Two Chinese pedigrees for adenomatous polyposis coli: new mutations at codon 1309 and predisposition to phenotypic variations

Familial adenomatous polyposis (FAP) is an autosomal dominant inherited disease caused by a mutation in the adenomatous polyposis coli (APC) gene. Some studies have attempted to correlate mutations at codon 1309 with classic FAP (≥100 colorectal polyps). We report two Chinese FAP pedigrees with new frameshift mutations at codon 1309, in which affected individuals manifest phenotypic variations. Comprehensive physical examinations were performed for all living individuals and the medical data of deceased patients were collected. Screening of the APC and human mutY homolog (MUTYH) genes for germline mutations was conducted by direct polymerase chain reaction (PCR) sequencing. In two pedigrees, a heterozygous deletion in exon 16 of the APC gene was present in all FAP patients but absent in the unaffected individuals. There were no changes to the MUTYH gene. The first pedigree, with a new frameshift mutation at c.3926_3930 del AAAAG (p. Glu1309Aspfs X4), exhibited obvious differences in the polyp number such that the proband manifested only three colorectal polyps, whereas another patients showed the symptoms of classic FAP. The second pedigree, also traced a new mutation at c.3922_3925 del AAAG (p. Glu1309Argfs X11). Although all of the patients presented with classic polyposis, one of them exhibited a delayed onset of colorectal cancer in his 50s. Two novel mutations at codon 1309 in two Chinese families suffering from FAP could enrich the germline mutation spectrum of the APC gene. Families of individuals might manifest different phenotypes, even with an identical codon 1309 mutation, unlike in previous studies.     

Refining the relation between 'first hits' and 'second hits' at the APC locus: the 'loose fit' model and evidence for differences in somatic mutation spectra among patients

The site of the 'first hit' in the APC tumour suppressor gene determines the type of the 'second hit', both in familial adenomatous polyposis (FAP) and sporadic colorectal tumours. Mutations near codon 1300 are associated with loss of heterozygosity (LOH) of the wild-type allele; other tumours tend to have two protein-truncating mutations. In this study, we have confirmed and refined the LOH-associated region in colorectal FAP: allelic loss in adenomatous polyps tended to occur when the germline mutation lay in the region of the APC gene between the first and second beta-catenin degradation repeats (codons 1285-1378). LOH generally occurred by mitotic recombination, leaving two identical alleles, each encoding a protein with one remaining beta-catenin degradation repeat. For patients with germline mutations that truncated the protein before the first repeat (codon 1264), LOH was very rare and tumours generally acquired a somatic mutation which left two, or less often one, repeats remaining in the protein. In our sample set, patients with germline mutations after the second beta-catenin degradation repeat tended to have undetectable, presumably cryptic, somatic mutations in their polyps. Exceptions to these rules were, however, not uncommon. Although the site of the germline mutation was the strongest determinant of the somatic mutation in FAP tumours and most patients showed no clear tendency to acquire specific types of truncating 'second hit', a minority of patients did have unusual somatic mutation spectra in their polyps. Thus, some individuals may be predisposed to particular types of 'second hit' (for example, frameshift rather than nonsense changes). Overall, disease severity (polyp number) did not vary with individuals' spectrum of somatic APC mutations, providing no clear evidence for modifier genes that influence disease severity in this fashion. Our data are consistent with the hypothesis that there exists an optimal level of beta-catenin signalling in colorectal tumours and that the APC mutation spectrum principally reflects this fact. The association between 'first hits' and 'second hits' at APC is not, however, so strong as to suggest that tumorigenesis only occurs if the genotype is optimum; we suggest 'relaxed' terminology, the 'loose fit' model, to describe this situation.     

Mutation analysis of the <Emphasis Type="Italic">APC</Emphasis> gene in Taiwanese FAP families: low incidence of <Emphasis Type="Italic">APC</Emphasis> germline mutation in a distinct subgroup of FAP families

Familial adenomatous polyposis (FAP) is an autosomal-dominant disease caused by germline mutations in the adenomatous polyposis coli (APC) gene. The affected individuals develop colorectal polyposis and show various extra-colonic manifestations. In this study, we aimed to investigate the genetic and clinical characteristics of FAP in Taiwanese families and analyze the genotype–phenotype correlations. Blood samples were obtained from 66 FAP patients registered in the hereditary colorectal cancer database. Then, germline mutations in the APC genes of these 66 polyposis patients from 47 unrelated FAP families were analyzed. The germline-mutation-negative cases were analyzed by performing multiplex ligation-dependent probe amplification (MLPA) and single-strand conformation polymorphism (SSCP) analysis of the MUTYH gene. Among the analyzed families, 79% (37/47) of the families showed 28 APC mutations, including 19 frameshift mutations, 4 nonsense mutations, 3 genomic deletion mutations, 1 missense mutation, and 1 splice-site mutation. In addition, we identified 15 novel mutations in 32% (15/47) of the families. The cases in which APC mutations were not identified showed significantly lower incidence of profuse polyposis (P = 0.034) and gastroduodenal polyps (P = 0.027). Furthermore, FAP families in which some affected individuals had less than 100 polyps showed significant association with low incidence of APC germline mutations (P = 0.002). We have added the APC germline-mutation data for Taiwanese FAP patients and indicated the presence of an FAP subgroup comprising affected individuals with nonadenomatous polyps or less than 100 adenomatous polyps; this form of FAP is less frequently caused by germline mutations of the APC gene.     

A Novel Exon Duplication Event Leading to a Truncating Germ-line Mutation of the APC Gene in a Familial Adenomatous Polyposis Family

Familial Adenomatous Polyposis (FAP) is an autosomal dominant condition predisposing to multiple adenomatous polyps of the colon. FAP patients frequently carry heterozygous mutations of the APC tumour suppressor gene. Affected individuals from a cohort of FAP families (n=22), where no germ-line APC mutation was detected by direct sequencing, were analysed by Multiplex Ligation-dependent Probe Amplification (MLPA). MLPA identified a previously unreported APC mutation involving duplication of exon 4. Subsequent analysis of cDNA from affected family members revealed expression of mutant mRNA species containing two copies of exon 4, resulting in a frameshift and premature stop codon. Bioinformatic analysis of the relevant APC genomic segment predicted a role for homologous recombination possibly involving Alu repeats in the generation of this genotype. Our results highlight the importance of MLPA as an adjunct to exon-by-exon sequencing in identifying infrequent mutational events in cancer predisposing genes. Amy McCart and Andrew Latchford contributed equally to this work.     

APC gene mutations and colorectal adenomatosis in familial adenomatous polyposis

Correlations between germline APC mutation sites and colorectal pathophenotypes, as evaluated by the direct count of adenomas at colectomy, were investigated analysing colectomy specimens from 29 FAP patients carrying one mis-sense (codon 208) and 14 frame-shift or non-sense APC mutations (codons 232, 367, 437, 623, 876, 995, 1061, 1068, 1075, 1112, 1114, 1309, 1324, 1556). The mis-sense mutation at codon 208 was associated with a relatively mild colorectal pathophenotype. The mutation at codon 367, subject to alternative splicing, was associated with attenuated FAP. The mutation at codon 1309 was associated with the profuse colorectal adenomatosis. For 13 mutations, predicted to result in null alleles or truncated APC proteins, we correlated density and distribution of colorectal adenomas with the predicted functional effects of the mutation. The most severe colorectal pathophenotype was significantly associated with the truncating mutation at codon 1309, which is located downstream to the I beta-catenin binding domain but upstream II beta-catenin-binding domain. Mutations between codons 867 and 1114, which affect the I beta-catenin binding domain, as well as mutations occurring in exons 6 and 9, predicted to result in null alleles, were associated with a less severe colorectal pathophenotype. Overall, the highest number of adenomas was detected in the right colon, followed by the left colon, transverse colon sigma and rectum. However, the highest density of adenomas was observed in the left colon, followed by the right colon, sigma, transverse colon and rectum. Colorectal carcinomas, observed in only five patients, were all in the left colon.     

Mutation analysis of the APC gene in unrelated Korean patients with FAP: four novel mutations with unusual phenotype

Germline mutations within the adenomatous polyposis coli (APC) gene are responsible for most cases of familial adenomatous polyposis (FAP), an autosomal dominantly inherited predisposition to colorectal cancer. To date more than 900 different APC germline mutations have been characterized worldwide demonstrating allelic heterogeneity. Here, we analyzed the APC gene in 23 DNA samples from unrelated Korean patients with the typical clinical symptoms of FAP by denaturing high-performance liquid chromatography (DHPLC) and direct sequencing. We identified 20 different APC sequence variants, including 9 truncating mutations, 1 missense mutation, 7 polymorphisms, and 3 intronic variants. Nine different truncating mutations, including four novel mutations (p.Leu180TyrfsX5, p.Gly567X, p.Ser1275PhefsX13, p.Leu1280CysfsX8), were detected. The most common mutation was a 5 bp deletion at codon 1,309 (p.Glu1309AspfsX4) as in Western studies. The next most common mutation was p.Ser1275PhefsX13 with a severe form of FAP with many extracolonic manifestations; this was a novel mutation identified in our study and may represent the second hot-spot mutation in a Korean population. Novel mutations are of particular interest because of the unusual phenotypic features shown by patients. In present study, we found new positions associated with thyroid cancer (codon 180) and desmoid tumor (codon 1,280), which have not been previously reported. The results of this molecular study have revealed the existence of novel pathogenic mutations in Korean patients with FAP. In addition to allowing phenotype–genotype correlations to be performed, these results are currently being used in genetic counseling and in patient care.     

<Emphasis Type="Italic">APC</Emphasis> mosaicism in a young woman with desmoid type fibromatosis and familial adenomatous polyposis

Familial adenomatous polyposis (FAP) is usually caused by germline mutations in the adenomatous polyposis coli (APC) gene. The classic form is characterized by hundreds to thousands of adenomas in the colorectum and early onset colorectal cancer (CRC) if left untreated. FAP is also associated with multiple extra-colonic manifestations such as gastroduodenal polyps, osteomas, epidermoid cysts, fibromas and desmoids. Most desmoid tumours in FAP patients occur intra-abdominally. Approximately 15–20% of the APC mutations are de novo mutations. Somatic mosaicism has been reported in some sporadic cases of polyposis but is probably an underestimated cause of the disease. This case report presents the detection of a mosaic APC mutation in a 26-year-old woman who as a child had been diagnosed with desmoid type fibromatosis. FAP was suggested when she presented with extensive extra abdominal fibromatosis. Our findings indicate that APC mutations may be suspected in patients presenting with a desmoid regardless of its location. If there is clinical evidence that the patient has FAP, adenomas and colonic mucosa in addition to leukocyte DNA should be included in the screening, preferably using methods that are more sensitive than Sanger sequencing.     

Using genotype to assist clinical surveillance: a retrospective study of Chinese familial adenomatous polyposis patients

Without treatment, familial adenomatous polyposis (FAP) patients will inevitably develop colorectal cancer (CRC) during lifetime. Yet, surgical trauma is a high risk of desmoid tumor (DT), one of the main causes of death in FAP patients. So far, the timing for colectomy is primarily based on the clinician’s experience and the patient’s preference; most patients undergo surgery at mid-20’s. In this study, we analyzed the germline mutation distribution in 35 FAP patients from different families, 16 of them diagnosed with DTs. We also investigated the association between the molecular alterations and the clinicopathological features. Capture-based targeted sequencing using a panel of 520 genes was performed on tumor tissue and paired normal mucosa or white blood cells from 18 FAP probands who were initially diagnosed with CRC. Of all 35 FAP patients, 30 (85.7%) of them harbored germline APC mutations scattered from codon 161 to 1578. The mutations in the 16 DT patients scattered from codon 457 to 1578. All three patients with the mutation at the 3’ of 1444 codon were diagnosed with DT. The percentage of high-risk DT (stage III or IV) harboring mutations at the 5’ of 1062 or 1062-1578 was 14.3% and 77.8%, respectively, and all three patients with 3’ of 1399 codon mutation had high risk. In addition, by using public database, we compared 140 FAP patients with DT to all 1880 FAP patients on the Leiden Open Variation Database and found that the odd ratio of DT in codon 159 to 495 was 0.34, while in codon 1310 to 2011 was 2.36. Compared to sporadic CRCs, the somatic spectrum of FAP CRCs was similar to the early onset CRCs, with higher TP53 (94.1%) and lower somatic APC mutations (65.7%), but the KRAS mutation rate was the highest (58.5%). One of the 18 FAP CRCs was identified as microsatellite instability-high (MSI-H), with tumor mutation burden (TMB) of 115.65 mut/Mb. Given that no TP53 mutations were detected in the low- and high-grade adenomas, ctDNA TP53 sequencing might be used for the close monitoring before FAP colectomy. In conclusion, except mutations at the 5’ end of APC (5’ to 495), all FAP patients need to consider the risk of DT after colectomy. The chance of life-threating DTs was higher in patients with 3’ 1062 codon mutation and peaked in patients with 3’ 1399 codon mutation. Scheduled monitoring of TP53 ctDNA is proposed to be a novel tool for optimizing the operation time.     

Mutational spectrum of <Emphasis Type="Italic">APC</Emphasis> and genotype-phenotype correlations in Greek FAP patients

Background  

Familial adenomatous polyposis, an autosomal dominant inherited disease caused by germline mutations within the APC gene, is characterized by early onset colorectal cancer as a consequence of the intrinsic phenotypic feature of multiple colorectal adenomatic polyps. The genetic investigation of Greek adenomatous polyposis families was performed in respects to APC and MUTYH germline mutations. Additionally, all available published mutations were considered in order to define the APC mutation spectrum in Greece.     

The relationship between frequencies of extracolonic manifestations and the position of APC germline mutation in patients with familial adenomatous polyposis

BACKGROUND: Familial adenomatous polyposis (FAP) patients develop various extracolonic lesions; however, the relationship between germline mutation of the APC gene and extracolonic manifestations is mostly unknown. To examine the genotype-phenotype relationship, we compared the APC mutation and clinical data. METHODS: Germline mutations from codon 157 to 1465 of the APC gene were identified in 39 families of FAP and clinical data were collected from 80 patients of these families. Germline mutations were classified into two groups: mutations from exon 4 to 9 (codon 157 to 416, Group 1) and those from exon 10 to 15H (codon 564 to 1465, Group 2). The complication rates of extracolonic manifestations were compared between these two groups. RESULTS: Frequencies of duodenal polyps and gastric adenomas in Group 2 were higher than those in Group 1 (p < 0.0001 and p < 0.0004, respectively) and development of osteoma was more frequent in Group 2 (p = 0.01). The number of colorectal polyps and retinal pigments also correlated with the germline mutation, which was consistent with previous reports. However, such correlations were less obvious with regard to gastric fundic polyps, desmoid tumors, soft tissue tumors and colorectal cancer. CONCLUSION: There are two types with regard to extracolonic manifestations of FAP: one is more severely affected according to the position of germline mutation of the APC gene and the other is not affected.     

Familial adenomatous polyposis of the colon

Familial adenomatous polyposis (FAP) is a well-defined autosomal dominant predisposition to the development of polyposis in the colon and rectum at unusually early ages. The first symptoms of FAP are diarrhea and blood in the stool. Weight loss and weaknesses occur after the development of advanced tumour. The incidence of the FAP disorder is one per 10000 newborns. There are high levels of heterogeneity with regard to the number and timing of the occurrence of polyps. The classical form of FAP is characterized by the presence of more than 100 polyps, which appear in the second decade of life. The average time of occurrence of polyps is 15 years. The earliest symptoms of polyposis have been observed in a three-year-old child. The polyps are characterized by large potential for the development towards malignant tumour. Malignancy can occur from late childhood onwards. Attenuated adenomatous polyposis coli is characterized by a more benign course of disease in contrast to classical FAP. The occurrence of FAP is associated with mutations in the APC tumour suppressor gene, which was described in 1991. The APC gene is located on chromosome 5q21 and is involved in cell proliferation control. A recessive form of adenomatous polyposis is caused by mutations in the base excision repair gene - MUTYH gene. The MUTYH gene is involved in repairing DNA lesions as a result of oxidative DNA damage. MUTYH associated polyposis (MAP) is a predisposition to the development of polyps of the colon but the number of polyps is lower in comparison to classical FAP. The high risks of cancer observed in these two diseases make them important medical issues. Molecular studies of colonic polyposis have been performed in Poland for over fifteen years. A DNA Bank for Polish FAP patients was established at the Institute of Human Genetics in Poznan in which DNA samples from 600 FAP families have been collected.     

Balance between endoscopic and genetic information in the choice of ileorectal anastomosis for familial adenomatous polyposis

BACKGROUND AND OBJECTIVES: The number of rectal polyps and the site of mutations in the APC (Adenomatous polyposis coli) gene have been used to guide the surgical management in patients with familial adenomatous polyposis (FAP). The aim of this study is to assess the utility of the APC mutation screening compared to the degree of the rectal polyposis in surgical decision making. METHODS: The post-surgical courses of 25 patients submitted to subtotal colectomy with ileorectal anastomosis (IRA) were reviewed. Preservation of the rectum was prospectively decided on the basis of well-defined endoscopic criteria. The number of rectal polyps was assessed preoperatively and every 6-12 months. APC gene was screened for mutations by heteroduplex analysis, single strand conformation polymorphism, in vitro synthesized protein (IVSP), and DNA sequencing. Patients negative for APC mutations were tested for MYH mutations. RESULTS: On the basis of preoperative polyp rectal count we categorized patients as follows: Group I, 5 or fewer adenomas; Group II, 6-9 adenomas; Group III, 10 or more adenomas. After a follow-up ranging from 12 to 225 months we have observed a significant difference of recurrent rectal adenomas between Groups I-II versus III. No difference was detected among patients of Group I and II. The mean number of adenomas/year/patient was 0.67, 1.62, and 9.29 for Group I, II, and III, respectively. Carpeting polyposis of the rectal stump developed in three patients with APC mutation at codon 1309 and two of them needed later proctectomy. Diffuse rectal polyposis was observed in one patient with mutation at exon 9 who had 10 small polyps at time of surgery. Mutation at the 5'-end of APC (codons 144-232), mutation of MYH and unknown APC or MYH mutation were correlated with a low number of polyps both at presentation and follow-up. No IRA patients developed rectal cancer. CONCLUSIONS: In our experience fewer than 10 rectal polyps at presentation can predict a favorable outcome after IRA. Identification of specific germ-line APC or MYH mutation can address the choice of surgical treatment.     

Multiple approach to the exploration of genotype-phenotype correlations in familial adenomatous polyposis.

PURPOSE: Familial adenomatous polyposis (FAP), caused by a mutation in the APC gene, is a colorectal cancer predisposition syndrome associated with several other clinical conditions. The severity of the FAP is related to the position of the inherited mutation in the APC gene. We analyzed a large series of FAP patients to identify associations among major clinical manifestations and to correlate the mutation site with specific disease manifestations. MATERIALS AND METHODS: APC mutations were identified in 953 FAP patients from 187 families. We used unconditional logistic regression models and a method involving generalized estimating equations to investigate the association between genotype and phenotype. We used multiple correspondence analysis to represent the interrelationships of a multiway contingency table of the considered variables. RESULTS: APC germline mutations were located between codons 156 and 2011 of the APC gene. Mutations spanning the region between codons 543 and 1309 were variable, but strongly associated with congenital hypertrophy of retinal pigment epithelium. Mutations between codons 1310 and 2011 were associated with a six-fold risk of desmoid tumors relative to the low-risk reference region (159 to 495). Mutations at codon 1309 were associated with early development of colorectal cancer. Mutations between codons 976 and 1067 were associated with a three- to four-fold increased risk of duodenal adenomas. The cumulative frequency of extracolonic manifestations was highest for mutations between codons 976 and 1067, followed by mutations between 1310 and 2011. CONCLUSION: Analysis of the relation between APC mutation site and phenotype identifies subgroups of FAP patients at high risk for major extracolonic disease, which is useful for surveillance and prevention.     

Analysis of Small Fragment Deletions of the APC gene in Chinese Patients with Familial Adenomatous Polyposis,a Precancerous Condition

Background: : Familial adenomatous polyposis (FAP) is an autosomal dominant inherited disease mainly causedby mutations of the adenomatous polyposis coli (APC) gene with almost complete penetrance. These colorectalpolyps are precancerous lesions that will inevitable develop into colorectal cancer at the median age of 40-yearold if total proctocolectomy is not performed. So identification of APC germline mutations has great implicationsfor genetic counseling and management of FAP patients. In this study, we screened APC germline mutationsin Chinese FAP patients, in order to find novel mutations and the APC gene germline mutation characteristicsof Chinese FAP patients. Materials and Methods: The FAP patients were diagnosed by clinical manifestations,family histories, endoscope and biopsy. Then patients peripheral blood samples were collected, afterwards,genomic DNA was extracted. The mutation analysis of the APC gene was conducted by direct polymerasechain reaction (PCR) sequencing for micromutations and multiplex ligation-dependent probe amplification(MLPA) for large duplications and/or deletions. Results: We found 6 micromutations out of 14 FAP pedigrees,while there were no large duplications and/or deletions found. These germline mutations are c.5432C>T(p.Ser1811Leu), two c.3926_3930delAAAAG (p.Glu1309AspfsX4), c.3921_3924delAAAA (p.Ile1307MetfsX13),c3184_3187delCAAA(p.Gln1061AspfsX59) and c4127_4126delAT (p.Tyr1376LysfsX9), respectively, and alldeletion mutations resulted in a premature stop codon. At the same time, we found c.3921_3924delAAAA andtwo c.3926_3930delAAAAG are located in AAAAG short tandem repeats, c3184_3187delCAAA is located in theCAAA interrupted direct repeats, and c4127_4128 del AT is located in the 5’-CCTGAACA-3’ ,3’-ACAAGTCC-5palindromes (inverted repeats) of the APC gene. Furthermore, deletion mutations are mostly located at condon1309. Conclusions: Though there were no novel mutations found as the pathogenic gene of FAP in this study,we found nucleotide sequence containing short tandem repeats and palindromes (inverted repeats), especiallythe 5 bp base deletion at codon 1309, are mutations in high incidence area in APC gene.     

A comparison of the phenotype and genotype in adenomatous polyposis patients with and without a family history

Objectives: Adenomatous polyposis of the colon is often secondary to an inherited mutation in adenomatous polyposis coli (APC) gene, however, approximately one third of patients have no family history of the disease. We studied the phenotype and genotype of adenomatous polyposis in patients without a family history. Methods: A cohort of 57 unrelated adenomatous polyposis patients were evaluated. Seventeen patients with no family history were compared with 40 patients who had a positive family history of the disease. Family history and medical records were collected and analyzed. Germline APC and Mut Y homologue (MYH) testing was undertaken. Results: Patients without a family history were diagnosed with polyposis at an older age (41 years vs. 32 years) and presenting more frequently with symptoms (76 vs 20, P < 0.05). The number of colonic polyps and frequency of extracolonic manifestation associated with adenomatous polyposis did not differ between the two groups. APC mutations were detected less frequently among patients without a family history of the disease (4 out of 17 vs 25 out of 40, P=0.007), even among those with greater than 100 colorectal adenomas (4 out of 12 versus 21 out of 29, P=0.03). One homozygous MYH mutation carrier (G382D) was detected among the six patients without a family history and without a germline APC mutation who were tested. Conclusions: Adenomatous polyposis patients without a family history are usually diagnosed with symptoms, and at a later age. Phenotypically, they are similar to those with a family history. However, germline APC mutations are detected far less frequently in patients without a family history. A small percentage of these cases may be secondary to biallelic germline MYH mutations.     

A novel pathogenic germline mutation in the adenomatous polyposis coli gene in a Tunisian family with FAP

Familial adenomatous polyposis (FAP) is an autosomal dominant disorder which typically presents with colorectal cancer in early adult life, secondary to extensive adenomatous polyps of the colon. In addition to the colonic manifestations, the syndrome presents several extracolonic features including, congenital hypertrophy of the retinal pigment, osteomata and desmoid tumors. In this study, we aimed to investigate the clinical and genetic features in a Tunisian family with FAP. Sequence of the APC gene (Adenomatous Polyposis Coli) revealed a novel mutation (c.2016-2017 del TA) in exon 15, present in all affected individuals in an heterozygous state. The frameshift mutation generates a premature stop codon at amino acid 677 of the APC protein (p. H672Qfs X5). The unaffected family members did not harbor this mutation, however, a first degree relative of the patient aged of 32 year-old was phenotypically normal but carries the c.2016-2017 del TA mutation. This discrepancy can be explained by the effect of modifier gene which can affect the expressivity of the disease.     

Colorectal cancer risk variants at 8q23.3 and 11q23.1 are associated with disease phenotype in <Emphasis Type="Italic">APC</Emphasis> mutation carriers

Familial adenomatous polyposis (FAP) is a dominantly inherited syndrome caused by germline mutations in the APC gene and characterized by the development of multiple colorectal adenomas and a high risk of developing colorectal cancer (CRC). The severity of polyposis is correlated with the site of the APC mutation. However, there is also phenotypic variability within families with the same underlying APC mutation, suggesting that additional factors influence the severity of polyposis. Genome-wide association studies identified several single nucleotide polymorphisms (SNPs) that are associated with CRC. We assessed whether these SNPs are associated with polyp multiplicity in proven APC mutation carriers. Sixteen CRC-associated SNPs were analysed in a cohort of 419 APC germline mutation carriers from 182 families. Clinical data were retrieved from the Dutch Polyposis Registry. Allele frequencies of the SNPs were compared for patients with <100 colorectal adenomas versus patients with ≥100 adenomas, using generalized estimating equations with the APC genotype as a covariate. We found a trend of association of two of the tested SNPs with the ≥100 adenoma phenotype: the C alleles of rs16892766 at 8q23.3 (OR 1.71, 95 % CI 1.05–2.76, p = 0.03, dominant model) and rs3802842 at 11q23.1 (OR 1.51, 95 % CI 1.03–2.22, p = 0.04, dominant model). We identified two risk variants that are associated with a more severe phenotype in APC mutation carriers. These risk variants may partly explain the phenotypic variability in families with the same APC gene defect. Further studies with a larger sample size are recommended to evaluate and confirm the phenotypic effect of these SNPs in FAP.     

Somatic mutations of LKB1 and beta-catenin genes in gastrointestinal polyps from patients with Peutz-Jeghers syndrome

Peutz-Jeghers syndrome (PJS) is characterized by multiple gastrointestinal hamartomatous polyps, mucocutaneous melanin deposition, and increased risk of cancer, mainly in the gastrointestinal tract. We examined mutations of the LKB1, beta-catenin, APC, K-ras, and p53 genes in 27 gastrointestinal hamartomatous polyps from 10 patients in nine PJS families. Of these hamartomatous polyps, one intestinal polyp had an adenomatous lesion, and one gastric polyp contained adenomatous and carcinomatous lesions. Germ-line mutations of the LKB1 gene were detected in six PJS families. Somatic mutations of the LKB1 gene were found in 5 polyps, whereas loss of heterozygosity (LOH) at the LKB1 locus at 19p was seen in 14 other polyps. In adenomatous lesions microdissected from hamartomatous polyps, both beta-catenin mutation and 19p LOH were detected. Furthermore, a carcinomatous lesion in a gastric hamartomatous polyp was found to contain a mutation of the p53 gene and LOH at the p53 locus in addition to LOH at the LKB1 locus and a beta-catenin mutation. K-ras mutations were detected in a few polyps, whereas no APC mutation or 5q LOH was detected in hamartomatous polyps. These results suggest that gastrointestinal hamartomatous polyps in PJS patients develop through inactivation of the LKB1 gene by germ-line mutation plus somatic mutation or LOH of the unaffected LKB1 allele, and that additional mutations of the beta-catenin gene and p53 gene convert hamartomatous polyps into adenomatous and carcinomatous lesions.