Schnyder Corneal Dystrophy-Associated UBIAD1 is Defective in MK-4 Synthesis and Resists Autophagy-Mediated Degradation.

Mutations in UbiA prenyltransferase domain-containing protein-1 (UBIAD1), a prenyltransferase that uses geranylgeranyl pyrophosphate (GGpp) to synthesize the vitamin K2 subtype menaquinone-4 (MK-4), cause Schnyder corneal dystrophy (SCD). This autosomal-dominant disorder is characterized by opacification of the cornea that results from abnormal accumulation of cholesterol in the tissue. We previously discovered that sterols trigger binding of UBIAD1 to endoplasmic reticulum (ER)-localized HMG CoA reductase, the rate-limiting enzyme in the synthesis of cholesterol and nonsterol isoprenoids, including GGpp. This binding inhibits sterol-accelerated, ER-associated degradation (ERAD) of reductase, permitting continued synthesis of GGpp in cholesterol-replete cells. GGpp triggers release of UBIAD1 from reductase, allowing for its maximal ERAD and ER-to-Golgi translocation of UBIAD1. SCD-associated UBIAD1 resists GGpp-induced release and remains sequestered in ER to inhibit reductase ERAD. Here, we report the development of a biochemical assay for UBIAD1-mediated synthesis of MK-4 in isolated membranes and intact cells. Using this assay, we compared the enzymatic activity of wild type UBIAD1 with that of SCD-associated variants of the enzyme. Our studies revealed that SCD-associated UBIAD1 exhibited reduced MK-4 synthetic activity, which may result from their reduced affinity for GGpp. Sequestration in the ER protects SCD-associated UBIAD1 from autophagy and allows intracellular accumulation of the mutant protein, which amplifies the inhibitory effect on reductase ERAD. These findings have important implications not only for the understanding of SCD etiology, but also for the efficacy of cholesterol-lowering statin therapy, which becomes limited in part because of UBIAD1-mediated inhibition of reductase ERAD.