Inactivation of promoter 1B of APC causes partial gene silencing: evidence for a significant role of the promoter in regulation and causative of familial adenomatous polyposis.

Familial adenomatous polyposis (FAP) is caused by germline mutations in the adenomatous polyposis coli (APC) gene. Two promoters, 1A and 1B, have been recognized in APC, and 1B is thought to have a minor role in the regulation of the gene. We have identified a novel deletion encompassing half of this promoter in the largest family (Family 1) of the Swedish Polyposis Registry. The mutation leads to an imbalance in allele-specific expression of APC, and transcription from promoter 1B was highly impaired in both normal colorectal mucosa and blood from mutation carriers. To establish the significance of promoter 1B in normal colorectal mucosa (from controls), expression levels of specific transcripts from each of the promoters, 1A and 1B, were examined, and the expression from 1B was significantly higher compared with 1A. Significant amounts of transcripts generated from promoter 1B were also determined in a panel of 20 various normal tissues examined. In FAP-related tumors, the APC germline mutation is proposed to dictate the second hit. Mutations leaving two or three out of seven 20-amino-acid repeats in the central domain of APC intact seem to be required for tumorigenesis. We examined adenomas from mutation carriers in Family 1 for second hits in the entire gene without any findings, however, loss of the residual expression of the deleterious allele was observed. Three major conclusions of significant importance in relation to the function of APC can be drawn from this study; (i) germline inactivation of promoter 1B is disease causing in FAP; (ii) expression of transcripts from promoter 1B is generated at considerable higher levels compared with 1A, demonstrating a hitherto unknown importance of 1B; (iii) adenoma formation in FAP, caused by impaired function of promoter 1B, does not require homozygous inactivation of APC allowing for alternative genetic models as basis for adenoma formation.

Periampullary adenomas and adenocarcinomas in familial adenomatous polyposis: cumulative risks and APC gene mutations.

BACKGROUND & AIMS: Patients with familial adenomatous polyposis (FAP) have a high prevalence of duodenal adenomas, and the region of the ampulla of Vater is the predilection site for duodenal adenocarcinomas. This study assessed the risk of stage IV periampullary adenomas according to the Spigelman classification and periampullary adenocarcinomas in Swedish FAP patients screened by esophagogastroduodenoscopy (EGD). The genotype of patients with stage IV periampullary adenomas and periampullary adenocarcinomas was also investigated. METHODS: A retrospective study of 180 patients screened by EGD in 1982-1999 was undertaken. Kaplan-Meier analysis was performed to evaluate cumulative risk. Mutation analysis was carried out in patients with periampullary adenocarcinomas diagnosed outside the screening program, in addition to patients in the screening group with stage IV periampullary adenomas and adenocarcinomas. RESULTS: Periampullary adenoma stage IV was diagnosed in 14 patients (7.8%), with a cumulative risk of 20% at age 60 years. Periampullary adenocarcinoma was diagnosed in 5 patients (2.8%), with a cumulative risk of 10% at age 60. Three of the adenocarcinomas occurred in patients with stage IV periampullary adenomas compared with 2 in patients with less severe periampullary adenomatosis at screening (odds ratio, 31; 95% confidence interval, 4.6-215). Fifteen (88%) of the APC gene mutations were detected; 12 of these were located downstream from codon 1051 in exon 15. CONCLUSIONS: The life time risk of severe periampullary lesions in FAP patients is high, and an association between stage IV periampullary adenomas and a malignant course of the periampullary adenomatosis is strongly suggestive. Mutations downstream from codon 1051 seem to be associated with severe periampullary lesions.

A large deletion disrupts the exon 3 transcription activation domain of the BRCA2 gene in a breast/ovarian cancer family.

We describe the identification of a large deletion in the BRCA2 gene as the disease-causing mutation in a Swedish breast/ovarian cancer family. The 5068-bp deletion encompassed the 3' region of exon 3, including the 3' splice site and most of intron 3, and it resulted on the mRNA level in an inframe exon 3 skipping. The junction site also included an insertion of 4 bp (CCAT). The mutation (nt504del5068insCCAT) resulted in a genotype absent of the two transcription activation regions localized to exon 3. The breast cancer phenotype associated with the described mutation resembled the phenotype of breast cancer found in both BRCA1 and BRCA2 mutation carriers. This is the first report of a large deletion as the disease-causing mutation in the BRCA2 gene.

Novel mutations in the APC gene and clinical features in Swedish patients with polyposis coli.

The adenomatous polyposis coli (APC) gene was investigated in Swedish patients with familial adenomatous polyposis (FAP). A combination of analyses including single stranded conformation polymorphism (SSCP), heteroduplex (HD), protein truncation test (PTT) and direct sequencing was used to enable optimal mutation detection. Three novel mutations in the gene were identified, i.e. nt2644C- > T (giving an Arg876Stop mutation), nt4025del173 (leading to premature truncation of the protein at codon 1337) and nt3526insG (giving truncation at codon 1178). In addition, one previously described mutation, i.e. the 5-bp-deletion nt3942del5(AAAGA) in codon 1309 (giving a premature termination of the protein at codon 1314) was detected. All four mutations were located in the 5'-half of exon 15. The two latter mutations were associated with the CHRPE (congenital hypertrophy of retina pigment epithelium) phenotype (CHRPE was not examined in the other two cases). The patients with mutations in codon 1309 and 1336 had a more severe FAP phenotype.

Novel germline mutations in Swedish von Hippel-Lindau disease patients.

We have used a combination of different mutation detection systems in analyzing two Swedish families with von Hippel-Lindau disease (VHL). The methods employed were single-stranded conformation polymorphism (SSCP), heteroduplex analysis, and direct sequencing. The families were both shown to carry constitutional mutations in the VHL gene not reported earlier. Both were frameshift mutations (i.e. nt761delC and nt732insAA) predicted to give a premature stop codon. The correlation of mutation data to different clinical features of VHL is discussed.

[von Hippel-Lindau hereditary tumor syndrome. Mutational analysis may improve prognosis]. / von Hippel-Lindaus ärftliga tumörsyndrom. Mutationsanalys kan förbättra prognosen.

The von Hippel-Lindau disease is a familial tumour syndrome characterised by greatly increased risks of developing central nervous haemangioma, renal cell carcinoma, retinal angioma and pheochromocytoma. Carriers have inherited a mutated tumour suppressor gene located at chromosome 3p25-26. The VHL gene has recently been cloned, three exons identified and the DNA sequence determined. A Swedish kindred comprising four generations and 41 individuals was investigated. Three living individuals with clinically verified VHL disease demonstrated a single base deletion of a cytosine residue at position 761 of the VHL gene, corresponding to amino acid 254. Among the remaining family members, two asymptomatic carriers of this VHL mutation were identified and offered a clinical follow-up programme for early detection and treatment of future VHL manifestations.