The human transmembrane 6 superfamily member 2 (TM6SF2) gene has been implicated in plasma lipoprotein metabolism, alcoholic and non-alcoholic fatty liver disease and myocardial infarction in multiple genome-wide association studies. To investigate the role of Tm6sf2 in metabolic homeostasis, we generated mice with elevated expression using adeno-associated virus (AAV)-mediated gene delivery. Hepatic overexpression of mouse Tm6sf2 resulted in phenotypes previously observed in Tm6sf2-deficient mice including reduced plasma lipid levels, diminished hepatic triglycerides secretion and increased hepatosteatosis. Furthermore, increased hepatic Tm6sf2 expression protected against the development of atherosclerosis in LDL-receptor/ApoB48-deficient mice. In cultured human hepatocytes, Tm6sf2 overexpression reduced apolipoprotein B secretion and resulted in its accumulation within the endoplasmic reticulum (ER) suggesting impaired ER-to-Golgi trafficking of pre-very low-density lipoprotein (VLDL) particles. Analysis of two metabolic trait-associated coding polymorphisms in the human TM6SF2 gene (rs58542926 and rs187429064) revealed that both variants impact TM6SF2 expression by affecting the rate of protein turnover. These data demonstrate that rs58542926 (E167K) and rs187429064 (L156P) are functional variants and suggest that they influence metabolic traits through altered TM6SF2 protein stability. Taken together, our results indicate that cellular Tm6sf2 level is an important determinant of VLDL metabolism and further implicate TM6SF2 as a causative gene underlying metabolic disease and trait associations at the 19p13.11 locus.

In addition to a novel truncating variant, we describe for the first time a missense variant functionally demonstrated affecting the lipase maturation function of LMF1. This is the first case in which compound heterozygous variants in LMF1 were functionally demonstrated as causative of severe hypertriglyceridemia.

BackgroundLipase Maturation Factor 1 (LMF1) is an ER-chaperone involved in the post-translational maturation and catalytic activation of vascular lipases including lipoprotein lipase (LPL), hepatic lipase (HL) and endothelial lipase (EL). Mutations in LMF1 are associated with lipase deficiency and severe hypertriglyceridemia indicating the critical role of LMF1 in plasma lipid homeostasis. The currently available mouse model of LMF1 deficiency is based on a naturally occurring truncating mutation, combined lipase deficiency (cld), which may represent a hypomorphic allele. Thus, development of LMF1-null mice is needed to explore the phenotypic consequences of complete LMF1 deficiency.FindingsIn situ hybridization and qPCR analysis in the normal mouse embryo revealed ubiquitous and high-level LMF1 expression. To investigate if LMF1 was required for embryonic viability, a novel mouse model based on a null-allele of LMF1 was generated and characterized. LMF1-/- progeny were born at Mendelian ratios and exhibited combined lipase deficiency, hypertriglyceridemia and neonatal lethality.ConclusionOur results raise the possibility of a previously unrecognized role for LMF1 in embryonic development, but indicate that LMF1 is dispensable for the viability of mouse embryo. The novel mouse model developed in this study will be useful to investigate the full phenotypic spectrum of LMF1 deficiency.

Background: Lipase maturation factor 1 (Lmf1) plays an important role in plasma lipid metabolism, but its regulation remains uncharacterized. Results: Endoplasmic reticulum (ER) stress induces Lmf1 expression in cell lines and mouse liver. Atf6␣ deficiency abolishes, whereas active Atf6␣ stimulates this response. Conclusion: Lmf1 is an unfolded protein response (UPR) target through Atf6␣ signaling. Significance: Lmf1 regulation by the UPR suggests a possible role in ER homeostasis.

Vascular lipases including lipoprotein lipase (LPL), hepatic lipase (HL) and endothelial lipase (EL) are important determinants of plasma lipid homeostasis and coronary artery disease risk. The ER-resident lipase-chaperone Lipase Maturation Factor 1 (LMF1) plays a key role in the biogenesis of active vascular lipases and plasma lipid metabolism. While mice and humans harboring loss-of-function mutations in LMF1 exhibit deficiencies in all three vascular lipases and hypertriglyceridemia, detailed metabolic characterization of LMF1 deficiency has been hampered by the neonatal lethality of LMF1-/- mice. The goal of the present study was to investigate the metabolic consequences of combined lipase deficiency in the adult mouse. To overcome the lethality associated with whole-body LMF1 deficiency, we pursued a transgene rescue strategy involving cross-breeding of Mck-LMF1 transgenic and LMF1-/- mice. Mice expressing LMF1 exclusively in muscle tissue (Mck-LMF1-/-) exhibited normal survival and plasma lipid levels indicating that restoration of LPL activity in muscle only is sufficient to normalize plasma lipid metabolism in LMF1-/- mice. However, Mck-LMF1-/- mice also demonstrated decreased body weight (BW) and adiposity, reduced hepatic steatosis and improved insulin sensitivity; phenotypes that have not been observed in single-lipase deficient mouse models. Mechanistic studies revealed that decreased energy expenditure was responsible for diminished BW in Mck-LMF1-/- mice. To investigate the role of adipose tissue in this phenotype, we generated adipose-specific LMF1 knock-out (Ad-LMF1-KO) mice. While Ad-LMF1-KO mice exhibited hypertriglyceridemia, BW and adiposity remained unaffected indicating that adipose tissue was not the driver of lean phenotype in LMF1 deficiency. Furthermore, normolipemic heterozygous LMF1+/- mice exhibited lower BW and adiposity relative to wild-type littermates. Finally, we observed direct correlation between hepatic LMF1 expression and adiposity across ~100 inbred mouse strains in the Hybrid Mouse Diversity Panel. In conclusion, our results implicate hepatic LMF1 expression in energy metabolism and suggest that LMF1 plays a novel, lipase-independent role in metabolic regulation.