The development of molecular studies was triggered in 1991 by the discovery of the relationship between APC mutations and FAP. Over the past 11 years, the development of various techniques followed by increased numbers of publications has occurred in a growing number of countries and ethnic groups.
Investigations performed over several years using such techniques as single-stranded conformational polymorphism (SSCP) analysis, heteroduplex (HD) analysis or denaturing gradient gel electrophoresis (DGGE) method and more latterly DNA sequencing allowed the establishment of mutation databases. At the present time, the best
APC gene mutation database resides at the Institute of Medical Genetics in Cardiff (
http://www.hgmd.cf.ac.uk/ac/index.php) as it provides a comprehensive list of mutations for the majority of reported genes [
24]. At present, information exists for 858 different
APC mutations have been reported to the HGMD database. Small deletions constitute the highest proportion of
APC mutations. A total of 356 mutations have been described and, in the majority, they lead to a change in the reading frame and the creation of a premature termination codon. Much rarer are large deletions (54), splice site mutations (49), small deletions combined with insertions (17), large insertions (7), complex rearrangements (6) and 3 mutations were observed in regulatory sequences. 235 mutations were represented as substitutions leading to changes in amino acids or to the development of the stop codon at the mutation site. The total of 131 small insertions have been described so far. In the case of substitutions, which constitute 30% of all mutations, the stop codon arises at the mutation site, whereas in the case of deletions or insertions, which constitute 68% of mutations, the STOP codon occurs downstream of the mutation as a result of a shift in the reading frame. There is a region in the gene characterised by an increased frequency of mutations called the MCR (
mutation cluster region) which encompasses codons 1055–1309 and 23% of all germline mutations occur in this region. In addition, among germline mutations, an increased frequency of the following three mutations has been observed: deletions of 5 bp in codone 1309 (10%), deletions of 5 bp in codone 1061 and deletions of 4 bp in codone 1064 [
18]. The majority of mutations can be found in the 5’ region of exon 15 of the
APC gene. A characteristic FAP attribute is loss of heterozygosity (LOH) in the
APC in adenomas developing as a result of somatic mutation in the second allele. The frequency distribution of these mutations differs from germline mutations. Up to 60% of somatic mutations are situated in the fragment, which encompasses 8% of the gene between codons 1286 and 1513. Somatic mutations occur in two hot spots in codon 1309 and codon 1450. Mutations in
APC also occur in cases of large intestinal tumours unrelated with polyposis. In “sporadic” colorectal cancer (CRC) these neoplasms occur as a result of a mutation in one of the
APC alleles followed by a loss of heterozygosity as a result of somatic mutations. In the case of the Lynch syndrome (HNPCC), the process is accelerated by inherited mutations in DNA repair genes [
25]. According to the latest investigations, it is not necessary for LOH to occur in
APC in order for the initiation of colorectal cancer to take place. In half of the sporadic cases of CRC in which no changes in
APC occur, tumour development is associated with heterozygote mutations in the β-catenin gene (
CTNNB1) [
26]. These mutations result in the switching off of the APC regulatory function leading to β-catenin accumulation and, hence, expression of the
MYC gene and development of CRC.
Attempts have been made to determine whether a genotype phenotype correlation exists that can be associated with the clinical course of the disease. It has been reported that the occurrence of the stop codon upstream of codon 157 is associated with a decreased number of polyps and a milder disease course. A typical disease course associated with hundreds to thousands of adenomas can be expected when the stop signal occurs between codons 169 and 1600. Mutations found between codons 1403 and 1587 lead to the diversification of symptoms that include osteomas, desmoid tumours and CHRPEs. The occurrence of desmoid tumours is also associated with mutations in the region between codons 1445 and 1578. Mutations at the 3’ end of APC result in a differing picture of disease both with regard to polyp numbers and the occurrence of extra-colonic symptoms.