Early Embryonic Lethality Caused by Targeted Disruption of the 3-Hydroxy-3-methylglutaryl-CoA Reductase Gene

Abstract

The endoplasmic reticulum (ER) enzyme 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, which converts HMG-CoA to mevalonate, catalyzes the ratelimiting step in cholesterol biosynthesis. Because this mevalonate pathway also produces several non-sterol isoprenoid compounds, the level of HMG-CoA reductase activity may coordinate many cellular processes and functions. We used gene targeting to knock out the mouse HMG-CoA reductase gene. The heterozygous mutant mice (Hmgcr؉/؊) appeared normal in their development and gross anatomy and were fertile. Although HMG-CoA reductase activities were reduced in Hmgcr؉/؊ embryonic fibroblasts, the enzyme activities and cholesterol biosynthesis remained unaffected in the liver from Hmgcr؉/؊ mice, suggesting that the haploid amount of Hmgcr gene is not rate-limiting in the hepatic cholesterol homeostasis. Consistently, plasma lipoprotein profiles were similar between Hmgcr؉/؊ and Hmgcr؉/؉ mice. In contrast, the embryos homozygous for the Hmgcr mutant allele were recovered at the blastocyst stage, but not at E8.5, indicating that HMG-CoA reductase is crucial for early development of the mouse embryos. The lethal phenotype was not completely rescued by supplementing the dams with mevalonate. Although it has been postulated that a second, peroxisome-specific HMG-CoA reductase could substitute for the ER reductase in vitro, we speculate that the putative peroxisomal reductase gene, if existed, does not fully compensate for the lack of the ER enzyme at least in embryogenesis.The mevalonate pathway produces isoprenoids that are essential for diverse cellular functions, ranging from cholesterol synthesis to growth control. The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) 1 reductase (EC 1.1.1.34), which catalyzes the conversion of HMG-CoA to mevalonate, is the rate-limiting enzyme in the mevalonate pathway (1). Because of its major role in cholesterol biosynthesis, the regulation of HMG-CoA reductase has been intensely studied. To ensure a steady mevalonate supply, the non-sterol and sterol end-products of mevalonate metabolism exert feedback regulation on the activity of this enzyme through multivalent mechanisms, including inhibition of transcription of the HMG-CoA reductase mRNA, blocking of translation, and acceleration of protein degradation, thus regulating the amount of reductase protein over a several hundred-fold range (reviewed in Refs.

Functional and association studies on the pig HMGCR gene, a cholesterol-synthesis limiting enzyme

“…Moreover, HMGCR mRNA levels are unaffected by changes in intronic splicing events (Medina et al, 2008). Consistent to this, the slight decrease in liver HMGCR mRNA levels of hemizygous HMGCR mice resulted in no differences in cholesterol or TG content in liver or blood (Ohashi et al, 2003). In this same line, divergent selection on pigs for serum total cholesterol (seven generations) was not associated with differences in hepatic or jejunum HMGCR activity or with HMGCR RFLP polymorphisms, at least in young animals, but rather with CYP7 polymorphisms (Schoknecht et al, 1994).…”

Robust Central Reduction of Amyloid-β in Humans with an Orally Available, Non-Peptidic β-Secretase Inhibitor

“…Similarly, the deficits in behavioral cognitive tests, e.g., Morris water maze, Y maze, etc., observed in BACE knock-outs were not present in BACE1 hemizygous mice (Laird et al, 2005), suggesting that partial reduction in BACE1 activity may be better tolerated even when it already occurs during development. Finally, as a cautionary note about over-interpreting theoretical safety concerns from KO data, it should be noted that genetic KO of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase results in embryonic lethality (Ohashi et al, 2003), but statins that target HMG-CoA reductase remain one of the most widely prescribed classes of drugs in the world (Simmons, 2003).…”

ABCA1 plays no role in the centripetal movement of cholesterol from peripheral tissues to the liver and intestine in the mouse

“…As a consequence, rates of cholesterol synthesis are very high in the growing organs of the developing fetus and newborn animal (3)(4)(5). Any mutation that limits the availability of this critical molecule during development leads either to intrauterine death of the embryo or to major developmental abnormalities in the newborn (6)(7)(8). Even after maturity is reached and organs attain their adult size, there is continuous replacement, or "turnover," of cholesterol in the membranes of cells.…”