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Stepwise deletions of polyA sequences in mismatch repair-deficient colorectal cancers
1Department of Pathology, University of Southern California School of Medicine, Los Angeles, California, USA.
Abstract:
PolyA simple repeat sequence deletions are common in tumors with microsatellite instability (MSI+). Such deletions occur one base at a time in DNA mismatch repair (MMR)-deficient yeast suggesting larger deletions in human MSI+ tumors represent multiple sequential stepwise losses. Sum total deletions in four polyA repeats were variable (between -17 to -45 bp) in 20 sporadic MSI+ colorectal cancers. Progressive but less extensive total deletions (maximum of -12 bp) occurred in similar polyA sequences in MMR-deficient mice (mlh1-/-) up to 478 days old. PolyA repeat lengths were relatively stable but already shortened in the MMR-deficient cell line HCT116. A transgene with 26 A's transfected into HCT116 shortened an average of 3.8 bases pairs after 469 days in culture, less than average deletions of BAT25 (-5.3) or BAT26 (-9.0) in MSI+ cancers. These findings further suggest that extensive polyA deletions common in MSI+ tumors likely reflect multiple stepwise smaller deletions that accumulate more than hundreds of divisions after loss of MMR.
Insights
Microsatellite instability (MSI+) in tumors involves common polyA repeat deletions. These extensive deletions likely result from multiple small, sequential losses accumulating over hundreds of cell divisions after DNA mismatch repair (MMR) deficiency.
Area of Science:
- Genetics
- Molecular Biology
- Cancer Research
Background:
- PolyA simple repeat sequence deletions are frequently observed in tumors with microsatellite instability (MSI+).
- DNA mismatch repair (MMR)-deficient yeast shows single-base deletions, suggesting larger deletions in human MSI+ tumors result from multiple sequential losses.
Purpose of the Study:
- To investigate the mechanism of polyA repeat sequence deletions in MSI+ tumors.
- To compare polyA deletion accumulation in human MSI+ cancers and MMR-deficient mouse models.
Main Methods:
- Analysis of polyA repeat deletions in sporadic MSI+ colorectal cancers.
- Assessment of polyA sequence changes in MMR-deficient mice (mlh1-/-).
- Transfection of a polyA transgene into MMR-deficient HCT116 cells for stability assessment.
Main Results:
- Variable total deletions (-17 to -45 bp) in polyA repeats of MSI+ colorectal cancers.
- Progressive but less extensive deletions (max -12 bp) in MMR-deficient mice.
- PolyA repeat lengths were stable but shortened in HCT116 cells; a transgene showed less shortening than observed in MSI+ tumor markers.
Conclusions:
- Extensive polyA deletions in MSI+ tumors are likely due to multiple stepwise deletions accumulating over numerous cell divisions after MMR loss.
- The findings support a model of cumulative, sequential deletions contributing to genomic instability in MSI+ cancers.