Mutational Analysis Of APC Gene In Patients With Familial Adenomatous Polyposis

Familial adenomatous polyposis （FAP） characterized by the numerousadenomatous polyps in the colon and rectum, is inherited in an autosomal dominantmanner. According to polyp number and age at onset, the phenotype is usuallyclassified as classical familial adenomatous polyposis （CFAP） and attenuated familialadenomatous polyposis （AFAP）. CFAP is characterized by more than100colorectaladenomas and early onset, and AFAP by fewer than100adenomas and a later age ofonset. FAP is mainly caused by the mutation in adenomatous polyposis coli （APC）gene. The APC gene spans108kb of genomic DNA, and is located at chromosome5q21-22. It contains15coding exons and encodes a protein containing2843aminoacids. The APC protein has multiple domains, through which it binds to variousproteins. It functions as a negative regulator of β-catenin levels and suppressescanonical Wnt signalling, which is essential for tumorigenesis. In the absence of theAPC protein, β-catenin accumulates in the nucleus and interacts with factors thatup-regulate the transcription of genes involved in cell cycle entry and progression,and accelerates the oncogene expression. APC mutations are found in approximately80%to90%of patients with CFAP, most of pathogenic alterations are nonsense orframeshift mutations, and large deletions and/or duplications, splice site mutations arealso pathogenic factors of FAP, which result in the formation of premature terminationof the protein of disordered function, and causes the disease.Gene mutation detection in six pedigrees with CFAP were studied at genomic DNA leavel in this study. Thirty-four fragments covering the entire coding sequenceof the APC gene were amplified by PCR, and the PCR products were detected bySanger sequencing. Gene mutations were found in all six pedigrees. The mutation ofc.2891T＞G（L964X） in APC exon15was detected in pedigree one, which changedcodon for leucine to termination codon at codon964. The mutation of c.4012C＞T（Q1338X）of exon15was detected in pedigree two, which altered the codon forGlutamine to termination codon at codon1338. The mutation of c.904C＞T（R302X）of exon8was detected in pedigree three, which resulted arginine codon to terminationcodon at codon302. The mutation of c.3927₃931delAAAGA of exon15wasdetected in pedigree four, leading to premature termination at codon1312. Themutation of c.2154₂157delGAAA of exon15was detected in pedigree five, leadingto premature termination at codon716. The mutation of c.646C＞T（R216X）of exon6was detected in pedigree six, changing the codon for arginine to termination codonat codon216. All mutations were searched for in the Human Gene Mutation Database（HGMD）, and mutations in pedigrees one and five were novel.Molecular diagnosis can reveal the mutions of APC gene and the genotypes ofthe family members. It can also provide reliable data for genetic consultation andprenatal diagnosis, and contributes clinically earlier diagnosis and more accuratetreatment to the affected individuals.