Schnyder corneal dystrophy-associated UBIAD1 is defective in MK-4 synthesis and resists autophagy-mediated degradation

Abstract

The autosomal dominant disorder Schnyder corneal dystrophy (SCD) is caused by mutations in UbiA prenyltransferase domain-containing protein-1 (UBIAD1), which uses geranylgeranyl pyrophosphate (GGpp) to synthesize the vitamin K2 subtype menaquinone-4 (MK-4). SCD is characterized by opacification of the cornea, owing to aberrant build-up of cholesterol in the tissue. We previously discovered that sterols stimulate association of UBIAD1 with endoplasmic reticulum (ER)-localized HMG CoA reductase, which catalyzes a rate-limiting step in synthesis of cholesterol and nonsterol isoprenoids, including GGpp. Binding to UBIAD1 inhibits sterol-accelerated, ER-associated degradation (ERAD) of reductase and permits continued synthesis of GGpp in cholesterol-replete cells. GGpp disrupts UBIAD1-reductase binding and thereby allows for maximal ERAD of reductase as well as ER-to-Golgi translocation of UBIAD1. SCD-associated UBIAD1 is refractory to GGpp-mediated dissocation from reductase and remains sequestered in ER to inhibit ERAD. Here, we report development of a biochemical assay for UBIAD1-mediated synthesis of MK-4 in isolated membranes and intact cells. Using this assay, we compared enzymatic activity of wild type UBIAD1 with that of SCD-associated variants. Our studies revealed that SCDassociated 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 cholesterollowering statin therapy, which becomes limited in part because of UBIAD1-mediated inhibition of reductase ERAD. Keywords: endoplasmic reticulum/Golgi apparatus/vesicular transport/menaquinone-4/autophagy/cholesterolby guest, on July 4, 2020 www.jlr.org Downloaded from

Murine deficiency of peroxisomal L-bifunctional protein (EHHADH) causes medium-chain 3-hydroxydicarboxylic aciduria and perturbs hepatic cholesterol homeostasis

“…4B). The increase in HMGCR protein was much larger than the increase in Hmgcr mRNA, which may be explained by well-documented posttranscriptional regulatory mechanisms of HMGCR (Goldstein & Brown 1990; Jun et al 2020). Mevalonate kinase (MVK, 1.5-fold) and phosphomevalonate kinase (PMVK, 1.9-fold) were also increased in Ehhadh KO livers (Fig.…”

“…4B). The increase in HMGCR protein was much larger than the increase in Hmgcr mRNA, which may be explained by well-documented posttranscriptional regulatory mechanisms of HMGCR (Goldstein & Brown 1990;Jun et al 2020).…”

Functional study of SCCD pathogenic gene UBIAD1 (Review)

“…The pathogenesis of SCCD was then investigated and clarified by consistently exploring the functions of UBIAD1. From research, it was identified that UBIAD1 regulates HMGCR through GGpp in cholesterol biosynthesis and metabolism of significant componential molecules ( 46 , 53 , 54 , 56 , 58 , 59 , 87 , 88 ). UBIAD1 is also a critical tumor suppressor in the urinary system ( 11 , 44 , 47 , 48 , 51 , 88 ) and regulates cell proliferation through the ras signaling pathway ( 88 ).…”

“…A SCCD-related mice model affirms the physiological significance of UBIAD1 in cholesterol homeostasis and demonstrates the inhibition of HMGCR ERAD, contributing to SCCD pathogenesis ( 46 , 58 ). Similarly, Jun et al ( 59 ) establishes a biochemical assay for UBIAD1-mediated synthesis of MK-4 in isolated membranes and intact cells. The results reveal that mutated UBIAD1 exhibited reduced Mk-4 biosynthetic activity compared with wild-type UBIAD1.…”

“…Sequestration in the ER, therefore, protects SCCD-associated UBIAD1 from autophagy and allows intracellular accumulation of the mutated protein, which magnifies the inhibition of HMGCR ERAD. The results of Jun et al ( 59 ) further broaden the understanding of SCCD pathogenesis and limit the efficacy of cholesterol-lowering statin therapies.…”