Loss of Heterozygosity on the Long Arm of Chromosome 6 in Breast Cancer: Possibly Four Regions of Deletion1

Deletions and rearrangements involving chromosome 6q have been reported in a number of human cancers such as ovarian and breast tumors as well as melanoma and hemopoietic malignancies. To gain insight into the regions undergoing deletions on the long arm of chromosome 6, we performed a survey ofloss of heterozygosity (LOH) at 1 1 CA repeat markers, mapping from 6q13 to 6q27 in 83 matched sets of tumor and blood DNAs from breast tumor patients. LOH was observed at all tested markers with frequencies ranging from 11.4 to 39.8% of the informative cases, whereas D2S123, a marker linked to the HMSH2 gene mapping in a region rarely presenting allele losses, showed 3.2%. LOH patterns were often complex, with a number of tumors presenting multiple interstitial losses, thus indicating that chromosome 6q can be severely rearranged in breast cancer. Patterns of losses and correlation between LOH occurring at adjacent markers suggested the existence of three (possibly four) distinct regions of allele losses. These were defined by: D6S251-D6S434 (6q13), D6S292-D6S310-D6S314-D6S311 (6q24-q25), and D6S441-D6S281 (6q27). The fourth and more hypothetical region was in the 6q21 region and defined by D6S287-D6S407. Interestingly, the region at 6q24-q25 defined by D6S292-D6S310-D6S314-D65311 was predominantly observed in evolved and aggressive breast tumors. LOH at D6S314 was correlated with PR tumors. All together, our data suggest the possible presence of several genes on 6q whose alteration may play a role in breast cancer formation and development. INTRODUCTION It is now well accepted that cancer progression builds up on the accumulation of genetic alterations in tumor cells. Identified Received 1/29/96: revised 5/15/96: accepted 5/I 7/96. I This work was supported by funds from the Fondation Nationale des Centres de Lutte Contre Ic Cancer and the Ligue Contre le Cancer, Comit#{233}de l’H#{233}rault. C. N. was supported in part by the European Economic Community and a short-term studentship from the Ligue Contre le Cancer, Comit#{233} de l’H#{233}rault. 2 To whom requests for reprints should be addressed, at lnstitut de G#{233}n#{233}tique Mol#{233}culaire de Montpellier, UMR CNRS 9942, 1919 route de Mende, 34033 Montpellier Cedex 1 , France. Phone: 33.67 61 36 31: Fax: 33.67 04 02 31; E-mail: [email protected]. consequences are due to either oncogene activation or loss of function of a tumor suppressor gene. According to Knudson’s two-hit model, inactivation of an antioncogene requires a biphasic process to eliminate both alleles. Most frequently one of these two events involves the loss of one allele due to chromosomal deletion ( 1 ). Homozygous deletions have been observed but seem to occur more rarely. Hence, the search for chromosomal deletions by means of LOH3 at polymorphic markers has yielded valuable data on the localizations of potential tumor suppressor genes. Up to 14 chromosomal arms (Ip, lq, 3p, 6q, 7q, 8p, 1 lp, 1 lq, l3q, 16q, l7p, l7q, 18q, and 22q) present LOH in breast cancer, and some of these have been shown to bear multiple regions of deletion (2, 3). Deletions on chromosome 6q were first reported in ovarian carcinoma (4-6). Additional results presenting partial losses on this chromosomal arm in breast tumors (7, 8), B-cell nonHodgkin’s lymphoma (9), acute T-cell lymphocytic leukemia ( 1 0), and melanoma ( I 1 ) came in support of the existence of one or several tumor suppressor genes on 6q. This hypothesis was further substantiated by chromosome transfer of the long arm of chromosome 6, which resulted in the reversion of the tumorigenicity of human melanoma cells (12). Studies on ovarian carcinoma pointed toward the existence of two sites of deletion at 6q12-q23 and at 6q24-q27 (13, 14), whereas LOH analyses on breast cancer were consistent with deletions centered around the MYB gene (6q24) extending distally to D6S37 (6q26-27; Ref. 8). These data prompted us to study allele losses at I I CA repeat polymorphic markers spanning from 6ql3-14 to 6q27. To this means, we analyzed 83 pairs of tumor and cognate PBL DNAs from sporadic breast cancer patients. Results show complex patterns of LOH, which suggest that the alteration of several genes in this region may be involved in breast cancer