Effect of the Asp298 Variant of Endothelial Nitric Oxide Synthase on Survival for Patients With Congestive Heart Failure
To the Editor:
McNamara et al1 recently reported a significant association between the Asp298 variant of endothelial NO synthase (NOS3) and poorer event-free survival in congestive heart failure (CHF) patients. This study was based on the premise that the Asp298 NOS3 variant would have a shorter half-life, thus implying a decreased in vivo generation of NO, which is deemed to play a protective role. To support their contention, the authors quoted a study that claimed the Asp298 variant to be more vulnerable to intracellular cleavage.2 However, this contention was subsequently disputed by a painstaking study that clearly demonstrated that the intracellular cleavage found in cells harboring the Asp298 NOS3 substitution was an in vitro artifact3 caused by the acidic pH used.2 According to both studies, the Asp298 replacement does not affect NOS3 biological activity.2,3
Thus, it remains controversial whether the Asp298 variant implies a blunted NOS3 activity; accordingly, the contention that the latter accounts for the poorer event-free survival in CHF observed by McNamara et al1 cannot be taken for granted.
At variance with the Asp298 variant, another single-nucleotide polymorphism located in the promoter region of the NOS3 gene, the T−786C was shown to bear functional consequences. The C−786 allele binds a replication protein A1 that acts as a repressor of NOS3 transcription. Therefore, it is not surprising that this allele has been associated with a significant reduction in NOS3 gene promoter activity, with ensuing reduction in the rate of transcription of NOS3 in response to hypoxia and also with vasospastic angina in Japanese patients.
We recently reported that the Asp298 NOS3 variant was in linkage disequilibrium with the T−786C substitution.4 More importantly, we also showed that this variant, and not the Asp,298 was associated with a blunted forearm blood flow response to acetylcholine in essential hypertensive patients5 and with multivessel coronary artery disease in the GENICA study.4 Therefore, we propose that the C−786 variant, which was not determined in the study by McNamara et al,1 can be the most relevant one, both functionally and as predictor of events in CHF patients.
McNamara DM, Holubkov R, Postava L, et al. Effect of the Asp298 variant of endothelial nitric oxide synthase on survival for patients with congestive heart failure. Circulation. 2003; 107: 1598–1602.
Tesauro M, Thompson WC, Rogliani P, et al. Intracellular processing of endothelial nitric oxide synthase isoforms associated with differences in severity of cardiopulmonary diseases: cleavage of proteins with aspartate vs glutamate at position 298. Proc Natl Acad Sci U S A. 2000; 97: 2832–2835.
Fairchild TA, Fulton D, Fontana JT, et al. Acidic hydrolysis as a mechanism for the cleavage of the Glu(298)→Asp variant of human endothelial nitric-oxide synthase. J Biol Chem. 2001; 276: 26674–26679.
We appreciate the point emphasized by Drs Maiolino and Rossi that other common polymorphisms exist in the NOS3 gene, including the T−786 C promoter polymorphism referenced in their 2 recent studies. We also agree that there is much debate about the functional significance of both the T−786 C and the Asp298Glu polymorphisms. Overall, clinical studies of these polymorphisms support the concept that genetic heterogeneity at the NOS3 locus influences both cardiovascular risk and clinical outcomes. Whether these specific mutations are causative, or in linkage disequilibrium with distinct and functionally more important polymorphisms, remains open to question.
Although not part of the published study, the impact of the T−786C polymorphism for heart failure outcomes has been investigated by M.B. in our laboratory. Although this analysis does confirm linkage disequilibrium between these 2 polymorphisms, the T−786C polymorphism does not appear to influence event-free survival. We cannot exclude the possibility raised by Drs Maiolino and Rossi that the impact of the Asp298 variant in our cohort was due to linkage to an additional polymorphism of NOS3. However, our data do not support the influential role for the T−786C polymorphism that they have proposed.