TGM2: a cell surface marker in esophageal adenocarcinomas

Abstract

Introduction: Esophageal adenocarcinomas (EAC) are aggressive cancers that are increasing in incidence and associated with a poor prognosis. The identification of highly expressed genes in EAC relative to metaplastic Barrett's esophagus (BE) may provide new targets for novel early cancer detection strategies using endoscopically administered, fluorescently labeled peptides.

Methods: Gene expression analysis of BE and EACs were used to identify the cell surface marker transglutaminase 2 (TGM2) as overexpressed in cancer. The expression of two major isoforms of TGM2 was determined by qRT-polymerase chain reaction in an independent cohort of 128 EACs. Protein expression was confirmed by tissue microarrays and immunoblot analysis of EAC cell lines. TGM2 DNA copy number was assessed using single nucleotide polymorphism microarrays and confirmed by qPCR. TGM2 expression in neoadjuvantly treated EACs and following small interfering RNA-mediated knockdown in cisplatin-treated EAC cells was used to determine its possible role in chemoresistance.

Results: TGM2 is overexpressed in 15 EACs relative to 26 BE samples. Overexpression of both TGM2 isoforms was confirmed in 128 EACs and associated with higher tumor stage, poor differentiation, and increased inflammatory and desmoplastic response. Tissue microarrays and immunohistochemistry confirmed elevated TGM2 protein expression in EAC. Single nucleotide polymorphism and qPCR analysis revealed increased TGM2 gene copy number as one mechanism underlying elevated TGM2 expression. TGM2 was highly expressed in resistant EAC after patient treatment with neoadjuvant chemotherapy/radiation suggesting a role for TGM2 in chemoresistance.

Conclusion: TGM2 may be a useful cell surface biomarker for early detection of EAC.

FIGURE 1

Expression of TGM2 during progression from Barrett’s esophagus (BE) to esophageal adenocarcinoma (EAC) and abundance of its long and short isoforms. A, Box plot of TGM2 mRNA expression levels (probe 201042_at; Affymetrix U133A) in BE, low-grade dysplasia, high-grade dysplasia, and EAC. TGM2 expression is significantly higher in EAC relative to BE samples (Student’s t test [p < 0.001; analysis of variance p= 0.001]). (B) Validation of TGM2 expression by qRT-PCR of the same samples shown in Figure 1A. The y-axis represents normalized expression to BE using the 2^−ΔΔCt algorithm and shows that TGM2 expression is significantly higher in EAC (Student’s t test p = 0.009; analysis of variance p = 0.003). Outlier samples are marked with an open circle. (C) qRT-PCR of the expression of the long (black) and short (gray) isoforms of TGM2 in 128 EACs normalized to normal esophagus and stomach mucosa using the 2^−ΔΔCt algorithm. The frequency of EAC expressing the TGM2 short and long isoforms greater than twofold is 70.3% and 57%, respectively. (D) The correlation coefficient between tumors expressing both isoforms r = 0.5826 and p > 0.001.

FIGURE 2

Increased TGM2 protein expression in EACs. Sections of esophageal adenocarcinoma tumors were stained with TGM2 antibody as described in materials and methods and shown here at 10× magnification. TGM2 expression is absent in normal esophageal squamous epithelium (A), Barrett’s esophagus shows only low-level expression in stroma (B), and higher expression is seen in EACs (C and D), respectively.

FIGURE 3

Amplification of the chromosomal 20q arm is associated with increased TGM2 expression in EACs. A) Amplification of individual chromosomal regions in 73 EACs was examined by single nucleotide polymorphism (SNP) array analysis and visualized using hg18 genome build data and IGV software. Increased DNA copy number is shown in red and reduced copy number in blue. (B) Magnified view of chromosome 20 where the y-axis shows a descending log₂ copy number ratio. Horizontal bars represent individual tumor samples. TGM2 is highlighted by a red line at 20q12 and the magnified 82 kb region of the TGM2 gene is on the right. (C) Copy number of TGM2 from SNP array where 16 of 73 (22%) EACs show doubling of copy number with the cutoff value at ≥ 1.8 and representing greater than 4N of the haploid genome. (D) qPCR analysis of intron 2 (gray) and intron 11 (black) was performed to determine the TGM2 copy number in 52 EACs. Data were normalized to normal tissue using the 2^−ΔΔCt algorithm. Highlighted samples are those demonstrating greater than a 1.8-fold increase in TGM2 DNA copy number. The frequency of tumors showing copy number increases using the intron 2 probe was eight of 52 (15%) and six of 52 (11.5%) with intron 11, with an average of 13.4%. (E) Correlation coefficient of TGM2 intron 11 and intron 2 copy number analyzed within the same tumors reveals an r value of 0.96 (p < 0.001).

FIGURE 4

Chemoresistant esophageal adenocarcinoma (EAC) tumors express elevated TGM2. qRT-PCR analysis of TGM2 expression in 21 EAC tumors that had minimal response to neoadjuvant chemoradiation treatment at the time of surgery. The y-axis represents the fold change in TGM2 expression relative to the average of normal esophagus tissues (N). Among the neoadjuvant-treated patients, four of 21 (19%) had greater than or equal to twofold greater TGM2 expression (p < 0.001 by Student’s t test) than two EACs demonstrating TGM2 gene amplification and not receiving neoadjuvant treatment (EAC).

FIGURE 5

TGM2 small interfering RNA (siRNA)-mediated knockdown reduces cell viability in esophageal adenocarcinoma (EAC) cells. A) OE33 EAC cells were transfected with either nontarget (NT) or 10pM TGM2 siRNA (siTGM2) for 24 hours. Total cell lysates were collected and used for western blot analysis of TGM2 and the loading control β–actin (ACTB). Second individual siRNAs showed similar TGM2 knockdown efficiency marked with an asterisk. (B) WST-1 assays performed in quadruplicate and normalized to the mock control of transfected OE33 with 10pM siRNA of nontarget or TGM2 were measured at 24, 48, and 72 hours. Student’s t test p values are marked when significant.