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Colonic glycoproteins in monozygotic twins with inflammatory bowel disease
GASTROEh’TEROLOGY
1991;100:419-423
Colonic Glycoproteins in Monozygotic Twins With Inflammatory Bowel Disea CURT TYSK, HERMANN RIEDESEL, EVA LINDBERG, BENOIT PANZINI, DANIEL PODOLSKY, and GUNNAR JhNEROT Division of Gastroenterology, Department of Medicine, &ebro Medical Center Hospital, drebro, Sweden, and the Gastrointestinal Unit, Massachusetts General Hospital, Boston, Massachusetts
Colonic glycoprotein composition was evaluated in monozygotic twins with inflammatory bowel disease using ion-exchange chromatography. Fifty-three individuals, 12 pairs and 1 single twin with ulcerative colitis and 14 pairs with Crohn’s disease, were evaluated. Seven twin pairs were concordant for the presence of ulcerative colitis or Crohn’s disease, whereas twin siblings of 10 ulcerative colitis probands and 9 Crohn’s disease probands were not known to have inflammatory bowel disease. Content of one chromatographically defined component of colonic mucin, designated HCM species IV, was reduced in both patients with ulcerative colitis (1040+ 300 cpm/lO,OOOcpmtotal HCM) and their apparently healthy twins (1340-+540 cpm/lO,OOO cpm totalHCM) compared with control subjects (4030+ 1,000cpm/10,000cpmtotal HCM).Composition of mucin in Crohn’s disease patients and their nonaffected twins was not significantly different than in controls. These observations suggest that altered profiles of mucin glycoprotein may be present before the onset of ulcerative colitis and may be genetically defined. Conversely, it appears that alterations in glycoproteins only are not sufficient to initiate mucosal inff ammation.
he normal colonic mucosa is covered with a viscoelastic mucus gel that may play an important role in providing both lubrication and protection against the colonic microflora and noxious agents. The major constituents of mucus include largemolecular-weight mucin glycoproteins as well as water, electrolytes, immunoglobulins, and sloughed epithelial cells. Although many features of mucin glycoprotein structure remain unclear, these glycoproteins consist of apoproteins to which a large number of oligosaccharide chains of variable length and structure are attached. The enormous variability in glyco-
T
sylation may be largely responsible for the apparent heterogeneity and complexity of these substances (1). A number of observations have suggested that mucin glycoproteins may be altered in association with ulcerative colitis (UC). Both quantitative and qualitative alterations in mucin glycoproteins have been described in UC. These include the presence of mannose-containing glycoproteins (2,3), reduced content of sulphomucins, and increased content of sialomucins in active disease (4), as well as reduced acetylation of sialic acid in C7C8 position (5) and abnormal lectin binding (6,7). Interestingly, in vitro mucus production was also quantitatively reduced in inactive UC but normal in active disease compared with both patients with Crohn’s disease (CD) and healthy controls (8). Earlier studies from this laboratory have described the apparent presence of six designated species (I-VI) of chromatographically distinguishable glycoprotein populations in normal colonic mucosa (9). In UC, a reduction in species IV was found during both active and inactive disease, which was also present both in inflamed and in uninvolved parts of the colon. Patients with CD, infectious colitis, ischemic colitis, or radiation colitis had a normal glycoprotein composition, indicating that the finding was not a nonspecific reflection of inflammatory activity (10). A recent study using a monoclonal antibody technique confirmed these observations (11). However, despite these observations, it remains unclear whether this alteration precedes onset of inflammation or reflects a subtle biochemical residue of past activity. This study evaluates the relationship of colonic glycoprotein profiles to onset of disease in
Abbreviations used in this paper: CD, Crohn’s disease; CRP, C-reactive protein; DZ, dizygotic; MZ, monozygotic. o 1991 by the American Gastroenterological Association 0016-5095/91/$3.00
GASTROENTEROLOGYVol. 100, No. 2
420 TYSK ET AL.
UC by analyzing colonic glycoproteins in monozygotic twins with inflammatory bowel disease (IBD).
Patients and Methods By matching the Swedish twin registry at the Department of Environmental Hygiene, Karolinska Institute, Stockholm, Sweden, with the central diagnosis register of hospital inpatients at the National Board of Health and Welfare, Stockholm, Sweden, a population of monozygotic (MZ) or dizygotic (DZ) twins of the same sex has been identified and described earlier (12). In that study, 34 MZ pairs with IBD were found. Those who were younger than 75 years with both twins in each pair still alive were invited to participate in a clinical investigation. Two MZ pairs recently diagnosed with IBD were also invited. Fifty-three of 66 total individuals agreed to participate. They constituted 14 pairs with CD and 12 pairs with UC and a single twin with UC, whose healthy twin brother did not want to attend. Two sets of MZ twins were concordant for UC; neither set had surgery. In the remaining 10 pairs of UC twins, 4 patients with UC had a proctocolectomy and therefore samples could not be obtained. Thus, in summary, samples were available from 10 patients with UC and 10 nonaffected siblings. In the CD group, five pairs were concordant for the disease. Three patients had a proctocolectomy. In one set of concordant twins, severe perianal disease prevented sigmoidoscopy in both twins. The zygosity classification of the Swedish Twin registry was used, which relies on questions on childhood resemblance. A correct classification of MZ twins is obtained in 99% in comparison to serological methods (13). The controls were five healthy subjects (one male, four female) who had a colonoscopy performed for investigation of occult gastrointestinal bleeding or suspected colonic adenomas. They had no actual gastrointestinal symptoms and colonoscopy showed a normal mucosa and histopathological investigation of their rectal biopsies was normal. Rectal biopsy specimens were taken lo-13 cm from the anus. Three subjects were never smokers, and the other two had stopped smoking l-10 years earlier. One was undergoing therapy with diuretics and one was undergoing treatment with 7.5 mg prednisolone daily. Biopsy specimens from one control subject could not be analyzed while only four controls remained. From each twin, where sigmoidoscopy could be performed, and from each control subject, two rectal mucosal biopsy samples were obtained for glycoprotein analysis and two biopsy samples were obtained for routine histopathologic investigation. Biopsy specimens for glycoprotein analysis were put in normal saline and immediately placed in frozen ice. Mucin glycoprotein heterogeneity was evaluated as previously described (10). All biopsy specimens were analyzed blindly and the code was broken first when all the analyses had been performed. The study was approved by the Ethical Committee, Orebro Medical Center Hospital, Orebro, Sweden, and the Subcommittee on Human Studies of the Massachusetts General Hospital, Boston, Massachusetts. The results are presented as means f SD. Student’s t test was used when comparing means of groups and a P < 0.05 was considered statistically significant.
Results Among the 14 twins with UC, the mean age at diagnosis was 27.7 (17-45) years and the actual mean age was 49.1 (24-74) years. In patients with CD, the mean age at diagnosis was 28.5 (20-45) years and the actual mean age was 42.9 (34-63) years. All patients with UC were in clinical remission at the time of this study and had normal levels of hemoglobin, C-reactive protein (CRP), and serum orosomucoid. Furthermore, all showed an inactive rectal mucosa on both macroscopic and microscopic assessment. Six patients were treated with sulphasalazine and one healthy twin with 5 mg prednisolone every second day for chronic hepatitis. Two patients with CD had mild diarrhea and slightly increased CW and orosomucoid levels. They were treated with sulphasalazine. Two patients had perianal manifestations preventing sigmoidoscopy. The others were inactive and no other therapy was given except vitamins or loperamide. Smoking habits were similar within the discordant twin pairs with UC. In 6 of 10 pairs, both were never smokers; in 2 of 10 pairs, both were ex-smokers; and in 1 of 10 pairs, both were smokers. In the only pair with differing smoking habits, the healthy twin had never been a smoker and his diseased twin brother was an ex-smoker. A thorough interview showed no symptoms suggestive of IBD in the healthy twins. All had a normal rectal mucosa and histopathology was normal as well. The healthy twins have remained healthy in mean 21.4 (B-40) years after diagnosis in the twins with UC and 14.9 (7-31) years in the twins with CD. The results of the glycoprotein analysis are shown in Table 1 and Figure 1. A significant reduction in species IV among twins with UC compared with normal controls was found (P < 0.001). The same alteration was also seen in the healthy twins to the patients with UC, and both the diseased twins and their healthy twin partners had the same glycoprotein composition (Figure 1). Patients with UC had an increase in species V as described earlier in patients in remission (9). Their healthy twin partners also showed the same finding (Figure 1). The increase in patients with UC was statistically significant compared with both the twins with CD and the control subjects (P < 0.01). Twins with CD and their healthy twin partners had a similar glycoprotein composition as the control subjects.
Discussion Both UC and CD are genetically influenced multifactorial disorders. However, so far, no specific genetic marker has been shown to be significantly associated with IBD. Studies of the HLA-system, immunoglobulin allotypes, complement allotypes, or
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GASTROENTEROLOGY
TYSK ET AL.
and are not secondary to the inflammation (10). This study supports this hypothesis in demonstrating the altered glycoprotein profile in the absence of disease. These observations indicate that the glycoprotein configuration may be genetically defined rather than reflecting some environmental effect. In summary, these findings indicate that altered colonic mucin glycoproteins may reflect a genetically defined factor that predisposes to UC. Although a mucin glycoprotein gene has recently been partially cloned from a human colon cancer-derived cell line (19), it is unclear whether all colonic mucin glycoprotein peptide backbones derive from a single gene or a family of genes. It is of interest that extensive allelic diversity has been described at the locus of this partially cloned cDNA. Alternatively, insofar as the diminished level of HCM IV was observed in the unaffected twin, it is possible that the alteration is associated but unrelated to the disease. Thus the genetic locus responsible for this biochemical feature may be near but distinct from the gene involved in the pathogenesis of UC or IBD. The absolute value of the glycoprotein species differed from previous results. In this study, lower values of species I-III and higher values of species IV and V was found both in patients with UC, CD, and healthy controls compared with the results reported from Podolsky and Isselbacher (9). The reason is unknown, but perhaps a different diet and/or a different colonic microflora might affect the results. The reduction found in species IV and the increase in species V might indicate problems in the separation technique. However, the divergence was found only in the UC group and not among the controls or twins with CD. However, it seems unlikely that the findings resulted from methodological problems. Alternatively, it should be noted that samples were obtained from rectal mucosa in contrast to previous studies that used samples taken from mid-sigmoid colon. The relative amounts of the glycoprotein fractions may differ with a greater preponderance of the fourth fraction in rectal mucosa. Smoking has been shown to affect glycoprotein synthesis. Using glucosamine incorporation, Cope et al. found in patients with UC a quantitatively normalized glycoprotein production among smokers compared with nonsmokers who had a reduced production compared with controls (20). In our study, very few patients with UC were smokers and their results did not differ from nonsmoking twins with UC. However, smoking habits among the discordant MZ twins with UC were similar except in one pair. Thus, it seems that the combination of similar smoking habits and the same aberration in glycoproteins is not sufficient to cause UC and additional factors are needed. The pathophysiological consequence of the changed glycoprotein composition is unknown. Theories on
Vol. 100, No. 2
reduced protective properties against the colonic microflora have been put forward. The changed glycoproteins could possibly be more susceptible for bacterial enzymatic degradation, making the mucus barrier less resistant to toxins or other noxious agents with increased risk of acquiring UC (21,22). This aberration in glycoproteins could be a predisposing biochemical factor increasing susceptibility to a possibly otherwise innocuous insult. References 1. Smith
AC, Podolsky DK. Colonic mucin glycoproteins in health and disease. Clin Gastroenterol 1986;15:815-837. 2. Teague RM, Fraser D, Clamp JR. Changes in monosaccharide content of mucin glycoproteins in ulcerative colitis. Br Med J 1973;2:645-646. 3. Fraser GM, Clamp JR. Changes in human colonic mucus in ulcerative colitis. Gut 1975;16:832-833. 4. Ehsanullah M, Filipe Ml, Gazzard B. Mucin secretion in inflammatory bowel disease: correlation with disease activity and dysplasia. Gut 1982;23:485-489. 5. Culling CFA, Reid PE, Dunn WL. A histochemical comparison of the 0-acetylated sialic acids of the epithelial mucins in ulcerative colitis, Crohn’s disease and normal controls. J Clin Path01 1979;32:1272-1277. 6. Boland CR, Lance P, Levin B, Riddell RH, Kim YS. Abnormal goblet cell glycoconjugates in rectal biopsies associated with an increased risk of neoplasia in patients with ulcerative colitis: early results of a prospective study. Gut 1984;25:13641371. mucus 7. Rhodes JM, Black R, Pate1 P, Savage A. Carcinomatype glycoprotein abnormalities in ulcerative colitis and Crohn’s disease demonstrated by altered lectin binding. Gut 1985;26: 578-579. 8. Cope GF, Heatley RV, Kelleher J, Axon ATR. In vitro mucus glycoprotein production by colonic tissue from patients with ulcerative colitis. Gut 1988;29:229-234. 9. Podolsky DK, Isselbacher KJ. Composition of human colonic mucin: selective alteration in inflammatory bowel disease. J Clin Invest 1983;72:142-153. 10. Podolsky DK, Isselbacher KJ. Glycoprotein composition of colonic mucosa. Specific alterations in ulcerative colitis. Gastroenterology 1984;87:991-998. 11. Podolsky DK, Fournier DA. Alterations in mucosal content of colonic glycoconjugates in inflammatory bowel disease defined by monoclonal antibodies. Gastroenterology 1988;95:379-387, 12. Tysk C, Lindberg E, Jarnerot G, Floderus-Myrhed B. Ulcerative colitis and Crohns disease in an unselected population of monozygotic and dizygotic twins. A study of heritability and the influence of smoking. Gut 1988;29:990-996. 13. Cederlof R, Friberg L, Jonsson E, Kaij L. Studies on similarity diagnosis in twins with the aid of mailed questionnaires. Acta Genet 1961;11:338-362. 14. Pena AS, Weterman IT, Lamers CBHW. Predisposing markers and regulating genes in inflammatory bowel disease. In: Jarnerot G, ed. Inflammatory bowel disease. New York: Raven, 1987:919. 15. Hollander D, Vadheim CM, Brettholz E, Petersen GM, Delahunty T, Rotter JI. Increased intestinal permeability in patients with Crohn’s disease and their relatives. Ann Intern Med 1986;105: 883-885. D, Vadheim CM, et al, Intestinal permeabil16. Katz KD, Hollander ity in patients with Crohn’s disease and their healthy relatives. Gastroenterology 1989;97:927-931.
February 1991
17. Evans JG, Acheson ED. An epidemiological
study of ulcerative colitis and regional enteritis in the Oxford area. Gut 1965;6:311-
324. 18. Garland
19.
20. 21.
22.
CF, Lilienfeld AM, Mendeloff AI, Markowitz JA, Terre11 KB, Garland FC. Incidence rates of ulcerative colitis and Crohn’s disease in fifteen areas of the United States. Gastroenterology 1981;81:1115-1124. Gum JR, Byrd JC, Hicks JN, Toribarn NN, Lampsrt DTA, Kina YS. Molecular cloning of human intestinal mucin cDNAs. J Biol Chem 1989;264:6489-6487. Cope GF, Heatley RV, Kelleher J. Smoking and colonic mucus in ulcerative colitis. Br Med J 1986;293:481. Rhodes JM, Gallimore R, Elias E, Allan RN, Kennedy JF. Faecal mucus degrading glycosidases in ulcerative colitis and Crohn’s disease. Gut 1985;26:761-765. Rhodes JM, Black RR, Gallimore R, Savage A. Histochemical
COLONIC GLYCOPROTEINS IN MZ TWINS WITH IBD
423
demonstration of desialation and desulphation of normal and inflammatory bowel disease rectal mucus by faecal extracts. Gut 1985:26:1312-1318.
Received January 29,199O. Accepted July 23,199O. Address requests for reprints to: Daniel Podolsky, M.D., Gastrointestinal Unit, Massachusetts General Hospital, Boston, Massachusetts 02114. This study was financially supported by the brebro County Research committee and a National Institutes of Health grant (DK34422). Curt Tysk and Eva Lindberg were financially supported for IBD studies by Pharmacia, Uppsala, Sweden. The authors gratefully acknowledge Wolfgang Kraaz at the Department of Pathology, &ebro, who made the histopathologic investigation.
1991;100:419-423
Colonic Glycoproteins in Monozygotic Twins With Inflammatory Bowel Disea CURT TYSK, HERMANN RIEDESEL, EVA LINDBERG, BENOIT PANZINI, DANIEL PODOLSKY, and GUNNAR JhNEROT Division of Gastroenterology, Department of Medicine, &ebro Medical Center Hospital, drebro, Sweden, and the Gastrointestinal Unit, Massachusetts General Hospital, Boston, Massachusetts
Colonic glycoprotein composition was evaluated in monozygotic twins with inflammatory bowel disease using ion-exchange chromatography. Fifty-three individuals, 12 pairs and 1 single twin with ulcerative colitis and 14 pairs with Crohn’s disease, were evaluated. Seven twin pairs were concordant for the presence of ulcerative colitis or Crohn’s disease, whereas twin siblings of 10 ulcerative colitis probands and 9 Crohn’s disease probands were not known to have inflammatory bowel disease. Content of one chromatographically defined component of colonic mucin, designated HCM species IV, was reduced in both patients with ulcerative colitis (1040+ 300 cpm/lO,OOOcpmtotal HCM) and their apparently healthy twins (1340-+540 cpm/lO,OOO cpm totalHCM) compared with control subjects (4030+ 1,000cpm/10,000cpmtotal HCM).Composition of mucin in Crohn’s disease patients and their nonaffected twins was not significantly different than in controls. These observations suggest that altered profiles of mucin glycoprotein may be present before the onset of ulcerative colitis and may be genetically defined. Conversely, it appears that alterations in glycoproteins only are not sufficient to initiate mucosal inff ammation.
he normal colonic mucosa is covered with a viscoelastic mucus gel that may play an important role in providing both lubrication and protection against the colonic microflora and noxious agents. The major constituents of mucus include largemolecular-weight mucin glycoproteins as well as water, electrolytes, immunoglobulins, and sloughed epithelial cells. Although many features of mucin glycoprotein structure remain unclear, these glycoproteins consist of apoproteins to which a large number of oligosaccharide chains of variable length and structure are attached. The enormous variability in glyco-
T
sylation may be largely responsible for the apparent heterogeneity and complexity of these substances (1). A number of observations have suggested that mucin glycoproteins may be altered in association with ulcerative colitis (UC). Both quantitative and qualitative alterations in mucin glycoproteins have been described in UC. These include the presence of mannose-containing glycoproteins (2,3), reduced content of sulphomucins, and increased content of sialomucins in active disease (4), as well as reduced acetylation of sialic acid in C7C8 position (5) and abnormal lectin binding (6,7). Interestingly, in vitro mucus production was also quantitatively reduced in inactive UC but normal in active disease compared with both patients with Crohn’s disease (CD) and healthy controls (8). Earlier studies from this laboratory have described the apparent presence of six designated species (I-VI) of chromatographically distinguishable glycoprotein populations in normal colonic mucosa (9). In UC, a reduction in species IV was found during both active and inactive disease, which was also present both in inflamed and in uninvolved parts of the colon. Patients with CD, infectious colitis, ischemic colitis, or radiation colitis had a normal glycoprotein composition, indicating that the finding was not a nonspecific reflection of inflammatory activity (10). A recent study using a monoclonal antibody technique confirmed these observations (11). However, despite these observations, it remains unclear whether this alteration precedes onset of inflammation or reflects a subtle biochemical residue of past activity. This study evaluates the relationship of colonic glycoprotein profiles to onset of disease in
Abbreviations used in this paper: CD, Crohn’s disease; CRP, C-reactive protein; DZ, dizygotic; MZ, monozygotic. o 1991 by the American Gastroenterological Association 0016-5095/91/$3.00
GASTROENTEROLOGYVol. 100, No. 2
420 TYSK ET AL.
UC by analyzing colonic glycoproteins in monozygotic twins with inflammatory bowel disease (IBD).
Patients and Methods By matching the Swedish twin registry at the Department of Environmental Hygiene, Karolinska Institute, Stockholm, Sweden, with the central diagnosis register of hospital inpatients at the National Board of Health and Welfare, Stockholm, Sweden, a population of monozygotic (MZ) or dizygotic (DZ) twins of the same sex has been identified and described earlier (12). In that study, 34 MZ pairs with IBD were found. Those who were younger than 75 years with both twins in each pair still alive were invited to participate in a clinical investigation. Two MZ pairs recently diagnosed with IBD were also invited. Fifty-three of 66 total individuals agreed to participate. They constituted 14 pairs with CD and 12 pairs with UC and a single twin with UC, whose healthy twin brother did not want to attend. Two sets of MZ twins were concordant for UC; neither set had surgery. In the remaining 10 pairs of UC twins, 4 patients with UC had a proctocolectomy and therefore samples could not be obtained. Thus, in summary, samples were available from 10 patients with UC and 10 nonaffected siblings. In the CD group, five pairs were concordant for the disease. Three patients had a proctocolectomy. In one set of concordant twins, severe perianal disease prevented sigmoidoscopy in both twins. The zygosity classification of the Swedish Twin registry was used, which relies on questions on childhood resemblance. A correct classification of MZ twins is obtained in 99% in comparison to serological methods (13). The controls were five healthy subjects (one male, four female) who had a colonoscopy performed for investigation of occult gastrointestinal bleeding or suspected colonic adenomas. They had no actual gastrointestinal symptoms and colonoscopy showed a normal mucosa and histopathological investigation of their rectal biopsies was normal. Rectal biopsy specimens were taken lo-13 cm from the anus. Three subjects were never smokers, and the other two had stopped smoking l-10 years earlier. One was undergoing therapy with diuretics and one was undergoing treatment with 7.5 mg prednisolone daily. Biopsy specimens from one control subject could not be analyzed while only four controls remained. From each twin, where sigmoidoscopy could be performed, and from each control subject, two rectal mucosal biopsy samples were obtained for glycoprotein analysis and two biopsy samples were obtained for routine histopathologic investigation. Biopsy specimens for glycoprotein analysis were put in normal saline and immediately placed in frozen ice. Mucin glycoprotein heterogeneity was evaluated as previously described (10). All biopsy specimens were analyzed blindly and the code was broken first when all the analyses had been performed. The study was approved by the Ethical Committee, Orebro Medical Center Hospital, Orebro, Sweden, and the Subcommittee on Human Studies of the Massachusetts General Hospital, Boston, Massachusetts. The results are presented as means f SD. Student’s t test was used when comparing means of groups and a P < 0.05 was considered statistically significant.
Results Among the 14 twins with UC, the mean age at diagnosis was 27.7 (17-45) years and the actual mean age was 49.1 (24-74) years. In patients with CD, the mean age at diagnosis was 28.5 (20-45) years and the actual mean age was 42.9 (34-63) years. All patients with UC were in clinical remission at the time of this study and had normal levels of hemoglobin, C-reactive protein (CRP), and serum orosomucoid. Furthermore, all showed an inactive rectal mucosa on both macroscopic and microscopic assessment. Six patients were treated with sulphasalazine and one healthy twin with 5 mg prednisolone every second day for chronic hepatitis. Two patients with CD had mild diarrhea and slightly increased CW and orosomucoid levels. They were treated with sulphasalazine. Two patients had perianal manifestations preventing sigmoidoscopy. The others were inactive and no other therapy was given except vitamins or loperamide. Smoking habits were similar within the discordant twin pairs with UC. In 6 of 10 pairs, both were never smokers; in 2 of 10 pairs, both were ex-smokers; and in 1 of 10 pairs, both were smokers. In the only pair with differing smoking habits, the healthy twin had never been a smoker and his diseased twin brother was an ex-smoker. A thorough interview showed no symptoms suggestive of IBD in the healthy twins. All had a normal rectal mucosa and histopathology was normal as well. The healthy twins have remained healthy in mean 21.4 (B-40) years after diagnosis in the twins with UC and 14.9 (7-31) years in the twins with CD. The results of the glycoprotein analysis are shown in Table 1 and Figure 1. A significant reduction in species IV among twins with UC compared with normal controls was found (P < 0.001). The same alteration was also seen in the healthy twins to the patients with UC, and both the diseased twins and their healthy twin partners had the same glycoprotein composition (Figure 1). Patients with UC had an increase in species V as described earlier in patients in remission (9). Their healthy twin partners also showed the same finding (Figure 1). The increase in patients with UC was statistically significant compared with both the twins with CD and the control subjects (P < 0.01). Twins with CD and their healthy twin partners had a similar glycoprotein composition as the control subjects.
Discussion Both UC and CD are genetically influenced multifactorial disorders. However, so far, no specific genetic marker has been shown to be significantly associated with IBD. Studies of the HLA-system, immunoglobulin allotypes, complement allotypes, or
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GASTROENTEROLOGY
TYSK ET AL.
and are not secondary to the inflammation (10). This study supports this hypothesis in demonstrating the altered glycoprotein profile in the absence of disease. These observations indicate that the glycoprotein configuration may be genetically defined rather than reflecting some environmental effect. In summary, these findings indicate that altered colonic mucin glycoproteins may reflect a genetically defined factor that predisposes to UC. Although a mucin glycoprotein gene has recently been partially cloned from a human colon cancer-derived cell line (19), it is unclear whether all colonic mucin glycoprotein peptide backbones derive from a single gene or a family of genes. It is of interest that extensive allelic diversity has been described at the locus of this partially cloned cDNA. Alternatively, insofar as the diminished level of HCM IV was observed in the unaffected twin, it is possible that the alteration is associated but unrelated to the disease. Thus the genetic locus responsible for this biochemical feature may be near but distinct from the gene involved in the pathogenesis of UC or IBD. The absolute value of the glycoprotein species differed from previous results. In this study, lower values of species I-III and higher values of species IV and V was found both in patients with UC, CD, and healthy controls compared with the results reported from Podolsky and Isselbacher (9). The reason is unknown, but perhaps a different diet and/or a different colonic microflora might affect the results. The reduction found in species IV and the increase in species V might indicate problems in the separation technique. However, the divergence was found only in the UC group and not among the controls or twins with CD. However, it seems unlikely that the findings resulted from methodological problems. Alternatively, it should be noted that samples were obtained from rectal mucosa in contrast to previous studies that used samples taken from mid-sigmoid colon. The relative amounts of the glycoprotein fractions may differ with a greater preponderance of the fourth fraction in rectal mucosa. Smoking has been shown to affect glycoprotein synthesis. Using glucosamine incorporation, Cope et al. found in patients with UC a quantitatively normalized glycoprotein production among smokers compared with nonsmokers who had a reduced production compared with controls (20). In our study, very few patients with UC were smokers and their results did not differ from nonsmoking twins with UC. However, smoking habits among the discordant MZ twins with UC were similar except in one pair. Thus, it seems that the combination of similar smoking habits and the same aberration in glycoproteins is not sufficient to cause UC and additional factors are needed. The pathophysiological consequence of the changed glycoprotein composition is unknown. Theories on
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reduced protective properties against the colonic microflora have been put forward. The changed glycoproteins could possibly be more susceptible for bacterial enzymatic degradation, making the mucus barrier less resistant to toxins or other noxious agents with increased risk of acquiring UC (21,22). This aberration in glycoproteins could be a predisposing biochemical factor increasing susceptibility to a possibly otherwise innocuous insult. References 1. Smith
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Received January 29,199O. Accepted July 23,199O. Address requests for reprints to: Daniel Podolsky, M.D., Gastrointestinal Unit, Massachusetts General Hospital, Boston, Massachusetts 02114. This study was financially supported by the brebro County Research committee and a National Institutes of Health grant (DK34422). Curt Tysk and Eva Lindberg were financially supported for IBD studies by Pharmacia, Uppsala, Sweden. The authors gratefully acknowledge Wolfgang Kraaz at the Department of Pathology, &ebro, who made the histopathologic investigation.