Infection with Helicobacter pylori (H. pylori) is a risk factor for the development of gastric cancer. Here we show that infection of gastric epithelial cells with 'cag' pathogenicity island (cagPAI)-positive H. pylori induced aberrant expression of activation-induced cytidine deaminase (AID), a member of the cytidine-deaminase family that acts as a DNA- and RNA-editing enzyme, via the IkappaB kinase-dependent nuclear factor-kappaB activation pathway. H. pylori-mediated upregulation of AID resulted in the accumulation of nucleotide alterations in the TP53 tumor suppressor gene in gastric cells in vitro. Our findings provide evidence that aberrant AID expression caused by H. pylori infection might be a mechanism of mutation accumulation in the gastric mucosa during H. pylori-associated gastric carcinogenesis.

Mutations in the Na؉ -HCO 3 ؊ co-transporter (NBC1) cause permanent proximal renal tubular acidosis (pRTA) with ocular abnormalities. However, little has been known about the relationship between the degree of NBC1 inactivation and the severity of pRTA. This study identified three new homozygous mutations (T485S, A799V, and R881C) in the common coding regions of NBC1. T he Na ϩ -HCO 3 Ϫ co-transporter (NBC1) has multiple functions (1,2). Whereas the kidney-type transporter (kNBC1) plays an essential role in bicarbonate absorption from renal proximal tubules, the pancreatic-type transporter (pNBC1) is involved in bicarbonate secretion from pancreatic duct cells (3-6). We showed recently that mutations in NBC1 cause proximal renal tubular acidosis (pRTA) with ocular abnormalities (7-9). Because NBC1 is widely expressed in several ocular tissues (10), its inactivation may potentially explain the occurrence of ocular abnormalities. However, only a limited number of NBC1 mutations have been identified so far (7-9,11-13), and several important questions remain unanswered. For example, the exact relationship between the degree of NBC1 inactivation and the severity of pRTA has not been established. A study in NHE3-deficient mice suggests that acid secretion from distal tubules is greatly enhanced in pRTA (14).How effectively such compensatory mechanism works in human, however, is largely unknown. In addition, the effects of individual mutations on the transport properties of NBC1 have not been investigated intensively. These issues would be important to clarify further the molecular mechanism of pRTA as well as the physiologic roles of NBC1. In our study, we identified three new missense mutations in kNBC1 from patients with pRTA and ocular abnormalities. The functional analysis of these new as well as the known mutants was performed in Xenopus oocytes and cultured cells. Materials and Methods PatientsPatient 1 (T485S) is a boy from consanguineous parents. He had a history of failure to thrive and received a diagnosis of severe pRTA and band keratopathy at 2 yr of age. At 3 yr of age, his height (90 cm) and weight (11.8 kg) both were less than the third percentile. BP was 79/49 mmHg. Bilateral corneas were cloudy, and a red reflex was bilaterally absent, suggesting the presence of cataracts. Bone survey was normal with no evidence of rickets. While he was taking sodium bicarbonate (9.75 g/d), serum analysis revealed Na ϩ 140 mEq/L, K ϩ 3.8 mEq/L, Cl Ϫ 116 mEq/L, creatinine 0.5 mg/dl, and HCO 3 Ϫ 13 mmol/L. He had a thyroid function test that was initially suggestive of mild hypothyroidism. However, during the follow-up, thyroid function tests were

Activation-induced cytidine deaminase (AID) is involved in somatic DNA alterations of the immunoglobulin gene for amplification of immune diversity. The fact that constitutive expression of AID in mice causes tumors in various organs, including lymphoid tissues and lungs, suggests the important role of the aberrant editing activity of AID on various tumor-related genes for carcinogenesis. AID expression, however, is restricted to activated B cells under physiological conditions. We demonstrate here that ectopic AID expression is induced in response to tumor necrosis factor-a stimulation in cultured human hepatocytes. The proinflammatory cytokine-mediated expression of AID is achieved by IjB kinase-dependent nuclear factor (NF)-jB signaling pathways. Hepatitis C virus, one of the leading causes of hepatocellular carcinoma (HCC), enhanced AID expression via NF-jB activation through expression of viral core protein. The aberrant expression of AID in hepatoma-derived cells resulted in accumulation of genetic alterations in the c-myc and pim1 genes, suggesting that inappropriate expression of AID acts as a DNA mutator that enhances the genetic susceptibility to mutagenesis in human hepatocytes. Our current findings indicate that the inappropriate expression of AID is induced by proinflammatory cytokine stimulation and may provide the link between hepatic inflammation and the development of HCC.

Activation-induced cytidine deaminase (AID) plays a role as a genome mutator in activated B cells, and inappropriate expression of AID has been implicated in the immunopathological phenotype of human B-cell malignancies. Notably, we found that the transgenic mice overexpressing AID developed lung adenocarcinoma and hepatocellular carcinoma (HCC), suggesting that ectopic expression of AID can lead to tumorigenesis in epithelial tissues as well. To examine the involvement of AID in the development of human HCC, we analyzed the AID expression and its correlation with mutation frequencies of the p53 gene in liver tissues from 51 patients who underwent resection of primary HCCs. The specific expression, inducibility by cytokine stimulation and mutagenic activity of AID were investigated in cultured human hepatocytes. Only trace amounts of AID transcripts were detected in the normal liver; however, endogenous AID was significantly upregulated in both HCC and surrounding noncancerous liver tissues with underlying chronic hepatitis or liver cirrhosis (p < 0.05). Most liver tissues with underlying chronic inflammation with endogenous AID upregulation already contained multiple genetic changes in the p53 gene. In both hepatoma cell lines and cultured human primary hepatocytes, the expression of AID was substantially induced by TGF-b stimulation. Aberrant activation of AID in hepatocytes resulted in accumulation of multiple genetic alterations in the p53 gene. Our findings suggest that the aberrant expression of AID is observed in human hepatocytes with several pathological settings, including chronic liver disease and HCC, which might enhance the genetic susceptibility to mutagenesis leading to hepatocarcinogenesis. ' 2006 Wiley-Liss, Inc.

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