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Familial occurrence and inheritance studies in inflammatory bowel disease
The Netherlands
JOURNAL OF MEDICINE ELSEVIER
Netherlands Journal of Medicine 48 (1996) 53-56
Familial occurrence and inheritance studies in inflammatory bowel disease V. Binder *, M. Orholm Herlev Hospital, University of Copenhagen, DK 2730 Herlev, Denmark
Abstract
A number of studies have demonstrated aggregation of cases of ulcerative colitis or Crohn's disease in families, and of cases of both diseases within the same families, suggesting that patients share a genetic background. Perhaps because of differences in the selection of patients, study design and diagnostic criteria, different patterns of occurrence of inflammatory bowel disease (IBD) have been found among relatives of patients with these disorders. In recent years, however, several studies have been carried out, aiming by epidemiological methods to reveal (1) the frequency of familial occurrence of IBD among patients with ulcerative colitis and Crohn's disease, and (2) the prevalence of IBD among 10 relatives to patients with these diseases. Results from these studies show a relatively uniform pattern of family occurrence in about 10% of patients with ulcerative colitis and Crohn's disease, and a prevalence among 1° relatives of about 10 times that of the background population. A twin study reported a significantly higher concordance rate for Crohn's disease than for ulcerative colitis in monozygotic twins. By use of complex segregation analyses in 3 different studies, a very similar model of inheritance was found to fit for ulcerative colitis, namely a major dominant or additive gene with a low penetrance. For Crohn's disease the best-fitting mode] was a major recessive gene, with a high penetrance. This difference strongly supports the concept of ulcerative colitis and Crohn's disease as two separate disease entities. The occurrence of both diseases within the same families in certain members of the affected families is difficult to explain. The search for distinct associations of HLA genes with inflammatory bowel disease has shown a positive correlation between DR2 and ulcerative colitis and a negative association with DR4 and DRw6, compared with ethnically matched controls. In contrast, in Crohn's disease a positive association with the combination of DR1 and DQw5 alleles was revealed, thus indicating genetically different disease susceptibility for the two disorders. In general, however, no consistent pattern has been revealed from studies of association of HLA-A or -B antigens or blood group and serum protein markers. In two French families with several members affected with Crohn's disease no evidence for an HLA haplotype association could be revealed. Possible inherited markers of ulcerative colitis or Crohn's disease have been sought but without convincing success. Increased intestinal permeability, presence of anticolon antibodies and presence of antineutrophil leukocyte antibodies have been proposed, but not proved. Thorough studies are now needed of multimember families with disease for linkage studies to identify loci which contribute to increased liability. Such studies are in progress in different centres. Keywords: Inflammatory bowel disease; Familial occurrence; Genetics; Epidemiology
* Corresponding author. 0300-2977/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0300-2977(95)00093-3
54
V. Binder, M. Orholm / Netherlands Journal of Medicine 48 (1996) 53-56
1. Introduction Does inflammatory bowel disease (IBD) occur more frequently in families of patients with IBD than in the population in general and, if so, how often? Is the frequency of familial occurrence similar in ulcerative colitis and Crohn's disease? How is the prevalence of IBD in siblings, parents and offsprings of patients with ulcerative colitis and Crohn's disease? And what about twins? Do patients with familial occurrence of IBD differ in other aspects from patients without? What are the current inheritance theories? These questions have formed the background for research in recent years in an attempt to reveal the genetic background of ulcerative colitis and Crohn's disease. The first description of Crohn's disease by Dr Crohn himself mentioned in fact a familial occurrence in two siblings, and it was stated that the concurrence could be purely accidental or could express a congenital predisposition or could point to a transmissible causative agent--exactly the possibilities we have to deal with [1]. In a large study of 838 patients with onset of IBD before the age of 20, Farmer and co-workers found a family occurrence as high as 35%; 15.8% and 16.6% of the patients with ulcerative colitis and Crohn's disease had at least one first-degree relative with the disease, and of the possible relationships sibling-sibling was the most frequent [2]. A recent publication from Northern France described two families, one where husband and wife plus all four children had Crohn's disease, and one large family with 11 children of whom 7 had Crohn's disease. In the Netherlands, Weterman and Pena from Leiden reported that among their 400 patients with Crohn's disease 32 (8%) had a familial occurrence of the disease, and also here the most frequent relationship was among siblings [3].
2. Epidemiological background Studies of familial occurrence carried out according to epidemiological methods have the advantage of avoiding bias in selection of patients,
which could possibly mean selection of patients with an extraordinarily high genetic predisposition. In Denmark we performed such a study a few years ago, where 504 patients with ulcerative colitis and 133 patients with Crohn's disease were thoroughly interviewed about their family relations so that pedigrees could be drawn up [4]. We chose patients with at least 9 years duration of disease; the median age was 52 years for ulcerative colitis and 47 years for Crohn's disease. Fifty-nine patients or 11.7% of the patients with ulcerative colitis were found to have at least one relative with IBD, and 41 (8.1%) had at least one first-degree relative with IBD. In patients with Crohn's disease the similar figures were 8.2 and 5.2%. Among all 637 patients with IBD, 11% had a familial occurrence and 7.5% had first-degree relatives with one of the diseases. Against the background of the full pedigrees for all patients, the prevalence of ulcerative colitis and Crohn's disease among first- and second-degree relatives could be calculated. At the same time from our other epidemiological study projects we had the prevalence of ulcerative colitis and Crohn's disease in the background population, which was 161 and 55 per 100000 inhabitants respectively for the two diseases. A relative risk of ulcerative colitis of 9.5 was found for first-degree relatives to a patient with ulcerative colitis, and a relative risk of 1.8 for Crohn's disease. First-degree relatives of a patient with Crohn's disease had a prevalence 10.3 times higher than expected for Crohn's disease, and furthermore a relative risk of ulcerative colitis of 4.4. For second-degree relatives the prevalence found was far less, but still significant for Crohn's disease. The age- and sex-standardized risks of IBD among relatives confirm the above-mentioned relative risks. The age- and sex-standardized risks were calculated for parents, siblings and children separately in ulcerative colitis, but no significant differences were found. Similarly, no difference was found for different sex-combinations in family relations. The age of the proband at diagnosis was found to have an influence, in that relatives of patients below 50 years at age at diagnosis had a higher population relative risk than relatives of patients
lZ Binder, M. Orholrn /Netherlands Journal of Medicine 48 (1996) 53-56
with a higher age at diagnosis. There was however no general correlation between age at diagnosis and familial occurrence. In Crohn's disease, the number of affected relatives was not large enough for statistical calculations, but siblings were affected more often than children and parents. Although two other reported studies from England and Sweden did not include simultaneous prevalence estimations in the background populations, very similar figures were obtained, both for frequency of familial occurrence and for prevalence of IBD among close relatives [5,6]. In all 3 studies the two diseases occurred within the same families. In our study cross-occurrence of the two diseases was found in 16% of the families. A very important twin study has been carried out in Sweden by Tysk et al., who compared the Swedish twin registry containing 25 000 twin pairs of same sex with the national diagnosis registry of hospital inpatients with IBD [7]. They found 36 twin pairs with ulcerative colitis in at least one twin, and 44 twin pairs with Crohn's disease in at least one. The concordance rate for Crohn's disease in the monozygotic twins was as high as 58%, compared to 3.9% in dizygotic twin pairs, whereas the concordance rate for ulcerative colitis was only 6.3% in monozygotic twins. These data strongly point to a different genetic background for the two diseases. Since a population relative risk of about 10 in close relatives strongly points to a genetic component in these disorders, at least in some of the patients, the next step was to analyze in depth the family relationships found. A so-called complex segregation analysis based on the mixed model has been used on 3 different patient groups with Crohn's disease, and on 2 patient groups with ulcerative colitis [8-11]. Segregation analysis involves the examination of the pattern of occurrence of diseases within families. It is only of interest when an excess of disease within families has already been shown. In looking for patterns of aggregation, we try to understand the type of aggregation and the possible explanation of that aggregation. As the observed patterns can be the result of numerous different mechanisms, we are
55
looking for statistical evidence preferring one model to another. Hypotheses concerning the mode of inheritance of a disease are then performed by fitting probability models to family data. The segregation analysis used is based on the so-called mixed model, which incorporates both Mendelian inheritance of a single major gene locus, non-Mendelian polygenic inheritance as well as transmissible and non-transmissible environmental factors. The complex segregation analysis showed in Crohn's disease that a major recessive gene, with a gene frequency in the population of about 1% and a high penetrance, between 50 and 100%, was the best-fitting model. In our study, the model did not reach the level of significance, however [10]. In ulcerative colitis, the best-fitting model was a major dominant or additive gene, with a low gene frequency of less than 1%, and a low penetrance of about 25%.
3. Genetic markers
It is obvious that a genetic marker or genetic markers would be very important in understanding the underlying disease processes in IBD. H L A studies have hitherto not been very successful, but recently it seems that a positive association with the HLA-DR2 allele does exist in ulcerative colitis, and a negative association with DR4 and DRw6, when compared with ethnically matched controls. In contrast, in Crohn's disease it was found that a combination of DR1 and DQw5 was associated with the disease, thus indicating a quite different genetic background for the two diseases as regards the HLA system [12]. Other studies, however, have not been able to confirm this; thus no H L A correlation could be revealed in the French study of two multimember affected families [13]. Anti-neutrophil cytoplasmic antibodies (ANCA) have recently been in focus as a possible genetic marker in ulcerative colitis. About 60% of ulcerative colitis patients have these antibodies, whereas they are only sporadically present in Crohn's disease. Shanahan et al. reported that an
56
1A Binder, M. Orholm /Netherlands Journal of Medicine 48 (1996) 53-56
increased prevalence of pANCA is found not only in patients with ulcerative colitis but also among healthy relatives of these patients [14]. The preponderance of HLA-DR2 was found exclusively among the ANCA-positive patients, and the Californian group of scientists proposed a heterogeneity in ulcerative colitis patients with this background [15]. Other centres have however not been able to confirm these findings [16]. Other possible genetic markers have been looked for such as immunoglobulin subclasses in mucosal biopsy specimens, where it seems that healthy twins to ulcerative colitis patients have a similar distribution, whereas no such finding was seen in twins to patients affected with Crohn's disease [17].
4. Conclusion
About 10% of both ulcerative colitis and Crohn's disease patients have a genetic background, and these genetic factors seem to be different in ulcerative colitis and Crohn's disease. No genetic factors have been depicted with certainty until now, and future linkage studies are needed for further elucidation of gene localization. Environmental factors, however, may be involved, especially in Crohn's disease where the increasing incidence cannot be explained by genetics.
References [1] Crohn BB. The broadening conception of regional ileitis. Am J Dig Dis 1934;1:979. [2] Farmer RG, Michener WM, Mortimer EA. Studies of family history among patients with inflammatory bowel disease. Clin Gastroenterol 1980;9:271-278. [3] Weterman IT, Pena AS. Familial incidence of Crohn's disease in the Netherlands and a review of the literature. Gastroenterology 1984;86:449-452. [4] Orholm M, Munkholm P, Langholz E, Nielsen OH, Sore-
nsen TIA, Binder V. Familial occurrence of inflammatory bowel disease. N Engl J Med 1991;324:84-88. [5] Mayberry JF, Rhodes J. Familial prevalence of inflammatory bowel disease in relatives of patients with Crohn's disease. Br Med J 1980;280:284. [6] Mons~n U, Bernell O, Johansson C, Hellers G. Prevalence of inflammatory bowel disease among relatives of patients with Crohn's disease. Scand J Gastroenterol 1991;26:302-306. [7] Tysk C, Lindberg E, J~irnerot G, Flod~rus-Myrhed B. Ulcerative colitis and Crohn's disease in an unselected population of monozygotic and dizygotic twins. A study of heritability and the influence of smoking. Gut 1988;29:990-996. [8] Kfister W, Pascoe L, Purrmann J, Funk S, Majewski F. The genetics of Crohn disease: complex segregation analysis of a family study with 265 patients with Crohn disease and 5,387 relatives. Am J Med Genet 1989;32: 105-108. [9] Mons~n U. Inflammatory bowel disease. An epidemiological and genetic study. Acta Chir Scand 1990;suppl. 559:1-42. [10] Orholm M, Iselius L, Sorensen TIA, Munkholm P, Langholz E, Binder V. Investigation of inheritance of chronic inflammatory bowel diseases by complex segregation analysis. Br Med J 1993;306:20-24. [11] Mons6n U, lselius L, Johansson C, Hellers G. Evidence for a major additive gene in ulcerative colitis. Clin Genet 1989;36:411-414. [12] Toyoda H, Wang S-J, Yang H-Y, et al. Distinct associations of HLA class II genes with inflammatory bowel disease. Gastroenterolgoy 1993;104:741-748. [13] Van Kruiningen HJ, Colombel JF, Cartun RW, et al. An in-depth study of Crohn's disease in two French families. Gastroenterology 1993;104:351-360. [14] Shanahan F, Duerr RH, Rotter JI, et al. Neutrophil autoantibodies in ulcerative colitis: familial aggregation and genetic heterogeneity. Gastroenterology 1992;103: 456-461, [15] Yang H, Rotter JI, Toyoda H, et al. Ulcerative colitis: a genetically heterogeneous disorder defined by genetic (HLA class II) and subclinical (antineutrophil cytoplasmic antibodies)markers. J Clin Invest 1993;92:1080-1084. [16] Reumaux D, Delecourt L, Colombel JF, Noel LH, Duthilleul P, Cortot A. Anti-neutrophil cytoplasmic autoantibodies in relatives of patients with ulcerative colitis. Gastroenterology 1992;103:1706. [17] Helgeland L, Tysk C, J~irnerot G, et al. IgG subclass distribution in serum and rectal mucosa of monozygotic twins with or without inflammatory bowel disease. Gut 1992;33:1358-1364.
JOURNAL OF MEDICINE ELSEVIER
Netherlands Journal of Medicine 48 (1996) 53-56
Familial occurrence and inheritance studies in inflammatory bowel disease V. Binder *, M. Orholm Herlev Hospital, University of Copenhagen, DK 2730 Herlev, Denmark
Abstract
A number of studies have demonstrated aggregation of cases of ulcerative colitis or Crohn's disease in families, and of cases of both diseases within the same families, suggesting that patients share a genetic background. Perhaps because of differences in the selection of patients, study design and diagnostic criteria, different patterns of occurrence of inflammatory bowel disease (IBD) have been found among relatives of patients with these disorders. In recent years, however, several studies have been carried out, aiming by epidemiological methods to reveal (1) the frequency of familial occurrence of IBD among patients with ulcerative colitis and Crohn's disease, and (2) the prevalence of IBD among 10 relatives to patients with these diseases. Results from these studies show a relatively uniform pattern of family occurrence in about 10% of patients with ulcerative colitis and Crohn's disease, and a prevalence among 1° relatives of about 10 times that of the background population. A twin study reported a significantly higher concordance rate for Crohn's disease than for ulcerative colitis in monozygotic twins. By use of complex segregation analyses in 3 different studies, a very similar model of inheritance was found to fit for ulcerative colitis, namely a major dominant or additive gene with a low penetrance. For Crohn's disease the best-fitting mode] was a major recessive gene, with a high penetrance. This difference strongly supports the concept of ulcerative colitis and Crohn's disease as two separate disease entities. The occurrence of both diseases within the same families in certain members of the affected families is difficult to explain. The search for distinct associations of HLA genes with inflammatory bowel disease has shown a positive correlation between DR2 and ulcerative colitis and a negative association with DR4 and DRw6, compared with ethnically matched controls. In contrast, in Crohn's disease a positive association with the combination of DR1 and DQw5 alleles was revealed, thus indicating genetically different disease susceptibility for the two disorders. In general, however, no consistent pattern has been revealed from studies of association of HLA-A or -B antigens or blood group and serum protein markers. In two French families with several members affected with Crohn's disease no evidence for an HLA haplotype association could be revealed. Possible inherited markers of ulcerative colitis or Crohn's disease have been sought but without convincing success. Increased intestinal permeability, presence of anticolon antibodies and presence of antineutrophil leukocyte antibodies have been proposed, but not proved. Thorough studies are now needed of multimember families with disease for linkage studies to identify loci which contribute to increased liability. Such studies are in progress in different centres. Keywords: Inflammatory bowel disease; Familial occurrence; Genetics; Epidemiology
* Corresponding author. 0300-2977/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0300-2977(95)00093-3
54
V. Binder, M. Orholm / Netherlands Journal of Medicine 48 (1996) 53-56
1. Introduction Does inflammatory bowel disease (IBD) occur more frequently in families of patients with IBD than in the population in general and, if so, how often? Is the frequency of familial occurrence similar in ulcerative colitis and Crohn's disease? How is the prevalence of IBD in siblings, parents and offsprings of patients with ulcerative colitis and Crohn's disease? And what about twins? Do patients with familial occurrence of IBD differ in other aspects from patients without? What are the current inheritance theories? These questions have formed the background for research in recent years in an attempt to reveal the genetic background of ulcerative colitis and Crohn's disease. The first description of Crohn's disease by Dr Crohn himself mentioned in fact a familial occurrence in two siblings, and it was stated that the concurrence could be purely accidental or could express a congenital predisposition or could point to a transmissible causative agent--exactly the possibilities we have to deal with [1]. In a large study of 838 patients with onset of IBD before the age of 20, Farmer and co-workers found a family occurrence as high as 35%; 15.8% and 16.6% of the patients with ulcerative colitis and Crohn's disease had at least one first-degree relative with the disease, and of the possible relationships sibling-sibling was the most frequent [2]. A recent publication from Northern France described two families, one where husband and wife plus all four children had Crohn's disease, and one large family with 11 children of whom 7 had Crohn's disease. In the Netherlands, Weterman and Pena from Leiden reported that among their 400 patients with Crohn's disease 32 (8%) had a familial occurrence of the disease, and also here the most frequent relationship was among siblings [3].
2. Epidemiological background Studies of familial occurrence carried out according to epidemiological methods have the advantage of avoiding bias in selection of patients,
which could possibly mean selection of patients with an extraordinarily high genetic predisposition. In Denmark we performed such a study a few years ago, where 504 patients with ulcerative colitis and 133 patients with Crohn's disease were thoroughly interviewed about their family relations so that pedigrees could be drawn up [4]. We chose patients with at least 9 years duration of disease; the median age was 52 years for ulcerative colitis and 47 years for Crohn's disease. Fifty-nine patients or 11.7% of the patients with ulcerative colitis were found to have at least one relative with IBD, and 41 (8.1%) had at least one first-degree relative with IBD. In patients with Crohn's disease the similar figures were 8.2 and 5.2%. Among all 637 patients with IBD, 11% had a familial occurrence and 7.5% had first-degree relatives with one of the diseases. Against the background of the full pedigrees for all patients, the prevalence of ulcerative colitis and Crohn's disease among first- and second-degree relatives could be calculated. At the same time from our other epidemiological study projects we had the prevalence of ulcerative colitis and Crohn's disease in the background population, which was 161 and 55 per 100000 inhabitants respectively for the two diseases. A relative risk of ulcerative colitis of 9.5 was found for first-degree relatives to a patient with ulcerative colitis, and a relative risk of 1.8 for Crohn's disease. First-degree relatives of a patient with Crohn's disease had a prevalence 10.3 times higher than expected for Crohn's disease, and furthermore a relative risk of ulcerative colitis of 4.4. For second-degree relatives the prevalence found was far less, but still significant for Crohn's disease. The age- and sex-standardized risks of IBD among relatives confirm the above-mentioned relative risks. The age- and sex-standardized risks were calculated for parents, siblings and children separately in ulcerative colitis, but no significant differences were found. Similarly, no difference was found for different sex-combinations in family relations. The age of the proband at diagnosis was found to have an influence, in that relatives of patients below 50 years at age at diagnosis had a higher population relative risk than relatives of patients
lZ Binder, M. Orholrn /Netherlands Journal of Medicine 48 (1996) 53-56
with a higher age at diagnosis. There was however no general correlation between age at diagnosis and familial occurrence. In Crohn's disease, the number of affected relatives was not large enough for statistical calculations, but siblings were affected more often than children and parents. Although two other reported studies from England and Sweden did not include simultaneous prevalence estimations in the background populations, very similar figures were obtained, both for frequency of familial occurrence and for prevalence of IBD among close relatives [5,6]. In all 3 studies the two diseases occurred within the same families. In our study cross-occurrence of the two diseases was found in 16% of the families. A very important twin study has been carried out in Sweden by Tysk et al., who compared the Swedish twin registry containing 25 000 twin pairs of same sex with the national diagnosis registry of hospital inpatients with IBD [7]. They found 36 twin pairs with ulcerative colitis in at least one twin, and 44 twin pairs with Crohn's disease in at least one. The concordance rate for Crohn's disease in the monozygotic twins was as high as 58%, compared to 3.9% in dizygotic twin pairs, whereas the concordance rate for ulcerative colitis was only 6.3% in monozygotic twins. These data strongly point to a different genetic background for the two diseases. Since a population relative risk of about 10 in close relatives strongly points to a genetic component in these disorders, at least in some of the patients, the next step was to analyze in depth the family relationships found. A so-called complex segregation analysis based on the mixed model has been used on 3 different patient groups with Crohn's disease, and on 2 patient groups with ulcerative colitis [8-11]. Segregation analysis involves the examination of the pattern of occurrence of diseases within families. It is only of interest when an excess of disease within families has already been shown. In looking for patterns of aggregation, we try to understand the type of aggregation and the possible explanation of that aggregation. As the observed patterns can be the result of numerous different mechanisms, we are
55
looking for statistical evidence preferring one model to another. Hypotheses concerning the mode of inheritance of a disease are then performed by fitting probability models to family data. The segregation analysis used is based on the so-called mixed model, which incorporates both Mendelian inheritance of a single major gene locus, non-Mendelian polygenic inheritance as well as transmissible and non-transmissible environmental factors. The complex segregation analysis showed in Crohn's disease that a major recessive gene, with a gene frequency in the population of about 1% and a high penetrance, between 50 and 100%, was the best-fitting model. In our study, the model did not reach the level of significance, however [10]. In ulcerative colitis, the best-fitting model was a major dominant or additive gene, with a low gene frequency of less than 1%, and a low penetrance of about 25%.
3. Genetic markers
It is obvious that a genetic marker or genetic markers would be very important in understanding the underlying disease processes in IBD. H L A studies have hitherto not been very successful, but recently it seems that a positive association with the HLA-DR2 allele does exist in ulcerative colitis, and a negative association with DR4 and DRw6, when compared with ethnically matched controls. In contrast, in Crohn's disease it was found that a combination of DR1 and DQw5 was associated with the disease, thus indicating a quite different genetic background for the two diseases as regards the HLA system [12]. Other studies, however, have not been able to confirm this; thus no H L A correlation could be revealed in the French study of two multimember affected families [13]. Anti-neutrophil cytoplasmic antibodies (ANCA) have recently been in focus as a possible genetic marker in ulcerative colitis. About 60% of ulcerative colitis patients have these antibodies, whereas they are only sporadically present in Crohn's disease. Shanahan et al. reported that an
56
1A Binder, M. Orholm /Netherlands Journal of Medicine 48 (1996) 53-56
increased prevalence of pANCA is found not only in patients with ulcerative colitis but also among healthy relatives of these patients [14]. The preponderance of HLA-DR2 was found exclusively among the ANCA-positive patients, and the Californian group of scientists proposed a heterogeneity in ulcerative colitis patients with this background [15]. Other centres have however not been able to confirm these findings [16]. Other possible genetic markers have been looked for such as immunoglobulin subclasses in mucosal biopsy specimens, where it seems that healthy twins to ulcerative colitis patients have a similar distribution, whereas no such finding was seen in twins to patients affected with Crohn's disease [17].
4. Conclusion
About 10% of both ulcerative colitis and Crohn's disease patients have a genetic background, and these genetic factors seem to be different in ulcerative colitis and Crohn's disease. No genetic factors have been depicted with certainty until now, and future linkage studies are needed for further elucidation of gene localization. Environmental factors, however, may be involved, especially in Crohn's disease where the increasing incidence cannot be explained by genetics.
References [1] Crohn BB. The broadening conception of regional ileitis. Am J Dig Dis 1934;1:979. [2] Farmer RG, Michener WM, Mortimer EA. Studies of family history among patients with inflammatory bowel disease. Clin Gastroenterol 1980;9:271-278. [3] Weterman IT, Pena AS. Familial incidence of Crohn's disease in the Netherlands and a review of the literature. Gastroenterology 1984;86:449-452. [4] Orholm M, Munkholm P, Langholz E, Nielsen OH, Sore-
nsen TIA, Binder V. Familial occurrence of inflammatory bowel disease. N Engl J Med 1991;324:84-88. [5] Mayberry JF, Rhodes J. Familial prevalence of inflammatory bowel disease in relatives of patients with Crohn's disease. Br Med J 1980;280:284. [6] Mons~n U, Bernell O, Johansson C, Hellers G. Prevalence of inflammatory bowel disease among relatives of patients with Crohn's disease. Scand J Gastroenterol 1991;26:302-306. [7] Tysk C, Lindberg E, J~irnerot G, Flod~rus-Myrhed B. Ulcerative colitis and Crohn's disease in an unselected population of monozygotic and dizygotic twins. A study of heritability and the influence of smoking. Gut 1988;29:990-996. [8] Kfister W, Pascoe L, Purrmann J, Funk S, Majewski F. The genetics of Crohn disease: complex segregation analysis of a family study with 265 patients with Crohn disease and 5,387 relatives. Am J Med Genet 1989;32: 105-108. [9] Mons~n U. Inflammatory bowel disease. An epidemiological and genetic study. Acta Chir Scand 1990;suppl. 559:1-42. [10] Orholm M, Iselius L, Sorensen TIA, Munkholm P, Langholz E, Binder V. Investigation of inheritance of chronic inflammatory bowel diseases by complex segregation analysis. Br Med J 1993;306:20-24. [11] Mons6n U, lselius L, Johansson C, Hellers G. Evidence for a major additive gene in ulcerative colitis. Clin Genet 1989;36:411-414. [12] Toyoda H, Wang S-J, Yang H-Y, et al. Distinct associations of HLA class II genes with inflammatory bowel disease. Gastroenterolgoy 1993;104:741-748. [13] Van Kruiningen HJ, Colombel JF, Cartun RW, et al. An in-depth study of Crohn's disease in two French families. Gastroenterology 1993;104:351-360. [14] Shanahan F, Duerr RH, Rotter JI, et al. Neutrophil autoantibodies in ulcerative colitis: familial aggregation and genetic heterogeneity. Gastroenterology 1992;103: 456-461, [15] Yang H, Rotter JI, Toyoda H, et al. Ulcerative colitis: a genetically heterogeneous disorder defined by genetic (HLA class II) and subclinical (antineutrophil cytoplasmic antibodies)markers. J Clin Invest 1993;92:1080-1084. [16] Reumaux D, Delecourt L, Colombel JF, Noel LH, Duthilleul P, Cortot A. Anti-neutrophil cytoplasmic autoantibodies in relatives of patients with ulcerative colitis. Gastroenterology 1992;103:1706. [17] Helgeland L, Tysk C, J~irnerot G, et al. IgG subclass distribution in serum and rectal mucosa of monozygotic twins with or without inflammatory bowel disease. Gut 1992;33:1358-1364.