Progranulin Replacement Therapy for FTD — Vector Development and Validation

Progranulin Replacement Therapy FTD

Experiment Overview

Target Knowledge Gap: FTD Gap #3: "What is the relationship between progranulin deficiency and TDP-43 aggregation?" (Score: 33/40) — Also addresses FTD Gap #1: pathology determination in GRN vs MAPT carriers

Scientific Value: 10 | Feasibility: 7 | Novelty: 9 | Disease Impact: 10 | Reach: 8 | Cost Efficiency: 6 | Time Efficiency: 6 | Evidence Base: 8 | Addresses Uncertainty: 9 | Translation Potential: 10

Composite Score: 83/140

Background

Heterozygous loss-of-function mutations in the GRN gene (encoding progranulin) cause approximately 5-10% of familial FTD cases and represent the most common genetic cause of frontotemporal lobar degeneration with TDP-43 pathology (FTLD-TDP). Patients with GRN mutations have:

• ~50% reduction in circulating progranulin (haploinsufficiency)
• TDP-43 pathology in affected brain regions
• Early onset (typically 45-65 years)
• Variable age of onset (35-75 years) suggesting modifier factors

Progranulin is a secreted growth factor involved in:

• Lysosomal function and autophagy
• Neuronal survival and neurite outgrowth
• Inflammation modulation (TNF signaling)
• TDP-43 metabolism and clearance

The hypothesis is that restoring progranulin levels to normal could prevent or slow TDP-43 pathology progression.

Hypothesis

AAV-mediated delivery of wild-type GRN to the CNS can restore progranulin levels, reduce TDP-43 pathology, and slow clinical progression in GRN mutation carriers.

Experimental Design

...

Progranulin Replacement Therapy FTD

Experiment Overview

Target Knowledge Gap: FTD Gap #3: "What is the relationship between progranulin deficiency and TDP-43 aggregation?" (Score: 33/40) — Also addresses FTD Gap #1: pathology determination in GRN vs MAPT carriers

Scientific Value: 10 | Feasibility: 7 | Novelty: 9 | Disease Impact: 10 | Reach: 8 | Cost Efficiency: 6 | Time Efficiency: 6 | Evidence Base: 8 | Addresses Uncertainty: 9 | Translation Potential: 10

Composite Score: 83/140

Background

Heterozygous loss-of-function mutations in the GRN gene (encoding progranulin) cause approximately 5-10% of familial FTD cases and represent the most common genetic cause of frontotemporal lobar degeneration with TDP-43 pathology (FTLD-TDP). Patients with GRN mutations have:

• ~50% reduction in circulating progranulin (haploinsufficiency)
• TDP-43 pathology in affected brain regions
• Early onset (typically 45-65 years)
• Variable age of onset (35-75 years) suggesting modifier factors

Progranulin is a secreted growth factor involved in:

• Lysosomal function and autophagy
• Neuronal survival and neurite outgrowth
• Inflammation modulation (TNF signaling)
• TDP-43 metabolism and clearance

The hypothesis is that restoring progranulin levels to normal could prevent or slow TDP-43 pathology progression.

Hypothesis

AAV-mediated delivery of wild-type GRN to the CNS can restore progranulin levels, reduce TDP-43 pathology, and slow clinical progression in GRN mutation carriers.

Experimental Design

Phase 1: Vector Engineering and Optimization (Months 1-12)

Approach: Design and optimize AAV vectors for CNS progranulin expression

| Component | Details |
|-----------|---------|
| Serotype | AAV9, AAV-PHP.B, AAV-v10 — compare CNS transduction |
| Promoter | hSyn (neuronal), gfaABC1D (astrocytes), CAG (ubiquitous) |
| Transgene | Wild-type human GRN cDNA, codon-optimized |
| Safety features | WPRE-free, minimal promoter elements |

In vitro screening:

• HEK293, iPSC-derived neurons: Expression level, secretion
• Primary neuron cultures: Toxicity screening (MTT, caspase-3)
• Lysosomal function: Cathepsin D activity assays

In vivo pilot (mouse):

• AAV-GRN vs AAV-Luc (control)
• Dose escalation: 1×10¹⁰, 1×10¹¹, 1×10¹² vg/mouse
• Route: IV vs ICV vs intrathecal
• Endpoint: GRN expression (IHC, ELISA), tolerability

Acceptance criteria: >5x human GRN expression in cortex, no toxicity at 4 weeks

Phase 2: GRN Knockout Mouse Validation (Months 12-24)

Approach: Test efficacy in GRN-/- mouse model of FTLD-TDP

| Model | Source | Background |
|-------|--------|------------|
| Grn-/- mice | Jackson Labs | C57BL/6J |
| Grn-/- x TDP-43 G298S | Cross | Hybrid |

Study design:

| Group | N | Treatment |
|-------|---|-----------|
| Wild-type + AAV-GRN | 15 | High-dose AAV-GRN |
| Wild-type + AAV-Luc | 15 | Control vector |
| Grn-/- + AAV-GRN | 20 | High-dose AAV-GRN |
| Grn-/- + AAV-Luc | 20 | Control vector |
| Grn-/- + AAV-GRN (low) | 15 | Low-dose AAV-GRN |

Endpoints (12 months):

• Behavioral: Open field, Morris water maze, nest building
• Biochemical: Plasma/cortical GRN (ELISA), TDP-43 pathology (IHC)
• Histopathology: Neuronal loss, gliosis, TDP-43 inclusions
• Biomarkers: NfL, p-tau181 (plasma)

Hypothesis: Grn-/- mice receiving AAV-GRN will show:

• Normalized cortical GRN levels
• Reduced TDP-43 pathology
• Improved behavioral performance
• Normalized lysosomal function

Phase 3: Non-Human Primate Safety and PK (Months 24-30)

Approach: GLP toxicology in cynomolgus macaques

| Cohort | N | Dose | Duration |
|--------|---|---|----------|
| Control | 6 | Vehicle | 13 weeks |
| Low-dose | 6 | 1×10¹⁴ vg | 13 weeks |
| Mid-dose | 6 | 3×10¹⁴ vg | 13 weeks |
| High-dose | 6 | 1×10¹⁵ vg | 13 weeks |

Endpoints:

• PK: Plasma/CSF GRN levels over time
• PD: Target engagement (GRN in CSF)
• Safety: Clinical observations, hematology, chemistry, histopathology
• Biodistribution: qPCR for vector genome in tissues
• Immunogenicity: Anti-AAV antibodies, neutralizing antibodies

Regulatory: Pre-IND meeting with FDA to discuss pivotal trial design

Phase 4: Human Biomarker Study (Months 30-36)

Approach: Measure target engagement in GRN mutation carriers

| Cohort | N | Description |
|--------|---|-------------|
| Pre-symptomatic GRN+ | 30 | At-risk carriers, no symptoms |
| Symptomatic GRN+ | 20 | Mild-moderate FTD |
| Non-carrier controls | 20 | Age-matched |

Endpoints:

• Plasma/CSF progranulin (ELISA)
• CSF TDP-43 species (seed amplification assay)
• Plasma NfL, p-tau181
• Clinical measures (CDR, FAB, MMSE)

Purpose: Validate biomarker endpoints for registrational trial

Expected Outcomes

Primary Outcomes

Optimized AAV-GRN vector — Ready for IND-enabling studies

Efficacy proof in Grn-/- mice — Significant reduction in TDP-43 pathology

NHP safety data — No unexpected toxicity at therapeutic doses

Biomarker validation — CSF/plasma progranulin as pharmacodynamic marker

Secondary Outcomes

Dose selection — Define minimal effective dose

Delivery route — Compare IV vs intrathecal

Clinical signal — Preliminary evidence of disease modification

Feasibility Assessment

Strengths

• Well-validated target — GRN haploinsufficiency is causative
• Precedent — AAV gene therapy approved for CNS (onasemnogene)
• Biomarker available — Plasma GRN easily measurable
• Patient population — Well-characterized via ARTFL/LEFFTDS

Challenges

• TDP-43 mechanism unclear — Does progranulin replacement reverse pathology?
• Delivery to frontal/temporal cortex — Requires widespread CNS distribution
• Immune response — Pre-existing anti-AAV antibodies in population
• Therapeutic window — Balance efficacy vs off-target effects

Resource Requirements

| Resource | Estimated Cost |
|----------|---------------|
| Phase 1 (vector) | $800K |
| Phase 2 (mouse) | $600K |
| Phase 3 (NHP) | $1.2M |
| Phase 4 (biomarker) | $400K |
| Total | ~$3M |

Timeline: 36 months to IND submission

Risk Mitigation

| Risk | Mitigation |
|------|------------|
| Limited CNS distribution | Test multiple serotypes, delivery routes |
| TDP-43 not reversible | Begin in pre-symptomatic carriers |
| Immune response | Screen for pre-existing antibodies, use steroid cover |
| Off-target expression | Use neuron-specific promoters |

References

[Baker et al., GRN mutations cause FTLD-TDP (2006)](https://pubmed.ncbi.nlm.nih.gov/16641999/)

[Ward et al., AAV-mediated GRN delivery in mice (2019)](https://pubmed.ncbi.nlm.nih.gov/31683156/)

[Le Guennec et al., Progranulin and TDP-43 in FTD (2022)](https://pubmed.ncbi.nlm.nih.gov/35828759/)

[Gijselinck et al., GRN mutations in FTD (2023)](https://pubmed.ncbi.nlm.nih.gov/36375040/)

Cross-Links

• [FTD Knowledge Gaps](/gaps/ftd) — Parent gap analysis
• [GRN Gene Page](/genes/grn) — Gene information
• [Progranulin Protein](/proteins/grn-protein) — Protein details
• [TDP-43 Proteinopathy Pathway](/mechanisms/tdp-43-proteinopathy) — Mechanism
• [C9orf72 Hexanucleotide Repeat Mechanism](/experiments/c9orf72-hexanucleotide-repeat-mechanism) — Related experiment