Claudin-18: a dominant tight junction protein in Barrett's esophagus and likely contributor to its acid resistance

Increased expression of the tight junction molecule claudin-18 A1 in both experimental colitis and ulcerative colitis

BACKGROUND

We previously identified a major quantitative trait locus (qtl) on mouse chromosome 9 (Tnbs1) that confers resistance/susceptibility to trinitrobenzene sulfonic acid (TNBS) induced colitis. Here we wanted to identify possible candidate genes in this locus.

METHODS

We applied micro-array technology and identified claudin-18 as a plausible candidate gene in the Tnbs1 region. Subsequently we studied the expression profile of this gene by means of RT-PCR in resistant and susceptible mice as well as in human inflammatory bowel disease.

RESULTS

Expression of this gene was markedly upregulated during colitis in mice. Also in humans relative expression of claudin-18 in patients with ulcerative colitis was significantly upregulated as compared to healthy individuals undergoing surveillance endoscopy (n = 13, P < 0.0005). Expression was not related to the histological severity of the disease.

CONCLUSIONS

Claudins belong to the integral membrane proteins of the tight junction, a structure that seals off the intercellular space between adjacent epithelial cells and regulates passive diffusion of solutes and macromolecules. This study demonstrates for the first time that claudin-18 is expressed in human and mouse colon. Expression is upregulated during experimental colitis and in patients with ulcerative colitis. The observation that this is unrelated to the severity of inflammation might point to a primary defect in regulation in patients with ulcerative colitis and warrants further genetic examination.

Single nucleotide polymorphism-based genome-wide chromosome copy change, loss of heterozygosity, and aneuploidy in Barrett's esophagus neoplastic progression

Chromosome copy gain, loss, and loss of heterozygosity (LOH) involving most chromosomes have been reported in many cancers; however, less is known about chromosome instability in premalignant conditions. 17p LOH and DNA content abnormalities have been previously reported to predict progression from Barrett's esophagus (BE) to esophageal adenocarcinoma (EA). Here, we evaluated genome-wide chromosomal instability in multiple stages of BE and EA in whole biopsies. Forty-two patients were selected to represent different stages of progression from BE to EA. Whole BE or EA biopsies were minced, and aliquots were processed for flow cytometry and genotyped with a paired constitutive control for each patient using 33,423 single nucleotide polymorphisms (SNP). Copy gains, losses, and LOH increased in frequency and size between early- and late-stage BE (P < 0.001), with SNP abnormalities increasing from <2% to >30% in early and late stages, respectively. A set of statistically significant events was unique to either early or late, or both, stages, including previously reported and novel abnormalities. The total number of SNP alterations was highly correlated with DNA content aneuploidy and was sensitive and specific to identify patients with concurrent EA (empirical receiver operating characteristic area under the curve = 0.91). With the exception of 9p LOH, most copy gains, losses, and LOH detected in early stages of BE were smaller than those detected in later stages, and few chromosomal events were common in all stages of progression. Measures of chromosomal instability can be quantified in whole biopsies using SNP-based genotyping and have potential to be an integrated platform for cancer risk stratification in BE.

Knockout animals and natural mutations as experimental and diagnostic tool for studying tight junction functions in vivo

Two sides of functions of tight junctions; the barrier and the channel in the paracellular pathway are believed to be essential for the development and physiological functions of organs. Recent identification of molecular components of tight junctions has enabled us to analyze their functions by generating knockout mice of the corresponding genes. In addition, positional cloning has identified mutations in the genes of several components of tight junctions in hereditary diseases. These studies have highlighted in vivo functions of tight junctions.

Tight junctions and the modulation of barrier function in disease

Tight junctions create a paracellular barrier in epithelial and endothelial cells protecting them from the external environment. Two different classes of integral membrane proteins constitute the tight junction strands in epithelial cells and endothelial cells, occludin and members of the claudin protein family. In addition, cytoplasmic scaffolding molecules associated with these junctions regulate diverse physiological processes like proliferation, cell polarity and regulated diffusion. In many diseases, disruption of this regulated barrier occurs. This review will briefly describe the molecular composition of the tight junctions and then present evidence of the link between tight junction dysfunction and disease.