The APC gene is localized on the long arm of the chromosome 5 in the region 5q21 - q22. It plays a significant role in the cell cycle control, intercellular comunication and contributes to the formation of intercellular junctions. It is the part of Wnt/Wingless signaling pathway where it controls an activity of the β-catenin protein. APC protein forms a complex with GSK3β and axin. This complex is bound with cytoplasmic β-catenin and facilitates its inactivation using proteolytic enzymes.
APC gene mutations are the most common cause of an autosomal dominant disease called familial adenomatous polyposis (FAP).
In the families with FAP there were so far identified more than 800 different mutations of the APC gene. The majority of them lead to the production of shortened - non-functional proteins. This abnormal protein can not attenuate the activity of B - catenin and it leads to the disruption of the cell cycle and to polyp formations from which, after the cascade of some genetic changes, tumors may occur. Polyps and tumors emerge mostly in the colon but may also develop in another place, e.g. in the upper digestive tract, brain or thyroid gland.
A typical feature of the classical FAP phenotype is the development of thousands of adenomatous polyps in the colon and rectum. The less serious FAP (AFAP) form is characterized by the presence of <100 polyps, lower risk of colorectal carcinoma (CRC) and its later onset.
In patients with a negative result of APC it is appropriate to consider the MUTYH gene examination.
Examination
In our laboratory, we perform the detection of the APC gene using PCR amplification and direct sequencing. We also provide the detection of large-scale changes of the APC gene (deletions, duplications) using the method MLPA - CE/FDA with certified diagnostic kit SALSA MLPA KIT P043-C1 APC (MRC Holland).
Clinical sensitivity
Classical FAP is caused by the mutation in the APC gene in ~85%, AFAP in 20% - 30%.
By the sequence analysis it is possible to capture 90 % of mutations and by the method MLPA ~ 10% of large-scale deletions/duplications.
Sequence analysis of coding sequence and flanking intron regions in combination with MLPA analysis covers 95% of known mutations in APC gene.
Analytical sensitivity and specificity of the sequencing of 99%.
The analytical sensitivity of MLPA: 90 % and specificity: 98%.
Limitations:
Mutations deeply in intron and regulatory sequences are not captured. Rare polymorphisms in the annealing site of primers or probes could cause a diagnostic error.
In the case of analysis of somatic mutations by sequencing, mutations will not be detected, as long as the altered cell line is not represented at least 20%.
References
- Yashima K, Nakamori S, Murakami Y, Yamaguchi A, Hayashi K, Ishikawa O, Konishi Y, Sekiya T. Mutations of the adenomatous polyposis coli gene in the mutation cluster region: comparison of human pancreatic and colorectal cancers. Int J Cancer. 1994 Oct 1;59(1):43-7.
- Pagenstecher, C et al., A Complex Rearrangement in the APC Gene Uncovered by Multiplex Ligation-Dependent Probe Amplification. 2007. J Mol Diagn. 9:122-126.
- Giardiello FM, Brensinger JD, Petersen GM. AGA technical review on hereditary colorectal cancer and genetic testing. Available online (registration or institutional access required). 2001; Accessed 9-26-12.
- Laken SJ, Papadopoulos N, Petersen GM, Gruber SB, Hamilton SR, Giardiello FM, et al. Analysis of masked mutations in familial adenomatous polyposis. Proc Natl Acad Sci USA. 1999; 96:2322–2326.
- Aretz S, Stienen D, Uhlhaas S, Pagenstecher C, Mangold E, Caspari R, Propping P, Friedl W. Large submicroscopic genomic APC deletions are a common cause of typical familial adenomatous polyposis. J Med Genet. 2005; 42:185–92.
- Claes K, Dahan K, Tejpar S, De Paepe A, Bonduelle M, Abramowicz M, et al. The genetics of familial adenomatous polyposis (FAP) and MutYH-associated polyposis (MAP). Acta Gastroenterol Belg. 2011; 74(3):421-6.
