BMC Cancer 2008, 8:19 http://www.biomedcentral.com/1471-2407/8/19 in WM1552C, the enrichment of chromosome 1 genes in the RGP library might be explained by losses involving this chromosome in the metastatic cell lines. Indeed, loss of the long arm and translocation involving chromosome 1 was observed in cell lines that are paired with the meta- static cell lines WM1617 and 1205Lu [6,46]. This is in agreement with the frequent rearrangements found in chromosome 1 in advanced melanomas [46-48]. Several of the genes identified in the RGP library have been reported as anti-tumorigenic or anti-metastatic, as for instance, DCN [43], ANLN [49], HMGB2 [50], CXCL11 [51]. Some of them are potentially involved in the KISS1 pathway, since KISS1 expression was detected only in WM1552C cells (Fig. 1). The KISS1 gene product, the secreted protein kisspeptin/metastin, plays an inhibi- tory role in chemotaxis and invasion of melanoma cells by a mechanism involving remodeling of the actin cytoskeleton [23]. Interestingly, many genes in the RGP library encode proteins associated to the actin cytoskele- ton (see Additional File 2, Table S1 and Additional File 4, Table S3). Besides its role as a metastasis suppressor [20], first revealed in a melanoma model using SSH approach, kisspeptin/metastin and its receptor GPR54/KISS1R were recently implicated as important triggers of the complex process of sexual maturation [52,53]. The hormone leptin and its receptor also play important role in this process since leptin is a permissive factor for pubertal develop- ment [54], and the possibility that leptin modulates KISS1 expression in the central nervous system is under investi- gation [55]. In this context, the identification of LEPR gene, which codes for the leptin receptor, in the RGP library (Additional File 2, Table S1) might bear some rel- evance. Another gene with expression pattern similar to the one shown for KISS1, i.e., expressed exclusively in WM1552C (Fig. 3), is DCN. Both gene products, kisspep- tin and decorin, besides having anti-migratory and anti- invasive roles in tumor cells are also implicated as regula- tors in the process of trophoblast invasion [56,57]. LUM, whose expression is down regulated in most metastatic melanomas (Fig. 5), is another member of the family of small proteoglycans that includes decorin for which an anti-invasive role was reported [58]. Therefore, novel genes with potential role in the maintenance of a status of low aggressiveness are likely to be represented in the RGP library, and some of them might be directly or indirectly associated to the KISS1 metastasis-suppressor pathway. Of note, detection of WNT5A in the RGP library is in marked contrast to previous evidence that WNT5A is strongly associated with aggressiveness in human melanoma [59] and also to a most recent finding that KISS1 expression is down-regulated by Wnt5a [60]. This raises the interesting possibility that WM1552C cells carry an inactivating mutation that affects the Wnt5a signaling pathway. Two of the RGP genes (ALS2CR7 and MBOAT1) validated by Northern blots encode proteins of unknown function, both highly expressed in WM1552C compared to VGP and metastatic cell lines making them interesting candi- dates for further investigations. This is the first evidence for the expression of these genes in melanocytic cells, and points towards a role in melanoma development. Potential oncogenic pathways Among the genes represented in the Met library, many are associated to tumor growth, invasiveness and metastasis, such as TM4SF1 [61], LAMA4 [62], G3BP2 [63], CD59 antigen [64], and SPP1 [65]. Genes encoding proteins involved in the control of cell adhesion and cell migration are enriched in the Met library (Table 3). Among these genes, we have several growth factor receptors and integrins, including ITGB8 whose relevance in cancer remains to be characterized. Among the validated Met genes (HLA-DRA, PLP1 and MITF), several lines of evi- dence suggest their involvement in tumorigenesis. HLA- DRA encodes the α chain of the HLA-DR, one of the MHC class II molecules that, in contrast to MHC class I mole- cules, are not normally expressed by nonprofessional anti- gen-presenting cells (APC). Functional MHC class II molecules, key initiators of an immune response by acti- vating CD4+ naïve T cells, are heterodimeric proteins composed of α and β chains encoded by separated genes (A and B). Melanocytes from normal skin and common nevus are negative for HLA class II molecules [66,67] while both primary and metastatic melanomas display heterogeneous levels of positive cells [67,68]. Although melanoma cells acquire HLA-DRA expression during tumor development, the prognostic value of this expres- sion has not been clarified [68-73]. In contrast to the over- expression of HLA-DR α chain in several cancers, the HLA- DR β chain is not frequently overexpressed in cancer, sug- gesting that cancer cells do not express a functional HLA- DR receptor [74]. Clones corresponding to the HLA-DRA gene were the most abundantly sequenced clones from the Met library. However, sequences corresponding to the HLA-DRB gene were not detected in the same library, sug- gesting that HLA-DRA may not form a functional antigen- presenting molecule in these cells. PLP1 is a transmembrane protein involved in myeliniza- tion [75] whose up regulation was detected in leiomyo- mas [76] and melanoma cell lines [77], and although no previous study addresses the role of PLP1 in melanoma development, remarkably, we show here that all vertical growth phase and metastatic cell lines exhibit high expres- sion levels of this gene. It is interesting to note that PLP1 expression is regulated by the transcription factor SOX10 Page 13 of 18 (page number not for citation purposes)