The results of our study suggest that levels of fibulin-3 in plasma and effusions may aid in determining the diagnosis and prognosis of pleural mesothelioma. The specificity and sensitivity of fibulin-3 in discriminating between asbestos-exposed persons, as well as patients with effusions not due to mesothelioma, and patients with mesothelioma are superior to those of other published markers, and fibulin-3 levels are not influenced by the duration of asbestos exposure. In addition, high levels of fibulin-3 in effusions have a high positive predictive value for the presence of mesothelioma and appear to reflect the prognosis.
Fibulin-3 is a highly conserved member of the extracellular glycoprotein fibulin family encoded by the gene epidermal growth factor–containing fibulin-like extracellular matrix protein 1 (EFEMP1
) on chromosome 2p16.15
Gene expression is low in normal tissues, with the highest expression in the thyroid.16
Fibulin-3 is expressed in condensing mesenchyme, giving rise to bony and cartilaginous structures.17
It mediates cell-to-cell and cell-to-matrix communication, is inversely related to cell growth, and has variable angiogenic effects.18,19
Inactivation of EFEMP1
due to DNA hypermethylation has been reported in lung,20,21
carcinomas. In contrast, fibulin-3 is up-regulated in pancreatic adenocarcinoma metastases,27
and there are conflicting opinions about whether the elevated expression of fibulin-3 enhances or suppresses invasion of glioblastomas.28,29
A single study of differences in gene expression between ovarian or primary peritoneal serous carcinomas and diffuse malignant peritoneal mesothelioma showed that EFEMP1
was overexpressed in all the mesotheliomas but in none of the ovarian or serous carcinomas.30
In hereditary maculopathy, overexpression of fibulin-3 is associated with a mutation in EFEMP1
We have not detected any mutations in the EFEMP1
-coding exons of 20 pleural mesotheliomas (data not shown).
The plasma fibulin-3 level was significantly elevated in patients with mesothelioma in the two separate geographic cohorts (Detroit and New York), and these elevations were confirmed in a blinded validation with the use of specimens from Toronto. The characteristics of the patients with mesothelioma in the Detroit and New York cohorts were similar, including their mean plasma fibulin-3 levels. Plasma fibulin-3 levels discriminated between stage I or II mesothelioma and asbestos exposure without mesothelioma, at a specificity of 94% and a sensitivity of 100%, and the similarity of plasma fibulin-3 levels in early- and late-stage disease suggests that fibulin-3 may be associated with early events in mesothelial transformation. Surprisingly, effusion fibulin-3 levels did not correlate with plasma levels. Cavitary levels of fibulin-3 may reflect the biology of mesothelioma more accurately than plasma levels, because an advanced stage of disease was associated with higher effusion fibulin-3 levels, and effusion fibulin-3 levels (in contrast to plasma levels) were independently prognostic in patients who underwent complete staging at the time of cytoreductive surgery. Only effusion levels of vascular endothelial growth factor (VEGF) have been associated with prognosis in patients with mesothelioma33
; however, VEGF levels cannot discriminate between effusions due to mesothelioma and effusions not due to mesothelioma.34
The data from this study cannot support a conclusion that fibulin-3 is an early detection marker for mesothelioma, owing to the lack of prospective, plasma-based longitudinal collections. Our CARET data suggest that plasma, not serum, should be used for fibulin measurement, because of the possibility of uncontrolled thrombin cleavage. Further validation studies of plasma fibulin-3, as well as the prognostic implications of an elevated fibulin-3 level, must be performed as part of an international effort to investigate the management of a rising plasma fibulin-3 level and to determine the number of years before clinical onset that plasma fibulin-3 can be used to detect mesothelioma. Moreover, the role of fibulin-3 as a monitoring biomarker after treatment for the disease should be prospectively validated, since we report that plasma fibulin-3 levels fell dramatically after surgical cytoreduction and rose at the time of progression. The precision of the cutoff points defined in this study for maximal sensitivity and specificity in the detection of disease will also need to be examined in further studies. Although the values for plasma fibulin-3 were similar in our cohorts, the cutoffs varied between the two geographic cohorts. This could be due to heterogeneity of the cohorts (specifically, the health of asbestos-exposed persons as compared with that of patients with cancer) or the effects of batching assays. Finally, fibulin-3 levels must be measured in more asbestos-exposed patients with benign effusions, which will require a prospective trial.
Future investigations should also explore why fibulin-3 is selectively elevated in mesothelioma as compared with other cancers and should address the question of whether this is an epigenetic-based phenomenon either through methylation or microRNA control. These studies could potentially clarify the role of fibulin-3 in mesothelioma growth, invasion, and metastasis formation and determine whether the molecule might be targeted for specific cytotoxic or biologic therapies.