Reiko Ogawa

Disruption of Dicer1 Induces Dysregulated Fetal Gene Expression and Promotes Hepatocarcinogenesis

BACKGROUND & AIMS Growing evidence suggests that microRNAs coordinate various biological processes in the liver. We describe experiments to address the physiologic roles of these new regulators of gene expression in the liver that are as of yet largely undefined. METHODS We disrupted Dicer, an enzyme essential for the processing of microRNAs, in hepatocytes using a conditional knockout mouse model to elucidate the consequences of loss of microRNAs. RESULTS The conditional knockout mouse livers showed the efficient disruption of Dicer1 at 3 weeks after birth. This resulted in prominent steatosis and the depletion of glycogen storage. Dicer1-deficient liver exhibited increased growth-promoting gene expression and the robust expression of fetal stage-specific genes. The consequence of Dicer elimination included both increased hepatocyte proliferation and overwhelming apoptosis. Over time, Dicer1-expressing wild-type hepatocytes that had escaped Cre-mediated recombination progressively repopulated the entire liver. Unexpectedly, however, two thirds of the mutant mice spontaneously developed hepatocellular carcinomas derived from residual Dicer1-deficient hepatocytes at 1 year of age. CONCLUSIONS Dicer and microRNAs have critical roles in hepatocyte survival, metabolism, developmental gene regulation, and tumor suppression in the liver. Loss of Dicer primarily impairs hepatocyte survival but can promote hepatocarcinogenesis in cooperation with additional oncogenic stimuli.

Frequent GNAS mutations in low-grade appendiceal mucinous neoplasms

Background:The molecular basis for the development of appendiceal mucinous tumours, which can be a cause of pseudomyxoma peritonei, remains largely unknown.Methods:Thirty-five appendiceal mucinous neoplasms were analysed for GNAS and KRAS mutations. A functional analysis of mutant GNAS was performed using a colorectal cancer cell line.Results:A mutational analysis identified activating GNAS mutations in 16 of 32 low-grade appendiceal mucinous neoplasms (LAMNs) but in none of three mucinous adenocarcinomas (MACs). KRAS mutations were found in 30 LAMNs and in all MACs. We additionally analysed a total of 186 extra-appendiceal mucinous tumours and found that GNAS mutations were highly prevalent in intraductal papillary mucinous tumours of the pancreas (88%) but were rare or absent in mucinous tumours of the colorectum, ovary, lung and breast (0–9%). The prevalence of KRAS mutations was quite variable among the tumours. The introduction of the mutant GNAS into a colorectal cancer cell line markedly induced MUC2 and MUC5AC expression, but did not promote cell growth either in vitro or in vivo.Conclusion:Activating GNAS mutations are a frequent and characteristic genetic abnormality of LAMN. Mutant GNAS might play a direct role in the prominent mucin production that is a hallmark of LAMN.

Dicer is required for proper liver zonation

A number of genes and their protein products are expressed within the liver lobules in a region‐specific manner and confer heterogeneous metabolic properties to hepatocytes; this phenomenon is known as ‘metabolic zonation’. To elucidate the roles of Dicer, an endoribonuclease III type enzyme required for microRNA biogenesis, in the establishment of liver zonation, we examined the distribution of proteins exhibiting pericentral or periportal localization in hepatocyte‐specific Dicer1 knockout mouse livers. Immunohistochemistry showed that the localization of pericentral proteins was mostly preserved in Dicer1‐deficient livers. However, glutamine synthetase, whose expression is normally confined to a few layers of hepatocytes surrounding the central veins, was expressed in broader pericentral areas. Even more striking was the observation that all the periportal proteins that were examined, including phosphoenolpyruvate carboxykinase, E‐cadherin, arginase 1, and carbamoyl phosphate synthetase‐I, lost their localized expression patterns and were diffusely expressed throughout the entire lobule. Thus, with regard to periportal protein expression, the consequences of Dicer loss were similar to those caused by the disruption of β‐catenin. An analysis of livers deficient in β‐catenin did not identify the down‐regulation of Dicer1 or any microRNAs, indicating that they are not directly activated by β‐catenin. Thus, the present study illustrates that Dicer plays a pivotal role in the establishment of liver zonation. Dicer is essential for the suppression of periportal proteins by Wnt/β‐catenin/TCF signalling, albeit it likely acts in an indirect manner. Copyright

Frequent GNAS and KRAS mutations in pyloric gland adenoma of the stomach and duodenum

Gastric and duodenal adenomas exhibit a significant morphological and phenotypical diversity and are classified into intestinal‐type, foveolar‐type and pyloric gland adenomas. We analysed the mutations in GNAS, KRAS, BRAF and CTNNB1 and the expressions of mismatch repair (MMR) proteins in 80 gastric and 32 duodenal adenomas with histologically distinct subtypes, as well as in 71 gastric adenocarcinomas. Activating GNAS mutations were found in 22 of the 35 pyloric gland adenomas (PGAs; 63%) but in none of the foveolar‐type or intestinal‐type adenomas or the adenocarcinomas. Fourteen PGAs (41%), two foveolar‐type adenomas (9%), five intestinal‐type adenomas (9%) and one adenocarcinoma (1%) had KRAS mutations. BRAF mutations were absent in all the adenomas and adenocarcinomas that were examined. CTNNB1 mutations were only found in two intestinal‐type adenomas (4%). Notably, 13 of the 14 KRAS‐mutated gastric and duodenal PGAs had concurrent GNAS mutations. The loss of the MMR proteins, which is indicative of microsatellite instability, was observed in one PGA (3%), 12 foveolar‐type adenomas (52%), one intestinal‐type adenoma (2%) and five adenocarcinomas (7%). These observations indicate that each histological subtype of gastric and duodenal adenomas has a distinct genetic background. In particular, the present study identified the frequent presence of activating GNAS mutations, which are often associated with KRAS mutations, as a characteristic genetic feature of PGAs of the stomach and duodenum.

Frequent activating GNAS mutations in villous adenoma of the colorectum

To elucidate the role of GNAS mutations in colorectal tumourigenesis, we performed a mutation analysis in a total of 234 colorectal tumours, including adenomas, serrated lesions and adenocarcinomas. Activating GNAS mutations were found in 20 of the 24 villous adenomas (83%) but were absent in all the other tumours, except for one tubulovillous adenoma (3%) and two adenocarcinomas (3%). KRAS and BRAF mutations were always mutually exclusive. KRAS mutations were frequent in villous (67%) and tubulovillous (60%) adenomas but were rare or absent in tubular adenomas (6%) and serrated lesions, including hyperplastic polyps, sessile serrated polyps/sessile serrated lesions and traditional serrated adenomas (0‐9%). BRAF mutations were found in four villous adenomas (17%) and in the large majority of serrated lesions (81‐92%), but were absent in tubular and tubulovillous adenomas. Seventeen villous adenomas (71%) harboured GNAS mutations concomitantly with KRAS or BRAF mutations. Immunohistochemically, all the villous adenomas retained mismatch repair protein expression, suggesting that they are microsatellite‐stable. The current study showed that the presence of activating GNAS mutations, in association with KRAS or BRAF mutations, is a characteristic genetic feature of colorectal villous adenoma. Copyright

Frequent MED12 mutations in phyllodes tumours of the breast

Background:Phyllodes tumours are rare fibroepithelial tumours of the breast, that include benign, borderline, and malignant lesions. Although the molecular basis of phyllodes tumours largely remains unknown, a recent exome study identified MED12 mutations as a sole recurrent genetic alteration in fibroadenoma, a common benign fibroepithelial tumour that shares some histological features with the phyllodes tumour.Methods:Forty-six phyllodes tumours and 58 fibroadenomas of the breast were analysed for MED12 mutations by using Sanger sequencing.Results:MED12 mutations were identified in 37 out of the 46 phyllodes tumours (80%). The prevalence of MED12 mutations was similar among benign (15/18, 83%), borderline (12/15, 80%), and malignant tumours (10/13, 77%). MED12 mutations were also identified in 36 of the 58 fibroadenomas (62%). The mutations were frequent among intracanalicular-type (24/32, 75%) and complex-type lesions (4/6, 67%), but were significantly less common among the pericanalicular-type lesions (8/20, 40%). A microdissection-based analysis showed that MED12 mutations were confined to the stromal components in both phyllodes tumours and fibroadenomas.Conclusions:MED12 mutations were frequent among the phyllodes tumours of the breast, regardless of the tumour grade. Phyllodes tumours and fibroadenomas share, at least in part, a common genetic background.

Lobular endocervical glandular hyperplasia is a neoplastic entity with frequent activating GNAS mutations.

To clarify the significance of GNAS mutations in cervical tumorigenesis, we performed mutational analyses in a total of 154 lesions and in 22 normal tissues of the uterine cervix. Activating GNAS mutations were found in 8 of the 19 lobular endocervical glandular hyperplasias (LEGH; 42%) and 4 of the 79 endocervical-type mucinous adenocarcinomas (5%) but were never seen in the normal endocervical tissue, minimal deviation adenocarcinomas, endometrioid adenocarcinomas, or squamous cell carcinomas. We further examined the presence of human papillomavirus (HPV) DNA and p16 expression to probe the relationship between GNAS mutations and HPV infection in LEGHs and carcinomas. All the GNAS-mutated LEGHs were negative for HPV DNA and p16 expression, whereas all the GNAS-mutated adenocarcinomas were positive for HPV DNA and/or p16 expression, implicating GNAS mutations in the development of LEGH and a minor subset of HPV-related cervical adenocarcinomas. Additional mutational analyses of LEGH identified KRAS and STK11 mutations in 1 and 2 cases, respectively. The GNAS, KRAS, and STK11 mutations were mutually exclusive; thus, a total of 11 LEGHs (58%) had 1 of these genetic alterations. Although LEGH has been regarded as a metaplastic lesion, the frequent presence of genetic alterations suggests a neoplastic nature.

Expression of SLCO1B3 is associated with intratumoral cholestasis and CTNNB1 mutations in hepatocellular carcinoma.

Recent studies have shown that intratumoral cholestasis is a hallmark of CTNNB1 mutations in hepatocellular carcinomas (HCC). Here, we analyzed the expressions of genes involved in bile acid and bilirubin metabolism and their correlation with the mutational status of CTNNB1 in a series of HCC. The expressions of CYP7A1 and CYP27A1, which encode rate‐limiting enzymes in bile acid synthesis, were unaltered or only marginally increased in CTNNB1‐mutated HCC compared with those in HCC with wild‐type CTNNB1. Among the genes involved in bile acid and bilirubin transport, the expression of SLCO1B3 was significantly elevated in HCC with CTNNB1 mutations, whereas the expression of ABCC4 was elevated in HCC with wild‐type CTNNB1. Immunohistochemistry confirmed the frequent expression of SLCO1B3 in CTNNB1‐mutated HCC at the protein level, but not in most HCC with wild‐type CTNNB1. Immunohistochemistry for MRP4 (encoded by ABCC4) partly agreed with ABCC4 expression, but most cases did not express detectable levels of MRP4. Notably, all HCC with bile accumulation, including those without CTNNB1 mutations, expressed SLCO1B3, suggesting that SLCO1B3 expression, rather than CTNNB1 mutation, is the critical determinant of intratumoral cholestasis. As SLCO1B3 is involved in the uptake of a number of chemotherapeutic and diagnostic agents, SLCO1B3 expression and the status of CTNNB1 mutation might need to be considered in the drug delivery to HCC. (Cancer Sci 2011; 102: 1742–1747)

Esophageal melanomas harbor frequent NRAS mutations unlike melanomas of other mucosal sites

Mucosal melanomas have genetic alterations distinct from those in cutaneous melanomas. For example, NRAS- and BRAF-activating mutations occur frequently in cutaneous melanomas, but not in mucosal melanomas. We examined 16 esophageal melanomas for genetic alterations in NRAS, BRAF, and KIT to determine whether they exhibit genetic features common to melanomas arising from other mucosal sites. A sequencing analysis identified NRAS mutations in six cases; notably, four of these mutations were located in exon 1, an uncommon mutation site in cutaneous and other mucosal melanomas. BRAF and KIT mutations were found in one case each. Immunohistochemistry showed KIT expression in four cases, including the tumor with a KIT mutation and two other intramucosal tumors. The low frequency of BRAF mutations and the presence of a KIT mutation-positive case are findings similar to those of mucosal melanomas of other sites, but the prevalence of NRAS mutations was even higher than that of cutaneous melanomas. The present study implies that esophageal melanomas have genetic alterations unique from those observed in other mucosal melanomas.

Familial adenomatous polyposis-associated and sporadic pyloric gland adenomas of the upper gastrointestinal tract share common genetic features

Familial adenomatous polyposis (FAP) is a hereditary cancer predisposition syndrome caused by a germline APC mutation. A recent study showed the enrichment of pyloric gland adenomas (PGAs) of the stomach, in addition to fundic gland polyps (FGPs) and foveolar‐type adenomas (FAs), in patients with FAP. In the present study, we analysed the genetic alterations in these FAP‐associated gastric lesions.