RESUMO
INTRODUCTION: A significant proportion of high-risk breast cancer families are not explained by mutations in known genes. Recent genome-wide searches (GWS) have not revealed any single major locus reminiscent of BRCA1 and BRCA2, indicating that still unidentified genes may explain relatively few families each or interact in a way obscure to linkage analyses. This has drawn attention to possible benefits of studying populations where genetic heterogeneity might be reduced. We thus performed a GWS for linkage on nine Icelandic multiple-case non-BRCA1/2 families of desirable size for mapping highly penetrant loci. To follow up suggestive loci, an additional 13 families from other Nordic countries were genotyped for selected markers. METHODS: GWS was performed using 811 microsatellite markers providing about five centiMorgan (cM) resolution. Multipoint logarithm of odds (LOD) scores were calculated using parametric and nonparametric methods. For selected markers and cases, tumour tissue was compared to normal tissue to look for allelic loss indicative of a tumour suppressor gene. RESULTS: The three highest signals were located at chromosomes 6q, 2p and 14q. One family contributed suggestive LOD scores (LOD 2.63 to 3.03, dominant model) at all these regions, without consistent evidence of a tumour suppressor gene. Haplotypes in nine affected family members mapped the loci to 2p23.2 to p21, 6q14.2 to q23.2 and 14q21.3 to q24.3. No evidence of a highly penetrant locus was found among the remaining families. The heterogeneity LOD (HLOD) at the 6q, 2p and 14q loci in all families was 3.27, 1.66 and 1.24, respectively. The subset of 13 Nordic families showed supportive HLODs at chromosome 6q (ranging from 0.34 to 1.37 by country subset). The 2p and 14q loci overlap with regions indicated by large families in previous GWS studies of breast cancer. CONCLUSIONS: Chromosomes 2p, 6q and 14q are candidate sites for genes contributing together to high breast cancer risk. A polygenic model is supported, suggesting the joint effect of genes in contributing to breast cancer risk to be rather common in non-BRCA1/2 families. For genetic counselling it would seem important to resolve the mode of genetic interaction.
Assuntos
Neoplasias da Mama/genética , Predisposição Genética para Doença/genética , Genoma Humano/genética , Estudo de Associação Genômica Ampla/métodos , Proteína BRCA1/genética , Proteína BRCA2/genética , Mapeamento Cromossômico , Cromossomos Humanos Par 14/genética , Cromossomos Humanos Par 2/genética , Cromossomos Humanos Par 6/genética , Saúde da Família , Feminino , Haplótipos , Humanos , Islândia , Escore Lod , Masculino , Modelos Genéticos , Herança Multifatorial , LinhagemRESUMO
BACKGROUND: Chromosomal aberrations in breast tumors from BRCA1 and BRCA2 germ-line mutation carriers are considerably more frequent than what is seen in sporadic breast tumors. According to Comparative Genomic Hybridisation analysis (CGH), deletions on chromosome 4 are one of the most frequent events in BRCA1-associated tumors, suggesting inactivation of specific tumor suppressor genes. MATERIALS AND METHODS: In the present study, 16 microsatellite markers covering chromosome 4 were used to map loss of heterozygosity (LOH) in tumors from BRCA1 (n=41) as well as in tumors from BRCA2 (n=66) mutation carriers and in tumors from unselected cases of breast cancer (n =68). RESULTS: The frequency of LOH in these groups ranged from 16-73% in BRCA1-associated tumors, 13-42% in BRCA2-associated tumors and 8-33% in unselected tumors. LOH was significantly more frequent in BRCA1-associated tumors as compared to BRCA2-associated tumors and unselected tumors, and particularly high (over 70%) at 4q35.2. Pathological variables that were found significantly associated (p< or =0.05) with LOH at specific markers were: high percentage of cells in S-phase, negative estrogen receptor status, young age at diagnosis and large tumors. Deletion mapping indicates the existence of seven non-overlapping regions at chromosome 4, which were identified in all three groups of tumors. Three of these seven regions, 4p16.3-p16.1, 4q27-q32.1 and 4q35.1-4qter, have not been reported in breast cancer previously. CONCLUSION: The results manifest the frequent alterations of chromosome 4 in BRCA1-associated breast tumors and indicate the location of several genes of potential importance in breast cancer development.
