Several recent studies indicate that loss of function (LoF) mutations in the ANGPTL3, 4 and 8 gene lower levels of triglyceride rich lipoproteins (TRLs) and ANGPTL3 and ANGPTL4 LoF variants offer protection from coronary heart disease (CHD). Inhibition of the ANGPTL3, 4 or 8 can therefore provide an attractive therapeutic mechanism to lower CHD risk. There are however many outstanding uncertainties that exist, pertaining to the impact of complete or partial genetic deficiency of these three proteins on: (i) the relative contribution of TRL vs LDL vs HDL in mediating CHD risk reduction; (ii) the impact on the range of apoB and apoA containing lipoproteins; (iii) the interrelationships of these three ANGPTL proteins; (iv) the dose-response relationship with CHD risk; (v) the relevance of complete or partial deficiency to glucose tolerance; and (vi) the safety implications. This proposal aims to address these gaps by leveraging natural human models of ANGPTL3, 4 and 8 deficiency, already enrolled in two population groups: (i) an Italian cohort (n = 613) that is enriched for ANGPTL3 human knockouts (n = 22); (ii) the Pakistan Genomic Resource (PGR) (n ~ 100,000) enriched for consanguinity. Specifically, in AIM-1, in 22 trios (22 ANGPTL3 null homozygous knockouts and an equal number of heterozygotes and non-carriers) from Italy, we will measure: lipoprotein particle concentration, size, and composition, relationship with ANGPTL8 and ANGPTL4 levels in fed and fasting states, metabolic parameters (e.g., oral glucose tolerance test [OGTT]), and liver function. We will also evaluate the impact of ANGPTL3 deficiency on kinetics of apoB containing lipoproteins, including VLDL apoB production rate, conversion rates of VLDL to IDL and LDL apoB, and clearance rates of VLDL, IDL, and LDL apoB. We will also assess disease mediation by using ANGPTL3 LoF genotypes and CHD risk with LDL-C, TRL and other risk factors (n = 613).
In AIM -2, will leverage the Pakistani bioresource, PGR, and in 22 ANGPTL4 LoF carriers and 22 non- carriers, we will measure: lipoprotein particle concentration, size, and composition, metabolic parameters (e.g., [OGTT]), and liver function. We will also assess kinetics of TRL-apoB and HDL apoA-I containing lipoproteins through isotope traced based studies and conduct OGTT, hepatic fat quantification and abdominal CT scans to test for lymphadenopathy. We will also conduct analyses to explore relative contribution of TG, LDL-C and HDL-C in mediating CHD risk conferred by ANGPTL4 LoF through large population based studies.
In AIM -3, we will measure plasma ANGPTL8 in 5,000 incident MI cases and 5,000 controls, which will enable (a) MI case-control analyses, (b) gene-discovery analyses in relation to ANGPTL8 levels, and (c) Mendelian Randomization analyses to assess causality of ANGPTL8 levels with CHD risk. In 22 ANGPTL8 LoF carriers and 22 non-carriers, we will measure OGTT and hepatic fat to assess safety profile and conduct lipoprotein kinetic studies to examine the relationship of ANGPTL8 LoF with lipid metabolism.
We and others have identified that individuals who carry loss of function (LoF) mutations in the ANGPTL3, ANGPTL4 and ANGPTL8 genes have a natural tendency to have lower plasma triglycerides and as a result lower risk of CHD; hence these three genes in the ANGPTL pathway are being considered as therapeutic targets in CHD. In this application, we will conduct extensive phenotyping studies in the participants who carry LoF mutations in the ANGPTL3, ANGPTL4 and ANGPTL8 genes and matched non-carriers to investigate biology and understand any adverse metabolic effects resulting due to natural deficiency of any of the three genetic products. We will also use data from large population-based studies to help understand the associations of both plasma ANGPTL8 and mutations in the ANGPTL8 gene with a range of cardio-metabolic disorders.
