A hypermutator is a tumour sample with an unusually high mutation count
— far more than is typical for its cancer subtype. In this dataset a sample is flagged
when its z score is greater than 2 (more than two standard deviations above the
cancer’s mean burden).
Biologically, hypermutators usually carry a defect that breaks DNA repair or proofreading
— for example mismatch-repair deficiency (Lynch syndrome), POLE/POLD1
proofreading mutations, or APOBEC over-activity. The cell can’t correct replication
errors, so mutations accumulate orders of magnitude faster than normal.
Clinically these are a distinct group: high mutation load creates many neoantigens, so
hypermutators often respond well to immune checkpoint inhibitors
(e.g. pembrolizumab, nivolumab). They sit outside the standard oncogene/SSL/untreatable
framework but may have very effective alternative options.