S Bevan

Contribution of the MHC region to the familial risk of coeliac disease

Susceptibility to coeliac disease is genetically determined by possession of specific HLA-DQ alleles, acting in concert with one or more non-HLA linked genes. The pattern of risk seen in sibs and twins in coeliac disease is most parsimonious with a multiplicative model for the interaction between the two classes of genes. Based on a sib recurrence risk for coeliac disease of 10% and a population prevalence of 0.0033, the sib relative risk is 30. To evaluate the contribution of the MHC region to the familial risk of coeliac disease, we have examined haplotype sharing probabilities across this region in 55 coeliac disease families. Based on these probabilities the sib relative risk of coeliac disease associated with the MHC region is 3.7. Combining these results with published data on allele sharing at HLA, the estimated sib relative risk associated with the MHC region is 3.3. Therefore, the MHC genes contribute no more than 40% of the sib familial risk of coeliac disease and the non-HLA linked gene (or genes) are likely to be the stronger determinant of coeliac disease susceptibility.

Analysis of genetic and phenotypic heterogeneity in juvenile polyposis

BACKGROUND Juvenile polyposis syndrome (JPS) is characterised by gastrointestinal (GI) hamartomatous polyposis and an increased risk of GI malignancy. Juvenile polyps also occur in the Cowden (CS), Bannayan-Ruvalcaba-Riley (BRRS) and Gorlin (GS) syndromes. Diagnosing JPS can be problematic because it relies on exclusion of CS, BRRS, and GS. Germline mutations in the PTCH, PTENand DPC4 (SMAD4)genes can cause GS, CS/BRRS, and JPS, respectively. AIMS To examine the contribution of mutations in PTCH,PTEN, and DPC4(SMAD4) to JPS. METHODS Forty seven individuals from 15 families and nine apparently sporadic cases with JPS were screened for germline mutations inDPC4, PTEN, andPTCH. RESULTS No patient had a mutation in PTEN orPTCH. Five different germline mutations were detected in DPC4; three of these were deletions, one a single base substitution creating a stop codon, and one a missense change. None of these patients had distinguishing clinical features. CONCLUSIONS Mutations in PTEN and PTCHare unlikely to cause juvenile polyposis in the absence of clinical features indicative of CS, BRRS, or GS. A proportion of JPS patients harbour DPC4 mutations (21% in this study) but there remains uncharacterised genetic heterogeneity in JPS.

CHEK2 variants in susceptibility to breast cancer and evidence of retention of the wild type allele in tumours.

We have recently shown that the CHEK2*1100delC mutation acts as a low penetrance breast cancer susceptibility allele. To investigate if other CHEK2 variants confer an increased risk of breast cancer, we have screened an affected individual with breast cancer from 68 breast cancer families. Five of these individuals were found to harbour germline variants in CHEK2. Three carried the 1100delC variant (4%). One of these three individuals also carried the missense variant, Arg180His. In the other two individuals, missense variants, Arg117Gly and Arg137Gln, were identified. These two missense variants reside within the Forkhead-associated domain of CHEK2, which is important for the function of the expressed protein. None of these missense variants were present in 300 healthy controls. Microdissected tumours with a germline mutation showed loss of the mutant allele suggesting a mechanism for tumorigenesis other than a loss of the wild type allele. This study provides further evidence that sequence variation in CHEK2 is associated with an increased risk of breast cancer, and implies that tumorigenesis in association with CHEK2 mutations does not involve loss of the wild type allele.

Screening SMAD1 , SMAD2 , SMAD3 , and SMAD5 for germline mutations in juvenile polyposis syndrome

BACKGROUND AND AIMS Juvenile polyps occur in several Mendelian disorders, whether in association with gastrointestinal cancer alone (juvenile polyposis syndrome, JPS) or as part of known syndromes (Cowden, Gorlin, and Bannayan-Zonana) in association with developmental abnormalities, dysmorphic features, or extraintestinal tumours. Recently, some JPS families were shown to harbour germline mutations in theSMAD4 (DPC4) gene, providing further evidence for the importance of the TGFβ signalling pathway in colorectal cancer. There remains, however, considerable, unexplained genetic heterogeneity in JPS. Other members of the SMAD family are excellent candidates for JPS, especiallySMAD2 (which, likeSMAD4, is mutated somatically in colorectal cancers), SMAD3 (which causes colorectal cancer when “knocked out” in mice),SMAD5, and SMAD1. METHODS SMAD1,SMAD2, SMAD3, andSMAD5 were screened for germline mutations in 30 patients with JPS and without SMAD4mutations. RESULTS No mutations were found in any of these genes. A G–A C89Y polymorphism with possible effects on protein function was found in SMAD3, but the frequencies of the G and A alleles did not differ between patients with JPS and controls. CONCLUSIONS It remains to be determined whether or not this polymorphism is involved in a minor predisposition to colorectal or other carcinomas.SMAD4 may be the only member of the SMAD family which causes JPS when mutant in the germline. The other genes underlying JPS remain to be identified.

Low frequency of germline E-cadherin mutations in familial and nonfamilial gastric cancer

SummaryLittle is known about the relative contributions of genetic and environmental factors to the development of gastric cancer. Mutations in the cell adhesion molecule E-cadherin are recognized to be associated with the development of undifferentiated, diffuse and invasive gastric cancers. A recent study of two gastric cancer families has shown that germline mutations in the E-cadherin gene can be causative (Guilford P et al, Nature 1998; : 402–405). We have examined the E-cadherin gene for constitutive mutations in a systematic series of 106 gastric cancer patients, 10 with a family history of the disease and 96 sporadic cases. No pathogenic mutations were observed in any of the 106 patients. The results indicate that germline mutations in E-cadherin will not account for more than 3% of gastric cancers.

Linkage analysis for ATM in familial B cell chronic lymphocytic leukaemia.

B cell chronic lymphocytic leukaemia (CLL) shows evidence of familial aggregation, but the inherited basis is poorly understood. Mutations in the ATM gene have been demonstrated in CLL. This, coupled with a possibly increased risk of leukaemia in relatives of patients with Ataxia Telangiectasia, led us to question whether the ATM gene is involved in familial cases of CLL. To examine this proposition we typed five markers on chromosome 11q in 24 CLL families. No evidence for linkage between CLL and ATM in the 24 families studied and the best estimates of the proportion of sibling pairs that share no, one or both haplotypes at ATMwere not different from their null expectations. This would imply that ATM is unlikely to make a significant contribution to the three-fold increase in risk of CLL seen in relatives of patients.

Genome screening of coeliac disease

Coeliac disease is caused by T cell sensitisation of the intestine to cereal prolamins, which results in a range of mucosal abnormalities that may lead to malabsorption.1 The population prevalence in western countries is ∼1 in 200.2–4 Evidence for an inherited predisposition to coeliac disease comes from studies of first degree relatives of patients and studies of twins.5,6 A strong association is seen between coeliac disease and the HLA-DQ (α1*05, β1*02) heterodimer (DQ2) which is present in approximately 95% of patients,7–9 compared with 20-30% of healthy subjects.10,11 The difference in concordance rates between monozygotic twins and HLA identical sibs (80-100% v 25%) implicates non-HLA genes in the genetic predisposition to coeliac disease.11 The overall relative risk in sibs is at least 20 and is therefore four-fold higher than that attributable to HLA alone under model of inheritance.7 Genome linkage searches carried out on Irish,12 Italian,13, and UK14 coeliac disease families have identified a number of potential sites for the location of non-HLA linked genes. The putative candidate loci detected in the three studies are, however, largely inconsistent and the findings have not been replicated in other populations.15–19 Here we report the results of a genome screen of 24 multiplex families with coeliac disease and discuss the findings of this study in relation to previously published analyses. Twenty-four families with two or more members affected with coeliac disease were recruited for this study (fig 1). Nine of these families (Nos 1, 3, 4, 6, 30, 32, 39, 43, and 44) have been used in a previous study of candidate regions.16 All the families were of northern European ancestry. Twelve of the families were recruited from the UK, nine from Sweden, two from Switzerland, …

PTEN mutations are uncommon in Proteus syndrome

Editor—Proteus syndrome (MIM 176920) is a rare, congenital, hamartomatous disorder, which is a member of a group of local overgrowth diseases. Happle1 proposed that some of these disorders are the result of the action of a lethal gene that can only survive in the mosaic state, which arises from an early somatic mutation or from a half chromatid mutation. Such a mechanism has been shown to be the underlying basis of McCune-Albright syndrome (MIM 174800).2 One of the mandatory diagnostic criteria for Proteus syndrome is a mosaic distribution of lesions and sporadic occurrence, entirely consistent with Happles hypothesis. Currently, little is known about the molecular causes of Proteus syndrome. It is, however, likely that the overgrowth of tissue involves all germ layers. This may be because of hyperproliferation, an absence of appropriate apoptosis, or alternatively cellular hypertrophy. There have been …

Analysis of the CTLA4 Gene in Swedish Coeliac Disease Patients

Background: A genetic susceptibility to coeliac disease is well established, involving HLA and nonHLA components. CTLA4 is an important regulator of T-cell function and some studies have suggested that sequence variation in the gene might be a determinant of disease susceptibility, although the evidence is conflicting. Methods: Sixty-two children with biopsy-proven coeliac disease attending a single centre in Sweden were studied. All were genotyped for presence of the HLA-DQA1*0501, B1*0201 alleles. Those who carried the HLA-DQ heterodimer (58/62) were genotyped for the +49 (A/G) exon 1 polymorphism. The transmission disequilibrium test (TDT) was used to test for association between coeliac disease and the A allele. The entire CTLA4 gene was screened for other sequence variants using a combination of conformation-sensitive gel electrophoresis and direct sequencing. Results: A significant association between the exon 1 polymorphism and coeliac disease was observed ( P = 0.02). No other sequence variants in CTLA4 were detected. Conclusions: This study provides further evidence that variation in CTLA4 is a determinant of coeliac disease susceptibility. If not mediated through the +49 (A/G) dimorphism directly, then the effect is likely to be mediated through linkage disequilibrium.

Relative power of linkage and transmission disequilibrium test strategies to detect non-HLA linked coeliac disease susceptibility genes

BACKGROUND Susceptibility to coeliac disease is genetically determined by possession of specific HLA DQ alleles, acting in concert with one or more non-HLA linked genes. The pattern of familial risk is most parsimonious with a multiplicative model for the interaction between these two classes of genes. Haplotype sharing probabilities across the HLA region in affected sibling pairs suggest that genes within the MHC complex contribute no more than 40% of the sibling familial risk of coeliac disease, making the non-HLA linked gene (or genes) the stronger determinant of coeliac disease susceptibility. Attempts to localise these non-HLA linked genes have been carried out using both linkage and association tests. AIMS To review the evidence for the involvement of non-HLA linked genes in coeliac disease, and to compare the relative merits of linkage and transmission disequilibrium tests (TDT) to detect the non-HLA linked gene (or genes) contributing to the development of coeliac disease. METHODS Under a range of genetic models the number of affected sibling pairs needed to detect linkage was compared with the number of families required to show a relation between marker and disease, adopting the TDT strategy. RESULTS AND CONCLUSIONS Power calculations show that, if there is a single major non-HLA linked susceptibility locus, a non-parametric linkage approach may well prove effective. However, if there are a number of non-HLA susceptibility genes, each with small effect, the sample size necessary for linkage studies will be prohibitive and a systematic search for allelic association should be a more effective strategy.