👤 Michael B Boffa

Elevated levels of lipoprotein(a) (Lp[a]) are a causal risk factor for atherosclerotic cardiovascular disease. Similarities between the apolipoprotein(a) (apo[a]) component of Lp(a) and plasminogen suggest that antifibrinolytic properties may account for the pathological effects of Lp(a). However, the antifibrinolytic effects of apo(a) do not appear to be retained by the complete Lp(a) particle. We evaluated the effects of Lp(a), apo(a), and various apo(a) variants on clot formation and lysis times, thrombin generation, plasminogen activation, and fibrin architectures in ex vivo plasma clots. We also constructed predictive protein models to gain insight into the apo(a)-plasminogen interaction. Apo(a) strongly inhibited fibrinolysis, an effect dependent on the presence of the apo(a) protease domain and mediated by Lys216 in this domain. Modeling of apo(a) suggests that Lys216 is blocked from binding to plasminogen in the Lp(a) particle by the presence of the apoB-containing lipoprotein. Lp(a) and apo(a) shortened plasma clot formation times, and accounting for this revealed a small but significant prolongation of fibrinolysis by Lp(a). The procoagulant effects involved the development of lysis-resistant clot architectures and were mediated through the strong lysine-binding site in apo(a) kringle IV type 10. In addition, Lp(a) (but not apo[a]) accelerated thrombin generation. The strong antifibrinolytic effects of apo(a) do not appear to be retained in the complete Lp(a) particle. However, Lp(a) and apo(a) displayed procoagulant effects, in part dependent on the kringle 4-like lysine-binding site. Further analysis is required to determine whether these reported procoagulant effects of Lp(a) impact thrombosis in vivo. Show less

Lipoprotein(a) (Lp(a)) is a genetically determined causal risk factor for cardiovascular disease, with approximately 20% of the population exhibiting elevated levels. While there are promising drugs in development, there are currently no approved therapies specifically designed to lower Lp(a) levels. For high-risk individuals with extreme levels of Lp(a), liver-directed genome editing could be an effective one-time solution. Genome editing approaches such as CRISPR and TALENs can reduce Lp(a) in LPA-transgenic mouse models, but they frequently induce large and potentially harmful genomic deletions. Here, we report the first application of TadA-derived cytosine base editing (CBE), delivered via helper-dependent adenovirus (HDAdV) and adeno-associated virus (AAV) vectors, to introduce premature stop codons into LPA. This strategy produced robust and durable lowering of circulating apolipoprotein(a) (apo(a)) in LPA-transgenic mice. Using SMRT-seq with single-molecule unique molecular identifiers, we quantified deletion events and found that CBE did not induce large deletions when targeting a single LPA site and produced only a small fraction (<4%) of large deletions when editing across multiple sites. In contrast, CRISPR-Cas9 cutting of LPA resulted primarily in large deletions. These findings demonstrate that CBE enables sustained reduction of circulating apolipoprotein(a) in an LPA-transgenic mouse model while largely preserving genomic integrity. Show less

Elevated plasma lipoprotein(a) [Lp(a)] is a causal and independent risk factor for atherosclerotic cardiovascular disease and an emerging therapeutic target. Over the past 15 years, many medical bodies from around the world have released scientific statements and clinical guidelines regarding Lp(a). This review tracks how recommendations on Lp(a) have evolved over this timeframe. Powerful studies demonstrating the independent association of elevated Lp(a) in large numbers of patients have been published. The data allowed a more precise formulation of risk categories for Lp(a) levels and of models for how a given level of Lp(a) in a moderate-risk to high-risk primary prevention patient might inform management of modifiable risk factors such as LDL cholesterol. Guidelines and statements have increasingly recommended universal screening for elevated Lp(a) and have identified elevated Lp(a) as a risk-enhancing or amplifying factor. However, some gaps and inconsistencies remain. Ongoing cardiovascular outcomes trials of potent Lp(a)-lowering therapies will inform clinical use of Lp(a) in the future. Presently, consensus is building for measurement of Lp(a) in all adults and for incorporation of Lp(a) levels into clinical decision-making for prevention of cardiovascular disease. However, caution is warranted as the evidence base underlying this consensus has several important missing pieces. Show less

Case-control, intervention and laboratory studies have demonstrated a link between apolipoprotein B (ApoB)-containing lipoproteins and clot structure and thrombosis. There is, however, limited evidence on a population level. We determined the cross-sectional relationship between lipoprotein(a) [Lp(a)], low-density lipoprotein cholesterol (LDL-C), and ApoB with fibrinogen and plasma clot properties in 1462 Black South Africans, a population with higher fibrinogen and Lp(a) levels compared with individuals of European descent. Data were obtained from participants in the South African arm of the Prospective Urban and Rural Epidemiology study. Clot properties analyzed included lag time, slope, maximum absorbance, and clot lysis time (turbidity). Lp(a) was measured in nM using particle-enhanced immunoturbidimetry. General linear models (GLM) were used to determine the associations between ApoB and ApoB-containing lipoproteins with fibrinogen and plasma clot properties. Stepwise regression was used to determine contributors to clot properties and Lp(a) variance. GLM and regression results combined, indicated fibrinogen concentration and rate of clot formation (slope) had the strongest association with Lp(a); clot density associated positively with both Lp(a) and LDL-C; time to clot formation associated negatively with ApoB; and clot lysis time (CLT) demonstrated strong positive associations with both ApoB and LDL-C, while its association with Lp(a) was fibrinogen concentration dependent. These findings suggest that ApoB and the lipoproteins carrying it contribute to prothrombotic clot properties in Africans on an epidemiological level and highlight potential novel prothrombotic roles for these (apo)lipoproteins to be considered for the development of targeted therapeutic approaches to address thrombotic conditions related to clot properties. Show less

Life-threatening hyperammonemia occurs in both inherited and acquired liver diseases affecting ureagenesis, the main pathway for detoxification of neurotoxic ammonia in mammals. Protein O-GlcNAcylation is a reversible and nutrient-sensitive post-translational modification using as substrate UDP-GlcNAc, the end-product of hexosamine biosynthesis pathway. Here we show that increased liver UDP-GlcNAc during hyperammonemia increases protein O-GlcNAcylation and enhances ureagenesis. Mechanistically, O-GlcNAcylation on specific threonine residues increased the catalytic efficiency for ammonia of carbamoyl phosphate synthetase 1 (CPS1), the rate-limiting enzyme in ureagenesis. Pharmacological inhibition of O-GlcNAcase, the enzyme removing O-GlcNAc from proteins, resulted in clinically relevant reductions of systemic ammonia in both genetic (hypomorphic mouse model of propionic acidemia) and acquired (thioacetamide-induced acute liver failure) mouse models of liver diseases. In conclusion, by fine-tuned control of ammonia entry into ureagenesis, hepatic O-GlcNAcylation of CPS1 increases ammonia detoxification and is a novel target for therapy of hyperammonemia in both genetic and acquired diseases. Show less