Hedgehog Pathway Activation Module

Module hedgehog_pathway_activation 5 disorders Pathograph 5

A conserved oncogenic mechanism module representing ligand-independent, constitutive activation of Hedgehog (Hh) signaling. Across Hedgehog-driven neoplasms, a genetic lesion removes the pathway's tonic restraint and locks the SMO-GLI axis in the "on" state: either loss-of-function of a negative regulator (the tumor suppressors PTCH1, PTCH2, or SUFU) or gain-of-function of the positive transducer SMO. Both gene-level directions converge on the same net effect - constitutive Smoothened activity and constitutive GLI transcriptional output - which drives Hedgehog-dependent proliferation and tumorigenesis. This module captures the shared causal chain and defines the mechanism-of-action pattern for SMO inhibitors (vismodegib, sonidegib): conforming disorder nodes declare conformance via conforms_to, and their SMO-inhibitor treatments link back through target_mechanisms to the "Constitutive Smoothened Activity" node they inhibit. This is deliberately the GAIN-of-pathway-activity arm of Hedgehog biology. It is the mechanistic INVERSE of the developmental Hedgehog LOSS-of-function disorders (holoprosencephaly, GLI3-repressor syndromes, and the ciliary transduction failures), where reduced Hh signal output causes malformation rather than cancer. Those loss-of-signal disorders are modeled separately (see ciliopathy_dysfunction#Impaired Hedgehog Signal Transduction and limb_digit_patterning_serial_homology); this module intentionally does NOT fold the two opposite directions into one effect-reversing chain, mirroring the deliberate split between cellular_senescence and senescence_tumor_suppression.

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Pathophysiology Nodes

4 shared nodes are defined in this module.

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Cell Types

No cell types are annotated for this module.

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Biological Processes

Notes

This is a mechanism module, not a specific disease. Disorder entries reference individual nodes via conforms_to (e.g., "hedgehog_pathway_activation#Constitutive Smoothened Activity"). Key conformance / treatment target: "Constitutive Smoothened Activity" - the node bound by the approved SMO antagonists. The "Constitutive GLI Transcriptional Output" node is the SMO-independent convergence point: SUFU loss and GLI2 amplification activate the pathway downstream of SMO, which is why SUFU-driven disease and acquired SMO-inhibitor resistance bypass the drug target. Worked conformers: Basal_Cell_Carcinoma (PTCH1 loss / SMO gain-of-function), Gorlin_Syndrome and its PTCH1-related / SUFU-related NBCCS entries, and Medulloblastoma_SHH_Activated. GENE-LEVEL DIRECTION: annotate tumor-suppressor lesions (PTCH1/PTCH2/SUFU) as LOSS_OF_FUNCTION and SMO lesions as GAIN_OF_FUNCTION on the conforming disorder node's genetic_context.functional_impact_category; the PATHWAY-LEVEL direction is activation in every case.

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Used By Disorder Entries

Basal Cell Carcinoma via PTCH1 Loss of Function → Loss of Hedgehog Pathway Negative Regulation , SMO Activating Mutations → Loss of Hedgehog Pathway Negative Regulation , Constitutive Hedgehog Pathway Activation → Constitutive Smoothened Activity , GLI Transcription Factor Activation → Constitutive GLI Transcriptional Output , Uncontrolled Basal Cell Proliferation → Hedgehog-Driven Proliferation and Tumorigenesis

Gorlin Syndrome via PTCH1 Germline Mutations → Loss of Hedgehog Pathway Negative Regulation , SUFU Germline Mutations → Loss of Hedgehog Pathway Negative Regulation , SMO Constitutive Activation → Constitutive Smoothened Activity , GLI Transcription Factor Activation → Constitutive GLI Transcriptional Output

Medulloblastoma, SHH-Activated via SHH Pathway Activation → Constitutive Smoothened Activity , GLI Transcription Factor Activation → Constitutive GLI Transcriptional Output , Cerebellar Granule Cell Proliferation → Hedgehog-Driven Proliferation and Tumorigenesis

PTCH1-related nevoid basal cell carcinoma syndrome via PTCH1 Loss with SMO Disinhibition → Loss of Hedgehog Pathway Negative Regulation

SUFU-related nevoid basal cell carcinoma syndrome via SUFU Loss with GLI Derepression → Loss of Hedgehog Pathway Negative Regulation

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence-backed metadata.

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Pathophysiology

Loss of Hedgehog Pathway Negative Regulation

trigger

In the resting state, the transmembrane receptor PTCH1 (and its paralog PTCH2) tonically suppresses Smoothened (SMO), and SUFU sequesters GLI transcription factors in the cytoplasm. Constitutive Hedgehog activation begins when this restraint is removed by one of two mechanistically opposite but convergent lesions: loss-of-function of a negative regulator (biallelic/Knudson two-hit inactivation of the tumor suppressors PTCH1, PTCH2, or SUFU) OR gain-of-function of the positive transducer SMO (activating mutation). Conforming disorder nodes substitute the tumor-type-specific driver and record the gene-level direction (functional_impact_category = LOSS_OF_FUNCTION for PTCH1/PTCH2/SUFU, GAIN_OF_FUNCTION for SMO); either way the net pathway-level consequence is activation.

negative regulation of smoothened signaling pathway GO:0045879 DECREASED

Used by disorders

Basal Cell Carcinoma as PTCH1 Loss of Function

Basal Cell Carcinoma as SMO Activating Mutations

Gorlin Syndrome as PTCH1 Germline Mutations

Gorlin Syndrome as SUFU Germline Mutations

PTCH1-related nevoid basal cell carcinoma syndrome as PTCH1 Loss with SMO Disinhibition

SUFU-related nevoid basal cell carcinoma syndrome as SUFU Loss with GLI Derepression

Downstream

Constitutive Smoothened Activity Loss of PTCH-mediated inhibition (or an activating SMO mutation) releases Smoothened into a constitutively active state at the primary cilium. The SUFU-loss branch bypasses this node and activates GLI directly downstream.
Constitutive GLI Transcriptional Output SUFU loss acts downstream of SMO, directly derepressing GLI and activating the pathway independently of Smoothened.

Constitutive Smoothened Activity

central effector

Freed from PTCH restraint (or rendered constitutively active by mutation), SMO accumulates in the primary ciliary membrane and transduces Hedgehog signal ligand-independently. This SMO-active state is the direct molecular target of the approved Hedgehog-pathway inhibitors vismodegib and sonidegib, which bind SMO and restore its inhibited conformation. Lesions acting downstream of SMO (SUFU loss, GLI2 amplification) leave this node bypassed and predict limited SMO-inhibitor efficacy.

smoothened signaling pathway GO:0007224 INCREASED

Used by disorders

Basal Cell Carcinoma as Constitutive Hedgehog Pathway Activation

Gorlin Syndrome as SMO Constitutive Activation

Medulloblastoma, SHH-Activated as SHH Pathway Activation

Downstream

Constitutive GLI Transcriptional Output Constitutively active SMO drives nuclear accumulation of GLI1/GLI2 and loss of the GLI3 repressor, activating Hedgehog target transcription.

Constitutive GLI Transcriptional Output

effector

Both PTCH loss (via SMO derepression) and SUFU loss (via direct release of GLI sequestration), as well as GLI2 amplification, converge on constitutive nuclear GLI1/GLI2 activity. GLI transcriptionally activates pro-proliferative and pro-survival targets (CCND1, MYCN, BCL2) and drives a positive-feedback loop by inducing PTCH1 and GLI1. Because this node sits below the SMO drug target, it is the SMO-independent convergence point that underlies both SUFU-driven disease and acquired resistance to SMO inhibitors, motivating direct GLI-directed strategies.

smoothened signaling pathway GO:0007224 INCREASED positive regulation of transcription by RNA polymerase II GO:0045944 INCREASED

Used by disorders

Basal Cell Carcinoma as GLI Transcription Factor Activation

Gorlin Syndrome as GLI Transcription Factor Activation

Medulloblastoma, SHH-Activated as GLI Transcription Factor Activation

Downstream

Hedgehog-Driven Proliferation and Tumorigenesis GLI target-gene transcription drives cell-cycle entry and sustained proliferation of the Hedgehog-responsive cell of origin.

Hedgehog-Driven Proliferation and Tumorigenesis

consequence

Constitutive GLI activity reactivates a developmental Hedgehog proliferative program in the tissue's Hedgehog-responsive progenitor (e.g., epidermal basal cells in basal cell carcinoma, cerebellar granule neuron precursors in SHH-activated medulloblastoma), producing unchecked proliferation and tumor formation. Conforming disorder nodes substitute the tumor-type-specific cell of origin.

cell population proliferation GO:0008283 INCREASED

Used by disorders

Basal Cell Carcinoma as Uncontrolled Basal Cell Proliferation

Medulloblastoma, SHH-Activated as Cerebellar Granule Cell Proliferation