CHAIN-TERMINATING MUTATIONS IN THE APC GENE LEAD TO ALTERATIONS IN APC RNA AND PROTEIN-CONCENTRATION

APC mutations introduce premature stop codons into the open reading fr ame of the gene, leading to the formation of truncated tumor suppresso r proteins. Both RNA and protein levels are likely to be profoundly al tered by such nonsense mutations. To test this hypothesis, Western blo tting and RT-PCR strategies were used to characterize mutant and norma l APC protein and APC RNA concentrations in lymphoblastoid cell lines from 22 unrelated polyposis patients carrying different APC mutations. Variable levels of truncated APC peptides were observed in 14 of 14 c ell lines with APC mutations within exon 15. No truncated APC protein was detected in six of eight cell lines with APC mutations located 5' of exon 15. Mutations located in exon 15 showed mutant RNA underrepres entation in four of eight cell lines, whereas mutations located 5' of exon 15 showed RNA reduction in five of six cell lines. These findings indicate that a two- to threefold decrease in RNA concentration is co mmon when APC alleles carry chain-terminating mutations. They also sug gest that the severe decrease of truncated APC protein observed in som e cell lines is due to mechanisms acting at the protein level. (C) 199 8 Wiley-Liss, Inc.