MISMATCH REPAIR IN ESCHERICHIA-COLI ENHANCES INSTABILITY OF (CTG)(N) TRIPLET REPEATS FROM HUMAN HEREDITARY-DISEASES

Long CTG triplet repeats which are associated with several human hered itary neuromuscular disease genes are stabilized in ColE1-derived plas mids in Escherichia coli containing mutations in the methyl-directed m ismatch repair genes (mutS, mutL, or mutH). When plasmids containing ( CTG)(180) were grown for about 100 generations in mutS, mutL, or mutH strains, 60-85% of the plasmids contained a full-length repeat, wherea s in the parent strain only about 20% of the plasmids contained the fu ll-length repeat. The deletions occur only in the (CTG)(180) insert, n ot in DNA flanking the repeat. While many products of the deletions ar e heterogeneous in length, preferential deletion products of about 140 , 100, 60, and 20 repeats were observed. We propose that the E. coli m ismatch repair proteins recognize three-base loops formed during repli cation and then generate long singlet stranded gaps where stable hairp in structures may form which can be bypassed by DNA polymerase during the resynthesis of duplex DNA. Similar studies were conducted with pla smids containing CGG repeats; no stabilization of these triplets was f ound in the mismatch repair mutants. Since prokaryotic and human misma tch repair proteins are similar, and since several carcinoma cell line s which are defective in mismatch repair show instability of simple DN A microsatellites, these mechanistic investigations in a bacterial cel l may provide insights into the molecular basis for some human genetic diseases.