targets and modifies the chromatin architecture at the ERCC1 gene, reducing its expression. Hypermethylation of the promoter for let-7a miRNA reduces its expression and this allows hyperexpression of HMGA2. Hyperexpression of HMGA2 can then reduce expression of ERCC1. Thus, there are three mechanisms that may be responsible for the low level of protein expression of ERCC1 in 84% to 100% of sporadic colon cancers. From results in gliomas and in cadmium carcinogenesis, methylation of the ERCC1 promoter may be a factor. One or more miRNAs that repress ERCC1 may be a factor. And epigenetically reduced let-7a miRNA allowing hyperexpression of HMGA2 could also reduce protein expression of ERCC1 in colon cancers. Which epigenetic mechanism occurs most frequently, or whether multiple epigenetic mechanisms reduce ERCC1 protein expression in colon cancers has not been determined. Accelerated aging DNA repair-deficient Ercc1 mutant mice show numerous features of accelerated aging, and have a limited lifespan. Accelerated aging in the mutant involves various organs. Ercc1 mutant mice are deficient in several DNA repair processes including transcription-coupled DNA repair. This deficiency prevents resumption of RNA synthesis on the template DNA strand subsequent to it receiving a transcription-blocking DNA damage. Such blockages of transcription appear to promote premature aging, particularly in non-proliferating or slowly proliferating organs such as the nervous system, liver and kidney (see DNA damage theory of aging). When Ercc1 mutant mice were subjected to dietary restriction their response closely resembled the beneficial response to dietary restriction of wild-type mice. Dietary restriction extended the lifespan of the Ercc1 mutant mice from 10 to 35 weeks for males and from 13 to 39 weeks for females. It appears that in Ercc1 mutant mice dietary restriction while delaying aging also attenuates accumulation of genome-wide DNA damage and preserves transcriptional output, likely contributing to improved cell viability. Spermatogenesis and oogenesis Both male and female Ercc1-deficient mice are infertile. The DNA repair function of Ercc1 appears to be required in both male and female germ cells at all stages of their maturation. The testes of Ercc1-deficient mice have an increased level of 8-oxoguanine in their DNA, suggesting that Ercc1 may have a role in removing oxidative DNA damages. Notes References Further reading External links GeneReviews/NIH/NCBI/UW entry on Xeroderma Pigmentosum