P4HA2

P4HA2 encodes the catalytic α-subunit of collagen prolyl 4-hydroxylase isoenzyme II (C-P4H-II), an α2β2 tetrameric enzyme that converts proline residues in collagen chains to 4-hydroxyproline, a modification required for collagen triple-helix formation and thermal stability (PMID:26001784). Together with P4HA1, P4HA2 accounts entirely for prolyl 4-hydroxylation of type I collagen, as the remaining isoform P4HA3 cannot hydroxylate type I collagen (PMID:41999892); C-P4H-I can largely compensate for loss of C-P4H-II in endochondral bone development, but compound loss produces chondrodysplasia, reduced 4-hydroxyproline content, and biomechanically impaired matrix (PMID:26001784). P4HA2-dependent hydroxylation is required for normal extracellular matrix composition in vivo: loss of P4HA2 disrupts scleral and corneal collagen fibril arrangement, lowers collagen thermal stability, elevates fibronectin, reduces collagen I, and compromises visual acuity (PMID:41756678), and a loss-of-function missense variant that reduces P4HA2 expression and collagen hydroxylation underlies myopic axial elongation (PMID:29364500). Transcriptionally, P4HA2 is induced by HIF-1 under hypoxia (PMID:16877351) and by wild-type p53 (PMID:20504876), and is regulated downstream of an IGF-II→IGF1R/IR hybrid receptor→SOX9 profibrotic axis in lung fibroblasts (PMID:37510994). Beyond these collagen-modifying and regulatory roles, the broader signaling context captured here is limited to individual cell-type studies.

1. 2006 Medium

   Established that P4HA2 is an oxygen-responsive gene, linking collagen post-translational modification capacity to hypoxic signaling.

   Evidence Primary human chondrocytes under 1% O2 with qPCR and pharmacological HIF-1 inhibition

   PMID:16877351

   Open questions at the time

   • No direct ChIP or promoter-binding evidence for HIF-1 occupancy
   • Pharmacological inhibitor may have off-target effects
   • Limited to chondrocytes
2. 2007 Medium

   Placed P4HA2 upstream of HIF-1 in cardiac inflammatory signaling, indicating its loss can feed back into HIF-driven transcription with functional cardioprotective consequences.

   Evidence siRNA knockdown in HL-1 cardiomyocytes and murine ischemia-reperfusion model with chemokine, infarct-size, and leukocyte readouts

   PMID:17545479

   Open questions at the time

   • Mechanism by which P4HA2 loss activates HIF-1 not resolved
   • Single lab
   • Relationship to collagen hydroxylation activity unclear
3. 2010 Medium

   Showed P4HA2 is a transcriptional target of wild-type p53, while revealing tumor-type-specific uncoupling of mRNA induction from protein output.

   Evidence p53 overexpression, shRNA knockdown, and chemical induction in glioblastoma cells with RT-PCR and western blot

   PMID:20504876

   Open questions at the time

   • Cause of the mRNA-to-protein uncoupling not identified
   • No direct p53 promoter-binding assay
   • Single tumor context
4. 2012 Low

   Reported a context-dependent role for P4HA2 in regulating breast cancer cell proliferation and migration.

   Evidence Overexpression and siRNA knockdown in Hs578T cells with proliferation and migration assays

   PMID:22813596

   Open questions at the time

   • No mechanistic pathway placement beyond phenotype
   • Single cell line, single lab
   • Direction of effect not linked to collagen hydroxylation
5. 2015 High

   Defined P4HA2's enzymatic identity and its functional redundancy with P4HA1, establishing C-P4H-II as the chondrocyte-predominant collagen prolyl 4-hydroxylase whose loss is largely compensable.

   Evidence P4ha2-/- and P4ha1+/-;P4ha2-/- mouse models with hydroxyproline quantification, collagen Tm, histology and atomic force microscopy

   PMID:26001784

   Open questions at the time

   • Degree of redundancy in non-chondrocyte tissues not fully mapped
   • Substrate selectivity between isoforms not defined
6. 2018 Medium

   Demonstrated that P4HA2 catalytic activity is required for normal collagen hydroxylation in humans, linking loss-of-function to myopic axial elongation.

   Evidence Whole-exome sequencing plus patient-derived fibroblast assays (RT-qPCR, western blot, collagen hydroxylation)

   PMID:29364500

   Open questions at the time

   • Single family/variant
   • No animal rescue of the specific variant
   • Mechanism connecting reduced hydroxylation to ocular axial growth not detailed
7. 2023 Medium

   Positioned P4HA2 within an IGF-II→IGF1R/IR hybrid receptor→SOX9 profibrotic axis in lung fibroblasts.

   Evidence Primary human lung fibroblasts with IGF-II treatment, receptor knockdown/inhibition, SOX9 siRNA, RT-qPCR and western blot

   PMID:37510994

   Open questions at the time

   • No direct SOX9 promoter-binding evidence at P4HA2
   • Single lab
   • In vivo fibrosis relevance not established
8. 2025 Low

   Proposed a pVHL-independent mechanism whereby P4HA1/2-driven collagen production sustains HIF-1α and HIF-2α levels in VHL-deficient ccRCC.

   Evidence P4HA1/P4HA2 knockdown/knockout in ccRCC cells with HIF-α western blots and collagen production assays (preprint)

   Open questions at the time

   • Preprint, not peer-reviewed
   • Mechanistic link between collagen production and HIF-α translation/transcription only partial
   • Single lab
9. 2025 Low

   Linked P4HA2 to PI3K/Akt/mTOR survival signaling and quercetin-induced apoptosis in hepatocellular carcinoma cells.

   Evidence siRNA knockdown with quercetin treatment and pharmacological pathway modulators in HCC cells; viability and apoptosis assays

   PMID:40347062

   Open questions at the time

   • Mechanistic link between P4HA2 and PI3K/Akt is indirect
   • No biochemical reconstitution
   • Single lab
10. 2026 Medium

    Established that P4HA2-mediated collagen hydroxylation is required for proper scleral/corneal ECM composition and refractive function in vivo.

    Evidence P4ha2-/- mice and CRISPR P4HA2-knockout HEK293 cells with electron microscopy, hydroxylation quantification and western blots

    PMID:41756678

    Open questions at the time

    • Mechanism connecting fibronectin elevation/collagen I reduction not fully resolved
    • Single lab
11. 2026 High

    Showed that prolyl 4-hydroxylation of type I collagen is mediated exclusively by P4HA1 and P4HA2, with P4HA3 unable to substitute.

    Evidence CRISPR P4ha3-/- and P4ha1-/-;P4ha2-/- cells with direct C-P4H activity assays, hydroxyproline, collagen Tm, pepsin-resistance and fibril assembly assays

    PMID:41999892

    Open questions at the time

    • Isoform-specific substrate preferences across collagen types not fully mapped
    • Single lab
12. 2026 Low

    Placed P4HA2 downstream of the circadian transcription factor BMAL1 in a glucose-responsive collagen synthesis pathway in chondrocytes.

    Evidence BMAL1 siRNA knockdown in primary human chondrocytes under high glucose with RT-qPCR, western blot and collagen II quantification

    PMID:41630778

    Open questions at the time

    • No direct BMAL1 promoter-binding evidence
    • Single method per readout
    • Single lab

• How the multiple upstream transcriptional inputs (HIF-1, p53, SOX9, BMAL1) are integrated to set P4HA2-dependent collagen hydroxylation in specific tissues, and the precise mechanism linking P4HA2/collagen production to HIF-α stabilization, remain unresolved.
• No unified model integrating the distinct transcriptional regulators
• pVHL-independent HIF-α mechanism unconfirmed in peer review
• Tissue-specific isoform redundancy incompletely mapped