A high level of genetic instability might cause mutations to accumulate in tumours. Microsatellite instability (MSI), due to defects of the DNA mismatch repair system, affects in particular repeat sequences (microsatellites) scattered throughout the genome. By scanning transcriptome databases, we found that microsatellites in the human genome are less numerous in coding DNA than in the 3′-untranslated region (UTR), known to mediate control of gene expression. By mutation analysis, we identified a 1 bp deletion in a (T)8 microsatellite embedded in the 1801 nucleotide long 3′-UTR of CEACAM1 gene, thought to be involved in tumour onset and progression. By Lentiviral Vector-mediated gene transfer, we showed that the wild-type but not the mutated CEACAM1 3′-UTR greatly decreased transgene expression at both mRNA and protein level. Messenger RNA abundance was fully regulated by the most 3′ region of CEACAM1 3′-UTR. This region includes the (T)8 microsatellite but not any known classified regulatory element. These data show that CEACAM1 3′-UTR contains non-canonical elements contributing to mRNA regulation, among which a short repeat sequence could play a critical regulatory function. This suggests that, in cancer cells, a single mutation in a 3′-UTR short microsatellite might strongly affect gene expression.
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