Increased tumorigenesis associated with loss of the tumor suppressor gene Cadm1
© van der Weyden et al.; licensee BioMed Central Ltd. 2012
Received: 18 February 2012
Accepted: 3 May 2012
Published: 3 May 2012
CADM1 encodes an immunoglobulin superfamily (IGSF) cell adhesion molecule. Inactivation of CADM1, either by promoter hypermethylation or loss of heterozygosity, has been reported in a wide variety of tumor types, thus it has been postulated as a tumor suppressor gene.
We show for the first time that Cadm1 homozygous null mice die significantly faster than wildtype controls due to the spontaneous development of tumors at an earlier age and an increased tumor incidence of predominantly lymphomas, but also some solid tumors. Tumorigenesis was accelerated after irradiation of Cadm1 mice, with the reduced latency in tumor formation suggesting there are genes that collaborate with loss of Cadm1 in tumorigenesis. To identify these co-operating genetic events, we performed a Sleeping Beauty transposon-mediated insertional mutagenesis screen in Cadm1 mice, and identified several common insertion sites (CIS) found specifically on a Cadm1-null background (and not wildtype background).
We confirm that Cadm1 is indeed a bona fide tumor suppressor gene and provide new insights into genetic partners that co-operate in tumorigenesis when Cadm1-expression is lost.
KeywordsCell adhesion molecule Tumor suppressor Transposon Glucocorticoid Cell junction
Cell adhesion molecule 1 (CADM1; also known as TSLC1, IGSF4, Necl-2, RA175 SgIGSF, SynCAM1) is member of the immunoglobulin superfamily of cell adhesion molecules (IGSF-CAMs) and is composed of an extracellular domain containing three immunoglobulin-like C2-type domains, a transmembrane domain and a short cytoplasmic tail . The extracellular domain of CADM1 mediates the formation of homodimers or heterodimers with other CAM members, including Necl-1, CRTAM and Nectin-3 to regulate cell adhesion. The cytoplasmic domain of CADM1 interacts with the tumour-suppressor gene DAL-1 and the group of membrane-associated guanylate kinase (MAGuK) homologues, as well as being able to modulate the activation of small Rho GTPases, thus acting as a vital bridge between extracellular adhesion and intracellular signaling cascades. In addition, CADM1 can also modulate cell cycle progression and apoptosis [2, 3].
Less than a decade since the discovery of CADM1, loss of its expression by promoter hypermethylation or loss of heterozygosity (LOH) has been reported in a wide variety of tumor types ( Additional file 2: Figure S1) and frequently correlates with advanced tumor stage (poor prognosis) and metastasis . Studies in nude mice have demonstrated that re-expression of CADM1 suppresses in vivo tumorigenicity of non-small cell lung cancer and nasopharyngeal carcinoma cell lines [1, 4, 5]. In contrast, studies using Cadm1 null (Cadm1 −/− ) mice have demonstrated important roles for Cadm1 in spermatogenesis/male fertility , behavior and motor performance , lens fiber cell architecture , and epidermal adhesion and wound repair . However, there are no reports on the incidence of cancer in these mice and thus whether Cadm1 is indeed a bona fide tumor suppressor gene.
The most statistically significant CIS was in the Nr3c1 gene, which encodes the glucocorticoid receptor (GR), and the insertions would be predicted to have a loss-of-function effect on Nr3c1, consistent with the finding of promoter hypermethylation or mono-allelic deletion of NR3C1 in several cancer types including colo-rectal cancer (CRC)  and leukemia , respectively. Glucocorticoids (GCs), which bind the GR and allow it to translocate to the nucleus and modulate gene expression, are effective inhibitors of proliferation and tumorigenesis and routinely used in treating T-cell acute lymphoblastic leukemia (T-ALL) . Interestingly, three of the other nine CIS genes identified are known interactors/regulators of the GR. St13 encodes the Hsp70-interacting protein that is involved in the assembly process of the GR, and ST13 mRNA and protein levels are down-regulated in CRC . The Ets1 proto-oncogene is a transcription factor that can act as a “molecular switch” for auto-regulation of the GR promoter, and high ETS1 expression predicts poor prognosis in patients with ovarian cancer . The Csf3r encodes the cell-surface granulocyte colony-stimulating factor (G-CSF) receptor, and activated GR can synergize with G-CSF signals .
Cell junctions including tight junctions, adherens junctions and desmosomes, consist of multi-protein complexes that provide contact between neighboring cells or between a cell and the extracellular matrix and as such play important roles in regulation of cell proliferation and differentiation, as well as cancer . Like CADM1, two of our CIS genes are part of these multi-protein complexes. Fchsd2 encodes the FCH and double SH3 domains protein 2 (FCHSD2), which binds to epithelial junction MAGuKs, specifically MAGI-1 and CASK ; CADM1 has been shown to interact with several MAGuK members, including CASK . Jup encodes junction plakoglobin (JUP), which complexes with numerous other desmosomal proteins (including cadherins, desmogleins and desmocollins) ), and was recently shown to be expressed on the surface of colorectal cancer cells associated with high metastatic potential .
Finally, it is interesting to note that in addition to ‘Cadm1 null-specific’ CIS, we also identified CIS that were only found in tumors from wildtype mice (i.e., not found in Cadm1 null tumors). These CIS represent loci that are mutated in the process of tumorigenesis in the presence of an intact Cadm1 signaling pathway, and whose contribution to tumorigenesis is potentially rendered obsolete in the absence of Cadm1. Some of these genes, such as Pik3r5 and Malt1, have also been identified as CIS in leukemia/lymphomas from wildtype mice in other Sleeping Beauty transposon screens we have performed (unpublished data). Several CIS genes including Pten Notch1 and Erg are mutated in both wildtype and Cadm1 null tumors suggesting that mutation of these genes can contribute to tumorigenesis regardless of Cadm1 status .
Thus we have shown that CADM1 is a bona fide tumor suppressor gene, and loss of Cadm1 results in an increased tumor incidence. Our insertional mutagenesis screen provides new insights into Cadm1-mediated tumor suppression by identifying genes that co-operate with loss of Cadm1 in lymphomagenesis, in particular those regulating glucocorticoid signaling and cell junctions.
LvdW was supported by the Kay Kendall Leukemia Fund. GP is a Pfizer Fellow of the Life Sciences Research Foundation. MJA was supported by Cancer Research UK. DJA was supported by Cancer Research UK and the Wellcome Trust.
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