Thrombospondin-4 is a putative tumour-suppressor gene in colorectal cancer that exhibits age-related methylation
© Greco et al; licensee BioMed Central Ltd. 2010
Received: 16 January 2010
Accepted: 16 September 2010
Published: 16 September 2010
Thrombospondin-4 (THBS4) is a member of the extracellular calcium-binding protein family and is involved in cell adhesion and migration. The aim of this study was to evaluate the potential role of deregulation of THBS4 expression in colorectal carcinogenesis. Of particular interest was the possible silencing of expression by methylation of the CpG island in the gene promoter.
Fifty-five sporadic colorectal tumours stratified for the CpG Island Methylator Phenotype (CIMP) were studied. Immunohistochemical staining of THBS4 protein was assessed in normal and tumour specimens. Relative levels of THBS4 transcript expression in matched tumours and normal mucosa were also determined by quantitative RT-PCR. Colony forming ability was examined in 8 cell lines made to overexpress THBS4. Aberrant promoter hypermethylation was investigated as a possible mechanism of gene disruption using MethyLight. Methylation was also assessed in the normal colonic tissue of 99 patients, with samples biopsied from four regions along the length of the colon.
THBS4 expression was significantly lower in tumour tissue than in matched normal tissue. Immunohistochemical examination demonstrated that THBS4 protein was generally absent from normal epithelial cells and tumours, but was occasionally expressed at low levels in the cytoplasm towards the luminal surface in vesicular structures. Forced THBS4 over-expression caused a 50-60% repression of tumour colony growth in all eight cell lines examined compared to control cell lines. Tumours exhibited significantly higher levels of methylation than matched normal mucosa, and THBS4 methylation correlated with the CpG island methylator phenotype. There was a trend towards decreased gene expression in tumours exhibiting high THBS4 methylation, but the correlation was not significant. THBS4 methylation was detectable in normal mucosal biopsies where it correlated with increasing patient age and negatively with the occurrence of adenomas elsewhere in the colon.
THBS4 shows increased methylation in colorectal cancer, but this is not strongly associated with altered gene expression, either because methylation has not always reached a critical level or because other factors influence THBS4 expression. THBS4 may act as a tumour suppressor gene, demonstrated by its suppression of tumour colony formation in vitro. THBS4 methylation is detectable in normal colonic mucosa and its level may be a biomarker for the occurrence of adenomas and carcinoma.
Thrombospondin-4 (THBS4) is a member of the extracellular calcium-binding protein family. It is a secreted pentameric globular protein that forms part of the extracellular matrix, and functions in calcium binding, cell attachment, cell migration, cellular proliferation, cytoskeletal organisation, neurite growth, binding of other extracellular matrix components and cell to cell interactions [1–6]. THBS4 is expressed at high levels in the heart and skeletal muscle, is detected in neuronal tissue in the brain and eye, and also in the skin, lung, pancreas, T-cells, breast and colon tissues [1, 3, 5, 7, 8]. It is a putative tumour suppressor gene, the hypermethylation of which has been linked to cutaneous T-cell lymphoma, breast cancer and colorectal cancer (CRC) [6, 8, 9].
DNA methylation is the addition of a methyl group to cytosine nucleotide bases via one of the DNA methyltransferase enzymes . CpG islands are most commonly found within the promoter region of genes, although they can occur in other coding and non-coding regions. More than 60% of gene promoters are found within CpG islands . CpG islands are usually not methylated regardless of the expression of the gene . However, if these promoter CpG islands become methylated, the associated gene may be permanently silenced, and this silencing is epigenetically inherited [10, 11]. The promoter region of THBS4 is typical of such a CpG island that is subject to epigenetic silencing. As a putative tumour-suppressor gene, methylation of the THBS4 promoter and silencing of its tumour-suppressor function is potentially pathogenic.
The CpG Island Methylator Phenotype (CIMP) has been used to describe a subset of colorectal tumours with a high frequency of methylation in genes known to be specifically methylated only in neoplasia and not in normal colon [12, 13]. While age-related (Type A) promoter methylation of some genes is common in both normal mucosa and neoplastic tissue, there is a subset of genes whose promoters are only found to be hypermethylated in CRC (Type C, cancer-related methylation) [12, 14]. However, age-related hypermethylation in normal colonic mucosa may contribute to the pre-malignant colorectal field [14, 15].
Sporadic colorectal cancer (CRC) can be divided into clinically relevant subgroups based on gene expression profiles that reflect pathways of tumour progression. The traditional pathway accounts for approximately 70% of all CRC . These are characterised by chromosomal instability, gene mutations and deletions, and are microsatellite stable (MSS). Adenomas are the precursor lesions to traditional pathway colorectal cancers . The serrated pathway accounts for approximately 30% of all sporadic CRC. These are characterised by a different spectrum of target gene alterations [15, 17, 18]. Mutations in the BRAF oncogene are common, along with CIMP . Serrated pathway tumours have high levels of microsatellite instability (MSI-H), which results from the hypermethylation and subsequent silencing of the mismatch repair gene MLH1. Serrated polyps precede the development of this tumour subgroup .
The aims of this study were to evaluate the deregulation of THBS4 in a series of colorectal cancers, to examine correlations with CIMP, to probe the effect of forcing THBS4 protein expression in CRC cell lines and to examine the mechanism of THBS4 deregulation.
Quantitative expression (qRTPCR) analysis was performed on 55 matched normal and tumour samples. All tumour samples collected in a fresh state were macroscopically dissected to remove contaminating normal mucosa. Quantitative methylation (qMSP) analysis was also conducted on this patient cohort as well as biopsies of normal mucosa taken during clinically indicated colonoscopy in a series of 99 patients. Biopsies were taken from each of 4 locations within the colon (caecum, transverse colon, sigmoid colon and rectum). All polyps were removed and submitted for histology. All patients gave written, informed consent, and the study was approved by the RBWH and QIMR Human Research Ethics Committees. DNA was extracted from tumour samples using a salt precipitation method as previously described , and from biopsy specimens using a DNA column method (Qiagen, Hilden, Germany). CIMP status was determined in tumours by normalising the methylation levels of each of the 5 Laird CIMP markers (CACNA1G, IGF2, NEUROG1, RUNX3, SOCS1)  by the methylation levels observed for the highly methylated ALU gene to generate the PMR, or Percentage of Methylated Reference. Tumours were classified as CIMP-negative (CIMP-neg) if 0/5 markers were methylated, CIMP-low (CIMP-L) if 1/5 or 2/5 markers were methylated, and CIMP-high (CIMP-H) if 3, 4 or 5/5 markers were methylated. The study cohort consisted of 14 CIMP-High, 11 CIMP-Low and 30 CIMP-Negative tumours.
Total RNA was extracted using an RNeasy MidiPrep kit (Qiagen, Hilden, Germany) and cDNA was synthesised using random hexamers and SUPERSCRIPT III (Invitrogen, Carlsbad, California). A Taqman® Gene Expression Assay (part# 4331182; assay ID Hs00170261_m1 Applied Biosystems, Carlsbad CA, USA) was performed on cDNA generating a 96 bp THBS4 product. Gene expression was normalised to β-actin (ACTB) expression using Taqman® Gene Expression Assay (part# 4331182; assay ID Hs99999903_m1 Applied Biosystems, Carlsbad CA, USA) generating a 171 bp product. The qPCR was performed in duplicate on a RotorGene3000 (Qiagen) using Absolute QPCR Mix (AB1133A; Integrated Sciences, NSW, Australia) and cycling of 40 cycles at 60°C annealing. The mathematical model described by Pffafl was used to determine the expression of THBS4 relative to the housekeeping gene ACTB .
THBS4 immunohistochemistry was performed on a subset of 40 patients. Briefly, fixed tumours were embedded in paraffin blocks and 0.2 μM sections were cut and mounted onto Superfrost Plus slides. They were deparaffinized in xylol and rehydrated by gradient alcohol before a 15 minute incubation with 0.5% hydrogen peroxide in phosphate buffered saline (PBS) to quench activity of endogenous peroxidases. After washes in PBS, the first containing 0.05% Triton X-100, sections were incubated in 10% goat serum and 0.01% acetylated BSA for 60 mins, then incubated overnight at 4°C in mouse anti-human THBS4 monoclonal antibody (MAB2390, R&D Systems, Minneapolis, MN) at 1:1500 in PBS/5% goat serum/1%BSA. Sections were again washed and incubated with rabbit anti-mouse Envision (Dako, Denmark) for 30 mins. The chromogenic substrate was 3,3-diaminobenzidine and sections were counterstained with Meyers' prior to dehydration and mounting of slides in Depex. Negative (no antibody) controls were included in all runs. Once the normal staining pattern for THBS4 was established, positive control normal tissue sections were also included in all runs for consistency.
Colony formation assay
A mammalian expression construct (pcDNA3.1::THBS4) was designed to examine the effect of over-expressing THBS4 in CRC cell lines. Briefly, 1 μg of either pcDNA3.1(+) (Invitrogen, Carlsbad, CA, USA) or pcDNA3.1::THBS4 was transfected into cells using FuGENE6 (Roche Applied Science, Indianapolis, IN) at a ratio of 3:1. Three null-expressing cell lines (Lim1215, SW48 and HT29), one low-expressing cell line (SW480) and four high-expressing cell lines (DLD1, LS174T, HCT116 and RKO) were transfected in triplicate at an initial density of approximately 50%. Cells were allowed to recover for 48 hrs before application of selective media at a final concentration of 700 ng/μL G418 (GibcoBRL Life Technologies, Invitrogen, CA) for 10-14 days. At this time there were no surviving untransfected control cells, and transfected colonies were stained with 0.25% crystal violet/80% methanol. The HT29 transfection was repeated and individual colonies were expanded to confirm THBS4 transcript re-expression.
In order to determine the methylation status of the THBS4 promoter, qMSP was performed for 55 paired normal and tumour samples, and 13 cell lines. Briefly, 1.5 μg of DNA was modified with sodium bisulfite using the EpiTect Bisulfite kit (Qiagen, Hilden, Germany), diluted 1/8 and subjected to real time PCR in duplicate on a Rotorgene6000 (Corbett Research, QIAGEN, Germany) using Absolute QPCR Mix (AB1133A; Integrated Sciences, NSW, Australia) and cycling of 40 cycles at 60°C annealing (400 nM of F 5'-CGTTGTCGCGGAGTTTAGTA-3'; 600 nM of R 5'-ACGACGACGACGTTAACC-3'; 50 nM of Probe 5'-[DFAM]-ACCTCGATCGACGCCCGAAC[DBHQ1]). The level of methylation was determined by normalising the THBS4 methylation levels by the methylation levels observed for the highly methylated ALU gene to generate the PMR, or Percentage of Methylated Reference (400 nM of ALU-F 5'-GGTTAGGTATAGTGGTTTATATTTGTAATTT-3'; 400 nM of ALU-R 5'-ATTAACTAAACTAATCTTAAACTCCTAACCT-3'; 100 nM of ALU-Probe [6FAM]-CCTACCTTAACCTCCC-[MGBNFQ]; cycling as described for THBS4 qMSP above). Analysis was performed using MethyLight .
The Wilcoxon signed rank test was used to compare THBS4 expression as well as methylation between the normal mucosal and tumour pairs. The Kruskal Wallis test was used to test for a relationship between tumour THBS4 methylation and CIMP status as well as the difference in normal mucosal THBS4 methylation in patients with and without colorectal pathology. A non-parametric trend test was applied to confirm an ordinal gradient across multiple groups. THBS4 methylation within the normal colonoscopic biopsy samples was calculated at each of the four sites sampled, as well as analysed according to an average proximal (mean of cecum and transverse colon), distal (mean of sigmoid colon and rectum) and pancolorectal (mean of all four sites) result. Correlations were analyzed by the Spearman's rank (ρ) coefficient. Distal and proximal levels of methylation were analyzed by a paired t-test. All tests were performed using Stata Statistical Software, version 10 [StataCorp].
Colony formation assay
Using a PMR of 10 as a cut off , tumours were separated into THBS4 methylation positive (PMR ≥10) and THBS4 methylation negative (PMR < 10). THBS4 promoter hypermethylation (PMR ≥10) was observed in 4/30 (13.3%) CIMP-NEG tumours, 4/11 (36.4%) CIMP-L tumours, and 5/14 (35.7%) CIMP-H tumours. Thus, while the average tumour PMR correlates well with an increasing number of markers positive for CIMP, there were an equal proportion of CIMP-L and CIMP-H tumours that showed high THBS4 methylation.
Correlation of Expression and Methylation of THBS4
THBS4 expression is not related to its promoter methylation in CRC cell lines
Methylation of THBS4 in Normal Colonic Biopsies
Thrombospondin-4 is a putative tumour-suppressor gene that plays an integral role in mediating cellular processes such as cell attachment and migration . Classes of tumour suppressor genes include those involved with DNA repair, cell growth, cell cycling, cell adhesion, cell migration, transcriptional regulation and apoptosis . These may be inactivated through gene mutation, chromosomal deletion or methylation . Tumour suppressor genes encode proteins which, upon loss of function, such as through epigenetic silencing, leads to a selective growth advantage for neoplastic cells. THBS4 has shown to be methylated in several tumour types, and this methylation is associated with transcriptional down-regulation [6, 8, 9].
Thrombospondin-4 is expressed at low levels in normal colonocytes, especially in the cytoplasm towards the luminal surface. Expression in tumours tends to be even lower than the levels observed in normal mucosa. Immunohistochemical localisation of THBS4 confirmed the absence of protein expression in the majority of tumours. However, some cancer cells within particular regions of the individual tumours have higher expression. A subset of tumours did express THBS4 and evidently secrete the protein, observed as more intense staining of vesicular structures towards the luminal surface. Indeed, THBS4 can been readily collected from culture supernatants of cells expressing THBS4 constructs , or display prominent ER and Golgi labelling , indicating that it is a secreted protein. While the secretion of the THBS4 protein may account for the low levels of staining observed, qPCR also demonstrated that gene expression is generally quite low. Thrombospondin-4 expression by qPCR is significantly higher in normal tissues than in matched tumour samples, supporting the notion that loss of THBS4 provides a selective growth advantage to cancer cells.
The forced over-expression of THBS4 in cancer cell lines significantly suppresses their colony forming abilities and growth, regardless of their baseline THBS4 expression levels. This was demonstrated in vitro for all 8 CRC cell lines tested, each with varying basal levels of THBS4 expression. A consistent and significant growth suppression of 50-60% was observed following the forced over-expression of THBS4. This demonstrates that a large amount of THBS4 protein correlates with reduced cell growth, and strengthens the evidence supporting its role as a tumour suppressor gene. Further experiments are now required to investigate the mechanism of this reduced growth, which may be a direct result of THBS4-mediated tumour suppression or toxicity due to higher than physiologically normal levels of THBS4 protein.
The Laird methylation marker panel used to determine CIMP status of colorectal cancers consists of the CACNA1G, IGF2, NEUROG1, RUNX3 and SOCS1 genes . THBS4 methylation in the tumours, as quantitatively measured by MethyLight, significantly increased with the increasing CIMP status of tumours. The greater the number of CIMP markers that were positive for methylation (PMR ≥10), the higher the level of THBS4 methylation, although the same proportion of CIMP-L and CIMP-H tumours demonstrated high THBS4 methylation. This suggests that THBS4 methylation may have a role in progression of CIMP positive CRC, regardless of level of CIMP.
We postulated that methylation of the THBS4 promoter region would cause reduction in gene expression level. However, although normal mucosa has significantly higher expression than tumour tissue and significantly lower methylation than tumour tissue, there was no statistically significantly correlation between high promoter methylation and reduced gene expression. Reduced THBS4 expression levels did not correlate with a THBS4 PMR of 10 or greater in either cancers or cell lines. Nevertheless, in another study, methyltransferase inhibition by 5-azadeoxycytidine in SW48 cells resulted in reactivation of silenced THBS4 . Thus, it appears that other factors may control expression of this gene, and that the moderate levels of THBS4 methylation observed in our study are not enough to significantly inhibit gene expression. It is also possible that other CpG sites within the THBS4 gene promoter that were not examined in this study are important for regulating gene expression
The genes methylated in CRC may be characterized as "type A" (Age-related) genes and "type C" (Cancer-specific) genes . Generally, "type A" genes are methylated in both normal and tumour tissue and their degree of methylation is proportional to the age of the normal tissue [14, 27, 28]. The methylation of "type C" genes, however, is more specific for neoplastic tissue [14, 27–30]. From this study, THBS4 behaved as a "type A" gene. THBS4 exhibited methylation in both normal and tumour tissue and displayed a positive correlation with age. Furthermore, as demonstrated in a recent study,  THBS4 behaved as a "type A" marker with respect to its association with neoplasia found elsewhere within the field. Methylation of the THBS4 promoter was highest in normal mucosa in patients with normal colonoscopies, and declined progressively as more advanced pathology was evident (Figure 7).
THBS4 may act as a tumour suppressor gene, evidenced by the dramatic repression of colony formation upon forced over-expression in CRC cell lines and its generally lower expression in cancers compared to normal mucosa. THBS4 methylation is higher in cancers that also exhibit the CIMP phenotype, which is demonstrated by the strong correlation with the Laird panel of methylation markers. However, it appears that methylation has not reached a critical level where it alters THBS4 gene expression, or that THBS4 expression is regulated by factors in addition to promoter hypermethylation. THBS4 promoter methylation occurs in a manner representative of a Type A gene, which is methylated in normal aged colon as part of the aging process. Additional studies are required to identify other genes that are involved in the regulation of THSB4.
List of Abbreviations
CpG Island Methylator Phenotype
This study was funded by the National Health and Medical Research Council of Australia (grant 442965) and Pathology Queensland, Clinical and Statewide Services, Queensland Health, Australia.
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