(Stroke. 2005;36:1821.)
© 2005 American Heart Association, Inc.
Letters to the Editor |
Protein Z Levels, Protein Z G79A Polymorphism, and Prothrombotic Conditions
Department of Medical and Surgical Critical Care, Thrombosis Centre, University of Florence and, Department of Heart and Vessels, Azienda Ospedaliero-Universitaria Careggi, Florence, Italy
To the Editor:
We read with great interest the article on protein Z gene polymorphisms, protein Z concentrations, and ischemic stroke published by Staton et al.1 The authors examine protein Z concentrations and 2 common polymorphisms of the protein Z gene in patients with a first-ever ischemic stroke and conclude that high levels of protein Z are prothrombotic and that the 79AA genotype of the G79A polymorphism is protective for the ischemic stroke.
The conclusions drawn by Staton et al seem questionable because of several flaws.
First, the hypothesis that high levels of protein Z are prothrombotic is, to date, not supported by the pathophysiology, because the role of protein Z has been demonstrated to be the inhibition of activated factor X through the formation of a complex with protein Z–dependent protease inhibitor. Consequently, low levels of protein Z are expected to be prothrombotic. In line with this theory, the majority of the studies in this field have reported low levels of protein Z in association with prothrombotic conditions. Actually, only 2 studies have reported that high levels of protein Z are prothrombotic, but methodological biases are detectable. In the former, the investigators modified the commercial assay and used a normal plasma pool.2 In the latter, the control group included subjects with a previous vascular event.3
Second, Staton et al reported lower levels of protein Z plasma in controls with respect to all the other case-control studies present in the literature, including reports with a greater number of subjects analyzed. This might be attributable to the presence of subjects with a previous vascular event (not better clarified) included in the control group. Actually, patients with vascular diseases have been reported to have low levels of protein Z.4
Third, the significant result of the 79A allele as a protective factor for the ischemic stroke has been performed by pooling data from a previous study. Actually, a pooled analysis in case-control studies, especially for the evaluation of genetic polymorphisms, is rather a questionable approach. To perform this kind of analysis, the 2 populations should be homogenous and similar for the main parameters investigated. Conversely, the Staton et al do not explain how they tested the homogeneity of the 2 populations, particularly with regard to genetic background and prevalence of the traditional cardiovascular risk factors. Too many differences (first of all, age) between such distant and different populations are detectable to correctly perform this analysis.
In fact, the analysis performed in their single population demonstrated no significant relationship between both the polymorphisms and the ischemic stroke; the only significant result derives from pooling data from the study by Lichy et al5 and is also determined by an overall low number of homozygotes for the AA genotype (n=32), thereby not reaching a sufficient statistical power.
In consideration of all these limitations, we do not believe that the conclusions stated by Staton et al, ie, high levels of protein Z prothrombotic and 79A allele protective for the ischemic stroke, can be drawn from this study. Because low protein Z plasma levels have been reported in several studies to be associated with different prothrombotic conditions, it can be hypothesized that protein Z is either simply a marker of the disease or influenced by the acute phase, but it is at least questionable, to date, to postulate that high levels of protein Z are associated with prothrombotic conditions and that 79AA genotype is protective for the ischemic stroke.
References
- Staton J, Sayer M, Hankey GJ, Cole V, Thom J, Eikelboom JW. Protein Z gene polymorphisms, protein Z concentrations, and ischemic stroke. Stroke. 2005; 36: 1123–1127.
[Abstract/Free Full Text] - Kobelt K, Biasiutti FD, Mattle HP, Lammle B, Wuillemin WA. Protein Z in ischaemic stroke. Br J Hematol. 2001; 114: 169–173.[CrossRef][Medline] [Order article via Infotrieve]
- McQuillan AM, Eikelboom JW, Hankey GJ, Baker R, Thom J, Staton J, Yi Q, Cole V. Protein Z in ischemic stroke and its etiologic subtypes. Stroke. 2003; 34: 2415–2419.
[Abstract/Free Full Text] - Fedi S, Sofi F, Brogi D, Tellini I, Cesari F, Sestini I, Gazzini A, Comeglio M, Abbate R, Gensini GF. Low protein Z plasma levels are independently associated with acute coronary syndromes. Thromb Haemost. 2003; 90: 1173–1178.[Medline] [Order article via Infotrieve]
- Lichy C, Kropp S, Dong-Si T, Genius J, Dolan T, Hampe T, Stoll F. Renner K, Grond-Ginsbach C, Grau A. A common polymorphism of the protein Z gene is associated with protein Z plasma levels and with risk of cerebral ischemia in the young. Stroke. 2004; 35: 40–45.
[Abstract/Free Full Text]
Response:
Department of Medicine, McMaster University, Hamilton, Canada
Department of Hematology, Royal Perth Hospital, Perth, Australia
School of Medicine and Pharmacology, University of Western Australia, Perth, Australia
Dr Sofi and colleagues question our conclusion that the consistency of the association between protein Z genotype, elevated blood concentrations of protein Z, and ischemic stroke strengthens the evidence that increased blood concentrations of protein Z concentrations are causally associated with the risk of ischemic stroke.1
First, they propose that our conclusion is not supported by pathophysiology, citing methodological bias as an explanation for previous reports of elevated blood concentrations protein Z in stroke patients. However, they provide no explanation of how the use of a normal plasma pool may have biased the results of the study by Kobelt et al.2 In the study by McQuillan et al,3 removal of control subjects with a previous vascular event from the analysis does not change the results or conclusions.
Second, as indicated by Sofi et al, blood concentrations of protein Z in our study were lower than previously reported in several other studies, including their own.4 The reason for this is unclear, although could potentially be related to prolonged storage of blood samples before measuring protein Z levels.3 However, irrespective of the cause, our within-study comparisons of protein Z concentrations in stroke cases and controls remain valid because the duration of storage of samples for cases and controls was similar and all laboratory samples were collected, processed, stored, and analyzed in the same manner.
Third, Sofi and colleagues question our approach of pooling results from case control studies to clarify the association between protein Z polymorphisms and ischemic stroke. However, this is a well-established technique that is commonly used for aggregating previous research when individual studies have insufficient power to detect an association.5 Taken together with the data by Lichy et al,6 we believe that our study results are consistent with the conclusion that elevated blood concentrations of protein Z are prothrombotic.
References
- Staton J, Sayer M, Hankey GJ, Cole V, Thom J, Eikelboom JW. Protein Z gene polymorphisms, protein Z concentrations, and ischemic stroke. Stroke. 2005; 36: 1123–1127.
[Abstract/Free Full Text] - Kobelt K, Biasiutti FD, Mattle HP, Lammle B, Wuillemin WA. Protein Z in ischemic stroke. Br J Haematol. 2001; 114: 169–173.[CrossRef][Medline] [Order article via Infotrieve]
- McQuillan AM, Eikelboom JW, Hankey GJ, Baker R, Thom J, Staton J, Ti Q, Cole V. Protein Z in ischemic stroke and its etiologic subtypes. Stroke. 2003; 34: 2415–2419.
[Abstract/Free Full Text] - Fedi S, Sofi F, Brogi D, Tellini I, Cesari F, Sestini I, Gazzini A, Comeglio M, Abbate R, Gensini GF. Low protein Z plasma levels are independently associated with acute coronary syndromes. Thromb Haemost. 2003; 90: 1173–1178.[Medline] [Order article via Infotrieve]
- Attia J, Phakkinstian A, D’Este C. Meta analysis of molecular association studies: methodological lessons for genetic epidemiology. J Clin Epidemiol. 2003; 56: 297–303.[CrossRef][Medline] [Order article via Infotrieve]
- Lichy C, Kropp S, Dong-Si T, Genius J, Dolan T, Hampe T, Stoll F. Renner K, Grond-Ginsbach C, Grau A. A common polymorphism of the protein Z gene is associated with protein Z plasma levels and with risk of cerebral ischemia in the young. Stroke. 2004; 35: 40–45.
[Abstract/Free Full Text]
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