Racial variation in the O-acetylation phenotype of human colonic mucosa

Modified Sialic Acids on Mucus and Erythrocytes Inhibit Influenza A Virus Hemagglutinin and Neuraminidase Functions

Sialic acids (Sia) are the primary receptors for influenza viruses and are widely displayed on cell surfaces and in secreted mucus. Sia may be present in variant forms that include O-acetyl modifications at C-4, C-7, C-8, and C-9 positions and N-acetyl or N-glycolyl at C-5. They can also vary in their linkages, including α2-3 or α2-6 linkages. Here, we analyze the distribution of modified Sia in cells and tissues of wild-type mice or in mice lacking CMP-N-acetylneuraminic acid hydroxylase (CMAH) enzyme, which synthesizes N-glycolyl (Neu5Gc) modifications. We also examined the variation of Sia forms on erythrocytes and in saliva from different animals. To determine the effect of Sia modifications on influenza A virus (IAV) infection, we tested for effects on hemagglutinin (HA) binding and neuraminidase (NA) cleavage. We confirmed that 9-O-acetyl, 7,9-O-acetyl, 4-O-acetyl, and Neu5Gc modifications are widely but variably expressed in mouse tissues, with the highest levels detected in the respiratory and gastrointestinal (GI) tracts. Secreted mucins in saliva and surface proteins of erythrocytes showed a high degree of variability in display of modified Sia between different species. IAV HAs from different virus strains showed consistently reduced binding to both Neu5Gc- and O-acetyl-modified Sia; however, while IAV NAs were inhibited by Neu5Gc and O-acetyl modifications, there was significant variability between NA types. The modifications of Sia in mucus may therefore have potent effects on the functions of IAV and may affect both pathogens and the normal flora of different mucosal sites.IMPORTANCE Sialic acids (Sia) are involved in numerous different cellular functions and are receptors for many pathogens. Sia come in chemically modified forms, but we lack a clear understanding of how they alter interactions with microbes. Here, we examine the expression of modified Sia in mouse tissues, on secreted mucus in saliva, and on erythrocytes, including those from IAV host species and animals used in IAV research. These Sia forms varied considerably among different animals, and their inhibitory effects on IAV NA and HA activities and on bacterial sialidases (neuraminidases) suggest a host-variable protective role in secreted mucus.

Enzymatic Adaptation of Bifidobacterium bifidum to Host Glycans, Viewed from Glycoside Hydrolyases and Carbohydrate-Binding Modules

Certain species of the genus Bifidobacterium represent human symbionts. Many studies have shown that the establishment of symbiosis with such bifidobacterial species confers various beneficial effects on human health. Among the more than ten (sub)species of human gut-associated Bifidobacterium that have significantly varied genetic characteristics at the species level, Bifidobacterium bifidum is unique in that it is found in the intestines of a wide age group, ranging from infants to adults. This species is likely to have adapted to efficiently degrade host-derived carbohydrate chains, such as human milk oligosaccharides (HMOs) and mucin O-glycans, which enabled the longitudinal colonization of intestines. The ability of this species to assimilate various host glycans can be attributed to the possession of an adequate set of extracellular glycoside hydrolases (GHs). Importantly, the polypeptides of those glycosidases frequently contain carbohydrate-binding modules (CBMs) with deduced affinities to the target glycans, which is also a distinct characteristic of this species among members of human gut-associated bifidobacteria. This review firstly describes the prevalence and distribution of B. bifidum in the human gut and then explains the enzymatic machinery that B. bifidum has developed for host glycan degradation by referring to the functions of GHs and CBMs. Finally, we show the data of co-culture experiments using host-derived glycans as carbon sources, which underpin the interesting altruistic behavior of this species as a cross-feeder.

Fixation and Spread of Somatic Mutations in Adult Human Colonic Epithelium

We investigated the means and timing by which mutations become fixed in the human colonic epithelium by visualizing somatic clones and mathematical inference. Fixation requires two sequential steps. First, one of approximately seven active stem cells residing within each colonic crypt has to be mutated. Second, the mutated stem cell has to replace neighbors to populate the entire crypt in a process that takes several years. Subsequent clonal expansion due to crypt fission is infrequent for neutral mutations (around 0.7% of all crypts undergo fission in a single year). Pro-oncogenic mutations subvert both stem cell replacement to accelerate fixation and clonal expansion by crypt fission to achieve high mutant allele frequencies with age. The benchmarking of these behaviors allows the advantage associated with different gene-specific mutations to be compared irrespective of the cellular mechanisms by which they are conferred.

Characterization of O-acetylation in sialoglycans by MALDI-MS using a combination of methylamidation and permethylation

O-Acetylation of sialic acid in protein N-glycans is an important modification and can occur at either 4-, 7-, 8- or 9-position in various combinations. This modification is usually labile under alkaline reaction conditions. Consequently, a permethylation-based analytical method, which has been widely used in glycomics studies, is not suitable for profiling O-acetylation of sialic acids due to the harsh reaction conditions. Alternatively, methylamidation can be used for N-glycan analysis without affecting the base-labile modification of sialic acid. In this report, we applied both permethylation and methylamidation approaches to the analysis of O-acetylation in sialic acids. It has been demonstrated that methylamidation not only stabilizes sialic acids during MALDI processing but also allow for characterization of their O-acetylation pattern. In addition, LC-MS/MS experiments were carried out to distinguish between the O-acetylated glycans with potential isomeric structures. The repeatability of methylamidation was examined to evaluate the applicability of the approach to profiling of O-acetylation in sialic acids. In conclusion, the combination of methylamidation and permethylation methodology is a powerful MALDI-TOF MS-based tool for profiling O-acetylation in sialic acids applicable to screening of N-glycans.

The Densely O-Glycosylated MUC2 Mucin Protects the Intestine and Provides Food for the Commensal Bacteria

All mucins are highly O-glycosylated by variable glycans depending on species, histoblood group and organ. This makes the intestinal main mucin MUC2 non-degradable by the host digestive system but well by both commensal and pathogenic bacteria. The MUC2 glycans are important for selection of the commensal bacteria and act as a nutritional source for the bacteria; this also helps the host to recover some of the energy spent on constantly renewing the protective mucus layer. Glycosylation is the most diverse and common posttranslational modification of cell surfaces and secreted proteins. N-Glycosylation is most well studied and predictable, whereas O-glycosylation is more diverse and less well understood. O-Glycosylation is also often called mucin-type glycosylation as it is typical for mucins that often have more than 80% of the mass as O-glycans. This review will discuss the mucin-type O-glycosylation and especially the O-glycosylation of human and mice intestinal mucin MUC2 in relation to bacteria and disease.

Incidence of polymerase chain reaction-diagnosed Clostridium difficile in a large high-risk cohort, 2011-2012

OBJECTIVE

To describe incidence rates (IRs) of polymerase chain reaction (PCR)-diagnosed Clostridium difficile infection (CDI) in a large high-risk cohort.

PATIENTS AND METHODS

Members of Kaiser Permanente Southern California 1 year or older who were admitted to any of 14 Kaiser Permanente hospitals from January 1, 2011, through December 31, 2012, were included in the study. The CDI cases were identified by PCR in the inpatient and outpatient settings. The CDI IRs per 10,000 inpatient-days are estimated by year, surveillance category, age, sex, race/ethnicity, and Charlson comorbidity index. Recurrence rates are presented by age, sex, and race/ethnicity. Death and colectomy in the 30 days after CDI diagnosis, white blood cell count, and serum creatinine level are assessed.

RESULTS

Among 268,655 patients, 4286 (1.6%) had CDI. Among these patients, 671 (15.7%) had recurrent infections. The IR was highest among community-onset, health care facility-associated infections (11.1 per 10,000 inpatient-days). The CDI IRs differed by age, sex, and race/ethnicity. Overall, 528 patients (12.3%) died within 30 days of a positive CDI test result. The CDI IRs increased 34% with implementation of PCR testing.

CONCLUSION

Increasingly, PCR is being used because of its higher diagnostic sensitivity. Reassessing the epidemic using PCR updates our understanding of CDI risk. Our capacity to identify patients presenting in the outpatient setting after discharge provides a more accurate picture of health care-associated CDI rates, particularly because the community appears to assume an increasing role in CDI onset and possibly transmission. The CDI burden differs by race, comorbidity, sex, and previous health care use. The detected increase in CDI incidence after transitioning to PCR diagnosis was modest compared with previous studies.

The human Cas1 protein: a sialic acid-specific O-acetyltransferase?

Sialic acids are important sugars at the reducing end of glycoproteins and glycolipids. They are among many other functions involved in cell-cell interactions, host-pathogen recognition and the regulation of serum half-life of glycoproteins. An important modification of sialic acids is O-acetylation, which can alter or mask the biological properties of the parent sialic acid molecule. The nature of mammalian sialate-O-acetyltransferases (EC 2.3.1.45) involved in their biosynthesis is still unknown. We have identified the human CasD1 (capsule structure1 domain containing 1) gene as a candidate to encode the elusive enzyme. The human CasD1 gene encodes a protein with a serine-glycine-asparagine-histidine hydrolase domain and a hydrophobic transmembrane domain. Expression of the Cas1 protein tagged with enhanced green fluorescent protein in mammalian and insect cells directed the protein to the medial and trans-cisternae of the Golgi. Overexpression of the Cas1 protein in combination with α-N-acetyl-neuraminide α-2,8-sialyltransferase 1 (GD3 synthase) resulted in an up to 40% increased biosynthesis of 7-O-acetylated ganglioside GD3. By quantitative real-time polymerase chain reaction, we found up to 5-fold increase in CasD1 mRNA in tumor cells overexpressing O-Ac-GD3. CasD1-specific small interfering RNA reduced O-acetylation in tumor cells. These results suggest that the human Cas1 protein is directly involved in O-acetylation of α2-8-linked sialic acids.

Is permethylation strategy always applicable to protein N-glycosylation study?: A case study on the O-acetylation of sialic acid in fish serum glycans

O-Acetylation is one of the major modifications of sialic acids that significantly alters biological properties of the parent molecule. These O-acetylated forms are components of the cellular membrane and can affect physiological and pathological responses. Understanding the role of N-glycans in physiology is of increasing relevance to cellular biologists in various disciplines who study glycoproteomics yet lack information regarding the function of the attached glycans. However, permethylation, the most common mass spectrometric analytical means, leads to the loss of O-linked acetyl groups in sialic acids. In this chapter, we demonstrated that O-acetylation of sialic acid in Atlantic salmon serum N-glycan can be well investigated by capillary electrophoresis-mass spectrometry.

Fetal-juvenile origins of point mutations in the adult human tracheal-bronchial epithelium: absence of detectable effects of age, gender or smoking status

Allele-specific mismatch amplification mutation assays (MAMA) of anatomically distinct sectors of the upper bronchial tracts of nine nonsmokers revealed many numerically dispersed clusters of the point mutations C742T, G746T, G747T of the TP53 gene, G35T of the KRAS gene and G508A of the HPRT1 gene. Assays of these five mutations in six smokers have yielded quantitatively similar results. One hundred and eighty four micro-anatomical sectors of 0.5-6x10(6) tracheal-bronchial epithelial cells represented en toto the equivalent of approximately 1.7 human smokers' bronchial trees to the fifth bifurcation. Statistically significant mutant copy numbers above the 95% upper confidence limits of historical background controls were found in 198 of 425 sector assays. No significant differences (P=0.1) for negative sector fractions, mutant fractions, distributions of mutant cluster size or anatomical positions were observed for smoking status, gender or age (38-76 year). Based on the modal cluster size of mitochondrial point mutants, the size of the adult bronchial epithelial maintenance turnover unit was estimated to be about 32 cells. When data from all 15 lungs were combined the log2 of nuclear mutant cluster size plotted against log2 of the number of clusters of a given cluster size displayed a slope of approximately 1.1 over a range of cluster sizes from approximately 2(6) to 2(15) mutant copies. A parsimonious interpretation of these nuclear and previously reported data for lung epithelial mitochondrial point mutant clusters is that they arose from mutations in stem cells at a high but constant rate per stem cell doubling during at least ten stem cell doublings of the later fetal-juvenile period. The upper and lower decile range of summed point mutant fractions among lungs was about 7.5-fold, suggesting an important source of stratification in the population with regard to risk of tumor initiation.

Significant increase of colonic mutated crypts correlates with age in sporadic cancer and diverticulosis cases, with higher frequency in the left- than right-side colorectum

Mild periodic acid-Schiff (mPAS) staining can discriminate non-O-acetylated (mPAS-positive) from O-acetylated (mPAS-negative) epithelial sialoglycoproteins in human colonic mucosa, allowing the three haplotypes expressed from a single polymorphic autosomal gene (oat) to be distinguished. In heterozygotes, we previously demonstrated wholly mPAS-positive (stem cell mutated) crypts and clusters of two or more mPAS-positive crypts to be significantly increased with duration of ulcerative colitis. To establish whether such an increase in the number of mutated crypts with age also occurs in normal individuals or in cases with diverticulosis, the O-acetylation phenotype in the non-cancerous colonic mucosa of 47 sporadic colorectal cancer patients who were heterozygotes for oat was tested with mild-PAS staining. PAS-positive crypts were assessed histologically in relation to age and compared between the left (sigmoid colon and rectum) and right (cecum and ascending colon) sides of the colorectum. Wholly mPAS-positive (stem cell mutated) crypts and foci in heterozygotes were found to be increased significantly (P < 0.0001) in the left side with aging (r = 0.598 and 0.643, respectively). Such a positive correlation with aging was also confirmed in 19 diverticulosis cases without cancer (r = 0.797 and 0.793, respectively). The frequency of mutated crypts and foci on the right side was significantly lower than on the left side in both spontaneous colorectal cancer and diverticulosis cases. The results provide support for an intimate relationship between accumulation of mutated crypts with aging, possibly with significance for colorectal cancer development. Furthermore, the environment in the right side of the colon may be different from that in the left side in this regard.

Distributions of five common point mutants in the human tracheal-bronchial epithelium

The mutations C742T, G746T, G747T in the TP53 gene and G35T in the KRAS gene have been repeatedly found in sectors of human tumors by direct DNA sequencing. The mutation G508A in the HPRT1 gene has been repeatedly found among peripheral T lymphocytes by clonal expansion under selective conditions. To discover if these mutations also occur frequently in normal tissues from which tumors arise, we have developed and validated allele-specific mismatch amplification mutation assays (MAMA) for each mutation. Reconstruction experiments demonstrated linearity in the range of 9-3000 mutant alleles among 3 x 10(6) wild-type alleles. The cumulative distributions of all negative controls established robust detection limits (P<0.05) of 34-125 mutants per 10(6) copies assayed depending on the mutation. One hundred and seventy-seven micro-anatomical samples of approximately (0.5-6)x10(6) tracheal-bronchial epithelial cells from nine non-smokers were assayed representing en toto the equivalent of approximately 1.6 human bronchial trees to the fifth bifurcation. Statistically significant mutant copy numbers were found in 257 of 463 assays. Clusters of mutant copies ranged from 10 to 1000 in 239/257 positive samples. As all five point mutations were detected at mutant fractions of >10(-5) in two or more lungs, we infer that they are mutational hotspots generated in lung epithelial stem cells. As the cancer-associated mutations did not differ in cluster size distribution from the HPRT1 mutation, we infer that none of the mutations conferred a growth advantage to somatic heterozygous clusters or maintenance turnover units. Specific mutants appeared in very large copy numbers, 1000-35,000, in 18/257 positive assays. Various hypotheses to account for the observed cluster size distributions are offered.

O-acetylation and de-O-acetylation of sialic acids in human colorectal carcinoma

A decrease in the level of O-acetylated sialic acids observed in colorectal carcinoma may lead to an increase in the expression of sialyl Lewis(X), a tumor-associated antigen, which is related to progression of colorectal cancer to metastasis. The underlying mechanism for this reduction is, however, not fully understood. Two enzymes are thought to be primarily responsible for the turnover of O-acetyl ester groups on sialic acids; sialate-O-acetyltransferase (OAT) and sialate-O-acetylesterase (OAE). We have previously reported the characterization of OAT activity from normal colon mucosa, which efficiently O-acetylates CMP-Neu5Ac exclusively in the Golgi apparatus prior to the action of sialyltransferase. In this report we describe the identification of a lysosomal and a cytosolic OAE activity in human colonic mucosa that specifically hydrolyses 9-O-acetyl groups on sialic acid. Utilizing matched resection margin and cancer tissue from colorectal carcinoma patients we provide strong evidence suggesting that the level of O-acetylated sialic acids present in normal and diseased human colon may be dependent on the relative activities of OAT to lysosomal OAE. Furthermore, we show that the level of free cytosolic Neu5,9Ac2 in human colon is regulated by the relative activity of the cytosolic OAE.

Genetic and epigenetic mosaicism in cancer precursor tissues

The concept of field effects in cancer is old, but recent molecular data have substantiated it. Clones of cells that carry well-defined genetic or epigenetic aberrations, but which have not yet acquired the morphological hallmarks of neoplasia, have been documented in the precursor tissues of some of the most common pediatric and adult malignancies. Here I review this evidence, focusing on loss of heterozygosity (LOH) and gain of DNA methylation.

Altered mucin expression in the gastrointestinal tract: a review

Early studies of changes in mucin expression in disorders of the gastrointestinal tract focused on alterations in the carbohydrate chain. This review briefly considers the various mechanisms by which such alterations may come about: (a) normal variation, (b) sialic acid alterations, (c) defective assembly of carbohydrate side-chains, (d) changed expression of core proteins and (e) epithelial metaplasia. The availability of monoclonal antibodies to mucin core proteins adds a new dimension to mucin histochemistry. It is now possible to offer explanations for traditional mucin histochemical findings on the basis of lineage-specific patterns of mucin core protein expression. Changes in core protein expression are described in inflammatory, metaplastic and neoplastic disorders of the gastrointestinal tract. The possibility that mucin change could be important in the aetiology of some diseases such as ulcerative colitis and H. pylori gastritis is considered. It is more probable, however, that changes in mucin expression are secondary to reprogramming of cellular differentiation and altered cell turnover. As such they may serve as markers to explain pathogenesis and provide novel diagnostic and prognostic information.

Crypt-restricted metallothionein immunopositivity in murine colon: validation of a model for studies of somatic stem cell mutation

The ability to visualize the cellular effects of a somatic mutation is relevant to studies of cell kinetics and carcinogenesis. In the colon, mutagen administration leads to scattered crypt-restricted loss of activity of the X-linked enzyme glucose-6-phosphate dehydrogenase (G6PD); it has been shown that this is due to somatic mutation in the G6PD gene. Mutagen-induced crypt-restricted immunopositivity for metallothionein (MT) has been reported in one study in the mouse colon; if this is also due to somatic mutation, it provides a simple method for studying the phenomenon which could be carried out on paraffin sections. This study shows that, as in the G6PD model, the frequency of crypt-restricted immunopositivity for MT is very low in untreated animals, but increases proportionately with the dose of mutagen administered. There is a good overall correlation of a range of MT-positive crypt frequencies with those derived from studies using G6PD. As with the G6PD model, the MT-positive crypt phenotype evolves over time after mutagen administration; initially individual crypts include both positive and negative phenotype cells, but later almost all involved crypts are composed entirely of MT-positive cells. The frequency of MT-positive crypts stabilizes after a few weeks and remains at the same level 6 months later. All these observations are qualitatively identical to those found using the G6PD model and provide strong evidence that stable, crypt-restricted immunopositivity for MT results from a mutation affecting expression of the metallothionein gene in a colonic stem cell. This model will provide a useful tool to study factors influencing stem cell mutation frequency and cell kinetics in the colon.

Tumor burden and clonality in multiple intestinal neoplasia mouse/normal mouse aggregation chimeras

Aggregation chimeras were formed between C57BL/6 mice heterozygous for the Apc(min) (Min) mutation and wild-type SWR mice, that differ in their Pla2g2a status, a modifier of Apc(min), and also in their resistance to intestinal polyp formation. Variation in the dolichos biflorus agglutinin-staining patterns of the intestines of these mouse strains was used to determine the chimeric composition of the intestine in individual mice and to examine the clonal composition of adenomas. Macroscopic adenoma numbers in chimeric mice were compared with the expected adenoma numbers based on the percentage of C57BL/6J-Apc(min/+) epithelium in individual mice. These results unexpectedly show that there was no apparent inhibitory effect of the SWR-derived (Pla2g2a wild-type) tissue on adenoma formation in the C57BL/6J-Apc(min/+) epithelium. This suggests that the main genetic modifiers of the Min phenotype act at a cellular or crypt-restricted level with no discernable systemic effect. All adenomas were seen to contain C57BL/6J-Apc(min/+)-derived epithelium, confirming that the germ-line mutation of the mApc gene is necessary to initiate tumorigenesis in this model system, and that the mApc gene acts in a cell autonomous fashion.

Reduction of sialic acid O-acetylation in human colonic mucins in the adenoma-carcinoma sequence

The oligo-O-acetylation of sialic acids found in normal colonic mucins is greatly reduced in colorectal cancer. Mucins prepared from cancer tissue in adenocarcinoma showed this reduction, while normal O-acetylation was detected in resection margin and control cases and total mucin sialic acid content was significantly decreased in cancer vs. control samples. A reduction of the O-acetyl transferase activity catalysing the O-acetylation reaction was also found. A series of cultured human colorectal cell lines derived from the same premalignant adenomatous line, and representative of the adenoma-carcinoma sequence were examined and revealed a depletion of oligo-O-acetylation in the original diploid premalignant line, re-expression in a further premalignant line and reduction in malignant mucinous and adenocarcinoma cell lines. Reduction of sialic acid O-acetylation appears as an early event in the process of malignant transformation in human colorectal cancer.

Increased stem cell somatic mutation in the non-neoplastic colorectal mucosa of patients with familial adenomatous polyposis

Colorectal tumorigenesis in familial adenomatous polyposis (FAP) results from somatic mutation of either the normal APC allele or another growth control gene in epithelial cells bearing a germline APC defect. The rate at which tumors develop is therefore dependent on the somatic mutation frequency; it is not known whether this is normal or elevated in FAP. We aimed to quantify stem cell somatic mutation in FAP, comparing it with hereditary nonpolyposis colorectal cancer (HNPCC) and Crohn's disease (CD). Stem cell somatic mutation frequency was studied in 47 FAP patients, 5 HNPCC patients, and 13 CD patients, all younger than 49 years, by quantifying crypt-restricted loss of O-acetyltransferase activity in sections of morphologically normal colonic mucosa from individuals heterozygous for this monogenically inherited polymorphism. Median stem cell somatic mutation frequency was significantly higher in FAP than HNPCC (4.2 x 10(-4) v 1.4 x 10(-4), Mann-Whitney U, P < .02). The level in CD (4.0 x 10(-4)) was similar to FAP. Mutated crypts occurred in groups more frequently in FAP (22%) than HNPCC (12%) or CD (10%), suggesting an increase in stem cell division associated with crypt fission in FAP. We conclude that stem cell somatic mutation frequency is raised in non-neoplastic colorectal mucosa in FAP. This is probably related to increased stem cell proliferation and contributes to the high rate of tumor formation in this condition.

No difference in stem cell somatic mutation between the background mucosa of right- and left-sided sporadic colorectal carcinomas

Epidemiological, morphological, and molecular differences exist between carcinomas of the right and left sides of the large bowel. To investigate whether this is reflected in differences in somatic mutation frequency in the background mucosa, mutation of the neutral O-acetyltransferase gene (oat) was quantified in histologically normal resection margins from 20 informative (heterozygous) patients with caecal or ascending colon cancer (11 males, median age 75 years) and 20 with sigmoid colon or rectal cancer (10 males, median age 70 years). Mutant discordant crypts lacking O-acetyltransferase activity were visualized by mPAS staining and classified as wholly or partially involved by the mutant phenotype; median frequencies (x10(-4) were compared (Mann-Whitney U-test) after assessing a sample of more than 10,000 crypts per case. No significant difference was found between the frequencies of wholly involved mPAS-positive crypts in background mucosa of left- and right-sided cancers (p = 0.4569), indicating that tumours on both sides of the colon are associated with similar levels of lifetime-accumulated stem cell mutational load. However, partially involved mPAS-positive crypts were significantly more frequent in mucosa from left-sided cancers (p < 0.04), indicating increased mutational activity during the previous 12 months. Analysis of mucosa proximal and distal to left-sided cancers showed that this increase was due to a statistically higher frequency of partially involved crypts in proximal mucosa, which probably resulted from the obstructive effects of the tumour causing increased exposure of the proximal mucosa to luminal carcinogens and/or epithelial regeneration in response to low-grade inflammation or ischaemia. The findings indicate that although left-sided colonic cancer is commoner than right-sided cancer in the British population, carcinomas on both sides of the large bowel arise in a background of similar levels of stem cell mutational activity.

O-acetylation of sialic acids

Sialic acids can be acetylated at the 4, 7, 8 and/or 9 position. Biological roles of these substitutions have been missed until recently because of their low abundance and lability to conventional purification methods. The recent advances in the analysis of sialic acids have allowed to demonstrate that O-acetylation has a selective but widespread distribution. The metabolism of acetylated sialic acids is under the control of two groups of enzymes, O-acetyl transferases and 9-O-acetyl esterases. O-acetyl transferases are difficult to purify, and furthermore, attempts at expression cloning have failed in isolating the true 9-O-acetyl transferase cDNA. This explains that the regulation of the selective expression of O-acetylated sialic acid in not completely understood. Acetylation of sialic acid is expressed on the outer most part of the carbohydrate moiety of membrane and secreted glycoconjugates. This particular location explains why this modification is involved in cell/cell interactions and in the non-immune protection of mucosa.

Somatic mutation of the glucose-6-phosphate dehydrogenase (g6pd) gene in colonic stem cells and crypt restricted loss of G6PD activity

The study of somatic mutation frequency, particularly stem cell somatic mutation, is important to the understanding of mechanisms of carcinogenesis. The models currently in use for studies in stem cell tissues such as the colon infer the presence of stem cell somatic mutation from alteration in enzyme function, when this has shown to be mutagen dose dependent, restricted to the unit of clonal architecture, and persistent. The present study identifies and characterises somatic mutations in the g6pd gene in individual mouse colonic crypts showing histochemically demonstrable loss of G6PD activity. Microdissection of single crypts, showing either normal or low G6PD activity by histochemistry was performed in mice treated with ethylnitrosourea (ENU), and the presence of point mutations sought by PCR and direct sequencing. Because of the limitation of the small amount of partially degraded (due to fixation) DNA available from each crypt, only about 20% of the coding region of the g6pd gene could be sequenced. Despite this, somatic mutations were identified in 3 of the 9 crypts analysed which showed loss of G6PD activity, but in none of the crypts with normal activity. Each of the mutations identified would be predicted to lead to a decrease in enzyme activity. We conclude that we have confirmed that the crypt restricted loss of G6PD activity is indeed due to stem cell somatic mutation in the g6pd gene, and suggest that the G6PD model can be used as a paradigm for other models where somatic mutation is inferred from a change in histochemically identifiable gene expression.

Post-irradiation somatic mutation and clonal stabilisation time in the human colon

BACKGROUND

Colorectal crypts are clonal units in which somatic mutation of marker genes in stem cells leads to crypt restricted phenotypic conversion initially involving part of the crypt, later the whole crypt. Studies in mice show that the time taken for the great majority of mutated crypts to be completely converted, the clonal stabilisation time, is four weeks in the colon and 21 weeks in the ileum. Differences in the clonal stabilisation time between tissues and species are thought to reflect differences in stem cell organisation and crypt kinetics.

AIM

To study the clonal stabilisation time in the human colorectum.

METHODS

Stem cell mutation can lead to crypt restricted loss of O-acetylation of sialomucins in subjects heterozygous for O-acetyltransferase gene activity. mPAS histochemistry was used to visualise and quantify crypts partially or wholly involved by the mutant phenotype in 21 informative cases who had undergone colectomy up to 34 years after radiotherapy.

RESULTS

Radiotherapy was followed by a considerable increase in the discordant crypt frequency that remained significantly increased for many years. The proportion of discordant crypts showing partial involvement was initially high but fell to normal levels about 12 months after irradiation.

CONCLUSIONS

Crypts wholly involved by a mutant phenotype are stable and persistent while partially involved crypts are transient. The clonal stabilisation time is approximately one year in the human colon compared with four weeks in the mouse. The most likely reason for this is a difference in the number of stem cells in a crypt stem cell niche, although differences in stem cell cycle time and crypt fission may also contribute. These findings are of relevance to colorectal gene therapy and carcinogenesis in stem cell systems.

Mucus glycoproteins and their role in colorectal disease

In this review, the nature and impact of progress in the study of mucins is outlined, emphasizing the current understanding of the structure and physiological function of these molecules in the colorectum. The use of new methods for preparation and separation has led to improvements in the analysis of mucins; these are detailed, as are their difficulties and pitfalls. Results obtained with these methods are correlated with long-established histochemical techniques and the use of chemical, lectin, and antibody reagents for general and specific detection of mucins in all procedures is described. Improvements in the detection and analysis of mucins in biopsy-size tissue samples and in larger numbers of individual clinical cases have now permitted a much wider approach to the pathological evaluation of mucin biology and progress with these techniques is outlined. The significance of the discovery of a family of mucin genes is presented and new concepts of mucin structure resulting from these studies are described. Bacterial degradation of the mucus layer at the surface of the colorectal mucosa is considered in line with the homeostatic relationship with mucosal mucin synthesis. Finally, the implications of abnormal mucins in colorectal disease are considered in the light of recent methodological advances.

Binding specificity of influenza C-virus to variably O-acetylated glycoconjugates and its use for histochemical detection of N-acetyl-9-O-acetylneuraminic acid in mammalian tissues

The specificity of influenza C-virus binding to sialoglycoconjugates was tested with various naturally O-acetylated gangliosides or synthetically O-acetylated sialic acid thioketosides, which revealed binding to 9-O-acetylated N-acetylneuraminic acid. Binding was also observed with a sample of Neu5,7Ac2-GD3, however at a lower degree. Sialic acids with two or three O-acetyl groups in the side chain of synthetic sialic acid derivatives are not recognized by the virus. In these experiments, bound viruses were detected with esterase substrates. Influenza C-virus was also used for the histological identification of mono-O-acetylated sialic acids in combination with an immunological visualization of the virus bound to thin-sections. The occurrence of these sialic acids was demonstrated in bovine submandibular gland, rat liver, human normal adult and fetal colon and diseased colon, as well as in human sweat gland. Submandibular gland and colon also contain significant amounts of glycoconjugates with two or three acetyl esters in the sialic acid side chain, demonstrating the value of the virus in discriminating between mono- and higher O-acetylation at the same site. The patterns of staining showed differences between healthy persons and patients with colon carcinoma, ulcerative colitis or Crohn's disease. Remarkably, some human colon samples did not show O-acetyl sialic acid-specific staining. The histochemical observations were controlled by chemical analysis of tissue sialic acids.

Conventional patterns of human intestinal proliferation in a severe-combined immunodeficient xenograft model

The present work describes the pattern of human intestinal proliferation in an immunodeficient murine xenograft model, which we have shown to closely mimic cell division in normal paediatric gut. Cellular proliferation was measured using a double-label technique combining MIB-1 immunohistochemistry and [3H]thymidine autoradiography, to critically compare values for the tissue growth fraction (G1, G2, S- and M-phase cells) and DNA synthesizing (S-phase) cells in xenograft epithelium, lamina propria, muscularis externa and intraepithelial lymphocytes. The MIB-1 monoclonal antibody (which recognises the cell-cycle dependent nuclear antigen Ki-67) specifically labelled proliferating human cells within the xenografts and did not cross-react with dividing murine cells. This was confirmed using ultrastructural in situ hybridisation with human- and mouse-specific DNA probes to identify the genetic origin of proliferating cells. In general, we found a good tissue correlation between MIB-1 and [3H]thymidine labelling, the only exception being an apparent dysregulation of Ki-67 antigen expression in regenerating xenograft epithelium. In developed xenograft intestine, the highest levels of proliferation were consistently recorded within the crypt epithelium, where 15.7%-26.7% of cells were actively cycling and S-phase occupied approximately half of the cell cycle. The frequency distribution of proliferating epithelial cells within small and large intestinal xenograft crypts was clearly tissue-specific, showing typical patterns of cell division. Therefore, the presence of functional pluripotent epithelial stem cells and conventional spatio-temporal patterns in cellular proliferation, migration, de-cycling, lineage commitment and cytodifferentiation now makes this an attractive experimental model with which to study human intestinal crypt responses to various types of tissue manipulation, e.g. cytotoxic, radiotherapeutic, dietary, endocrine and gene-targeting therapy.

Normal colonic mucosa in hereditary non-polyposis colorectal cancer shows no generalised increase in somatic mutation

Hereditary non-polyposis colorectal cancer (HNPCC) has recently been linked to germline defects of DNA repair genes. Colorectal tumours in HNPCC frequently show DNA microsatellite instability, but it is not certain whether this mutator phenotype occurs throughout the morphologically normal colonic mucosa. We have previously used the mPAS histochemical technique in human colorectal mucosa to identify a polymorphism for O-acetyltransferase activity that shows monogenic inheritance and to show that crypt-restricted loss of O-acetyltransferase activity in heterozygotes is due to somatic mutation. We have now used this histochemical technique to measure the somatic mutation frequency in the uninvolved colon of 12 heterozygous patients with HNPCC, 15 with ileocaecal Crohn's disease and 16 with sporadic colorectal cancer (CRC). HNPCC patients showed a significant increase in mutation frequency with age (Mann-Whitney U, P = 0.02). In HNPCC patients aged < 49 years the mean stem cell mutation frequency was significantly lower than in the slightly younger group of patients with Crohn's disease (0.8 +/- 0.9 x 10(-4) vs 3.5 +/- 3.3 x 10(-4), P < 0.01), probably reflecting an increased mutation rate relating to chronic mucosal damage in Crohn's disease. Although not statistically significant, the stem cell mutation frequency was slightly less in HNPCC patients > 50 years than in sporadic CRC cases (4.9 +/- 3.4 x 10(-4) vs 5.9 +/- 3.6 x 10(-4), P > 0.5). We conclude that germline defects in HNPCC do not result in a generalised increase in liability to mutation in normal colonic mucosa but that a second, somatic, event is required. We postulate that this second event occurs in crypt stem cells at low frequency, giving rise to scattered individual crypts composed of mutation-prone cells. The cells in these crypts are then at high risk of acquiring the mutations that lead to adenomas, and to rapid progression to carcinoma.

Human colonic stem cell mutation frequency with and without irradiation

Mild periodic acid-Schiff (mPAS) staining distinguishes O-acetylated from non-O-acetylated sialoglycoproteins. In human colonic mucosa, individuals possess one of three phenotypes: uniformly mPAS-positive (non-O-acetylated), uniformly mPAS-negative (O-acetylated), and negative with infrequent scattered positive crypts. This is due to a polymorphism in a single autosomal gene (oat). Discordant crypts have not been found in children's colons, suggesting that they result from somatic mutation in heterozygous individuals. We now present evidence to confirm this based on a study of radiation-induced changes. Comparison of mPAS staining of large intestinal mucosa from patients given radiation 4 weeks before surgery for carcinoma of the rectum with matched controls receiving surgery alone showed a similar phenotype distribution, but information irradiated patients showed an increased frequency of discordant crypts (irradiated vs. non-irradiated 14.5 +/- 8.2 x 10(-4) vs. 6.1 +/- 4.2 x 10(-4)). When these were classified as wholly or partially involved by the aberrant phenotype, the increase was most marked in partially involved crypts (7.5 +/- 4.5 x 10(-4) vs. 0.3 +/- 0.5 x 10(-4), Mann-Whitney U, P < 0.005). Two patients receiving radiotherapy many years before colectomy showed a very high total discordant crypt frequency but relatively few partially affected crypts. Studies of somatic mutation in colonic or small intestinal crypts following a single dose of mutagen in mice have shown early partial crypt involvement by the mutated phenotype and later complete crypt involvement.(ABSTRACT TRUNCATED AT 250 WORDS)