Hypothesis

Autophagy in aged retinal pigment epithelium (RPE) is not merely declining—it is actively suppressed through complement-driven epigenetic repression of autophagy genes, creating a self-reinforcing cycle that accelerates pathology in age-related macular degeneration (AMD).

Background and Mechanistic Insight

Recent evidence shows autophagy is transcriptionally repressed in aging, with downregulation of PER2 acting as a key suppressor [https://www.aging-us.com/article/101018/text]. In the retina, complement overactivation—particularly C3 deposition and membrane attack complex (MAC/C5b-9) internalization—directly impairs lysosomal function, blocking autophagic flux [https://doi.org/10.1002/sctm.20-0211]. This creates a feed-forward loop: damaged mitochondria and lipofuscin accumulate, activate more complement, and further suppress autophagy [https://pmc.ncbi.nlm.nih.gov/articles/PMC8195907/].

But why does this loop persist? I propose that complement components themselves directly inhibit autophagy at the transcriptional level via epigenetic mechanisms. Specifically, internalized MAC or soluble C3a/C5a fragments could recruit histone deacetylases (HDACs) or DNA methyltransferases to promoters of autophagy-related genes (e.g., ATG5, BECN1, LC3), silencing them. This would explain why simply boosting autophagy capacity fails—the suppressors aren't just damaging lysosomes; they're rewriting the genetic program.

Novel Predictions

This hypothesis yields testable, falsifiable predictions:

1. Complement receptor activation triggers epigenetic changes. In human RPE cells, exposure to C3a or MAC should increase HDAC binding to ATG gene promoters, detectable by ChIP-seq. Blocking complement receptors (e.g., C3aR) should prevent this.
2. Epigenetic drugs rescue autophagy despite complement stress. HDAC inhibitors (e.g., vorinostat) should restore autophagy markers (LC3-II, p62 clearance) in complement-challenged RPE cells, even with ongoing MAC deposition.
3. In vivo, complement-driven epigenetic silencing correlates with AMD progression. Retinal tissue from AMD donors should show hypermethylation or repressive histone marks (H3K9me3) at ATG loci, correlating with C3d/MAC levels.
4. Blocking complement early prevents autophagy gene silencing. In mouse models of AMD (e.g., Cfh-/-), early complement inhibition should maintain ATG gene expression and autophagic flux with age, reducing drusen formation.

Implications and Next Steps

If correct, this shifts therapeutic focus from merely enhancing autophagy to disrupting the complement-epigenetic axis. Complement inhibitors (e.g., anti-C5 antibodies) combined with epigenetic modulators could break the cycle. Moreover, it raises the question: is autophagy suppression a maladaptive response to chronic complement activation, or a failed attempt to limit inflammation? The answer lies in testing these predictions—and fast, before more patients lose vision to a preventable loop.