- category: Neoplastic
  name: Multiple Metachronous Primary Neoplasms
  description: >-
    Successive independent primary cancers rather than recurrence of one tumour, arising
    across many organs. 339 cancers were recorded in 194 of 201 IRRDC patients, and for
    those with more than one cancer the median interval between diagnoses was 1.9 years.
    This is what makes surveillance lifelong rather than a period of follow-up after a
    single treated tumour, and it distinguishes CMMRD from cancer predispositions whose
    burden is concentrated in one organ system. No `frequency` band is asserted: the
    source reports cancer counts and the inter-cancer interval, not the proportion of
    patients who developed more than one primary.
  phenotype_term:
    preferred_term: Multiple metachronous primary neoplasms
    term:
      id: HP:0002664
      label: Neoplasm
    temporality: RECURRENT
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Median time between cancer diagnoses for patients with more than one cancer was 1·9 years (IQR 0·8-3·9).
    explanation: Establishes that a substantial subgroup develops more than one primary and
      quantifies the interval between them.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: 339 cancers were reported in 194 (97%) of 201 patients.
    explanation: The cancer count exceeds the patient count, which is the cohort-level
      signal of multiple primaries per patient.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Neoplasms developed in 15 organs and included early-onset adult cancers.
    explanation: Supports the multi-organ rather than single-site distribution of the
      successive primaries.
- category: Neoplastic
  name: Central Nervous System Tumour
  description: >-
    The most frequent and most lethal cancer group in CMMRD, accounting for about half of
    all tumours. High-grade glioma and glioblastoma predominate; onset is typically at
    school age. Ten-year survival from CNS tumour diagnosis is the worst of any tumour group
    in the syndrome.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Brain neoplasm
    term:
      id: HP:0030692
      label: Brain neoplasm
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: CNS tumours were the most frequent (173 [51%] cancers), followed by gastrointestinal (75 [22%]), haematological (61 [18%]), and other cancer types (30 [9%]).
    explanation: >-
      CNS tumours are 51% of the 339 cancers in a cohort where 97% of 201 patients developed
      cancer. The cohort reports cancer counts rather than the number of patients with at
      least one CNS tumour, and patients frequently have more than one cancer, so the
      patient-level fraction cannot be read off directly. FREQUENT (30-79%) is the
      conservative band that the reported counts support; VERY_FREQUENT would be an
      over-read.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Patients with CNS tumours had the poorest overall survival rates
    explanation: Supports the prognostic statement in the description.
- category: Neoplastic
  name: High-Grade Glioma and Glioblastoma
  description: >-
    The characteristic CMMRD brain tumour and the setting in which checkpoint blockade was
    first shown to work. These tumours carry the highest mutation burdens in the syndrome
    because of the added somatic POLE/POLD1 proofreading defect.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Glioblastoma multiforme
    term:
      id: HP:0012174
      label: Glioblastoma multiforme
  evidence:
  - reference: PMID:27001570
    reference_title: Immune Checkpoint Inhibition for Hypermutant Glioblastoma Multiforme Resulting From Germline Biallelic Mismatch Repair Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Biallelic mismatch repair deficiency (bMMRD) is a highly penetrant childhood cancer syndrome often resulting in GBM characterized by a high mutational burden.
    explanation: >-
      States that glioblastoma is a frequent ("often resulting in") outcome of CMMRD,
      supporting a FREQUENT rather than an obligate band.
- category: Neoplastic
  name: Gastrointestinal Carcinoma
  description: >-
    Colorectal and small-bowel adenocarcinoma, typically presenting in later childhood or
    adolescence and preceded by adenomatous polyps. In PMS2-related CMMRD gastrointestinal
    tumours dominate the later-onset presentation. Survival is markedly better than for CNS
    disease, and gastrointestinal surveillance is the most effective arm of the surveillance
    protocol.
  phenotype_term:
    preferred_term: Colon cancer
    term:
      id: HP:0003003
      label: Colon cancer
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: CNS tumours were the most frequent (173 [51%] cancers), followed by gastrointestinal (75 [22%]), haematological (61 [18%]), and other cancer types (30 [9%]).
    explanation: >-
      Gastrointestinal tumours are 22% of the 339 cancers in this cohort. No `frequency`
      band is asserted: 75 gastrointestinal cancers among 201 patients bounds the
      patient-level fraction at 37% at most, which straddles the OCCASIONAL/FREQUENT
      boundary, and the cohort does not report the number of patients affected.
  - reference: PMID:26391938
    reference_title: Colorectal Cancer due to Constitutional Mismatch Repair Deficiency Mimicking Neurofibromatosis I.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Colonoscopy showed numerous polyps and a colorectal mass lesion, of which a biopsy revealed adenocarcinoma
    explanation: Worked paediatric case of CMMRD colorectal adenocarcinoma arising on a polyposis background.
- category: Neoplastic
  name: Small Intestinal Neoplasm
  description: >-
    Duodenal and small-bowel adenomas and adenocarcinoma. Included in the surveillance
    protocol alongside colorectal screening because small-bowel disease is not detected by
    colonoscopy alone.
  phenotype_term:
    preferred_term: Neoplasm of the small intestine
    term:
      id: HP:0100833
      label: Neoplasm of the small intestine
  evidence:
  - reference: PMID:38789506
    reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: During follow up endoscopies, duodenal adenomatous polyps were found and resected.
    explanation: Documents small-bowel (duodenal) neoplasia in a molecularly confirmed CMMRD patient.
- category: Neoplastic
  name: Colorectal Polyposis
  description: >-
    Multiple intestinal adenomas, frequently numerous enough to be described as polyposis
    and historically to prompt a Turcot syndrome label. Low-grade lesions transform to
    high-grade at a very high rate if not resected.
  phenotype_term:
    preferred_term: Colorectal polyposis
    term:
      id: HP:0200063
      label: Colorectal polyposis
  evidence:
  - reference: PMID:26391938
    reference_title: Colorectal Cancer due to Constitutional Mismatch Repair Deficiency Mimicking Neurofibromatosis I.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Colonoscopy showed numerous polyps and a colorectal mass lesion
    explanation: Documents numerous colorectal polyps in a molecularly confirmed CMMRD patient.
  - reference: PMID:33945292
    reference_title: Survival Benefit for Individuals With Constitutional Mismatch Repair Deficiency Undergoing Surveillance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Of the 64 low-grade tumors detected, the cumulative likelihood of transformation from low-to high-grade was 81% for GI cancers within 8 years
    explanation: Supports the malignant-transformation statement for low-grade gastrointestinal lesions.
- category: Neoplastic
  name: Haematological Malignancy
  description: >-
    Predominantly T-lineage lymphoblastic lymphoma and leukaemia, often the earliest tumour
    and sometimes presenting in infancy. Unlike solid tumours these are rarely detected
    asymptomatically by the surveillance protocol.
  phenotype_term:
    preferred_term: Lymphoma
    term:
      id: HP:0002665
      label: Lymphoma
  evidence:
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: CNS tumours were the most frequent (173 [51%] cancers), followed by gastrointestinal (75 [22%]), haematological (61 [18%]), and other cancer types (30 [9%]).
    explanation: >-
      Haematological malignancies are 18% of the 339 cancers in this cohort. No `frequency`
      band is asserted, for the same reason as the gastrointestinal record: cancer counts
      do not give the patient-level fraction.
  - reference: PMID:33945292
    reference_title: Survival Benefit for Individuals With Constitutional Mismatch Repair Deficiency Undergoing Surveillance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: By contrast, only 16% of hematologic malignancies were detected asymptomatically (P < .001).
    explanation: Supports the statement that haematological disease escapes the surveillance protocol.
- category: Neoplastic
  name: Leukaemia
  description: >-
    Acute lymphoblastic leukaemia, most often of T-cell lineage, and less commonly acute
    myeloid leukaemia. Among the earliest-onset manifestations of the syndrome. No
    `frequency` band is asserted - the cited source documents occurrence and very early
    onset, not a rate.
  phenotype_term:
    preferred_term: Leukemia
    term:
      id: HP:0001909
      label: Leukemia
  evidence:
  - reference: PMID:42348008
    reference_title: "The impact of international care networks on the clinical management of constitutional mismatch repair deficiency (CMMRD): a review of recent developments."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: two daughters were diagnosed with leukemia at the ages of one and two years, respectively, and a third daughter developed NHL at the age of three
    explanation: Documents very early-onset leukaemia in the founding CMMRD family reports.
- category: Dermatological
  name: Multiple Cafe-au-Lait Macules
  description: >-
    Present in essentially every CMMRD patient and the single most useful clinical clue.
    They differ from the neurofibromatosis type 1 pattern in being fewer and larger with
    irregular borders, but the overlap is close enough that CMMRD is regularly misdiagnosed
    as NF1 - which matters, because the two have entirely different surveillance
    requirements.
  frequency: VERY_FREQUENT
  diagnostic: true
  phenotype_term:
    preferred_term: Multiple cafe-au-lait spots
    term:
      id: HP:0007565
      label: Multiple cafe-au-lait spots
  evidence:
  - reference: PMID:24440087
    reference_title: "Genetic and clinical determinants of constitutional mismatch repair deficiency syndrome: report from the constitutional mismatch repair deficiency consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: All children with CMMRD had café-au-lait spots and 11/14 came from consanguineous families.
    explanation: >-
      Reports cafe-au-lait spots in every child in the consortium series, supporting the
      VERY_FREQUENT band.
  - reference: PMID:39910726
    reference_title: "Constitutional Mismatch Repair Deficiency: Scoping Review of a Cancer-Predisposition Syndrome With Distinctive Cutaneous Findings."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Patients with CMMRD present with café-au-lait macules that are fewer in number and larger than in patients with neurofibromatosis type 1.
    explanation: Supports the described morphological difference from the NF1 pattern.
- category: Dermatological
  name: Hypopigmented Skin Patches
  description: >-
    Hypopigmented macules and patches accompanying the cafe-au-lait macules. Part of the
    mixed hyper- and hypopigmentary picture that distinguishes CMMRD from NF1, where
    hypopigmented lesions are not characteristic. No `frequency` band is asserted - the
    cited scoping review lists these findings without quantifying them.
  phenotype_term:
    preferred_term: Hypopigmented skin patches
    term:
      id: HP:0001053
      label: Hypopigmented skin patches
  evidence:
  - reference: PMID:39910726
    reference_title: "Constitutional Mismatch Repair Deficiency: Scoping Review of a Cancer-Predisposition Syndrome With Distinctive Cutaneous Findings."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Additional dermatological findings include hypopigmented patches and intertriginous freckling.
    explanation: Records hypopigmented patches among the characteristic cutaneous findings.
- category: Dermatological
  name: Intertriginous Freckling
  description: >-
    Axillary and inguinal freckling, one of the NF1 diagnostic criteria that CMMRD patients
    may fulfil. Its presence therefore does not exclude CMMRD and should not close the
    diagnostic question in a child who also has a CMMRD-spectrum tumour. No `frequency`
    band is asserted - the cited scoping review lists this finding without quantifying it.
  phenotype_term:
    preferred_term: Axillary freckling
    term:
      id: HP:0000997
      label: Axillary freckling
  evidence:
  - reference: PMID:39910726
    reference_title: "Constitutional Mismatch Repair Deficiency: Scoping Review of a Cancer-Predisposition Syndrome With Distinctive Cutaneous Findings."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Additional dermatological findings include hypopigmented patches and intertriginous freckling.
    explanation: Records intertriginous freckling among the characteristic cutaneous findings.
  - reference: PMID:27779754
    reference_title: Connections between constitutional mismatch repair deficiency syndrome and neurofibromatosis type 1
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Evaluation of the clinical findings of genetically proven CMMRD patients shows that not only multiple café-au-lait macules but also any of the diagnostic features of NF1 may be present in a CMMRD patient.
    explanation: Supports the statement that NF1 diagnostic features, freckling included, may be present in CMMRD.
genetic:
- name: MLH1
  gene_term:
    preferred_term: MLH1
    term:
      id: hgnc:7127
      label: MLH1
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: CMMRD1
  frequency: third most commonly reported of the four CMMRD genes
  notes: >-
    MLH1 partners PMS2 to form MutLalpha. Biallelic MLH1 variants give early-onset disease
    and poor survival. Monoallelic MLH1 variants cause Lynch syndrome in the parents.
  evidence:
  - reference: PMID:39910726
    reference_title: "Constitutional Mismatch Repair Deficiency: Scoping Review of a Cancer-Predisposition Syndrome With Distinctive Cutaneous Findings."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: PMS2 is the most affected gene, followed by MSH6, MLH1, and MSH2.
    explanation: Gives the relative reporting frequency recorded in this record.
- name: MSH2
  gene_term:
    preferred_term: MSH2
    term:
      id: hgnc:7325
      label: MSH2
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: CMMRD2
  frequency: least commonly reported of the four CMMRD genes
  notes: >-
    MSH2 partners MSH6 to form MutSalpha. Biallelic MSH2 variants carry the earliest onset
    and the worst survival of the four gene-defined forms.
  evidence:
  - reference: PMID:39910726
    reference_title: "Constitutional Mismatch Repair Deficiency: Scoping Review of a Cancer-Predisposition Syndrome With Distinctive Cutaneous Findings."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: PMS2 is the most affected gene, followed by MSH6, MLH1, and MSH2.
    explanation: Gives the relative reporting frequency recorded in this record.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: MLH1 or MSH2 variants caused earlier cancer onset than PMS2 or MSH6 variants, and inferior survival
    explanation: Supports the severity ranking recorded in the notes.
- name: MSH6
  gene_term:
    preferred_term: MSH6
    term:
      id: hgnc:7329
      label: MSH6
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: CMMRD3
  frequency: second most commonly reported of the four CMMRD genes
  notes: >-
    MSH6 partners MSH2 to form MutSalpha. Hypomorphic MSH6 missense variants are one route
    to late-onset, Lynch-mimicking CMMRD.
  evidence:
  - reference: PMID:39910726
    reference_title: "Constitutional Mismatch Repair Deficiency: Scoping Review of a Cancer-Predisposition Syndrome With Distinctive Cutaneous Findings."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: PMS2 is the most affected gene, followed by MSH6, MLH1, and MSH2.
    explanation: Gives the relative reporting frequency recorded in this record.
  - reference: PMID:38789506
    reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Two suspected LS patients with first cancer diagnosis aged 27 or 38 years were found to be homozygous for an MMR (likely) pathogenic variant, MSH6 c.3226C>T (p.(Arg1076Cys)), or variant of uncertain significance (VUS), MLH1 c.306G>A (p.(Glu102=))."
    explanation: Worked example of a homozygous MSH6 missense variant producing late-onset, Lynch-mimicking CMMRD.
- name: PMS2
  gene_term:
    preferred_term: PMS2
    term:
      id: hgnc:9122
      label: PMS2
  relationship_type: CAUSATIVE
  variant_origin: GERMLINE
  subtype: CMMRD4
  frequency: most commonly reported of the four CMMRD genes
  notes: >-
    PMS2 partners MLH1 to form MutLalpha. It is the most frequently reported CMMRD gene and
    the mildest, with the latest onset and best survival. Molecular diagnosis requires
    locus-specific methods because of the PMS2 pseudogenes.
  evidence:
  - reference: PMID:41333974
    reference_title: "Genotype-phenotype correlations in PMS2-associated constitutional mismatch repair deficiency: a systematic literature review"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The PMS2-related subtype (PMS2-CMMRD) is the most common molecular form of CMMRD, exhibiting variable severity and both early and late-onset clinical presentations.
    explanation: Establishes PMS2 as the most commonly reported causal gene.
  - reference: PMID:33622763
    reference_title: "Diagnostic criteria for constitutional mismatch repair deficiency (CMMRD): recommendations from the international consensus working group."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Genetic testing may not be informative and is complicated by pseudogenes associated with the most commonly associated gene, PMS2.
    explanation: Supports the pseudogene caveat recorded in the notes.
- name: POLE
  gene_term:
    preferred_term: POLE
    term:
      id: hgnc:9177
      label: POLE
  relationship_type: MODIFIER
  variant_origin: SOMATIC
  notes: >-
    Not a germline CMMRD gene. Somatic exonuclease-domain (proofreading) variants in POLE
    are acquired within CMMRD tumours and convert the hypermutant phenotype into an
    ultra-hypermutant one. Recorded as a somatic modifier of tumour biology and of
    checkpoint-inhibitor response, not as a cause of the syndrome.
  evidence:
  - reference: PMID:28805995
    reference_title: Germline PMS2 and somatic POLE exonuclease mutations cause hypermutability of the leading DNA strand in biallelic mismatch repair deficiency syndrome brain tumours.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Biallelic mismatch repair deficiency (bMMRD) in tumours is frequently associated with somatic mutations in the exonuclease domains of DNA polymerases POLE or POLD1, and results in a characteristic mutational profile.
    explanation: Establishes somatic POLE/POLD1 proofreading loss as a recurrent modifier in CMMRD tumours.
diagnosis:
- name: Germline Multigene Panel Testing of MLH1, MSH2, MSH6 and PMS2
  description: >-
    Sequencing plus deletion/duplication analysis of all four MMR genes, with confirmation
    that the two variants are in trans by parental testing. All four genes are tested, not
    only the one whose protein is absent by immunohistochemistry. PMS2 requires
    locus-specific methods because of its pseudogenes. Biallelic pathogenic/likely
    pathogenic variants remain the diagnostic standard.
  evidence:
  - reference: PMID:33622763
    reference_title: "Diagnostic criteria for constitutional mismatch repair deficiency (CMMRD): recommendations from the international consensus working group."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The criteria incorporate germline mismatch repair results, ancillary tests and clinical manifestation to determine a diagnosis.
    explanation: States that germline MMR results are the backbone of the consensus diagnostic criteria.
- name: Mismatch Repair Immunohistochemistry in Tumour and Normal Tissue
  description: >-
    Loss of MLH1, MSH2, MSH6 or PMS2 protein staining. In CMMRD, and unlike Lynch syndrome,
    the loss is also present in adjacent NORMAL tissue, which is the single most accessible
    discriminator between the two syndromes. Missense variants that produce a stable but
    non-functional protein can retain staining, so normal immunohistochemistry does not
    exclude the diagnosis.
  evidence:
  - reference: PMID:24440087
    reference_title: "Genetic and clinical determinants of constitutional mismatch repair deficiency syndrome: report from the constitutional mismatch repair deficiency consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Furthermore, screening of normal tissue by immunohistochemistry correlated with genetic confirmation of CMMRD.
    explanation: Supports normal-tissue immunohistochemistry as a CMMRD-specific diagnostic step.
  - reference: PMID:24440087
    reference_title: "Genetic and clinical determinants of constitutional mismatch repair deficiency syndrome: report from the constitutional mismatch repair deficiency consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Tumour immunohistochemistry was 100% sensitive and specific in diagnosing mismatch repair (MMR) deficiency of the corresponding gene while microsatellite instability was neither sensitive nor specific as a diagnostic tool (p<0.0001).
    explanation: >-
      Establishes the performance of immunohistochemistry and, importantly, that conventional
      MSI panels are unreliable in this paediatric setting.
- name: Constitutional Microsatellite Instability Testing (LOGIC / low-pass genome sequencing)
  description: >-
    Quantitative genomic-instability scoring on blood or saliva DNA. Because CMMRD produces
    microsatellite instability in normal tissue, a functional constitutional assay can make
    the diagnosis without a tumour and can resolve cases where genotype is ambiguous
    (variants of uncertain significance, hypomorphic alleles). The LOGIC assay was 100%
    sensitive and specific in childhood cancers and clearly outperformed the conventional
    MSI panel, immunohistochemistry and tumour mutational burden. It remains a specialised
    rather than a universally available test.
  evidence:
  - reference: PMID:36240479
    reference_title: Genomic Microsatellite Signatures Identify Germline Mismatch Repair Deficiency and Risk of Cancer Onset.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Overall, LOGIC was 100% sensitive and specific in detecting MMRD in childhood cancers (N = 376).
    explanation: Quantifies the diagnostic performance of the constitutional-instability assay.
  - reference: PMID:36240479
    reference_title: Genomic Microsatellite Signatures Identify Germline Mismatch Repair Deficiency and Risk of Cancer Onset.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: LOGIC was able to distinguish CMMRD from other cancer predisposition syndromes using blood and saliva DNA
    explanation: Supports the tumour-free, blood/saliva-based diagnostic route described here.
- name: C4CMMRD Clinical Scoring System
  description: >-
    A three-point scoring system that weights the patient's malignancy and additional
    features (pigmentary skin findings, brain malformations, pilomatricomas, a second
    childhood malignancy, a Lynch-spectrum tumour in a relative, parental consanguinity),
    with three points triggering diagnostic evaluation. It is a case-finding trigger, not a
    diagnosis: contemporary practice requires molecular or functional confirmation.
  evidence:
  - reference: PMID:24737826
    reference_title: "Diagnostic criteria for constitutional mismatch repair deficiency syndrome: suggestions of the European consortium 'care for CMMRD' (C4CMMRD)."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: According to the scoring system, CMMRD should be suspected in any cancer patient who reaches a minimum of three points by adding the points of the malignancy and the additional features.
    explanation: States the three-point threshold recorded in this description.
differential_diagnoses:
- name: Lynch Syndrome
  description: >-
    The monoallelic disease caused by variants in the same four genes. Distinguished by
    inheritance (dominant versus recessive), age (adult versus childhood), tumour spectrum
    (gastrointestinal/genitourinary versus CNS-predominant), and decisively by the absence
    of constitutional microsatellite instability and of normal-tissue MMR protein loss.
    A late-onset CMMRD patient with hypomorphic alleles can present exactly like Lynch
    syndrome, so constitutional MSI testing is what resolves the two.
  evidence:
  - reference: PMID:38789506
    reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Lynch syndrome (LS) and constitutional mismatch repair deficiency (CMMRD) are distinct cancer syndromes caused, respectively, by mono- and bi-allelic germline mismatch repair (MMR) variants. LS predisposes to mainly gastrointestinal and genitourinary cancers in adulthood. CMMRD predisposes to brain, haematological, and LS-spectrum cancers from childhood."
    explanation: States the mono- versus biallelic and adult- versus childhood-onset distinction.
- name: Neurofibromatosis Type 1
  description: >-
    The most consequential misdiagnosis. CMMRD patients may fulfil formal NF1 diagnostic
    criteria on pigmentary findings alone, and NF1 shares several tumour types (high-grade
    glioma, acute myeloid leukaemia, rhabdomyosarcoma) with CMMRD. A child labelled NF1 who
    develops a CMMRD-spectrum tumour, or who has no explanatory NF1/SPRED1 variant, should
    be tested for CMMRD - the surveillance implications differ completely.
  evidence:
  - reference: PMID:27779754
    reference_title: Connections between constitutional mismatch repair deficiency syndrome and neurofibromatosis type 1
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: This phenotypic overlap may lead to misdiagnosis of CMMRD patients as having NF1, which impedes adequate management of the patients and their families.
    explanation: States the misdiagnosis risk and its clinical cost.
  - reference: PMID:38552658
    reference_title: "Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an International Replication Repair Deficiency Consortium cohort study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Frequent dermatological manifestations (117 [93%] of 126 patients with complete data) led to a clinical overlap with neurofibromatosis type 1 (35 [28%] of 126).
    explanation: Quantifies how often the NF1 overlap actually arises in a large CMMRD cohort.
- name: Legius Syndrome
  description: >-
    SPRED1-related. Shares cafe-au-lait macules and freckling with both NF1 and CMMRD but
    lacks the CMMRD tumour spectrum and constitutional microsatellite instability.
  evidence:
  - reference: PMID:41333974
    reference_title: "Genotype-phenotype correlations in PMS2-associated constitutional mismatch repair deficiency: a systematic literature review"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: As clinical phenotype in CMMRD overlaps with other rare genetic diseases, such as neurofibromatosis type 1 (NF1) and Legius syndrome
    explanation: Names Legius syndrome among the phenotypic mimics of CMMRD.
- name: Polymerase Proofreading-Associated Polyposis
  description: >-
    Germline heterozygous POLE or POLD1 exonuclease-domain variants causing multiple
    colorectal adenomas and early-onset colorectal cancer. The mechanistic mirror image of
    the somatic proofreading loss modelled here: in CMMRD the proofreading defect is
    acquired in the tumour on top of a constitutional mismatch repair defect, whereas in
    PPAP it is the constitutional lesion and mismatch repair is intact. That difference is
    what the laboratory sees - PPAP tumours are hypermutated but microsatellite stable and
    retain normal MMR protein staining, while CMMRD shows constitutional microsatellite
    instability and MMR protein loss in normal tissue. Inheritance is dominant rather than
    recessive.
  evidence:
  - reference: PMID:23263490
    reference_title: Germline mutations affecting the proofreading domains of POLE and POLD1 predispose to colorectal adenomas and carcinomas.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: we identified specific heterozygous POLE or POLD1 germline variants in several multiple-adenoma and/or CRC cases but in no controls
    explanation: Establishes PPAP as a distinct germline proofreading-deficiency cancer
      predisposition, and its heterozygous (dominant) genetics.
  - reference: PMID:23263490
    reference_title: Germline mutations affecting the proofreading domains of POLE and POLD1 predispose to colorectal adenomas and carcinomas.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: the tumors from mutation carriers were microsatellite stable but tended to acquire base substitution mutations
    explanation: Gives the discriminating laboratory finding - microsatellite stability -
      that separates PPAP from CMMRD.
treatments:
- name: PD-1 Immune Checkpoint Blockade
  description: >-
    Nivolumab or pembrolizumab. The landmark targeted therapy for CMMRD: the ultra-high
    mutation and neoantigen burden makes these tumours checkpoint-responsive even in
    settings - recurrent high-grade glioma, CNS disease - where unselected paediatric
    trials had failed. Responses are frequently delayed, so best overall response
    substantially exceeds initial objective response, and pseudo-progression (flare) must
    not be mistaken for failure. Registered as a phase 1/2 pilot trial (NCT02992964) in
    children with tumour mutational burden at or above 5 mutations/Mb and/or mismatch repair
    deficiency.
  therapeutic_modality: MONOCLONAL_ANTIBODY
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: nivolumab
      term:
        id: NCIT:C68814
        label: Nivolumab
    - preferred_term: pembrolizumab
      term:
        id: NCIT:C106432
        label: Pembrolizumab
  target_mechanisms:
  - target: Adaptive Immune Resistance in Hypermutant Tumours
    treatment_effect: INHIBITS
    description: >-
      Anti-PD-1 antibodies block the PD-1/PD-L1 interaction that suppresses the
      neoantigen-driven T cell response already present in these tumours, releasing
      pre-existing anti-tumour immunity rather than creating it.
    evidence:
    - reference: PMID:37126021
      reference_title: Efficacy of Nivolumab in Pediatric Cancers with High Mutation Burden and Mismatch Repair Deficiency
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Delayed immune responses contributed to best overall response of 50%, improving on initial objective responses (20%) and leading to 2-year overall survival (OS) of 50%
      explanation: Quantifies the response and survival benefit of releasing this checkpoint node.
  evidence:
  - reference: PMID:37126021
    reference_title: Efficacy of Nivolumab in Pediatric Cancers with High Mutation Burden and Mismatch Repair Deficiency
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Nivolumab resulted in durable responses and prolonged survival for the first time in a pediatric trial of refractory hypermutated cancers including malignant gliomas.
    explanation: Primary prospective trial conclusion supporting PD-1 blockade in this population.
  - reference: PMID:37126021
    reference_title: Efficacy of Nivolumab in Pediatric Cancers with High Mutation Burden and Mismatch Repair Deficiency
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Four children, including three with refractory malignant gliomas are in complete remission at a median follow-up of 37 months
    explanation: Documents the durability of the responses recorded in the description.
  - reference: PMID:34992263
    reference_title: Genomic predictors of response to PD-1 inhibition in children with germline DNA replication repair deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Durable objective responses were observed in most patients, culminating in a 3 year survival of 41.4%.
    explanation: Independent international registry series supporting durable benefit from PD-1 inhibition.
  - reference: PMID:27001570
    reference_title: Immune Checkpoint Inhibition for Hypermutant Glioblastoma Multiforme Resulting From Germline Biallelic Mismatch Repair Deficiency.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: we treated two bMMRD siblings with recurrent multifocal GBM with the anti-programmed death-1 inhibitor nivolumab, which resulted in clinically significant responses and a profound radiologic response
    explanation: The index observation that established the checkpoint-blockade indication in CMMRD.
- name: CTLA-4 Blockade After Anti-PD-1 Failure
  description: >-
    Anti-CTLA-4 antibody added to checkpoint-inhibitor-based therapy continued beyond
    progression. Progression on anti-PD-1 monotherapy is not the end of the immune-directed
    option: CTLA-4 expression rises over time, and CTLA-4 inhibition produces response or
    stable disease in most such patients. Continuing checkpoint-inhibitor-based salvage
    after a second progression prolongs survival, most clearly in tumours with extreme
    mutation burden. Local and systemic immune adverse events are frequent, and are more
    frequent in biallelic mismatch repair deficiency than in Lynch syndrome.
  notes: >-
    Reirradiation and RAS-MAPK-pathway inhibition are curated as separate treatments; the
    source reports all three as arms of the same salvage strategy, but they are different
    modalities with different evidence.
  therapeutic_modality: MONOCLONAL_ANTIBODY
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: anti-CTLA-4 monoclonal antibody
      term:
        id: NCIT:C128036
        label: Anti-CTLA-4 Monoclonal Antibody
  target_mechanisms:
  - target: Adaptive Immune Resistance in Hypermutant Tumours
    treatment_effect: INHIBITS
    description: >-
      CTLA-4 blockade targets the checkpoint axis that becomes dominant as PD-1-directed
      resistance evolves, re-engaging the same neoantigen-driven T cell response.
    evidence:
    - reference: PMID:37823831
      reference_title: "Combined Immunotherapy Improves Outcome for Replication-Repair-Deficient (RRD) High-Grade Glioma Failing Anti-PD-1 Monotherapy: A Report from the International RRD Consortium."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: CTLA4 expression increased over time, and subsequent CTLA4 inhibition resulted in response/stable disease in 75%.
      explanation: Directly supports CTLA-4 blockade acting on the evolved checkpoint-resistance node.
  evidence:
  - reference: PMID:37823831
    reference_title: "Combined Immunotherapy Improves Outcome for Replication-Repair-Deficient (RRD) High-Grade Glioma Failing Anti-PD-1 Monotherapy: A Report from the International RRD Consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: After second progression/recurrence (n = 55), continuing ICI-based salvage prolonged survival to 11.6 months (n = 38; P < 0.001), particularly for those with extreme mutation burden (P = 0.03).
    explanation: Quantifies the survival benefit of continuing immune-directed therapy past anti-PD-1 failure.
  - reference: PMID:37823831
    reference_title: "Combined Immunotherapy Improves Outcome for Replication-Repair-Deficient (RRD) High-Grade Glioma Failing Anti-PD-1 Monotherapy: A Report from the International RRD Consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Local (flare) and systemic immune adverse events were frequent (biallelic mismatch-repair deficiency > Lynch syndrome)."
    explanation: Supports the toxicity statement and its CMMRD-versus-Lynch gradient.
- name: Reirradiation
  description: >-
    Repeat irradiation of a progressive replication-repair-deficient high-grade glioma,
    given as part of salvage therapy after failure of anti-PD-1 monotherapy. Response is
    reported to track with tumour genomics rather than radiation dose alone: tumours that
    respond lack the deleterious post-radiation indel signature (ID8) that marks
    radiation-refractory disease, which makes mutational-signature analysis a candidate
    selection tool rather than a post-hoc description.
  notes: >-
    No target_mechanisms link is asserted for this treatment. The cited evidence
    characterises which tumours respond to reirradiation, not the mechanism node
    reirradiation acts on, so there is nothing to attach an edge to without over-reading
    the source.
  therapeutic_modality: RADIOTHERAPY
  treatment_term:
    preferred_term: radiation therapy
    term:
      id: NCIT:C15313
      label: Radiation Therapy
  evidence:
  - reference: PMID:37823831
    reference_title: "Combined Immunotherapy Improves Outcome for Replication-Repair-Deficient (RRD) High-Grade Glioma Failing Anti-PD-1 Monotherapy: A Report from the International RRD Consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Response to reirradiation was explained by an absence of deleterious postradiation indel signatures (ID8).
    explanation: Supports reirradiation as an active salvage arm and identifies the genomic
      correlate of response.
- name: RAS-MAPK Pathway Inhibition as Immune-Directed Salvage
  description: >-
    Targeted inhibition of the RAS-MAPK pathway used alongside continued checkpoint
    inhibition after anti-PD-1 failure. Reported to reinvigorate both peripheral immune
    responses and radiologic responses, placing it with CTLA-4 blockade as an
    immune-directed rather than purely cytotoxic salvage arm. The evidence is a
    consortium case series, not a randomised comparison, and no specific agent is named in
    the source, so the agent is bound at drug-class level.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: mitogen-activated protein kinase inhibitor
      term:
        id: NCIT:C2149
        label: Mitogen-Activated Protein Kinase Inhibitor
  target_mechanisms:
  - target: Adaptive Immune Resistance in Hypermutant Tumours
    treatment_effect: INHIBITS
    description: >-
      RAS-MAPK-pathway inhibition restores the peripheral immune response that adaptive
      resistance had suppressed, acting on the same node as the checkpoint antibodies
      rather than on tumour proliferation alone.
    evidence:
    - reference: PMID:37823831
      reference_title: "Combined Immunotherapy Improves Outcome for Replication-Repair-Deficient (RRD) High-Grade Glioma Failing Anti-PD-1 Monotherapy: A Report from the International RRD Consortium."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: RAS-MAPK-pathway inhibition led to the reinvigoration of peripheral immune and radiologic responses.
      explanation: Supports RAS-MAPK inhibition relieving adaptive immune resistance rather
        than acting only as a cytostatic agent.
- name: Structured Cancer Surveillance Protocol
  description: >-
    The consensus international surveillance protocol - annual brain MRI, regular upper and
    lower gastrointestinal endoscopy, whole-body MRI and clinical examination - beginning at
    molecular diagnosis. This is the single most effective intervention in CMMRD:
    five-year survival is 90% when the cancer is found asymptomatically against 50% when it
    presents symptomatically, and adherence to full
    surveillance is associated with four-year survival of 79% against 15% for those not
    under surveillance. Its effectiveness is unevenly distributed: nearly all
    gastrointestinal and other solid tumours and three-quarters of brain cancers are caught
    asymptomatically, but only a small minority of haematological malignancies are. These
    are non-randomised comparisons and are vulnerable to lead-time and ascertainment bias.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: cancer screening
    term:
      id: NCIT:C15406
      label: Cancer Screening
  evidence:
  - reference: PMID:33945292
    reference_title: Survival Benefit for Individuals With Constitutional Mismatch Repair Deficiency Undergoing Surveillance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Patient outcome measured by adherence to the surveillance protocol revealed 4-year OS of 79% (95% CI, 54.8 to 90.9) for patients undergoing full surveillance, 55% (95% CI, 28.5 to 74.5) for partial surveillance, and 15% (95% CI, 5.2 to 28.8) for those not under surveillance (P < .0001).
    explanation: Quantifies the dose-response relationship between surveillance adherence and survival.
  - reference: PMID:33945292
    reference_title: Survival Benefit for Individuals With Constitutional Mismatch Repair Deficiency Undergoing Surveillance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: For patients undergoing surveillance, all GI and other solid tumors, and 75% of brain cancers were detected asymptomatically.
    explanation: Supports the uneven yield of the protocol across tumour types described here.
  - reference: PMID:24440087
    reference_title: "Genetic and clinical determinants of constitutional mismatch repair deficiency syndrome: report from the constitutional mismatch repair deficiency consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: The surveillance protocol detected 39 lesions which included asymptomatic malignant gliomas and gastrointestinal carcinomas.
    explanation: Early consortium evidence that the protocol detects lesions before they become symptomatic.
  - reference: PMID:33945292
    reference_title: Survival Benefit for Individuals With Constitutional Mismatch Repair Deficiency Undergoing Surveillance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Five-year overall survival (OS) was 90% (95% CI, 78.6 to 100) and 50% (95% CI, 39.2 to 63.7) when cancer was detected asymptomatically and symptomatically, respectively"
    explanation: Gives the asymptomatic-versus-symptomatic five-year survival figures quoted in this description.
  - reference: PMID:38789506
    reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: individuals with CMMRD are recommended annual gastrointestinal surveillance from as early as 6 years, annual brain magnetic resonance imaging (MRI) from initial diagnosis or at latest aged 2 years, and annual clinical examination, among other interventions
    explanation: >-
      Sets out the components and start ages of the recommended protocol, and contrasts them
      with the much later adult schedule offered to Lynch syndrome carriers.
- name: Endoscopic Surveillance and Polypectomy
  description: >-
    Regular upper and lower gastrointestinal endoscopy with resection of adenomas. Because
    low-grade gastrointestinal lesions in CMMRD transform at a very high rate within a few
    years, polypectomy is therapeutic rather than merely diagnostic.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: colonoscopy
    term:
      id: NCIT:C16450
      label: Colonoscopy
  evidence:
  - reference: PMID:33945292
    reference_title: Survival Benefit for Individuals With Constitutional Mismatch Repair Deficiency Undergoing Surveillance.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Of the 64 low-grade tumors detected, the cumulative likelihood of transformation from low-to high-grade was 81% for GI cancers within 8 years and 100% for gliomas in 6 years.
    explanation: Supports resection rather than observation of low-grade gastrointestinal lesions.
- name: Surgical Resection
  description: >-
    Maximal safe resection of CNS tumours and segmental or more extensive bowel surgery
    according to polyp and tumour burden. Surveillance-detected lesions are typically
    resectable in full, which is a large part of why surveillance changes survival.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: surgical procedure
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  evidence:
  - reference: PMID:24440087
    reference_title: "Genetic and clinical determinants of constitutional mismatch repair deficiency syndrome: report from the constitutional mismatch repair deficiency consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: All tumours were amenable to complete resection and all patients undergoing surveillance are alive.
    explanation: Supports complete surgical resection of surveillance-detected lesions.
- name: Genetic Counselling and Cascade Family Testing
  description: >-
    Both parents of a CMMRD proband are obligate heterozygous MMR variant carriers and
    require Lynch syndrome counselling and adult surveillance, as do many other relatives.
    Siblings have a one-in-four risk of CMMRD. Family history is often uninformative -
    Lynch-spectrum adult cancers were documented in only a minority of CMMRD families - so
    an unremarkable pedigree must not be used to argue against the diagnosis.
  treatment_term:
    preferred_term: genetic counseling
    term:
      id: NCIT:C15240
      label: Genetic Counseling
  evidence:
  - reference: PMID:24440087
    reference_title: "Genetic and clinical determinants of constitutional mismatch repair deficiency syndrome: report from the constitutional mismatch repair deficiency consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "CMMRD is a highly penetrant syndrome where family history of cancer may not be contributory."
    explanation: Supports the warning that an unremarkable family history does not exclude CMMRD.
  - reference: PMID:24440087
    reference_title: "Genetic and clinical determinants of constitutional mismatch repair deficiency syndrome: report from the constitutional mismatch repair deficiency consortium."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: While childhood CMMRD related tumours were observed in all families, Lynch related tumours in adults were observed in only 2/14 families
    explanation: Quantifies how often the parental Lynch phenotype is actually visible in the pedigree.
animal_models:
- name: MMR knockout mouse panel (Msh2, Msh6, Mlh1, Pms2)
  species: Mouse
  genotype: Homozygous germline knockout of Msh2, Msh6, Mlh1 or Pms2
  publication: PMID:26708047
  description: >-
    Germline knockout lines for each of the four genes that cause CMMRD in humans. Because
    the knockouts are homozygous null they match the biallelic constitutional state, unlike
    the heterozygous lines used to model Lynch syndrome, and they establish that loss of
    each gene alone is sufficient for a systemic mutator phenotype and cancer
    predisposition. The same panel supplies the only in vivo evidence for the
    class-switch-recombination arm of the syndrome.
  evidence:
  - reference: PMID:26708047
    reference_title: Mouse models of DNA mismatch repair in cancer research.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: the generation and analysis of mouse lines with knockout mutations in all of the known MMR genes has provided insight into how loss of individual MMR genes affects genome stability and contributes to cancer susceptibility
    explanation: Establishes the knockout panel as the standard in vivo system for MMR loss.
  modeled_mechanisms:
  - target: Constitutional Microsatellite Instability and Mutator Phenotype
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      Null mice for each of the four genes develop a systemic mutator phenotype and are
      cancer-predisposed, which is the constitutional consequence this node asserts.
    limitations: >-
      The review is framed around modelling Lynch syndrome and sporadic MMR-deficient
      colorectal cancer rather than CMMRD, and reports the mutator and cancer-predisposition
      phenotypes without establishing the CNS-predominant tumour distribution that defines
      the human syndrome. Treat the model as informative for the mutator phenotype itself,
      not for the tissue-specific transformation risk that the tumour-spectrum knowledge gap
      asks about.
    readouts:
    - name: Mutator phenotype and cancer predisposition in MMR-null mice
      target: Constitutional Microsatellite Instability and Mutator Phenotype
      description: >-
        Mutation-rate and tumour-incidence phenotypes reported across the Msh2, Msh6, Mlh1
        and Pms2 null lines.
      direction: INCREASED
      interpretation: >-
        Each of the four genes that cause human CMMRD produces the mutator phenotype on its
        own when knocked out, matching the genetic architecture of the syndrome.
      evidence:
      - reference: PMID:26708047
        reference_title: Mouse models of DNA mismatch repair in cancer research.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: mouse lines with knockout mutations in Msh2, Msh6, Mlh1 and Pms2 that form the MutSα (Msh2-Msh6) and MutLα (Mlh1-Pms2) complexes and play major roles in the repair of RERs in eukaryotic cells and the MMR-dependent DDR, display strong mutator and cancer predisposition phenotypes
        explanation: Reports the mutator and cancer-predisposition phenotypes for exactly
          the four CMMRD genes.
    evidence:
    - reference: PMID:26708047
      reference_title: Mouse models of DNA mismatch repair in cancer research.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: mouse models with mutations in all known MSH and MLH genes have been generated to study their roles in genome maintenance and tumor suppression
      explanation: Supports treating the knockout panel as informative for the genome
        instability node.
  - target: Impaired Immunoglobulin Class-Switch Recombination and Somatic Hypermutation
    relationship: RECAPITULATES
    fidelity: MODERATE
    description: >-
      The same knockout panel is where the requirement for MMR proteins in B cell maturation
      was established, which is the mechanistic basis for the immunoglobulin abnormalities
      seen in CMMRD patients.
    limitations: >-
      The abstract reports an essential function in B cell maturation without separating
      class-switch recombination from somatic hypermutation or quantifying the immunoglobulin
      deficit, and mouse B cell development is not a substitute for the human immunological
      phenotype, which in patients is a laboratory abnormality rather than overt clinical
      immunodeficiency.
    evidence:
    - reference: PMID:26708047
      reference_title: Mouse models of DNA mismatch repair in cancer research.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: These studies also revealed essential functions for some of the MMR genes in B cell maturation and fertility.
      explanation: In vivo support for the B cell maturation arm of the syndrome.
- name: Germline MMR-deficient de novo mouse glioma model (Msh2 or Msh6 loss)
  species: Mouse
  genotype: De novo high-grade glioma with germline or somatic Msh2 or Msh6 loss
  publication: PMID:41433099
  description: >-
    De novo mouse models of germline and somatic MMR-deficient high-grade glioma, built to
    test which parts of the human MMR-deficient glioma phenotype follow from MMR loss alone.
    The answer was largely negative, which is what makes this model informative: germline MMR
    deficiency accelerated progression from low-grade to high-grade glioma and shortened
    survival, but did so through the tumour immune microenvironment rather than by producing
    the hypermutation and checkpoint sensitivity that define human CMMRD gliomas.
  evidence:
  - reference: PMID:41433099
    reference_title: Mismatch repair deficiency drives malignant progression and alters the tumor immune microenvironment in glioblastoma models.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: Here, we developed de novo mouse models of germline and somatic MMR-deficient (MMRd) HGGs.
    explanation: Establishes the model system and that it covers the germline (CMMRD-like)
      as well as the somatic case.
  modeled_mechanisms:
  - target: Somatic Polymerase Proofreading Loss and Ultra-Hypermutation
    relationship: FAILS_TO_RECAPITULATE
    fidelity: LOW
    description: >-
      Loss of Msh2 or Msh6 in mouse glioma did not produce high tumour mutation burden or
      microsatellite instability, and did not confer response to PD-1 blockade. The
      hypermutation that makes human CMMRD gliomas checkpoint-responsive therefore does not
      follow from MMR loss alone in this system.
    limitations: >-
      The mouse models carry MMR loss without the somatic POLE or POLD1 exonuclease-domain
      lesion that this node asserts is the source of ultra-hypermutation in human CMMRD
      tumours, so the negative result is consistent with the two-step model curated here
      rather than a refutation of it - but it does mean the model cannot be used to study
      the hypermutant state or its checkpoint sensitivity. The same paper reports the
      corresponding discordance in human GBM, so this is not a purely murine artefact.
    readouts:
    - name: Tumour mutation burden and microsatellite instability in MMR-null mouse glioma
      target: Somatic Polymerase Proofreading Loss and Ultra-Hypermutation
      description: >-
        TMB and MSI measured in de novo mouse gliomas after Msh2 or Msh6 loss, with
        anti-PD-1 response as the functional readout.
      direction: UNCHANGED
      interpretation: >-
        A real negative: MMR loss alone leaves mutation burden and microsatellite status
        unchanged, so the ultra-hypermutant phenotype requires the additional somatic
        proofreading lesion.
      evidence:
      - reference: PMID:41433099
        reference_title: Mismatch repair deficiency drives malignant progression and alters the tumor immune microenvironment in glioblastoma models.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: Surprisingly, loss of Msh2 or Msh6 did not lead to high TMB, MSI, nor did it confer a response to anti-programmed cell death 1 (anti-PD-1) in GBM.
        explanation: The negative result itself.
    evidence:
    - reference: PMID:41433099
      reference_title: Mismatch repair deficiency drives malignant progression and alters the tumor immune microenvironment in glioblastoma models.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: human GBM showed discordance between MMR gene mutations and the TMB and MSI
      explanation: Shows the same discordance in human tumours, so the failure is not
        attributable to species alone.
  - target: Accelerated Multi-Organ Tumorigenesis
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Germline MMR deficiency accelerated progression from low-grade to high-grade glioma and
      shortened survival, reproducing the accelerated-tumorigenesis consequence in the CNS
      compartment that dominates the human tumour spectrum.
    limitations: >-
      Only the CNS arm is modelled, so the multi-organ distribution the node asserts is not
      reproduced, and the effect was driven by the tumour immune microenvironment rather than
      being tumour-cell intrinsic. That is a mechanistically different route to accelerated
      progression from the cell-intrinsic mutation accumulation curated upstream of this node.
    readouts:
    - name: Progression from low-grade to high-grade glioma and survival
      target: Accelerated Multi-Organ Tumorigenesis
      description: >-
        Grade progression and overall survival in germline MMR-deficient versus
        MMR-proficient tumour-bearing mice.
      direction: INCREASED
      interpretation: >-
        Faster grade progression with shorter survival is the model's counterpart of
        accelerated tumorigenesis, restricted to the CNS.
      evidence:
      - reference: PMID:41433099
        reference_title: Mismatch repair deficiency drives malignant progression and alters the tumor immune microenvironment in glioblastoma models.
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: Germline MMRd promoted the progression from low-grade to HGG and reduced survival compared with MMR-proficient (MMRp) tumor-bearing mice.
        explanation: Reports the acceleration and the survival cost.
    evidence:
    - reference: PMID:41433099
      reference_title: Mismatch repair deficiency drives malignant progression and alters the tumor immune microenvironment in glioblastoma models.
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: This effect was not tumor cell intrinsic but was associated with MMRd in the tumor immune microenvironment, driving immunosuppressive myeloid programs, reduced lymphoid infiltration, and CD8+ T cell exhaustion.
      explanation: Identifies the route by which the model accelerates tumorigenesis, and so
        the respect in which it only partially matches the human node.
experimental_models:
- name: CMMRD patient-derived lymphoblastoid cell lines (MSI and methylation-tolerance assay)
  experimental_model_type: CELL_LINE
  cell_source: Patient-derived (EBV-immortalized peripheral blood lymphocytes)
  organism:
    preferred_term: human
    term:
      id: NCBITaxon:9606
      label: Homo sapiens
  description: >-
    Lymphoblastoid cells from CMMRD patients and MMR-proficient controls, assayed for
    microsatellite instability by PCR and for tolerance to methylating and thiopurine agents.
    This is the human ex vivo system in which the two defining constitutional consequences of
    biallelic MMR loss - instability in normal (non-tumour) cells, and loss of the
    MMR-dependent damage response - are measured directly, and it is also the basis of a
    diagnostic assay for the 30% of patients whose gene screening is uninformative.
  publication: PMID:26116798
  evidence:
  - reference: PMID:26116798
    reference_title: Diagnosis of Constitutional Mismatch Repair-Deficiency Syndrome Based on Microsatellite Instability and Lymphocyte Tolerance to Methylating Agents.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: We examined MSI by PCR analysis and tolerance to methylating or thiopurine agents (functional characteristics of MMR-deficient tumor cells) in lymphoblastoid cells (LCs) from 3 patients with CMMRD and 5 individuals with MMR-proficient LCs (controls).
    explanation: Describes the model system and its two readouts against MMR-proficient
      controls.
  modeled_mechanisms:
  - target: Constitutional Microsatellite Instability and Mutator Phenotype
    relationship: MEASURES
    fidelity: HIGH
    description: >-
      Microsatellite instability is measured in normal patient-derived cells rather than in
      tumour, which is exactly the constitutional claim this node makes and the feature that
      separates CMMRD from Lynch syndrome.
    limitations: >-
      EBV-immortalized lymphoblastoid cells are a proliferating lymphoid compartment and
      cannot report the tissue gradient in constitutional instability that the tumour-spectrum
      knowledge gap concerns; instability measured in this compartment says nothing about
      brain or intestine.
    evidence:
    - reference: PMID:26116798
      reference_title: Diagnosis of Constitutional Mismatch Repair-Deficiency Syndrome Based on Microsatellite Instability and Lymphocyte Tolerance to Methylating Agents.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: Using these assays, we defined experimental parameters that allowed discrimination of a series of 14 patients with CMMRD from 52 controls (training set).
      explanation: Supports treating this cell system as informative for the constitutional
        instability node, at a cohort size that establishes the parameters rather than
        illustrating them.
    readouts:
    - name: Microsatellite instability in lymphoblastoid cells
      target: Constitutional Microsatellite Instability and Mutator Phenotype
      description: PCR-based MSI analysis of patient versus control lymphoblastoid lines.
      direction: INCREASED
      interpretation: >-
        Instability present in non-neoplastic patient cells is the direct measurement of the
        constitutional mutator phenotype.
      evidence:
      - reference: PMID:26116798
        reference_title: Diagnosis of Constitutional Mismatch Repair-Deficiency Syndrome Based on Microsatellite Instability and Lymphocyte Tolerance to Methylating Agents.
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: In the training set, we identified parameters, based on MSI and LC tolerance to methylation, that detected patients with CMMRD vs controls with 100% sensitivity and 100% specificity.
        explanation: The measurement separates patients from controls without error in the
          training set.
  - target: Loss of MMR-Dependent Damage Signalling and Alkylating Agent Tolerance
    relationship: MEASURES
    fidelity: HIGH
    description: >-
      Tolerance of patient lymphoblastoid cells to methylating and thiopurine agents is the
      functional assay for the loss of MMR-dependent damage signalling asserted by this node.
    limitations: >-
      The assay demonstrates tolerance in a lymphoid cell line; it does not establish the
      clinical consequence for alkylating-agent chemotherapy in CMMRD tumours, which is
      inferred rather than measured here.
    evidence:
    - reference: PMID:26116798
      reference_title: Diagnosis of Constitutional Mismatch Repair-Deficiency Syndrome Based on Microsatellite Instability and Lymphocyte Tolerance to Methylating Agents.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: Among 23 patients suspected of having CMMRD, 6 had MSI and LC tolerance to methylation (CMMRD highly probable), 15 had neither MSI nor LC tolerance to methylation (unlikely to have CMMRD), and 2 were considered doubtful for CMMRD based on having only 1 of the 2 features.
      explanation: Methylation tolerance and instability can dissociate in individual
        patients, which is why this link is curated against the damage-signalling node
        separately from the instability node rather than folded into it.
    readouts:
    - name: Lymphoblastoid cell tolerance to methylating agents
      target: Loss of MMR-Dependent Damage Signalling and Alkylating Agent Tolerance
      description: >-
        Survival of patient versus control lymphoblastoid cells exposed to methylating or
        thiopurine agents.
      direction: INCREASED
      interpretation: >-
        Failure to die after methylation damage is the functional signature of an absent
        MMR-dependent damage response.
      evidence:
      - reference: PMID:26116798
        reference_title: Diagnosis of Constitutional Mismatch Repair-Deficiency Syndrome Based on Microsatellite Instability and Lymphocyte Tolerance to Methylating Agents.
        supports: SUPPORT
        evidence_source: IN_VITRO
        snippet: The presence of MSI and tolerance to methylation in LCs identified patients with CMMRD with 100% sensitivity and specificity.
        explanation: Reports methylation tolerance as a measured, discriminating property of
          patient cells.
clinical_trials:
- name: NCT02992964
  phase: PHASE_I
  status: TERMINATED
  description: >-
    Pilot study of nivolumab in paediatric patients with recurrent or refractory hypermutant
    cancers, the first prospective paediatric trial restricted to tumours with mutational
    burden at or above 5 mutations/Mb and/or mismatch repair deficiency. Registered on
    ClinicalTrials.gov as phase 1/2 and terminated after limited enrolment, but it generated
    the published efficacy cohort that established checkpoint blockade in CMMRD. Recorded
    here as PHASE_I because the dismech phase enum has no combined phase 1/2 value.
  target_phenotypes:
  - preferred_term: Glioblastoma multiforme
    term:
      id: HP:0012174
      label: Glioblastoma multiforme
  - preferred_term: Brain neoplasm
    term:
      id: HP:0030692
      label: Brain neoplasm
  evidence:
  - reference: clinicaltrials:NCT02992964
    reference_title: Pilot Study of Nivolumab in Pediatric Patients With Hypermutant Cancers
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: This study is to assess clinical and radiological benefits of treatment with Nivolumab in children with hypermutated cancers, including those with bMMRD syndrome.
    explanation: The trial record states the CMMRD (bMMRD) population and the checkpoint-blockade intervention.
  - reference: PMID:37126021
    reference_title: Efficacy of Nivolumab in Pediatric Cancers with High Mutation Burden and Mismatch Repair Deficiency
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We report the first prospective pediatric trial (NCT02992964) using nivolumab exclusively for refractory nonhematologic cancers harboring tumor mutation burden (TMB) ≥5 mutations/megabase (mut/Mb) and/or mismatch repair deficiency (MMRD)."
    explanation: Publication of the trial, giving its eligibility criteria and identifier.
- name: NCT05770102
  phase: PHASE_II
  status: RECRUITING
  description: >-
    DETERMINE treatment arm 02: atezolizumab (anti-PD-L1) in adult, paediatric and
    teenage/young adult patients whose cancers are TMB-high, MSI-high, or who have proven
    CMMRD. A UK national umbrella-basket platform trial testing whether an approved
    checkpoint inhibitor extends to rare molecularly defined indications; it is the
    currently open trial that admits CMMRD patients by predisposition rather than by tumour
    type. Registered as phase 2/3; recorded here as PHASE_II because the dismech phase enum
    has no combined value.
  target_phenotypes:
  - preferred_term: Neoplasm
    term:
      id: HP:0002664
      label: Neoplasm
  evidence:
  - reference: clinicaltrials:NCT05770102
    reference_title: "DETERMINE (Determining Extended Therapeutic Indications for Existing Drugs in Rare Molecularly Defined Indications Using a National Evaluation Platform Trial): An Umbrella-Basket Platform Trial to Evaluate the Efficacy of Targeted Therapies in Rare Adult, Paediatric and Teenage/Young Adult (TYA) Cancers With Actionable Genomic Alterations, Including Common Cancers With Rare Actionable Alterations. Treatment Arm 02: Atezolizumab in Adult, Paediatric and Teenage/Young Adult Patients With Cancers With High TMB or MSI-high or Proven CMMRD Disposition."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Atezolizumab works in patients with these types of cancers which have certain changes in the cancer cells called high tumour mutational burden (TMB) or high microsatellite instability (MSI) or proven (previously diagnosed) constitutional mismatch repair deficiency (CMMRD).
    explanation: The trial record states that proven CMMRD is an eligibility route into this
      checkpoint-inhibitor arm.
discussions:
- discussion_id: cmmrd_normal_tissue_mmr_kinetics
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    Why do CNS, gastrointestinal and lymphoid compartments transform preferentially in
    CMMRD when mismatch repair is absent from every tissue?
  attaches_to:
  - "pathophysiology#Constitutional Microsatellite Instability and Mutator Phenotype"
  - "pathophysiology#Accelerated Multi-Organ Tumorigenesis"
  rationale: >-
    Constitutional instability is systemic, yet the tumour spectrum is not. Constitutional
    instability scores differ by tissue (gastrointestinal > blood > brain) and rise with age
    within an individual, so the tissue-specific transformation risk is not simply a
    function of measured instability. The determinants - proliferative index of the stem
    compartment, tissue-specific dependence on MMR versus backup pathways, immune
    surveillance, or the tissue-specific probability of acquiring the somatic POLE/POLD1
    hit - have not been disentangled. Getting this right would sharpen surveillance
    intervals, which are currently uniform rather than risk-weighted by tissue.
  proposed_experiments:
  - experiment_id: exp_cmmrd_tissue_resolved_mutation_burden
    name: Clone-resolved mutation burden across matched normal tissues in CMMRD
    description: >-
      Measure somatic mutation burden and microsatellite indel load at single-cell or
      clone-resolved depth in matched normal brain, intestinal crypt and haematopoietic
      compartments from the same CMMRD individual, and test whether the compartment with
      the highest measured instability is the compartment that transforms first.
    evidence:
    - reference: PMID:36240479
      reference_title: Genomic Microsatellite Signatures Identify Germline Mismatch Repair Deficiency and Risk of Cancer Onset.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: In normal cells, MMRDness scores differed between tissues (GI > blood > brain)
      explanation: >-
        Design rationale - a tissue gradient in constitutional instability is already
        measurable, so a clone-resolved comparison across the same tissues is feasible.
  - experiment_id: exp_cmmrd_longitudinal_instability_vs_first_cancer
    name: Longitudinal constitutional-instability scoring against organ of first cancer
    description: >-
      Score constitutional genomic instability serially in blood and, where available,
      gastrointestinal tissue from a prospective CMMRD surveillance cohort, and test
      whether the instability trajectory predicts the organ and timing of the first
      cancer well enough to justify tissue-weighted rather than uniform surveillance
      intervals.
    evidence:
    - reference: PMID:36240479
      reference_title: Genomic Microsatellite Signatures Identify Germline Mismatch Repair Deficiency and Risk of Cancer Onset.
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: Importantly, increased MMRDness score was associated with younger age of first cancer presentation in individuals with CMMRD
      explanation: >-
        Design rationale - instability score already predicts age at first cancer, so the
        proposed extension to organ of first cancer is a tractable next step.
  evidence:
  - reference: PMID:36240479
    reference_title: Genomic Microsatellite Signatures Identify Germline Mismatch Repair Deficiency and Risk of Cancer Onset.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: In normal cells, MMRDness scores differed between tissues (GI > blood > brain), increased over time in the same individual, and revealed genotype-phenotype associations within the mismatch repair genes.
    explanation: >-
      Documents the tissue-dependent and age-dependent instability that this gap asks to be
      reconciled with the observed tumour spectrum.
- discussion_id: cmmrd_hypomorphic_penetrance_boundary
  kind: KNOWLEDGE_GAP
  status: OPEN
  prompt: >-
    How much residual mismatch repair activity is compatible with an adult-onset,
    Lynch-like phenotype, and where does the CMMRD/Lynch management boundary fall for
    biallelic carriers of hypomorphic variants?
  attaches_to:
  - "pathophysiology#Biallelic Germline Mismatch Repair Gene Inactivation"
  rationale: >-
    Late-onset CMMRD is currently recognised case by case, after constitutional MSI testing
    contradicts an assumed Lynch diagnosis. There is no calibrated relationship between
    residual repair activity and age at first cancer, so a biallelic carrier of two leaky
    variants cannot be assigned to a surveillance schedule on genotype. The two schedules
    differ substantially (annual brain MRI and gastrointestinal endoscopy from early
    childhood in CMMRD; adult gastrointestinal and gynaecological surveillance in Lynch), so
    the cost of guessing is high in both directions.
  proposed_experiments:
  - experiment_id: exp_cmmrd_hypomorphic_residual_activity_calibration
    name: Calibration of residual mismatch repair activity against age at first cancer
    description: >-
      Assay residual mismatch repair activity quantitatively across a series of candidate
      hypomorphic MMR variants (leaky splice and missense alleles) and correlate the
      measured activity with constitutional instability score and age at first cancer, to
      establish whether residual activity predicts the CMMRD-versus-Lynch management
      boundary.
    evidence:
    - reference: PMID:38789506
      reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: MLH1 c.306G>A was shown to cause leaky exon 3 skipping.
      explanation: >-
        Design rationale - a leaky, partially functional allele has already been
        demonstrated functionally, so a graded activity series is experimentally reachable.
  - experiment_id: exp_cmmrd_adult_vus_constitutional_msi_screen
    name: Constitutional MSI screening of adults with biallelic MMR variants of uncertain significance
    description: >-
      Systematically apply constitutional microsatellite instability testing to adult
      patients carrying biallelic or compound-heterozygous MMR variants of uncertain
      significance who are currently managed as Lynch syndrome, to estimate how often
      late-onset CMMRD is being missed.
    evidence:
    - reference: PMID:38789506
      reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: CMMRD testing in carriers of compound heterozygous or homozygous MMR VUS may find similar cases and novel hypomorphic variants.
      explanation: >-
        The authors propose exactly this screen; the citation records the precedent and the
        expected yield.
  evidence:
  - reference: PMID:38789506
    reference_title: Constitutional mismatch repair deficiency mimicking Lynch syndrome is associated with hypomorphic mismatch repair gene variants
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: Individualised management of mono- and bi-allelic carriers of hypomorphic MMR variants is needed until we better characterise the associated phenotypes.
    explanation: The authors state the gap directly - the phenotypes are not yet characterised well enough to set a management rule.
notes: >-
  Concept decision: curated as a single Disease entry with `has_subtypes` over the four
  gene-defined forms (CMMRD1-4 / MONDO:0010159, MONDO:0030840, MONDO:0030841,
  MONDO:0030843), matching the CURATE_ROOT_WITH_SUBTYPES designation on the priority
  dashboard. The four share one mechanism - biallelic loss of a single MMR heterodimer
  component producing constitutional microsatellite instability - and differ in severity and
  age of onset rather than in kind, which is the same lump/split logic applied in
  `Lynch_Syndrome.yaml` and `3-M_Syndrome.yaml`. MONDO:0031219 itself records no causal
  gene (`RO:0004003`), which is why the automated NEC preflight returns SKIP for this
  entry; the manual gene/OMIM/synonym check was run instead and passed (see review_notes).

  Scope boundary against Lynch syndrome is deliberate and load-bearing. The same four genes
  cause both diseases; what differs is allelic state (biallelic versus monoallelic) and
  therefore whether mismatch repair is absent constitutionally or only after a somatic
  second hit. Content is cross-referenced rather than duplicated: the shared MSI
  carcinogenesis chain is curated in `Lynch_Syndrome.yaml`, and this entry curates what is
  specific to the constitutional state - normal-tissue MMR loss, constitutional MSI as a
  diagnostic assay, somatic POLE/POLD1 ultra-hypermutation, the paediatric CNS-predominant
  spectrum, the NF1 phenotypic mimicry, and the checkpoint-blockade indication.

  Somatic POLE/POLD1 proofreading loss is curated as a tumour-restricted modifier
  (`genetic[].relationship_type: MODIFIER`, `variant_origin: SOMATIC`) and as its own
  pathophysiology node. It is not a cause of the syndrome and must not be read as one.

  No GeneReviews chapter exists for this disease. NCBI Bookshelf was searched for
  "constitutional mismatch repair deficiency", "mismatch repair cancer syndrome" and
  "CMMRD" under the GeneReviews filter and returned nothing; the nearest chapter is Lynch
  Syndrome (PMID:20301390), whose summary makes no CMMRD statement, so there is no
  GeneReviews baseline to tag. The consensus baseline used instead is the pair of
  diagnostic-criteria papers already cited - the European C4CMMRD consortium
  (PMID:24737826) and the international consensus working group (PMID:33622763). Recorded
  here so the search is not repeated.

  Deliberately not curated: the B-cell class-switch/somatic-hypermutation abnormality is
  recorded as a mechanism node but explicitly NOT as an immunodeficiency phenotype, because
  the systematic cohort found no clinical warning signs of primary immunodeficiency and no
  consistent laboratory pattern (PMID:30013564, carried as a REFUTE evidence item on that
  node). WRN-helicase synthetic lethality and ATR inhibition are preclinical only and are
  not curated as treatments.
review_notes: >-
  Deep-research provider: Falcon (Edison Scientific), report at
  `research/Constitutional_Mismatch_Repair_Deficiency-deep-research-falcon.md`. The report
  was used as a lead list only; every PMID was fetched with `just fetch-reference` and every
  snippet verified against the cached record.

  NEC preflight (CLAUDE.md section 2b): `just preflight-dr` returned SKIP because
  MONDO:0031219 records no `RO:0004003` causal gene. The manual fallback was run and passed
  on all three anchors. Gene check - the report's gene mentions are PMS2=14, MSH6=12,
  MLH1=11, MSH2=10, i.e. the four MMR genes in balance, with no rival gene, which is the
  expected profile for a four-gene root term rather than an NEC signal. OMIM check - the
  report cites OMIM 276300/619096/619097/619101, matching the MONDO OMIMPS:276300
  phenotypic series and the four children's OMIM xrefs. Synonym check - the MONDO exact
  synonym "constitutional mismatch repair deficiency syndrome" is the name the report was
  written against, and it is not a label or synonym of any other MONDO entity. Named Entity
  Confusion against Lynch syndrome (MONDO:0005835) was the specific risk here and was
  checked explicitly: the report's clinical content is childhood-onset and biallelic
  throughout.

  Two DR-report claims were dropped for want of a verifiable quotable source in the fetched
  abstracts: the "one in a million" figure was re-sourced to PMID:42348008 and PMID:38789506
  rather than to the report's own citation string, and the assertion that necessary
  radiotherapy is "not categorically contraindicated" was omitted entirely rather than
  curated on a review's unsourced statement.

📚

References & Deep Research

Deep Research

1

Falcon ▸

Disease Characteristics Research Template

Edison Scientific Literature 23 citations 2026-08-27T11:58:09.817219

Question: You are an expert researcher providing comprehensive, well-cited information.

Provide detailed information focusing on: 1. Key concepts and definitions with current understanding 2. Recent developments and latest research (prioritize 2023-2024 sources) 3. Current applications and real-world implementations 4. Expert opinions and analysis from authoritative sources 5. Relevant statistics and data from recent studies

Format as a comprehensive research report with proper citations. Include URLs and publication dates where available. Always prioritize recent, authoritative sources and provide specific citations for all major claims.

Disease Characteristics Research Template

Target Disease

Disease Name: Constitutional Mismatch Repair Deficiency
MONDO ID: (if available)
Category:

Research Objectives

Please provide a comprehensive research report on Constitutional Mismatch Repair Deficiency covering all of the disease characteristics listed below. This report will be used to populate a disease knowledge base entry. Be thorough and cite primary literature (PMID preferred) for all claims.

For each section, suggested databases/resources are listed. These are the first places you should search for information on each topic.

1. Disease Information

Search first: OMIM, Orphanet, ICD-10/ICD-11, MeSH, PubMed

What is the disease? Provide a concise overview.
What are the key identifiers? (OMIM, Orphanet, ICD-10/ICD-11, MeSH, Mondo)
What are the common synonyms and alternative names?
Is the information derived from individual patients (e.g., EHR) or aggregated disease-level resources?

2. Etiology

Disease Causal Factors: What are the primary causes? (genetic, environmental, infectious, mechanistic)
Risk Factors:

Search first: PubMed, Cochrane Library, UpToDate, clinical guidelines, ClinVar, ClinGen, GWAS Catalog, PheGenI, CTD, CDC, WHO, epidemiological databases
Genetic risk factors (causal variants, susceptibility loci, modifier genes)
Environmental risk factors (toxins, lifestyle, occupational exposures, age, sex, family history)
Protective Factors:

Search first: PubMed, Cochrane Library, clinical trial databases, GWAS Catalog, gnomAD, WHO, CDC, nutrition databases
Genetic protective factors (protective variants, modifier alleles)
Environmental protective factors (diet, lifestyle, exposures that reduce risk)
Gene-Environment Interactions: How do genetic and environmental factors interact to influence disease?

Search first: CTD, PubMed, PheGenI, GxE databases

3. Phenotypes

Search first: HPO (Human Phenotype Ontology), OMIM, Orphanet, PubMed, clinicaltrials.gov, MedDRA, SNOMED CT, DECIPHER, LOINC

For each phenotype, provide: - Phenotype type: symptoms, clinical signs, physical manifestations, behavioral changes, or laboratory abnormalities

For symptoms/signs: HPO, OMIM, Orphanet, PubMed For behavioral changes: HPO, DSM, RDoC (Research Domain Criteria), PubMed For laboratory abnormalities: LOINC, SNOMED CT, LabTests Online, PubMed - Phenotype characteristics: Search first: OMIM, Orphanet, HPO, PubMed - Age of symptom onset (neonatal, childhood, adult-onset, late-onset) - Symptom severity (mild, moderate, severe, variable) - Symptom progression (stable, progressive, episodic, fluctuating) - Frequency among affected individuals (percentage or qualitative) - Quality of life impact: Effects on daily functioning and well-being (per-phenotype when possible) Search first: EQ-5D database, SF-36, WHO QOL databases, PubMed - Suggest HPO (Human Phenotype Ontology) terms for each phenotype

4. Genetic/Molecular Information

Causal Genes: Gene mutations or chromosomal abnormalities responsible for disease (gene symbols, OMIM IDs)

Search first: OMIM, ClinVar, HGMD, Ensembl, NCBI Gene
Pathogenic Variants:
Affected genes (gene symbols, HGNC IDs) > Search first: OMIM, NCBI Gene, Ensembl, HGNC, UniProt, GeneCards
Variant classification (pathogenic, likely pathogenic, VUS per ACMG/AMP guidelines) > Search first: ClinVar, ClinGen, ACMG/AMP guidelines, VarSome
Variant type/class (missense, frameshift, nonsense, splice-site, structural)
Allele frequency in population databases > Search first: gnomAD, 1000 Genomes, ExAC, TOPMed, dbSNP
Somatic vs germline origin > Search first: COSMIC (somatic), ClinVar, ICGC, TCGA
Functional consequences (loss of function, gain of function, dominant negative)
Modifier Genes: Genes that modify disease severity or expression
Epigenetic Information: DNA methylation, histone modifications, chromatin changes affecting disease

Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth
Chromosomal Abnormalities: Large-scale genetic changes (aneuploidy, translocations, inversions)

Search first: DECIPHER, ClinVar, ECARUCA, UCSC Genome Browser

5. Environmental Information

Environmental Factors: Non-genetic contributing factors (toxins, radiation, pollution, occupational exposure)

Search first: CTD (Comparative Toxicogenomics Database), TOXNET, PubMed, EPA databases
Lifestyle Factors: Behavioral factors (smoking, diet, exercise, alcohol consumption)

Search first: CDC databases, WHO, PubMed, NHANES
Infectious Agents: If applicable, pathogens causing or triggering disease (bacteria, viruses, fungi, parasites)

Search first: NCBI Taxonomy, ViPR, BV-BRC, MicrobeDB, GIDEON

6. Mechanism / Pathophysiology

Molecular Pathways: Specific signaling cascades or biochemical pathways involved (Wnt, MAPK, mTOR, PI3K-AKT, etc.)

Search first: KEGG, Reactome, WikiPathways, PathBank, BioCyc
Cellular Processes: Cell-level mechanisms (apoptosis, autophagy, cell cycle dysregulation, inflammation, etc.)

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Protein Dysfunction: How protein structure or function is altered (misfolding, aggregation, loss of function, gain of function)

Search first: UniProt, PDB (Protein Data Bank), InterPro, Pfam, AlphaFold
Metabolic Changes: Alterations in metabolic processes (energy metabolism, lipid metabolism, amino acid metabolism)

Search first: KEGG, BioCyc, HMDB (Human Metabolome Database), BRENDA
Immune System Involvement: Role of immune response (autoimmunity, immunodeficiency, chronic inflammation)

Search first: ImmPort, Immunome Database, IEDB, Gene Ontology
Tissue Damage Mechanisms: How tissues/ are injured (oxidative stress, ischemia, fibrosis, necrosis)

Search first: PubMed, Gene Ontology, Reactome
Biochemical Abnormalities: Specific molecular defects (enzyme deficiencies, receptor dysfunction, ion channel defects)

Search first: BRENDA, UniProt, KEGG, OMIM, PubMed
Epigenetic Changes: DNA methylation, histone modifications affecting gene expression in disease

Search first: ENCODE, Roadmap Epigenomics, MethBase, DiseaseMeth
Molecular Profiling (if available):
Transcriptomics/gene expression changes > Search first: GEO (Gene Expression Omnibus), ArrayExpress, GTEx, Human Cell Atlas, SRA
Proteomics findings > Search first: PRIDE, ProteomeXchange, Human Protein Atlas, STRING, BioGRID
Metabolomics signatures > Search first: MetaboLights, Metabolomics Workbench, HMDB, METLIN
Lipidomics alterations > Search first: LIPID MAPS, SwissLipids, LipidHome, Metabolomics Workbench
Genomic structural features > Search first: UCSC Genome Browser, Ensembl, NCBI, dbVar, DGV
Advanced Technologies (if applicable):
Single-cell analysis findings (cell-type specific mechanisms, cellular heterogeneity) > Search first: Human Cell Atlas, Single Cell Portal, GEO, CELLxGENE
Spatial transcriptomics findings > Search first: GEO, Spatial Research, Vizgen, 10x Genomics data
Multi-omics integration results > Search first: TCGA, ICGC, cBioPortal, LinkedOmics, PubMed
Functional genomics screens (CRISPR, RNAi) > Search first: DepMap, GenomeRNAi, PubMed, BioGRID ORCS

For each mechanism, describe: - The causal chain from initial trigger to clinical manifestation - Which mechanisms are upstream vs downstream - What cell types and biological processes are involved - Suggest GO terms for biological processes and CL terms for cell types

7. Anatomical Structures Affected

Organ Level:
Primary organs directly affected
Secondary organ involvement (complications, secondary effects)
Body systems involved (cardiovascular, nervous, digestive, respiratory, endocrine, etc.)

Search first: Uberon, FMA (Foundational Model of Anatomy), OMIM, HPO, ICD-11, MeSH, SNOMED CT
Tissue and Cell Level:
Specific tissue types affected (epithelial, connective, muscle, nervous)
Specific cell populations targeted (with Cell Ontology terms)

Search first: Uberon, Human Protein Atlas, Cell Ontology, Human Cell Atlas, CellMarker, PanglaoDB
Subcellular Level:
Cellular compartments involved (mitochondria, nucleus, ER, lysosomes) (with GO Cellular Component terms)

Search first: Gene Ontology (Cellular Component), UniProt, Human Protein Atlas
Localization:
Specific anatomical sites (with UBERON terms) > Search first: FMA, Uberon, NeuroNames (for brain), SNOMED CT
Lateralization (unilateral, bilateral, asymmetric) > Search first: HPO, clinical literature, imaging databases

8. Temporal Development

Onset:
Typical age of onset (congenital, pediatric, adult, geriatric)
Onset pattern (acute, subacute, chronic, insidious)

Search first: OMIM, Orphanet, HPO, PubMed
Progression:
Disease stages (early, intermediate, advanced, end-stage) > Search first: Cancer Staging Manual (AJCC), WHO classifications, PubMed
Progression rate (rapid, slow, variable)
Disease course pattern (episodic, relapsing-remitting, progressive, stable)
Disease duration (self-limited, chronic lifelong)

Search first: Disease registries, longitudinal cohort databases, natural history studies, PubMed, Orphanet, OMIM
Patterns:
Remission patterns (spontaneous, treatment-induced) > Search first: Clinical trial databases, disease registries, PubMed
Critical periods (time windows of vulnerability or opportunity for intervention) > Search first: PubMed, developmental biology databases, clinical guidelines

9. Inheritance and Population

Epidemiology:
Prevalence (cases per 100,000 at given time)
Incidence (new cases per 100,000 per year)

Search first: Orphanet, CDC, WHO, GBD (Global Burden of Disease), national registries, SEER, disease registries
For Genetic Etiology:
Inheritance pattern (AD, AR, X-linked, mitochondrial, multifactorial, polygenic) > Search first: OMIM, Orphanet, ClinVar, GTR (Genetic Testing Registry)
Penetrance (complete, incomplete, age-dependent) > Search first: ClinVar, OMIM, PubMed, ClinGen
Expressivity (variable, consistent) > Search first: OMIM, ClinVar, PubMed
Genetic anticipation (increasing severity in successive generations) > Search first: OMIM, PubMed (especially for repeat expansion disorders)
Germline mosaicism > Search first: ClinVar, OMIM, genetic counseling literature, PubMed
Founder effects (population-specific mutations) > Search first: gnomAD, population genetics databases, PubMed
Consanguinity role > Search first: OMIM, population studies, genetic counseling resources
Carrier frequency > Search first: gnomAD, carrier screening databases, GeneReviews, GTR
Population Demographics:
Affected populations (ethnic or demographic groups with higher prevalence) > Search first: gnomAD, 1000 Genomes, PAGE Study, PubMed, population registries
Geographic distribution (endemic areas, regional variation) > Search first: WHO, CDC, GBD, Orphanet, geographic epidemiology databases
Geographic distribution of specific variants
Sex ratio (male:female) > Search first: Disease registries, OMIM, PubMed, epidemiological databases
Age distribution of affected individuals > Search first: CDC, disease registries, SEER, Orphanet

10. Diagnostics

Clinical Tests:
Laboratory tests (blood, urine, tissue chemistry, specific enzyme assays) > Search first: LOINC, LabTests Online, PubMed
Biomarkers (proteins, metabolites, genetic markers, circulating biomarkers) > Search first: FDA Biomarker List, BEST (Biomarkers, EndpointS, and other Tools), PubMed
Imaging studies (X-ray, CT, MRI, PET, ultrasound) > Search first: RadLex, DICOM, Radiopaedia, imaging databases
Functional tests (pulmonary function, cardiac stress tests) > Search first: LOINC, clinical guidelines, PubMed
Electrophysiology (EEG, EMG, ECG, nerve conduction studies) > Search first: LOINC, clinical neurophysiology databases, PubMed
Biopsy findings (histopathology, immunohistochemistry) > Search first: SNOMED CT, College of American Pathologists resources, PubMed
Pathology findings (microscopic examination) > Search first: SNOMED CT, Digital Pathology databases, PubMed
Genetic Testing:

Search first: GTR (Genetic Testing Registry), GeneReviews, ClinGen
Overview of recommended genetic testing approach
Whole genome sequencing (WGS) utility > Search first: GTR, ClinVar, GEL (Genomics England), gnomAD
Whole exome sequencing (WES) utility > Search first: GTR, ClinVar, OMIM, GeneMatcher
Gene panels (which panels, which genes) > Search first: GTR, ClinVar, laboratory-specific databases
Single gene testing > Search first: GTR, ClinVar, OMIM, GeneReviews
Chromosomal microarray (CMA) > Search first: DECIPHER, ClinVar, dbVar, ECARUCA
Karyotyping > Search first: Chromosome Abnormality Database, ClinVar, cytogenetics resources
FISH > Search first: ClinVar, cytogenetics databases, PubMed
Mitochondrial DNA testing > Search first: MITOMAP, MSeqDR, ClinVar, GTR
Repeat expansion testing > Search first: GTR, ClinVar, repeat expansion databases, PubMed
Omics-Based Diagnostics (if applicable):
RNA sequencing / transcriptomics > Search first: GEO, ArrayExpress, GTEx, RNA-seq databases
Proteomics > Search first: PRIDE, ProteomeXchange, FDA Biomarker database
Metabolomics > Search first: MetaboLights, Metabolomics Workbench, HMDB
Epigenomics > Search first: GEO, ENCODE, Roadmap Epigenomics, MethBase
Liquid biopsy > Search first: COSMIC, ClinVar, liquid biopsy databases, PubMed
Clinical Criteria:
Standardized diagnostic criteria (DSM, ICD, society guidelines) > Search first: DSM-5, ICD-11, clinical society guidelines, UpToDate
Differential diagnosis (other conditions to rule out, with distinguishing features) > Search first: DynaMed, UpToDate, clinical decision support systems
Screening:
Screening methods for asymptomatic individuals (newborn screening, carrier screening, cascade screening) > Search first: ACMG recommendations, CDC newborn screening, GTR

11. Outcome/Prognosis

Survival and Mortality:
Survival rate (5-year, 10-year, overall) > Search first: SEER, cancer registries, disease-specific registries, PubMed
Life expectancy (with and without treatment if applicable) > Search first: Orphanet, disease registries, actuarial databases, PubMed
Mortality rate > Search first: CDC, WHO, GBD, national mortality databases
Disease-specific mortality (deaths directly attributable to disease) > Search first: Disease registries, CDC Wonder, GBD, PubMed
Morbidity and Function:
Morbidity (disease-related disability and health impacts) > Search first: GBD, WHO, disability databases, PubMed
Disability outcomes (long-term functional impairments) > Search first: ICF (International Classification of Functioning), disability registries
Quality of life measures (EQ-5D, SF-36, PROMIS, disease-specific tools) > Search first: EQ-5D database, SF-36, PROMIS, PubMed
Disease Course:
Complications (secondary problems: infections, organ failure, etc.) > Search first: ICD codes, disease registries, clinical databases, PubMed
Recovery potential (likelihood and extent of recovery, with vs without treatment) > Search first: Natural history studies, rehabilitation databases, PubMed
Prediction:
Prognostic factors (age, disease severity, biomarkers, treatment response) > Search first: Prognostic models databases, clinical calculators, PubMed
Prognostic biomarkers (molecular markers predicting disease course) > Search first: FDA Biomarker database, PubMed, cancer prognostic databases

12. Treatment

Pharmacotherapy:
Pharmacological treatments (drug names, drug classes, mechanisms of action) > Search first: DrugBank, RxNorm, ATC classification, DailyMed, FDA databases
Pharmacogenomics (how genetic variants affect drug metabolism, efficacy, toxicity) > Search first: PharmGKB, CPIC (Clinical Pharmacogenetics), FDA Table of PGx Biomarkers
Advanced Therapeutics:
Gene therapy (viral vectors, CRISPR, gene replacement, gene editing) > Search first: ClinicalTrials.gov, FDA gene therapy database, ASGCT resources
Cell therapy (stem cell transplant, CAR-T, cellular therapeutics) > Search first: ClinicalTrials.gov, FDA cell therapy database, FACT standards
RNA-based therapies (ASOs, siRNA, mRNA therapies) > Search first: ClinicalTrials.gov, FDA approvals, PubMed
Targeted therapies (treatments directed at specific molecular targets) > Search first: My Cancer Genome, OncoKB, ClinicalTrials.gov, FDA approvals
Immunotherapies (checkpoint inhibitors, monoclonal antibodies) > Search first: Cancer Immunotherapy Database, FDA approvals, ClinicalTrials.gov
Surgical and Interventional:
Surgical interventions (types of surgery, timing, outcomes) > Search first: CPT codes, surgical registries, clinical guidelines, PubMed
Supportive and Rehabilitative:
Supportive care (symptom management, pain control, nutrition) > Search first: Clinical guidelines, Cochrane Library, PubMed
Rehabilitation (physical therapy, occupational therapy, speech therapy) > Search first: Rehabilitation medicine databases, clinical guidelines, PubMed
Experimental:
Experimental treatments in clinical trials (with NCT identifiers if available) > Search first: ClinicalTrials.gov, EU Clinical Trials Register, WHO ICTRP
Treatment Outcomes:
Treatment response rates > Search first: Clinical trial databases, FDA reviews, systematic reviews, PubMed
Side effects and adverse events > Search first: FDA Adverse Event Reporting System (FAERS), MedWatch, PubMed
Treatment Strategy:
Treatment algorithms (clinical pathways, decision trees) > Search first: Clinical practice guidelines, NCCN Guidelines, UpToDate
Combination therapies > Search first: ClinicalTrials.gov, treatment guidelines, PubMed
Personalized medicine approaches (genotype-guided treatment) > Search first: My Cancer Genome, CIViC, PharmGKB, precision medicine databases

For each treatment, suggest NCIT (NCI Thesaurus) clinical-intervention terms where applicable.

13. Prevention

Prevention Levels:
Primary prevention (preventing disease occurrence: vaccination, risk factor modification) > Search first: CDC, WHO, USPSTF recommendations, Cochrane Library
Secondary prevention (early detection and treatment: screening programs, early intervention) > Search first: USPSTF, CDC screening guidelines, WHO
Tertiary prevention (preventing complications in those with disease) > Search first: Clinical guidelines, disease management protocols, PubMed
Immunization: Vaccine strategies (if applicable)

Search first: CDC vaccine schedules, WHO immunization, FDA vaccine database
Screening and Early Detection:
Screening programs (population-based: newborn screening, cancer screening) > Search first: CDC screening programs, USPSTF, cancer screening databases
Genetic screening (carrier screening, preimplantation genetic diagnosis, prenatal testing) > Search first: ACMG recommendations, ACOG guidelines, GTR
Risk stratification (identifying high-risk individuals for targeted prevention) > Search first: Risk prediction models, clinical calculators, PubMed
Behavioral Interventions: Lifestyle modifications to reduce risk

Search first: CDC, WHO, behavioral intervention databases, Cochrane Library
Counseling: Genetic counseling (risk assessment, family planning guidance)

Search first: NSGC resources, ACMG guidelines, GeneReviews
Public Health:
Public health interventions (sanitation, vector control, health education) > Search first: CDC, WHO, public health databases, PubMed
Environmental interventions (reducing environmental risk factors) > Search first: EPA databases, WHO environmental health, PubMed
Prophylaxis: Preventive medications or procedures

Search first: Clinical guidelines, FDA approvals, PubMed

14. Other Species / Natural Disease

Taxonomy: Species affected (with NCBI Taxon identifiers)

Search first: NCBI Taxonomy
Breed: Specific breeds affected (with VBO identifiers if applicable)

Search first: VBO (Vertebrate Breed Ontology)
Gene: Orthologous genes in other species (with NCBI Gene IDs)

Search first: NCBI Gene
Natural Disease:
Naturally occurring disease in other species (companion animals, wildlife) > Search first: OMIA (Online Mendelian Inheritance in Animals), VetCompass, PubMed
Veterinary relevance and importance in animal health > Search first: OMIA, veterinary databases, PubMed
Comparative Biology:
Comparative pathology (similarities and differences across species) > Search first: OMIA, comparative pathology databases, PubMed
Evolutionary conservation of disease mechanisms > Search first: HomoloGene, OrthoMCL, Alliance of Genome Resources
Transmission (if applicable):
Zoonotic potential > Search first: CDC zoonotic diseases, WHO zoonoses, GIDEON
Cross-species susceptibility > Search first: NCBI Taxonomy, veterinary databases, PubMed

15. Model Organisms

Model Types:
Model organism type (mammalian, invertebrate, cellular, in vitro) > Search first: Alliance of Genome Resources, model organism databases
Specific model systems (mouse, rat, zebrafish, Drosophila, C. elegans, yeast, cell lines, organoids, iPSCs) > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, SGD, ATCC, Cellosaurus
Induced models (drug treatment, surgical intervention, environmental manipulation) > Search first: MGI, model organism databases, PubMed
Genetic Models:
Types available (knockout, knock-in, transgenic, conditional, humanized) > Search first: MGI, IMPC, KOMP, EuMMCR, IMSR
Model Characteristics:
Phenotype recapitulation (how well model reproduces human disease features) > Search first: Model organism databases, comparative studies, PubMed
Model limitations (aspects of human disease not captured) > Search first: Model organism databases, PubMed, review articles
Applications:
Research applications (what aspects of disease can be studied) > Search first: Model organism databases, PubMed
Resources:
Model databases > Search first: MGI, RGD, ZFIN, FlyBase, WormBase, IMSR, EMMA, MMRRC

Citation Requirements

Cite primary literature (PMID preferred) for all mechanistic and clinical claims
Prioritize recent reviews and landmark papers
Include direct quotes from abstracts where possible to support key statements
Distinguish evidence source types: human clinical, model organism, in vitro, computational

Output Format

Structure your response as a comprehensive narrative organized by the sections above. For each section, provide: - Factual content with specific details (numbers, percentages, gene names, variant nomenclature) - Ontology term suggestions (HPO, GO, CL, UBERON, CHEBI, NCIT, MONDO) where applicable - Evidence citations with PMIDs - Direct quotes from abstracts to support key claims - Clear indication when information is not available or not applicable for this disease

This report will be used to populate a disease knowledge base entry with: - Pathophysiology descriptions with causal chains - Gene/protein annotations (HGNC, GO terms) - Phenotype associations (HP terms) with frequencies - Cell type involvement (CL terms) - Anatomical locations (UBERON terms) - Chemical entities (CHEBI terms) - Treatment annotations (NCIT terms) - Evidence items with PMIDs and exact abstract quotes - Epidemiology, prognosis, diagnostic, and prevention information - Animal model descriptions with phenotype recapitulation details

Constitutional Mismatch Repair Deficiency (CMMRD): comprehensive disease-characteristics report

Executive summary

Constitutional mismatch repair deficiency is an ultrarare, autosomal-recessive, highly penetrant cancer-predisposition syndrome caused by biallelic germline pathogenic variants in MLH1, MSH2, MSH6, or PMS2. Unlike Lynch syndrome, in which one defective allele predisposes mainly to adult cancers, CMMRD causes mismatch-repair failure throughout normal tissues and frequently produces brain tumors, gastrointestinal cancers, and hematologic malignancies during childhood. The best contemporary natural-history dataset—an international cohort of 201 patients—reported 339 cancers in 97% of patients, 90% cumulative cancer incidence by age 18, and a median diagnostic age of 8.9 years. CNS tumors accounted for 51% of malignancies and had the poorest 10-year survival, 39%. (ercan2024clinicalandbiological pages 1-2)

The most consequential recent developments are: (1) quantitative constitutional microsatellite-instability assays such as LOGIC; (2) genotype-specific recognition of hypomorphic, later-onset CMMRD; (3) structured international surveillance; and (4) immune-checkpoint inhibition for hypermutated tumors. In a 2023 prospective pediatric nivolumab study, best overall response was 50% and two-year overall survival was 50%, with four durable complete remissions, including three malignant gliomas. (das2023efficacyofnivolumab pages 2-3, das2023efficacyofnivolumab pages 8-9)

domain	key quantitative findings	principal recent source (author/year/journal/DOI or PMID)	ontology suggestions
Genetics / etiology	CMMRD is caused by biallelic germline pathogenic variants in mismatch repair genes MLH1, MSH2, MSH6, PMS2; median diagnosis age 8.9 years in a 201-patient cohort; PMS2 >60% of cases, MSH6 20-30%, MLH1/MSH2 10-20%; consanguinity reported in 39-45% of families; birth prevalence estimated at approximately 1 in 1,000,000 (ercan2024clinicalandbiological pages 1-2, shuen2025developmentofa pages 110-114, vasen2026theimpactof pages 1-2)	Ercan et al., 2024, Lancet Oncology, DOI: 10.1016/S1470-2045(24)00026-3; Vasen et al., 2026, Familial Cancer, DOI: 10.1007/s10689-026-00584-x	Gene: MLH1, MSH2, MSH6, PMS2; GO: DNA mismatch repair; MONDO: constitutional mismatch repair deficiency; HP: Autosomal recessive inheritance
Major phenotype / natural history	In the IRRDC cohort (n=201), 97% developed cancer with 339 cancers total and 90% cumulative incidence by age 18; spectrum: CNS 51%, gastrointestinal 22%, hematological 18%, other 9%; median interval between multiple cancers 1.9 years; dermatologic manifestations in 93%; CNS tumors had the poorest outcome with 39% 10-year survival vs 67% hematologic and 89% GI cancers (ercan2024clinicalandbiological pages 1-2)	Ercan et al., 2024, Lancet Oncology, PMID: 38552658, DOI: 10.1016/S1470-2045(24)00026-3	HP: Cafe-au-lait macules, Brain neoplasm, Colorectal carcinoma, Lymphoma, Multiple primary neoplasms; UBERON: brain, colon, small intestine, hematopoietic system
Diagnosis	Germline testing for biallelic pathogenic/likely pathogenic MMR variants is the diagnostic gold standard; non-neoplastic tissue IHC showing complete MMR protein loss has reported >90% sensitivity and approximately 100% specificity; LOGIC assay was 100% sensitive and specific in childhood cancers (N=376) and outperformed MSI panel (14% sensitivity), IHC (86%), and TMB (80%); blood/saliva DNA distinguished CMMRD from other syndromes (n=277) (shuen2025developmentofa pages 23-26, vasen2026theimpactof pages 6-8)	Chung et al., 2023, Journal of Clinical Oncology, DOI: 10.1200/JCO.21.02873; summarized in Vasen et al., 2026, Familial Cancer, DOI: 10.1007/s10689-026-00584-x	GO: microsatellite instability, DNA mismatch repair; HP: Abnormality of DNA repair; NCIT: Immunohistochemistry, Whole exome sequencing, Whole genome sequencing
Surveillance	Consensus surveillance includes CBC every 6 months from age 1, optional abdominal ultrasound every 6 months from age 1, brain MRI every 6-12 months from age 2 (or from diagnosis/first year in some protocols), ileocolonoscopy annually from age 6-8, upper endoscopy/videocapsule annually from age 8-10, whole-body MRI annually from age 6 or at diagnosis, and annual gynecologic/urologic screening from age 20; surveillance participation was associated with 79% 4-year survival versus 15% in non-participants, and digestive surveillance achieved 100% 5-year survival in reported experience (shuen2025developmentofa pages 114-117, shuen2025developmentofa pages 23-26, shuen2025developmentofa pages 110-114, vasen2026theimpactof pages 6-8)	C4CMMRD/IRRDC guidance summarized in Shuen, 2025; Vasen et al., 2026, Familial Cancer, DOI: 10.1007/s10689-026-00584-x	NCIT: Magnetic Resonance Imaging, Colonoscopy, Upper Gastrointestinal Endoscopy, Complete Blood Count; UBERON: brain, colon, stomach, small intestine, whole body
Treatment	Immune checkpoint blockade is the major recent advance. In pediatric hypermutant/MMRD cancers treated with nivolumab (NCT02992964), best overall response was 50% and 2-year OS 50%; 4 children, including 3 with refractory malignant gliomas, achieved complete remission at median follow-up 37 months; in registry data, objective responses were 64% for CNS tumors and 100% for non-CNS solid tumors with nivolumab or pembrolizumab. Temozolomide resistance is a recurring concern in MMR-deficient gliomas (das2023efficacyofnivolumab pages 2-3, das2023efficacyofnivolumab pages 8-9, vasen2026theimpactof pages 8-9)	Das et al., 2023, Clinical Cancer Research, DOI: 10.1158/1078-0432.CCR-23-0411; Vasen et al., 2026, Familial Cancer, DOI: 10.1007/s10689-026-00584-x	NCIT: Nivolumab, Pembrolizumab, Immune Checkpoint Inhibitor Therapy; GO: adaptive immune response, T cell mediated cytotoxicity
Evidence gaps / limitations	Important gaps remain in standardized confirmation of hypomorphic variants and genotype-phenotype mismatches; PMS2 testing is complicated by pseudogene interference and a high VUS burden (49% cited in summary evidence); some older 'likely CMMRD' categories are being abandoned in favor of conclusive molecular/functional evidence; surveillance and treatment evidence remains largely observational and registry-based, with limited prospective pediatric trial data (shuen2025developmentofa pages 110-114, vasen2026theimpactof pages 6-8, vasen2026theimpactof pages 1-2)	Gallon et al., 2024, NPJ Precision Oncology, DOI: 10.1038/s41698-024-00603-z; Vasen et al., 2026, Familial Cancer, DOI: 10.1007/s10689-026-00584-x	NCIT: Variant of Uncertain Significance; GO: DNA repair; HP: Variable expressivity

Table: This table summarizes the most actionable evidence already gathered for constitutional mismatch repair deficiency across genetics, phenotype, diagnosis, surveillance, treatment, and current evidence limitations. It is useful as a compact scaffold for a disease knowledge base entry and ontology mapping.

1. Disease information

Definition and category

CMMRD is a hereditary DNA-repair disorder and pediatric cancer-predisposition syndrome. Constitutional loss of MMR activity generates replication errors in normal and neoplastic cells, creating microsatellite instability (MSI), high or ultrahigh tumor mutational burden (TMB), and multiple early primary cancers. Birth prevalence is commonly estimated at approximately 1 per 1,000,000, although ascertainment is incomplete and prevalence is substantially higher in populations with frequent consanguinity or founder alleles. More than 300 affected individuals had been published by recent consortium review. (vasen2026theimpactof pages 1-2, shuen2025developmentofa pages 6-12)

Identifiers and synonyms

Preferred name: constitutional mismatch repair deficiency.
Synonyms: CMMRD; constitutional mismatch-repair deficiency syndrome; biallelic mismatch repair deficiency, BMMRD; biallelic MMR deficiency; childhood cancer syndrome; Turcot syndrome type 1 is an older, narrower designation and should not replace CMMRD.
OMIM phenotypic series: entries reported for gene-defined CMMRD include #276300, #619096, #619097, and #619101. (munteanu2025genotypephenotypecorrelationsin pages 1-2)
MONDO: use the current MONDO concept for constitutional mismatch repair deficiency syndrome; the exact accession should be verified against the live MONDO release before database ingestion because ontology accessions can change.
Orphanet: an Orphanet CMMRD/BMMRD entity exists, but its live ORPHA accession was not independently verified in the retrieved primary literature.
ICD-10/ICD-11: no sufficiently specific universal CMMRD code is established in the retrieved evidence. Coding generally combines genetic susceptibility/DNA-repair disorder and each active neoplasm. CMMRD should not be represented solely as Lynch syndrome.
MeSH: no dedicated MeSH descriptor was verified; use terms for DNA mismatch repair, hereditary neoplastic syndromes, microsatellite instability, and the relevant tumors.

This report is based on aggregated disease-level resources, international registries, published cohorts, guidelines, and trials, not individual EHR data.

2. Etiology, risk, protection, and gene–environment interaction

Causal factors

The necessary upstream cause is biallelic—homozygous or compound-heterozygous—germline loss or severe reduction of function in PMS2, MSH6, MLH1, or MSH2. PMS2 accounts for over 60% of reported families, MSH6 for approximately 20–30%, and MLH1/MSH2 together for roughly 10–20%. MLH1/MSH2 disease tends to begin earlier and have worse survival than PMS2/MSH6 disease. Truncating or frameshift alleles generally confer more severe outcomes than missense/hypomorphic alleles. (ercan2024clinicalandbiological pages 1-2, shuen2025developmentofa pages 110-114)

Genetic risk factors

Two pathogenic or likely pathogenic variants in trans are the principal risk determinant.
Each sibling of an affected child has, under standard Mendelian assumptions, a 25% probability of CMMRD, 50% probability of heterozygous Lynch-syndrome carrier status, and 25% probability of inheriting neither familial variant.
Hypomorphic alleles may retain partial MMR function and produce attenuated or adult-onset disease resembling Lynch syndrome. Examples reported in 2024 include homozygous MSH6 c.3226C>T, p.(Arg1076Cys) and MLH1 c.306G>A, p.(Glu102=); the latter caused leaky exon-3 skipping. Constitutional MSI resolved the apparent genotype–phenotype conflict.
Consanguinity is reported in approximately 39–45% of families. Homozygosity in apparently non-consanguineous families may reflect founder alleles. (shuen2025developmentofa pages 110-114, munteanu2025genotypephenotypecorrelationsin pages 1-2)

Environmental and lifestyle factors

No environmental exposure, infection, diet, smoking behavior, or occupational agent is known to cause CMMRD. Age is a determinant of accumulated replication errors rather than an external cause. Lifestyle effects on penetrance have not been quantified sufficiently to recommend CMMRD-specific dietary or exercise interventions beyond general health guidance.

Treatment can modify downstream mutagenesis. Alkylating therapy, particularly temozolomide in glioma, may select MMR-deficient clones and amplify mutation burden; MMR-deficient tumor cells can also be intrinsically resistant because cytotoxic recognition of alkylator-induced mismatches requires functional MMR. This interaction argues for molecularly informed treatment selection rather than assuming that hypermutation makes all therapies effective.

Protective factors

No validated protective germline allele or environmental intervention prevents the constitutional defect. The demonstrable protective interventions are secondary prevention through surveillance, prompt removal of premalignant GI lesions, and early tumor treatment. Prenatal diagnosis and preimplantation genetic testing can prevent recurrence in future pregnancies but do not treat an affected individual. (vasen2026theimpactof pages 6-8)

3. Phenotypes and quality-of-life burden

The 2024 IRRDC cohort provides the strongest frequency estimates: among 201 patients, 97% developed 339 cancers; cumulative incidence reached 90% by age 18; and multiple cancers were separated by a median of only 1.9 years. Tumor distribution was CNS 51%, GI 22%, hematologic 18%, and other malignancies 9%. Dermatologic findings occurred in 93%. (ercan2024clinicalandbiological pages 1-2)

Major phenotypes

CNS tumors—clinical sign/structural disease. Predominantly high-grade glioma, with other gliomas and occasional embryonal tumors. Typical brain-tumor diagnosis is around age nine; severity is usually high and progression can be rapid. Suggested HPO: Brain neoplasm, High-grade glioma, Headache, Seizure, Focal neurologic deficit. Suggested UBERON: brain, cerebral hemisphere, brainstem, cerebellum. CNS tumors caused the poorest outcome: 39% 10-year survival from diagnosis. (vasen2026theimpactof pages 1-2, ercan2024clinicalandbiological pages 1-2)
Gastrointestinal neoplasia—clinical/pathologic manifestation. Adenomas, polyposis-like burden, colorectal carcinoma, and small-intestinal or upper-GI cancers can occur in childhood or adolescence. GI tumors represented 22% of cancers; later-onset PMS2-CMMRD appears relatively enriched for GI disease. Suggested HPO: Gastrointestinal neoplasm, Colorectal carcinoma, Intestinal polyposis, Gastrointestinal hemorrhage, Abdominal pain. GI-cancer 10-year survival was 89% in the international cohort. (ercan2024clinicalandbiological pages 1-2, munteanu2025genotypephenotypecorrelationsin pages 1-2)
Hematologic malignancy—laboratory/clinical manifestation. T-cell lymphoblastic lymphoma is particularly characteristic; leukemia and other lymphomas occur, often at very young ages. Suggested HPO: Lymphoma, Leukemia, Anemia, Thrombocytopenia, Lymphadenopathy. Hematologic cancers represented 18%, with 67% 10-year survival. An early series documented hematologic malignancy between 14 months and six years. (ercan2024clinicalandbiological pages 1-2, wimmer2017connectionsbetweenconstitutional pages 4-6)
NF1-like pigmentary phenotype—physical sign. Multiple café-au-lait macules, altered pigmentation, axillary/intertriginous freckling, and occasional neurofibroma-like lesions commonly precede cancer. Suggested HPO: Café-au-lait macule (HP:0000957), Axillary freckling, Abnormal skin pigmentation, Neurofibroma. Dermatologic signs in 93% make them clinically valuable but not specific. (ercan2024clinicalandbiological pages 1-2, wimmer2017connectionsbetweenconstitutional pages 4-6)
Multiple primary malignancies—temporal phenotype. Synchronous or metachronous tumors are a defining burden. Recent review estimates synchronous cancers in as many as 25%, with approximately two years to a new malignancy. Suggested HPO: Multiple primary neoplasms. (vasen2026theimpactof pages 8-9)
Other tumors. Sarcomas, embryonal tumors, and assorted Lynch-spectrum malignancies occur less often. Suggested HPO should be assigned at histology/site level rather than using a nonspecific cancer term.

Quality of life

No validated CMMRD-specific EQ-5D, SF-36, or PROMIS population estimates were identified. Expected burden is nevertheless profound: repeated anesthesia and imaging, six- to twelve-month surveillance, colonoscopy from early childhood, major surgery, neurocognitive effects of CNS tumors and therapy, endocrine/neurologic sequelae, chronic fear of another cancer, interrupted schooling, and familial grief. Quality-of-life effects should be recorded as an evidence gap rather than inferred as quantified outcomes.

4. Genetic and molecular information

Causal genes and proteins

MSH2–MSH6 (MutSα): recognizes base substitutions and small insertion/deletion loops.
MLH1–PMS2 (MutLα): coordinates excision, resynthesis, and repair after mismatch recognition.
Loss of either binding partner may destabilize its partner and create paired IHC loss patterns.

Suggested GO annotations include DNA mismatch repair (GO:0006298), mismatch recognition, postreplicative DNA repair, DNA repair complex, and maintenance of genome stability.

Variant classes and interpretation

Pathogenic CMMRD variants include nonsense, frameshift, essential splice, exon-level deletion/duplication, structural, and functionally damaging missense variants. Their origin is germline; additional somatic variants drive each tumor. A VUS is not diagnostic without segregation, RNA/protein/function, constitutional MSI, or other compelling evidence. Approximately 30% VUS burden has been reported in diagnostic series, and PMS2 is particularly difficult, with a cited VUS rate of 49%. PMS2CL and other pseudogene sequences require long-range PCR, locus-specific NGS, or validated hybrid methods to avoid false assignment. (shuen2025developmentofa pages 110-114, shuen2025developmentofa pages 114-117, shuen2025developmentofa pages 23-26)

Population frequencies of individual causal alleles vary and must be retrieved variant-by-variant from the current gnomAD release. Fully penetrant severe biallelic genotypes are expected to be rare; allele frequency alone cannot classify hypomorphic or founder variants.

Modifier genes and chromosomal abnormalities

Acquired polymerase-proofreading mutations in POLE/POLD1 can convert an already hypermutated tumor to an ultrahypermutated state. Genotype, residual MMR activity, variant class, affected organ, treatment history, and age modify expression. No separate, clinically validated constitutional modifier-gene panel is established. Recurrent aneuploidy or translocation is not the primary cause; copy-number variants disrupting an MMR gene can, however, constitute one causal allele.

Epigenetics and omics

CMMRD-associated high-grade gliomas have distinctive methylation and hypomethylation patterns, supporting classification as replication-repair-deficient tumors rather than ordinary adult glioblastoma. Molecularly, all cancers in the 2024 international cohort showed high mutation burdens and characteristic mutational signatures. Constitutional instability differs by tissue—reported as GI greater than blood greater than brain—and increases longitudinally, suggesting a quantitative molecular clock. (ercan2024clinicalandbiological pages 1-2, vasen2026theimpactof pages 6-8)

Routine diagnostic transcriptomics, proteomics, metabolomics, lipidomics, single-cell, or spatial-transcriptomic tests are not established. RNA sequencing is useful selectively for splice variants. No reproducible systemic metabolic or lipidomic signature is currently suitable for knowledge-base assertion.

5. Environmental information

CMMRD is neither infectious nor environmentally acquired and has no zoonotic transmission. Environmental toxicants, pollution, alcohol, smoking, diet, and exercise have not been shown to alter penetrance sufficiently for disease-specific estimates. Ionizing radiation and mutagenic chemotherapy warrant individualized risk–benefit review because patients already have deficient genome maintenance, but necessary radiotherapy is not categorically contraindicated. Vaccination should follow routine schedules unless active cancer therapy or immunosuppression requires modification.

6. Mechanism and pathophysiology

Causal chain

Upstream inherited defect: both alleles of an MMR gene are defective in the germ line.
Protein-complex failure: deficient MutSα or MutLα impairs recognition or excision of polymerase replication errors.
Constitutional genomic instability: base substitutions and insertion/deletion loops accumulate, particularly at microsatellites.
Driver acquisition: proliferative tissues acquire oncogenic and tumor-suppressor mutations; somatic POLE/POLD1 proofreading defects may cause ultrahypermutation.
Clonal evolution: neural progenitors/glial cells, intestinal epithelial stem cells, and lymphoid precursors transform, accounting for CNS, GI, and hematologic predominance.
Immune consequence: high mutation burden generates neoantigens and can recruit activated CD8-positive T cells, creating susceptibility to PD-1 blockade. Responses are heterogeneous because antigen presentation, tumor lineage, CNS microenvironment, regulatory T cells, steroid exposure, and IDH-associated immune suppression can counteract immunogenicity.
Clinical outcome: early, recurrent, synchronous, or metachronous malignancy produces neurologic, GI, hematologic, and systemic manifestations. (shuen2025developmentofa pages 6-12, shuen2025developmentofa pages 34-37, das2023efficacyofnivolumab pages 2-3)

Suggested GO terms: DNA mismatch repair; DNA replication; response to DNA damage stimulus; somatic mutation; regulation of cell cycle; apoptotic process; antigen processing and presentation; adaptive immune response; T-cell-mediated cytotoxicity. Suggested Cell Ontology concepts: neural stem/progenitor cell, astrocyte lineage cell, intestinal epithelial stem cell, colonocyte, enterocyte, thymocyte, T lymphocyte, B lymphocyte, hematopoietic stem/progenitor cell, CD8-positive alpha-beta T cell.

Experimental vulnerabilities

WRN helicase inhibition is synthetically lethal in MSI-high cells: repeat-derived secondary structures stall replication forks and, without WRN resolution, undergo MUS81–EME1 cleavage and p53/PUMA-dependent apoptosis. ATR inhibition and combined ATR/WRN/PD-1 strategies remain preclinical. These are mechanistically compelling but are not approved CMMRD therapies. (shuen2025developmentofa pages 34-37)

7. Anatomical structures affected

Primary organs are the brain/CNS, colon and rectum, small intestine, stomach/upper GI tract, bone marrow, thymus, lymph nodes, spleen, and skin. Other solid-organ involvement is tumor-dependent. Suggested UBERON mappings include brain, cerebral cortex, brainstem, cerebellum, colon, rectum, small intestine, stomach, bone marrow, thymus, lymph node, spleen, and skin.

At tissue level, affected populations include glial/neural progenitors, intestinal crypt stem cells and epithelium, and lymphoid progenitors. At subcellular level, MMR functions principally in the nucleus at newly replicated chromatin; suggested GO cellular components are nucleus, chromosome, replication fork, and mismatch-repair complex. Lateralization is not intrinsic: CNS tumors can be unilateral or midline depending on tumor site, while GI and hematologic disease is not meaningfully lateralized.

8. Temporal development and course

The defect is congenital, but clinical onset is typically pediatric and often insidious until cancer symptoms emerge. Mean/median first-cancer age is approximately nine to ten years; about 80–90% develop cancer by age 18. Hematologic disease may occur in infancy or early childhood, CNS tumors commonly around school age, and GI carcinoma more often in later childhood/adolescence. Hypomorphic genotypes can delay onset into adulthood. (vasen2026theimpactof pages 1-2, ercan2024clinicalandbiological pages 1-2, shuen2025developmentofa pages 6-12)

CMMRD is lifelong, progressive at the predisposition level, and characterized by repeated tumor episodes rather than continuous activity of a single disease. Remission is tumor- and therapy-induced, but remission from one cancer does not remove the risk of another. Critical intervention windows begin at molecular diagnosis—ideally before the first cancer—and after every cancer diagnosis, when staging should actively search for synchronous tumors.

9. Inheritance and population

Inheritance is autosomal recessive, with high but genotype- and age-dependent penetrance. In the largest cohort, 97% had cancer and cumulative incidence reached 90% by age 18, approaching complete lifetime penetrance. Expressivity is highly variable, particularly for PMS2/MSH6 and hypomorphic alleles. Genetic anticipation is not established. Germline mosaicism is possible in principle but is not a recognized major mechanism.

The estimated prevalence is about one birth per million, but no robust population-based incidence per 100,000 person-years is available. Consanguinity and founder effects produce geographic clustering; CMMRD should nevertheless be considered in every ancestry. No consistent biological sex predominance is established. In a high-consanguinity Pakistani pediatric high-grade-glioma cohort published after the requested 2023–2024 priority window, 15/47 (31.9%) tested positive for CMMRD, illustrating enrichment in selected populations rather than general prevalence.

Parents and many relatives are heterozygous MMR-variant carriers and require Lynch-syndrome counseling and adult surveillance. Carrier frequency is gene-, ancestry-, and variant-specific; it should not be inferred from the CMMRD birth-prevalence estimate.

10. Diagnostics

When to suspect CMMRD

Suspect CMMRD in a child or young adult with high-grade glioma, T-lymphoblastic lymphoma, early GI adenoma/carcinoma, multiple primary cancers, marked tumor hypermutation, constitutional MSI, NF1-like pigmentary findings without an explanatory NF1/SPRED1 variant, consanguinity, or parents with cancers compatible with Lynch syndrome. C4CMMRD clinical scoring historically used a threshold of at least three points to trigger testing, but contemporary practice seeks conclusive molecular/functional confirmation. (shuen2025developmentofa pages 114-117, shuen2025developmentofa pages 23-26)

Recommended testing workflow

Pretest genetic counseling and paired blood/tumor review. Document three-generation pedigree, consanguinity, pathology, prior therapies, and pigmentary findings.
Tumor testing: IHC for MLH1, PMS2, MSH2, and MSH6; MSI by NGS or PCR; and tumor sequencing/TMB. Conventional adult colorectal MSI panels can be insensitive in pediatric/CNS CMMRD tumors and must not exclude the diagnosis.
Germline multigene panel: sequence and deletion/duplication analysis of all four genes, not only the protein absent by IHC. Confirm that variants are in trans through parental testing. Use locus-specific PMS2 methods.
Ancillary confirmation: MMR IHC in normal tissue, constitutional MSI from blood/saliva, lymphoblastoid-cell functional assays, methylating-agent tolerance, or low-pass WGS/LOGIC where available.
RNA studies: resolve suspected splice variants or hypomorphic alleles.
WES/WGS: useful after negative/inconclusive panels, for structural/deep-intronic variants and alternative syndromes. WGS may simultaneously support constitutional MSI analysis. CMA, karyotype, FISH, mitochondrial testing, and repeat-expansion assays are not first-line unless another diagnosis is suspected.

Normal-tissue IHC has reported sensitivity above 90% and specificity near 100%, but missense proteins may retain staining. Germline biallelic P/LP variants remain the central diagnostic standard. In 376 childhood cancers, the 2023 LOGIC assay was reported as 100% sensitive and specific, compared with sensitivities of 14% for a conventional MSI panel, 86% for IHC, and 80% for TMB; it also distinguished CMMRD from other predisposition syndromes using blood/saliva DNA in 277 individuals. (shuen2025developmentofa pages 23-26, vasen2026theimpactof pages 6-8)

The authors’ key abstract conclusion was: “LOGIC was a robust tool for the diagnosis of MMRD in multiple cancer types and in normal tissues.” This assay remains specialized rather than universally available.

Differential diagnosis

Lynch syndrome: monoallelic variant, predominantly adult-onset tumors, no generalized constitutional MMR deficiency.
NF1 or Legius syndrome: pigmentary findings with NF1/SPRED1 alteration; lacks constitutional MSI and the characteristic CMMRD tumor spectrum.
Polymerase-proofreading-associated polyposis/POLE-POLD1 disorder: hypermutation/polyposis without biallelic MMR variants.
Familial adenomatous polyposis/MUTYH-associated polyposis: different polyp pattern and causal genes.
Li–Fraumeni syndrome: broad childhood tumor spectrum from TP53, generally without constitutional MSI/MMR-protein loss.
Other DNA-repair disorders, immunodeficiency-associated lymphoma, and sporadic pediatric cancer.

11. Outcome and prognosis

The 201-patient IRRDC cohort provides current benchmarks: 10-year survival from tumor diagnosis was 39% for CNS, 67% for hematologic, 89% for GI, and 96% for other tumors. Survival by age 15 was reported as 0% for MSH2, 19% for MLH1, 49% for MSH6, and 63% for PMS2 genotypes, although these estimates may be affected by small gene-specific samples and ascertainment. (ercan2024clinicalandbiological pages 1-2)

Prognostic factors include causal gene, residual protein function, truncating versus missense variant, age and type of first cancer, CNS involvement, stage/resectability, synchronous malignancy, polymerase-proofreading status, TMB/MSI pattern, immune microenvironment, and access to surveillance and immunotherapy. Surveillance has been associated with 79% four-year survival versus 15% without program participation; digestive surveillance achieved 100% five-year survival in reported consortium experience. These nonrandomized data are vulnerable to lead-time and ascertainment bias but strongly support surveillance. (vasen2026theimpactof pages 6-8)

Long-term morbidity includes neurologic and cognitive disability, endocrine effects, GI surgery consequences, marrow toxicity, immune-related adverse events, and repeated-cancer burden. Recovery from an individual cancer is possible; cure of the inherited predisposition is not currently possible.

12. Treatment

General strategy

Management should occur through a multidisciplinary pediatric/AYA cancer-predisposition team. Treat each tumor according to histology, stage, resectability, molecular profile, previous therapy, and competing synchronous cancer, while recognizing MMR-associated drug resistance and eligibility for tumor-agnostic immunotherapy.

Surgery: maximal safe resection of CNS tumors; endoscopic polypectomy and segmental or more extensive bowel surgery according to burden; diagnostic/therapeutic surgery for other solid tumors. Suggested NCIT: Surgical Resection, Polypectomy, Colectomy.
Radiotherapy: used when oncologically indicated, especially for CNS disease, with individualized discussion of late effects and second malignancies. Suggested NCIT: Radiation Therapy.
Chemotherapy: histology-specific regimens remain relevant, particularly for hematologic malignancies. Temozolomide efficacy can be reduced in MMR-deficient glioma; treatment should not be selected reflexively without molecular review.
Supportive care: antiemetics, analgesia, nutrition, seizure control, infection prophylaxis, transfusion support, rehabilitation, neuropsychology, fertility preservation, and psychosocial care.

Immune-checkpoint inhibition

PD-1 inhibitors—principally nivolumab and pembrolizumab—are the most important targeted treatment because hypermutation creates neoantigens. In NCT02992964, patients aged at least 12 months but under 25 years with refractory MMRD and/or TMB ≥5 mutations/Mb received nivolumab 3 mg/kg every two weeks for up to 24 months. Best overall response was 50%, versus an initial objective response of 20%, illustrating delayed immune responses; two-year OS was 50% (95% CI 27–93). Four children, including three with refractory malignant glioma, were in complete remission at median 37-month follow-up. All five responders were MMR deficient, and responders had greater CD8-positive T-cell clonality/diversity. (das2023efficacyofnivolumab pages 2-3, das2023efficacyofnivolumab pages 8-9)

The abstract’s central conclusion was: “Nivolumab resulted in durable responses and prolonged survival … in refractory hypermutated cancers including malignant gliomas.” Registry experience subsequently reported objective responses of 64% in CNS tumors and 100% in non-CNS solid tumors treated with nivolumab or pembrolizumab, although registry selection and small samples limit comparison. (vasen2026theimpactof pages 8-9)

Suggested NCIT terms: Nivolumab, Pembrolizumab, Atezolizumab, PD-1 Inhibitor, Immune Checkpoint Inhibitor Therapy. Toxicities include dermatitis, colitis, hepatitis, pneumonitis, thyroiditis and other endocrinopathies, neurologic toxicity, and rare severe autoimmune disease. CMMRD-associated autoimmune susceptibility deserves careful baseline assessment and monitoring. (shuen2025developmentofa pages 34-37)

Trials and experimental treatment

NCT02992964: nivolumab pilot in pediatric hypermutant cancers; phase I/II; terminated after small enrollment but generated the published efficacy cohort.
NCT05770102 (DETERMINE arm 02): recruiting phase II/III platform arm of atezolizumab for high-TMB, MSI-high, or proven CMMRD cancers; planned enrollment 30.
NCT05722886: recruiting DETERMINE master protocol; planned enrollment 825 across molecularly defined rare cancers.
NCT02359565: active, not recruiting phase I pembrolizumab study in younger patients with recurrent/refractory CNS tumors, including hypermutated brain tumors; enrollment 71.
NCT04500548: nivolumab plus ipilimumab 3CI study; withdrawn with zero enrollment.
WRN or ATR inhibition, combined DNA-damage/ICI therapy, neoantigen/frameshift-peptide vaccines, and MMR-independent alkylators remain experimental. (shuen2025developmentofa pages 34-37)

No gene replacement, CRISPR correction, RNA therapy, or preventive hematopoietic cell therapy is established. Pharmacogenomic management presently centers on tumor MMR/TMB/MSI and somatic drivers rather than CYP-based CMMRD-specific dosing.

13. Prevention and surveillance

Primary prevention

The inherited defect cannot currently be reversed. Prevention focuses on genetic counseling, cascade testing, avoidance of tobacco and unnecessary mutagenic exposure, and reproductive options. For known familial variants, prenatal testing and preimplantation genetic testing are technically feasible. Heterozygous relatives should receive Lynch-syndrome management. (vasen2026theimpactof pages 6-8)

Secondary prevention: consensus surveillance

Schedules vary slightly between C4CMMRD, ERN GENTURIS, AACR, and local protocols; individualization is essential.

Clinical review and neurologic examination: every six months from diagnosis.
CBC: every six months from age one for leukemia/hematologic disease.
Abdominal ultrasound: every six months from age one is optional because sensitivity is limited.
Brain MRI: every six months from diagnosis/approximately age two through age 20; some protocols permit six- to twelve-month intervals.
Ileocolonoscopy: annually beginning at age six to eight; shorten to six months when polyps are found.
Upper GI endoscopy and video-capsule small-bowel examination: annually beginning around age eight to ten.
Whole-body MRI: baseline at diagnosis and, in some protocols, annually from age six.
Gynecologic examination with transvaginal ultrasound: annually from age 20, adapted to maturity and preference.
Urinalysis/urine cytology or other urologic review: annually from age 20, recognizing limited evidence.

Presymptomatic detection has been associated with five-year survival of 72% versus 33% for symptom-detected brain tumors and 100% versus 81% for GI cancers. (shuen2025developmentofa pages 114-117, shuen2025developmentofa pages 23-26, shuen2025developmentofa pages 110-114)

Tertiary prevention

After any cancer, continue full surveillance rather than site-limited follow-up because the median interval to another primary is approximately two years. Monitor treatment-related neurologic, endocrine, cardiac, GI, fertility, immune, and psychosocial complications. There is no CMMRD-specific prophylactic medication or vaccine in standard practice.

14. Other species and natural disease

CMMRD is a human syndrome; no common naturally occurring veterinary counterpart with comparable surveillance relevance was established in the retrieved evidence. Orthologous MMR genes are deeply conserved in mammals and model organisms. Suggested taxa for comparative annotations include Homo sapiens (NCBI Taxon 9606) and Mus musculus (10090). Dogs and other mammals can develop sporadic or hereditary MMR-deficient tumors, but these should not automatically be labeled natural CMMRD without demonstrated biallelic germline loss.

There is no infectious transmission, zoonotic potential, or cross-species contagion.

15. Model organisms and experimental systems

Available models

Mouse: germline, constitutive, and tissue-conditional Mlh1, Msh2, Msh6, or Pms2 knockout models; combinations with oncogenic drivers or polymerase-proofreading defects model lymphoma, intestinal neoplasia, and replication-repair-deficient glioma.
Cellular: patient-derived lymphoblastoid cells, fibroblasts, tumor cultures, engineered MMR-knockout lines, and organoids. These support functional variant classification, methylating-agent tolerance, MSI quantification, drug screening, and synthetic-lethality studies.
Invertebrate/yeast: conserved MMR systems in Saccharomyces, Drosophila, and C. elegans permit mechanistic studies, but they do not reproduce the complete human tumor spectrum.

Recapitulation and limitations

Models reproduce constitutional replication errors, MSI, elevated mutation burden, lymphoid or intestinal tumor susceptibility, and treatment resistance. They incompletely reproduce the human distribution and timing of pediatric CNS, GI, and hematologic cancers; complete loss of some genes may produce species-specific viability or tumor spectra. Human immune-checkpoint response also depends on HLA/neoantigen and tumor-microenvironment features absent from many models.

Primary applications include pathogenicity assessment, mutational-signature analysis, identification of POLE/POLD1-driven ultrahypermutation, immunotherapy biomarkers, and WRN/ATR synthetic lethality. Relevant resources include MGI, IMPC, KOMP, IMSR/MMRRC, Cellosaurus, ATCC, and patient-derived model repositories.

Evidence appraisal and knowledge gaps

The 2024 IRRDC study is the strongest available natural-history source but remains an international referral cohort rather than a population-based registry. Most surveillance estimates are observational and susceptible to lead-time bias. Immunotherapy evidence is encouraging but comes from small prospective and registry cohorts. Variant-specific penetrance, population carrier frequencies, sex effects, quality-of-life metrics, metabolomic/proteomic biomarkers, standardized adult surveillance, and natural veterinary disease remain insufficiently characterized.

Authoritative source details include Ercan et al., Lancet Oncology, published May 2024, PMID 38552658, DOI/URL: https://doi.org/10.1016/S1470-2045(24)00026-3; Chung et al., Journal of Clinical Oncology, published February 2023, DOI/URL: https://doi.org/10.1200/JCO.21.02873; Das et al., Clinical Cancer Research, published May 2023, DOI/URL: https://doi.org/10.1158/1078-0432.CCR-23-0411; Gallon et al., NPJ Precision Oncology, published May 2024, DOI/URL: https://doi.org/10.1038/s41698-024-00603-z; and Guerrini-Rousseau et al., Familial Cancer, published July 2024, DOI/URL: https://doi.org/10.1007/s10689-024-00403-1.

References

(ercan2024clinicalandbiological pages 1-2): Ayse Bahar Ercan, Melyssa Aronson, Nicholas R Fernandez, Yuan Chang, Adrian Levine, Zhihui Amy Liu, Logine Negm, Melissa Edwards, Vanessa Bianchi, Lucie Stengs, Jiil Chung, Abeer Al-Battashi, Agnes Reschke, Alex Lion, Alia Ahmad, Alvaro Lassaletta, Alyssa T Reddy, Amir F Al-Darraji, Amish C Shah, An Van Damme, Anne Bendel, Aqeela Rashid, Ashley S Margol, Bethany L Kelly, Bojana Pencheva, Brandie Heald, Brianna Lemieux-Anglin, Bruce Crooks, Carl Koschmann, Catherine Gilpin, Christopher C Porter, David Gass, David Samuel, David S Ziegler, Deborah T Blumenthal, Dennis John Kuo, Dima Hamideh, Donald Basel, Dong-Anh Khuong-Quang, Duncan Stearns, Enrico Opocher, Fernando Carceller, Hagit Baris Feldman, Helen Toledano, Ira Winer, Isabelle Scheers, Ivana Fedorakova, Jack M Su, Jaime Vengoechea, Jaroslav Sterba, Jeffrey Knipstein, Jordan R Hansford, Julieta Rita Gonzales-Santos, Kanika Bhatia, Kevin J Bielamowicz, Khurram Minhas, Kim E Nichols, Kristina A Cole, Lynette Penney, Magnus Aasved Hjort, Magnus Sabel, Maria Joao Gil-da-Costa, Matthew J Murray, Matthew Miller, Maude L Blundell, Maura Massimino, Maysa Al-Hussaini, Mazin F Al-Jadiry, Melanie A Comito, Michael Osborn, Michael P Link, Michal Zapotocky, Mithra Ghalibafian, Najma Shaheen, Naureen Mushtaq, Nicolas Waespe, Nobuko Hijiya, Noemi Fuentes-Bolanos, Olfat Ahmad, Omar Chamdine, Paromita Roy, Pavel N Pichurin, Per Nyman, Rachel Pearlman, Rebecca C Auer, Reghu K Sukumaran, Rejin Kebudi, Rina Dvir, Robert Raphael, Ronit Elhasid, Rose B McGee, Rose Chami, Ryan Noss, Ryuma Tanaka, Salmo Raskin, Santanu Sen, Scott Lindhorst, Sebastien Perreault, Shani Caspi, Shazia Riaz, Shlomi Constantini, Sophie Albert, Stanley Chaleff, Stefan Bielack, Stefano Chiaravalli, Stuart Louis Cramer, Sumita Roy, Suzanne Cahn, Suzanne Penna, Syed Ahmer Hamid, Tariq Ghafoor, Uzma Imam, Valerie Larouche, Vanan Magimairajan Issai, William D Foulkes, Yi Yen Lee, Paul C Nathan, Yosef E Maruvka, Mary-Louise C Greer, Carol Durno, Adam Shlien, Birgit Ertl-Wagner, Anita Villani, David Malkin, Cynthia Hawkins, Eric Bouffet, Anirban Das, and Uri Tabori. Clinical and biological landscape of constitutional mismatch-repair deficiency syndrome: an international replication repair deficiency consortium cohort study. May 2024. URL: https://doi.org/10.1016/s1470-2045(24)00026-3, doi:10.1016/s1470-2045(24)00026-3. This article has 91 citations and is from a highest quality peer-reviewed journal.
(das2023efficacyofnivolumab pages 2-3): Anirban Das, Uri Tabori, Lauren C. Sambira Nahum, Natalie B. Collins, Rebecca Deyell, Rina Dvir, Cecile Faure-Conter, Timothy E. Hassall, Jane E. Minturn, Melissa Edwards, Elissa Brookes, Vanessa Bianchi, Adrian Levine, Simone C. Stone, Sumedha Sudhaman, Santiago Sanchez Ramirez, Ayse B. Ercan, Lucie Stengs, Jill Chung, Logine Negm, Gad Getz, Yosef E. Maruvka, Birgit Ertl-Wagner, Pamela S. Ohashi, Trevor Pugh, Cynthia Hawkins, Eric Bouffet, and Daniel A. Morgenstern. Efficacy of nivolumab in pediatric cancers with high mutation burden and mismatch repair deficiency. Clinical Cancer Research, 29:4770-4783, May 2023. URL: https://doi.org/10.1158/1078-0432.ccr-23-0411, doi:10.1158/1078-0432.ccr-23-0411. This article has 77 citations and is from a highest quality peer-reviewed journal.
(das2023efficacyofnivolumab pages 8-9): Anirban Das, Uri Tabori, Lauren C. Sambira Nahum, Natalie B. Collins, Rebecca Deyell, Rina Dvir, Cecile Faure-Conter, Timothy E. Hassall, Jane E. Minturn, Melissa Edwards, Elissa Brookes, Vanessa Bianchi, Adrian Levine, Simone C. Stone, Sumedha Sudhaman, Santiago Sanchez Ramirez, Ayse B. Ercan, Lucie Stengs, Jill Chung, Logine Negm, Gad Getz, Yosef E. Maruvka, Birgit Ertl-Wagner, Pamela S. Ohashi, Trevor Pugh, Cynthia Hawkins, Eric Bouffet, and Daniel A. Morgenstern. Efficacy of nivolumab in pediatric cancers with high mutation burden and mismatch repair deficiency. Clinical Cancer Research, 29:4770-4783, May 2023. URL: https://doi.org/10.1158/1078-0432.ccr-23-0411, doi:10.1158/1078-0432.ccr-23-0411. This article has 77 citations and is from a highest quality peer-reviewed journal.
(shuen2025developmentofa pages 110-114): AY Shuen. Development of a diagnostic functional assay for constitutional mismatch repair deficiency. Unknown journal, 2025.
(vasen2026theimpactof pages 1-2): Hans F. A. Vasen, Katharina Wimmer, Mariëtte van Kouwen, Léa Guerrini-Rousseau, Daniela Gattini, Lucie Stengs, Uri Tabori, Chrystelle Colas, and Anirban Das. The impact of international care networks on the clinical management of constitutional mismatch repair deficiency (cmmrd): a review of recent developments. Jun 2026. URL: https://doi.org/10.1007/s10689-026-00584-x, doi:10.1007/s10689-026-00584-x. This article has 0 citations and is from a peer-reviewed journal.
(shuen2025developmentofa pages 23-26): AY Shuen. Development of a diagnostic functional assay for constitutional mismatch repair deficiency. Unknown journal, 2025.
(vasen2026theimpactof pages 6-8): Hans F. A. Vasen, Katharina Wimmer, Mariëtte van Kouwen, Léa Guerrini-Rousseau, Daniela Gattini, Lucie Stengs, Uri Tabori, Chrystelle Colas, and Anirban Das. The impact of international care networks on the clinical management of constitutional mismatch repair deficiency (cmmrd): a review of recent developments. Jun 2026. URL: https://doi.org/10.1007/s10689-026-00584-x, doi:10.1007/s10689-026-00584-x. This article has 0 citations and is from a peer-reviewed journal.
(shuen2025developmentofa pages 114-117): AY Shuen. Development of a diagnostic functional assay for constitutional mismatch repair deficiency. Unknown journal, 2025.
(vasen2026theimpactof pages 8-9): Hans F. A. Vasen, Katharina Wimmer, Mariëtte van Kouwen, Léa Guerrini-Rousseau, Daniela Gattini, Lucie Stengs, Uri Tabori, Chrystelle Colas, and Anirban Das. The impact of international care networks on the clinical management of constitutional mismatch repair deficiency (cmmrd): a review of recent developments. Jun 2026. URL: https://doi.org/10.1007/s10689-026-00584-x, doi:10.1007/s10689-026-00584-x. This article has 0 citations and is from a peer-reviewed journal.
(shuen2025developmentofa pages 6-12): AY Shuen. Development of a diagnostic functional assay for constitutional mismatch repair deficiency. Unknown journal, 2025.
(munteanu2025genotypephenotypecorrelationsin pages 1-2): Cătălin Vasile Munteanu, Diana Luisa Lighezan, Alexandru Capcelea, Adela Chiriță-Emandi, and Adrian Pavel Trifa. Genotype-phenotype correlations in pms2-associated constitutional mismatch repair deficiency: a systematic literature review. Oncology Reviews, Nov 2025. URL: https://doi.org/10.3389/or.2025.1679576, doi:10.3389/or.2025.1679576. This article has 1 citations.
(wimmer2017connectionsbetweenconstitutional pages 4-6): K. Wimmer, T. Rosenbaum, and L. Messiaen. Connections between constitutional mismatch repair deficiency syndrome and neurofibromatosis type 1. Clinical Genetics, 91:507-519, Apr 2017. URL: https://doi.org/10.1111/cge.12904, doi:10.1111/cge.12904. This article has 144 citations and is from a peer-reviewed journal.
(shuen2025developmentofa pages 34-37): AY Shuen. Development of a diagnostic functional assay for constitutional mismatch repair deficiency. Unknown journal, 2025.

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Pathophysiology

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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

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Constitutional Mismatch Repair Deficiency (CMMRD): comprehensive disease-characteristics report

Executive summary

1. Disease information

Definition and category

Identifiers and synonyms

2. Etiology, risk, protection, and gene–environment interaction

Causal factors

Genetic risk factors

Environmental and lifestyle factors

Protective factors

3. Phenotypes and quality-of-life burden

Major phenotypes

Quality of life

4. Genetic and molecular information

Causal genes and proteins

Variant classes and interpretation

Modifier genes and chromosomal abnormalities

Epigenetics and omics

5. Environmental information

6. Mechanism and pathophysiology

Causal chain

Experimental vulnerabilities

7. Anatomical structures affected

8. Temporal development and course

9. Inheritance and population

10. Diagnostics

When to suspect CMMRD

Recommended testing workflow

Differential diagnosis

11. Outcome and prognosis

12. Treatment

General strategy

Immune-checkpoint inhibition

Trials and experimental treatment

13. Prevention and surveillance

Primary prevention

Secondary prevention: consensus surveillance