Inactivation of promoter 1B of APC causes partial gene silencing: evidence for a significant role of the promoter in regulation and causative of familial adenomatous polyposis

Abstract

Familial adenomatous polyposis (FAP) is caused by germline mutations in the adenomatous polyposis coli (APC) gene. Two promoters, 1A and 1B, have been recognized in APC, and 1B is thought to have a minor role in the regulation of the gene. We have identified a novel deletion encompassing half of this promoter in the largest family (Family 1) of the Swedish Polyposis Registry. The mutation leads to an imbalance in allele-specific expression of APC, and transcription from promoter 1B was highly impaired in both normal colorectal mucosa and blood from mutation carriers. To establish the significance of promoter 1B in normal colorectal mucosa (from controls), expression levels of specific transcripts from each of the promoters, 1A and 1B, were examined, and the expression from 1B was significantly higher compared with 1A. Significant amounts of transcripts generated from promoter 1B were also determined in a panel of 20 various normal tissues examined. In FAP-related tumors, the APC germline mutation is proposed to dictate the second hit. Mutations leaving two or three out of seven 20-amino-acid repeats in the central domain of APC intact seem to be required for tumorigenesis. We examined adenomas from mutation carriers in Family 1 for second hits in the entire gene without any findings, however, loss of the residual expression of the deleterious allele was observed. Three major conclusions of significant importance in relation to the function of APC can be drawn from this study; (i) germline inactivation of promoter 1B is disease causing in FAP; (ii) expression of transcripts from promoter 1B is generated at considerable higher levels compared with 1A, demonstrating a hitherto unknown importance of 1B; (iii) adenoma formation in FAP, caused by impaired function of promoter 1B, does not require homozygous inactivation of APC allowing for alternative genetic models as basis for adenoma formation.

APC promoter 1B deletion in seven American families with familial adenomatous polyposis

“…The MLPA analysis reveals that this promoter 1B mutation is distinct from that reported in families from Sweden (6); we would expect the Swedish mutation to show a deletion of only the 5′ most (260nt) probe by MLPA. Our reported deletion at approximately 34 kb is smaller than the 132 kb deletion from Finland (7).…”

Epigenetic regulation of APC in the molecular pathogenesis of gallbladder cancer

“…Both somatic10 and germline11 mutations of APC play a key role in familial adenomatous polyposis (FAP) and colorectal cancer. Inactivation of 1B has been observed in FAP due to deletion mutation in the promoter region12. Lower frequency of APC promoter methylation was earlier reported in Chilean GBC patients1314.…”

The Mystery of Mismatch Repair Deficiency: Lynch or Lynch-like?

“…Our understanding gene promoter function is still primitive. For example, loss of portions of the APC promoter 1B, which is almost 55 kb from the start site of the gene, causes familial adenomatous polyposis 6 . Who knows what might be going on in the promoters of the DNA MMR genes?…”