This study provides a first description of the patient and tumour characteristics of incident cases of malignant melanoma in the prospective, population-based cohort Malmö Diet and Cancer Study, diagnosed until Dec 31st
, 2008. In addition, it is demonstrated that the investigative biomarker RBM3 is down-regulated in metastatic deposits, associated with favourable histopathological parameters in primary melanomas and an independent predictor of a prolonged overall survival. In a translational context, these findings are quite in line with a previous study, where RBM3 was demonstrated to be one of five down-regulated genes in an in vitro
model of melanoma progression [20
]. Moreover, as RBM3 has been demonstrated to be a good prognostic biomarker in several other cancer forms, e.g. breast cancer [14
] and ovarian cancer [15
], its clinical utility in stratification of melanoma patients should be validated in future studies.
According to current clinical guidelines in Sweden, sentinel node biopsy is performed in melanomas > 1 mm, but as an increase in thin melanomas (< = 1 mm) seems to make up for most of the increasing incidence of malignant melanomas [25
], there is an unmet need for prognostic biomarkers in this category [26
]. In this study, RBM3 was not significantly associated with prognosis in thin (< = 1 mm) melanomas but was an independent favourable prognostic factor for OS in melanomas > 1 mm. The reason for this remains unclear and further studies in larger patient cohorts are needed to determine the prognostic value of RBM3 in thin melanomas. However, the observation that RBM3 remained an independent factor for overall survival in the cohort as a whole, which represented tumours of less advanced clinical stages than in the average population [2
], indicates its potential utility as a biomarker for prognostic stratification of patients with early-stage melanoma.
In the light of the above, a methodological aspect that needs further attention is the bias related to the use of the TMA technique in malignant melanoma biomarker studies, e.g. the technical difficulty in sampling small tumours. In this study, we attempted to sample melanomas < 0.5 mm if the diameter was > 10 mm, and in several cases, sampling was successful. The mean Breslow depth of invasion in the TMA cohort was only slightly higher than in the full cohort (1.66 mm compared to 1.57 mm). In addition, as determined by comparison with full-face sections for a subset of the tumours, RBM3 did not seem to display a heterogeneous expression pattern.
In this study we used a monoclonal antibody against RBM3, which was also used in our previous study on ovarian cancer [15
]. In the first paper, describing the prognostic value of RBM3 in breast cancer, we used a polyclonal antibody generated within the HPA project [14
]. Both antibodies have been extensively validated using siRNA techniques in breast cancer cell lines [14
] and ovarian cancer cell lines [15
] and similar results have been obtained regarding the staining distribution in various normal and cancerous tissues (data not shown). Although being a semi-quantitative method, IHC has several advantages since it allows for assessment of protein expression in different sub-cellular compartments, which might have important prognostic implications. In the case of RBM3, previous findings indicate that its nuclear rather than cytoplasmic localization is the most relevant parameter for prognostication [14
], which is also demonstrated here for melanoma.
As the MDCS is a population-based cohort study, a potential selection bias compared to the general population must be taken into consideration [22
]. Since all participants were > 40 years at study entry, the mean age among melanoma cases was higher than in the average population. Notably, since older melanoma patients often present with more advanced disease [27
], the relatively low proportion of cases with advanced disease reported here is somewhat unexpected. This could in part be explained by the fact that data necessary for staging could not be obtained for all cases. Nevertheless, clinical stage, as well as the prognostic impact of other established clinicopathological characteristics fell out as expected, which validates the cohort as a platform for future studies of lifestyle and tumour biology in relation to melanoma risk and prognosis.
Given the previously demonstrated association between RBM3 and cisplatin sensitivity in ovarian cancer cell lines [15
], the potential value of RBM3 as a predictor of response to platinum-based chemotherapy in patients with metastatic malignant melanoma could be of interest to investigate in future studies. However, in contrast to the situation in ovarian cancer, where RBM3 showed a consistent expression pattern in primary tumours and omental deposits [15
], the data presented here, and previous in vitro data [20
], show that RBM3 is down-regulated in the majority of metastatic melanomas. Hence, in the predictive setting in melanoma patients, thorough sampling and immunohistochemical analysis of metastatic deposits would be required in order to identify a comparatively small number of patients with RBM3 positive metastases.