: Our findings confirm that the majority of CDKN2A-associated melanoma families in Sweden are due to a single founder mutation. They also show that families with the CDKN2A 113insArg mutation have an increased risk not only of multiple melanomas and pancreatic carcinoma but also of breast cancer.

Almost half (48%) of women in southern Sweden with early-onset breast cancer have some family history of breast or ovarian cancer, and 9.0% of early-onset breast cancer cases are associated with a germline mutation in BRCA1 or BRCA2. Mutation carriers were more prevalent among young women, women with at least one first- or second-degree relative with breast or ovarian cancer, and women with bilateral breast cancer.

Introduction Germline mutations in the BRCA1 and BRCA2 genes account for a considerable fraction of familial predisposition to breast cancer. Somatic mutations in BRCA1 and BRCA2 have not been found and the involvement of these genes in sporadic tumour development therefore remains unclear.

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A significant proportion of familial breast cancers cannot be explained by mutations in the BRCA1 or BRCA2 genes. We applied a strategy to identify predisposition loci for breast cancer by using mathematical models to identify early somatic genetic deletions in tumor tissues followed by targeted linkage analysis. Comparative genomic hybridization was used to study 61 breast tumors from 37 breast cancer families with no identified BRCA1 or BRCA2 mutations. Branching and phylogenetic tree models predicted that loss of 13q was one of the earliest genetic events in hereditary cancers. In a Swedish family with five breast cancer cases, all analyzed tumors showed distinct 13q deletions, with the minimal region of loss at 13q21-q22. Genotyping revealed segregation of a shared 13q21 germ-line haplotype in the family. Targeted linkage analysis was carried out in a set of 77 Finnish, Icelandic, and Swedish breast cancer families with no detected BRCA1 and BRCA2 mutations. A maximum parametric two-point logarithm of odds score of 2.76 was obtained for a marker at 13q21 (D13S1308, ‫؍‬ 0.10). The multipoint logarithm of odds score under heterogeneity was 3.46. The results were further evaluated by simulation to assess the probability of obtaining significant evidence in favor of linkage by chance as well as to take into account the possible influence of the BRCA2 locus, located at a recombination fraction of 0.25 from the new locus. The simulation substantiated the evidence of linkage at D13S1308 (P < 0.0017). The results warrant studies of this putative breast cancer predisposition locus in other populations. Breast Cancer Linkage Consortium data on 237 breastovarian cancer families showed that 52% were linked to BRCA1 and 32% to BRCA2 (1). Recent reports indicate that the proportion of breast cancer families attributable to the BRCA1 and BRCA2 genes may be smaller than initially thought, especially in studies that have been based on population-based family materials. For instance, in Finnish breast cancer families with three or more affected cases, mutations of the BRCA1 gene were seen in only 10% and those of BRCA2 in only 11% of the families (2). In southern Sweden, the corresponding percentages were 23% and 11% (3). These studies suggest that in the Nordic populations, a significant proportion of familial breast cancer is not explained by the two known major susceptibility genes. Therefore, identification of additional breast cancer susceptibility genes is an important goal.According to the two-hit model of cancer development (4), hereditary cancers arise as a result of a germ-line mutation in a recessive tumor suppressor gene, followed by the somatic deletion of the wild-type allele of the gene. Somatic deletions detected from tumor tissues of patients with a genetic predisposition therefore may pinpoint those loci that harbor recessive germ-line mutations. Such somatic deletions detected by comparative genomic hybridization (CGH) recently were used to assign the locus for the Peutz-Jeghers' cancer syndrome, an intesti...