A novel pathogenic splice acceptor site germline mutation in intron 14 of the APC gene in a Chinese family with familial adenomatous polyposis

Abstract

Familial adenomatous polyposis (FAP) is an autosomal dominant precancerous condition, clinically characterized by the presence of multiple colorectal adenomas or polyps. Patients with FAP has a high risk of developing colorectal cancer (CRC) from these colorectal adenomatous polyps by the mean age of diagnosis at 40 years. Germline mutations of the APC gene cause familial adenomatous polyposis (FAP). Colectomy has recommended for the FAP patients with significant polyposis. Here, we present a clinical molecular study of a four generation Chinese family with FAP. Clinical diagnosis of FAP has been done according to the phenotype, family history and medical records. Patient's blood samples were collected and genomic DNA was extracted. In order to identify the pathogenic mutation underlying the disease phenotype targeted next-generation sequencing and confirmatory sanger sequencing has undertaken. Targeted next generation sequencing identified a novel heterozygous splice-acceptor site mutation [c.1744-1G>A] in intron 14 of APC gene, which is co-segregated with the FAP phenotypes in the proband and amongst all the affected family members. This mutation is not present in unaffected family members and in normal healthy controls of same ethnic origin. According to the LOVD database for Chinese colorectal cancer patients, in Chinese population, 60% of the previously reported APC gene mutations causes FAP, are missense mutations. This novel splice-acceptor site mutation causing FAP in this Chinese family expands the germline mutation spectrum of the APC gene in the Chinese population.

Keywords: APC gene; colorectal cancer; familial adenomatous polyposis; splice acceptor site mutation; targeted next-generation sequencing.

Figure 1. Pedigree of the Chinese family with familial adenomatous polyposis

Family members with FAP are indicated with Shading. Squares and circles denoted males and females respectively. Individuals labeled with a solidus were deceased. Roman numerals indicate generations. Arrow indicates the proband (II-9).

Figure 2. Clinical description

(A–B) Colonoscopy and histology of the proband (II-9). Polyps of colon is High grade intraepithelial neoplasia (dysplasia), cancer invade the submucosa in proband (II-9). A. Protrude type mass in retucm, the mass take up the whole enteric cavity. (B) Moderately differentiated adenocarcinoma. (C–D) Colonoscopy and histology of the other affected members (III-25). Polyps of colon, cancer invade the submucosa (III-25). (C) Multiple polyps. (D) Tubular adenoma. (E–F) No polyps in the colon of unaffected member with normal histology (III-12). (E) Smooth intestinal tract. (F) Chronic inflammation of mucosa.

Figure 3. Sanger sequencing of the APC gene identified a germline novel heterozygous splice acceptor site mutation

(GenBank Accession: NM_000038). (A) DNA sequence derived from unaffected family member (III-12). (B) The equivalent region from proband (II: 9) arrow showing the novel heterozygous splice acceptor site substitution mutation (c.1744-1G>A).

Figure 4. Schematic diagram of the APC protein structure with functional domains