Genetic testing by cancer site: stomach

ABSTRACT: Gastric cancer is a global public health concern, ranking as the fourth leading cause of cancer mortality, with a 5-year survival of only 20%. Approximately 10% of gastric cancers appear to have a familial predisposition, and about half of these can be attributed to hereditary germline mutations. We review the genetic syndromes and current standards for genetic counseling, testing, and medical management for screening and treatment of gastric cancer. Recently, germline mutations in the E-cadherin/CDH1 gene have been identified in families with an autosomal dominant inherited predisposition to gastric cancer of the diffuse type. The cumulative lifetime risk of developing gastric cancer in CDH1 mutation carriers is up to 80%, and women from these families also have an increased risk for developing lobular breast cancer. Prophylactic gastrectomies are recommended in unaffected CDH1 mutation carriers, because screening endoscopic examinations and blind biopsies have proven inadequate for surveillance. In addition to this syndrome, gastric cancer risk is elevated in Lynch syndrome associated with germline mutations in DNA mismatch repair genes and microsatellite instability, in hereditary breast and ovarian cancer syndrome due to germline BRCA1 and BRCA2 mutations, in familial adenomatous polyposis caused by germline APC mutations, in Li-Fraumeni syndrome due to germline p53 mutations, in Peutz-Jeghers syndrome associated with germline STK11 mutations, and in juvenile polyposis syndrome associated with germline mutations in the SMAD4 and BMPR1A genes. Guidelines for genetic testing, counseling, and management of individuals with hereditary diffuse gastric cancer are suggested. A raised awareness among the physician and genetic counseling communities regarding these syndromes may allow for increased detection and prevention of gastric cancers in these high-risk individuals.     

Reproductive factors in hereditary breast cancer

Background: An early age at menarche, a short menstrual cycle length, and a high age at first full term pregnancy or nulliparity are known risk factors for breast cancer. These risk factors have previously been reported to differ between breast cancer patients with and without a family history of breast cancer and also between breast cancer patients and controls. Methods: Self-administered questionnaires were filled out by 95 women belonging to 24 families with known BRCA1 mutations, 16 women belonging to nine families with known BRCA2 mutations, and 95 women belonging to 65 families with hereditary breast cancer where no BRCA1 or BRCA2 mutations could be detected. Thirty-nine women were BRCA1 mutation carriers and 56 women were BRCA1 negative, 11 women were BRCA2 carriers and five BRCA2 negative. All women were born between 1905 and 1979. Results: Age at menarche, physiological menstrual cycle length at age 30 or at current age in younger women (when not using oral contraceptives), age at first full term pregnancy, and nulliparity did not significantly differ between BRCA1 mutation carriers and BRCA1 negative women. Too few women were BRCA2 negative to serve as a control group. BRCA2 mutation carriers were therefore compared with BRCA1 negative and BRCA2 negative women. None of the above reproductive factors did significantly differ between BRCA2 mutation carriers and from BRCA1 and BRCA2 families. Women from non-BRCA1/BRCA2 hereditary breast cancer families had a higher age at menarche, but this was no longer significant after adjustment for other factors in a multivariate model. Conclusion: Our results suggest that reproductive risk factors of breast cancer are not related to BRCA1 or BRCA2 carrier status. There was also no indication that these factors differ in carriers of unknown susceptibility genes compared with non-carriers from BRCA1 and BRCA2 families.     

Mutation analysis of the CHK2 gene in families with hereditary breast cancer.

Recently CHK2 was functionally linked to the p53 pathway, and mutations in these two genes seem to result in a similar Li-Fraumeni syndrome (LFS) or Li-Fraumeni-like syndrome (LFL) multi-cancer phenotype frequently including breast cancer. As CHK2 has been found to bind and regulate BRCA1, the product of one of the 2 known major susceptibility genes to hereditary breast cancer, it also more directly makes CHK2 a suitable candidate gene for hereditary predisposition to breast cancer. Here we have screened 79 Finnish hereditary breast cancer families for germline CHK2 alterations. Twenty-one of these families also fulfilled the criteria for LFL or LFS. All families had previously been found negative for germline BRCA1, BRCA2 and TP53 mutations, together explaining about 23% of hereditary predisposition to breast cancer in our country. Only one missense-type mutation, Ile(157)-->Thr(157), was detected. The high Ile(157)--> Thr(157)mutation frequency (6.5%) observed in healthy controls and the lack of other mutations suggest that CHK2 does not contribute significantly to the hereditary breast cancer or LFL-associated breast cancer risk, at least not in the Finnish population. For Ile(157)--> Thr(157)our result deviates from what has been reported previously.     

Incidence of BRCA1 and BRCA2 mutations in 54 Chilean families with breast/ovarian cancer, genotype–phenotype correlations

Summary Our aim was to analyze the incidence of mutations in BRCA1 and BRCA2 genes in 54 families with breast/ovarian cancer. Families were selected from three Institutions following the standard criteria for hereditary breast/ovarian cancer. PCR amplification of all exons was performed, followed by SSCP, heteroduplex, PTT and sequencing analysis. We identified eight truncation mutations, three in the BRCA1 gene and five in the BRCA2 gene. Three of these mutations have not been reported previously by other groups: 308insA in one family, 3936 C>T in two families, for BRCA1, and 4970insTG in one family for BRCA2. In addition two families having Ashkenazi Jewish ancestors present the well known mutations 185delAG and 6174delT. Interestingly, 5 out of 11 families have mutations recurrent in Spanish families. Among the 54 families selected, seven have breast and ovary cancer cases, and only two presented a mutation in BRCA1 or BRCA2 genes. Other cancers as prostate and stomach are frequent among relatives carrying the mutation. Five cases of very early onset (<31 years old) breast cancer were detected. The frequencies of BRCA1 (0.074) and BRCA2 (0.13) mutations in our families is low but similar to the incidence found in other populations, like in Spain. Since is widely known that risk factors that modulate the development of breast cancer such as lifestyle risk factors, geographic location, country of origin and socioeconomic status, besides a familial history of breast cancer our findings suggest that the history of colonization and immigrations is very relevant when studying hereditary factors associated to breast cancer.     

Inherited predisposition to pancreatic adenocarcinoma: role of family history and germ-line p16, BRCA1, and BRCA2 mutations

Susceptibility to pancreatic adenocarcinoma appears to be linked to germ-line mutations in genes causing various familial cancer syndromes. The objectives of this study were to estimate the proportion of unselected pancreatic cancer patients belonging to hereditary cancer syndrome families and to determine the frequency ofp16, BRCA1, BRCA2, hMSH2, and hMLH1 germ-line mutations in patients with a personal or family history of cancer. The study population consisted of 102 patients with histologically verified pancreatic adenocarcinoma, unselected for age, sex, family history, or ethnic origin. Patients completed a family history questionnaire and provided blood for mutation analysis. Three-generation pedigrees were constructed and classified as high risk/familial, intermediate risk/ familial, intermediate risk/nonfamilial, or low risk according to defined criteria. High- and intermediate-risk cases were analyzed for germ-line mutations using a combination of methods. Thirty-eight of 102 (37%) patients were characterized as high or intermediate risk, and the remainder were classified as low risk. Germ-line mutations were identified in five (13%) of these cases [one in p16 (I49S); one in BRCA1 (5382 insC); and three in BRCA2 (6174delT)]. The BRCA1 and BRCA2 mutations were identified in Ashkenazi Jewish patients. Four of the mutation carriers had strong family histories of the syndromes associated with the mutated genes. No mutations were identified in patients in whom the sole risk factor was a family history of pancreatic cancer, and only one mutation was found among patients with early-onset disease. We conclude that known causes of genetic predisposition are an important risk factor in a small proportion of pancreatic cancer patients, especially if associated with a strong family history of syndromes associated with the disease. The lack of detectable germ-line mutations in most high- and intermediate-risk cases suggests that there are probably additional, as yet unidentified genes predisposing to this disease.     

Childhood cancer in families with and without BRCA1 or BRCA2 mutations ascertained at a high-risk breast cancer clinic

BACKGROUND: Germline mutations in the BRCA1 and BRCA2 genes are associated with breast cancer, ovarian cancer and other malignancies. Biallelic mutations of BRCA2 are a cause of Fanconi anemia and characteristic childhood cancers. We undertook this study to evaluate the contribution of familial BRCA mutations to childhood cancer in hereditary breast cancer families. PATIENTS AND METHODS: We compared the prevalence of childhood cancers in 379 families with BRCA1 or BRCA2 mutations and 426 families without mutations. All families were ascertained at a high-risk breast cancer clinic. Our study included first- through fourth-degree relatives of BRCA mutation carriers and cancer-affected individuals with negative testing for BRCA mutations. The primary endpoint was any case of childhood cancer (diagnosed < age 21). RESULTS: 20 cases of childhood cancer occurred in 379 families with BRCA1 or BRCA2 mutations and 35 cases of childhood cancer occurred in 426 families with negative mutation testing (p = 0.12). Nine childhood cancers occurred in 240 families with BRCA1 mutations, and 11 childhood cancers occurred in 141 families with BRCA2 mutations (p = 0.1). 13 of 18 families with childhood cancer and BRCA1 or BRCA2 mutations (72%) and 13 of 31 families with childhood cancer and negative mutation testing (42%) met the Birch criteria for Li-Fraumeni like syndrome (LFL). CONCLUSIONS: In this retrospective analysis, heterozygous BRCA1 and BRCA2 mutations were not a risk factor for childhood cancer in hereditary breast cancer families. These data support the current practice of delaying BRCA mutation testing until adulthood.     

Molecular cloning of BRCA1: a gene for early onset familial breast and ovarian cancer

Summary Molecular analyses allow one to determine genetic lesions occurring early in the development of tumors. With positional cloning approaches we are searching for a gene involved in the development of early onset familial breast and ovarian cancer that maps to human chromosome 17q21 and is termed BRCA1. This involves localizing the region genetically within families with multiply affected members, capturing the region identified by genetic analyses in YACs (yeast artificial chromosomes), converting those YACs to smaller manipulable pieces (such as cosmids), and searching for genes via a variety of approaches such as direct screening of cDNA libraries with genomic clones, direct selection by hybridization, exon trapping, and CpG island rescue. Once identified, candidate genes will be screened for mutations in affected family members in whom breast cancer segregates with the locus on 17q21. The frequency of this gene has been calculated to be 0.0033; from this the incidence of carriers, i.e. those carrying such a predisposition, is one in 150 women. The isolation of BRCA1 and the elucidation of the mutations resulting in breast and ovarian cancer predisposition will allow identification of women who have inherited germ-line mutations in BRCA1. In families known to harbor a germ-line BRCA1 mutation, diagnosis of affected members will be rapid. It is possible that one will also be able to detect alterations of the second copy of this gene early in tumor development in individuals carrying a germ-line mutation. It is not yet known how frequently somatic BRCA1 mutations predispose to breast and ovarian carcinoma in the general female population. If, as in other genetic diseases, new germ-line mutations occur in some women and thus contribute to the development of breast cancer, it may be feasible to screen women in the general population for predisposing mutations. In addition, if acquired genetic mutations of the BRCA1 gene are involved as early events in the development of non-familial forms of the disease, early detection of possible breast carcinoma may become feasible in biopsy of breast tissue.     

The prevalence of BRCA2 mutations in familial pancreatic cancer.

Mutations in the BRCA2 gene have been implicated in pancreatic cancer susceptibility through studies of high-risk breast and ovarian cancer families. To determine the contribution of mutations in BRCA2 to familial pancreatic cancer, we screened affected probands from 151 high-risk families identified through pancreatic cancer clinics for germ-line BRCA2 mutations. Of these families, 118 had two or more first- and second-degree relatives with pancreatic cancer, and an additional 33 had two or more affected second-degree relatives. The average age of onset for pancreatic cancer was 62.8 years. Five BRCA2 truncating mutations were identified, three in families with two or more first- and second-degree relatives with pancreatic cancer. Three of the families with mutations had a history of breast cancer but not ovarian cancer. Four of five families with mutations were identified through probands with early-onset (<55 years) pancreatic cancer. The results of this study were combined with those from a BRCA2 mutation study of 29 other families from the same Johns Hopkins University National Familial Pancreatic Tumor Registry to estimate the frequency of BRCA2 mutations. A total of 10 carriers from 180 families were identified, suggesting that BRCA2 mutations account for 6% of moderate and high-risk pancreatic cancer families.     

BRCA1/BRCA2 rearrangements and CHEK2 common mutations are infrequent in Italian male breast cancer cases

Male breast cancer (MBC) is a rare and poorly known disease. Germ-line mutations of BRCA2 and, to lesser extent, BRCA1 genes are the highest risk factors associated with MBC. Interestingly, BRCA2 germ-line rearrangements have been described in high-risk breast/ovarian cancer families which included at least one MBC case. Germ-line mutations of CHEK2 gene have been also implicated in inherited MBC predisposition. The CHEK2 1100delC mutation has been shown to increase the risk of breast cancer in men lacking BRCA1/BRCA2 mutations. Intriguingly, two other CHEK2 mutations (IVS2+1G>A and I157T) and a CHEK2 large genomic deletion (del9-10) have been associated with an elevated risk for prostate cancer. Here, we investigated the contribution of BRCA1, BRCA2 and CHEK2 alterations to MBC predisposition in Italy by analysing a large series of MBC cases, unselected for breast cancer family history and all negative for BRCA1/BRCA2 germ-line mutations. A total of 102 unrelated Italian MBC cases were screened for deletions/duplications of BRCA1, BRCA2 and CHEK2 by multiplex ligation-dependent probe amplification. No BRCA1, BRCA2 and CHEK2 genomic rearrangements, including the CHEK2 del9-10, were found in the series analysed. Furthermore, none of the MBC cases and 263 male population controls, also included in this study, carried the CHEK2 1100delC, IVS2+1G>A and I157T common mutations. Overall, our data suggest that screening of BRCA1/2 rearrangements is not advantageous in MBC cases not belonging to high-risk breast cancer families and that common CHEK2 mutations play an irrelevant role in MBC predisposition in Italy.     

Population-based estimate of the average age-specific cumulative risk of breast cancer for a defined set of protein-truncating mutations in BRCA1 and BRCA2. Australian Breast Cancer Family Study.

The average breast cancer risk for carriers of a germ-line mutation in BRCA1 or BRCA2 (penetrance) has been estimated from the multiple-case families collected by the Breast Cancer Linkage Consortium (BCLC) to be approximately 80% to age 70. However, women now being tested for these mutations do not necessarily have the intense family history of the BCLC families. Testing for protein-truncating mutations in exons 2, 11, and 20 of BRCA1 and exons 10 and 11 of BRCA2 was conducted in a population-based sample of 388 Australian women with breast cancer diagnosed before age 40. Onset of breast cancer was analyzed in the known and potential mutation-carrying first- and second-degree female relatives of cases found to carry a mutation. Of the 18 mutation-carrying cases (9 BRCA1 and 9 BRCA2), only 5 (1 BRCA1 and 4 BRCA2) had at least one affected relative, so family history of breast cancer was not a strong predictor of mutation status in this setting. The risk in mutation carriers was, on average, 9 times the population risk [95% confidence interval (CI), 4-23; P < 0.001]. Penetrance to age 70 was 40% (95% CI, 15-65%), about half that estimated from BCLC families. By extrapolation, approximately 6% (95% CI, 2-20%) of breast cancer before age 40 may be caused by protein-truncating mutations in BRCA1 or BRCA2. Breast cancer risk in BRCA1 or BRCA2 mutation carriers may be modified by other genetic or environmental factors. Genetic counselors may need to take into account the family history of the consultand.     

Clinical aspects of familial ovarian cancer - current status and issues in Japan

Familial ovarian cancer occurs as part of two genetically distinct syndromes: hereditary breast and ovarian cancer (HBOC) and hereditary nonpolyposis colorectal cancer (HNPCC). HBOC caused by inherited mutations of BRCA1/2 and HNPCC caused by mismatch-repair genes are considered responsible for about 65 to 75% and 10 to 15% of familial ovarian cancers, respectively. Germline mutations of BRCA1 are considered responsible for about 50% of ovarian cancer families and 80% of breast-ovarian cancer families. BRCA2 mutations are less common than BRCA1 mutations in ovarian cancer families. A high proportion of serous adenocarcinomas at an advanced stage has been reported with BRCA-related ovarian cancers in several studies. It is controversial whether BRCA-related ovarian cancer patients carry a better prognosis despite the aggressive tumor-pathological characteristics of their disease, compared to sporadic cases. However, a good therapeutic response may be attributable to platinum-based chemotherapy. Recently in Japan, gene testing of BRCA1/2 has been available as a routine clinical test for diagnosing ovarian cancer families. Because the mutation spectrum of BRCA1/2 in Japanese was different from that of non-Ashkenazi individuals, the clinical application of BRCA1/2 gene testing for Japanese has been advocated. Approximately 1-5% of ovarian cancer patients in Japan are thought to have a family history of breast and/or ovarian cancer. The prevalence of deleterious mutations of BRCA1/2 in Japanese was reportedly significantly higher than that of non-Ashkenazi individuals despite the low frequency of familial cases in Japan. Although the age at diagnosis of ovarian cancers with BRCA1/2 mutation in the United States was earlier than those of the sporadic cases, there were no differences among Japanese. These results suggest that clinical and genetic aspects of BRCA-related ovarian cancer of the Japanese are different from those of Caucasians. A serious issue in this field is how the results will lead to a basis for the clinical application of a cancer prevention strategy targeting BRCA mutation carriers in Japanese.     

Germline mutations of TP53 and BRCA2 genes in breast cancer/sarcoma families

The genetic aetiology of familial aggregations of breast cancer and sarcomas has been elucidated only in part. In this study, 23 unrelated individuals from families with one case of sarcoma and at least one case of breast cancer were screened for mutations in the TP53, BRCA1 and BRCA2 genes. Families were classified according to their conformity to the criteria defining the Li-Fraumeni syndrome (LFS), Li-Fraumeni-like (LFL) syndrome and hereditary breast/ovarian cancer (HBOC). Germline TP53 mutations were identified in three instances (13%), including one LFS and two LFL families, while none of the non-LFS/non-LFL families had a TP53 mutation. Three cases (13%), including one with a TP53 mutation, carried BRCA2 mutations. Of these, two were observed in LFL/HBOC families and the other one in a non-LFS/non-LFL/HBOC family, while none of the non-HBOC families tested positive. These findings suggest that the screening of BRCA2, in addition to TP53, may be appropriate for the molecular characterisation of breast cancer/sarcoma families, with practical implications for counselling and clinical management.     

Heredit?rer Brustkrebs

Breast cancer in women under 40 years old is associated with an elevated risk of cancer also in first degree relatives. The term hereditary breast and ovarian cancer syndrome (HBOC) is derived from the frequent occurrence of both forms of cancer in these families. Approximately 25?C55% of these hereditary cancers can be explained by germline mutations of the susceptibility genes BRCA 1 and BRCA2 and 5?C10% by other syndromes of cancer susceptibility. The remaining diseases are explained by mutations in other genes. BRCA1 and BRCA2 mutations are autosomal dominant with reduced penetrance and coding for tumor suppressor genes. Mutations in these genes often lead to loss of the allele and loss of the second allele leads to loss of function of the corresponding protein and facilitates malignant transformation. Knowledge about genetic reasons has implications for counselling and therapy of individuals seeking advice. Employing individually adjusted measures, early detection can be optimized and the risk of cancer can be decreased. In this article the current data and recommendations with regard to risk of hereditary breast and ovarian cancer are reviewed.     

Prophylactic surgery in hereditary breast/ovarian cancer syndrome

The hereditary breast/ovarian cancer syndrome is responsible for approximately 5% of all breast cancers and 10% of all ovarian cancers. Although this accounts for a small portion of these diseases, much attention has been focused on this syndrome because of the abundance of research in this area. The majority of the hereditary breast/ovarian syndrome can be attributed to germ-line mutations in the BRCA1 and BRCA2 genes. Reliable screening techniques for these mutations have been developed and are readily available in clinical practice. For patients who are thought to have the hereditary breast/ovarian cancer syndrome based on family history or genetic testing, options exist for either intensive screening or prophylactic surgery. This review will discuss the mechanisms by which mutations in the BRCA genes lead to the development of cancer, the limitations of currently available screening techniques, and the efficacy of prophylactic surgery. In general, prophylactic oophorectomy can be performed laparoscopically as an outpatient procedure, carrying as its main drawback the associated consequence of surgical menopause. Prophylactic mastectomy is quite effective in reducing the risk of breast cancer but is a more extensive surgical procedure and results in disfigurement. For any given patient, the best estimates of individual risk of breast or ovarian cancer should be weighed against the benefits of prophylactic surgery and the patient's personal wishes.     

Systemic treatment for hereditary cancers: a 2012 update

The history of specific therapy for hereditary tumors dates back to mid 1980s and involves a number of reports demonstrating regression of familial colon polyps upon administration of sulindac. Virtually no clinical studies on other hereditary cancer types were available until the year 2009, when Byrski et al. presented the data on unprecedented sensitivity of BRCA1-associated breast malignancies to cisplatin. This breakthrough has revived interest to the treatment of cancer in germ-line mutation carriers. Recent trials and clinical observations have confirmed the efficacy of platinating agents and PARP inhibitors in BRCA1/2-driven breast, ovarian and pancreatic carcinomas. Pegylated liposomal doxorubicin may be considered as a promising treatment option for BRCA1/2-related ovarian cancer after the failure of platinum-containing therapy. Several novel drugs have been recently introduced in the management of rare familial tumor syndromes. Vandetanib, a low-molecular weight RET kinase inhibitor, demonstrated substantial efficacy in the treatment of hereditary and sporadic medullary thyroid cancer. Vismodegib, an inhibitor of SMO oncoprotein, caused regression of basal-cell carcinomas in patients with Gorlin syndrome. Down-regulation of mTOR kinase by everolimus has been successfully used for the therapy of subependymal giant-cell astrocytomas in patients with tuberous sclerosis. The achievements in the prevention, diagnostics and treatment of hereditary cancers may serve as an excellent example of triumph of translational medicine.     

Drug therapy for hereditary cancers

Tumors arising in patients with hereditary cancer syndromes may have distinct drug sensitivity as compared to their sporadic counterparts. Breast and ovarian neoplasms from BRCA1 or BRCA2 mutation carriers are characterized by deficient homologous recombination (HR) of DNA, that makes them particularly sensitive to platinum compounds or inhibitors of poly (ADP-ribose) polymerase (PARP). Outstandingly durable complete responses to high dose chemotherapy have been observed in several cases of BRCA-related metastatic breast cancer (BC). Multiple lines of evidence indicate that women with BRCA1-related BC may derive less benefit from taxane-based treatment than other categories of BC patients. There is virtually no reports directly assessing drug response in hereditary colorectal cancer (CRC) patients; studies involving non-selected (i.e., both sporadic and hereditary) CRC with high-level microsatellite instability (MSI-H) suggest therapeutic advantage of irinotecan. Celecoxib has been approved for the treatment of familial adenomatous polyposis (FAP). Hereditary medullary thyroid cancers (MTC) have been shown to be highly responsive to a multitargeted tyrosine kinase inhibitor vandetanib, which exerts specific activity towards mutated RET receptor. Given the rapidly improving accessibility of DNA analysis, it is foreseen that the potential predictive value of cancer-associated germ-line mutations will be increasingly considered in the future studies.     

Müllerian intra-abdominal carcinomatosis in hereditary breast ovarian cancer syndrome: implications for risk-reducing surgery

More than 40 years ago Lynch et al. described several multigenerational breast cancer family pedigrees which demonstrated autosomal dominant inheritance of a trait(s) that increased risks for both breast and ovarian cancers. Mutation carriers in at least 90 % of these hereditary breast ovarian cancer (HBOC) syndrome families have been linked to cancer-associated mutations in the genes BRCA1 and BRCA2. This review focuses on the contributions of Lynch, colleagues and collaborators and pertinent literature, toward defining the HBOC syndrome, the cancer risks that the inherited adverse mutations convey, the gynecologic tissues and organs from which the malignancy may arise to disseminate throughout the pelvic and abdominal organs and peritoneum and how this information can be used to reduce the risk and morbidities of intra-abdominal carcinomatosis in effected individuals.     

Frequent somatic loss of BRCA1 in breast tumours from BRCA2 germ-line mutation carriers and vice versa.

Breast cancer susceptibility genes BRCA1 and BRCA2 are tumour suppressor genes the alleles of which have to be inactivated before tumour development occurs. Hereditary breast cancers linked to germ-line mutations of BRCA1 and BRCA2 genes almost invariably show allelic imbalance (AI) at the respective loci. BRCA1 and BRCA2 are believed to take part in a common pathway in maintenance of genomic integrity in cells. We carried out AI and fluorescence in situ hybridization (FISH) analyses of BRCA2 in breast tumours from germ-line BRCA1 mutation carriers and vice versa. For comparison, 14 sporadic breast tumours were also studied. 8 of the 11 (73%) informative BRCA1 mutation tumours showed AI at the BRCA2 locus. 53% of these tumours showed a copy number loss of the BRCA2 gene by FISH. 5 of the 6 (83%) informative BRCA2 mutation tumours showed AI at the BRCA1 locus. Half of the tumours (4/8) showed a physical deletion of the BRCA1 gene by FISH. Combined allelic loss of both BRCA1 and BRCA2 gene was seen in 12 of the 17 (71%) informative hereditary tumours, whereas copy number losses of both BRCA genes was seen in only 4/14 (29%) sporadic control tumours studied by FISH. In conclusion, the high prevalence of AI at BRCA1 in BRCA2 mutation tumours and vice versa suggests that somatic events occurring at the other breast cancer susceptibility gene locus may be selected in the cancer development. The mechanism resulting in AI at these loci seems more complex than a physical deletion.