The key conformance target is "Impaired ER Quality Control and Secretion of TSR-Containing Proteins". Peters plus syndrome enters through the B3GLCT-specific "Absent Glucose-beta-1,3-Fucose Disaccharide" node. Geleophysic dysplasia caused by the ADAMTSL2 p.Ser641Leu TSR3 variant enters through the "Absent O-Fucose" node because that variant eliminates O-fucosylation of the substrate itself; ADAMTSL2 secretion is POFUT2-dependent but was not reduced in B3GLCT-null cells. This asymmetry is why the module spans O-glycosylation broadly rather than treating POFUT2 and B3GLCT deficiency as equivalent. C-mannosylation and cell-specific chaperones can partly compensate, so conformance requires substrate- and tissue-relevant evidence rather than the mere presence of a TSR. Weill-Marchesani syndrome is intentionally not wired: ADAMTS17 secretion falls in POFUT2-null HEK293T cells but was minimally affected in Pofut2-deficient developing bone, so its human disease dependence remains unresolved. This module is distinct from congenital_disorder_of_glycosylation, whose shared hub is broad N-glycan hypoglycosylation, and from loss_of_proteostasis and er_protein_storage_disease, which model canonical recognition or retention of unfolded/misfolded proteins. Here POFUT2 recognizes the correctly folded TSR and glycosylation stabilizes that fold.
Which TSR-containing proteins actually require POFUT2- and B3GLCT-dependent glycosylation for secretion or extracellular function in the human tissues affected by disease, and when do C-mannosylation or tissue-specific chaperones compensate?
HUMAN MODEL MISMATCH
OPEN
gap_tsr_o_glycosylation_substrate_and_tissue_specificity
Attached to:
Impaired ER Quality Control and Secretion of TSR-Containing Proteins
Selective Depletion or Dysfunction of TSR-Containing Extracellular Proteins
Evidence for glycosylation-dependent secretion exists, but the model system changes the answer. POFUT2-null HEK293T assays reduce secretion of multiple ADAMTS clients, whereas conditional loss in developing mouse bone had minimal effects on ADAMTS17 and CCN2 secretion despite a strong skeletal phenotype. Endogenous platelet THBS1 trafficking is preserved in both Pofut2- and B3glct-null mice and its TSRs are highly C-mannosylated. These findings make direct extrapolation from a generic cell line or one mouse tissue to human disease unsafe and are the reason Weill-Marchesani syndrome is not yet declared a conformer.
Proposed experiments:
Human tissue-specific TSR glycosylation and secretion matrix
Absent O-Fucose on Folded Thrombospondin Type-1 Repeats
trigger
The first arm loses O-fucose from a correctly folded TSR, either because POFUT2 activity is absent or because a substrate variant disrupts its O-fucosylation site. This removes the glycan that stabilizes the folded domain and also prevents the downstream B3GLCT glucose extension. In the worked human substrate example, ADAMTSL2 p.Ser641Leu eliminates O-fucosylation of TSR3 and reduces secretion.
Downstream
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Impaired ER Quality Control and Secretion of TSR-Containing Proteins
Without O-fucose, the folded TSR loses its stabilizing quality-control mark and cannot receive the glucose extension, reducing efficient folding and ER exit for susceptible substrates.
Show evidence (2 references)
"In contrast, secretion was significantly reduced for ADAMTSL2 carrying GPHYSD1 mutations (S641L in TSR3 and G817R in TSR6), and S641L eliminated O-fucosylation of TSR3."
A disease-causing human ADAMTSL2 substrate variant directly removes the O-fucose modification and reduces secretion, establishing the substrate-level entry into this arm.
"In addition, POFUT2 is localized to the endoplasmic reticulum (ER) and only modifies folded TSRs, stabilizing their structures."
Establishes that the first glycosylation step recognizes and stabilizes the folded TSR rather than tagging an unfolded domain.
Absent Glucose-beta-1,3-Fucose Disaccharide on Thrombospondin Type-1 Repeats
trigger
The second arm retains the POFUT2-added O-fucose but loses B3GLCT-dependent glucose extension, leaving a fucose monosaccharide in place of the mature glucose-beta-1,3-fucose disaccharide. The added glucose supplies further fold stabilization, but this step is required by only a subset of POFUT2 substrates.
Downstream
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Impaired ER Quality Control and Secretion of TSR-Containing Proteins
Loss of the terminal glucose removes additional fold stabilization and reduces ER exit for the B3GLCT-dependent subset of TSR proteins.
Show evidence (2 references)
"O-fucose is added to cysteine-rich domains called thrombospondin type 1 repeats (TSRs) by protein O-fucosyltransferase 2 (POFUT2) and is elongated with glucose by β3-glucosyltransferase (B3GLCT)."
Establishes the sequential reaction and the B3GLCT-dependent glucose-extension step interrupted in Peters plus syndrome.
"Overall, our data supports the hypothesis that loss of glucose from B3GLCT substrate proteins is responsible for the defects observed in PTRPLS patients, but not for those observed in PTRPLS-like patients."
Ties the glucose-extension lesion specifically to disease-causing B3GLCT loss rather than to any variant in the enzyme.
Impaired ER Quality Control and Secretion of TSR-Containing Proteins
central effector
POFUT2 and B3GLCT form a noncanonical ER quality-control pathway: POFUT2 recognizes a correctly folded TSR, and sequential glycosylation stabilizes that fold and favors efficient secretion. Loss of the relevant sugar makes susceptible TSR domains less stable and delays or prevents ER exit. The effect is selective rather than universal because dependence varies by TSR, substrate, cell type, and compensating modifications such as C-mannosylation.
Downstream
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Selective Depletion or Dysfunction of TSR-Containing Extracellular Proteins
Susceptible client proteins are secreted less efficiently or reach the extracellular space with altered glycan-dependent function.
Show evidence (3 references)
"While known ER quality-control machinery rely on identifying and tagging unfolded proteins, we find that POFUT2 and B3GLCT mediate a noncanonical ER quality-control mechanism that recognizes folded TSRs and stabilizes them by glycosylation."
Defines the conserved folded-state-recognizing quality-control hub and distinguishes it from canonical unfolded-protein quality control.
"Here we show that while POFUT2 is required for secretion of all targets tested, B3GLCT only affects the secretion of a subset, consistent with the observation that B3GLCT mutant phenotypes in PPS patients are less severe than embryonic lethal phenotypes of Pofut2-null mice."
Demonstrates the experimentally important asymmetry between the O-fucose and glucose-extension steps and the selective effect of B3GLCT loss.
"Here, we conducted molecular dynamics (MD) simulations and provide crystallographic and NMR evidence that the Glucose-Fucose disaccharide interacts with specific amino acids in the TSR3 domain in thrombospondin-1 that are within proximity to the O-fucosylation modification site resulting in..."
Supplies a structural mechanism for glycan-mediated TSR stabilization at the central quality-control step.
Extracellular Matrix Remodeling and Growth-Factor Signaling Dysregulation
consequence
Altered abundance or extracellular function of ADAMTS/ADAMTSL proteins changes fibrillin microfibril composition, matrix remodeling, and the availability of matrix-regulated BMP, IHH, and TGF-beta signals. The resulting anatomical phenotype is disorder- and tissue-specific; this module stops at the conserved matrix/signaling consequence rather than treating eye, skeletal, cardiac, or CNS malformations as interchangeable.
Show evidence (1 reference)
"Limb shortening was evident at embryonic day 14.5 where loss of O-fucosylation led to an accumulation of fibrillin 2 (FBN2), decreased BMP and IHH signaling, and increased TGF-β signaling."
Connects loss of TSR O-fucosylation to fibrillin-matrix and opposing growth-factor-signaling changes in developing tissue.