P16/INK4A up-regulation mediated by SIX6 defines retinal ganglion cell pathogenesis in glaucoma

Glaucoma, a blinding neurodegenerative disease, whose risk factors include elevated intraocular pressure (IOP), age and genetics, is characterized by accelerated and progressive retinal ganglion cell (RGC) death. Despite decades of research, the mechanism of RGC death in glaucoma is still unknown. Here, we demonstrate that the genetic effect of the SIX6 risk-variant (rs33912345, His141Asn) is enhanced by another major POAG risk gene P16/INK4A (cyclin-dependent kinase inhibitor 2A). We further show that the upregulation of homozygous SIX6 risk alleles (CC) leads to an increase in P16/INK4A expression with a subsequent cellular senescence, as evidenced in a mouse model of elevated IOP and in human POAG eyes. Our data indicate that SIX6 and/or IOP promotes POAG by directly increasing P16/INK4A expression, leading to RGC senescence in adult human retinas. Our study provides important insights linking genetic susceptibility to the underlying mechanism of RGC death and provides a unified theory of glaucoma pathogenesis.