PMM2-Congenital Disorder of Glycosylation (PMM2-CDG): Comprehensive Research Report

1. Disease Information

Overview

PMM2-CDG (formerly known as CDG-Ia or "carbohydrate-deficient glycoprotein syndrome type I") is an autosomal recessive inborn error of metabolism caused by biallelic pathogenic variants in PMM2, the gene encoding phosphomannomutase 2. It is the most common and best-characterized disorder of N-linked protein glycosylation in humans, accounting for roughly 60% of all diagnosed CDG cases, with more than 1,000 patients reported worldwide across >900 published cases (Frontiers in Endocrinology, 2025; Orphanet). PMM2-CDG is a multisystem disease: because essentially all secreted and membrane glycoproteins require N-glycosylation for correct folding, stability, and trafficking, defective glycosylation produces a broad, variable phenotype spanning neurological, gastrointestinal, hepatic, cardiac, renal, coagulation, endocrine, and skeletal systems.

Key Identifiers

Source: OMIM #212065; GeneReviews — PMM2-CDG

Synonyms / Alternative Names

• CDG-Ia / CDG1A / CDGS1
• Congenital disorder of glycosylation type Ia
• Phosphomannomutase 2 deficiency
• Carbohydrate-deficient glycoprotein syndrome type I (historical)
• Jaeken syndrome (historical eponym, after Jaak Jaeken who first described the disorder in 1980)

Data Source Character

Most published information derives from aggregated case series and multicenter cohort studies (e.g., the 96-patient French cohort, the 50-patient coagulation cohort, and international registries such as the Frontiers in Congenital Disorders of Glycosylation Consortium (FCDGC) natural history study), rather than raw individual EHR mining. Orphanet and OMIM entries synthesize published cohort and case-report literature.

2. Etiology

Disease Causal Factors

PMM2-CDG is caused exclusively by biallelic (compound heterozygous or homozygous) pathogenic variants in PMM2 (chromosome 16p13.2), which encodes phosphomannomutase 2, the enzyme that isomerizes mannose-6-phosphate (M6P) to mannose-1-phosphate (M1P) — an essential precursor for GDP-mannose and dolichol-phosphate-mannose synthesis required for N-glycan assembly (GeneReviews; PMID 22956764). There is no known environmental or infectious causal pathway — this is a purely monogenic mechanistic disease.

Genetic Risk Factors

• Causal variants: >120 pathogenic PMM2 variants have been reported (missense predominating, with fewer nonsense, splice-site, and small indel variants). Missense predominance reflects that complete loss of PMM2 activity is embryonic lethal in humans (consistent with mouse data below).
• Most common pathogenic alleles:
• c.422G>A (p.Arg141His / R141H) — the single most frequent PMM2-CDG allele. It has no detectable residual enzymatic activity and has never been observed in homozygosity in a living patient, implying that R141H homozygosity is embryonic/fetal lethal. gnomAD heterozygote carrier frequency is ~0.39% overall (891/224,376), rising to ~0.84% in the Finnish subpopulation (ClinVar VCV000007706).
• c.357C>A/G (p.Phe119Leu / F119L) — retains ~25% residual enzymatic activity, likely due to impaired dimerization; among 18 Danish CDG-Ia patients, F119L together with R141H accounted for 88% of alleles.
• The compound heterozygote genotype R141H/F119L is the most frequent genotype worldwide and is the genotype modeled in the leading mouse model (see Section 15).
• Genotype–phenotype correlation is imperfect but broadly: genotypes combining two severe/null alleles are rarer (often embryonic lethal or very severe), while a null allele paired with a hypomorphic (partial-activity) allele like F119L is compatible with survival across the full clinical spectrum (PMC12042452, "genotype–phenotype correlations in PMM2-CDG").

Protective/Modifier Factors

No validated protective genetic variants are established. Hypomorphic alleles with higher residual PMM2 activity (vs. null alleles) act as de facto "protective" modifiers of severity in trans-heterozygotes.

Environmental Risk Factors

None established — this is a purely genetic (autosomal recessive, biallelic) disease with no known environmental, occupational, or lifestyle contributors to disease occurrence. (Environmental factors can modulate the severity of acute decompensations — e.g., febrile illness, surgery, and fasting can precipitate stroke-like episodes, bleeding, or hypoglycemic crises — but do not cause the underlying disease.)

Gene-Environment Interactions

Not a primary feature of this monogenic disorder; the closest analog is that intercurrent physiologic stress (infection, fasting, surgery) interacts with baseline coagulopathy and hypoglycemia susceptibility to precipitate acute complications.

3. Phenotypes

PMM2-CDG phenotypes span a wide severity spectrum from neonatal death to mild adult presentation. Below are phenotype categories with suggested HPO terms.

Neurological

Source: PMC12042452; PMC10530657 (coagulation cohort); Neurology case report on unusual eye movements.

Dysmorphic / Physical

• Inverted/hypoplastic nipples and abnormal fat distribution/lipodystrophy (suprapubic fat pads, "orange-peel" skin over buttocks) — considered classic, near-pathognomonic early signs (HP:0003186 Inverted nipples; HP:0009026 Abnormal subcutaneous fat tissue distribution).
• Facial dysmorphism (large ears, high forehead) — HP:0000238 etc.

Gastrointestinal/Hepatic

• Failure to thrive, feeding difficulty/vomiting, enteropathy — HP:0001508, HP:0011968
• Hepatomegaly, elevated transaminases, hepatic fibrosis — HP:0002240, HP:0002910
• Recurrent pancreatitis (subset) — HP:0001733

Cardiac

• Pericardial effusion, hypertrophic cardiomyopathy — HP:0001636, HP:0001700

Renal

• Nephrotic-range proteinuria/renal cysts in a subset — HP:0000100

Hematologic (laboratory abnormalities)

• Coagulopathy from hypoglycosylated clotting factors/inhibitors — see Section 6/10. Antithrombin deficiency is the single most common lab abnormality, found in 83.3% of patients, with activity <50% (normal 80–130%) in 62.5% of the cohort (PMC10530657). Also common: Factor XI deficiency, protein C deficiency, protein S and Factor IX deficiency (less common).
• Bleeding symptoms in 16%; thrombosis in 10% of a large cohort.

Endocrine (laboratory/clinical)

• Hypothyroidism (glycosylation-dependent TSH/thyroglobulin dysfunction)
• Hyperinsulinemic hypoglycemia — reported in ~2.5% of a 933-patient PMM2-CDG review, with hyperinsulinism confirmed in 43% of the hypoglycemic subgroup; diazoxide-responsive in most treated cases (PMC7012739; PMC9680396)
• Hypogonadotropic hypogonadism / abnormal FSH-LH glycoforms in females (elevated FSH from birth, later ovarian failure)
• Growth hormone axis abnormalities (short stature) — reflecting IGFBP-3 hypoglycosylation (Frontiers Endocrinology 2025)

Phenotype Characteristics

• Onset: Congenital/neonatal-infantile in the classic (and most common) presentation; a distinct late-onset/adult phenotype is increasingly recognized, sometimes presenting primarily with cerebellar ataxia or stroke-like episodes without the classic infantile multisystem picture (Orphanet Journal of Rare Diseases, 29 French adult patients).
• Severity/progression: Highly variable — ranges from infants who die in the first year of life (severe multisystem/hydrops-like presentation with cardiac/renal/hepatic failure) to mildly affected adults with only ataxia and mild cognitive involvement. Three recognized natural-history stages: (1) infantile multisystem type, (2) late-infantile/childhood ataxia–intellectual disability type (ages 3–10), (3) adult stable disability type (GeneReviews).
• Course: Neurological (cerebellar) disease is generally non-progressive after early childhood plateau in survivors, though cerebellar atrophy on imaging can progress radiographically even as clinical function stabilizes (PMC8360885, activities of daily living correlates).

Quality of Life

A cross-sectional adaptive functioning study found PMM2-CDG substantially impacts adaptive functioning across the lifespan and imposes significant parental stress, particularly around motor and communication domains (Scientific Reports, 2023, PMC10739927). A patient-reported outcomes study (Orphanet J Rare Dis, 2022) identified fatigue, gastrointestinal symptoms, and mobility/ataxia as top patient/caregiver concerns not always captured by clinician-rated instruments (OJRD 2022).

4. Genetic/Molecular Information

Causal Gene

• PMM2 (phosphomannomutase 2), HGNC:9115, chromosome 16p13.2, OMIM *601785. Encodes a cytosolic enzyme that functions as a homodimer.

Variant Classification and Type

• Predominantly missense variants (reflecting embryonic lethality of complete null/null genotypes); fewer nonsense, frameshift, and splice-site alleles.
• ClinVar/ACMG classification: Both R141H and F119L are classified Pathogenic (ClinVar RCV000008145).
• Functional consequences are predominantly hypomorphic loss-of-function — destabilizing the protein fold/reducing dimerization/catalytic efficiency rather than classic null alleles, consistent with the observation that complete biallelic null genotypes are not observed in living patients.

Allele Frequency

• R141H heterozygote frequency in gnomAD: ~0.39% overall population, ~0.84% in Finnish subpopulation.
• Population-based carrier-frequency modeling (using gnomAD allele frequencies across all known pathogenic PMM2 alleles) estimates a theoretical birth prevalence as high as 1:20,000, though observed/diagnosed birth prevalence is substantially lower (1:77,000–1:286,000 in some analyses; see Section 9) — a gap attributed to embryonic lethality of severe genotype combinations and underdiagnosis (PMC8383291).

Somatic vs. Germline

Exclusively germline — PMM2-CDG is a classic Mendelian recessive disorder with no somatic mosaicism subtype reported as clinically significant.

Modifier Genes

MPI (mannose phosphate isomerase) activity ratio relative to PMM2 modulates response to mannose-based therapies (the PMM2:MPI ratio determines whether supplemental mannose is shunted productively into the pathway or diverted) — this is a pharmacologically relevant "modifier" relationship rather than a classic genetic modifier locus (treatment reviews, ScienceDirect).

Chromosomal Abnormalities

Not applicable — PMM2-CDG is caused by point mutations/small indels in a single gene, not large structural/chromosomal rearrangements.

Molecular/Structural Insights

A 2025 comprehensive update integrates molecular dynamics and structural analysis of PMM2 mutant proteins to refine genotype-phenotype correlations, showing that many pathogenic variants destabilize the dimer interface or active site rather than abolishing catalysis outright (PMC12042452).

5. Environmental Information

• Environmental/toxin factors: None causally implicated; this is a purely monogenic disease.
• Lifestyle factors: Not causal, though diet (mannose/galactose supplementation trials) is relevant therapeutically (Section 12).
• Infectious agents: Not causal of the underlying disease, but febrile/infectious illness is a recognized trigger of acute decompensation — stroke-like episodes, coagulopathic bleeding events, and metabolic crises are frequently precipitated by intercurrent infection (PMC10530657).

6. Mechanism / Pathophysiology

Core Molecular Defect

PMM2 catalyzes the reversible isomerization of mannose-6-phosphate (M6P) → mannose-1-phosphate (M1P) in the cytosol. M1P is the substrate for GDP-mannose pyrophosphorylase, generating GDP-mannose, which is required both directly (for cytosolic-face LLO assembly) and via conversion to dolichol-phosphate-mannose (Dol-P-Man) (for luminal-face assembly) in construction of the dolichol-linked oligosaccharide (LLO) precursor, Glc3Man9GlcNAc2-PP-Dolichol, in the endoplasmic reticulum. Deficient PMM2 activity lowers the M1P pool, causing truncated/incomplete LLO synthesis, which the oligosaccharyltransferase (OST) complex then transfers inefficiently — or not at all — onto nascent glycoproteins' Asn-X-Ser/Thr sequons (PMID 22956764; GeneReviews).

Cellular Processes / Protein Dysfunction

The resulting global hypoglycosylation compromises protein folding, ER quality control, stability, and trafficking across essentially every secreted/membrane glycoprotein — explaining the multisystem phenotype. Recent work has extended understanding to specific immune signaling pathways: hypoglycosylation impairs the TNFα–TNFR1 signaling axis, implicating aberrant cytokine signaling in PMM2-CDG immunopathology (PMC12488661, "Immunopathology in PMM2-CDG").

Suggested Ontology Terms

• GO:0004615 (phosphomannomutase activity)
• GO:0006487 (protein N-linked glycosylation)
• GO:0006506 (GPI anchor biosynthetic process, related pathway)
• GO:0097502 (mannosylation)
• Cellular component: GO:0005783 (endoplasmic reticulum), GO:0005793 (ER-Golgi intermediate compartment)

Endocrine Mechanism

Multiple hormone-axis glycoproteins are directly affected: thyroglobulin, TSH, prolactin, FSH, LH, and IGFBP-3 are all N-glycosylated and functionally impaired by hypoglycosylation, explaining hypothyroidism, gonadal dysfunction, and growth abnormalities as direct downstream consequences of the core enzymatic defect rather than secondary organ damage (Frontiers in Endocrinology 2025).

Coagulation Mechanism

Antithrombin, protein C, protein S, and several clotting factors (XI, IX) are glycoproteins whose hypoglycosylation directly reduces their circulating activity/half-life, producing a mixed pro- and anti-thrombotic coagulopathy that can manifest as either spontaneous bleeding or thrombosis/stroke-like episodes depending on which factor imbalance predominates at a given time (de la Morena-Barrio et al., J Thromb Haemost, PMID referenced in search; PMC10530657).

Causal Chain Summary (for pathograph modeling)

1. Biallelic PMM2 pathogenic variants → decreased phosphomannomutase 2 enzymatic activity (molecular scale; LOSS_OF_FUNCTION/GAIN... not applicable — hypomorphic)
2. → Decreased M1P / GDP-mannose / Dol-P-Man pools (molecular)
3. → Truncated lipid-linked oligosaccharide (LLO) synthesis (molecular)
4. → Global protein N-hypoglycosylation (cellular)
5. → Downstream organ-specific consequences: cerebellar granule cell/neuronal developmental disruption (neurological), hypoglycosylated coagulation factors (coagulopathy), hypoglycosylated hormone/hormone receptors (endocrinopathy), hepatocyte dysfunction (hepatic), etc.

Molecular/Omics Profiling

A 2025 mouse model study (see Section 15) using single-cell/bulk transcriptomic profiling of cerebellar tissue revealed a neurodevelopmental origin of PMM2-CDG brain pathology — i.e., cerebellar granule cell development is disrupted prenatally/perinatally rather than purely via later neurodegeneration (bioRxiv/PMC12157701, "Novel mouse model reveals neurodevelopmental origin of PMM2-CDG brain pathology").

7. Anatomical Structures Affected

Organ Level

• Primary: Cerebellum/CNS, peripheral nerves, eyes; skin/subcutaneous fat; liver; heart; GI tract; kidney; endocrine glands (thyroid, gonads, pancreatic islets)
• Body systems: Nervous, endocrine, digestive, cardiovascular, renal, hematologic/coagulation, integumentary, skeletal
• Suggested UBERON: cerebellum (UBERON:0002037), liver (UBERON:0002107), heart (UBERON:0000948), kidney (UBERON:0002113), thyroid gland (UBERON:0002046)

Tissue and Cell Level

• Cerebellar granule neurons and Purkinje cells (developmental/degenerative target)
• Hepatocytes (fibrosis, dysfunction)
• Vascular endothelium (coagulation factor synthesis)
• Suggested Cell Ontology: cerebellar granule cell (CL:0000643), hepatocyte (CL:0000182)

Subcellular Level

• Endoplasmic reticulum (site of N-glycosylation/LLO assembly) — GO:0005783
• Cytosol (site of PMM2 enzymatic reaction) — GO:0005829
• Golgi apparatus (downstream glycan processing) — GO:0005794

Localization

Bilateral, symmetric cerebellar hypoplasia/atrophy (not lateralized); systemic (multi-organ) involvement rather than focal.

8. Temporal Development

• Onset: Congenital in the classic form (dysmorphic features and hypotonia apparent at or shortly after birth); a late-onset/adult-presenting phenotype exists with initial symptoms (ataxia, peripheral neuropathy) emerging in adolescence or adulthood.
• Onset pattern: Chronic/insidious for the baseline multisystem disease, punctuated by acute stroke-like episodes, bleeding, and hypoglycemic crises that are typically provoked by infection, fasting, or surgical stress.
• Progression / stages (per GeneReviews):
• Infantile multisystem stage (birth–~3 years): failure to thrive, hypotonia, coagulopathy, hepatic/cardiac/renal involvement; highest mortality risk period.
• Late-infantile/childhood ataxia–intellectual disability stage (ages 3–10): cerebellar ataxia and cognitive delay become the dominant clinical picture as acute multisystem crises recede.
• Adult stable disability stage: chronic, largely non-progressive neurological disability (ataxia, peripheral neuropathy, mild cognitive impairment) with continued risk of episodic coagulopathic/stroke-like events.
• Rate: Highly variable across patients — from neonatal death to stable lifelong mild disability.
• Remission patterns: No spontaneous remission of the underlying enzymatic defect; individual acute complications (hypoglycemia, effusions) can resolve with supportive/targeted treatment.
• Critical periods: Fetal/early neonatal period is critical for cerebellar developmental injury (per the neurodevelopmental-origin mouse data); early childhood is the period of highest risk for life-threatening multisystem crises.

9. Inheritance and Population

Epidemiology

Estimates vary substantially by methodology: - A 2025 ScienceDirect analysis calculated an incidence estimate of 1 in 33,576 for North America and Europe combined (1 in 40,375 in North America; 1 in 29,043 in Europe), predicting ~303 live births/year across both regions (ScienceDirect, "Incidence and prevalence of PMM2-CDG: Past, present, and future"). - Allele-frequency-based (gnomAD) theoretical birth prevalence estimates run as high as 1:20,000, but later empirical estimates from diagnosed cohorts are lower, 1:77,000 to 1:286,000 (PMC8383291) — the gap likely reflects embryonic lethality of severe genotypes and under-ascertainment/underdiagnosis. - Regional variation: most commonly reported/diagnosed in Denmark and other Scandinavian countries; estimated combined CDG prevalence in the Saudi population ~14/million; in Poland ~1/million.

Inheritance Pattern

Autosomal recessive. Both parents are obligate heterozygous carriers; recurrence risk is 25% per pregnancy for unaffected-carrier parents.

Penetrance / Expressivity

Full penetrance for the biochemical/glycosylation defect in biallelic carriers, but highly variable clinical expressivity — genotype only partially predicts phenotype severity, and even patients sharing the identical genotype (e.g., R141H/F119L) can show a wide range of clinical severity, implicating additional genetic/epigenetic/stochastic modifiers not yet fully characterized.

Genetic Anticipation

Not reported — not a repeat-expansion disorder.

Germline Mosaicism

Not specifically documented as a recurring feature of PMM2-CDG in the literature reviewed.

Founder Effects

R141H shows elevated carrier frequency in the Finnish population (gnomAD), consistent with a founder or drift effect in Northern European populations; the disease is disproportionately reported in Scandinavian cohorts.

Consanguinity

Increases risk in populations/families with elevated consanguinity rates, as for any autosomal recessive disorder, though PMM2-CDG is not specifically enriched in classically consanguineous populations relative to Northern European populations where it is most reported.

Carrier Frequency

Estimated from gnomAD population allele frequencies; R141H alone carrier frequency ~0.39% (general), ~0.84% (Finnish).

Sex Ratio / Age Distribution

No strong sex bias reported for disease occurrence (autosomal, so expected 1:1), though certain endocrine manifestations (hypergonadotropic hypogonadism, elevated FSH) are specifically described in affected females. Age distribution spans neonatal death through adulthood as described above.

10. Diagnostics

First-Line Biochemical Screening

• Serum transferrin isoelectric focusing (IEF) / carbohydrate-deficient transferrin (CDT) analysis: the standard first-line screening test for N-glycosylation disorders including PMM2-CDG. PMM2-CDG produces a characteristic Type I transferrin isoform pattern (loss of entire N-glycan chains, distinguishing it from Type II patterns seen in Golgi-processing CDGs) (Mayo Clinic Labs test catalog).
• Apolipoprotein C-III isoform analysis: used as a complementary/confirmatory first-line screen alongside transferrin isoform analysis, particularly useful in cases where transferrin results are equivocal or in liver disease (which can confound transferrin glycoform interpretation).
• Sensitivity: a 2024 study reported 94% overall sensitivity of transferrin isoform analysis for PMM2-CDG detection (PMID 39216211) — meaning ~6% of cases could be missed by this screen alone, reinforcing the need for molecular confirmation when clinical suspicion is high despite normal/equivocal screening.

Enzyme Assay

Phosphomannomutase enzymatic activity assay in leukocytes or cultured fibroblasts, used to confirm pathogenicity when genetic variants are of uncertain significance.

Molecular Genetic Testing

• Definitive diagnosis: identification of biallelic pathogenic/likely pathogenic PMM2 variants by sequencing (single-gene sequencing, CDG-focused gene panel, or exome/genome sequencing).
• Recommended approach per GeneReviews: given the broad, nonspecific multisystem presentation, a metabolic/CDG gene panel or exome sequencing is often the practical first-tier molecular test alongside biochemical screening.

Imaging

• Brain MRI: cerebellar hypoplasia at birth progressing to cerebellar atrophy — a key supportive radiological finding correlating with ataxia severity (PMC8360885).

Differential Diagnosis

Other CDG subtypes (particularly MPI-CDG/CDG-Ib, which is treatable with mannose and must be distinguished), other congenital ataxias/cerebellar hypoplasia syndromes, other causes of neonatal coagulopathy/hepatopathy, and other lipodystrophy syndromes.

Screening

No universal newborn screening program currently exists for PMM2-CDG (unlike some other IEMs); diagnosis is typically clinically triggered. Carrier screening panels for PMM2 exist commercially (e.g., Myriad Foresight Carrier Screen) for reproductive risk assessment in at-risk populations.

11. Outcome/Prognosis

Mortality

Historically, mortality in the first years of life has been reported at roughly 20% in the infantile-onset multisystem form, primarily from cardiac, hepatic, or coagulopathic/infectious complications, though outcomes have improved with modern supportive care. Long-term follow-up cohorts (e.g., the French cohort of 96 patients) demonstrate that many patients who survive the high-risk infantile period stabilize into the chronic ataxia/intellectual disability phenotype with a near-normal lifespan, though episodic life-threatening events (stroke-like episodes, severe coagulopathy) remain a lifelong risk (Genetics in Medicine, "Long-term follow-up in PMM2-CDG").

Morbidity

Chronic ataxia, peripheral neuropathy, and mild-to-moderate intellectual disability are the dominant sources of long-term morbidity and reduced adaptive functioning/quality of life, as documented in cross-sectional adaptive functioning studies (PMC10739927).

Complications

Recurrent stroke-like episodes (18% of a large cohort), spontaneous bleeding (16%), thrombosis (10%), pericardial effusion, hyperinsulinemic hypoglycemia, hypothyroidism, and hepatic fibrosis are recognized long-term/recurrent complications requiring ongoing surveillance.

Prognostic Factors

Genotype (presence of two null/severe alleles vs. a hypomorphic allele) partially correlates with severity; degree of residual PMM2 enzymatic activity is a key biochemical prognostic correlate. Early recognition and aggressive management of acute coagulopathic/metabolic crises appears to improve survival through the highest-risk infantile period.

12. Treatment

There are currently no FDA/EMA-approved disease-modifying therapies for PMM2-CDG (Frontiers search summary; EMA orphan designation EU/3/18/2047 reflects ongoing drug development, not an approved product). Management is currently supportive and symptomatic, with several investigational disease-modifying approaches in active clinical trials.

Supportive/Symptomatic Care (current standard of care)

• Multidisciplinary management: physical/occupational/speech therapy for ataxia and developmental delay (NCIT:C15302 Physical Therapy; NCIT:C159273 Speech Therapy; NCIT:C121351 Occupational Therapy)
• Nutritional support/feeding interventions for failure to thrive (NCIT:C15447 Dietary Intervention)
• Coagulopathy management: fresh frozen plasma or factor/antithrombin concentrate replacement during acute bleeding, thrombotic, or peri-surgical periods
• Diazoxide for hyperinsulinemic hypoglycemia (successful in 7/10 treated patients in one series) (NCIT:C15986 Pharmacotherapy; therapeutic_agent CHEBI diazoxide)
• Thyroid hormone replacement for hypothyroidism (NCIT:C15986 Pharmacotherapy)
• Cardiac monitoring/management of pericardial effusion and cardiomyopathy
• Genetic counseling for families (NCIT:C15240)

Investigational Disease-Modifying Therapies

1. Epalrestat (repurposed aldose reductase inhibitor) — furthest along in clinical development: - Originally developed for diabetic neuropathy in Japan; repurposed by Perlara/collaborators based on the hypothesis that aldose reductase inhibition redirects glucose flux to increase mannose/GDP-mannose availability. - A Phase III, randomized, double-blind, placebo-controlled trial in pediatric PMM2-CDG patients (Mayo Clinic-led) enrolled 38 subjects, closed enrollment November 2023, and as of March 2024 all placebo subjects were permitted to cross over to open-label epalrestat at their 15-month visit (PR Newswire, March 2024; Mayo Clinic trial page). - Trial design assesses safety, tolerability, and clinical/metabolic improvement (oral, three-times-daily dosing). - Therapeutic modality: SMALL_MOLECULE; NCIT treatment_term: Pharmacotherapy (NCIT:C15986).

2. GLM101 (mannose-1-phosphate replacement therapy) — Glycomine: - A glycoprotein-based mannose-1-phosphate replacement therapy designed to bypass the deficient PMM2 enzymatic step by delivering M1P directly into cells, restoring downstream GDP-mannose/Dol-P-Man pathway flux. - Phase 1 study: NCT05549219 ("24-Week Study to Assess the PD, Safety, Tolerability, and PK of GLM101"). - Phase 2a open-label results: among 9 adult/adolescent patients, treatment produced an average 11.9-point improvement on the ICARS (International Cooperative Ataxia Rating Scale) over 24 weeks, with a favorable safety profile (no serious adverse events; only mild-moderate AEs) (BioSpace, Glycomine Phase 2 results). - Phase 2b "POLAR" trial: global, randomized, double-blind, placebo-controlled study; enrollment of 43 patients across 15 sites completed as of April 2026, with topline data expected Q4 2026 (Glycomine press release, April 2026). - Therapeutic modality: PROTEIN_REPLACEMENT (or classify as small-molecule/metabolite replacement depending on schema fit).

3. Dietary mannose supplementation: - Corrects hypoglycosylation in PMM2-deficient fibroblasts in vitro, but short-term oral/IV mannose monotherapy trials in patients have shown inconsistent/largely unsuccessful clinical results (Orphanet J Rare Dis, "Unsuccessful intravenous D-mannose treatment in PMM2-CDG"). One study found that after >1 year of dietary mannose supplementation, a majority of patients showed improved glycosylation biomarkers, suggesting a longer treatment horizon may be needed (Orphanet J Rare Dis 2020, "Dietary mannose supplementation"); efficacy is thought to depend on the individual patient's PMM2:MPI enzymatic activity ratio.

4. Pharmacological chaperones/proteostasis regulators (preclinical): - Screening identified compounds (8 candidates, 4 confirmed as functional chaperones) that increase thermal stability of destabilized/oligomerization-defective PMM2 mutant proteins and increase residual PMM enzymatic activity in cell models — proof-of-concept for a chaperone therapy strategy, not yet in clinical trials (ResearchGate, "Pharmacological Chaperoning: A Potential Treatment for PMM2-CDG").

5. AAV gene replacement therapy (preclinical): - A 2025 study in the novel Pmm2 mouse model showed AAV-based gene replacement therapy prevented and halted manifestation of abnormal neurological phenotypes when administered appropriately, providing strong preclinical proof-of-concept for gene therapy in PMM2-CDG (Gene Therapy (Nature), 2025). Therapeutic modality: GENE_THERAPY.

Treatment Algorithm

No formal consensus treatment algorithm exists beyond symptomatic/supportive management protocols and acute-crisis management guidance for coagulopathy (multicentric study on hemostasis anomalies and acute management, ScienceDirect).

13. Prevention

• Primary prevention: Not applicable in the classic sense (no modifiable environmental cause); the main "primary prevention" lever is reproductive genetic counseling and carrier screening for at-risk couples/families, with options for prenatal diagnosis (chorionic villus sampling/amniocentesis with molecular PMM2 testing) or preimplantation genetic testing in known-carrier couples.
• Secondary prevention: No population newborn screening program currently exists (unlike some IEMs on standard newborn screening panels); early clinical recognition based on classic phenotype (inverted nipples, abnormal fat pads, cerebellar hypoplasia, hypotonia) and prompt biochemical/molecular testing shortens diagnostic delay.
• Tertiary prevention: Proactive multidisciplinary surveillance (coagulation panels, thyroid function, echocardiography, renal function, glucose monitoring) to catch and manage complications (thrombosis, effusions, hypoglycemia) before they become life-threatening is standard practice in specialized CDG centers.
• Carrier screening: Commercially available (e.g., Myriad Foresight Carrier Screen) for reproductive planning.
• Genetic counseling: Central to family management given 25% recurrence risk in each subsequent pregnancy for carrier couples.

14. Other Species / Natural Disease

PMM2-CDG is not known to occur as a naturally-occurring inherited disease in non-human species (unlike some other Mendelian disorders with veterinary counterparts in OMIA). PMM2 orthologs are broadly conserved (mouse Pmm2, zebrafish pmm2), which supports engineered animal modeling (Section 15) rather than natural disease occurrence.

15. Model Organisms

Mouse Models

• Original hypomorphic mouse (Pmm2^R137H/F118L^): An earlier attempt at a mouse model harboring the mouse-orthologous equivalent of the human R141H/F119L compound heterozygous genotype resulted in complete embryonic lethality, making it unsuitable for postnatal disease study (HMG, Oxford Academic).
• Viable hypomorphic model (Pmm2^R137H/F115L^): A subsequent, refined hypomorphic mouse line was generated that is viable and recapitulates multiple PMM2-CDG disease features, corresponding to the common human R141H/F119L genotype.
• Tamoxifen-inducible conditional knockout: A newer, widely tissue-deficient Pmm2 knockout mouse (inducible) was developed and characterized to reveal distinct neurological phenotypes relevant to human PMM2-CDG.
• 2025 novel mouse model — neurodevelopmental origin study: Demonstrated that PMM2-CDG cerebellar/brain pathology has a neurodevelopmental origin (disrupted early brain development) rather than purely progressive neurodegeneration, and the same model was used to show that AAV-based gene replacement therapy prevents/halts abnormal neurological phenotypes when given at the appropriate developmental window (PMC12157701; Gene Therapy 2025).
• Prior hypomorphic alleles tested have historically fallen into two unhelpful extremes: too mild (no discernible phenotype) or too severe (embryonic lethal) — underscoring the difficulty of modeling a disease where complete loss-of-function is not compatible with survival.

Zebrafish Models

• Morpholino knockdown model (pmm2 morphants): Reproduces PMM2-CDG-relevant developmental abnormalities including craniofacial defects and impaired motility linked to altered motor neurogenesis in the spinal cord; global N-glycosylation and LLO levels are reduced, directly recapitulating the human biochemical defect (PMID 22956764; Molecular Biology of the Cell). This model specifically proposed a substrate-accumulation mechanism (in addition to simple substrate deficiency) contributing to altered neurogenesis.
• Zebrafish are noted as valuable complementary models given that >70% of human proteins (and ~82% of disease-associated human genes) have zebrafish orthologs.

Yeast Models

• Yeast (S. cerevisiae) models of phosphomannomutase deficiency have been used to study fundamental enzymatic and structural consequences of PMM2 pathogenic variants at the cell-biology level (bioRxiv preprint, "Yeast Models Of Phosphomannomutase 2 Deficiency").

Cellular/In Vitro Models

• HepG2 PMM2-CDG knockout cell line: A CRISPR-engineered hepatocyte-derived knockout model described as "a versatile platform for variant and therapeutic evaluation," useful for testing pharmacological chaperones and other small-molecule interventions in a human cellular context (ScienceDirect, 2024).
• Patient-derived fibroblasts: Widely used historically for enzyme activity confirmation and mannose-supplementation proof-of-concept studies.

Model Applications and Limitations

• Mouse and zebrafish models each recapitulate specific disease facets (neurodevelopmental/cerebellar pathology in mouse; craniofacial and motor neurogenesis defects in zebrafish) but no single model fully recapitulates the human multisystem phenotype (coagulopathy, endocrinopathy, hepatic, cardiac involvement together).
• The species-specific difficulty in generating a viable "null/null"-equivalent model mirrors the human observation that severe biallelic null genotypes are not viable, reinforcing the mechanistic inference of embryonic lethality for complete PMM2 loss across species.

Summary Table: Suggested Ontology Bindings for KB Curation

Sources

• Frontiers in Endocrinology (2025) — N-glycosylation and endocrine axes in PMM2-CDG
• Orphanet: PMM2-CDG (ORPHA:79318)
• 29 French adult patients with PMM2-CDG — Orphanet J Rare Dis
• Patient reported outcomes for PMM2-CDG — Orphanet J Rare Dis (2022)
• GARD — PMM2-CDG
• GeneReviews — PMM2-CDG
• OMIM #212065
• EMA Orphan Designation EU/3/18/2047
• A zebrafish model of PMM2-CDG — PMID 22956764
• Immunopathology in PMM2-CDG: TNFα-TNFR1 signalling — PMC12488661
• A comprehensive update of genotype–phenotype correlations in PMM2-CDG — PMC12042452
• Suspected central adrenal insufficiency in PMM2-CDG — PMC12511663
• Untangling adaptive functioning of PMM2-CDG — Scientific Reports (2023)
• Clinical and radiological correlates of ADLs in cerebellar atrophy in PMM2-CDG — PMC8360885
• Child Neurology case: PMM2-CDG with unusual eye movements — Neurology
• Perlara/Mayo Clinic — Phase III epalrestat trial crossover, PR Newswire (2024)
• Mayo Clinic — Epalrestat Phase I/III trial page
• ClinicalTrials.gov NCT05549219 — GLM101 PD/Safety Study
• ClinVar VCV000007706.122 (PMM2 R141H)
• ClinVar RCV000008145 (PMM2 c.422G>A / p.Arg141His)
• Incidence and prevalence of PMM2-CDG: Past, present, and future — ScienceDirect (2025)
• The Estimated Prevalence of N-Linked CDGs Across Populations — PMC8383291
• A mouse model of PMM2-CDG — Human Molecular Genetics
• HepG2 PMM2-CDG knockout model — ScienceDirect (2024)
• Novel mouse model reveals neurodevelopmental origin of PMM2-CDG brain pathology — PMC12157701
• AAV-based gene replacement therapy in PMM2-CDG mouse model — Gene Therapy (Nature, 2025)
• Dietary mannose supplementation in PMM2-CDG — Orphanet J Rare Dis (2020)
• Unsuccessful intravenous D-mannose treatment in PMM2-CDG — Orphanet J Rare Dis
• Pharmacological Chaperoning: A Potential Treatment for PMM2-CDG — ResearchGate
• Sensitivity of transferrin isoform analysis for PMM2-CDG — PMID 39216211
• Mayo Clinic Labs — CDG Overview/CDT test catalog
• Coagulation abnormalities in a prospective cohort of 50 patients with PMM2-CDG — PMC10530657
• Association between acute complications in PMM2-CDG and haemostasis anomalies — ScienceDirect
• Long-term follow-up in PMM2-CDG: are we ready to start treatment trials? — Genetics in Medicine (2018)
• Glycomine — Phase 2 GLM101 efficacy data, BioSpace
• Glycomine — Phase 2b POLAR study enrollment complete (April 2026)
• Hypoglycemia in CDG patients due to PMM2 mutations: hyperinsulinemic follow-up — PMC7012739
• Hyperinsulinemic Hypoglycemia Due to PMM2 Mutation with ARPKD — PMC9680396

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