Homocysteine influences blood clot properties alone and in combination with total fibrinogen but not with fibrinogen γ' in Africans

The Relationship of Circulating Homocysteine with Fibrinogen, Blood Pressure, and Other Cardiovascular Measures in African Adolescents

OBJECTIVES

To evaluate the associations between homocysteine (Hcy) and cardiovascular health in South African adolescents.

STUDY DESIGN

Circulating Hcy concentrations of 172 South African adolescents (105 girls, ages 13 to <18 years) were measured. Anthropometric and cardiovascular factors were also included and cross-sectionally analyzed through general linear models.

RESULTS

Hcy correlated positively with body weight (P = .03; after adjusting for multiple testing, it was not regarded as significant) and muscle mass (P = .01), but negatively with fibrinogen concentrations (P = .001). Across Hcy tertiles, blood pressure produced approximating U-shaped curves, with differences between the middle and upper tertiles (all P < .02). Forty percent of the adolescents had elevated blood pressure, of whom 37% fell in the lowest and 38% in the highest Hcy tertiles. Hcy differed between the sexes (with boys having higher Hcy), but not between subgroups based on puberty, weight, stunting, smoking, or alcohol consumption.

CONCLUSIONS

Both high and low Hcy could be early contributing risk factors to cardiovascular health. The associations between Hcy and blood pressure suggest that dietary and lifestyle manipulation, to achieve the optimal range of Hcy, may be beneficial in preventing Hcy-related hypertension in adulthood. The inverse relationship between Hcy and fibrinogen remains to be clarified.

Effects of Hyperhomocysteinemia on the Platelet-Driven Contraction of Blood Clots

Hyperhomocysteinemia (HHcy) is associated with thrombosis, but the mechanistic links between them are not understood. We studied effects of homocysteine (Hcy) on clot contraction in vitro and in a rat model of HHcy. Incubation of blood with exogenous Hcy for 1 min enhanced clot contraction, while 15-min incubation led to a dose-dependent suppression of contraction. These effects were likely due to direct Hcy-induced platelet activation followed by exhaustion, as revealed by an increase in fibrinogen-binding capacity and P-selectin expression determined by flow cytometry. In the blood of rats with HHcy, clot contraction was enhanced at moderately elevated Hcy levels (10–50 μM), while at higher Hcy levels (>50 μM), the onset of clot contraction was delayed. HHcy was associated with thrombocytosis combined with a reduced erythrocyte count and hypofibrinogenemia. These data suggest that in HHcy, platelets get activated directly and indirectly, leading to enhanced clot contraction that is facilitated by the reduced content and resilience of fibrin and erythrocytes in the clot. The excessive platelet activation can lead to exhaustion and impaired contractility, which makes clots larger and more obstructive. In conclusion, HHcy modulates blood clot contraction, which may comprise an underappreciated pro- or antithrombotic mechanism.

Natural Selection on Genes Related to Cardiovascular Health in High-Altitude Adapted Andeans

The increase in red blood cell mass (polycythemia) due to the reduced oxygen availability (hypoxia) of residence at high altitude or other conditions is generally thought to be beneficial in terms of increasing tissue oxygen supply. However, the extreme polycythemia and accompanying increased mortality due to heart failure in chronic mountain sickness most likely reduces fitness. Tibetan highlanders have adapted to high altitude, possibly in part via the selection of genetic variants associated with reduced polycythemic response to hypoxia. In contrast, high-altitude-adapted Quechua- and Aymara-speaking inhabitants of the Andean Altiplano are not protected from high-altitude polycythemia in the same way, yet they exhibit other adaptive features for which the genetic underpinnings remain obscure. Here, we used whole-genome sequencing to scan high-altitude Andeans for signals of selection. The genes showing the strongest evidence of selection-including BRINP3, NOS2, and TBX5-are associated with cardiovascular development and function but are not in the response-to-hypoxia pathway. Using association mapping, we demonstrated that the haplotypes under selection are associated with phenotypic variations related to cardiovascular health. We hypothesize that selection in response to hypoxia in Andeans could have vascular effects and could serve to mitigate the deleterious effects of polycythemia rather than reduce polycythemia itself.