METHYLATION OF THE HIC-1 CANDIDATE TUMOR-SUPPRESSOR GENE IN HUMAN BREAST-CANCER

HIC-1 (hypermethylated in cancer) is a candidate tumor suppressor gene which is located at 17p13.3, a region which frequently undergoes alle lic loss in breast and other human cancers, HIC-1 is proposed to be co mmonly inactivated in human cancers by hypermethylation of a normally unmethylated dense CpG island which encompasses the entire gene, To st udy whether HIC-1 inactivation may be important to the development of breast cancer, we first measured methylation of the HIC-1 gene in norm al breast ductal tissues from microdissected frozen breast tissues and from epithelial cells purified from mammoplasty specimens. Surprising ly, in all normal breast ductal tissues we found approximately equal a mounts of densely methylated HIC-1 and completely unmethylated HIC-1, This is in contrast to most normal tissues, in which all copies of HIC -1 are completely unmethylated. We then evaluated 39 primary breast ca ncer tissues and found virtually complete methylation of the HIC-1 gen e in 26 (67%) of the cases. We also found loss of heterozygosity at th e telomeric portion of chromosomal arm 17p in 22 of the 26 cases with strongly methylated HIC-1, suggesting that loss of an unmethylated HIC -1 allele mag contribute to the inactivation of HIC-1 in cells with a pre-existing methylated allele, Finally, by RNase protection analysis, HIC-1 was found to be expressed in microdissected normal breast ducta l tissues and unmethylated tumors but not in tumors with hypermethylat ion of the HIC-1 gene. These results indicate that hypermethylation of HIC-1 and associated loss of HIC-1 expression is common in primary br east cancer. Furthermore, the HIC-1 gene is densely methylated in appr oximately one-half of the alleles in normal breast epithelium, which m ay predispose this tissue to inactivation of this gene by loss of hete rozygosity.