The expansion of triplet repeat microsatellite sequences is the molecular correlate of anticipation in a number of rare Mendelian neurodegenerative disorders. This finding prompted us to study these sequences in primary breast cancer in which there is evidence of genetic anticipation. We used a PCR/silver stain method to determine whether triplet-repeat instability (TRI) was present in DNA from malignant breast tumors, and analyzed microsatellite instability (MSI) in triplets SCA1, SCA2, SCA3, SCA6, HD, DRPLA and X25-GAA. We studied 54 consecutive primary breast cancers previously analyzed for dinucleotide instability (DI) at 9 loci. Microsatellite instability (TRI and/or DI) was found in 28/54 (52%) cases, ranging from 0 to 56% in each patient. Dinucleotide instability occurred at ≥2 loci in 19/54 (35%) cases and TRI in 6/54 (11%). Considering single locus instability, we found DI in 26/54 (48%) tumors and TRI in 13/54 (24%). Triplets DRPLA and X25-GAA were most frequently unstable (14% of cases); SCA2 instability was not detected. Interestingly, most tumors with TRI had DI (11/13, 85%). There was a correlation between TRI and DI in the same tumor (42 vs 7% in DI+ and DI− tumors respectively, p=0.0028). Furthermore, TRI appears more frequently associated with lymph node metastases and more advanced clinical stages and more frequent in patients <50 years old, with positive steroidal hormone receptor status, positive p185 and negative p53. These findings are of interest because they demonstrate a relationship between TRI and the clinicopathological characteristics of cancer aggressiveness. Triplet repeat alterations can interfere with gene expression and proteomic functions, which suggests they can play a role in the neoplastic progression of mammary cells. Furthermore, the association of TRI and DI in the same tumor suggests that alterations in the DNA repair gene could culminate in selective phenotypes and breast cancer progression in a considerable number of patients.

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