MUTATION ANALYSIS OF THE THRA1 GENE IN BREAST-CANCER - DELETION FUSION OF THE GENE TO A NOVEL SEQUENCE ON 17Q IN THE BT474 CELL-LINE

We have previously described a common region of deletion and allele lo ss on chromosome 17q in sporadic breast cancers that is likely to cont ain a tumor suppressor gene. The region, mapped to 17q12-q21, was bord ered by D17S250 and D17S579 on the centromeric and telomeric sides, re spectively. This deletion region overlaps the BRCA1 locus, which predi sposes to familial breast and ovarian cancer. The most frequent loss o f heterozygosity was observed at the thyroid hormone receptor alpha (T HRA1) locus. Southern analysis revealed a rearrangement of THRA1 in th e BT474 breast cancer cell line. This rearrangement represented a dele tion of exons 8-10 of one THRA1 allele that was also coamplified with ERBB2. Northern blots showed two mutant transcripts in BT474 cells. An alysis of the mutant transcripts revealed fusion of the THRA1 exon 7 b y splicing to a novel sequence designated BTR for ''BT474 transcribed rearrangement.'' BTR was found to be highly conserved and mapped to 17 q. The deletion in BT474 cells spans the entire BRCAI region. To searc h for additional mutations in the THRA1 gene, all nine protein-encodin g exons of THRA1 were examined for point mutations via single strand c onformation analysis in a series of primary breast tumors, breast canc er cell lines, and lymphoblastoid cell lines derived from the youngest affected members of several German breast cancer families. No point m utations were detected, including the unrearranged THRA1 allele in BT4 74. We have thus excluded THRA1 as a commonly mutated sporadic breast cancer tumor suppressor gene and as the BRCA1 gene.