rs80338701 — PMM2 F119L

PMM2 F119L — The Scandinavian CDG Allele and Its Compound Heterozygous Consequences

Every protein passing through the secretory pathway must be N-glycosylated¹¹ N-glycosylated
The attachment of sugar chains to asparagine residues in proteins destined for the endoplasmic reticulum lumen. N-glycosylation is required for correct folding, quality control, secretion, and function of hundreds of proteins including coagulation factors, hormones, enzymes, and membrane receptors before it can fold correctly and exit the cell. The enzyme phosphomannomutase 2 (PMM2) is indispensable for this process: it converts mannose-6-phosphate to mannose-1-phosphate, the precursor that feeds into GDP-mannose and ultimately into the lipid-linked oligosaccharide core. When PMM2 fails, hundreds of glycoproteins are synthesized in aberrant, incompletely glycosylated forms — and the consequences ripple through virtually every organ system.

The F119L variant (rs80338701, c.357C>A, p.Phe119Leu) is the second most common pathogenic PMM2 allele worldwide and the dominant CDG-causing allele in Scandinavian populations. In Danish PMM2-CDG patients, F119L accounts for approximately 43% of all disease-causing alleles²² approximately 43% of all disease-causing alleles
Briones P et al. J Inherit Metab Dis, 2002. F119L was entirely absent in the Spanish cohort, confirming strong Northern European geographic clustering. It is almost always found in compound heterozygosity³³ compound heterozygosity
Carrying two different pathogenic mutations — one on each chromosome copy — rather than two copies of the same mutation. In PMM2-CDG, F119L on one chromosome is almost always paired with R141H on the other. This is the single most common PMM2-CDG genotype globally with the R141H allele (rs28936415); homozygous F119L is presumed lethal, mirroring the embryonic lethality of R141H homozygosity.

The Mechanism

Phenylalanine 119 sits within the HAD-like catalytic domain of PMM2, a domain conserved across all known phosphomannomutase orthologs. The phenylalanine-to-leucine substitution removes an aromatic side chain and replaces it with a smaller aliphatic one, disrupting local hydrophobic packing within the protein core.

Andreotti et al. 2015⁴⁴ Andreotti et al. 2015
Andreotti G et al. Heterodimerization of Two Pathological Mutants Enhances the Activity of Human Phosphomannomutase2. PLoS One, 2015 made a critical discovery about the F119L/R141H heterodimer: the enzyme complex formed between these two mutant subunits retains catalytic activity approximately equal to wild-type/R141H heterodimers (the asymptomatic carrier state). However, the F119L/R141H heterodimer is substantially less stable both in vitro and in cellular conditions than wild-type heterodimers. This means the primary defect is not loss of catalytic function per se — it is accelerated protein degradation, leaving cells with too little PMM2 protein to sustain adequate glycosylation flux. This finding directly motivates pharmacological chaperone and proteostasis-modulating strategies as therapeutic approaches.

The Evidence

PMM2-CDG (formerly CDG type Ia, or Jaeken syndrome) was the first congenital disorder of glycosylation described and remains the most common, with over 1,000 documented cases. Matthijs et al. 1997⁵⁵ Matthijs et al. 1997
Matthijs G et al. Mutations in PMM2, a phosphomannomutase gene on chromosome 16p13. Nat Genet, 1997 identified PMM2 as the disease gene in the original cohort, with F119L among the founding mutations.

The most detailed clinical picture of the R141H/F119L compound heterozygous genotype comes from Kjaergaard et al. 2001⁶⁶ Kjaergaard et al. 2001
Kjaergaard S et al. Congenital disorder of glycosylation type Ia (CDG-Ia): phenotypic spectrum of the R141H/F119L genotype. Arch Dis Child, 2001: 25 patients, all compound heterozygous for R141H and F119L, showed severe neonatal presentations including hypotonia, failure to thrive, liver dysfunction, inverted nipples, and abnormal subcutaneous fat distribution. By infancy, cerebellar atrophy was present in 18 of the patients examined radiologically; supratentorial atrophy in 10. Strabismus was common. The mortality rate was 18%, with death occurring primarily in early childhood.

Systemic complications are now well-characterized. A prospective coagulation study De Graef et al. 2023⁷⁷ De Graef et al. 2023
De Graef D et al. Coagulation abnormalities in a prospective cohort of 50 patients with PMM2-CDG. Mol Genet Metab, 2023 found antithrombin deficiency in 83.3% of PMM2-CDG patients (antithrombin is a glycoprotein whose glycosylation is required for normal function), with Factor IX, Factor XI, Protein C, and Protein S also frequently reduced. Sixteen percent of patients experienced spontaneous bleeding; 10% developed thrombotic episodes. These abnormalities do not resolve over time and require lifelong monitoring.

Cardiac involvement carries the highest acute mortality risk. A 2024 surveillance study Zemet et al. 2024⁸⁸ Zemet et al. 2024
Zemet R et al. Cardiomyopathy in congenital disorders of glycosylation. Mol Genet Metab, 2024 documented hypertrophic cardiomyopathy in 10 of 17 CDG cardiac cases, with pericardial effusions progressing to cardiac tamponade causing two deaths. One patient required cardiac transplantation. Neurological follow-up data Muthusamy et al. 2024⁹⁹ Muthusamy et al. 2024
Genetic Medicine, 2024 confirm ataxia in 90.2%, myopathy in 82.4%, seizures in 56.9%, and peripheral neuropathy in 52.9%, with progressive cerebellar atrophy as the characteristic neuroimaging finding.

Practical Actions

For carriers (CA genotype): One functional PMM2 allele provides sufficient phosphomannomutase 2 activity for normal physiology. Carriers are clinically healthy. The significance is reproductive: if both partners carry any pathogenic PMM2 variant, each pregnancy carries a 25% risk of compound heterozygous CDG. F119L is predominantly a Northern European variant (virtually absent in African, East Asian, and Latino populations), so partner carrier risk depends strongly on ancestry.

For compound heterozygotes (disease state): PMM2-CDG requires multidisciplinary care. Transferrin isoelectric focusing (IEF) is the standard screening and monitoring test — it detects the characteristic Type I underglycosylated transferrin pattern. Surveillance must cover coagulation, cardiac function, thyroid, liver enzymes, and neurological development. There is no approved curative therapy, but emerging approaches include epalrestat — an aldose reductase inhibitor approved for diabetic neuropathy in Japan that was found to increase PMM2 enzyme activity 30–400% in patient fibroblasts¹⁰¹⁰ 30–400% in patient fibroblasts
Iyer S et al. Dis Model Mech, 2019 by redirecting glucose toward glucose-1,6-bisphosphate, an endogenous PMM2 stabilizer. Mannose supplementation has been trialed but clinical benefit is limited. Pharmacological chaperone strategies to stabilize the F119L/R141H heterodimer are under active investigation, motivated by the Andreotti 2015 finding that the heterodimer retains catalytic competence if it can be kept from degrading.

Interactions

F119L almost never acts as a standalone allele. Its clinical significance derives entirely from compound heterozygosity with a second pathogenic PMM2 allele — most commonly R141H (rs28936415). The Andreotti 2015 biochemical study showed that the F119L/R141H heterodimer is kinetically active but thermodynamically unstable, making protein stabilization the primary therapeutic target. Any individual who tests carrier-positive for F119L should have their partner tested for PMM2 variants, particularly R141H, since this compound combination is the single most common PMM2-CDG genotype in European populations.