P16INK4a Upregulation Mediated by SIX6 Defines Retinal Ganglion Cell Pathogenesis in Glaucoma

Abstract

SUMMARY 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.

New insights into the genetics of primary open-angle glaucoma based on meta-analyses of intraocular pressure and optic disc characteristics.

“…Although the overlap between IOP-loci and the optic disc parameters-loci is not large, this is the first study showing a genome-wide significant evidence of the genetic correlation between IOP and VCDR; we expect that larger sample sizes and improved imputation accuracy may help to find more of the loci underlying the genetic correlation between these two endophenotypes. Of the novel associations, CDKN1A is strongly associated with POAG, This finding is in line with other studies (26), pointing to the CDK-inhibitor genes as key players in the development of POAG. The p53 pathway has been implicated in POAG and interact in different cellular contexts with four of the new genes pointed out by this study (GADD45A, PDZD2, RREB1 and PSCA).…”

“…A potential functional interaction between POAG-loci (i.e. SIX6 and CDKN genes) has been shown previously (26,27). We performed in vivo studies in embryonic zebrafish eye and found that knockdown of six6b upregulates both cdkn2a/cdkn2b and cdkn1a.…”

“…Previous studies have shown that the transcription factor SIX6 alters the expression of CDKN2A and CDKN2B (26,27). All three genes (SIX6, CDKN2A and CDKN2B) are in loci associated with POAG, suggesting a possible functional link between these POAG-loci.…”

“…The mechanism behind the genetic variation in SIX6, and the CDKN genes remains obscure. However, in a recent study, Skowronska-Krawczyk et al showed that SIX6 regulates the expression of CDKN2A in the context of IOP elevation (26). More comprehensive studies at the individual tissue level e.g.…”

Correction

“…The current study provides a solid molecular foundation on which to integrate these findings. A more complete understanding of the biological underpinnings of glaucoma will no doubt also help to identify new targets for intervention, and might reveal mechanistic insights into the molecular basis of other age- 4 report that these risk factors converge on a single molecular cascade in which the transcription factor SIX6 binds to and activates the gene p16INK4a. Increased p16INK4a expression causes RGC senescence and, eventually, RGC degradation and death.…”