EWS/PNET is a small, blue, round cell tumour with a very characteristic t(11,22) chromosomal rearrangement, which results in fusion of the EWS and FLI‐1 genes. The resultant fusion protein promotes oncogenesis, which is necessary for continued growth of tumour cell lines. Studies show that loss of the COOH‐terminal domain can attenuate the ability of EWS/FLI‐1 to promote anchorage‐independent growth. Furthermore, cells with EWS/FLI‐1 COOH deletion mutants show a progressive decrease in tumour oncogenesis and loss of round cell morphology on histological examination. The above data led to the suggestion that the C‐terminus of FLI‐1 seems to have a crucial functional role in EWS/FLI‐1 oncogenicity.3
Since the commercial availability of FLI‐1, few comprehensive studies on its expression in benign and malignant tissues have been published. In 1999, Nilsson et al11
were the first to demonstrate FLI‐1 expression using sc‐356 polyclonal antibody in EWS/PNET cell lines and in all five formalin‐fixed paraffin‐wax‐embedded EWS/PNET cases. Soon after, Folpe et al4
explored its expression in a series of SRCTs. They showed that FLI‐1 was expressed not only in 71% EWS/PNET but also in various SRCTs, including 7/8 (88%) lymphoblastic lymphomas, 1/1 DSRCT, 0/1 poorly differentiated synovial sarcoma, 0/32 RMS, 0/3 neuroblastoma, 0/3 Wilms' tumour, 0/8 ONB and 0/1 mesenchymal chondroblastoma, indicating that FLI‐1 can be expressed in a variety of SRCTs but its expression is still highly specific (90%) for EWS/PNET. Since then, FLI‐1 has been used as a marker for EWS/PNET, and its use has been repeatedly described in numerous case reports.12,13,14,15
Another work, by Llombart‐Bosch et al
found FLI‐1 expression in 16/19 (84%) EWS/PNETs, 4/5 (80%) NHLs (large cell type, unspecified lineage), 2/9 (22.2%) neuroblastomas and in 3/6 (50%) undifferentiated synovial sarcomas. In our series of SRCTs, FLI‐1 was found in 46/62 (74.2%) EWS/PNETs, 2/3 (66.7%) ONBs, 2/3 (66.6%) DSRCTs, 1/14 (7.1%) RMS and 7/102 (6.9%) small cell carcinomas of the lung. In addition, and similar to the results by Folpe et al
, we found FLI‐1 to be almost always negative in Wilms' tumours (n
47) and neuroblastomas (n
31), findings that are both important and very useful in the differential diagnosis of SRCTs in children. Finally, FLI‐1 was negative in our series of 148 cases of germ cell tumours, which might enter into the differential diagnosis with EWS/PNET. From these data, we concluded that FLI‐1 is a highly sensitive and specific marker in distinguishing EWS/PNET from other SRCTs, as well as from germ cell tumours. Further, we showed that FLI‐1 has high positive predictive and negative predictive values in distinguishing EWS/PNET from other SRCTs, as well as from germ cell tumours. Also, 19/132 (13.1%) NHLs expressed FLI‐1, and it was seen in different NHL subtypes, including follicular, Burkitt's, diffuse large B‐cell and peripheral T‐cell lymphoma. Thus, FLI‐1 is seen not only in lymphoblastic lymphoma but also in other lymphoma subtypes. FLI‐1 is expressed in normal endothelial cells, and the FLI‐1
gene has been reported to play an important role in the embryological development of blood vessels.16
The purpose of using FLI‐1 in the diagnosis of vascular tumours was explored, and FLI‐1 seemed to be expressed in 94% of cases.5,17
In the present study, we found FLI‐1 to be expressed in 71.6% of all vascular neoplasms.
There are two studies in the literature discussing FLI‐1 expression in tumours other than SRCTs and vascular neoplasms. In the first report, all 60 cases of non‐vascular tumours were negative for FLI‐1 expression. These cases were 0/16 sarcoma, 0/7 melanoma and 0/45 overall carcinoma, including 3 squamous cell carcinomas (SCC), 12 breast ductal carcinomas, 21 adenocarcinomas (pancreatic (n
4), pulmonary (n
5), ovarian papillary serous (n
5), uterine endometrioid (n
6) and colonic (n
1)), 3 renal cell carcinomas, 1 hepatocellular carcinoma, 3 salivary mucoepidermoids and 2 pulmonary carcinoids. In the second report, however, FLI‐1 monoclonal antibody (GI‐46‐222, BD Pharmingen) was used, and the results were as follows: all EWS/PNET (n
15) and vascular tumour (n
45) cases, 2/5 MCCs and 1/10 malignant melanoma (MM) were strongly positive for FLI‐1, weak expression was seen in 3/5 MCCs, 3/10 synovial sarcomas, 5/10 malignant melanomas, 6/10 lung adenocarcinomas, 1/10 breast carcinoma, and all DSRCTs (n
5), RMS (n
10), high‐grade pleomorphic sarcomas (n
10) and colon carcinomas (n
10) were negative for FLI‐1.18
On exploring FLI‐1 expression in a large number of tumours, we found it to be present in a small percentage of a variety of solid tumours, such as 27/508 (6.9%) SCCs, 19/837 (2.3%) adenocarcinomas, 10/400 (2.5%) urothelial bladder carcinomas, 9/29 (31.0%) medullary carcinomas of the breast, 1/40 (2.5%) glioblastoma multiforme and 1/40 (2.5%) basal cell carcinomas of the skin. Despite its expression in a variety of malignant tumours, FLI‐1 is still a highly sensitive and specific marker to distinguish EWS/PNET from all types of malignancies. Furthermore, and in this context, it did not have a high positive predictive value but still showed a high negative predictive value. Thus, our study is the first to evaluate a large series of types of tumours using FLI‐1 polyclonal antibody, and its expression by the tumours described above should be kept in mind when using FLI‐1 as a marker to find the primary origin of a metastatic poorly differentiated tumour.
Finally, although Llombart et al found FLI‐1 expression in 18/20 (90%) MCCs, we found it in 10/34 (29.4%) MCCs. The difference between the two studies could not be due to difference in techniques; it might be because FLI‐1 was carried out on whole sections, and because the samples were interpreted by three pathologists blinded to the tumour types being evaluated. After nuclear expression of FLI‐1 was scored by the pathologists, the samples were re‐evaluated in light of the discrepancy between Llombart's study and ours. In our results, the majority of MCC cases showed negative nuclear staining for FLI‐1 (fig 2).
We found FLI‐1 polyclonal antibody in a variety of tumours including EWS/PNET, ONB, small cell carcinoma of the lung, NHL (B and T cell), RMS, DSRCT, vascular tumours and MCB, and subsets of cases of MCC, SCC, adenocarcinomas, urothelial carcinomas and basal cell carcinoma. In conclusion, and despite the expression of FLI‐1 in a variety of malignancies, it is still considered to be highly specific and sensitive marker in distinguishing EWS/PNET from other tumours. However, we should be aware of its expression in the above tumour types whenever this marker is used.