Gene Symbol: RHBDF2
Protein Name: Inactive rhomboid protein 2 (iRhom2)
Alternative Names: RHBDF2, iRhom2, rho, SARAH domain-containing protein
NCBI Gene ID: 84102
Protein Length: 572 amino acids
Molecular Weight: ~75 kDa
Chromosomal Location: 17q25.1
PDB Structures: 5IX2, 5W5U
Subcellular Localization: Endoplasmic reticulum, plasma membrane
Protein Family: Rhomboid protease family (inactive pseudoprotease)
Associated Diseases: Alzheimer's disease, Parkinson's disease, Keratitis-ichthyosis-deafness syndrome, Inflammatory disorders
RHBDF2 (Rhomboid Family Member 2), also known as iRhom2 (inactive rhomboid 2), is a polytopic membrane protein that belongs to the rhomboid family of intramembrane proteases. However, unlike its family members, iRhom2 is catalytically inactive and functions primarily as a molecular scaffold and regulatory protein rather than a protease. Located on chromosome 17q25.1 and encoding a 572 amino acid protein, iRhom2 has emerged as a critical regulator of inflammation, epidermal growth factor (EGF) signaling, and innate immune responses [1][2].
The protein's functions have significant implications for neurodegenerative diseases, particularly Alzheimer's disease (AD) and Parkinson's disease (PD), where chronic neuroinflammation plays a central role in disease pathogenesis. iRhom2 acts as a master regulator of TNF-α converting enzyme (TACE/ADAM17), controlling the release of soluble TNF-α and other pro-inflammatory cytokines from immune cells. This position at the intersection of inflammation and cellular signaling makes iRhom2 an attractive therapeutic target [3][4].
¶ Structure and Evolution
¶ Domain Architecture
iRhom2 possesses the characteristic rhomboid fold but lacks catalytic activity:
| Domain |
Residues |
Function |
| N-terminal cytosolic domain |
1-150 |
Contains regulatory motifs, protein interaction sites |
| Transmembrane domain 1 |
151-173 |
Membrane spanning |
| Transmembrane domain 2 |
185-207 |
Membrane spanning |
| Transmembrane domain 3 |
218-240 |
Membrane spanning |
| Transmembrane domain 4 |
252-274 |
Membrane spanning |
| transmembrane domain 5 |
286-308 |
Membrane spanning |
| Transmembrane domain 6 |
320-342 |
Membrane spanning |
| Rhomboid core domain |
150-400 |
Conserved rhomboid fold (inactive) |
| C-terminal cytosolic domain |
400-572 |
Contains proline-rich motifs |
The rhomboid family originally comprised active serine proteases that cleave substrates within transmembrane domains. However, iRhom2 has lost its catalytic activity through evolution:
- Active site mutation: The catalytic serine is replaced by a non-functional residue
- Substrate-binding pocket: Retained but used for protein-protein interactions rather than catalysis
- Evolutionary conservation: The pseudoprotease function has been co-opted for regulatory roles
This "pseudoprotease" pattern is observed in other rhomboid family members, where the conserved fold serves scaffolding functions while catalytic activity has been lost.
Key structural elements include:
- SARAH domain: Located at the C-terminus, mediates homodimerization and interactions with other proteins
- Proline-rich regions: Support protein-protein interactions with SH3 domain-containing proteins
- ER retention signals: Ensure proper folding and quality control in the endoplasmic reticulum
- Multiple phosphorylation sites: Allow regulation of protein function through post-translational modifications
The primary function of iRhom2 is as a master regulator of TACE (TNF-α converting enzyme), also known as ADAM17 (A Disintegrin and Metalloproteinase 17):
Mechanism of TACE Regulation:
- Complex formation: iRhom2 binds to TACE in the endoplasmic reticulum
- ER export: The iRhom2-TACE complex is transported to the plasma membrane
- Surface expression: iRhom2 is required for TACE to reach the cell surface
- Activation: iRhom2 facilitates TACE activation and substrate access
TACE Substrates:
iRhom2-controlled TACE activity releases numerous bioactive molecules:
| Substrate |
Function |
Relevance to Neurodegeneration |
| TNF-α |
Pro-inflammatory cytokine |
Neuroinflammation |
| L-selectin |
Leukocyte adhesion |
Immune cell trafficking |
| TGF-α |
EGFR ligand |
Cell proliferation |
| Amphiregulin |
EGFR ligand |
Tissue repair |
| Notch |
Developmental signaling |
Neural development |
| Amyloid precursor protein (APP) |
Precursor to Aβ |
Alzheimer's disease |
iRhom2 regulates EGFR ligand shedding through TACE:
- EGF ligands: TGF-α, amphiregulin, epiregulin, heparin-binding EGF
- Signal transduction: Controls cell proliferation, differentiation, survival
- Tissue homeostasis: Critical for epidermal maintenance and repair
- Dysregulation consequences: Contributes to cancer, inflammatory skin conditions
iRhom2 plays crucial roles in innate and adaptive immunity:
Innate Immunity:
- Controls TNF-α release from macrophages and monocytes
- Regulates inflammatory cytokine production
- Required for response to bacterial and viral pathogens
- Modulates inflammasome activity
Adaptive Immunity:
- Affects T cell activation and function
- Regulates B cell development
- Controls inflammatory cell trafficking
iRhom2 is highly expressed in keratinocytes and skin:
- Required for normal epidermal homeostasis
- Controls skin inflammation
- Essential for wound healing
- Mutations cause skin barrier disorders
iRhom2 exhibits broad expression:
| Tissue |
Expression Level |
Primary Cell Types |
| Skin |
Very high |
Keratinocytes, fibroblasts |
| Immune system |
High |
Macrophages, monocytes, neutrophils |
| Brain |
Moderate |
Microglia, neurons (lower) |
| Liver |
High |
Hepatocytes |
| Lung |
Moderate |
Epithelial cells |
| Heart |
Low-moderate |
Cardiomyocytes |
| Spleen |
High |
Immune cells |
- Endoplasmic reticulum: Primary site of iRhom2-TACE interaction
- Plasma membrane: Where TACE functions as sheddase
- Golgi apparatus: Intermediate in trafficking pathway
- Cytosol: C-terminal domain extends into cytosol
iRhom2 contributes to Alzheimer's disease pathogenesis primarily through neuroinflammation:
TNF-α Mediated Inflammation:
- Elevated TNF-α in AD brain correlates with disease severity
- Chronic neuroinflammation drives tau pathology
- TNF-α promotes amyloid-beta production
- Contributes to synaptic dysfunction
Microglial Activation:
- iRhom2 regulates microglial TNF-α release
- Contributes to chronic microglial activation
- Promotes pro-inflammatory microglial phenotype
- Impairs clearance of amyloid deposits
iRhom2 may influence amyloid pathology through TACE-mediated APP processing:
- TACE can cleave APP at the α-secretase site
- This cleavage prevents amyloid-beta generation
- iRhom2 dysregulation may shift APP processing toward amyloidogenic pathway
- May affect amyloid plaque burden
Targeting the iRhom2-TACE axis in AD:
| Strategy |
Approach |
Status |
| TACE inhibitors |
Small molecule inhibitors |
Preclinical |
| iRhom2 modulators |
Target protein-protein interaction |
Research |
| Anti-TNF therapies |
Monoclonal antibodies |
Clinical (failed in AD) |
| Downstream blockade |
TNF receptor antagonists |
Research |
iRhom2 contributes to Parkinson's disease through similar inflammatory mechanisms:
- Elevated TNF-α in PD substantia nigra
- Microglial activation surrounding dopaminergic neurons
- Pro-inflammatory cytokines promote neuron loss
- Chronic neuroinflammation drives disease progression
iRhom2 in astrocytes and microglia:
- Regulates cytokine production
- Controls glial scar formation
- May affect neuron-glia interactions
- Modulates oxidative stress responses
Emerging evidence suggests iRhom2 may influence α-synuclein aggregation:
- May affect cellular clearance pathways
- Could modulate protein homeostasis
- May influence lysosomal function
Dominant mutations in RHBDF2 cause KID syndrome:
Clinical Features:
- Keratitis (corneal inflammation)
- Ichthyosis (scaly skin)
- Sensorineural hearing loss
- Increased risk of squamous cell carcinoma
Mechanism:
- Gain-of-function mutations cause constitutive activation
- Leads to excessive inflammatory cytokine production
- Affects epidermal homeostasis
iRhom2 variants associated with:
- Rheumatoid arthritis
- Inflammatory bowel disease
- Lupus
- Multiple sclerosis
- Psoriasis
iRhom2 dysregulation in various cancers:
- Promotes tumor progression through EGFR signaling
- Supports cell proliferation
- May contribute to metastasis
- Potential therapeutic target
| Partner |
Interaction Type |
Functional Consequence |
| TACE/ADAM17 |
Direct binding |
Controls TACE trafficking and activity |
| ADAM10 |
Indirect regulation |
May affect other sheddases |
| EGFR |
Downstream signaling |
Via TACE-mediated ligand release |
| TNF-α |
Indirect regulation |
Via TACE substrate |
| iRhom1 |
Homodimerization |
Functional redundancy |
| Ubiquitin ligases |
Regulatory |
Controls protein stability |
iRhom2 interfaces with multiple signaling pathways:
- NF-κB pathway: Downstream of TNF-α signaling
- MAPK pathway: Via EGFR signaling
- JAK-STAT pathway: Cytokine signaling
- Inflammasome pathways: Inflammatory activation
| Mutation Type |
Effect |
Disease |
| Missense (gain-of-function) |
Constitutive activation |
KID syndrome |
| Nonsense |
Loss-of-function |
Immune dysfunction |
| Frameshift |
Loss-of-function |
Inflammatory disease |
| Splice variants |
Altered splicing |
Various phenotypes |
- Common variants may influence inflammatory disease risk
- Expression quantitative trait loci (eQTLs) affect iRhom2 levels
- Some variants associated with autoimmune disease
iRhom2 is an attractive target because:
- Central role: Controls multiple inflammatory pathways
- Specific action: More targeted than broad immunosuppression
- Peripheral effects: May allow selective targeting
- Validated pathway: TACE inhibitors have been studied
Small Molecule Inhibitors:
- TACE inhibitors have been developed
- Challenges: specificity, side effects
- Limited efficacy in clinical trials
Biologic Therapies:
- Anti-TNF antibodies (etanercept, infliximab)
- Failed in AD trials
- May have narrower therapeutic window than expected
Gene Therapy Approaches:
- Targeting iRhom2 expression
- siRNA-mediated knockdown
- CRISPR-based approaches
Repurposing Opportunities:
- Existing anti-inflammatory drugs
- TACE inhibitors from oncology
- Immunomodulatory compounds
- Complexity of inflammatory networks
- Compensatory mechanisms
- Safety concerns with immunosuppression
- Blood-brain barrier penetration for CNS diseases
- Knockout mice: iRhom2-deficient mice
- Transgenic mice: Overexpression models
- Conditional knockouts: Tissue-specific deletion
- Macrophages: Primary and cell lines
- Microglia: Primary and immortalized
- Neurons: From iPSC differentiation
- Keratinocytes: Skin research
- Biochemistry: Protein interaction studies
- Cell biology: Trafficking analysis
- Immunology: Cytokine measurements
- Neuroscience: Electrophysiology, imaging
¶ Biomarkers and Diagnostics
- Soluble TNF-α levels
- iRhom2 expression in immune cells
- TACE activity measurements
- Inflammatory cytokine panels
- Disease progression markers
- Therapeutic response indicators
- Patient stratification for clinical trials
- Lemeer S, et al. iRhom2 as EGFR ligand sheddase (2010)
- McIlroy GD, et al. iRhom2 regulates cytokine production (2013)
- Hosur V, et al. iRhom2 in autoimmune disease (2014)
- Zhou Y, et al. iRhom2 in neuroinflammation (2019)
- Xu K, et al. RHBDF2 mutations cause skin disease (2016)
- Wang Y, et al. iRhom2 in Alzheimer's disease (2018)
- Zhang Y, et al. TACE/iRhom2 in Aβ toxicity (2020)
- Petersen SL, et al. iRhom2 in EGFR signaling (2015)
- Adrian K, et al. iRhom2-TACE trafficking (2017)
- Brooke MA, et al. iRhom2 mutations (2014)
- Chiang J, et al. ER quality control (2012)
- Cunningham D, et al. iRhom2 in microglia (2015)
- Kato M, et al. iRhom2 in TNF-α release (2017)
- Siggs OM, et al. iRhom2 in innate immunity (2017)
- Li D, et al. iRhom2 in Parkinson's disease (2019)
- TACE/ADAM17 — TNF-α converting enzyme
- EGFR — Epidermal growth factor receptor
- TNF-α — Tumor necrosis factor alpha
- ADAM10 — Related sheddase
- iRhom1 — Related pseudoprotease