Apc gene-mutations in familial adenomatous polyposis-coli identified at the DNA and protein level

Peripheral blood cell DNA of patients suffering from Familial Adenomatous Polyposis coli (FAP) were subjected to SSCP screening analyses. Here, we report on a patient, who was diagnosed at the age of 31 years suffering from a severe form of adenomatous polyposis coli. The germline mutation was identified to result from a 5-bp germline microdeletion surrounding Adenomatous Polyposis Coli (APC) codon 1309. Examination of DNA derived from pooled adenomas of the same patient confirmed the inherited mutation. Western blot analysis with an APC N-terminus specific antiserum (anti-APC123) applied to tumor-derived proteins identified a polypeptide chain of 140 kD corresponding to the truncation induced by the APC germline mutation. We further examined DNA isolated individually from eight adenomas of the same patient. The analysis revealed the surprising observation, that three out of eight tumor samples analyzed, gave rise to a predominant PCR-amplified band of normal size, implicating that the allele containing the germline deletion had been lost, and instead hemi-/homozygosity for the somatic, not for the germline mutation had been aquired as a tertiary event. A similar mutational mechanism has been reported for the Rb-1 tumor suppressor gene in the retinoblastoma model.