Circulating concentrations of asymmetric dimethylarginine (ADMA) strongly predict CV outcomes [1] however causal relationships have been difficult to establish. Local regulation of ADMA levels is achieved through metabolism by three enzymes dependent on the tissue and cell type: the two isoforms of dimethylarginine dimethylaminohydrolase (DDAH) and the poorly characterised enzyme alanine-glyoxylate aminotransferase-2 (AGXT2, which is mainly restricted to kidney).

Recent reports demonstrate a low-frequency coding polymorphism (V140I) in the AGXT2 gene alters urinary metabolite levels [2]. I have now demonstrated that AGXT2 plays a physiological role in ADMA metabolism by phenotyping AGXT2 null mice and quantifying the association between kidney tissue AGXT2 expression and circulating methylarginines in human transplant recipients (accounting for ~20% of ADMA variation). Furthermore I have shown disruption of the AGXT2 gene leads to hypertension in mice and, by association with V140I, diastolic blood pressure in humans [3]. The increased circulating ADMA observed with AGXT2 disruption may lead to hypertension by inhibiting production of endothelial-derived nitric oxide (NO) with consequent endothelial dysfunction.However local NO production has also been shown to inhibit renal sodium reabsorption along the nephron [4] and this is an alternative mechanism by which AGXT2 disruption might mediate hypertension.

As part of an intermediate fellowship application, which includes a number of epidemiological, human physiological and animal studies, I plan to examine the mechanisms underlying AGXT2 mediated hypertension. Specifically I hope to examine in collaboration with Professor John Deanfield the association between V140I and flow-mediated dilatation in humans to investigate if AGXT2 disruption leads to hypertension through endothelial dysfunction in humans.

Hypothesis: Functional variants in methylarginine metabolising enzymes are associated with impaired endothelial function

Exposure: Common variation in the genes encoding AGXT2 (and DDAH1) at loci previously reported to associate with an intermediate phenotype e.g. rs37369 (AGXT2 - V140I).

Outcomes: Per cent flow mediated dilatation (%FMD).

Adjustment for confunding by ethnicity (reported ethnic origin).

Power calculations: There will be 90% (alpha=0.05)power to detect an absolute difference of 0.5% in percent FMD (mean FMD: 8%; SD:4%) in those carrying the minor alle (assuming a MAF 0.1) at rs37369.

The opportunity to investigate the mechanism underlying the ADMA-CVD relationship using genetic tools will help us to establish the causal relationships in this area. Results of these studies may lead to the development of biomarkers for risk prediction as well as the potential to develop strategies to target this pathway.

1. Aucella, F., et al., Methylarginines and mortality in patients with end stage renal disease: A prospective cohort study. Atherosclerosis, 2009. 207(2): p. 541-545.

2. Suhre, K., et al., A genome-wide association study of metabolic traits in human urine. Nature Genetics, 2011. 43(6): p. 565-9.

3. Caplin, B., et al., Alaninie-Glyoxylate Aminotransferase-2 Metabolises Endogenous Methylarginines, Regulates Nitric Oxide and Controls Blood Pressure. . Arterioscler Thromb Vasc Biol, In Revision.

4. Garvin, J.L., M. Herrera, and P.A. Ortiz, Regulation of renal NaCl transport by nitric oxide, endothelin, and ATP: clinical implications. Annual review of physiology, 2011. 73: p. 359-76.