APC and the three-hit hypothesis.

Area of Science:

• Oncology
• Cancer Genetics
• Molecular Biology

Background:

• The 'two-hit hypothesis' traditionally assumes bi-allelic tumor suppressor gene (TSG) mutations lead to complete loss of protein function.
• For the Adenomatous Polyposis Coli (APC) gene, mutant proteins may retain partial function, with two hits co-selected for optimal Wnt activation.
• Tumor progression may involve altered Wnt signaling, potentially selecting for more than two hits at the APC locus.

Purpose of the Study:

• To investigate whether colorectal tumors acquire more than two genetic hits at the APC locus during progression.
• To determine the mechanisms and consequences of additional APC hits on Wnt signaling and tumor growth.

Main Methods:

• Comprehensive screening of colorectal cancer (CRC) cell lines and primary CRCs for alterations at the APC locus.
• Analysis of third hits, including copy number variations and protein-truncating mutations.
• Assessment of the impact of third hits on APC mRNA and protein levels.

Main Results:

• Some colorectal cancers (CRCs) possess third hits at the APC locus, primarily copy number gains/deletions or truncating mutations.
• Third hits were less frequent when the second APC hit involved copy-neutral loss of heterozygosity.
• These additional hits altered APC mRNA and protein levels, impacting Wnt signaling and potentially tumor growth.

Conclusions:

• The two-hit model requires modification for the APC TSG in colorectal cancer, as additional hits can be selected.
• Repeated targeting of the APC gene, beyond two hits, can occur during tumor progression, influencing Wnt pathway activity.
• This suggests that in some cancers, stepwise progression involves repeated targeting of the same gene or pathway.