Two-hit inactivation of FHIT by loss of heterozygosity and hypermethylation in breast cancer.

Purpose: The FHIT gene, which spans the FRA3B fragile site at chromosome 3p14.2, is a candidate tumor suppressor gene in breast carcinomas. In this study, we would like to delineate more precisely its role in breast tumorigenesis.

Experimental Design: To confirm the tumorigenic role of FHIT, 46 sporadic invasive ductal carcinomas of the breast were tested for the "two hits" required to inactivate this gene. Microsatellite loss of heterozygosity (LOH) was considered as the first hit. To examine the possibility that hypermethylation serves as the second hit for FHIT inactivation, methylation of 5'-CpG islands of FHIT was analyzed by methylation-specific PCR.

Results: LOH was detected in 8 of 40 informative tumors, and hypermethylation was observed in 22 of 46 (48%) cases. Aberrant FHIT protein expression was found in 31 of 46 (67%) cases examined. All seven tumors showing both LOH and hypermethylation showed complete loss of Fhit protein expression. In addition, a significant positive association was found between the existence of LOH and 5'-CpG island hypermethylation (P = 0.04), which was consistent with the two-hit model.

Conclusions: To our knowledge, this study provides the first evidence that biallelic inactivation of FHIT by LOH and hypermethylation leads to the complete inactivation of FHIT gene in patients with breast cancer. Silencing of the FHIT gene by promoter hypermethylation occurs in primary breast carcinomas, especially those with LOH. These findings support a role for this tumor suppressor gene in sporadic breast tumorigenesis.