Targeting HIF1A and HIF2A for the treatment of dry AMD

Lynn Ebner and Gabriele Wögenstein

Vision loss in the dry form of age-related macular degeneration (AMD) is caused by the degeneration of retinal pigment epithelium (RPE) and photoreceptor cells. Although various exogenous, systemic and genetic risk factors have been identified for disease development or progression, a therapy for dry AMD is still an unmet medical need.

During normal ageing, the posterior ocular tissue undergoes several changes including accumulation of autofluorescent material in RPE cells, formation and accumulation of Drusen deposits, thickening of Bruch’s membrane and reduction of choroidal blood flow. These changes reduce the oxygen availability in RPE and photoreceptor cells and may lead to mild but chronic tissue hypoxia imposing cellular stress. Since photoreceptors have an extraordinarily high energy demand, and maintenance of photoreceptor homeostasis and function requires sufficient tissue oxygenation, chronic hypoxia may lead to disease development when crossing critical threshold levels. Thus, it seems plausible that some of the tissue changes discussed above may be more pronounced in eyes with dry AMD.

Reduced tissue oxygenation leads to a well defined molecular response. Key factors in this response are hypoxia-inducible factors (HIFs), which are transcription factors activated at low oxygen concentrations. HIFs control expression of a large number of genes required for adaptation to low oxygen conditions. Activation of HIF1 and HIF2 is controlled by the von Hippel Lindau (VHL) protein complex that targets hydroxylated HIF-alpha subunits to the proteasomal degradation pathway in normoxic conditions. In hypoxia, HIF-alpha subunits are not hydroxylated and thus not recognised by VHL. Therefore, HIF-alpha proteins are stabilised and can function in gene regulation.

Evidence suggests that chronic activation of HIF1 in photoreceptors and of HIF2 in the RPE leads to a change in cellular metabolism, RPE atrophy and photoreceptor degeneration. Thus, we develop a gene therapy-based RNA interference approach to preserve cellular integrity and function in such conditions. This approach will deliver anti-HIF1a shRNA to photoreceptors and anti-HIF2a shRNA to the RPE using a single AAV vector injected into the subretinal space. Cell type-specific expression of the shRNAs will be achieved through specific Pol II promoters and embedding of the shRNAs into a miR-scaffold.