"The CUX1 gene has been characterized genetically as a haploinsufficient tumour suppressor gene. Paradoxically, CUX1 overexpression is observed in the majority of human cancers and is associated with shorter patient survival. While many transcriptional targets may explain how CUX1 overexpression promotes tumour development and/or progression, the biochemical function(s) of CUX1 that explain its role in tumour suppression remain(s) to be defined. Recent studies have established a role for the p200 CUX1 protein as an auxiliary factor in DNA repair. Specifically, the CUT domains within CUX1 were found to stimulate the enzymatic activities of three enzymes of the base excision repair pathway. This new function of CUX1 led us to hypothesize that diminished CUX1 expression may impair DNA repair efficiency and accelerate the accumulation of mutations, thereby increasing the risk of tumour initiation. As a first test of this hypothesis, we sought to measure the effect of CUX1 knockdown on mutation frequency using the hypoxanthine phosphoribosyltransferase (HPRT) mutation assay on the human lymphoblast cell line TK6. In multiple independent experiments we observed an increase in mutation frequency upon CUX1 knockdown. The increase in mutation frequency was exacerbated by treatment of cells with ionizing radiation. Using reverse-transcription polymerase chain reaction (RT-PCR), we analyzed the mutation spectrum of 135 HPRT-negative mutant clones. Consistent with the role of CUX1 in base excision repair, we observed an increase in the number of point mutations in the irradiated cell population following CUX1 knockdown. Surprisingly, however, we observed a greater increase in the number of large DNA deletions and rearrangements in both irradiated and non-irradiated cells. This finding suggests that CUX1 may also be involved in another DNA repair pathway." --