Analysis of losses of heterozygosity of the candidate tumour suppressor gene DMBT1 in melanoma resection specimens

Generation of a vector system facilitating cloning of DMBT1 variants and recombinant expression of functional full-length DMBT1

Deleted in malignant brain tumours 1 (DMBT1) codes for a approximately 340kDa glycoprotein with highly repetitive scavenger receptor cysteine-rich (SRCR) domains. DMBT1 was implicated in cancer, defence against viral and bacterial infections, and differentiation of epithelial cells. Recombinant expression and purification of DMBT1 is an essential step for systematic standardized functional research and towards the evaluation of its therapeutical potential. So far, DMBT1 is obtained from natural sources such as bronchioalveolar lavage or saliva, resulting in time consuming sample collection, low yields, and protein preparations which may substantially vary due to differential processing and genetic polymorphism, all of which impedes functional research on DMBT1. Cloning of DMBT1 cDNAs is hampered because of the size and the 13 highly homologous SRCR exons. In this study, we report on the setup of a vector system that facilitates cloning of DMBT1 variants. We demonstrate applicability of the vector system by expression of the largest DMBT1 variant in a tetracycline-inducible mammalian expression system using the Chinese hamster ovary cell line. Yields up to 30 mg rDMBT1 per litre of cell culture supernatant could be achieved with an optimized production procedure. By harnessing the specific bacteria-binding property of DMBT1 we established an affinity purification procedure which allows the isolation of more than 3 mg rDMBT1 with a purity of about 95%. Although the glycosylation moieties of rDMBT1 are different from DMBT1(SAG) isolated from saliva, we demonstrate that rDMBT1 is functionally active in aggregating Gram-positive and Gram-negative bacteria and binding to C1q and lactoferrin, which represent two known endogenous DMBT1 ligands.

B-raf Exon 15 Mutations Are Common in Primary Melanoma Resection Specimens but Not Associated with Clinical Outcome

Downstream of Ras, the serine/threonine kinase B-raf has recently been reported to be mutated, among other carcinomas, in a majority of melanoma cell lines with a preponderance of mutations within the kinase domain including the activating V599E transition. We therefore investigated a representative number of 50 primary melanoma resection specimens for the presence of mutations within the activation segment (exon 15) of the B-raf kinase domain. Applying polymerase chain reaction and single-strand conformation polymorphism gel electrophoresis, followed by DNA cloning and sequencing, we found 19 cases (38%) to harbor somatic B-raf exon 15 mutations. With respect to the B-raf protein sequence, the V599E mutation was predicted in 63% of these positive melanomas, followed in frequency by the V599K transition (31%). Detection of B-raf exon 15 mutations or prediction of the activating mutation V599E were not statistically associated with the risk for subsequent metastasis in the follow-up of patients. Altogether, the B-raf oncogene is affected in a substantial subset of melanoma resection specimens. As B-raf alterations possibly affect melanocyte-specific pathways controlling proliferation and differentiation, activation of this oncogene may contribute to the development of melanoma.

A candidate tumor suppressor HtrA1 is downregulated in ovarian cancer

We report here that HtrA1, a candidate tumor suppressor, is downregulated in ovarian cancer. Expression of HtrA1 is downregulated in five of seven ovarian cancer cell lines. In total, 59% of primary ovarian tumors have either a complete absence or markedly reduced levels of HtrA1 expression compared to the brushings of ovarian surface epithelium. Primary ovarian tumors show high frequencies of loss of an allele at microsatellite markers near htrA1 locus on 10q26. Downregulation of HtrA1 in SKOV3 by antisense transfection promotes anchorage-independent growth, while exogenous expression of HtrA1 in OV202 induces cell death. HtrA1-induced cell death is not inhibited by the broad caspase inhibitor, zVAD(OMe)fmk, but instead reflects serine protease activity associated with HtrA1. These observations raise the possibility of HtrA1 as a candidate tumor suppressor involved in promoting serine-protease-mediated cell death and that downregulation of HtrA1 in ovarian cancer may contribute to malignant phenotype.

Frequent downregulation of DMBT1 and galectin-3 in epithelial skin cancer

DMBT1 and galectin-3 are potential interacting proteins with presumably complex roles in tumorigenesis. While at present a variety of mechanisms are discussed for DMBT1 and its participation in cancer, galectin-3 is commonly known to exert tumor-promoting effects. However, in vitro studies in a rodent system have suggested that DMBT1/galectin-3 interaction in the ECM triggers epithelial differentiation, which would point to tumor-suppressive properties. To improve the understanding of DMBT1/galectin-3 action in cancer, we carried out studies in skin cancer of different origins. Mutational analyses of DMBT1 identified a missense mutation in 1 of 13 melanoma cell lines. It led to an exchange of an evolutionary conserved proline residue for serine and located within the second CUB domain of DMBT1. Immunohistochemical analyses demonstrated absence of DMBT1/galectin-3 expression from melanocytes but induction of DMBT1 expression in 1 of 8 nevi and 1 of 11 melanomas and of galectin-3 expression in 3 of 8 nevi and 4 of 8 melanomas. These data suggest that DMBT1 and galectin-3 are unlikely to act as classical tumor suppressors in melanomas. DMBT1 and galectin-3 appear to be secreted to the ECM by epithelial cells within the epidermis and the hair follicle. Compared to the flanking normal epidermis, skin tumors of epithelial origin frequently displayed downregulation of DMBT1 (18 of 19 cases) and galectin-3 (12 of 12 cases). Thus, loss of DMBT1/galectin-3 expression may play a role in the genesis of epithelial skin cancer. This would support the view that galectin-3 can exert tumor-suppressive effects in certain scenarios, and DMBT1/galectin-3-mediated differentiation represents a candidate mechanism for this effect.

DMBT1, a regulator of mucosal homeostasis through the linking of mucosal defense and regeneration?

DMBT1 (deleted in malignant brain tumor 1), which encodes a large scavenger receptor cysteine rich (SRCR) B protein, has been proposed to be a tumor suppressor gene, due to the high frequency of its homozygous deletion and the lack of expression in a variety of cancers. However, studies on its physiological functions and its relationship with tumorigenesis are still at an initial stage. Two mucosal defense-related molecules, gp-340 and salivary agglutinin, have been identified to be alternatively spliced products of DMBT1, which suggests that DMBT1 is a pattern recognition receptor in innate immunity. Meanwhile, results from immunohistochemical staining and studies at the cellular level, began to associate DMBT1 with a proliferation to differentiation switching process in gastrointestinal epithelial cells. Together with its up-regulation in inflammation, these findings suggest that DMBT1 might be a local regulator of homeostasis, possibly through linking mucosal inflammation to the modulation of epithelial regeneration, and whose abnormality is a frequent cause of malignancy.